jd_thesis

Formation and evolution of the Sudbury

impact melt sheet

James R. Darling

A dissertation submitted to the University of Bristol in accordance

with the requirements of the degree of Doctor of Philosophy

in the Faculty of Science.

Department of Earth Sciences

University of Bristol

January 2010

Advisors

Prof. Chris Hawkesworth

Dr. Craig Storey

Dr. Peter Lightfoot (Vale Inco)

Word count

ca. 35000

c© James R. Darling

All Rights Reserved, 2010

Abstract

A unique terrestrial large impact melt sheet is located within the Sudbury Structure in

Ontario, Canada. This study addresses problems relating to the formation and subse-

quent evolution of the melt sheet, by characterising isotopic variations throughout the

various igneous units present.

The Pb isotopic compositions of sulphides and feldspars from a range of rock types

and sulphide ores were measured by MC-ICPMS. Significant and systematic vertical and

lateral isotopic heterogeneity has been identified throughout the Main Mass of the melt

sheet, as well as between early formed Offset Dykes. These findings demonstrate that

the melt sheet was a dynamic system, with chemical variability present from shortly

after the impact event through to the final products of silicate crystallisation, despite

vigorous thermal convection.

Importantly, the Pb isotope systematics of Offset Dyke quartz diorites, together

with trace element and Sr-Nd isotope data, demonstrate that such heterogeneity can be

accounted for by mixing of locally exposed target lithologies. As such, and in contrast to

many impact model predictions, an upper to mid crustal origin for the igneous complex is

apparent. The results have significant implications for the understanding of melt sheet-

footwall interactions, as well as the origins of Sudbury’s vast Ni-Cu-PGE sulphide ore

deposits.

The melt sheet also offers a unique analogue for such features on the early Earth.

Characterisation of the trace element compositions and inclusion populations of zircons

from throughout the melt sheet stratigraphy revealed that large variations in zircon

crystallisation temperature and composition will be an inevitable consequence of the

evolution of such magmatic systems. Furthermore, it is clear that zircons in mafic

rocks crystallise in residual liquids of granitic composition. The findings have significant

implications for the interpretation of the Hadean detrital zircon record.

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0 Abstract

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Author’s Declaration

I declare that the work in this dissertation was carried out in accordance with the re-

quirements of the University’s Regulations and Code of Practice for Research Degree

Programmes and that it has not been submitted for any other academic award. Except

where indicated by specific reference in the text, the work is the candidate’s own work.

Work done in collaboration with, or with the assistance of, others, is indicated as such.

Any views expressed in the dissertation are those of the author.

James R. Darling

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0 Author’s Declaration

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Acknowledgments

This study benefited from the expertise, advice and assistance of many people, all of

whom are thanked for their contribution to the scientific content of this theses and my

personal development as a scientist. In particular, Chris Hawkesworth, Craig Storey and

Peter Lightfoot are thanked for their expert guidance and support throughout the last

three years.

Many members of Bristol Isotope Group (BIG) contributed to my understanding of

geochemical methods and analytical techniques: Chris Coath, Matthias Willbold, Corey

Archer, Kirsty Crocket, Bob Steele, Gavin Foster, Phillip Pogge von Strandmann and

Tim Elliot. Riccardo Avanzinelli is thanked in particular for his invaluable help and

support with the measurement of Pb isotopes.

Stuart Kearns is thanked for his help with electron microbeam techniques. Chung

Choi for assistance with sample preparation and ICP-OES analyses. Gemma Sherwood

and Matthew Cross for their assistance with sample preparation.

Fieldwork benefited greatly from the financial support of Vale Inco and the logistical

support of the Sudbury Basin Group of Vale Inco Exploration. Extensive assistance from

Enrick Tremblay and Peter Lightfoot is gratefully acknowledged.

This project was funded by NERC Studentship NER/S/A/2006/14061, Vale Inco

and the Eugene M. Shoemaker Impact Cratering Award.

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0 Acknowledgments

viii

Table of Contents

Abstract iii

Author’s Declaration v

Acknowledgments vii

List of Figures xii

1 Introduction 1

1.1 The Sudbury Igneous Complex - a unique terrestrial melt sheet . . . . . . 1

1.2 Thesis aims . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.3 Thesis structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.4 Author contributions to published papers . . . . . . . . . . . . . . . . . . 6

1.5 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2 The geology of the Sudbury Structure 9

2.1 Regional geological setting . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.1.1 The Superior Province . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.1.2 The Southern Province . . . . . . . . . . . . . . . . . . . . . . . . 11

2.2 Geology of the Sudbury Structure . . . . . . . . . . . . . . . . . . . . . . 14

2.2.1 The Sudbury Igneous Complex . . . . . . . . . . . . . . . . . . . . 14

2.2.2 Breccias of the Sudbury Structure . . . . . . . . . . . . . . . . . . 24

2.2.3 Sulphide ore deposits . . . . . . . . . . . . . . . . . . . . . . . . . . 26

2.2.4 The Whitewater Group . . . . . . . . . . . . . . . . . . . . . . . . 27

2.2.5 Deformation of the Sudbury Structure . . . . . . . . . . . . . . . . 29

2.3 Geochemistry of the Sudbury Igneous Complex . . . . . . . . . . . . . . . 30

2.3.1 Major and trace elements . . . . . . . . . . . . . . . . . . . . . . . 31

2.3.2 Isotopic studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

3 Methodology 37

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TABLE OF CONTENTS

3.1 Sample processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

3.2 Pb isotope analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

3.2.1 Reagents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

3.2.2 Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . 40

3.2.3 Digestion protocols . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

3.2.4 Pb ion exchange chromatography . . . . . . . . . . . . . . . . . . . 42

3.2.5 Procedural blanks . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

3.2.6 Pb isotope mass spectrometry . . . . . . . . . . . . . . . . . . . . . 45

3.2.7 Standards summary . . . . . . . . . . . . . . . . . . . . . . . . . . 49

3.2.8 U/Th/Pb ratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

3.2.9 Calculation of initial Pb isotope ratios . . . . . . . . . . . . . . . . 55

3.3 Zircon major and trace element analysis . . . . . . . . . . . . . . . . . . . 57

3.3.1 Electron microprobe protocols . . . . . . . . . . . . . . . . . . . . 58

3.3.2 Laser Ablation ICPMS protocols . . . . . . . . . . . . . . . . . . . 60

3.3.3 Standards summary . . . . . . . . . . . . . . . . . . . . . . . . . . 61

4 Isotopic heterogeneity in the Sudbury impact melt sheet 64

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

4.2 Darling J.R. et. al. (2009) Isotopic heterogeneity in the Sudbury impact

melt sheet. Earth and Planetary Science Letters . . . . . . . . . . . . . . 65

4.3 Pb isotopes as a tool for sulphide ore exploration? . . . . . . . . . . . . . 80

4.3.1 Sampling strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

4.3.2 Pb isotope results . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

4.3.3 Comparison of Pb isotopes and whole rock geochemistry . . . . . . 81

5 Pb isotope systematics of the Offset Dykes 88

5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

5.2 Darling J.R. et. al. Shallow impact: isotopic insights into crustal contri-

butions to the Sudbury impact melt sheet; in review . . . . . . . . . . . . 89

6 Impact melt sheet zircons 109

6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

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TABLE OF CONTENTS

6.2 Darling J.R. et al (2009) Impact melt sheet zircons and their implications

for Hadean crustal processes; Geology v.37; no.10; p. 927-930 . . . . . . . 111

7 Summary and future directions 117

7.1 Melt sheet evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

7.2 Origin of the melt sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

7.3 Implications for sulphide ore formation . . . . . . . . . . . . . . . . . . . . 119

7.3.1 Distinct sources for ores in different Offset Dykes? . . . . . . . . . 120

7.4 Zircons in impact melts . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

Bibliography 123

8 Appendices 135

8.0.1 Supplementary materials for Darling et al. (2009) Impact melt

sheet zircons and their implications for Hadean crustal processes;

Geology; v. 37; no.10; p. 927-930 . . . . . . . . . . . . . . . . . . . 135

8.0.2 Supplementary material to Chapter 7 . . . . . . . . . . . . . . . . 141

xi

List of Figures

2.1 Regional geological setting of the Sudbury Structure . . . . . . . . . . . . 10

2.2 Summarised stratigraphy and depositional setting of the Huronian Super-

group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.3 Simplified geological map of the Sudbury Structure . . . . . . . . . . . . . 15

2.4 Schematic representation of the stratigraphic relationships in the Sudbury

Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.5 Petrology of the North and South Ranges of the Main Mass . . . . . . . . 17

2.6 Photographs of thin sections from the main units of the SIC . . . . . . . . 20

2.7 Schematic representation of geological relationships in the Worthington

Offset Dyke . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

2.8 Photographs of thin sections from the main quartz diorite phases of the

Worthington Offset Dyke . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

2.9 Photographs of Sudbury Breccia, Footwall Breccia and Sublayer exposures 25

2.10 Cross-section through the Levack Mine highlighting the difference between

contact and footwall ore deposits . . . . . . . . . . . . . . . . . . . . . . . 27

2.11 Photograph of the Basal Member of the Onaping Formation . . . . . . . . 29

2.12 Cross section through the Sudbury Structure . . . . . . . . . . . . . . . . 31

2.13 Selected major and trace element, and trace element ratio variations through-

out the Main Mass and Sublayer . . . . . . . . . . . . . . . . . . . . . . . 33

2.14 Comparison of model initial Pb isotope ratios from the SIC and Sudbury

Breccias . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

3.1 Plagioclase feldspar step leaching procedure, and results for sample FOQD1 41

3.2 Comparison of the matrices of elution steps from Pb ion exchange chro-

matography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

3.3 Representative examples of the drift in uncorrected 206Pb/204Pb ratios of

the NIST SRM 981 standard throughout analytical sessions . . . . . . . . 50

3.4 Natural logarithm plots of raw NIST SRM 981 Pb isotope ratios . . . . . 51

3.5 Individual measurements of the (a) 206Pb/204Pb, (b) 207Pb/206Pb, and

(c) 208Pb/206Pb ratios of NIST SRM 612 from this study . . . . . . . . . 52

xii

LIST OF FIGURES

3.6 An example of reproducibility for processed samples. . . . . . . . . . . . . 54

3.7 Schematic representation of model initial Pb isotope calculation . . . . . . 57

3.8 (a) Standard deviation of individual measurements versus counting times

in EPMA analyses. (b) Traverse across standard Kim-5 . . . . . . . . . . 59

4.1 Plots of uranogenic and thorogenic Pb isotope ratios of ore samples from

throughout the Creighton embayment . . . . . . . . . . . . . . . . . . . . 81

4.2 Model initial 207Pb/204Pb plotted against whole rock assay data (from

Vale Inco Exploration) for samples from the Creighton embayment . . . . 83

4.3 Model initial 207Pb/204Pb plotted against the Ni content of pyrrhotite

separates from the Creighton embayment . . . . . . . . . . . . . . . . . . 84

6.1 Comparison of the record of early Earth processes with models of mass

flux to the Moon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

7.1 Sampling localities of Offset Dyke ores . . . . . . . . . . . . . . . . . . . . 120

7.2 Histogram of model initial Pb isotope ratios of Offset Dyke ore samples . 121

8.1 Cathodoluminescence images of representative zircons from each unit of

the melt sheet. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

8.2 Chondrite normalised REE profiles of Sudbury impact melt sheet zircons. 140

Note that this list of figures does not include those within the three published and

submitted papers.

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LIST OF FIGURES

xiv

1

Introduction

1.1 The Sudbury Igneous Complex - a unique terrestrial

melt sheet

Few geological features have been more intensively studied than the Sudbury Structure in

Ontario, Canada. Although the first ore from Sudbury’s vast nickel and copper sulphide

deposits was mined in 1886 (see Giblin, 1984), the origin of the structure remained enig-

matic throughout more than a century of research. While early investigations recognised

that many of the geological features of the area were unusual, including widespread brec-

ciation of country rocks and the unique nature of the igneous suite, the Sudbury Igneous

Complex (SIC) was widely regarded as a magmatic intrusive body that was linked with

explosive volcanism (models reviewed in Giblin, 1984). However, following the discovery

of shatter cones in the region by Dietz and Butler (1964), the involvement of a meteorite

impact in the formation of the Sudbury Structure was widely recognised. There is now

an overwhelming body of evidence in support of an impact origin. In addition to shatter

cones, which are indicative of shock pressures of ∼2-10 GPa (Milton, 1977; Roddy and

Davis, 1977), many other characteristic features of impact basins have been reported.

These include shock deformation and metamorphic features in minerals throughout the

footwall (e.g. Dressler 1984a), impact diamonds (Masaitis et al., 1999), and the similar-

ity of pseudotachylites and suevitic breccias to rocks found in well constrained terrestrial

impact structures such as Reis and Vredefort (See Peredery and Morrison, 1984).

For many years, impact models attributed the SIC to mantle derived magmatism

triggered by the impact event (Naldrett, 1984b; and references therein). Not until the

application of isotope geochemistry to the various rock types of the SIC, did the intimate

association between the impact and the igneous complex become clear. The osmium,

neodymium, strontium and lead isotope systematics of the SIC all require an overwhelm-

ingly crustal source (Dickin et al., 1992, 1996; Faggart et al., 1985; Hurst and Wetheril,

1974; Walker et al., 1991), precluding significant mantle contribution, and consistent

with the origin of the SIC as an impact melt sheet. Further developments in the under-

standing of the physical and geological effects of impact processes, such as the scaling

of impact melt volumes (e.g. Grieve and Cintala, 1992), together with more recent ge-

ological, geochemical and geophysical studies in Sudbury, have led to the recognition

that all of the main features of the Sudbury Structure can be accounted for by impact

(e.g. Naldrett, 1999; Zieg and Marsh, 2005; Grieve and Therriault, 2000; and references

therein).

1

1 Introduction

The Sudbury impact structure had an original diameter of ca. 250 km (Spray et al.,

2004), making it one of the largest on Earth. The two other terrestrial impact basins of

comparable size, the ∼ 300 km Vredefort impact structure in South Africa and the ∼ 180

km Chicxulub crater in Mexico, are either deeply eroded or deeply buried (see Grieve

and Therriault, 2000). As such, Sudbury offers a unique opportunity to study many of

the processes and geological effects of large meteorite impacts, a fact that will ensure

many more decades of intensive investigation of this enigmatic structure. Of particular

interest in this study is the well preserved, if deformed, impact melt sheet, which is by far

the largest recognised on Earth. This offers a unique opportunity to study the processes

of melt sheet formation and evolution, which are of great importance to crustal evolution

on planetary bodies (e.g. Grieve et al., 2006).

The Sudbury Structure is also famous for hosting a vast resource of Ni-Cu-Platinum

Group Element (PGE) sulphide ore. The historic resource exceeds 1500 million tonnes

with an average grade of around 1.2 wt% Ni, 1.1 wt% Cu and 0.4 g/t of Pt and Pd

(Keays and Lightfoot, 2004). Long held as a type locality for the study of magmatic

sulphide deposits, it is now clear that the processes leading to ore formation in Sudbury

were exceptional. Recent developments have begun to understand ore forming processes

in the context of an impact melt sheet (Keays and Lightfoot, 2004), however many key

issues, such as the distribution of ore around the complex, remain unresolved.

This investigation will build upon the recent geological and, in particular, geochemical

developments made in the understanding of the Sudbury impact melt sheet. Particular

emphasis will be placed on understanding how the melt sheet formed and evolved over

time, and the affect of these processes on ore formation. The melt sheet also offers a

unique terrestrial proxy for the affects of intense post-accretionary bombardment on the

early Earth (Kring and Cohen, 2002), and this will also be explored.

1.2 Thesis aims

The identification of a crustal source for the Sudbury melt sheet was accompanied by

the recognition that there are systematic compositional differences between the two sides

of the Sudbury Igneous Complex (SIC), termed the North and South Ranges. These

differences are manifest in the concentrations of certain trace elements (Lightfoot et al.,

1997a), however they are most clearly defined in isotopic investigations. Dickin et al.

(1996) attributed these differences to impact melting of different target lithologies, and

found a strong match between the Pb and Nd isotope ratios of ores and basal silicate

rocks with those of nearby pseudotachylites in the footwall. Further investigations of

the Os, Sr and Pb isotope systematics of the SIC have confirmed that distinct crustal

2

1.2. THESIS AIMS

sources are required to explain the composition of melt sheet lithologies from each side

of the SIC (Dickin et al., 1999; Morgan et al., 2002).

Theoretical considerations on the violent movement and mixing of superheated, low

viscosity shock melt (Phinney and Simonds, 1977; Simonds and Kieffer, 1993) suggest

that the initial products of impact melting would have been homogenised during crater

formation, within minutes to hours of impact (Melosh and Ivanov, 1999). Yet it is

clear that the spatial distribution of target lithologies played some part in controlling

the variations in composition throughout the SIC. In order to further understand the

evolution of the melt sheet, from crater formation to final crystallisation, it is important

to characterise the extent and scales of isotopic heterogeneity. This pertains to the

crustal sources of the impact melts, and the degree to which they were homogenised.

There is also considerable uncertainty surrounding the controls on the location of

sulphide ore deposits in the SIC. Sulphide mineralisation is attributed to early sulphur

saturation under superheated conditions. Dense immiscible sulphide liquids accumulated

towards the base of the melt sheet by efficient gravitational settling (Keays and Lightfoot,

2004). However, the distribution of ores along the basal margin is highly variable. While

concentration by convection and gravitational flow (e.g. Zieg and Marsh, 2005), together

with a connection between the thickness of the melt sheet and the mineral potential of the

basal margin (Lightfoot et al., 2001), go some way to explain the distribution of ores, the

current models are not entirely satisfactory. Chemical heterogeneity could potentially

be a significant, and as yet unexplored, control on the sulphur saturation history of the

melt sheet.

Sudbury is also a unique analogue for impact melt sheets on the early Earth. The

geological record of crustal processes during the first 600 Myr of Earth’s history is al-

most exclusively limited to detrital zircons from the Yilgarn craton. Numerous studies

have sought to elucidate the source rocks from which these zircons crystallised. Based

upon low crystallisation temperatures, inclusion assemblages dominated by granitic min-

erals and trace element compositions, a predominantly granitic source has been proposed

(Hopkins et al., 2008; Maas et al., 1992; Menneken et al., 2007; Watson and Harrison,

2006, 2005). In contrast to predictions of a long-lived mafic crust following magma ocean

crystallisation (Kamber, 2007; Kramers, 2007), this suggests that felsic continental crust

was widespread on the early Earth. Our ability to interpret the sources of these detri-

tal grains relies upon knowledge of zircon populations from well constrained magmatic

and metamorphic rocks. No such “ground truth” data currently exists for zircons crys-

tallised from impact melt sheets. Given the likely importance of impact processes during

intense post accretionary bombardment of the early Earth (e.g. Kring and Cohen, 2002;

Grieve et al., 2006), such melts are a potentially significant source of the ancient zircons.

3

1 Introduction

Furthermore, whilst inclusions in zircon are often utilised in metamorphic studies, very

little data is available for magmatic rocks. By characterising zircons from throughout

the SIC, the importance of impact melt sheets in the source of these ancient grains can

be addressed.

The principal aims of this thesis are therefore to:

• Understand how the melt sheet evolved, from crater formation through to final

crystallisation.

• Identify the crustal sources of the melt sheet and the depth of shock melting in the

Sudbury impact event.

• Characterise the extent to which chemical variability in the melt sheet controls

sulphide ore formation and explore the implications for exploration strategy.

• Identify whether impact melt sheet zircons are common in the Hadean detrital

zircon record.

In Sudbury, the Pb isotope system has proven to be more sensitive to target rock

variations than other isotopic systems such as Nd. This is because the two main groups

of target rocks are Archean gneisses of the Superior Province and Huronian metasedi-

mentary rocks, which were largely derived from the Superior Province (McLennan et al.,

1979, 2000). Due to the differences in chemical properties of parent and daughter iso-

topes, fractionation of U and Th from Pb occurs during sedimentary processes. This

has resulted in different Pb isotope evolution in each target rock group over the time

between sedimentation (<2450 Ma; Bennet et al., 1991; Young et al., 2001) and the time

of impact (1850 Ma; Krogh et al., 1982).

As such, the more specific objectives of this thesis are to:

• Characterise the scales of Pb isotope heterogeneity throughout the melt sheet

• Identify variations in metal depletion signatures throughout the melt sheet

• Test the degree of coherence between the Pb isotope compositions of sulphide ores

and the overlying melt sheet

• Characterise the Pb isotope variability in early formed impact melts from the Offset

Dykes

• Explore isotopic and trace element constraints upon the source of the melt sheet

and the metals contained within Sudbury’s ores

4

1.3. THESIS STRUCTURE

• Characterise zircons from throughout the melt sheet stratigraphy, with particular

emphasis on inclusion populations and trace element compositions.

1.3 Thesis structure

At the core of this thesis are three papers that are either published or under review.

These form the majority of the scientific contribution of this doctoral study and are

included in chapters 4, 5 and 6, together with some additional results and discussion.

Chapter 2 reviews key aspects of the geology of the Sudbury Structure, providing a

framework for later discussion. A detailed account of methods employed, in particular

for Pb isotope analysis, is provided in chapter 3.

In chapter 4 the scales of Pb isotope heterogeneity, and variations in metal depletion

signatures, throughout the melt sheet are investigated. The implications for melt sheet

evolution and ore forming processes will be explored. This chapter includes the following

paper:

Darling J.R., Hawkesworth C.J., Lightfoot P.C., Storey C.D. and Tremblay E. (2009) Isotopic

heterogeneity in the Sudbury impact melt sheet. Earth and Planetary Science Letters. doi:10.1016

/j.epsl.2009.11.023

Chapter 5 presents the results of an investigation into the Pb isotope systematics of

early formed impact melts in Sudbury, allowing for an assessment of the crustal sources

of the melt sheet. The implications for melt sheet formation will be discussed. This

chapter is comprised of the following paper:

Darling J.R., Hawkesworth C.J., Storey C.D., and Lightfoot P.C. Shallow impact; isotopic in-

sights into crustal contributions to the Sudbury impact melt sheet. Geochemica et Cosmochimica

Acta, in-review

In chapter 6 the characteristics of impact melt sheet zircons are investigated, and

the implications for the interpretation of detrital zircon populations are explored. This

chapter consists mainly of the following paper:

Darling J.R., Storey C.D. and Hawkesworth C.J. (2009) Impact melt sheet zircons and their

implications for Hadean crustal processes; Geology, vol. 37, no. 10, p. 927-930

As a result of including the three papers, chapters 4, 5 and 6 stand alone, and include

detailed discussion of results. The findings of this study are summarised in Chapter 7,

along with identified directions for future research.

There are a number of local terms for aspects of the geology of the Sudbury Structure,

which will be referred to throughout this thesis. To aid the reader, a glossary is provided

at the end of this introductory chapter.

5

1 Introduction

1.4 Author contributions to published papers

Chapters 4, 5 and 6 contain published papers, for each of which I am the primary author.

The research for all three papers has been undertaken during my doctoral studies at the

University of Bristol and I have made the predominant contribution to each. The details

of author contributions to the papers are as follows:

Darling J.R. et al. (2009) Isotopic heterogeneity in the Sudbury impact melt sheet. Earth and

Planetary Science Letters. doi:10.1016/j.epsl.2009.11.023

Building upon the initial PhD project outline, written by C. Hawkesworth and P.

Lightfoot, the design of this investigation, including sampling strategy and analytical

methodology, was done by J. Darling. Fieldwork was carried out by J. Darling, with

assistance from E. Tremblay, P. Lightfoot and the Sudbury Basin Group of Vale Inco

Exploration. The preparation and analysis of samples for Pb isotope ratio composi-

tions was undertaken by J. Darling, with technical assistance from C. Storey and R.

Avanzinelli. Data evaluation and interpretation, together with the preparation of the

manuscript was done by J. Darling, and benefited from discussion with co-authors. The

final paper was written by J. Darling and includes comments, mostly on the clarification

of methods and ideas, from co-authors and reviewers.

Darling J.R. et al., Shallow impact; isotopic insights into crustal contributions to the Sudbury

impact melt sheet. Geochemical et Cosmochimica Acta, in-review

Similarly to the EPSL paper, the project was designed and implemented by J. Dar-

ling. The manuscript was written by J. Darling, incorporating suggestions from co-

authors.

Darling J.R. et. al. (2009) Impact melt sheet zircons and their implications for Hadean crustal

processes; Geology, vol. 37, no. 10, p. 927-930

The project was designed by J. Darling, following discussions with C. Storey about

mineral inclusions in zircon, and was based largely on a proposal written by J. Darling

for the Eugene M. Shoemaker impact cratering award. Sampling and analytical work

were undertaken by J. Darling. The paper was written by J. Darling, and benefited

from discussion with co-authors and H. Marschall. The modelling of residual liquid

compositions was suggested by reviewer L. Coogan.

James Darling

6

1.5. GLOSSARY

1.5 Glossary

Embayment Trough like structures in the basal contact of the melt sheet that

are often associated with major ore deposits.

Footwall Breccia Polymict, matrix supported, contact metamorphosed breccia that

occurs between the Sudbury Igneous Complex and unbrec-

ciated footwall rocks (McCormick et al., 2002).

Lower Unit The Lower Unit of the Main Mass consist typically of quartz

monzogabbro to quartz gabbro. Throughout much of the previ-

ous literature on Sudbury, this sequence was referred to as the

Sudbury ‘norite’.

Main Mass The Sudbury impact melt sheet. Typically ∼ 2.5 km thick (Keays

and Lightfoot, 2004), the Main Mass is divided into the Lower

Unit and Upper Unit, separated by the Middle Unit, with the

approximate proportions 30:60:10 respectively.

Middle Unit A transitional unit between the Lower Unit and Upper Unit

of the Main Mass. Typically consists of quartz monzogabbro

to granodiorite and is often termed the ‘quartz gabbro transition

zone’ in previous literature.

North Range The northern exposed section of the Sudbury Igneous Com-

pex, underlain by Archean gneisses (the Levack Gneiss Complex)

and granitic intrusions of the Superior Province.

Offset Dykes Radial and concentric dykes of the Sudbury Igneous Com-

plex, containing quartz diorites with variable inclusion and sul-

phide content. These units were emplaced prior to differentiation

of the melt sheet (e.g. Lightfoot et al., 1997b) and may be rep-

resentative of the bulk crustal melt (Lightfoot and Farrow, 2002;

Lightfoot et al., 1997b; Mungall et al., 2004).

Onaping Formation A ∼ 1400 m thick sequence of impact generated pyroclastic like

fall, flow and debris flow deposits, hydroclastic breccias and melt

bodies (Ames et al., 2002, 1998; Muir and Peredery, 1984), which

immediately overlies the Sudbury Igneous Complex.

Onwatin & Chelmsford

Formations

A series of predominantly mudstones and greywackes, deposited

within the crater, that overlie the Onaping Formation (Rousell,

1984a).

South Range The southern exposed section of the Sudbury Igneous Com-

plex, underlain by metasedimentary and metavolcanic rocks of

the Huronian Supergoup, together with the Nipissing gabbro suite

and felsic intrusions. Seperated from the North Range by zones

of faulting.

7

1 Introduction

Sublayer Discontinuous inclusion and sulphide rich norite to monzogabbro

that occurs at the melt sheet - footwall contact and is locally

underlain by footwall breccias.

Sudbury Basin Sedimentary basin containing crater filling rocks of the Onap-

ing, Onwatin and Chelmsford Formations of the Whitewater

Group.

Sudbury Breccia Pseudotachylitic breccia within the footwall to the melt sheet, of-

ten occurring as veins or crudely tabular bodies. Formed by com-

munition and melting of target rocks during excavation and modi-

fication stages of crater formation (Dressler, 2004, 1984a; LaFrance

et al., 2008; Riller, 2005; Thompson and Spray, 1994).

Sudbury Igneous Com-

plex (SIC)

Grouping of the igneous rock types in the Sudbury Structure.

Incorporating the Main Mass, Sublayer and Offset Dykes.

Sudbury Structure Geological feature now recognised as the Sudbury impact struc-

ture. Term includes three main elements; the Sudbury Igneous

Complex, the Sudbury Basin and brecciated footwall rocks.

Upper Unit The Upper Unit of the Main Mass is granitic, with abundant

granophyric intergrowths. This unit is often referred to as the

Sudbury granophyre throughout previous literature.

8

2

The geology of the Sudbury Structure

As a result of over a hundred years of geological research and mineral exploration there

is a vast literature relating to the Sudbury Structure. There are many fascinating rock

types and geological relationships in Sudbury, which are described in detail in volumes

edited by Lightfoot and Naldrett (1994) and, in particular, Pye (1984). The aim of this

chapter is provide detail of the key geological relationships of the area in order to lay

the foundations for further discussion in the following chapters. Important developments

in the understanding of the Sudbury Structure from the last twenty years will also be

highlighted. During this time significant advancements have been made in many aspects

of Sudbury geology.

2.1 Regional geological setting

The Sudbury Structure is located near the junction of three major geological and struc-

tural provinces in the southern Canadian Shield. Straddling the contact between the

Archean Superior Province and the Paleoproterozoic Southern Province, the structure

also lies within 10 km of the Grenville Front, a zone of shear and thrust faults which

marks the edge of the Grenville Province (Figure 2.1).

Previous studies have shown that the difference in footwall rocks on each exposed

side of the SIC was an important factor in the evolution of the melt sheet. The northern

exposed section of the SIC, termed the North Range, is underlain by Archean gneisses

and felsic intrusions of the Superior Province. In contrast, the South Range footwall

consists of Paleoproterozoic metasedimentary and metavolcanic rocks of the Huronian

Supergroup, together with mafic and felsic intrusive bodies.

2.1.1 The Superior Province

The Archaean Superior Province north of Sudbury consists of several major lithostrati-

graphic components of the Abitibi Subprovince, including metavolcanic and metased-

imentary belts, felsic plutons and gneissic terrains (Card, 1972). The massive felsic

Algoman plutons (Van Schmus 1965) form much of the Abitibi Subprovince in the Sud-

bury area. These include the Cartier Batholith, adjacent to the Sudbury Structure, and

intrude tonalite-granodiorite-mafic gneisses such as those of the Levack Gneiss Complex.

East-west trending greenschist to amphibolite facies (e.g. Easton, 2000a) metavolcanic

and metasedimentary belts, such as the Benny and Abitibi greenstone belts, also occur.

9

2 The geology of the Sudbury Structure

Grenvil

le Fr

ont

SRSZ

Lake Huron46º

47º

46º

47º

82º

82º

Canada

USA

300 km

50 km

Levack Gneiss ComplexFelsic plutons, gneisses and migmatites

Huronian Supergroup

Metavolcanics and metasedimetary rocksAB - Abitibi Greenstone Belt

Nipissing Mafic suiteFelsic plutons

Sudbury Igneous ComplexWhitewater Group

Sudb

ury

Stru

ctur

eSo

uthe

rnPr

ovin

ceSu

perio

rPr

ovin

ce

Archean

Palaeoproterozoic

AB

Grenville Province

Phanerozoic cover

Synform

Antiform

Thrust fault SRSZ - South Range Shear Zone

Fault zone

Figure 2.1: Regional geological setting of the Sudbury Structure. The simplified geological mapshows the distribution of the main geological provinces and highlights the general trends of majorfold and fault structures. SRSZ - South Range Shear Zone. Simplified after Card et al. (1984).

10

2.1. REGIONAL GEOLOGICAL SETTING

Levack Gneiss Complex

The footwall at the northern margin of the SIC predominantly consists of high-grade

metamorphic rocks, collectively termed the Levack Gneiss Complex (Langford, 1960).

These upper amphibolite to granulite facies rocks consist of banded and migmatitic

tonalitic, granodioritic and dioritic gneisses, with interspersed metapyroxenite and gab-

broic bodies, xenoliths and minor paragneisses (Dressler, 1984b). Furthermore, foliated

tonalitic, granodioritic, mafic and ultramafic intrusions also occur (Card et al., 1984).

The complex has primary ages ranging from 2711 to 2647 Ma, with the peak of upper

amphibolite to granulite metamorphism occuring at ca. 2645 Ma (Krogh, et al. 1984)

at 6-8 kbar and 750-800 C (James et al., 1992). Shock deformation features associated

with the Sudbury event extend for up to 17 km from the base of the SIC (Dressler,

1984a), and within 2 km of the SIC the granulites are overprinted by thermal contact

metamorphism, with conditions of 4-6 kbar and 800-950 C reported (James et al., 1992).

Cartier Batholith

The Cartier Batholith forms part of the immediate footwall to the SIC in the west of

the Sudbury Structure and is extensive north of the Levack Gneiss Complex. These

granitoids are predominantly monzogranitic to granodioritic, generally undeformed and

provide a zircon U-Pb age of 2642 ± 1 Ma (Meldrum et al., 1997), marginally younger

than the inferred peak of granulite facies metamorphism in the Levack Gneiss Complex.

The rocks are metaluminous to mildly peraluminous and enriched in the light rare earth

elements (LREE), U and Th (Meldrum et al., 1997).

2.1.2 The Southern Province

The Southern Province represents an exposed remnant of a Paleoproterozoic mobile belt,

the Penokean Fold Belt, along the southern margin of the Superior Province (e.g. Card

et al., 1972). In the Sudbury region, and forming the footwall to the South Range of

the SIC, the Southern Province is comprised of the Huronian Supergroup and various

intrusive rock types. There is considerable complexity in the geology of the Southern

Province, which is outlined in Card et al. (1972), Card et al. (1984) and Dressler (1984b).

An overview of the main lithostratigraphic units is provided below.

Huronian Supergroup

The Huronian Supergoup comprises a southward and eastward thickening wedge of mainly

sedimentary supracrustal rocks, which were deformed during the 1.89-1.80 Ga Penokean

orogeny (Young et al., 2001). The sedimentary sequence has a maximum thickness of ∼

11


12 km near Lake Huron and ∼8 km to the north east of the Sudbury structure (Card

et al., 1984; Dressler, 1984b; Young et al., 2001), and consists of cyclical deposits of con-

glomerate, mudstone, greywacke and quartz-feldspar arenites (Figure 2.2). The lower

Huronian, which is divided into the Elliot Lake, Hough Lake and Quirke Lake Groups,

was likely deposited in a rift basin (Roscoe and Card, 1992; Young et al., 2001). In con-

trast, the Cobalt Group of the upper Huronian is thought to represent a passive margin

sequence (e.g. Young et al., 2001). The sedimentary rocks are constrained to have been

deposited between 2450 Ma and 2219 Ma on the basis of U-Pb zircon and baddeleyite

ages of lower Huronian mafic volcanic rocks and cross cutting Nipissing gabbro intrusions

(Corfu and Andrews, 1986; Krogh et al., 1982).

The lowermost Huronian sediments rest unconformably on Archean basement that

commonly exhibits a well developed palaeosol (McLennan et al., 1979). On the basis

of paleocurrent directions, REE concentrations and Nd and Pb isotope systematics, a

provenance for the sedimentary rocks dominated by the Archean Superior Province has

been identified (Card et al., 1972; McLennan et al., 2000, 1979; Young et al., 2001).

Intrusive igneous bodies

A number of intrusions occur within the Huronian Supergroup south of the Sudbury

Structure. Three Palaeoproterozoic granitic plutons, the Creighton, Murray and Skead

plutons, occur along the southern margin of the SIC, cutting the lower part of the

Huronian succession. Zircon U-Pb ages constrain the time of intrusion of these granites

to between 2.47 and 2.33 Ga (Krogh et al., 1984).

Several 2480-2450 Ma mafic intrusive suites, including the large leucogabbroic East

Bull Lake and Shakespeare-Dunlop intrusions (Figure 2.1), together with a number of

smaller mafic sills of similar age also occur (Prevec and Baadsgaard, 2005). Several of the

smaller mafic sills, the Drury Township, Falconbridge Township and Joe Lake intrusions,

occur adjacent to the SIC contact (see Figure 2.3).

The most voluminous intrusive rocks in the Southern Province are the 2.2 Ga Nipiss-

ing gabbro sills and dykes. Individual intrusions range in thickness from a few hundred

meters to over a thousand meters, and consist of dominantly tholeiitic to calc-alkaline

rocks ranging from diabase through gabbronorite, gabbro, quartz diorite and granodior-

ite. The intrusions are considered by Lightfoot et al. (1993) to represent a large igneous

gabbroic province that formed the roots to flood type basaltic eruptions. In the Sudbury

area a more mafic variety occurs, known as the Sudbury Gabbro. These rocks have up

to 18 wt% MgO (Lightfoot, 2002).

12

2.1. REGIONAL GEOLOGICAL SETTING

Fig

ure

2.2:

Sum

mar

ised

stra

tigr

aphy

ofth

eH

uron

ian

Supe

rgro

up,s

impl

ified

cros

sse

ctio

nan

din

terp

reta

tion

sof

depo

siti

onal

envi

ronm

ents

.Fr

omFe

doet

al.

(199

7).

13


2.2 Geology of the Sudbury Structure

The Sudbury Structure includes several related geological components:

• The Sudbury Igneous Complex (SIC): A term encompassing the igneous rock types

of the Sudbury Structure. The SIC is comprised of the Main Mass, now recognised

as an impact melt sheet, the radial and concentric Offset Dykes, which protrude

from the Main Mass into the footwall, and the Sublayer, a discontinuous mafic unit

that occurs at the Main Mass - footwall contact. Based upon differences in footwall

lithologies, the petrology of the Main Mass and the elliptical outcrop pattern, the

SIC is divided into North, East and South Ranges.

• The Sudbury Basin: the area enclosed by the SIC and containing rocks of the

Whitewater Group. The Whitewater group is comprised of three formations: the

Onaping, Onwatin and Chelmsford Formations. Stratigraphically, these formations

overlie the SIC, and are interpreted as suevitic (fallback) breccias and post impact

basin filling siliciclastic rocks (Ames et al., 2002; Dressler et al., 1996; Gibbins

et al., 1997; Muir and Peredery, 1984).

• Breccias: Two distinct types of breccia that are intimately linked to the formation

of the Sudbury Strucure occur in the footwall to the SIC. Footwall Breccias are

composed of shattered and crushed country rocks that form a layer 10 to 50 m

thick at the Main Mass or Sublayer - footwall contact. Pseodotachylitic breccias,

locally termed Sudbury Breccias, are extensive throughout the footwall, occuring

as dyke like bodies or irregular masses.

The distrubution of each of these features in shown in Figure 2.3 and a representation

of the stratigraphic relationships provided in Figure 2.4.

2.2.1 The Sudbury Igneous Complex

Main Mass

The Main Mass of the Sudbury Igneous Complex has crystallised with a highly regular

stratigraphy, and is accordingly divided into the Lower, Middle and Upper Units, with

the approximate proportions 30:10:60 respectively. The Main Mass varies considerably in

thickness. In areas between embayment structures (see Figure 2.4), thickness varies from

5000 m down to as little as 300 m (Keays and Lightfoot 2004). Despite these variations,

the ratio of the Lower Unit to Upper Unit remains relatively constant at nearly 40:60,

although the proportion of the Middle Unit varies significantly.

14

2.2. GEOLOGY OF THE SUDBURY STRUCTURE

CH

ELM

SFO

RD

Cop

per

Cliff

Offs

et

SUD

BURY

Wan

apite

iLa

ke

Park

inO

ffset

Foy

Offs

et

Hes

s O

ffset

Min

istic

Offs

et

Wor

thin

gton

Offs

et

GRENVILLE FR

ONT

TRIL

L

Froo

dSt

obie

46º 4

5'

46º 3

0'

81º 3

0'81

º 00'

10 K

m

SO

UT

H R

AN

GE

NO

RT

H R

AN

GE

Faul

t sys

tem

s

Gre

nville

Fro

nt -

front

al th

rust

Leva

ck G

neis

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ompl

ex

Tona

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and

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tier b

atho

lith

Mas

sive

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orite

, gra

nite

Gra

nitic

plu

tons

East

Bul

l Lak

e ty

pe m

afic

intru

sion

s

Ona

ping

For

mat

ion

Onw

atin

For

mat

ion

Che

lmsf

ord

Form

atio

n

WhitewaterGroup

Ellio

t Lak

e G

roup

Hou

gh &

Qui

rke

Gro

ups

Cob

alt G

roup

HuronianSupergroup Superior Province

Neo

arch

ean

Met

avol

cani

c an

d m

etas

edim

enta

ry ro

cks

(Ben

ny B

elt)

Upp

er U

nit

Mid

dle

Uni

tLo

wer

Uni

tQ

uartz

dio

rite

(Offs

et D

ykes

)

SIC

Nip

issi

ng m

afic

sui

te

Pale

opro

tero

zoic

Fig

ure

2.3:

Sim

plifi

edge

olog

ical

map

ofth

eSu

dbur

ySt

ruct

ure.

Mod

ified

afte

rA

mes

etal

.(2

005)

15


DM

Fractured andshocked

basement

Plag richupper unit

Sudbury Breccia

Offset Dykes

Mai

n M

ass

Whi

tew

ater

Gp.

Foot

wal

l

Post-impactbasin fill

Impactites(~1.4 km)

Melt sheet(~2.5 km)

SM

Lower Unit

Middle Unit

Upper Unit

Onaping Fm.

Onwatin Fm.

Chelmsford Fm.

SublayerFootwallBreccia Embayment

Figure 2.4: Schematic representation of the stratigraphic relationships in the Sudbury Structure.Simplified from Ames and Farrow (2007).

Before discussing the petrology of the Main Mass, it is important to note that the

terminology and subdivisions of the Main Mass used in this thesis differ from much of the

literature on Sudbury geology. Traditionally, the Lower, Middle and Upper Units have

been termed the “norite”, “quartz gabbro” and “granophyre” respectively. These terms

date back to petrographic studies by Williams (1891) and are somewhat misleading. For

example, as shown by Therriault et al. (2002), the Lower Unit (“norite”) samples do

not conform to the IUGS classification of norite (Le Maitre et al., 2005) and are more

correctly classified as quartz monzogabbro to quartz gabbro. The Middle Unit typically

consists of quartz monzogabbro to granodiorite and the Upper Unit rocks are granites.

Differences in petrology are observed between the North and South Ranges, in par-

ticular within the Lower Unit. Previously, the North Range Lower Unit was divided

into a “mafic norite” and a “felsic norite”. This distinction was made on the basis of an

increase in orthopyroxene relative to clinopyroxene, and a change from hypidiomorphic

granular to poikilitic textures, in the lowermost 300 m of the Lower Unit stratigraphy,

although these changes are gradational (Figure 2.5). In contrast, the South Range Lower

Unit has typically been divided into the “South Range norite” and basal “quartz rich

norite”, on the basis of an increase in quartz content (Figure 2.5) and change in texture

16


0

500

1000

1500

2000

0 50 100 0 50 100

UpperUnit

?

NORTH RANGE SOUTH RANGE

True

Thi

ckne

ss (m

)

Modal % Modal %

Oxides + apatite + titaniteBiotite + primary amphiboleClinopyroxene

OrthopyroxenePlagioclaseQuartz + granophyric intergrowths

LowerUnit

Middle Unit

Figure 2.5: Petrology of the North and South Ranges of the Main Mass. Modified after Naldrettet al. (1970).

towards the basal contact. Again, these changes are gradational over hundreds of meters.

The main petrological distinctions between the North Range and South Range Lower

Unit rocks are highlighted in Figure 2.5. The South Range rocks are variably altered

and towards the top of the Lower Unit little fresh pyroxene is present. Throughout this

thesis these rocks will be referred to as the North Range Lower Unit or South Range

Lower Unit, with distinction of the basal rock types by use of the terms basal Lower

Unit etc.

Various petrological features clearly distinguish the Main Mass from other well known

igneous bodies such as the Bushveld, Stillwater and Skaergaard complexes. These in-

clude: the lack of conspicuous modal layering, the large amount of silicic relative to mafic

rock, the lack of upper and lower chilled margins, a thick sandwich zone of unusual mafic

material (the Middle Unit), the uniformity of Lower Unit textures over large distances,

and lateral stratigraphic continuity over tens of kilometers (Zieg and Marsh, 2005).

South Range petrology

Upper Unit rocks in the South Range are, in general, sheared and altered along the

South Range Shear Sone (SRSZ). Where least deformed the rocks are seen to consist

17


predominantly of coarse grained quartz and alkali feldspar granophyric intergrowths (60-

70 %), plagioclase (15-20 %), amphibole (<10 %) and biotite, with minor oxides, apatite

and titanite. Plagioclase is developed as tabular grains set in a matrix of granophyric

intergrowths. Along the upper contact of the Main Mass a more plagioclase rich variant

is exposed (Figure 2.4), and contains fragments of country rocks similar to the Onaping

Formation (Muir and Peredery, 1984; Naldrett and Hewins, 1984). The Upper Unit

grades, with increasing plagioclase, clinopyroxene, biotite, magnetite and ilmenite into

the granodioritic to quartz monzogabbroic Middle Unit. This gradation typically occurs

over hundreds of meters, and is often referred to as the Transition Zone (e.g. Therriault

et al., 2002). Titaniferous magnetite of the Middle Unit constitutes up to ten percent of

the rock.

The contact between the Middle Unit and the underlying Lower Unit is also gra-

dational, with decreasing clinopyroxene, titanomagnetite and apatite into the Lower

Zone. Also, where least altered this boundary marks the entry of orthopyroxene. For

the most part, the predominantly quartz monzogabbroic Lower Unit has a medium to

coarse grained hypidiomorphic granular texture, consisting of plagioclase and orthopy-

roxene (En65-70; Naldrett et al., 1970) with clinopyroxene, quartz, biotite, amphibole,

magnetite and ilmenite. Clinopyroxene and plagioclase occur as subhedral tabular crys-

tals, whereas orthopyroxene occurs as anhedral masses, and the alignment of plagioclase

crystals forms a distinct planar igneous lamination. Although secondary alteration is

locally intensive, the hydrous phases biotite and amphibole (hornblende) are considered

to be primary (Naldrett et al., 1970). Towards the base of the Lower Unit there is an

increase in quartz (up to 20 %) and biotite (up to 20 %) content, as well as a decrease in

average grain size, and a loss of igneous lamination. The thickness of this predominantly

quartz gabbroic zone varies from tens to hundreds of meters.

North Range petrology

Upper Unit rocks in the North Range are less deformed and altered than those in the

South Range. The granophyric rocks are very similar to the least deformed South Range

samples, although occasional clinopyroxenes may be present (Naldrett et al., 1970). It

is composed mainly of granophyric intergrowths of quartz and microcline, with albite

laths up to 4 mm in length, anhedral quartz, perthite, epidote, biotite and amphibole.

The contact between the Upper and Middle Units is gradational, typically over 100-200

m. The Middle Unit is also similar to that of the South Range, consisting of plagioclase,

clinopyroxene, amphibole, biotite, titanite and magnetite, together with interstitial gra-

nophyric intergrowths. Oxide minerals may be slightly more abundant than in the South

Range, with ilmenite commonly occurring as exsolution lamellae rather than discrete

18


grains.

The contact between the Middle Unit and the Lower Unit is again gradational, typ-

ically over tens of meters. The Lower Unit is a coarse grained hypidiomorphic granular-

textured rock comprised of clinopyroxene and orthopyroxene, plagioclase, biotite, inter-

stitial micrographic intergrowths and minor apatite, ilmenite and ulvospinel. The ratio

of clinopyroxene to orthopyroxene is typically around 2:1, although in the lowermost

ca. 300 m there is a significant, gradational, increase in orthopyroxene (up to 50 % of

the rock). Similarly to the basal Lower Unit samples in the South Range, the basal

rocks do not show igneous lamination, while plagioclase crystals in the overlaying quartz

monzogabbros do have a conspicuous alignment.

Sublayer

The Sublayer contains almost 50 % of the known and historic sulphide ore resource in

Sudbury (Keays and Lightfoot, 2004). It occurs as discontinuous lenses along the contact

between the Main Mass and the footwall, with thicknesses greatest (hundreds of meters)

in trough like radial depressions called embayments (Figure 2.4). Field relationships with

the overlying Main Mass are often conflicting. For example, inclusions of the quartz rich

Lower Unit have been observed in the Sublayer and inclusions of Sublayer have been

observed in the Lower Unit (Naldrett 1984, Lightfoot 1995). The contacts with the

Lower Unit vary from gradational over tens of meters (Therriault et al., 2002) to sharp,

and are never seen to be chilled (Lightfoot et al., 1997c).

The Sublayer is comprised of an inclusion and sulphide rich noritic to gabbroic rock

with an igneous texture, underlain by local Footwall Breccia with a metamorphic tex-

tured matrix (Pattison, 1979). The matrix consists of orthopyroxene and plagioclase,

with varying amounts of clinopyroxene and minor quartz, granophyric intergrowths and

iron oxides, and has a medium to coarse grained hypidiomorphic to subophitic texture.

The sulphide content varies from <1 % to over 60 modal %. Inclusion populations are

dominated by locally derived footwall rocks and exotic clasts of ultramafic rock (Light-

foot et al., 1997c; Rae, 1975).

Offset Dykes

When propagated to depth the quartz diorite Offset Dykes have an estimated total

volume of 100 km3 (Keays and Lightfoot, 2004). The Offset Dykes group into two main

types: radial and concentric. Radial Offsets intrude up to 30 km into the footwall rocks,

typically following domains of Sudbury Breccia (e.g. the Worthington, Copper Cliff,

Ministic, Parkin and Foy Offset Dykes; see Figure 2.3) and are linked to embayment

structures at the base of the Main Mass. Varying in thickness from hundreds to tens

19


(a) NR Sublayer (b) SR Sublayer (2 mm)

(c) NR basal Lower Unit (d) NR Lower Unit

(e) Middle Unit (f) Upper Unit

Figure 2.6: Photographs of thin sections from the main units of the SIC. NR - North Range, SR- South Range. Field of view is 4 mm wide unless otherwise stated. (a) Sublayer norite from theTrill Embayment; (b) Sublayer gabbro from the Creighton Embayment; (c) and (d) North RangeLower Unit samples from the 81138 drill core, showing the difference between orthopyroxene richbasal monzogabbro and overlying monzogabbros; (e) altered Middle Unit quartz monzogabbofrom the North Range (plane polarized light); (f) North Range Upper Unit granophyre

20


of meters, the dykes generally thin away from the Main Mass. Concentric Offset Dykes

form ring-like structures centred on the Main Mass. They include ca. 1 km wide Sudbury

Breccia belts, which contain discontinuous lenses of quartz diorite (e.g. the Frood-Stobie

breccia belt; Scott and Spray, 2000), and more continuous dykes of quartz diorite (e.g.

the Hess and Manchester Offset Dykes). The shape and orientation of the Offset Dykes

are thought to have been controlled by pre-existing structures and lithological variations

in the footwall (Murphy and Spray, 2002; Lightfoot and Farrow, 2002; Lightfoot and

Naldrett, 1997). The dykes commonly pinch and swell, and are often discontinuous at

the surface.

The mineralogy of Offset Dyke quartz diorite is variable, and previous workers clas-

sified different types based upon their primary mafic phases (Grant and Bite, 1984).

These include orthopyroxene and clinopyroxene, amphibole and biotite. The most com-

mon phase is amphibole-quartz diorite, with the amphiboles considered pseodomorphic

after pyroxene (Grant and Bite, 1984) . The rocks are massive, fine- to medium-grained

and exhibit granular to sub-granular textures. Mafic minerals comprise 45 to 55 % of

the rocks, with 30-40 % feldspar, 5-15 % quartz and minor granophyric intergrowths and

oxides. According to variations in mineralogy, field relationships, sulphide content and

inclusions, two main phases of quartz diorite can be discerned in many Offset Dykes: (1)

an inclusion and sulphide poor marginal phase (QD; Figure 2.8a) that is often chilled

against the margins of the dykes; and (2) a fine to medium grained quartz diorite con-

taining sulphide, biotite and inclusions ranging from millimeters to meters in size (IQD;

Figure 2.8b).

The two phases of quartz diorite are perhaps best defined in the Worthington Offset

Dyke. The general field relationships along the dyke are shown in Figure 2.7. Marginal

chilled QD is transitional into medium grained QD. Locally the dyke contains a core

of IQD, which can be choked with inclusions surrounded by semi-massive to massive

sulphide. The inclusions are predominantly amphibolites, which are petrologically and

geochemically similar to Sudbury Gabbros (Lightfoot and Farrow, 2002). The lack of

sulphide in the marginal QD is evidence that this unit was emplaced prior to sulphur

saturation of the melt sheet, which likely occurred at temperatures in excess of the Lower

Unit liquidus (Keays and Lightfoot, 2004).

Geochronology of the SIC

A summary of the geochronological investigations of the SIC is provided in Table 2.1.

The Main Mass, Sublayer and Offset Dykes have indistinguishable magmatic ages of

1849-1850 Ma.

21


Sudbury breccia

Fine grained quartz dioritewith inclusions of greywacke

IQD (< 20 % inclusions)

IQD (> 20 % inclusionsdominated by amphibolite)

Amphibolite IQD(> 90 % inclusions)

Offset Dyke

ArkoseAmphibolite Metasedimentary rocks

Sulphides

Blebby to heavilydisseminated

Finely disseminated

Semi-massive

Medium-coarse grainedquartz diorite

20 mCountry rocks

Proximal to theMain Mass

Distal from Main Mass

Figure 2.7: Schematic representation of geological relationships in the Worthington Offset Dyke.Modified after Lightfoot et al. (1997a). IQD - Inclusion bearing quartz diorite. Note thatgreywacke inclusions in the marginal quartz diorite are rare.

(a) Quartz diorite (QD) (b) Inclusion bearing quartz diorite (IQD)

Figure 2.8: Photographs of thin sections from the main quartz diorite phases of the WorthingtonOffset Dyke. (a) Medium grained biotite-quartz diorite from the Totten Mine; (b) Fine-mediumgrained IQD with amphibolite inclusions from the Totten Mine. Width of field of view in 2 mmand both photos were taken with crossed polarizers

22


Gro

up

Lit

hol

ogy

Loca

tion

Age

(Ma)

Met

hod

Sou

rce

Mai

nM

ass

Low

erU

nit

Nor

thR

ange

1848

.9+

4.0/−

2.7

zirc

onU

-Pb

(TIM

S)K

rogh

etal

.(1

984)

Mai

nM

ass

Low

erU

nit

Nor

thR

ange

1850

+3.

4/−

2.4

zirc

onU

-Pb

(TIM

S)K

rogh

etal

.(1

984)

Mai

nM

ass

Low

erU

nit

Nor

thR

ange

1849

.53±

0.21

zirc

onU

-Pb

(TE

-TIM

S)D

avis

(200

8)M

ain

Mas

sL

ower

Uni

tN

orth

Ran

ge18

49.6

+3.

4/−

3zi

rcon

U-P

b(T

IMS)

Kro

ghet

al.

(198

2)M

ain

Mas

sU

pper

Uni

tN

orth

Ran

ge18

50.5±

3.0

badd

eley

ite

U-P

b(T

IMS)

Kro

ghet

al.

(198

4)M

ain

Mas

sL

ower

Uni

tSo

uth

Ran

ge18

49.7±

1.1

zirc

onU

-Pb

(TIM

S)K

rogh

etal

.(1

984)

Mai

nM

ass

Low

erU

nit

Sout

hR

ange

1849

.11±

0.19

zirc

onU

-Pb

(TE

-TIM

S)D

avis

(200

8)M

ain

Mas

sL

ower

Uni

tSo

uth

Ran

ge18

49.4

+1.

9/−

1.8

zirc

onU

-Pb

(TIM

S)K

rogh

etal

.(1

982)

Offs

etD

yke

quar

tzdi

orit

eC

oppe

rC

liff18

49.8±

2.0

zirc

onU

-Pb

(TIM

S)C

orfu

and

Lig

htfo

ot(1

996)

Offs

etD

yke

quar

tzdi

orit

eFo

y18

52+

4/−

3zi

rcon

U-P

b(T

IMS)

Cor

fuan

dL

ight

foot

(199

6)Su

blay

erno

rite

Whi

stle

Em

baym

ent

1848

.1±

1.8

zirc

onan

dba

ddel

eyit

eU

-P

b(T

IMS)

Cor

fuan

dL

ight

foot

(199

6)

Subl

ayer

oliv

ine

mel

anor

ite

in-

clus

ion

Whi

stle

Em

baym

ent

1849

.1±

1.1

zirc

onan

dba

ddel

eyit

eU

-P

b(T

IMS)

Cor

fuan

dL

ight

foot

(199

6)

Subl

ayer

met

apyr

oxen

ite

incl

u-si

onW

hist

leE

mba

ymen

t18

48.4±

1.4

zirc

onU

-Pb

(TIM

S)C

orfu

and

Lig

htfo

ot(1

996)

Subl

ayer

mel

anor

ite

pod

Whi

stle

Em

baym

ent

1848

.3±

1.0

zirc

onU

-Pb

(TIM

S)C

orfu

and

Lig

htfo

ot(1

996)

Subl

ayer

mel

anor

ite

pod

Whi

stle

Em

baym

ent

1848

.3±

1.0

zirc

onan

dba

ddel

eyit

eU

-P

b(T

IMS)

Cor

fuan

dL

ight

foot

(199

6)

Subl

ayer

diab

ase

Whi

stle

Em

baym

ent

1848

.7±

1.1

zirc

onU

-Pb

(TIM

S)C

orfu

and

Lig

htfo

ot(1

996)

Ona

ping

Form

atio

nam

phib

ole

vein

(hy-

drot

herm

al)

1848

.4+

3.8/−

1.8

tita

nite

U-P

b(T

IMS)

Am

eset

al.

(199

8)

Tab

le2.

1:Su

mm

ary

ofge

ochr

onol

ogic

alin

vest

igat

ions

ofSI

Can

das

soci

ated

litho

logi

es.

TIM

S;T

herm

alIo

nisa

tion

Mas

sSp

ectr

omet

ry.

TE

-TIM

S;T

herm

alE

xtra

ctio

nT

IMS.

23


2.2.2 Breccias of the Sudbury Structure

Sudbury Breccia

The most prominent feature of the Sudbury event in the footwall to the Main Mass are

zones of Sudbury Breccia. The similarity of these pseudotachylitic breccias (Thompson

and Spray, 1994) to the type locality of such rocks, in the Vredefort Structure in South

Africa (Shand, 1916), has long been recognised (Speers, 1957). They occur as irregular

shaped bodies or dikes that range in size from a few millimeters, to zones hundreds of

meters wide, such as the 45 km long South Range Breccia Belt (SRBB; Scott and Spray,

2000). The breccias are most pervasive within 10 km of the North and 15 km of the

South Range of the Main Mass. There is also a 10-15 km wide zone of brecciation 20 to

25 km north of the Main Mass and isolated bodies up to 80 km from the SIC (Dressler,

1984a).

The breccias are spatially associated with discontinuities such as faults, bedding

planes and lithological contacts (Dressler, 1984a; Rousell et al., 2003) and have sharp

contacts with their host rocks. The rocks consist of subrounded fragments of predomi-

nantly locally derived country rocks, in a fine matrix (∼100 µm; Scott and Spray, 2000),

which may be fragmental, recrystallised or locally igneous textured (Rousell et al., 2003).

Clast sizes range from milimeters to ten’s of meters.

Geochemical investigations have shown that the composition of the matrix and clasts

is similar (Dressler, 1984a) and that the breccias are mostly derived from local host rocks.

While there has been significant debate surrounding the mechanisms that produced

the breccias, there is growing consensus that cataclastic milling and frictional melting

were the dominant processes (Thompson and Spray, 1994; Rousell et al., 2003; Muller-

Mohr, 1992; Dressler, 1984a). Indeed, preferential communition (Muller-Mohr, 1992)

or frictional melting (Thompson and Spray, 1994) has resulted in the Sudbury Breccia

matrix being slightly more mafic than its local clast population.

Footwall Breccia

One of the main sulphide hosting units of the Sudbury Structure, the Footwall Breccia

is a polymict, matrix-supported, contact metamorphosed breccia that occurs between

the SIC and its footwall rocks. Thickness varies from meters up to hundreds of meters

in embayment structures. The matrix contains shocked mineral and lithic fragments,

clasts of pseudotachylitic breccia and has an igneous texture that has been annealed at

temperatures exceeding 1000 C (Lakomy, 1990).

24


(a) (b)

(c) (d)

Figure 2.9: Photographs of Sudbury Breccia, Footwall Breccia and Sublayer exposures. (a)Clast rich Sudbury Breccia from near the Manchester Offset Dyke, with meter scale subroundedclasts of quartzite. Sharp contact with country rocks is seen at the bottom of the picture. (b)Huronian arkose hosted clast poor Sudbury Breccia, again showing sharp contact with coutryrocks. (c) Footwall Breccia in the Levack area (North Range), with angular clasts of local SuperiorProvince felsic gneisses, norite enclave and igneous textured matrix. The scale on the card is incentimeters. (d) The Sublayer norite - Footwall Breccia interface exposed on the Highway 144Bypass section near Gertrude open pit.

25


2.2.3 Sulphide ore deposits

The Ni-Cu-PGE sulphide ore mineralisation occurs in five distinct environments: (1) the

Sublayer, (2) Footwall Breccias immediately beneath the Sublayer, (3) within veins in

the footwall, (4) the Offset Dykes and (5) zones of breccia that host the quartz diorite

(Lightfoot et al., 2001; Keays and Lightfoot, 2004).

There are a diverse range of mineralisation styles throughout these different environ-

ments (see Naldrett, 1984a and following chapters). However, three different types of

orebody can be defined. Contact orebodies, including ores hosted by the Sublayer and

Footwall Breccias, occur at the base of the Main Mass of the SIC. The sulphides gener-

ally change from massive, semi-massive, blebby and disseminated with height above the

footwall contact, over scales of meters to hundreds of meters. Typical sulphide mineral

assemblages are pyrrhotite (Fe1-xS), which is the principal host for all other sulphide

minerals, pentlandite ((Fe,Ni)9S8) and chalcopyrite (CuFeS2). The largest contact de-

posit is located in the Creighton embayment. Offset Dyke deposits tend to be associated

with discontinuities along the radial offsets, including swell zones, Sudbury Breccia and

changes in country rock (Figure 2.7; Lightfoot and Farrow, 2002). Massive, semi-massive

and blebby sulphides occur in association with inclusion bearing quartz diorite, and have

mineralogies similar to the contact ore deposits. Footwall deposits, such as the McCreedy,

Coleman and Levack mines, are characterised by chalcopyrite rich assemblages hosted by

a complex network of veins within brecciated footwall rocks (Morrison, 1984). Genetic

models for footwall mineralisation include the mobilisation of metals in the presence of

volatiles (Farrow and Watkinson, 1992, 1996; Molnar et al., 1997, 2001; Hanley et al.,

2005; Hanley and Mungall, 2003) and the injection of fractionated sulphide (e.g. Li et al.,

1992 and Naldrett, 1984a).

Sulphide fractionation has been proposed as the primary control on compositional

zonation at many orebodies around the SIC. Sulphide minerals undergo changes in the

solid state down to relatively low temperatures compared to silicates and oxides. These

changes occur because at high temperatures there are extensive fields of solid solution

that shrink upon cooling. In the Ni-Cu-Fe-S system, at high temperatures (1190 C;

Fleet, 2006) a pyrrhotite-like phase, termed the monosulphide solid solution (mss) crys-

tallises. As cooling continues the mss can no longer accommodate the Ni and Cu and

a series of exolutions takes place (Fleet, 2006), with a chalcopyrite-like phase, the inter-

mediate solid solution (iss), formed at 500-400 C (Fleet, 2006).

26


Contact orebodies

Craig ShaftLevack #2

N S

Footwall orebodies

Footwall rocks

Sudbury BrecciaMain Mass

Footwall Breccia Sulphide oreMine shaft/drift

Figure 2.10: Cross-section through the Levack Mine highlighting the difference between contactand footwall ore deposits. Modified after Ames and Farrow (2007).

2.2.4 The Whitewater Group

Onaping Formation

Directly overlying the Main Mass, the Onaping Formation is a stratified, ∼ 1400 m

thick, succession of heterolithic breccias and igneous rocks. The formation is typically

subdivided from bottom to top into a Basal Member, a Gray Member and a Black

Member (Muir and Peredery, 1984). Each member contains a variety of breccias that

are comprised of a combination of variably shock metamorphosed country rock fragments,

devitrified glasses, fluidal material, minor sulphide mineralisation and finely comminuted

matrix material (Muir and Peredery, 1984; Dressler et al., 1996; Ames et al., 2002).

Discordant igneous textured inclusion-bearing melt bodies also occur, and are typically

found between the Basal and Gray Members (Muir and Peredery, 1984). It should be

noted that an alternative subdivision for the Onaping Formation exists, and is based

upon the morphology and percentage of glass shards (Gibbins et al., 1997; Ames et al.,

2002), rather than colour and carbon content (Muir and Peredery, 1984). Accordingly,

Gibbins et al. (1997) define a lower Sandcherry Member (equivalent to the Basal and

Gray Members) and an upper Dowling Member (Black Member).

27


The Basal Member is discontinuous, with a thickness typically <150 m and up to

∼300 m. In the North and East Ranges, it contains a wide variety of fragments derived

from the Proterozoic and Archean country rocks set in a fine grained, heterogeneous,

matrix composed of recrystalised country rock particles and igneous textured minerals

(Muir and Peredery, 1984; Dressler et al., 1996). In the South Range, the Basal Member

consists mainly of clasts of quartz arenite, arkose and Proterozoic granite fragments

(Muir and Peredery, 1984). The matrix is typically recrystallised and moderately to

strongly foliated (Figure 2.11). The contact between the Basal Member and the Upper

Unit of the Main Mass is sharp to gradational over tens of meters.

The Gray Member is laterally continuous and ranges from 200-700 m in thickness.

It has a sharp contact with the Basal Member, or the Main Mass where the Basal

Member is absent, and a gradational, to locally sharp, transition into the overlying Black

Member. Breccia units of the Gray Member have significant lithological variety, forming

discontinous lens-shaped bodies which range from highly heterolithic to monolithic and

from matrix-supported to fragment-supported (Muir and Peredery, 1984). The breccias

consist of angular to rounded shock metamorphosed fragments of country rocks, crystal

fragments, recrystallised glasses, fluidal textured material and minor sulphides. Breccias

of the 800-1200 m thick Black Member have similar field characteristics, clast types and

recrystallised glasses, and are distinguished by the presence of: granular carbon, a basal

unit rich in chloritised shards, less altered glass fragments, planar bedding towards the

top of the member and different feldspar clast mineralogies (Dressler et al., 1991).

Melt bodies throughout the Onaping Formation are lens shaped to irregular and are

typically <35 m long, although clusters of melt bodies are up to 1 km in length. Chilled

margins are common, while crystalline cores are commonly fine grained and consist

of various proportions of plagioclase, K-feldspar, clinopyroxene, amphibole, quartz and

minor ilmenite, titanite, apatite and zircon. Inclusions of country rock account for

between <5 and 80 % of the volume of the melt bodies (Muir and Peredery, 1984).

A major circulation system of hydrothermal fluid pervasively altered the Onaping

Formation, resulting in zones of silicification, albitisation, chloritisation, calcitisation

and feldspathisation of glass shards and breccia matrices. These zones are associated

with Zn-Cu-Pb massive sulphide deposits and have a U-Pb titanite age of 1848.4 +3.8/-

1.8 Ma (Ames et al., 1998), which is indistinguishable from the age of the Main Mass

(Krogh et al., 1982).

Onwatin and Chelmsford Formations

The geology of the Onwatin and Chelmsford Formations is reviewed in detail by Rousell

(1984a), and much of the following brief summary of these rocks is taken from that

28


(a)

Figure 2.11: Photograph of the Basal Member of the Onaping Formation. The outcrop is atthe nothern end of the 144 Highway Bypass traverse in the South Range.

paper. The 600-1000 m thick Onwatin Formation predominantly consists of pelagic

sediments, with massive to laminated claystone, siltstone and minor graywacke. In

addition, carbonates, cherts and claystones of the Vermillion Member occur near the base

of the formation. The sediments are interpreted to have been deposited in a resricted

basin, with abundant carbonaceous material and pyrite suggestive of anoxic bottom

water conditions.

In contrast, the∼ 850 m thick Chelmsford Formation is comprised of layered greywackes

and siltstones that are characteristic of turbidite deposits, with paleocurrents indicating

a predominantly south-westerly flow direction. Whereas both the Onaping and Onwatin

Formations contain shocked clasts and mineral grains, no shock metamorphic features

have been described from the Chelmsford Formation. Furthermore, the source region of

the formation was dominated by tonalites, such as those exposed to the north-west of

the Sudbury Structure (Rousell, 1972).

2.2.5 Deformation of the Sudbury Structure

Orogenic deformation has affected the rocks of the Sudbury area both preceding and

following the impact event. In this section the major structural characteristics of the

structure are laid out. More detailed reviews are provided by Rousell (1984b) and Riller

(2005).

The lower contact of each segment of the Main Mass dips inwards towards the centre

of the Sudbury Basin (Figure 2.12), defining the elliptical outcrop pattern. In the North

Range, the basal contact has a dip of around 30, the East Range ∼ 70 and the South

Range varies from 50 to overturned in the south-east and south-west corners (Rousell,

1984b). The rocks of the Sudbury Structure have been heterogeneously deformed under

29


greenschist and, locally, lower amphibolite facies metamorphic conditions (Thompson

et al., 1985; Fleet et al., 1987). The metamorphic grade generally decreases to the

north, such that the North and East Ranges, as well as much of the Lower and Middle

Units of the South Range, have not been affected by the mesocopic ductile strain that

is prominent elsewhere. These fabrics are generally attributed to the Paleoproterozoic

Penokean orogeny (e.g. Sims et al., 1989).

The most prominent Penokean structure is the South Range Shear Zone (SRSZ:

Shanks and Schwerdtner, 1991; Rousell; 1975). This southeast dipping ductile thrust

system has fabrics that indicate a top to north-west sense of movement (Shanks and

Schwerdtner, 1991) and has resulted in the displacement of the South Range by more

than 8 km (Figure 2.12). The SRSZ cuts the South Range of the Main Mass in the

south-east and southwest corners and has deformed and altered much of the Upper Unit

throughout the South Range. The Main Mass has also been deformed by folding, with

the prominent north-east lobe having an origin by Penokean folding (Klimczak et al.,

2007; Cowan, 1999).

The North Range is cut by north striking faults of the Onaping Fault System (Figure

2.3). While these faults are traceable for many tens of kilometers, the vertical component

of movement is typically only 100-200 m (Rousell, 1984b). In contrast south-east dipping

faults, that run sub-parallel to the SRSZ, have strike separations on the order of 4 km

in the south-west corner of the Main Mass. These faults have produced the pronounced

thickness variations in the Main Mass in this area (Figure 2.3).

The timing of the Penokean orogeny (1.89-1.80 Ga; Sims et al., 1989) overlaps the age

of the impact event (1.85 Ga). There is also geochronological evidence that the SRSZ was

active during the Mazatzal event 1.7-1.6 Ga ago (Bailey et al., 2004). The later Grenville

orogeny (1.1-0.9 Ga; e.g. Easton, 2000b) also imprinted a regional metamorphism in the

area. However, based on paleomagnetic overprints in Huronian rocks, Hyodo et al. (1986)

estimated that the temperatures of this metamorphic event decreased to less than 400 C

beyond a distance of 2 km from the Grenville Front. There is no evidence for Grenville

age deformation of the Sudbury Structure.

2.3 Geochemistry of the Sudbury Igneous Complex

Numerous geochemical studies have sought to elucidate both the origin and processes of

differentiation of the SIC. In this section some of the key geochemical characteristics of

the Main Mass and Offset Dykes, and important constraints upon their origin, will be

laid out. As previously discussed, models have converged upon an impact melting origin

30

2.3. GEOCHEMISTRY OF THE SUDBURY IGNEOUS COMPLEX

10 km

NW SENorth Range South Range

Huronian Supergroup

Lower &Middle Units

Upper UnitOnaping Fm.Levack GneissComplex Onaping Fm.

Lower &Middle Units

Upper UnitSRSZ

??

?

?

5

10 km

Chelmsford &Onwatin Fm.

Figure 2.12: Summarised cross section through the Sudbury Structure, based upon the seismicreflection profiling by Milkereit et al. (1992). SRSZ - South Range Shear Zone. Modified afterRiller (2005).

for the SIC and particular emphasis will be placed on explaining the evidence for such

models.

2.3.1 Major and trace elements

It has long been recognised that the rocks of the SIC are compositionally unusual when

compared to other layered igneous bodies such as the Stillwater, Skaergaard and Bushveld

complexes. For example, they are anomalously rich in SiO2, have elevated Rb and

K2O and have fractionated rare earth element (REE) patterns (Chai and Eckstrand,

1994; Collins, 1934). A number of key investigations have presented detailed chemical

variations throughout the Main Mass (Collins, 1934; Therriault et al., 2002; Naldrett

and Hewins, 1984; Lightfoot et al., 1997c; Lightfoot and Zotov, 2005; Lightfoot et al.,

1997a; Cooper, 2000) and the Offet Dykes (Lightfoot and Farrow, 2002; Lightfoot et al.,

1997b,a) in order to resolve the petrogenesis of the SIC.

The Main Mass

Various models have been proposed to explain the highly regular stratigraphy of the

Main Mass. These include simple crystal fractionation (Naldrett, 1984a), the intrusion of

multiple magmas (Chai and Eckstrand, 1994) and crystallisation from a compositionally

zoned magma (Golightly, 1994; Zieg and Marsh, 2005). Constraints upon these models

can be drawn from geochemical variations throughout the Main Mass, and selected data

are presented in Figure 2.13.

31


A number of compositional breaks are apparant at the base of the Middle Unit and

the base of the Upper Unit, particularly in SiO2, MgO, TiO2 and trace elements. An up-

wards decline in incompatible elements and SiO2 (more pronounced from other traverses)

throughout the Upper Unit also occurs, which could be explained by crystallisation from

the roof downwards. In the North Range, SiO2 increases and MgO decreases from the

base of the Lower Unit upwards, indicating that the rocks become more evolved up-

section. In the South Range (Figure 2.13) the opposite trends occur. The compositional

breaks in the stratigraphy, together with the often conflicting major and trace element

trends between North and South Ranges, indicate that the chemostratigraphy cannot

easily be reconciled with simple in-situ differentiation by fractional crystallisation.

While the concentrations of incompatible elements change significantly throughout

the stratigraphy (Figure 2.13d and 2.13e, the ratios of incompatible elements (e.g. La/Y

- Figure 2.13g, Ce/Yb) are relatively constant (Lightfoot and Zotov, 2005; Lightfoot

et al., 1997a). Based on this similarity, models invoking distinct sources for the Upper

and Lower Units (e.g. Chai and Eckstrand, 1994) have been dismissed. This leaves

open the possibility that the Main Mass was compositionally stratified from base to

top (Golightly, 1994; Zieg and Marsh, 2005). The mechanisms of this process are not

clear, but may have been driven by viscosity and density variations (Golightly, 1994;

Zieg and Marsh, 2005). Irrespective of the details, in a two layer model crystallisation

would produce the Lower Unit from the base upwards and the Upper Unit from the top

downwards (Zieg and Marsh, 2005).

Naldrett et al. (1986) provided evidence for a significant crustal contribution to the

SIC, including the high SiO2, K2O and Rb concentrations, together with steep REE

profiles, and suggested that the complex originated by crustal contamination of a mantle

derived mafic melt. Using major element mixing models, Grieve et al. (1991) showed

that a mantle component was not necessary, and that the SIC was consistent with a

melt generated by in-situ crustal melting during an impact event. Further observations

of crustal trace element concentrations (Keays and Lightfoot, 2004; Lightfoot et al., 2001;

Prevec et al., 2000) and Rb/Sr (Hawkesworth et al., 1999) ratios are consistent with an

origin by wholesale crustal melting. Lightfoot et al. (1997a) demonstrate that a ∼ 20 %

component of mantle derived mafic rocks could be accomodated in the SIC, however it

is not required to explain the chemical variations throughout the Main Mass (Lightfoot

et al., 2001). Indeed no unambiguous evidence for significant mantle input to the SIC

has been reported, from either geochemical, field or petrological studies.

32


55 60 65 70 75

0

1000

2000

3000

4000

5000

RSP

(m)

UU

MU

TZ

LU

SL

SiO2 (wt %)

(a)

0 2 4 6 8 10

0

1000

2000

3000

4000

5000

RSP

(m)

MgO (wt%)

(b)

0 1 2 3

0

1000

2000

3000

4000

5000

RSP

(m)

TiO2 (wt %)

(c)

0 5 10 15 20 25

0

1000

2000

3000

4000

5000

RSP

(m)

UU

MU

TZ

LU

SL

Nb (ppm)

(d)

0 20 40 60 80 100

0

1000

2000

3000

4000

5000

RSP

(m)

La (ppm)

(e)

0 200 400 600

0

1000

2000

3000

4000

5000

RSP

(m)

Sr (ppm)

(f)

0 0.5 1 1.5 2 2.5 3

0

1000

2000

3000

4000

5000

RSP

(m)

UU

MU

TZ

LU

SL

La/Y

(g)

0 50 100 150 200

0

1000

2000

3000

4000

5000

RSP

(m)

Cu (ppm)

(h)

0 50 100 150 200

0

1000

2000

3000

4000

5000

RSP

(m)

Ni (ppm)

(i)

Figure 2.13: Selected major and trace element, and trace element ratio variations throughoutthe Main Mass and Sublayer. The data is from a traverse of the South Range (Lightfoot andZotov, 2005). SL, Sublayer; LU, Lower Unit; MU, Middle Unit; TZ; UU, Upper Unit.

33


The Offset Dykes

There are broad compositional differences between Offset Dykes from the North and

South Ranges, which appear not to be linked to the major mafic mineral phase content

(Lightfoot et al., 1997a). These differences are particularly apparent in TiO2, Sr and

REE ratios, and could represent assimilation of country rocks (Lightfoot et al., 1997a)

or primary differences in the composition of the SIC at the time of injection (Keays and

Lightfoot, 1999).

From the geological relationships of the Offset Dyke quartz diorite phases (Section

2.2.1) it is clear that the marginal, inclusion free, quartz diorite (QD) was emplaced at

temperatures in excess of the Main Mass Lower Unit liquidus. These phases have many

compositional traits similar to the Main Mass, including incompatible element ratios,

and have compositions intermediate between those of the Lower Unit (“felsic norite”)

and the Upper Unit (Lightfoot and Farrow, 2002; Lightfoot et al., 1997b,a; Tuchscherer

and Spray, 2002). As such they are considered to be representative of the bulk SIC

melt, emplaced prior to differentiation of the Main Mass (Lightfoot and Farrow, 2002;

Lightfoot et al., 1997b; Tuchscherer and Spray, 2002; Mungall et al., 2004). Despite the

high abundance of inclusions in the later, sulphide bearing, phase of quartz diorite (IQD),

this unit has very similar geochemical characteristics to QD (Lightfoot and Farrow, 2002).

This indicates that it too was likely emplaced prior to significant differentiation of the

Main Mass, and that assimilation of inclusions did not significantly affect the composition

of this phase.

2.3.2 Isotopic studies

A crustal source of the SIC has been confirmed in a number of investigations of the isotope

geochemistry of Main Mass and Sublayer units. These studies include measurements of:

Nd isotopes in Main Mass traverses (Faggart et al., 1985), Sublayer norites (Dickin et al.,

1996; Prevec et al., 2000) and the Onaping Formation (Deutsch, 1994); Sr isotopes in

Main Mass samples (Dickin et al., 1999; Hawkesworth et al., 1999; Hurst and Wetheril,

1974), Sublayer norites (Rao et al., 1985) and the Onaping Formation (Deutsch, 1994) ;

Pb isotopes in Main Mass traverses (Dickin et al., 1999) and Sublayer norites and sulphide

ores (Dickin et al., 1996); O isotopes in Main Mass samples (Ding and Schwarcz, 1984);

Os isotopes in sulphide ores (Dickin et al., 1992; Morgan et al., 2002; Walker et al., 1991).

The specific findings of a number of these studies will be discussed throughout Chapters

4, 5 and 6. All found that either a dominant crustal source was required to explain the

isotope systematics, or that data were consistent with a crustal source for the SIC.

As such, the Main Mass is now widely regarded as an impact melt sheet, formed from

34


2468

0

1

2

Sublayer &ores

SudburyBreccia

15.1 15.2 15.3 15.4 15.5 15.6 15.7

15.1 15.2 15.3 15.4 15.5 15.6 15.7

10

207Pb/204Pbm

207Pb/204Pbm

Freq

uenc

y

Figure 2.14: Comparison of model initial Pb isotope ratios of sulphide ores and associatedSublayer norites with Sudbury Breccia from either side of the complex. Grey bars are NorthRange samples, white bars are South Range. From Dickin et al. (1996).

shock melting of the crust. Little, if any, mantle input can be accommodated into the

SIC. Only the high Ni, Cu and PGE concentrations (80 ppm Ni, 100 ppm Cu and 1 ppb

Pd; Lightfoot and Farrow, 2002) relative to the bulk crust (59 ppm Ni, 27 ppm Cu and

1.5 ppb Pd; Rudnick and Gao, 2003) are now regarded as a geochemical feature of the

SIC that cannot readily be explained by crustal melting, although Mungall et al. (2004)

show that a lower crustal source for the melt sheet can account for the concentrations of

these elements.

A significant development in the understanding of the SIC in the context of an impact

origin was made by Dickin et al. (1996). These authors found systematic differences in

the Pb isotope ratios of sulphide ores and associated silicate rocks between the North

and South Ranges. Pb isotope ratios were found to cluster close to 1850 Ma reference

lines on plots of 206Pb/204Pb versus 207Pb/204Pb, with small degrees of scatter inferred to

represent minor isotopic heterogeneity inherited during the Sudbury event. Furthermore,

the model initial isotope ratios of samples from each side of the complex were found to

fall close to the Pb isotope composition of country rocks and pseudoachylites from each

respective range (Figure 2.14). As such significant differences in the source of North and

South Range melts were identified, consistent with impact melting of different target

rocks.

Distinct crustal sources have also been recognised for ores through studies of Os

isotopes (Dickin et al., 1992; Morgan et al., 2002; Walker et al., 1991). Further inves-

tigations of the Pb isotope systematics of Main Mass samples also revealed significant

35


differences in the model initial Pb isotope ratios of Lower Unit samples from each side

of the Complex (Dickin et al., 1999). These studies have further demonstrated that sig-

nificant variations in isotopic composition occur on the scale of the North versus South

Range.

36

3

Methodology

In Sudbury, the Pb isotope system has proven to be more sensitive to target rock vari-

ations than other isotopic systems such as Nd (Dickin et al., 1999, 1996; Faggart et al.,

1985). This is because the Huronian metasedimentary sequence, which underlies the

South Range of the SIC, was derived from the Superior Province (McLennan et al.,

1979, 2000), which underlies the North Range. Due to the differences in chemical prop-

erties of parent and daughter isotopes, fractionation of U and Th from Pb occurs during

sedimentary processes. For example U is redox sensitive, and in oxidised environments

is transported as dissolved uranyl complexes because U6+ is highly soluble in aque-

ous solutions (e.g. Langmuir, 1978). The Huronian supergroup records a change from

anoxic subaerial weathering in lower stata, with detrital uraninite present in the Elliot

Lake Group, to distinctly oxidising conditions in the Cobalt Group (Prasad and Roscoe,

1996). As a result, different Pb isotope evolution occured over the time between sedi-

mentation (<2450 Ma; Bennet et al., 1991; Young et al., 2001) and the time of impact

(1850 Ma; Krogh et al., 1982) in the North and South Range target rocks. Other iso-

topic systems, such as Lu-Hf and Sm-Nd, do not fractionate significantly in this way

(McCulloch and Wasserburg, 1978; Patchett et al., 1984; Vervoort et al., 1999), result-

ing in similar isotopic ratios in each province (Prevec et al., 2000; Naldrett et al., 1986;

McLennan et al., 2000).

As such, the principal isotopic system of interest in this investigation has been Pb.

While whole rock analyses have been undertaken in previous studies on Sudbury, there

is evidence for variable recent disturbance of such samples, and significant in-growth

of radiogenic Pb (Cooper, 2000; Dickin et al., 1999). In order to track Pb isotope

variations throughout the melt sheet at the time of formation, it would be preferable

to analyse phases that preserve close to initial magmatic Pb isotope ratios. Of the

common mineral phases present in SIC lithologies, sulphides and feldspars offer the

most appealing targets. Both of these mineral groups have low partition ceofficients

for U and Th, and can incorporate appreciable quantities of Pb (plagioclase felspars

typically ppm to hundreds of ppm; Ni-Cu-Fe sulphides typically ppm to thousands of

ppm). The details of mineral separation procedures used are presented in Section 3.1,

while the ion exchange chromotography technique used to chemically isolate and purify

Pb from these matrices is presented in Section 3.2.4.

The total range of expected Pb isotope variations throughout the SIC can be es-

timated from previous investigations. Dickin et al. (1996) found a 2-4 % difference in

model initial 207Pb/204Pb between Sudbury Breccia samples from each side of the com-

37

3 Methodology

plex. Given the crustal origin of the melt sheet, and the fact that Sudbury Breccia

samples provide an average composition of local country rocks (e.g. Dressler, 1984a),

this range can be taken as the maximum possible variation. However, variations in the

Main Mass are significantly smaller, with a transect of the North Range having a range

of ∼ 4 h , and the South Range ∼ 9 h (Dickin et al., 1999). Ideally therefore an analyt-

ical technique is required that provides high precision measurements, while allowing for

high sample throughput in order to characterise a large number of samples throughout

the melt sheet.

Accordingly, in this study Pb isotope ratios were measured by multicollector-inductively

coupled plasma mass spectrometry (MC-ICPMS), using a sample-standard bracketing

technique to correct for instrument induced mass fractionation of Pb isotopes. Discussion

on alternative methodologies and the details of the measurement protocols are presented

in section 3.2.6, along with details of the correction for instrumental mass bias and data

from a variety of international standards with different matrices. Details of mass spec-

trometry techniques used to measure U/Th/Pb ratios in the samples are presented in

Section 3.2.8.

Chapter 6 presents the results of an investigation into the trace element composition

and inclusion populations of zircons from throughout the SIC. Trace element composi-

tions were measured by laser ablation (LA) ICPMS, following major element determina-

tion by electron microprobe. Details of the analytical protocols employed in this study

are presented in Section 3.3.

3.1 Sample processing

Sulphides, feldspars and zircons were seperated from rock samples using a range of

techniques. Collected samples were prepared by splitting and cutting, to remove weath-

ered surfaces and inclusions where neccesary, and then crushed using a steel plated jaw

crusher. The crushate was sieved to isolate the <500 µm fraction and was then passed

over a wilfley table to separate mineral phases on the basis of density. Further separation

techniques included magnetic methods (hand magnet and isodynamic magnetic separa-

tor), and additional density separation using heavy liquids (lithium heteropolytungstates

- LST; 2.7-2.9 gcm-3). Selected physical properties of mineral phases of interest in this

study are shown in Table 3.1.

Sulphide ore samples were simply crushed, using ceramic or agate pestle and mortars,

with different sulphide phases separated via hand magnet (pyrrhotite rich fractions mag-

netic, chalocpyrite and pyrite rich fractions non magnetic). Zircon, plagioclase feldspar

38

3.2. PB ISOTOPE ANALYSIS

Mineral Specific gravity (gcm-3) χ (m3kg-1) AmperageFe-Ti Oxides 4.7 - 5.3 10 - 110000 handZircon 4.68 0 - 1 >4Pyrrhotite 4.58 - 4.65 10 - 30000 handChalcopyrite 4.1 - 4.3 0.55 - 10 >4Titanite 3.4 - 3.55 paramagnetic ∼ 3.5Pyroxenes 3.2 - 3.5 43 - 50 (opx) ∼ 1.0Apatite 3.15 - 3.2 diamagnetic >4Hornblende 3.0 - 3.4 paramagnetic ∼ 1.2Biotite 2.8 - 3.2 52 - 98 ∼ 0.8Plagioclase 2.62 - 2.76 diamagnetic >4Quartz 2.65 ∼ -0.5 >4

Table 3.1: Physical properties of common minerals in SIC lithologies. Specific gravity datafrom Klein and Hurlburt (1999). χ is the magnetic susceptibility, defined as the ratio of materialmagnetisation J (per unit mass) to the weak external magnetic field H (data from Hunt et al.,1995). Diamagnetic minerals have very low or negative magnetic susceptibility; Paramagneticminerals have small and positive susceptibility. Amperage is the current at which minerals wereseparated using the isodynamic magnetic separator at the Univeristy of Bristol, with a sidewaysslope of 10. Hand - separated with hand magnet.

and sulphide fractions for analysis were hand picked under a binocular microscope, en-

suring that grains with the least altered appearance and no inclusions (except zircons)

were taken.

3.2 Pb isotope analysis

3.2.1 Reagents

All lab work was carried out in class 100 (ISO 5) clean hoods in Class 10000 (ISO 7)

laboratories using Savillex perflouroalkoxy (PFA) labware. All reagents were prepared

with purified water (MQ) from a Millipore milliQ system, which has a resistivity of

∼18 MΩcm. HCl and HNO3 acids of various concentrations were prepared from VWR

AnalaR grade reagents, that were purified by distillation in a sub-boiling quartz still

(HCL only) followed by sub-boiling distillation in PFA elbow stills (2∆). Initially HCl

and HNO3 were double distilled in the PFA elbow stills, but after assessing the Pb

blank contributions of these reagents it was decided to triple distill them. Concentrated

HF and HBr were sourced from ROMIL, and of SpA (sub-ppb) and UpA (sub-ppt)

grade respectively. The Pb blanks of reagents used to prepare all samples were regularly

checked, by comparing signal intensities on the Element 2 ICPMS with those from the

MQ water.

All reagents were stored in cleaned FEP bottles. The cleaning procedure involved

fluxing in ∼7 M reagent grade HNO3 for at least 3 days on the hotplate, followed by rins-

39

3 Methodology

Step Reagent Hotplate1 6 M HCl (reagent grade) >2 hours at 130C2 Detergent (Extran MA 03) overnight at 100C3 7 M HNO3 (reagent grade) overnight at 140C4 6 M HCl (reagent grade) overnight at 130C5 7 M HNO3 (reagent grade) overnight at 140C6 6 M HCl (QD - in vial) overnight at 130C7 6 M HCl (2∆ - in vial) overnight at 140C

Table 3.2: Cleaning procedure for PFA vials. In between each step vials were thoroughly rinsedwith MQ. QD; quartz distilled.

ing 3 times with MQ, overnight refluxing in ∼2 M HNO3 and further rinsing. Cleaning

procedures for PFA screw-top vials are outlined in Table 3.2. Polypropylene (PP) lab-

ware, such as pipette tips and centrifuge tubes, was refluxed in 3 M HCl for a minimum

of 3 days before rinsing thoroughly with MQ.

3.2.2 Sample preparation

Prior to dissolution the picked mineral fractions were cleaned to remove surface contam-

inant and loosely bound Pb. Sulphide samples were washed with MQ after picking in

isopropanol, and subsequently cleaned in an ultrasonic cleaner using alternate steps of

MQ and 2 % HNO3. A more aggressive protocol was used for feldspar separates. Grains

were first cleaned in the same way as sulphides and then, following Housh and Bowring

(1991), were leached in 4 ml of 6M HCl and 7M HNO3 for 20 minutes each at 125 C.

The composition of these leaches is discussed below.

3.2.3 Digestion protocols

Following cleaning, sulphide samples of typically 10-20 mg were dissolved in 7 ml screw-

top PFA vials. 1 ml of 7 M HNO3 was added to the beaker and placed on the hotplate

at 140 C for 24 hours. After drying down, the sample was converted to chloride by

adding 0.5 ml of 6 M HCl and the drying down, then 1 ml of HCl was added and the

sample returned to the hotplate overnight. Finally samples were converted to bromide

(3 x 0.5 ml 0.4 M HBr) in preparation for Pb column chromatrography.

Typical samples sizes for feldspar dissolution were 100-200 mg. Following the leaching

steps, feldspar fractions were dissolved in 3 ml of 9 M HF and 1 ml of 7 M HNO3 over

a 2 day period in closed 15 ml PFA vials on the hotplate at 150 C. Subsequently, the

samples were dried down, then converted to nitrate (3 × 0.5 ml conc. HNO3) and then

taken up in 3 ml of 6 M HCl. An aliquot (0.5 ml) for U/Th/Pb ratio determination was

taken at this stage. After drying down, the samples were converted to bromide form by

40


Step Procedure Mass leached (g) Cum. wt %1 20 mins 4 ml 7 M HNO3 at 125C 0.019 1.92 20 mins 4 ml 6 M HCl at 125C 0.004 2.43 20 mins 4 ml 5 % HF + 0.5 ml 7 M

HNO3 at 125C0.109 13.2

4 repeat step 3 0.098 22.95 repeat step 3 0.116 34.46 20 mins 4 ml 10 % HF + 0.5 ml 7 M

HNO3 at 125C0.240 58.2

7 repeat step 6 0.207 78.78 repeat step 6 0.192 97.89 48 hours 3 ml 29 M HF + 1 ml 14 M

HNO3 at 160C0.021 99.8

0!

20!

40!

60!

80!

100!

120!

0! 20! 40! 60! 80! 100!

0!

1!

2!

3!

4!

5!

6!

7!

0! 20! 40! 60! 80! 100!

12!

16!

20!

24!

28!

32!

36!

40!

0! 20! 40! 60! 80! 100!

30!

35!

40!

45!

50!

55!

0! 20! 40! 60! 80! 100!

Pb!

(ng)!

U!

(ng)!

206Pb/204Pb! 208Pb/204Pb!

Figure 3.1: Plagioclase felspar step leaching procedure (upper), and results for sample FOQD1(lower). Between each leaching step, the leachate was pipetted off and the sample rinsed with3 ml of MQ (added to the leachate). The total sample mass in this experiment was 1.00914 g.Cum. wt % is the cumulative weight percent of sample dissolved. Lower: plots of the Pb andU abundance (excluding step 9), 206Pb/204Pb ratio and 208Pb/204Pb ratio of each step versuscumulative wt %.

41

3 Methodology

repeatedly drying down in 0.4 M HBr in preperation for Pb column chemistry.

The dilute HF concentrations were used in the feldspar dissolution to avoid dissolu-

tion or leaching of <20 µm zircon inclusions, which were observed in the residues of two

samples. Furthermore, small quantities of U and Th are common in feldspars, which are

commonly situated along grain boundaries and distorted lattice sites (Sinha, 1969), or

within inclusions (Zartmann and Wasserburg, 1969). Several studies have shown that

it is possible to remove Pb produced by in-situ decay of this U and Th, through acid

leaching experiments (Bolhar et al., 2007; Frei and Kamber, 1995; Housh and Bowring,

1991). Step leaching experiments, similar to those of Housh and Bowring (1991), were

undertaken in order to examine the distribution of U and Th, as well as the homogene-

ity of Pb isotope compositions, throughout the feldspars. The step leaching procedure,

together with results from plagioclase felspar sample FOQD1, are shown in Figure 3.1.

The results show that the initial cleaning leaching steps (steps 1 and 2), remove a sig-

nificant amount of Th (∼ 82 % of total mass), U (∼ 30 % of total mass) and Pb (∼ 10

% of total mass) with highly radiogenic Pb isotope compositions. While all of the HF

leaches (excluding step 9) fall along an 1850 Ma isochron (the age of the SIC), the HCl

and HNO3 leaches do not plot along this isochron. These observations are consistent

with this Th, U and Pb being “foreign”, possibly residing along grain boundaries and

fractures. In the residue after all steps a ∼ 15 µm zircon grain was observed, and this

is the likely source of the highly radiogenic Pb isotope compositions seen in the final 29

M HF step. HF leaching steps 4 to 8 (84.6 % of total sample mass) all have Pb isotope

compositions, U/Pb ratios and Th/U ratios within analytical uncertanties, indicating

that no Pb was leached from the zircon, that the grains are homogenous and that these

leaches likely reflect initial magmatic U/Pb ratios. 5 % HF leaching step 3 has higher Pb

isotope ratios, and likely includes a component of the radiogenic Pb removed by steps 1

and 2.

3.2.4 Pb ion exchange chromatography

Resin and column setup

The ion exchange chromatography procedure used in this investigation is adapted from

Strelow (1978), who outlined a method that efficiently separates Pb from various chal-

cophile elements. Columns of 50 µl volume were made from shrink-fit STFE Teflon,

and used with the anion exchange resin AG1-X8 (Bio-Rad Laboratories). The resin is

a quaternary amine anion exchanger on a polystyrene base, and in this investigation a

resin of 200-400 mesh size and wet bead diameter of 45-106 µl was used. A fresh batch

of resin that had been converted to chloride form and cleaned was used for each sample.

42


Step Procedure1 load resin (50 µl) in MQ2 wash resin - 400 µl 6 M HCl3 wash resin - 400 µl MQ4 wash resin - 200 µl 6 M HCl5 wash resin - 400 µl MQ6 precondition - 200 µl 0.4 M HBr7 load sample in 500 µl 0.4 M HBr8 elute with 250 µl of 0.2 M HBr + 0.5 N HNO3

9 repeat step 8 three times12 collect Pb - 400 µl 0.03 M HBr + 0.5 N HNO3

13 collect Pb - 150 µl 0.03 M HBr + 0.5 N HNO3

Table 3.3: Column preparation and elution scheme for Pb ion exchange chromatography. Pro-cedure after C. Taylor and R. Avanzinelli (2007, pers. comm.).

Cleaning procedures involved repeatedly cycling (6 times) in 6 M HCl for 24 hours, fol-

lowed by rinsing with MQ. The resin was stored in MQ to avoid potential degredation

in concentrated HCl. Columns were stored in 6 M HCl when not in use.

Elution scheme

Prior to loading on the columns, the sample Pb was complexed to bromide form by

repeatedly drying down with HBr, and finally taken up in 0.4 M HBr. Following Strelow

(1978) matrix elution in a HBr-HNO3 solution was undertaken, since it is preferable to

pure HBr elution when sample matrices have high concentrations of chalcophile elements

such as Bi, Cd, Zn, In and Cu. This is because the tailing effects on the elution peaks

disappear almost completely when 0.5-2.0 M nitric acid is added to the eluting agent.

The drawback of adding HNO3 to the eluting agent is that Pb is less strongly absorbed

by the resin, an effect which increases with higher nitric acid concentrations. The elution

and column preparation scheme used in this investigation is shown in Table 3.3.

Comparison of the composition of elution steps in Figure 3.2 shows that efficient

separation of Pb, from complex chalcophile element matrices, is achieved. The sample

shown was a mixed pyrrhotite-pentlandite-chalcopyrite sulphide ore, and contains wt %

of Ni and Cu and Co.

Investigations of Pb isotope fractionation during ion exchange chromatography have

revealed an enrichment in lighter isotopes by up to 200 ppm/amu in the first Pb eluted

from the columns, and an equivalent depletion in the final eluted fraction (Baker et al.,

2004). While these authors used a different elution scheme to this study, their results

highlight the need for a high yield from the columns. Fractionation was not significant

in this study, relative to measurement uncertainties, for a number of reasons: Firstly

Baker et al. (2004) show that fractionation is greater for larger resin bed sizes and in this

43

3 Methodology

1.E+00!

1.E+01!

1.E+02!

1.E+03!

1.E+04!

1.E+05!

1.E+06!

1.E+07!

1.E+08!

1.E+09!

Co! Ni! Cu! Zn! Se! Ru! Rh! Pd! Cd! Te! Re! Os! Ir! Pt! Au! Hg! Tl! Pb! Bi! Th! U!

Inte

nsity (

cps)!

Sample!

Elution steps 8-11!

Elution steps 12-13!

Figure 3.2: Comparison of the blank corrected signal intensities measured on solutionsof a sulphide sample and elution steps from passing the same sample through columns.20 mg of sample was dissolved and split equally prior to column chemistry. Intensitieswere measured on an Element 2 ICPMS.

investigation bed sizes are much smaller than those used by these authors; secondly, tests

with the standard NIST SRM 981 show that the column yields, obtained by comparing208Pb signal intensities of unprocessed and processed standards, were >98 %.

3.2.5 Procedural blanks

The measured Pb blanks for the full sulphide and feldspar procedure, together with

the column blank contribution are shown in Table 3.4. The blanks were measured by

comparing total Pb signal intensities with standards of known concentration. The full

procedural blanks are always less than 100 pg, with a Pb contribution from the ion

exchange chromatography averaging 22 pg. The higher blank levels for the sulphide pro-

cedure are attributed in part to the distillation procedures for HCl and HNO3. Changing

to triple distillation of these reagents reduced the Pb concentrations in measured aliquots

from ∼11 to ∼5 pg/g and ∼20 to ∼3 pg/g for 12 M HCl and 14 M HNO3 respectively

(concentrations measured by external calibration ICPMS). The difference in the theoret-

ical Pb blank, calculated from the Pb concentrations of reagents, of the two procedures

is <20 pg. Irrespective, these Pb blanks are at least three orders of magnitude less than

the typical minimum sample or standard mass of Pb processed through columns (100

ng).

Table 3.5 shows the deviation that these blank contributions would have on theoret-

ical samples of 100 ng of Pb of varying isotopic compositions. The deviations are small,

especially for the column and feldspar procedure where they are significantly less than

the analytical uncertainty for 206Pb/204Pb measurements (>150 ppm). The larger blank

44


contribution to the sulphide procedure could have a significant affect on samples with

high values of 206Pb/204Pb (e.g. 25). However, it should be noted that for most sulphide

samples more than 100 ng of Pb was processed (up to ∼1000 ng), and hence the devia-

tions shown represent maximum possible deviations from the true sample composition.

3.2.6 Pb isotope mass spectrometry

Double focusing multi-collector ICPMS (MC-ICPMS) has become a widely used tool for

high precision isotope ratio measurements since its introduction (Walder and Freedman,

1992). The main advantages that the technique offers over thermal ionisation mass

spectrometry (TIMS) are: (1) an increased sample throughput, (2) the ability to ionise

almost all elements, irrespective of their ionisation potential and (3) higher ionisation

efficiencies. Various studies have demonstrated that the external reproducibility of Pb

isotope ratio measurements by routine MC-ICPMS techniques (<100 ppm; e.g. Baker

et al., 2004) are at least as good as TIMS (∼100 ppm; e.g. Thirlwall, 2000), and also have

the advantage that they are not susceptible to differential loading blanks or anomalous207Pb behaviour dependant on the temperature of ionisation (Doucelance and Manhes,

2001; Thirlwall, 2002).

Pb isotopes offer a number of analytical challenges for both methods of mass spec-

trometry. Unlike other radioactive decay systems (e.g. Sr, Nd, Os), Pb only has one

non-radiogenic isotope (204Pb), and so there is no invariant ratio with which to correct

for instrumental mass bias. The precision and accuracy of Pb isotope measurements de-

pends upon the ability to correct for such fractionation. This is particluarly important

for MC-ICPMS, where mass bias can be on the percent level (Heumann et al., 1998).

Several different methods have been proposed in order to correct for Pb isotope mass bias,

these include; double spiking, external element doping and sample standard bracketing.

Double spiking involves measuring the relative amounts of four isotopes, two of which

are enhanced by the addition of enriched isotopic spikes to the sample. It is thought

to be the most accurate method (Baker et al., 2004), but requires the preperation of a

spike solution and is time consuming. For example, the sample Pb concentration should

be known in advance in order to calculate the correct spike aliquot, and an additional

mass spectrometer run is required.

External element doping involves the addition of Tl to the sample, and the correction

of Pb isotope ratios based on the observed fractionation of the natural Tl isotope ratio

(Collerson et al., 2002; Rehkamper and Halliday, 1998; Rehkamper and Mezger, 2000). A

critical assumption of this method is that Tl isotopes are fractionated in an identical way

to Pb. However, there is some uncertainty that this is the case, with poor reproducibility

45

3 Methodology

nP

bblank

(pg)2

se206P

b/204P

b2

se207P

b/204P

b2

se208P

b/204P

b2

se208P

b/206P

b2

seT

otalprocedural

(sulphide)6

7226

17.580.30

15.270.31

37.010.69

2.110.01

Total

procedural(feldspar)

325

2017.92

0.7415.72

0.5838.05

1.422.12

0.01C

olumn

722

1317.09

0.8714.99

0.7636.39

1.792.13

0.022

%H

NO

3blanks

2162.6

0.310.27

0.198.70

0.1629.54

3.102.90

0.30

Table

3.4:M

eanm

easuredblank

Pb

concentrationsand

isotopiccom

positions.T

otalprocedural

blanksfor

thesulphide

andfeldspar

proceduresare

shown,

togetherw

iththe

blankcontribution

fromcolum

nchem

istry.T

heaverage

composition

of2

%H

NO

3m

easurement

blanksare

alsoshow

n.

Pb

blank(pg)

206P

b/204P

bppm

deviation(19)

ppmdeviation

(25)T

otalprocedural

(sulphide)72

17.5854

213T

otalprocedural

(feldspar)25

17.9222

70C

olumn

2217.09

1470

Table

3.5:T

hedeviation,in

partsper

million

(ppm),that

theblank

contributionw

ouldhave

onhypotheticalsam

plesof

100ng

Pb

206P

b/204P

bratios

of19

and25

46


of Tl corrected Pb isotope ratios attributed to changes in the relative fractionation

behaviour of Tl and Pb induced by instrumental stability and matrix effects (Baker et al.,

2004; Barling and Weis, 2008). Indeed, Albarede et al. (2004) demonstrate a difference

in Tl and Pb mass fractionation factors on the order of 5-10 %, which is equivalent to 160

ppm/amu mass bias. In addition, it has been identified that Tl undergoes photochemical

oxidation from Tl+ to Tl3+ on exposure to UV light, and as a result behaves differently

during desolvation (Kamenov et al., 2004). Accordingly, the reproducibility of routine

Tl corrected Pb isotope ratio measurements is typically ± 300 ppm (Collerson et al.,

2002; Rehkamper and Halliday, 1998; Rehkamper and Mezger, 2000).

In preference to external element doping, a sample-standard bracketing approach

was adopted in this study. This method involves correcting for isotopic fractionation

by bracketing the measurement of samples with analyses of a standard solution of well

characterised composition. Fractionation is then corrected by interpolating the mass bias

factors of the standards to the sample, following the formula (Albarede et al., 2004):

(Rt)smp = (Rt)std(Rm)smp

(Rm)1−θstd1 ∗ (Rm)θstd2

Where θ is a constant analogous to time and chosen between 0 and 1, Rt and Rm are

the true and measured isotopic ratios respectively and the subscripts smp and std refer

to the sample and standard. Values of θ are generally chosen to be 0.5 when samples

and standards alternate regularly.

Sample-standard bracketing has been shown to provide accurate Pb isotope data

that is comparable in precision to double-spike approaches (Baker et al., 2005; Elburg

et al., 2005). The main advantage of this methodology is very high sample throughput

compared to double spiking and, in particular, TIMS methods. However, there are two

prerequisites for the application of the sample-standard bracketing approach. Firstly,

mass fractionation must change smoothly and predictably over the timescale between

analysis of the bracketing standards. Secondly, given that the mass bias behaviour of

samples and standards is assumed to be identical, very efficient seperation of Pb from

the sample is required. This is because the sample matrix can significantly change

fractionation behaviour (e.g. Woodhead, 2002).

Instrumentation and instrumental protocol

All analyses were performed on a Thermo Scientific Neptune double focusing MC-ICPMS

at the Bristol Isotope Group (BIG), University of Bristol. A Cetac Aridus desolvating

nebuliser was used in all analytical sessions. The typical instrumental operating pa-

rameters are shown in Table 3.6 and the cup configuration in Table 3.7. Isotopes were

47

3 Methodology

Instrument settings: accelerating voltage ∼ 10 kVextraction voltage -2000 Vfaraday cups connected to 1011 Ω resistorsresolution low ( ∼400)rf power ∼1200 Wsample cone Niskimmer cone Ni (x-geometry)

Argon gas flow rates: cool gas 15 l min-1

auxillary gas 0.8-0.9 l min-1

sample gas ∼ 1.0 l min-1

Sample introduction: Cetac Aridus desolatingnebulisersweep gas (Ar) from 4-7 l min-1 depending on

Aridus usedN2 4-7 ml min-1

spray chamber PFA at 105 Cdesolvator temperature 160 Cnebuliser flow rate ∼ 50 µl min-1

Table 3.6: Instrumental operating conditions for the Thermo Scientific Neptune MC-ICPMSand Cetac Aridus desolvating nebuliser.

measured simultaneously in static mode using a multi-Faraday cup array. Data collection

consisted of 50 x 4 s integrations. Sequences consisted of alternate analyses of standard

NIST SRM 981 with either other standards, samples or blanks. A three minute wash

of 2 % HNO3 proceeded all measurements. All samples and standards were introduced

in 2 % HNO3 and blank measurements of this reagent were performed every sixth mea-

surement. The average composition of these blanks is shown in Table 3.4 and typically

contributed <60 parts per million of the 208Pb signal.

The concentration of standards and samples was closely matched at 50 ng/g, which

resulted in 208Pb signals between 15 and 32 V and an average sensitivity of 840 V/ppm

Pb. The standard NIST SRM 982 was measured at the start, middle and end of each se-

quence and used to monitor the accuracy and precision of the sample standard bracketing

approach (discussed below).

In addition to the isotopes of Pb, 202Hg was measured to allow for correction of the

isobaric interference of 204Hg on 204Pb. Although no Tl was added to samples or stan-

dards, the 203Tl and 205Tl signals were also monitored. Other potential minor interfer-

ences are listed in Table 3.7. Of these, 186W16O and 186Os/16O are the most important,

because of the effect that they could have on the Hg interference correction. However

these were not found to be significant for a number of reasons: a) the use of N2 during

sample introduction minimised the formation of oxides (238U16O 238U 1 %); b) the

W content of samples and standards was negligible and c) the ion exchange chromato-

48


L3 L2 L1 C H1 H2 H3Mass 202 203 204 205 206 207 208Isotope 202Hg 203Tl 204Pb 205Tl 206Pb 207Pb 208Pb

Interference 186W16O 187Re16O Hg 187Re18O 190Os16O 191Ir16O 192Os16O186Os16O 187Os16O 188Os16O 189Os16O 166Er40Ar 167Er40Ar 192Pt16O162Dy40Ar 163Dy40Ar 164Dy40Ar 165Ho40Ar 168Yb40Ar

164Er40Ar 168Er40Ar

Table 3.7: Faraday cup configuration for the measurement of Pb isotope ratios. Potentialinterferences on each measured isotope are shown, although in this investigation a correction wasonly required for the isobaric interference of Hg on 204Pb.

graphic procedure used efficiently isolated Pb from Os (Figure 3.2), and furthermore the186Os constitutes only 1.59 % of Os.

Following off-line wash correction of the raw measured signals, corrections for the

isobaric interference of Hg on 204Pb were performed using the ratio of the natural abun-

dances of 204Hg and 202Hg (6.87/29.86) and a reverse mass bias correction. The contri-

bution of calculated 204Hg to the total 204 ion beam was negligible in most samples, with

a wash corrected 202Hg of zero. The maximum contribution in this study was 220 ppm.

Instrumental mass bias was also corrected off-line by external normalisation of sample

data to the bracketing NIST SRM 981 standard. The double spike values of Baker et al.

(2004) were used for normalisation.

3.2.7 Standards summary

In order for the sample-standard bracketing method to correct accurately and precisely

for mass bias, it is important that instrumental mass bias is stable over the duration of an

analytical session, and that standards and samples have identical fractionation behaviour.

The latter point is particularly important if samples and standards have very different

matrices, although efficient separation of Pb during ion exchange chromatography should

negate this affect. Mass bias in MC-ICPMS is very large relative to TIMS, with typical

ranges of 0.1 to 1 % and on the order of per-mil respectively. Comparing the average raw

(wash corrected but not mass bias corrected) ratios of NIST SRM 981 with the reference

values of Baker et al. (2004), an average mass bias of 0.54 % per amu is seen for this

study (Table 3.8). All bar four of the 30 run sequences were undertaken on Neptune 2

in the Bristol Isotope Group. The four sequences run on the older Neptune 1 have an

average mass bias of 0.56 % per amu.

The stability of mass bias during individual analytical sessions is represented in Figure

3.3. Average drift of the 981 standard over the course of all sequences was 410 ppm (2

relative standard deviations - RSD), but varied between 44 and 1001 ppm per analytical

49

3 Methodology

206Pb/204Pb 207Pb/204Pb 208Pb/204Pb 207Pb/206Pb 208Pb/206PbMean (n = 679) 17.1252 15.7524 37.5263 0.9218 2.19132 sd 0.0146 0.0204 0.0656 0.0004 0.00202 sd (%) 0.170 0.259 0.350 0.093 0.185Reference value 16.9416 15.4998 36.7249 0.91489 2.16775% difference 1.08 1.63 2.18 0.54 1.09ratio amu units 2 3 4 1 2% bias/amu 0.54 0.54 0.55 0.54 0.54

Table 3.8: Pb isotope mass bias in this investigation. The uncorrected NIST SRM 981 data (n= 679) are compared to reference values from Baker et al. (2004).

17.100

17.105

17.110

17.115

17.120

17.125

17.130

17.135

17.140

17.145

206 Pb/204 Pb

07/03/2008 25/04/2008 03/06/2008 27/09/2008 04/10/2008 03/07/2009

Figure 3.3: Representative examples of the drift in uncorrected 206Pb/204Pb ratios of the NISTSRM 981 standard throughout analytical sessions.

session. The absolute differences between 2 adjacent measurements of NIST SRM 981

ranged from 0.2 to 488 ppm. Previous sample-standard Pb isotope investigations in

Bristol have proposed a relationship between instrument sensitivity and the magnitude

of drift (K. Crocket, 2009, pers. comms). However, no such correlation has been observed

in this study.

The mass bias behaviour of the MC-ICPMS is investigated in Figure 3.4, by plotting

the raw NIST SRM 981 data in ln-ln space. The defined arrays demonstrate that the

mass bias behaviour over the course of this study has been normal and constant. The

slopes of the arrays are more closely matched by theoretical power law gradients than

those for the exponential law (see caption of Figure 3.4). However, the small difference

in gradients indicates that the power law does not fully describe the observed mass bias.

NIST SRM 982 was run throughout each analytical session to monitor accuracy and

50


2.837 2.838 2.839 2.840 2.841 2.842

2.752

2.754

2.756

2.758

2.760

2.839 2.840 2.841 2.842 2.843

3.620

3.622

3.624

3.626

3.628

ln (206Pb/204Pb) ln (206Pb/204Pb)

ln (2

07Pb

/204 P

b)

ln (2

08Pb

/204 P

b)

y = 1.5111x -1.5355R2 = 0.99928

y = 2.0251x -2.1275R2 = 0.99898

Figure 3.4: Natural logarithm plots of raw NIST SRM 981 Pb isotope ratios. Theoretical powerlaw gradients are 1.5003 and 2.003 (left and right graphs respectively), and more closely matchthe data than theoretical exponential law gradients of 1.4967 and 1.9906.

precision of the normalisation to NIST SRM 981. 107 mass bias corrected measurements

of this standard were made over the course of 30 months, and the results are shown in

Figure 3.5. The Pb isotope ratios are all within uncertainty of the double spike values

of Baker et al. (2004), with external reproducibility of ±160 ppm on 206Pb/204Pb, ±137

ppm on 207Pb/204Pb, ±186 ppm on 208Pb/204Pb and ±168 ppm on 207Pb/206Pb (all 2

RSD). The data are presented in Table 3.9.

A range of other standards and standard processing methods were also undertaken

in order to investigate possible: (a) differences in mass bias between pure standards and

samples of complex matrices, (b) interference issues relating to samples of complex ma-

trices and (c) fractionation of Pb isotopes during chemical separation of Pb. The results

of NIST SRM 981 and 982 standards that have been passed though columns, together

with basal standards BHVO-2 and BCR-2 are shown in Table 3.9. The standards are all

within uncertainty of reference values. Uncertainties in the BHVO-2 and BCR-2 data

are significantly higher than those for the NIST SRM 981 and 982 standards. Baker

et al. (2004) attribute this to heterogeneous distribution of contaminant in these rock

powders. This is consistent with the observation that different dissolved aliquots of these

standards have different measured Pb isotope ratios, resulting in an overall large uncer-

tainty. For example six analyses of a single aliquot of the BHVO-2 standard gave an

external reproducibility of 190 ppm (2 RSD) for 206Pb/204Pb.

In order to fully test the reproducibility of sample measurements, a sulphide sample

was arbitrarily selected for multiple analyses over a six month duration. The results

are shown in Figure 3.6. Four aliquots of sulphide sample 1206010A were dissolved and

51

3 Methodology

(a)

(b)

(c)

Figure 3.5: Individual measurements of the (a) 206Pb/204Pb, (b) 207Pb/206Pb, and (c)208Pb/206Pb ratios of NIST SRM 612 from this study (closed circles). The mean and 2 stan-dard deviation uncertainty of all measurements is shown (closed squares) and compared to thedouble-spiked values of Baker et al (2004; open squares).

52


Standard 206Pb/204Pb

2σ 207Pb/204Pb

2σ 208Pb/204Pb

2σ n

NIST SRM 981This study 16.9418 0.0021 15.4983 0.0027 36.7220 0.0075 6Baker et al. (2004) 16.9416 0.0013 15.5000 0.0013 36.7258 0.0031 119

NIST SRM 982This studya 36.7496 0.0059 17.1647 0.0024 36.7550 0.0068 107This studyb 36.7479 0.0069 17.1654 0.0034 36.7575 0.0050 6Baker et al. (2004) 36.7432

BCR-2This study 18.754 0.013 15.628 0.008 38.739 0.032 14Baker et al (2004) 18.765 0.011 15.628 0.005 38.752 0.022 8Woodhead and Hergt (2000) 18.754 0.021 15.623 0.007 38.716 0.042Collerson et al. (2002) 18.757 0.012 15.624 0.009 38.723 0.018

BHVO-2This study 18.633 0.018 15.538 0.019 38.234 0.030 16Baker et al (2004) 18.649 0.019 15.540 0.015 38.249 0.022 5Woodhead and Hergt (2000) 18.645 0.039 15.545 0.017 38.248 0.042 4Collerson et al. (2002) 18.645 0.039 15.545 0.017 38.248 0.042 2

Table 3.9: Summary of standard data obtained in this investigation, compared to double-spikereference values. Data for NIST SRM 982 includes aliquots not processed through Pb columns(This studya).

measured at least 3 times each. The resultant external reproducibility on the 15 analyses

was 151, 183 and 238 ppm for 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb respectively.

The internal precision on individual measurements, calculated by propagating the un-

certainty on bracketing standards and the sample measurement (2 standard error; tens

of ppm), clearly underestimates the true sample reproducibility. While the data from

1206010A could be used as an estimate of reproducibility of sulphide measurements, it is

apparent that there are no matrix or column fractionation effects that influence precision

or accuracy. As such the external reproducibility of the standard NIST SRM 982 (n =

107) was taken to be the best representation of true uncertainty in individual sample

measurements.

3.2.8 U/Th/Pb ratios

In order to determine the initial Pb isotope ratios of samples it is necessary to correct

for in-growth of radiogenic Pb. Sulphides and plagioclase feldspars generally have low

U/Pb ratios; for example chalcopyrites from the Creighton mine in Sudbury have U/Pb

ratios of ∼ 0.031 and two feldspar aliquots from the South Range Main Mass have ratios

of 0.025 and 0.087 (Dickin et al., 1996). However, over the 1.85 Gyr since the impact

event, significant in-growth of Pb will occur through U and Th decay.

In order to accurately measure U/Pb and Th/Pb ratios by ICPMS, it is necessary

to correct for instrumental drift throughout an analytical session and to calibrate the

53

3 Methodology

16.438!

16.443!

16.448!

16.453!

206Pb/204Pb!

15.530!

15.535!

15.540!

15.545!

207Pb/204Pb!

37.090!

37.100!

37.110!

37.120!

37.130!

37.140!

208Pb/204Pb!

Dissolution!

1!

Dissolution!

2!

Dissolution!

3!

Dissolution!

4!

Figure 3.6: An example of reproducibility for processed samples. Data are shown for sulphidesample 1206010 A. Four aliquots of handpicked sulphide were dissolved over the course of 18months (dissolutions 1-4). Of these dissolutions, multiple aliquots were processed through Pbcolumns. Uncertainties on individual measurements are 2 standard errors. The mean of allmeasurements is shown (closed square), with 2 standard deviation uncertainties (151, 183 and238 ppm for 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb respectively).

54


U, Th and Pb signal response against standards of known concentration. Absolute U,

Th and Pb concentrations of many of the analysed samples were not determined, due

to considerable uncertainty in taking an aliquot of the dissolved sample after dissolution

procedures (Section 3.2.3). In order to correct for drift, samples and standards were

spiked with 5 ng of internal standards In and Re. An external calibration procedure

was adopted, and 25, 5 and 0.5 ng g-1 calibration standards prepared. The internal and

calibration standards were made from certified concentration pure element standards,

using a Sartorius analytical balance with a readability of 0.01 mg.

The U-Th-Pb ratios of samples were measured on a Thermo Scientific Element 2

ICPMS in low resolution (300-400). A tandem quartz glass spray chamber (Cyclonic +

Scott double pass) and PFA 100 µl/min nebuliser were used to introduce samples, as they

provide excellent signal stability. At the beginning of each sequence the 25, 5 and 0.5 ng

g-1 calibration standards, along with a 2 % HNO3 blank, were measured in order to define

the relative response (response relative to internal standard Re) versus concentration.

These linear calibration curves were used to determine the concentrations of U, Th and

Pb in samples and quality control standards. Two quality control standards and up

to five samples were then analysed. Using values for the gradient and intercept of the

calibration curves the relative response was then used to calculate analyte concentrations.

Calibration curves, blank subtraction, internal standard corrections and concentration

calculations were performed online using the Element 2 software.

Employing an external calibration, precision of U/Pb ratios on repeat analyses of

feldspar samples (Table 3.10) was <0.5 % (2 RSD). Accuracy was monitored using basalt

standards BCR-2, BHVO-2 and in-house calibration standards. The average U/Pb and

Th/Pb ratios measured in this study are compared to reference values in Table 3.10.

Similarly to the findings of Kamber and Gladu (2009), the two basalt standards were

found to be heterogeneous. Individual aliquots of processed sample powder have repro-

ducibilities similar to those of the feldspar samples.

3.2.9 Calculation of initial Pb isotope ratios

In order to resolve Pb isotope variability in igneous rocks it is necessary to calculate initial

isotopic ratios, corrected for in-growth of radiogenic Pb since the time of formation. It

would be preferable to use U/Pb and Th/Pb ratios to age correct the measured Pb

isotope ratios. For example, the initial 206Pb/204Pb ratio ([206Pb/204Pb]i) is calculated

as follows: [206Pb204Pb

]i

=[

206Pb204Pb

]meas

−[

238U204Pb

]meas

(eλt − 1)

55

3 Methodology

n U/Pb 2σ Th/Pb 2σBHVO-2

This study 7 0.251 0.049 0.714 0.115Kamber and Gladu (2009) 0.272 0.060 0.755 0.120

BCR-2This study 9 0.158 0.009 0.538 0.024Kamber and Gladu (2009) 0.176 0.010 0.600 0.025

WOIQD1 Feldspar 3 0.02342 0.00018 0.00635 0.00006FSOIQD1 Feldspar 3 0.02079 0.00014 0.01616 0.00014POQD1 Feldspar 3 0.05101 0.00058 0.00745 0.00004WOQD1 Feldspar 3 0.02091 0.00008 0.01346 0.00006

Table 3.10: Summary of measured U/Pb and Th/Pb ratios for basalt standards BHVO-2 andBCR-2. Data are compared with the long term average values of Kamber and Gladu (2009),measured by external normalisation ICPMS. Also shown are examples of repeat analyses offeldspar samples from this study.

Whereby subscript meas refers to measured ratios and t is the time (years) since

crystallisation. The 238U/204Pb ratio is often referred to as µ and the 232Th/238U ratio

as κ. Uncertainty in initial ratios includes propagated uncertanty in the measured Pb

isotope ratio, µ (or κ), decay constants (Jaffey et al., 1971) and t. An example of this

error propagation, using the example of [206Pb/204Pb]i, is as follows:

A»206Pb

204Pb

–i

=

√√√√(a»206Pb

204Pb

–meas

)2

+ (1− eλt)2aµ2 + t2aλ2 + λ2at2µ2eλt

Where A is the propagated uncertainty and a is the uncertainty on individual com-

ponents.

Alteration of the U/Pb and Th/Pb ratios, particularly during recent weathering,

limit the use of this approach. Pb isotope ratios are considered more resilient to such

processes (e.g. Dickin et al., 1996); as such coupled 206Pb/204Pb and 207Pb/204Pb isotope

ratios can be used to model the initial Pb isotope composition. For the Sudbury data,

present day Pb isotope compositions are projected back parallel to 1850 Ma reference

lines, to a selected 206Pb/204Pb value of 15.464, which is the corresponding value of a

two stage Pb isotope evolution model at 1850 Ma (Stacey and Kramers, 1975). This

provides a robust measure of relative variation in initial 207Pb/204Pb, which is reported

as [207Pb/204Pb]m. These model initials are not directly comparable to initial ratios

calculated using measured U/Pb and Th/Pb ratios. A schematic representation of the

calculation of [207Pb/204Pb]m is provided in Figure 3.7. The formula is as follows:

[207Pb204Pb

]m

=[

207Pb204Pb

]meas

− 0.11311([

206Pb204Pb

]meas

− 15.4635)

56

3.3. ZIRCON MAJOR AND TRACE ELEMENT ANALYSIS

16.0

15.8

15.6

15.4

15.2

15.0

14 16 18 20 22

207Pb/204Pbm

1850 Ma isochrongradient = 0.11311

Measured samples

206Pb/204Pb = 15.464

207 P

b/20

4 Pb

206Pb/204Pb

Figure 3.7: Schematic representation of model initial Pb isotope calculation. The measured Pbisotope composition of samples is projected along a 1.85 Ga isochron to a 206Pb/204Pb value of15.464, which is the corresponding value of the Stacey and Kramers (1975) model at 1.85. AStacey and Kramers (1975) Pb isotope evolution model curve is shown from 0 to 2.4 Ga, with0.5 Ga tick marks.

Because all samples are projected back along parallel 1.85 Ga isochrons, uncertainty

in [207Pb/204Pb]m is only a function of the uncertainties in measured 206Pb/204Pb and206Pb/204Pb, such that:

A»207Pb

204Pb

–m

=

√√√√(a»206Pb

204Pb

–meas

)2

+(a»

207Pb

204Pb

–meas

)2

3.3 Zircon major and trace element analysis

Chapter 6 presents the findings of an investigation into the composition and inclusion

populations of zircons from throughout the melt sheet. A brief account of the analytical

protocols is provided in that paper, but more detailed analytical procedures are provided

here.

Zircons were separated from rock samples following the density and magnetic proce-

dures discussed in Section 3.1. Grains for analysis were then handpicked under a binoc-

ular microscope, ensuring that a representative sample of grain morphologies, colours

and inclusion abundances was taken, and subsequently mounted in epoxy resin. Pol-

ished grain mounts were photographed under reflected light and then carbon coated and

imaged by secondary electron and cathodoluminescence techniques to reveal internal

morphology and textures. These images were used to select measurement sites for the

various in-situ analytical techniques employed.

57

3 Methodology

In order to quantify laser ablation measurements of trace elements, it is necessary to

know the concentration of an element to which all others measured can be normalised

(an internal standard). There are two predominant approaches that have been used for

zircon: a) the measurement of a minor isotope of Si (29Si) and normalisation to the

stoichiometric SiO2 composition of zircon (32.8 wt %). b) the use of Hf as an internal

standard, by first measuring the Hf concentration by electron microprobe (EPMA). There

are several problematic issues with the Si approach: (1) because Si comprises tens of wt

% of zircons, even when measuring the minor isotopes 29Si and 30Si (4.68 and 3.08

% of Si respectively), signal intensities may overwhelm secondary electon multiplier

(SEM) detectors. This is particularly problematic on the Element 2. (2) The large

difference in signal intensities between internal standard and trace elements of interest

would cause erroneous corrections if the detector did not behave in a perfectly linear

fashion. (3) The use of a low mass internal standard results in long scan times when

using a magnetic sector ICPMS to measure the concentration of high mass elements. (4)

There are potentially significant interferences. For example 29Si cannot be resolved from12C16O1H, 14N2

1H and 58Fe++ at low resolution. (5) The SiO2 content of zircon is not

invariable, and ranges from 30.9 to 36 % in igneous zircon (Belousova et al., 2002).

In this study the measured Hf content was used as the internal standard. Because

individual zircon crystals are often strongly zoned in Hf content (see Hoskin and Schal-

tegger, 2003), a key requirement of this approach is that the position of measurements

of the Hf content by EPMA and the laser ablation spot are accurately matched on the

sample. As such, major element analyses by EPMA were undertaken for all laser ab-

lation analysis positions, often with multiple probe spots around larger laser spot ares

in order to ensure homogeneity, using the CL and SE images to guide the location of

analyses.

3.3.1 Electron microprobe protocols

Major elements were analysed with a Cameca SX-100 electron microprobe at the Univer-

sity of Bristol. The instrument is equipped with 5 wavelength dispersive spectrometers

(WDS), and the typical spectrometer setup is shown in Table 3.11. Calibration standards

were polished and re-carbon coated along with samples for each analytical session, in

order to minimize differential x-ray energy loss and absorbtion (Armstrong, 1993). The

following calibration standards were used: Kim 5 zircon (Si and Zr), HfO2 (Hf), Du-

rango apatite (Ca and P). Calibration involved measuring on peak for 60 seconds, with

background offsets of ± 800, and background measurements for 30 s. Five calibration

points for each element were measured.

58


Spec. 1 Spec. 2 Spec. 3 Spec. 4 Spec. 5Crystal LPET PET LIF TAP LLIFElement P (Kα, 3.171) Zr (Lα, 2.04) Ca (Kα, 3.69) Si (Kα, 1.74) Hf (Kα, 7.99)

Y (Lα, 1.922) Al (Kα, 1.49)

Table 3.11: Typical spectrometer setup for EPMA analysis of zircons. The analysedx-ray lines and corresponding wavelengths (A) are also shown.

0.00!

0.02!

0.04!

0.06!

0.08!

0.10!

0! 100! 200! 300!

Std

ev (

wt%

)!

Counting time (s)!

(a)

0.80!

0.85!

0.90!

0.95!

1.00!

1.05!

1.10!

0! 3! 6! 9! 12! 15!H

f (w

t %

)!

Distance (mm)!

(b)

Figure 3.8: (a) Standard deviation of individual measurements versus counting times. Thisexperiment was done with beam currents of 50 nA, and it was found that better precision wasachieved with 100 nA. Note that the y-axis is two standard deviations. (b) Traverse acrossstandard Kim-5. Error bars are 2 standard deviations. Horizontal line represents the long termaverage (see text).

An accelerating voltage of 17 keV was used in order to avoid interference of the Hf

Kα line by second order Zr Kα x-rays, which have an excitation energy of 17.997 keV

(e.g. Agrawal et al., 1994). Beam currents of 100 nA were used to maximise count rates,

and spot sizes were 2 µm. Corrections for x-ray absorption, secondary fluorescence,

electron backscattering and the electron stopping power of the sample were performed

online using the PAP model (Pouchou and Pichoir, 1985). Achievable precision of the Hf

concentration is limited by counting statistics. Figure 3.8a shows that at counting times

longer than 120 s the precision of individual measurements is minimised. Once operating

conditions were optimised, internal precision of Hf measurements was typically 1.7 % (2

RSD) at 1 wt % Hf.

The shard of Kim-5 zircon standard used for calibration was found to be relatively

homogenous in major element composition, and was used to monitor accuracy and pre-

cision. Over a two year period 112 analyses of the shard were undertaken, with averages

of 15.454 ± 0.234 (2sd), 48.491 ± 0.225 and 1.008 ± 0.038 for the wt % content of Si, Zr

and Hf respectively. Few studies have published Hf concentration data for this standard,

although our Hf results are consistent with the 1.068 ± 0.201 wt % (2sd) measured by

Cavosie et al. (2006). A traverse over the ∼1.5 mm grain emphasises its homogeneity

(Figure 3.8), although lower Hf values occur at the edges of the grain.

59

3 Methodology

Isotope (%) Interference Comments49Ti (5.4) 96Zr++ and 48Ca interferences on 48Ti (73.7 %)139La (99.9)140Ce (88.5)141Pr (100)146Nd (17.9) 130Ba16O isobaric int. on 142Nd and 144Nd (27.2 and 23.8 %)149Sm (13.8) isobaric int. on 152Sm and 154Sm (26.8 and 22.8 %)153Eu (52.2) 137Ba16O 113In40Ar157Gd (15.7) 141Pr16O 139La18O isobaric int. on 156Gd 158Gd 160Gd (21, 25, 22 %)159Tb (100) 143Nd16O 141Ba18O161Dy (18.9) 130Ba16O isobaric int. on 162Dy and 164Dy (25.5 and 28.2 %)165Ho (100) 149Sm16O166Er (33.6) 150Sm16O 150Nd16O169Tm (100) 153Eu16O172Yb (21.9) 156Gd16O isobaric interference on 174Yb (31.8 %)175Lu (97.4) 159Tb16O 135Ba40Ar179Hf (13.6) 163Dy16O minor interference free isotope232Th (100)238U (99.3)

Table 3.12: Isotopes selected for measurement and the major associated interferences. % is therelative abundance of the particular isotope. Low abundance 179Hf selected because of the highconcentrations of Hf in zircon. Note that there are also various Ru, Rh, Te, Sb, Sn and Cs oxideinterferences on the REE.

Although shown in the spectometer setup (Table 3.11) P, Y and Ca were generally

not measured, as the concentrations of these elements in the Kim-5 standard and in

Sudbury zircons were typically close to or below detection limits.

3.3.2 Laser Ablation ICPMS protocols

Analyses were undertaken with a New Wave 193 nm excimer laser and a Thermo Scien-

tific Element 2 ICPMS. Measured elements were the 14 rare earth elements (REE; La,

Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu), Ti, U and Th, as well as

internal standard Hf. A key consideration when designing laser ablation (LA) ICPMS

methods is the minimisation of interferences. Table 3.12 displays the isotopes selected

for analysis and potential interferences. The formation of light REE oxides, which cre-

ate interferences on the heavy REE, is of particular concern. These interferences can

be resolved at medium (∼4000) or high (∼10000) resolution on the Element 2, however

there is a corresponding reduction of ion transmission of ∼80 % and 95 % respectively.

In order to maximise detection limits, low resolution (∼400) analyses were undertaken,

and instrument setup and tuning focused upon the reduction of oxide formation.

Typical laser and ICPMS operating conditions are shown in Table 3.13. The use of

H-geometry skimmer cones and increased torch z-position (i.e. increased distance from

the cones), in combination with careful tuning of the sample Ar and He gas flow rates,

60


Laser settings carrier gas He (0.7-0.8 l min-1)energy density 3-4.2 Jcm-2

repetition rate 4 Hzspot size 20-30 µm (zircons) and 20-65 µm (glasses)tubing FEP lined; mixing bulb at He-Ar connection

Instrument: resolution lowrf power ∼1350 Wsample cone Niskimmer cone Ni (H-geometry)scanning mode mass accuracyacquisition mode time-resolved analysisanalysis duration 90 s (30 s blank and 60 s ablation)torch z-pos. 0-1 mm

Ar gas flow rates: cool gas ∼ 17 l min-1

auxillary gas ∼ 0.8 l min-1

sample gas 1.0-1.1 l min-1

Table 3.13: Typical laser ablation and ICPMS operating conditions for trace element analysisof zircons.

reduces oxide formation significantly. Oxide formation was quantified by monitoring of

the 238U16O/238U ratio, and was typically <0.5 %.

Sequences involved the bracketing of four samples by analyses of the NIST SRM

612 glass standard (∼ 40 µg g-1 of most elements), which was used to normalise sample

analyses. The NIST SRM 612 values of Pearce et al. (1997) were used for normalisa-

tion, which was performed off-line, along with blank subtractions and internal standard

normalisation, using the Glitter software (Macquarie Research Ltd., 2001).

For the majority of analytical sessions all of the elements shown in Table 3.12 were

analysed. However, over the course of 18 months, in some analytical sessions Ti, U and

Th were measured without simultaneous determination of the REE.

3.3.3 Standards summary

In order to monitor the accuracy and precision of analyses, a range of standard reference

materials were analysed. These included the NIST SRM 614 and 612 glass standards,

USGS basalt glass standard BCR-2G and the zircon standards 91500 and Temora-2. The

measured concentrations of standards are displayed in Table 3.14. All of the standards

are within uncertainty of reference values. However, zircon standard 91500 was found

to be heterogeneous, with large differences between analysed grains, as well as different

domains of individual grains.

61

3 Methodology

Mean

2 sdRef.1

2 sdM

ean2 sd

Ref.12 sd

Mean

2 sdRef.1

2 sdM

ean2 sd

Ref.22 sd

La25.61

1.3325.26

0.2235.75

1.0235.89

0.220.68

0.0825.26

0.220.02

0.200.01

0.01Ce

56.102.37

53.750.76

38.171.47

38.660.26

0.780.18

53.750.76

2.860.78

2.560.51

Pr6.85

0.316.88

0.0837.14

1.7838.01

0.300.73

0.136.88

0.080.03

0.040.02

0.03Nd

29.171.31

28.820.34

35.182.36

35.630.24

0.770.19

28.820.34

1.130.51

0.240.08

Sm6.64

0.286.57

0.0836.90

1.4037.29

0.260.74

0.156.57

0.080.63

0.630.50

0.15Eu

2.040.08

1.950.02

34.361.87

35.410.20

0.730.13

1.950.02

0.290.22

0.240.06

Gd

7.310.50

6.790.06

36.872.56

38.320.30

0.740.17

6.790.06

2.661.85

2.210.49

Tb0.95

0.03-

-36.12

1.43-

-0.66

0.08-

-0.95

0.660.86

0.14Dy

6.070.25

6.450.08

35.880.74

36.170.04

0.740.13

6.450.08

13.919.38

11.801.65

Ho1.20

0.05-

-37.87

1.47-

-0.67

0.06-

-5.01

3.304.84

0.68Er

3.360.19

3.180.05

37.641.28

39.000.20

0.670.16

3.180.05

25.7516.69

24.604.92

Tm0.49

0.02-

-37.72

0.69-

-0.68

0.10-

-8.05

5.246.89

0.69Yb

3.450.19

3.340.04

40.242.67

38.600.08

0.770.18

3.340.04

121.865.08

73.907.39

Lu0.48

0.030.50

0.0137.99

1.8736.50

1.160.67

0.070.50

0.0111.26

6.8613.10

2.10

Mean

2 sdRef.3,4

2 sdM

ean2 sd

Ref.52 sd

Mean

2 sdRef.6

2 sdTi

9.050.44

9.10.6

47.531.30

48.116.02

3.820.35

3.771.60

U223.8

60.8174

14037.73

2.5637.23

1.440.84

0.210.83

0.08Th

118.425.8

7153

36.973.12

37.152.46

0.700.10

0.740.10

BCR-2G (n =

18)NIST 612 (n =

9)NIST 614 (n =

12)91500 (n =

18)

Temora 2 (n =

12)NIST 612 (n =

5)NIST 614 (n =

35)

Table

3.14:Sum

mary

ofm

eanm

easuredtrace

element

concentrationsof

zirconand

glassstandards,com

paredto

referencevalues.

References:

1-

Kent

etal.

(2004),2

-W

iedenbecket

al.(2004),

3-

Hiess

etal.

(2008),4

-B

lacket

al.(2004),

5-

Pearce

etal.

(1997),6

-G

aoet

al.(2002).

62


.

63

4

Isotopic heterogeneity in the Sudbury impact

melt sheet

4.1 Introduction

Central to resolving how the Sudbury melt sheet has evolved over time is an understand-

ing of the scales at which chemical heterogeneity has been preserved. As previously dis-

cussed, it has been shown that Pb isotopic differences are present between ores and basal

silicate rocks of the North and South Range Main Mass (Dickin et al., 1996). However,

there is currently little understanding of isotopic or trace element variability within each

range.

In order to characterise such heterogeneity, a Pb isotope investigation of the South

Range Main Mass was undertaken. The details and results of which are presented in in

the following paper, which is included in this chapter:

Darling J.R., Hawkesworth C.J., Lightfoot P.C., Storey C.D. and Tremblay E. (2009)

Isotopic heterogeneity in the Sudbury impact melt sheet. Earth and Planetary

Science Letters. doi:10.1016/j.epsl.2009.11.023

The results of this paper have some significant implications for models of Ni-Cu-PGE

sulphide ore formation in Sudbury. Additional results are presented after the paper that

relate to the potential use of Pb isotopes as an exploration tool in Sudbury.

64

Isotopic heterogeneity in the Sudbury impact melt sheet

Darling J.R.a,1, Hawkesworth C.J.a,2, Lightfoot P.C.b, Storey C.D.c, Tremblay E.b

a Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UKb Vale Inco Exploration, Highway 17 West, Copper Cli!, Ontario, P0M 1N0, Canada

c School of Earth and Environmental Sciences, University of Portsmouth, Portsmouth PO1 3QL, UK

Abstract

A unique terrestrial large impact melt sheet is preserved in the 1850 Ma Sudbury Structure, Ontario. Wehave undertaken a Pb isotope investigation of the southern limb of the melt sheet, termed the South RangeMain Mass. The model initial Pb isotope ratios (207Pb/204Pbm) vary stratigraphically through the predom-inantly quartz monzogabbroic Lower Unit, varying from 15.40 to 15.45 at the base to ca. 15.35 at the topof the sequence. Lateral variations of similar range occur in basal Lower Unit samples over scales of lessthan 5 km. The range of these variations is similar to those of locally exposed upper crustal target rocks,and it is evident that the melt sheet has e!ciently preserved inherited variability. During the violent phasesof crater formation superheated impact melts are expected to be well-mixed mechanically, therefore signif-icant post impact melting of target rocks, fallback material and entrained clasts is required to explain suchheterogeneity.

The Sudbury Structure hosts world class Ni-Cu-PGE sulphide ore deposits. Systematic variation in207Pb/204Pbm occurs throughout sulphide ores within the Creighton Embayment, from massive (15.42-15.45) to interstitial (ca. 15.40-15.41) and disseminated (ca. 15.39) sulphide. Linking the Pb isotopecomposition of these ores to the immediately overlying Lower Unit stratigraphy, a protracted sulphidesegregation history is apparent. Massive sulphides segregated early, prior to or during initial silicate crys-tallisation, although the total time involved in sulphide accumulation spanned much of the crystallization ofthe Lower Unit. It is also shown that lateral variations in Ni depletion throughout the Main Mass correlatewith Pb isotopes. Those segments with the strongest chalcophile element depletion signatures, reflectingthe accumulation of significant basal sulphides, have high initial Pb isotope values, consistent with earlysulphide segregation. The characterisation of Pb isotopic heterogeneity has therefore provided insights intothe evolution and scales of mixing of the melt sheet, with the identified chemical variability between meltcells having a significant influence on ore forming processes.

Key words: Sudbury, impact melt, Pb isotopes, sulphide

1. INTRODUCTION

Impact melts have typically been considered tobe homogeneous. This view originated in largepart from theoretical considerations on the violentmovement and mixing of superheated, low viscos-ity shock melt during crater evolution (Phinneyand Simonds, 1977; Simonds and Kie"er, 1993),and from studies of melts in some terrestrial im-pact structures (Floran et al., 1978; See et al.,1998). However, di"erentiation and footwall as-similation are known to result in variable majorand trace element chemistry (Spray and Thomp-son, 2008) and melts in the Popagai crater, Siberia(Kettrup et al., 2003), and from the Sudbury struc-ture, Ontario (Dickin et al., 1999, 1996) preserveisotopic heterogeneities associated with variable

1Correspondence: [email protected] address: O!ce of the Principal, University of St

Andrews, North Street, St Andrews, Fife, KY16 9AJ, Scotland

target lithologies. Despite these advances, thescale of heterogeneity preserved in larger meltsheets is poorly understood. The 1850 Ma Sud-bury Igneous Complex (SIC) is the largest pre-served terrestrial impact melt sheet, with an origi-nal melt volume of at least 8000 km3 (Grieve andCintala, 1992), and it has been intensively stud-ied, partly because of its extensive Ni-Cu-PGE sul-phide ore deposits. The SIC therefore providesa well-constrained example in which to investi-gate the scale of heterogeneity in large impact meltsheets.

Here we present Pb isotope and whole rock geo-chemical variations in the southern exposed limbof the Sudbury impact melt sheet. Constrainingthe scale of heterogeneity is important for the de-velopment of impact melting models for the Sud-bury event, and for the subsequent evolution of themelt sheet. Moreover, understanding the controlson sulphide formation and accumulation in theseareas is key both to models of sulphide ore forma-

EPSL doi:10.1016/j.epsl.2009.11.023 February 12, 2010

Grenvill

e Fron

tCopper Cliff

Offset

LakeWanapitei

Figure 2

Sudbury

NO

RT

H R

AN

GE

SO

UT

H R

AN

GE

Whitewater Group

Quartz dioriteLower UnitMiddle UnitUpper Unit

The Sudbury Igneous ComplexFootwall

Superior ProvinceSouthern Province

Fault

81°0

0'

81°3

0'

46°30'

46°45'

10kmM

ain

Mas

s

Figure 1: Simplified geological map of the Sudbury Igneous Complex (SIC), showing the distribution of the Main Mass and O"setDykes (after Ames et al., 2005), no Sublayer is shown. The location of the study area is outlined. Footwall rocks to the North Rangeof the complex are Archean Superior Province orthogneisses, and Huronian sedimentary rocks and basalts of the Southern Provinceto the South Range. Significant north - south shortening has occurred along shear zones, which cut the centre of current exposure ofthe complex, resulting in the elliptical outcrop pattern.

tion in Sudbury, and to our understanding of gen-eral controls on magmatic sulphide deposits.

1.1. The Sudbury Igneous Complex (SIC)

The SIC straddles the contact of Archean or-thogneisses of the Superior Province and Protero-zoic Huronian supracrustal rocks of the SouthernProvince (Figure 1), and formed as a result ofimpact melting of crustal lithologies at 1850 Ma(Dietz, 1964; Krogh et al., 1982). The associ-ated impact basin, known as the Sudbury Basin,had an original diameter of ca. 250 km (Sprayet al., 2004), making it one of the largest and old-est known terrestrial impact structures. In additionto containing one of the principal reserves of Ni-Cu-Platinum Group Element (PGE) sulphides onEarth, Sudbury is of great importance in the studyof impact processes due to the preservation of mul-tiple features of the impact structure. The highlydi"erentiated melt sheet (the Main Mass), togetherwith radial and concentric dykes, termed O"setDykes, comprise the SIC. Overlying the SIC isa sequence of crater filling breccias and clasticsediments of the Whitewater Group. Addition-ally, within the footwall rocks extensive pseudo-tachylitic breccia belts occur, known as SudburyBreccias.

Quartz diorites of the O"set Dykes are thoughtto have been injected into fractures created byreadjustment of the crater floor prior to di"eren-tiation of the melt sheet (Lightfoot and Farrow,2002; Tuchscherer and Spray, 2002), and maybe representative in composition of the bulk melt

(Lightfoot et al., 1997c). The Main Mass is di-visible into the Lower and Upper Unit, separatedby the Middle Unit, with the approximate propor-tions 30:60:10 respectively. The Lower Unit, of-ten termed the Sudbury norite, typically consistsof quartz monzogabbro to quartz gabbro, whilstthe Upper Unit is granitic. Although highly vari-able, the Main Mass has an average thickness ofca. 2.5 km (Keays and Lightfoot, 2004). Thediscontinuous Contact Sublayer occurs at the meltsheet-footwall contact and consists of inclusionsand sulphide-rich norite to monzogabbro, locallyunderlain by footwall breccias. Detailed accountsof the local geology of the SIC are given in thevolume edited by Pye (1984).

Investigations of the Sr, Nd, Os and Pb isotopiccharacteristics of SIC lithologies have revealed astrong crustal signature for all units, consistentwith generation by impact melting of the crust(Dickin et al., 1999, 1996, 1992; Faggart et al.,1985; Hurst and Wetheril, 1974; Morgan et al.,2002; Walker et al., 1991). Lightfoot et al. (1997d)suggested that a mantle contribution of 20 % canbe accommodated in the SIC, however it is not re-quired to explain the chemical variations through-out the Main Mass (Lightfoot et al., 2001). Indeedno unambiguous evidence for significant mantleinput into the melt sheet has been reported, fromeither geochemical, field or petrological studies.The relatively constant incompatible element ra-tios throughout the entire Main Mass stratigraphyhave been taken as evidence for a single parentmagma for all SIC lithologies (Lightfoot et al.,1997b).

66

488000 492000472000 476000 480000 484000

488000 492000476000 480000 484000 496000

514

0000

UTM E (m)

UT

M N

(m

)

5148000

5152000

1 23 4 5 6 7 8

910 11

12 14 15 1617 18

19

2021

2223

Upper Unit

Middle Unit

Lower Unit

Sublayer

Quartz diorite

144 Bypass

traverse

Abandoned Railway

North (ABRN)

traverse

Copper Cliff

Mouth (CCM)

traverse

Murray

traverse

Creighton

Embayment Copper Cliff

Offset Dyke

1206010 core

(402 orebody)Vermillion River

Figure 2: Simplified geological map of the study area, showing sample localities for whole rock geochemistry (small grey circles, n =729) and Pb isotopes (open circles, n = 82). Southern Province footwall rocks (unshaded) are undi"erentiated. Main Mass traversesare numbered for reference (see Table 1). The approximate surface projection of borehole 1206010 is shown. Note that this boreholewas collared from underground at a depth of ca. 2.3 km. The Creighton Embayment is a type example of trough like structures,known as embayments, which occur at irregular intervals along the basal contact (Morrison, 1984). Universal Transverse Mercator(UTM) grid lines shown (zone 17N; scale in meters).

While trace element ratios show only very sub-tle variations around the Main Mass, isotopic in-vestigations have revealed significant di"erencesbetween the two exposed limbs of the complex,termed the North and South Ranges. While theNorth Range is underlain by the ca. 2700 MaLevack Gneiss Complex (Krogh et al., 1984),the South Range country rocks are predominantlyPaleoproterozoic Huronian metasedimentary andmafic igneous rocks. Useful tools in assessingthe isotopic composition of these country rocksare pseudotachylitic breccias (Sudbury Breccias)found throughout the footwall. Since the majorityof clasts within the Sudbury Breccia are derivedfrom their host rock (Rousell et al., 2003), thecrystalline matrices of these breccias approximatethe isotopic composition of local country rocks.

Data from Dickin et al. (1996) indicatethat Huronian basalts and mixed basalt-sedimentsources have model initial Pb isotope signatures(207Pb/204Pbm) of 15.34 to 15.42, whereas theRamsay Lake and McKim sedimentary sequenceshave higher 207Pb/204Pbm (15.4 to 15.6). Incontrast, the equivalent matrices from Archeanorthogneiss-hosted breccias have much lower ini-tial ratios of 15.0 to 15.2. Dickin et al. (1996) alsofound systematic Pb isotope di"erences betweenNorth and South Range ores, and similarities inthe initial Pb isotope ratios of Sublayer samplesand Sudbury Breccias from each side of the com-plex. Pb isotope ratios of ore samples were foundto cluster close to 1850 Ma reference lines on plotsof 206Pb/204Pb versus 206Pb/204Pb, with small de-grees of scatter inferred to represent minor isotopicheterogeneity inherited during the Sudbury event.An origin for the ores cogenetic with basal silicaterocks was proposed, and a marked contrast in theisotopic signatures of the North and South Rangesidentified.

1.2. Ni-Cu-PGE sulphide ores

Sulphide mineralisation within the SIC is at-tributed to early sulphur saturation under super-heated conditions. Dense immiscible sulphide liq-uids accumulated towards the base of the meltsheet by e!cient gravitational settling, leavingit depleted in chalcophile elements (Keays andLightfoot, 2004). Sulphide ores are predominantlyfound along the basal contact of the melt sheet, inclose association with Sublayer norites, within theO"set Dykes and in footwall breccias beneath theSublayer. Major ore deposits are typically asso-ciated with depressions or troughs, known as em-bayment structures (see Figure 2). It has, however,proven di!cult to assess why the spatial distribu-tion of ores along the basal margin is so variable.Three mechanisms have been proposed: 1) con-vection resulting in the concentration of sulphidesin embayment structures. 2) The mineral potentialof the basal margin may vary with the thicknessand extent of metal depletion of the overlying sil-icate rocks (Keays and Lightfoot, 2004). 3) Theexistence of magma cells of di"ering compositioncould lead to di"erent sulphur saturation historiesaround the melt sheet.

2. Sampling

Sampling focused upon a segment of the MainMass in the South Range, centred on the CreightonEmbayment (Figure 2). This area was selectedfor a number of reasons. Significant litholog-ical variability occurs in South Range countryrocks, which have di"erent isotope ratios fromtheir North Range counterparts, o"ering greaterpotential for measurable variations. These foot-wall lithologies can be grouped into four potentialcompositional end members; i) Huronian metased-iments largely derived from the Neoarchean Supe-rior Province (McLennan et al., 1979), ii) Huro-

67

Traverse

Number Traverse Name

MgO

(wt%)

Ni

(ppm)

Ni/

Ni*

Cu

(ppm)

Zr

(ppm)

Cu/

Zr

1 Crean Hill 5.99 35.92 0.44 21.50 43.60 0.49

2 Crean Hill East 5.96 34.85 0.44 20.00 37.70 0.53

3 Lockerby Mine 6.19 40.60 0.47 26.20 52.54 0.50

4 ConWest 6.30 45.47 0.51 27.83 60.11 0.46

5 Graham West 5.36 40.16 0.63 27.40 52.71 0.52

6 Vermillion River East 5.66 42.27 0.59 38.57 63.27 0.61

7 Abandoned Railway North 5.39 34.08 0.53 29.28 57.73 0.51

8 Emma Lake South 5.26 34.92 0.57 31.79 60.83 0.52

9 Gertrude Echo 6.13 56.60 0.67 40.32 56.45 0.71

10 Gertrude West 5.95 35.88 0.45 28.14 63.43 0.44

11 Gertrude Pit 5.83 33.30 0.44 25.90 65.09 0.40

12 144 Bypass 6.08 40.37 0.48 29.72 60.44 0.49

13 Creighton 5.70 39.31 0.54 34.00 64.00 0.53

15 Creighton North East 5.83 41.74 0.55 41.72 65.59 0.64

16 North Star 5.98 50.80 0.63 33.98 58.13 0.58

17 North West Tailing 6.24 46.77 0.53 30.62 57.40 0.53

18 Tam O'Shanter West 6.01 46.14 0.57 34.82 63.42 0.55

19 Tam O'Shanter 5.58 37.06 0.53 31.41 58.20 0.54

20 Whitewater Lake South 5.71 47.53 0.65 31.94 62.21 0.51

21 Pump Lake West 5.90 47.54 0.61 31.70 52.96 0.60

22 Copper Cliff Mouth 5.90 43.17 0.55 29.30 44.82 0.65

23 Murray 6.33 47.66 0.52 31.39 55.33 0.57

Table 1: Mean MgO, Ni, Ni/Ni*, Cu, Zr and Cu/Zr for the studied traverses. Mean values were calculated from the thickness weightedmean of Lower Unit samples, in which sulphides do not control the nickel budget (monitored with S and Ni concentrations).

472000 476000 480000 484000 488000 492000

5140

000

51520005148000

476000 496000480000 484000 488000 492000UTM E (m)

UTM

N (m

)

Ni/ N i*0.00 - 0.400.41 - 0.500.51 - 0.600.61 - 0.700.71 - 1.00> 1.00

Creighton Embayment

Gertrude Mine

Crean HillMine

Copper CliffOffset Dyke

2 4 6 8 10 121

w-p basalt array

B

MgO (wt %)

Ni (

ppm

) 100

10

1000

475000 480000 485000 490000 495000

0 .4 5

0 .5 0

0 .5 5

0 .6 0

0 .6 5

UTM E (m)

Cu/Zr MeanNi/Ni* Mean

C

A

Qtz monzogabbro - qtz gabbroQuartz gabbro (basal Lower Unit)Sublayer norite

Figure 3: Metal depletion signatures in the Main Mass Lower Unit. A) Spatial variation in Ni/Ni* in the Lower Unit. For simplicitythe Lower Unit is un-shaded and no Sublayer is shown. B) Ni versus MgO content of Lower Unit and Sublayer samples from thisstudy. The curve shows a power law fit to the MgO-Ni array, i.e. Ni*, for within-plate (w-p) basalts (Lightfoot et al., 1997a). C)Calculated mean Ni/Ni* and non-model dependant Cu/Zr, plotted against mean east UTM grid reference for each respective traverse(data in Table 1).

68

nian mafic volcanics of the Elliot Lake Group, iii)the ca. 2220 Ma Nipissing mafic intrusive suite(Corfu and Andrews, 1986), and iv) other Paleo-proterozoic intrusions including the intermediate-felsic Creighton and Murray plutons, which areadjacent to the SIC contact. The study area con-tains some of the least deformed sections in theSouth Range, and previous workers have charac-terised traverses in the same region for whole rockgeochemistry, Sr, Nd and Pb isotopes, providinga framework for this investigation (Dickin et al.,1999, 1996; Faggart et al., 1985; Lightfoot andZotov, 2005). In the context of ore forming pro-cesses, the area is interesting because of the con-trast between segments of the basal contact thathost large ore deposits, such as the Copper Cli"O"set and Creighton Embayment, and adjacentsections that contain relatively little economic sul-phide ore.

Surface sampling for whole rock geochemistrywas undertaken by Vale Inco Limited as part of aprogram designed to establish whether the MainMass geochemistry records the signature of oreformation. Three sets of additional samples werecollected for Pb isotope analysis, with the loca-tions shown in Figure 2:

1) Four traverses normal to the basal contact,one throughout the entire Main Mass stratigra-phy, and 3 of the Lower Unit stratigraphy, all ofwhich were previously sampled for whole rockgeochemistry. 2) Basal Lower Unit quartz gab-bros and quartz monzogabbros were sampled totest lateral variability along the southern margin.Two samples were taken at each east-west loca-tion at di"ering distances from the basal contact,allowing for assessment of the sensitivity of Pbisotopes to height above the contact. 3) Sulphideores from the Creighton Embayment were sam-pled from Vale Inco diamond drill hole 1206010.The borehole was drilled from the footwall into theCreighton orebody and provides excellent exam-ples of di"erent styles of sulphide mineralisation.

3. Methods

3.1. Sulphide Pb isotopes

Sulphides were separated from the di"erentsample sets using a number of methods. MainMass samples were crushed and sieved to <500µm. Sulphides were separated utilising a wil-fley table, hand magnet (for pyrrhotite-rich frac-tions) and magnetic separator (for chalcopyrite-and pentlandite-rich sulphides). Ore samples weresimply crushed using an agate pestle and mor-tar. Sulphide aliquots for analysis were hand-picked under a binocular microscope, ensuringthat only the least altered crystals were analysed,then cleaned in an ultrasonic bath with 18 M#

H2O and 2 % HNO3 to remove surface contami-nation prior to dissolution.

Approximately 5 to 10 mg of sulphide was dis-solved in screw top Teflon beakers using 7 MHNO3 then dried down and repeatedly convertedinto chloride and bromide form. Samples weretaken up in 0.4 M HBr for chromatographic ion-exchange chemistry, and the solutions were loadedonto 50 µl Teflon columns filled with cleanedDowex AG-1X8 (200-400 mesh) anion exchangeresin. After eluting with a solution of 0.2 M HBrand 0.5 M HNO3, Pb was collected with a solu-tion of 0.03 M HBr and 0.5 M HNO3. This HBr-HNO3 chemistry is preferable to pure HBr elutionschemes when separation of Pb from high concen-trations of other chalcophile elements is required,because it reduces the tailing e"ects on elutionpeaks (Strelow, 1978). The total procedural blankswere always less than 100 pg and had no e"ect onthe reported numbers.

The isotopic composition of Pb was determinedon a Thermo Finnigan Neptune multi-collectorMC-ICP-MS system at the Department of EarthSciences, University of Bristol. Samples were in-troduced using a Cetac Aridus desolvating unit. Asample-standard bracketing method was utilised,since Pb does not have a pair of invariant isotopesthat can be used to correct for instrumental massbias during analysis. Although mass bias correc-tions can be performed by thallium doping, it hasbeen argued that this procedure can yield inaccu-rate results (Baker et al., 2004; Thirlwall, 2002).NIST SRM 981 was used as the bracketing stan-dard and the double-spike values of Baker et al.(2004) were used for normalisation. NIST SRM982 was used as a consistency standard, whichyielded averages, over a 9 month period (n =83), of 36.7484 ± 50 (137 ppm), 17.1649 ± 31and 36.7557 ± 88 for 206Pb/204Pb, 207Pb/204Pb and208Pb/204Pb respectively (uncertainties are 2 stan-dard deviations). Concentrations of analysed stan-dard and sample solutions were closely matchedat 50 ppb. In order to test the reproducibility ofthe sulphide data repeat analyses were made ofsample 1206010A2, a massive sulphide ore fromCreighton Mine. Four sample aliquots were pro-cessed and measured multiple times, yielding av-erages of 16.445 ± 0.003, 15.538 ± 0.003 and37.116 ± 0.009 for 206Pb/204Pb, 207Pb/204Pb and208Pb/204Pb respectively (n = 16).

3.2. Trace elements

Major element oxides and selected trace ele-ments were determined by XRF on glass discs andpowder pellets; minor and trace elements includ-ing Ni, Cu, and Co were determined by ICP-OES.Accuracy and precision were monitored relative toboth Standard Reference Materials (SRMs) and in-house reference materials.

69

Sam

ple

UT

M E

UT

M N

Lith

olo

gy

Su

lph

ide

Tra

v n

o.1

RS

P2 (m

)206P

b/204P

b2!

207P

b/204P

b2!

208P

b/204P

b2!

207P

b/204P

b1850

32!

Mo

del µ

Main

Mass T

ravers

es

Abandoned R

ailw

ay N

orth

Tra

vers

e

RX

371976

479779

5144015

SR

Norite

Cpy

750

20.8

02

0.0

03

15.9

90

0.0

03

44.6

48

0.0

13

15.3

87

0.0

05

16.1

RX

371974

479768

5144097

SR

Norite

Cpy

7112

22.0

06

0.0

03

16.1

28

0.0

03

44.4

89

0.0

13

15.3

88

0.0

05

19.7

RX

371972

479729

5144162

SR

Norite

Cpy

7170

31.0

12

0.0

05

17.1

67

0.0

04

58.2

75

0.0

17

15.4

09

0.0

06

46.8

RX

371895

479531

5144737

SR

Norite

Cpy

7691

23.6

79

0.0

04

16.3

04

0.0

03

50.2

03

0.0

14

15.3

75

0.0

05

24.7

RX

371900

479456

5145085

SR

Norite

Cpy

71006

20.9

75

0.0

03

16.0

09

0.0

03

46.6

01

0.0

13

15.3

85

0.0

05

16.6

RX

371966

479339

5145346

SR

Norite

Cpy

71243

18.1

99

0.0

03

15.7

36

0.0

03

40.8

23

0.0

12

15.4

27

0.0

04

8.2

RX

371962

479247

5145617

SR

Norite

Cpy

71489

18.3

80

0.0

03

15.7

04

0.0

03

40.9

50

0.0

12

15.3

74

0.0

04

8.8

RX

371958

479095

5145866

SR

Norite

Cpy

71715

22.6

48

0.0

04

16.2

01

0.0

03

50.0

64

0.0

14

15.3

88

0.0

05

21.6

Hig

hw

ay 1

44 B

ypass T

ravers

e

144IB

NR

1484485

5144962

Subla

yer N

orite

Po P

n C

py

12

018.4

12

0.0

03

15.6

52

0.0

03

38.8

68

0.0

11

15.3

19

0.0

04

8.9

144B

NR

484448

5145026

Qtz

Norite

Po +

Pn

12

100

20.9

86

0.0

03

16.0

92

0.0

03

44.3

61

0.0

13

15.4

67

0.0

05

16.6

JD

SIC

0701

484424

5145198

SR

Norite

Cpy

12

250

18.5

12

0.0

03

15.7

91

0.0

03

41.2

66

0.0

12

15.4

46

0.0

04

9.2

JD

SIC

0703

SR

Norite

Cpy

12

500

20.8

54

0.0

04

16.0

36

0.0

03

44.9

20

0.0

15

15.4

26

0.0

05

16.2

JD

SIC

0702

484346

5146123

SR

Norite

Cpy

12

874

20.6

51

0.0

03

16.0

00

0.0

03

42.7

68

0.0

12

15.4

14

0.0

05

15.6

144N

R1

484285

5146398

SR

Norite

Cpy

12

1070

23.0

50

0.0

04

16.2

44

0.0

03

49.4

32

0.0

14

15.3

86

0.0

05

22.8

144M

LN

R1

484215

5146825

Mela

norite

Cpy

12

1384

16.9

90

0.0

03

15.5

78

0.0

03

38.4

49

0.0

11

15.4

05

0.0

04

4.6

JD

SIC

0708

484163

5147319

SR

Norite

Cpy

12

1615

23.4

44

0.0

04

16.2

68

0.0

03

51.1

74

0.0

15

15.3

65

0.0

05

24.0

144Q

GA

BQ

uartz

Gabbro

Cpy

12

1848

20.4

86

0.0

03

15.9

38

0.0

03

46.4

86

0.0

13

15.3

70

0.0

05

15.1

144G

ran1

484239

5148937

Gra

nophyre

Cpy

12

2942

17.0

61

0.0

03

15.4

98

0.0

03

37.1

45

0.0

11

15.3

17

0.0

04

4.8

Copper C

liff Mouth

Tra

vers

eR

X374755

493459

5149416

Qtz

Norite

Cpy

22

-335

15.9

84

0.0

03

15.4

45

0.0

03

36.0

22

0.0

10

15.3

87

0.0

04

1.6

RX

374752

493426

5149543

Qtz

Norite

Cpy

22

-217

15.8

39

0.0

02

15.4

17

0.0

03

36.0

30

0.0

10

15.3

75

0.0

04

1.1

RX

374751

493446

5149589

Qtz

Norite

Cpy

22

-174

16.0

31

0.0

03

15.4

40

0.0

03

36.5

44

0.0

10

15.3

76

0.0

04

1.7

RX

374746

493352

5149807

Qtz

Norite

Cpy

22

29

37.5

56

0.0

06

17.9

00

0.0

04

72.6

05

0.0

21

15.4

02

0.0

07

66.5

RX

374742

493338

5150120

SR

Norite

Cpy

22

322

19.4

18

0.0

03

15.8

43

0.0

03

40.3

76

0.0

11

15.3

96

0.0

05

11.9

RX

374732

493274

5150649

SR

Norite

Cpy

22

815

17.0

58

0.0

03

15.5

58

0.0

03

39.0

05

0.0

11

15.3

77

0.0

04

4.8

RX

374737

493079

5150949

SR

Norite

Cpy

22

1096

20.8

12

0.0

03

15.9

64

0.0

03

45.0

52

0.0

13

15.3

59

0.0

05

16.1

RX

374741

493081

5151184

SR

Norite

Cpy

22

1314

20.1

38

0.0

03

15.9

03

0.0

03

43.7

86

0.0

12

15.3

75

0.0

05

14.1

RX

374808

492947

5151540

SR

Norite

Cpy

22

1647

21.1

18

0.0

03

16.0

18

0.0

03

48.6

60

0.0

14

15.3

79

0.0

05

17.0

RX

374805

492863

5151725

SR

Norite

Cpy

22

1820

21.5

79

0.0

03

16.0

55

0.0

03

47.5

02

0.0

13

15.3

63

0.0

05

18.4

RX

374802

492791

5151972

SR

Norite

Cpy

22

2051

17.4

71

0.0

03

15.5

79

0.0

03

39.2

96

0.0

11

15.3

52

0.0

04

6.0

RX

374814

492831

5152251

SR

Norite

Cpy

22

2311

18.0

45

0.0

03

15.5

90

0.0

03

39.6

72

0.0

11

15.2

98

0.0

04

7.8

70

Sam

ple

UT

M E

UT

M N

Lit

ho

log

yS

ulp

hid

eT

rav n

o.1

RS

P2 (

m)

206P

b/2

04P

b2!

207P

b/2

04P

b2!

208P

b/2

04P

b2!

207P

b/2

04P

b1850

32!

Mo

del µ

Murr

ay T

ravers

eR

X361042

494189

5150893

Subla

yer

Norite

Po C

py P

n23

32

18.9

44

0.0

03

15.7

96

0.0

03

42.3

87

0.0

12

15.4

02

0.0

05

10.5

RX

361045

494322

5150951

Qtz

Norite

Cpy

23

75

21.6

80

0.0

03

16.1

03

0.0

03

44.9

52

0.0

13

15.4

00

0.0

05

18.7

RX

361050

494295

5151199

SR

Norite

Cpy

23

260

30.4

88

0.0

05

17.0

86

0.0

04

60.8

33

0.0

11

15.3

87

0.0

06

45.2

RX

361128

494037

5152236

SR

Norite

Cpy

23

1031

18.2

95

0.0

03

15.6

98

0.0

03

41.9

86

0.0

12

15.3

78

0.0

04

8.5

RX

361125

493974

5152446

SR

Norite

Cpy

23

1187

17.2

96

0.0

03

15.5

78

0.0

03

39.2

63

0.0

11

15.3

71

0.0

04

5.5

RX

361122

493954

5152642

SR

Norite

Cpy

23

1333

22.1

13

0.0

03

16.1

09

0.0

03

47.8

78

0.0

14

15.3

57

0.0

05

20.0

RX

361120

493946

5152783

SR

Norite

Cpy

23

1437

22.8

00

0.0

04

16.1

73

0.0

03

49.9

05

0.0

14

15.3

43

0.0

05

22.1

Basal N

ori

tes

RX

361020

472419

5142299

Qtz

Norite

Cpy

1290

18.2

00

0.0

03

15.7

21

0.0

03

37.0

48

0.0

11

15.4

12

0.0

04

8.2

RX

361020_b

472419

5142299

Qtz

Norite

Po P

n1

290

18.1

94

0.0

03

15.7

22

0.0

03

37.0

51

0.0

11

15.4

13

0.0

04

8.2

RX

374696

474065

5142192

Qtz

Norite

Cpy

2240

22.8

18

0.0

04

16.2

29

0.0

03

45.2

05

0.0

13

15.3

97

0.0

05

22.1

RX

374629

475718

5142373

SR

Norite

Cpy

3208

18.5

10

0.0

03

15.7

45

0.0

03

40.5

71

0.0

12

15.4

00

0.0

04

9.2

RX

374717

476108

5142259

Qtz

Norite

Cpy

471

22.1

60

0.0

03

16.1

47

0.0

03

49.4

61

0.0

14

15.3

89

0.0

05

20.1

RX

374719

476086

5142353

SR

Norite

Cpy

4155

19.8

33

0.0

03

15.8

78

0.0

03

44.4

44

0.0

13

15.3

84

0.0

05

13.1

RX

374608

477919

5142948

Qtz

Norite

Cpy

587

20.0

25

0.0

03

15.8

83

0.0

03

46.7

89

0.0

13

15.3

67

0.0

05

13.7

RX

374609

477927

5142905

SR

Norite

Cpy

5162

16.8

56

0.0

03

15.5

27

0.0

03

38.0

46

0.0

11

15.3

69

0.0

04

4.2

JD

07S

IC10

478276

5143228

Qtz

Norite

Cpy

696

19.6

75

0.0

03

15.8

40

0.0

03

43.1

45

0.0

12

15.3

64

0.0

05

12.7

JD

07S

IC10_r

478276

5143228

Qtz

Norite

Po P

n6

96

19.6

12

0.0

03

15.8

33

0.0

03

42.9

80

0.0

12

15.3

64

0.0

05

12.5

JD

SIC

0711

478456

5143852

SR

Norite

Cpy

7150

20.5

86

0.0

03

15.9

63

0.0

03

44.6

73

0.0

13

15.3

84

0.0

05

15.4

JD

SIC

0714

481740

5144722

Qtz

Norite

Cpy

9121

24.8

41

0.0

04

16.3

95

0.0

04

50.3

06

0.0

14

15.3

35

0.0

05

28.2

JD

SIC

0712

482564

5144871

Qtz

Norite

Cpy

10

99

17.0

66

0.0

03

15.6

10

0.0

03

37.7

60

0.0

11

15.4

29

0.0

04

4.8

RX

357179

483670

5144994

Qtz

Norite

Cpy

11

65

20.3

08

0.0

03

15.9

50

0.0

03

41.5

38

0.0

12

15.4

02

0.0

05

14.6

JD

SIC

0736

487370

5147688

Qtz

Norite

Cpy

15

110

17.4

05

0.0

03

15.6

03

0.0

03

38.1

20

0.0

11

15.3

83

0.0

04

5.8

JD

SIC

0735

487398

5147498

Qtz

Norite

Cpy

16

85

20.0

31

0.0

03

15.8

76

0.0

03

42.7

43

0.0

12

15.3

59

0.0

05

13.7

JD

SIC

0734

487802

5147789

Qtz

Norite

Cpy

17

64

22.7

71

0.0

04

16.2

05

0.0

03

55.5

48

0.0

16

15.3

78

0.0

05

22.0

JD

SIC

0732

488609

5148253

Qtz

Norite

Cpy

18

76

29.5

77

0.0

05

16.9

61

0.0

04

63.9

35

0.0

18

15.3

64

0.0

06

42.5

JD

SIC

0731

489517

5148618

SR

Norite

Cpy

18

187

22.5

75

0.0

04

16.1

75

0.0

03

48.2

81

0.0

14

15.3

71

0.0

05

21.4

JD

SIC

0728

490424

5148727

Qtz

Norite

Cpy

19

101

18.2

23

0.0

03

15.6

83

0.0

03

39.9

45

0.0

11

15.3

70

0.0

04

8.3

RX

374772

491392

5149821

Qtz

Norite

Cpy

20

65

18.3

55

0.0

03

15.6

96

0.0

03

42.9

05

0.0

12

15.3

69

0.0

04

8.7

RX

374774

491371

5149886

SR

Norite

Cpy

20

130

20.1

96

0.0

03

15.9

08

0.0

03

43.7

14

0.0

12

15.3

72

0.0

05

14.2

RX

374791

492216

5150279

Qtz

Norite

Cpy

21

127

17.8

09

0.0

03

15.5

92

0.0

03

37.8

89

0.0

11

15.3

27

0.0

04

7.1

RX

374792

492227

5150231

SR

Norite

Cpy

21

163

19.7

45

0.0

03

15.8

46

0.0

03

43.6

20

0.0

12

15.3

62

0.0

05

12.9

71

Drill C

ore

1206010

Co

re D

ep

th4 (m

)206P

b/204P

b2!

207P

b/204P

b2!

208P

b/204P

b2!

207P

b/204P

b1850

32!

Mo

del µ

1206010_A

A1

31

Po C

py

Aliq

uot 1

16.3

82

0.0

02

15.5

35

0.0

02

37.1

97

0.0

07

15.4

31

0.0

03

2.8

1206010_A

A1_r

31

Po C

py

Aliq

uot 2

16.3

82

0.0

02

15.5

33

0.0

02

37.1

94

0.0

07

15.4

30

0.0

03

2.8

1206010_B

B37

Po P

nA

liquot 1

16.0

35

0.0

02

15.4

82

0.0

02

36.2

58

0.0

07

15.4

17

0.0

03

1.7

1206010_B

B_r1

37

Po P

nA

liquot 2

16.0

87

0.0

02

15.4

92

0.0

02

36.3

26

0.0

07

15.4

21

0.0

03

1.9

1206010_B

B_r2

37

Cpy

Aliq

uot 2

16.0

44

0.0

02

15.4

87

0.0

02

36.2

83

0.0

07

15.4

22

0.0

03

1.7

1206010_C

C1

76

Po C

py

Aliq

uot 1

17.4

09

0.0

02

15.6

27

0.0

02

36.4

35

0.0

07

15.4

07

0.0

03

5.9

1206010_C

C1_r

76

Po C

py

Aliq

uot 2

16.0

66

0.0

02

15.4

78

0.0

02

36.2

12

0.0

07

15.4

10

0.0

03

1.8

1206010_D

D1

82

Po P

nA

liquot 1

15.8

80

0.0

02

15.4

59

0.0

02

35.9

63

0.0

06

15.4

12

0.0

03

1.3

1206010_D

D1_r

82

Po P

nA

liquot 1

15.9

26

0.0

02

15.4

68

0.0

02

36.0

07

0.0

06

15.4

16

0.0

03

1.4

1206010_D

D2

82

Cpy

Aliq

uot 2

15.9

63

0.0

02

15.4

68

0.0

02

36.0

30

0.0

06

15.4

12

0.0

03

1.5

1206010_E

E108

Po C

py P

n15.9

50

0.0

02

15.4

60

0.0

02

35.9

96

0.0

06

15.4

05

0.0

03

1.5

1206010_F

F112

Po C

py P

n15.8

19

0.0

02

15.4

43

0.0

02

35.9

27

0.0

06

15.4

03

0.0

03

1.1

1206010_G

G128

Po P

n15.8

36

0.0

02

15.4

30

0.0

02

36.0

93

0.0

06

15.3

88

0.0

03

1.1

1206010_A

A2_1

31

Po C

py

Aliq

uot 1

16.4

46

0.0

03

15.5

39

0.0

03

37.1

19

0.0

11

15.4

28

0.0

04

3.0

1206010_A

A2_2

31

Po C

py

Aliq

uot 1

16.4

43

0.0

03

15.5

38

0.0

04

37.1

16

0.0

14

15.4

27

0.0

05

2.9

1206010_A

A2_3

31

Po C

py

Aliq

uot 1

16.4

45

0.0

03

15.5

37

0.0

03

37.1

14

0.0

11

15.4

26

0.0

04

3.0

1206010_A

A2_4

31

Po C

py

Aliq

uot 1

16.4

43

0.0

03

15.5

35

0.0

03

37.1

06

0.0

11

15.4

24

0.0

04

2.9

1206010_A

A2_5

31

Po P

nA

liquot 2

16.4

47

0.0

03

15.5

40

0.0

03

37.1

21

0.0

11

15.4

29

0.0

04

3.0

1206010_A

A2_6

31

Po P

nA

liquot 2

16.4

46

0.0

03

15.5

38

0.0

03

37.1

16

0.0

11

15.4

27

0.0

04

3.0

1206010_A

A2_7

31

Po P

nA

liquot 2

16.4

44

0.0

03

15.5

37

0.0

03

37.1

11

0.0

11

15.4

26

0.0

04

2.9

1206010_A

A2_8

31

Po P

nA

liquot 2

16.4

45

0.0

03

15.5

39

0.0

03

37.1

20

0.0

11

15.4

28

0.0

04

3.0

1206010_A

A2_9

31

Cpy

Aliq

uot 3

16.4

44

0.0

03

15.5

37

0.0

03

37.1

15

0.0

11

15.4

26

0.0

04

2.9

1206010_A

A2_10

31

Cpy

Aliq

uot 3

16.4

45

0.0

03

15.5

38

0.0

03

37.1

17

0.0

11

15.4

27

0.0

04

3.0

1206010_A

A2_11

31

Cpy

Aliq

uot 3

16.4

47

0.0

03

15.5

40

0.0

03

37.1

22

0.0

11

15.4

29

0.0

04

3.0

1206010_A

A2_12

31

Cpy

Aliq

uot 3

16.4

46

0.0

03

15.5

38

0.0

03

37.1

18

0.0

11

15.4

27

0.0

04

3.0

1206010_A

A2_13

31

Po C

py P

nA

liquot 4

16.4

47

0.0

03

15.5

40

0.0

03

37.1

22

0.0

11

15.4

29

0.0

04

3.0

1206010_A

A2_14

31

Po C

py P

nA

liquot 4

16.4

44

0.0

03

15.5

37

0.0

03

37.1

15

0.0

11

15.4

26

0.0

04

2.9

1206010_A

A2_15

31

Po C

py P

nA

liquot 4

16.4

42

0.0

03

15.5

36

0.0

03

37.1

14

0.0

11

15.4

25

0.0

04

2.9

Mean

16.4

45

15.5

38

37.1

16

15.4

27

2!

0.0

03

0.0

03

0.0

09

0.0

03

Table2:

Pbisotope

resultsfrom

Main

Mass

traverses,basalLow

erU

nitsamples

andores

fromthe

Creighton

Em

bayment.

The

Low

erU

nitconsistsof

quartzgabbro

toquartz

monzogabbro,w

itha

gradualincreasein

quartzcontent(up

toca.20

%)in

thelow

ermost!

200m

ofstratigraphy.Abbreviations;C

py(chalcopyrite),Po

(pyrrhotite),Pn(pentlandite).Sam

plelocations

provided,UT

Mzone

17N,N

AD

27.1

Traverselocations

providedin

Figure2.

2R

SP-relative

stratigraphicposition.

3M

odelinitialPbisotope

ratios.

72

4. Results

4.1. Whole rock geochemistry

The samples have a wide range of Ni and MgOcontents (Table 1). The Ni/MgO ratio, which isan index of sulphide control in mafic magmas(Lightfoot et al., 1994), falls systematically up-ward through the stratigraphy. In the Sublayer andlowermost Lower Unit, the ratio is controlled bythe modal abundance of sulphide. Higher in theLower Unit stratigraphy the Ni/MgO ratio falls to<6, indicative of magmas that have been strippedof Ni (Lightfoot et al., 2001). A measure of thisdepletion is provided by comparison of Ni con-tents with those expected for rocks of similar MgOcontents (Table 1). Expected Ni concentrations(Ni*) were taken from a power law correlation ofNi and MgO from sulphur undersaturated within-plate basalts (Figure 3b), following Lightfoot et al.(2001). The Lower Unit rocks of the SIC all showNi depletion signatures (low Ni/Ni*) except in themost basal samples where sulphide accumulationhas occurred. New data from this study show thatthe magnitude of depletion varies laterally alongthe South Range (Figure 3a).

An overview of this lateral variability is pro-vided by the mean Ni/Ni* of samples for each indi-vidual traverse (Figure 3c). Strikingly, the highermean Ni/Ni* values (0.6-0.7), i.e. from the leastdepleted rocks, overlie relatively barren segmentsof the basal SIC contact, and lower mean Ni/Ni*traverses (0.4-0.5) overlie the heavily mineralisedCreighton embayment region. The Cu/Zr ratio isan additional sensitive, non-model dependant in-dicator of chalcophile element depletion, becausetypically Cu and Zr behave incompatibly in theabsence of sulphide. Comparison of the Ni/Ni*data with Cu/Zr is shown in Figure 3c, with simi-lar trends apparent.

4.2. Pb isotopes

The Pb isotope compositions of all analysedsamples are given in Table 2. The samples define awide range of present day 206Pb/204Pb (15.8 to 30)and 208Pb/204Pb (36 to 72). In plots of uranogenicand thorogenic Pb, samples cluster around an 1850Ma reference line (the age of the complex), anda 206Pb/204Pb versus 207Pb/204Pb isochron of allLower Unit samples gives an age of 1859 ± 32 Ma(2sd), providing confidence that Pb isotope sys-tematics have not been significantly a"ected byprocesses after the emplacement of the SIC. Sub-tle variations occur, represented by displacementfrom the 1850 Ma reference lines. In order toresolve these di"erences it is useful to calculateinitial lead isotope ratios. It would be preferableto use measured U/Pb and Th/Pb to correct for

in-growth of radiogenic Pb, however there is ev-idence for variable recent disturbance of U and Thin sulphide ores (Dickin et al., 1996).

Pb isotope ratios are less susceptible to distur-bance, and coupled 206Pb/204Pb and 206Pb/204Pbcan be used to estimate model lead isotope ratios at1850 Ma. A modified version of the methodologyof Dickin et al (1996) is used here. Present day Pbisotope compositions are projected back parallel to1850 Ma reference lines, to a selected 206Pb/204Pbvalue of 15.464, which is the corresponding valueof a two stage Pb isotope evolution model at 1850Ma (Stacey and Kramers, 1975). Such model ra-tios at 1850 Ma are di"erent from initial Pb iso-tope ratios age calculated using measured U andTh, but they o"er a robust measure of the relativevariation in 207Pb/204Pb at 1850 Ma, and are re-ported as 207Pb/204Pbm.

4.2.1. Basal Main Mass

Significant variations are seen in the model ini-tial Pb isotope ratios of the basal Main MassLower Unit samples along strike (Figure 4a).Whilst the total range of 207Pb/204Pbm is small(15.32 to 15.44), the di"erences are significantgiven that uncertainty in the model ratios is 0.003to 0.005 at 2 standard deviations. Gradual changesin 207Pb/204Pbm occur over scales of ca. 5 km,with two exceptions at ca. 482000 m east andca. 492000 m east, where sharp decreases areobserved. The most radiogenic 207Pb/204Pbm ra-tios occur in three sections of the basal contact,in the Crean Hill - Lockerby mine, Gertrude -Creighton and Copper Cli" areas. The scale oflateral variation is similar to that shown by meanNi/Ni*. There is a strong negative correlation be-tween 207Pb/204Pbm and Ni/Ni*, with sulphidesat the base of more Ni depleted traverses havinghigher 207Pb/204Pbm ratios than those associatedwith less depleted traverses (Figure 4b).

The two samples taken at each east-west lo-cation at di"erent stratigraphic heights have in-distinguishable Pb isotope ratios, indicating thatthe east-west trend is not sensitive to strati-graphic position within the lowermost 200-300 mof the stratigraphy. In samples where chalcopy-rite and pyrrhotite could both be separated thetwo fractions were found to have indistinguishable207Pb/204Pbm.

4.2.2. Traverses

Model initial Pb isotope variations along thefour analysed traverses are shown in Figure 5.In three of the traverses a Pb isotope stratigra-phy is clearly defined, with a progressive changefrom more radiogenic compositions of 15.40 to15.45 in basal Lower Unit samples to ca. 15.35in the uppermost Lower Unit. No systematic

73

20

7P

b/

20

4P

bm

UTM E (m)

Mean Ni/Ni*

15.34

15.36

15.38

15.40

15.42

15.44

470000 475000 480000 485000 490000 495000

0.4 0.45 0.5 0.55 0.6 0.65 0.7

15.32

15.34

15.36

15.38

15.40

15.42

15.44

r2 = 0.72

B

15.32

Crean

Hill

Gertrude

144 BypassCopper

Cliff

A

Figure 4: Pb isotope variation in basal Lower Unit samples. A) Model initial Pb isotope ratios plotted against easting (top). Uncer-tainties at 2 standard deviations are within the data points. Names of associated traverse locations are shown for reference. Broadly,more radiogenic values occur around large orebodies (e.g. Gertrude, Creighton, Copper Cli" and Crean Hill). B) Correlation betweenNi/Ni* and model initial Pb isotopes.

variation is discernable in the Abandoned Rail-way North (ABRN) traverse, although deviationsoutside of analytical uncertainty do occur. Themost radiogenic values measured in this study of207Pb/204Pbm (ca. 15.47) occur at the base of the144 Bypass traverse.

Model initial Pb isotope variations through theentire South Range stratigraphy are shown forthe 144 Bypass traverse. The Pb isotope com-positions of trace sulphides separated from theMain Mass are indistinguishable from spatiallyassociated feldspar values (Dickin et al., 1999).207Pb/204Pbm values in the Middle Unit are slightlylower than those at the top of the Lower Unit. Thegranophyre samples have even lower 207Pb/204Pbmvalues, similar to those of the North Range. Majorelement trends (Lightfoot and Zotov, 2005) ruleout mixing of Upper Unit granitic and Lower Unitmonzogabbroic liquids as a mechanism to createeither the general decrease in initial Pb isotope ra-tios up-section, or the intermediate values in theMiddle Unit.

4.2.3. Creighton ores

A general decrease in 207Pb/204Pbm is seenthrough the 1206010 drill hole (Figure 6), withfour distinct groupings. The highest values, nearthe footwall contact, are from a massive chalcopy-rite rich (cpy) vein, followed by massive pyrrhotite(po) and pentlandite (pn), which are intersectedover a 10 m core length. A group with lower val-ues includes Cpy and Po+Pn veins, as well as in-terstitial sulphide in a norite. Disseminated sul-phide from a sample of norite at 130 m has thelowest model initial value. The total range of

207Pb/204Pbm from the 1206010 core (ca. 15.38to 15.43) is similar to that of Lower Unit sam-ples from the overlying 144 Bypass traverse (15.37to 15.47). At issue is the cause of these varia-tions in Pb isotopes, and the links between the Pbisotope values in the sulphides and the overlyingMain Mass.

5. Discussion

5.1. Heterogeneity in the SIC

The new Pb isotope data have resolved small,but significant, isotopic variations both laterallyand vertically throughout the SIC. The Pb iso-tope stratigraphy defined in the traverses indicatesvariable proportions of melt from di"ering targetlithologies. Lateral variations therefore o"er po-tential for constraining the scale of preserved het-erogeneity.

Lateral variability in the initial Pb isotope ratiosof basal Lower Unit samples may reflect the exis-tence of cells of magma of di"erent compositions,or be a result of sampling at di"erent heights inthe melt sheet because of the marked relief alongthe basal contact. In the latter case, given thestratigraphic changes in Pb isotopes identified, theheight in the stratigraphy would determine the iso-tope composition of the samples analysed. Sev-eral lines of evidence suggest that this is not thecase; i) the thickness of the Lower Unit does notappear to be the principal control on the total rangeof Pb isotope stratigraphy (Figure 5). ii) SouthRange traverses show systematic trends in majorelement concentrations throughout the Lower Unit

74

Copper Cliff Mouth (CCM)

Murray

Abandoned Railway N. (ABRN)

144 Bypass 144 Bypass

144 Bypass

Lower Unit traverses Upper and Middle Units

Sublayer norite

Copper Cliff Mouth

0

500

1000

1500

2000

2500

3000

15.30 15.35 15.40 15.45

-500

Rel

ativ

e St

ratig

raph

ic P

ositi

on (m

)

207Pb/204Pbm

Typical uncertainty(2!)

Figure 5: Model initial Pb isotope variation through Main Masstraverses. Samples in the Copper Cli"Mouth traverse that plotat negative stratigraphic height are sulphide ores hosted by Sub-layer norites. The 144 Bypass traverse includes Middle andUpper Unit samples. Shaded grey squares are feldspar datafrom Dickin et al. (1999).

stratigraphy (Lightfoot and Zotov, 2005), yet nocorrelation is observed between whole rock ma-jor element concentrations and 207Pb/204Pbm. iii)The initial Pb isotope ratios at each sampling loca-tion are not sensitive to distance from the contactwithin the first ca. 200 m of stratigraphy (Figure4).

It has been suggested that local, late stage, as-similation strongly influenced Sublayer composi-tions (Lightfoot et al., 1997d; Prevec et al., 2000;Rao et al., 1985) and that this was the main controlon Pb isotope compositions (Dressler and Sharp-ton, 1998). However, it remains di!cult to linkthe Pb isotope values in the basal Lower Unit sam-ples with the spatial distribution of locally ex-posed country rocks. There are also examplesin which the Pb isotope ratios of Sublayer rocksare markedly di"erent from overlying Lower Unitquartz monzogabbros. For example in the 144 By-pass traverse low 207Pb/204Pbm values (15.3) occurin the Sublayer, whereas much more radiogenicvalues are recorded from the immediately overly-ing Lower Unit (Fig. 5).

The Lower Unit rocks of the SIC are LREE en-riched, and measures such as (Ce/Yb)CHUR are auseful tool to discriminate between crustal sourceend member compositions (Lightfoot et al., 1997b;Naldrett and Hewins, 1984; Prevec et al., 2000).The low (Ce/Yb)CHUR value of the 144 BypassSublayer (ca. 6) is consistent with a sourcestrongly influenced by locally exposed Huronian

0

20

40

60

80

100

120

207Pb/204Pbm

Rel

ativ

e St

ratig

raph

ic P

ositi

on(m

)

15.38 15.40 15.4415.42 15.46

Sulphide blebs in Sublayer noriteG -Interstitial sulphide in Sublayer noriteE & F -Massive po + pnD -Chalcopyrite rich vein in SublayerC -Massive po + pnB -Massive sulphide vein in footwall (po)A -Massive sulphide vein in footwall (cpy)A -

Figure 6: Model initial Pb isotope variation in the 1206010drill core, Creighton 402 orebody. Note that the borehole wascollared at a depth of ca. 2.3 km and drilled northerly from thefootwall into the orebody at the base of the melt sheet. Heightis the stratigraphic height in the Sublayer norite sequence abovethe basal contact.

basalts. Such assimilation cannot be responsiblefor the radiogenic initial Pb ratios in the adjacentoverlying Lower Unit quartz gabbros, which have(Ce/Yb)CHUR ratios of 8.9 to 9.1. As indicated byPrevec et al. (2000), it is evident that the compo-sition of the Sublayer magma is chemically dis-tinct from that of the lower Main Mass, and thatlocalised, late stage, assimilation of country rocksdoes not appear to be the dominant control on thePb isotope variations in the overlying Lower Unit.

Lateral variations in basal Lower Unit samplestherefore o"er insight into the scale of heterogene-ity preserved in the SIC, which we interpret to re-flect the existence of cells of di"ering Pb isotopecomposition. Trends in the mean Ni/Ni* valuesof sampled traverses and initial Pb isotope ratiosof basal Lower Unit samples are very similar, andbroad changes in both datasets occur over intervalsof 6-8 km (Figures 3 and 4). Superimposed on thisbroad trend are significant variations between ad-jacent basal Lower Unit samples and traverses. Forexample, at ca. 482000 m east a sample with low207Pb/204Pbm appears to be an outlier. However, itis matched by a traverse with high average Ni/Ni*.This implies that lateral heterogeneity is preservedon scales of <2 km, a conclusion supported by thedi"erences between the Copper Cli" and Murraytraverses, which are approximately 1.2 to 1.3 kmapart.

5.2. Physical constraints on the scale of convec-tion

From calculated Raleigh numbers Zieg andMarsh (2005) show that vigorous thermal convec-tion was likely in the SIC at temperatures abovethe liquidus. Mathematical constraints on the scaleof convection in the melt sheet can therefore besimply approximated, assuming that the Lower

75

4000m15.3

5

15.4

0

15.4

5

ABRN 144 Bypass CCM Murray MeanNi/Ni*

Creighton Embayment Copper Cliff Offset

0.46

0.50

0.54

0.58

0.62

0.66

0.42

Figure 7: Schematic representation of the preservation of cells of di"ering isotopic composition and metal depletion in the LowerUnit of the South Range. The shading of cells represents the mean Ni/Ni* values for sampled traverses, as shown in Figure 3.The diameters of cells were taken from lateral variations in the Ni/Ni* and Pb isotope datasets, together with the expected scale ofconvection. Also shown are model initial Pb isotope profiles for the studied traverses (as Figure 5), the scales for each profile areidentical. ABRN; Abandoned Railway North traverse: CCM; Copper Cli"Mouth traverse. Uncertainties in model initial Pb isotopesare within the thickness of the line. Black areas of the basal contact are known heavily mineralised regions.

Unit monzogabbroic liquid behaves in a Newto-nian manner at such temperatures. Given a systemin which melt is convecting between two parallelrigid boundaries, driven by a temperature gradient,the horizontal wavelength of velocity and temper-ature fluctuations about the static state should beabout twice the melt thickness. In the study regionthickness varies between 1500 and 2200 m. Ac-cordingly, the horizontal wavelength of convectionwould have been in the region of 3 to 4.5 km, sim-ilar to the scale of lateral geochemical variationsidentified. Therefore, such a convective regimewas clearly not e!cient at laterally homogenisingthe melt sheet. The scale of preserved heterogene-ity is illustrated schematically in Figure 7.

The lack of conspicuous modal layering, ther-mal constraints and interpretations of chemicalprofiles, have led to models invoking in situ crys-tallisation of a density stratified Sudbury meltsheet, with crystallisation of the Lower Unit oc-curring from the base upwards via the inwardpropagation of solidification fronts (e.g. Zieg andMarsh, 2005). It might therefore be expected thatthe uppermost Lower Unit samples would tend to-wards homogeneity. Interestingly, whilst the up-permost samples in each traverse have the least ra-diogenic values, the 207Pb/204Pbm ratios of 15.388,15.365, 15.352 and 15.343 in the ABRN, 144 By-pass, Copper Cli" Mouth (CCM) and Murray tra-verses respectively, di"er significantly. The im-plication of this west to east decrease is di!cultto constrain, although it may be related to generaltrends in melt contribution from di"erent footwalllithologies.

5.3. The origin of heterogeneity

Heterogeneities in the Lower Unit rocks of theSouth Range reflect varying contributions fromdi"erent Southern Province target lithologies. Assuggested by Simonds and Kie"er (1993), it isdi!cult to envisage a scenario whereby super-heated, low viscosity melt produced during the vi-olent contact/compression and excavation stagesof crater formation would retain heterogeneity onsuch a scale. Collapse of the transient cavity

would have taken a few minutes (Melosh andIvanov, 1999). Mixing in this environment ismechanical and driven by shear stresses that areinevitable in the sub-spherical geometry and ra-dial flow typical of impact craters (Melosh, 1989;Phinney and Simonds, 1977; Simonds and Kief-fer, 1993). The chemical di"erences between theNorth and South Ranges of the SIC, which werelikely separated by tens of kilometres, may havebeen preserved throughout these processes, but itis much less likely that the sub 5 km lateral hetero-geneities identified in this study would have doneso. The melt was initially superheated to 2000 K(Ivanov and Deutsch, 1999) and, depending uponthe e!ciency of convective heat loss, may not havecooled to liquidus temperatures for between sev-eral years to ca. 10 kyr (Zieg and Marsh, 2005). Itis therefore likely that the heterogeneity preservedin the Lower Unit rocks of the SIC relates to con-tinued melting of footwall rocks, fallback materialand entrained clasts after crater formation.

5.4. Impact melting models for the Sudbury event

The predominance of Huronian Pb isotope com-positions for the entire stratigraphy of the SouthRange (Figure 8) is surprising in the context ofsome estimates of the depth of melting in the Sud-bury event. During impact, the immense kineticenergy of the impactor is transferred to the targetby a shock wave. The majority of melted materialoriginates from an approximately spherical vol-ume below the maximum depth of penetration ofthe impacting body (Melosh, 1989). According tothe scaling relations of Grieve and Cintala (1992),a chondritic impactor for the Sudbury event wouldhave been around 8-14 km in diameter, travellingat 20-25 kms-1. The maximum depth of penetra-tion of such an impactor is approximately equal toits diameter (Grieve and Cintala, 1992). Given to-tal thickness of the Huronian Supergroup of 8-15km (Dressler, 1984; Young et al., 2001), most ofthe melting should therefore have taken place be-low these cover rocks, in the underlying Archeanbasement (Deutsch et al., 1995; Grieve et al., 1991;Mungall et al., 2004).

76

15.1 15.2 15.3 15.4 15.5 15.6 15.7

0

1

2

4

8

12

10

20

30

40

1

2

3

4

15.0

207Pb/

204Pbm

Fre

qu

en

cy

Sudbury BrecciaNorth Range South Range

South Range Ores

South Range

Lower Unit

South Range Upper and Middle Units

Figure 8: Histograms of the model initial Pb isotope composi-tion of Main Mass units and ores, compared to Sudbury Brec-cia (pseudotachylite) data from Dickin et al. (1996). SudburyBreccias approximate a regional average of the country rocks(Dressler, 1984).

Major element mixing models have been shownto be consistent with this conclusion. Estimates ofthe bulk SIC composition can be produced from95 % Archean basement and 5 % Huronian sedi-ments (Grieve et al., 1991; Mungall et al., 2004),implying that Huronian supracrustal material wasvaporised and expelled from the crater. However,neither the South Range Upper Unit nor LowerUnit have initial Pb isotope compositions compat-ible with sources dominated by Archean basementrocks (Figure 8). Hence, in contrast to the conclu-sions of Mungall and Hanley (2004), a significantupper to mid crustal component must be invokedin order to explain the isotopic compositions of theSouth Range.

5.5. Controls on sulphide forming processes

The identification of both lateral and verticalPb isotope variability in the South Range sug-gests that the formation and preservation of meltcells may be important for ore forming processes.Sulphide ores, accumulated at the base of themelt sheet, will preserve the isotopic compositionof the Lower Unit melts from which they seg-regated. Sulphides along the basal margin withhigh 207Pb/204Pbm are linked to Lower Unit mon-zogabbroic rocks lower in the sequence, and sam-ples with low 207Pb/204Pbm to higher levels in thestratigraphy. Given that the Lower Unit crys-tallised from the base upwards (e.g. Zieg andMarsh, 2005), and that the Pb isotope ratios of

trace sulphides from traverses are identical to as-sociated feldspar analyses, this essentially meansthat Pb isotopes chart the relative timing of sul-phide segregation from the Lower Unit. Whilstthis model is complicated by the di"erences in Pbisotope stratigraphy from di"erent traverses, thegeneral trend of each traverse is to higher valuesat the base.

Linking the varying Pb isotope ratios of di"er-ent mineralisation styles in the Creighton 402 ore-body to the overlying stratigraphy, it is clear thatthese sulphides were not derived from a single, in-stantaneous event (Figure 6). Massive ores, withhigh 207Pb/204Pbm, formed early, consistent withfindings that the Lower Unit was grossly over-saturated in sulphur at its silicate liquidus tempera-ture of ca. 1200 "C (Keays and Lightfoot, 2004; Liand Naldrett, 1993; Naldrett, 1989). The ranges ofPb isotope ratios in the 402 orebody, and Sublayerore samples from the Copper Cli"Mouth traverse,cover much of the defined Lower Unit Pb isotopestratigraphy. The total time involved in the genesisof these sulphides must therefore be similar to thatinvolved in the crystallisation of the Lower Unit,consistent with predictions that the melt remainedstrongly S-saturated throughout the crystallisationof the Lower Unit (Keays and Lightfoot, 2004).

It has also been shown that Pb isotopes vary sys-tematically with Ni depletion in the Main Mass(Fig. 4). Those segments in which accumulatedbasal sulphide has high Pb isotope values, consis-tent with early segregation, also have the strongestchalcophile element depletion signatures. In addi-tion, these segments correlate spatially with sec-tions of the basal contact that are known to havethe largest accumulations of sulphide. The oc-currence of this radiogenic Pb isotope componentseems integral to early sulphur saturation and max-imum metal depletion of the Lower Unit. Giventhe currently available dataset of Pb isotopes infootwall rocks, the likely sources of this melt com-ponent are Huronian metasedimentary rocks.

The results of this investigation suggest a pro-tracted S-saturation history of the Main Mass.However, given the lateral variability in basalLower Unit sulphides, it is apparent that the earlysaturation events are not ubiquitous, and that somecells probably did not reach sulphur saturation un-til nearer the onset of silicate crystallisation. Thesecells also have the lowest metal depletion signa-tures (e.g. Ni/Ni* and Cu/Zr).

6. Conclusions

Isotopic heterogeneity is present throughout theSudbury impact melt sheet. Cells of di"eringchemical composition were generated on lateralscales of less than a few kilometres, and a Pb

77

isotope stratigraphy formed during crystallisationof the Lower Unit. The range of isotopic vari-ations throughout the Lower Unit of the MainMass is similar to those of locally exposed foot-wall rocks. Although the initial products of im-pact melting may have been homogenised withinminutes of impact by violent mechanical mixingduring crater development, the superheated melthad significant potential for continued post-impactmelting of the footwall, fallback breccia and en-trained clasts. Chemical heterogeneity was thusformed on a scale similar to the estimated scale ofconvection cells.

The relatively radiogenic isotopic signature ofall South Range lithologies analysed indicates thatHuronian metasedimentary and mafic rocks were adominant source component for the South Rangeof the melt sheet. Our results indicate a significantmelt contribution from the upper crust, reflectingimpact melting at shallower levels than previouslysuggested, or that large volumes of melt may havebeen generated post impact. As such there is aneed to reconcile the geochemical characteristicsof SIC lithologies with impact melting models.

Pb isotopes provide a relative sense of timing ofsulphide segregation from the Main Mass. Earlysulphide saturation, at temperatures likely in ex-cess of the Lower Unit liquidus, formed muchof the massive O"set Dyke and Contact Sublayermineralisation, as highlighted by data from theCreighton 402 orebody in the Creighton embay-ment. However, given that the range of initialPb isotopes in ores have a similar range to thefull Lower Unit stratigraphy, it is apparent thatsulphide segregation occurred over a protractedperiod. Variable chalcophile element depletionthroughout the South Range suggests that cells ofthe Lower Unit have di"ering sulphur saturationhistories. Correlation with Pb isotopes indicatesthat melt derived from di"erent crustal sources hasa fundamental control on such processes.

Although convection and gravitationally drivenflow of sulphide melts may be responsible for thelocalisation of ores, the preservation of cells ofmelt of di"ering composition appears to have astrong influence on the location of major ore de-posits. No attempt has been made in this investi-gation to ascertain why apparently having highermelt contribution from Huronian sediments leadsto earlier sulphur saturation, and further investiga-tions should do so. Furthermore, it is logical thatearlier formed sulphides will have access to themost chalcophile element-rich melts, and thereforemay be expected to contain the highest metal con-centrations (tenor).

7. acknowledgements

We thank Vale Inco for their logistical and fi-nancial support throughout this investigation. TheSudbury Basin Group of Vale Inco Exploration arethanked for their scientific contributions and as-sistance with fieldwork. This project was fundedby a Natural Environment Research Council stu-dentship to J. Darling. The manuscript benefitedfrom constructive reviews by John Spray, RichardWalker and an anonymous reviewer, as well ashelpful comments from editor Rick Carlson.

References

Ames, D. E., Davidson, A., Buckle, J. L., Card, K. D., 2005.Geology, Sudbury bedrock compilation, Ontario. Vol. OpenFile 4570. Geological Survey of Canada.

Baker, J., Peate, D., Waight, T., Meyzen, C., 2004. Pb isotopicanalysis of standards and samples using a Pb-207-Pb-204double spike and thallium to correct for mass bias with adouble-focusing MC-ICP-MS. Chemical Geology 211 (3-4), 275–303.

Corfu, F., Andrews, A. J., 1986. A U-Pb Age for MineralizedNipissing Diabase, Gowganda, Ontario. Canadian Journalof Earth Sciences 23 (1), 107–109.

Deutsch, A., Grieve, R., Avermann, M., Bischo", L., Brock-meyer, P., Buhl, D., Lakomy, R., MullerMohr, V., Oster-mann, M., Sto$er, D., 1995. The Sudbury Structure (On-tario, Canada): A tectonically deformed multi-ring impactbasin. Geol Rundsch 84 (4), 697–709.

Dickin, A. P., Artan, M. A., Crocket, J. H., 1996. Isotopicevidence for distinct crustal sources of North and SouthRange ores, Sudbury Igneous Complex. Geochimica et Cos-mochimica Acta 60 (9), 1605–1613.

Dickin, A. P., Nguyen, T., Crocket, J. H., 1999. Isotopic evi-dence for a single impact melting origin of the Sudbury Ig-neous Complex. In: Dressler, B., Sharpton, V. (Eds.), LargeMeteorite Impacts and Planetary Evolution II. Vol. SpecialPaper 339. Geological Society of America, Boulder, Col-orado, pp. 361–371.

Dickin, A. P., Richardson, J. M., Crocket, J. H., Mcnutt, R. H.,Peredery, W. V., 1992. Osmium isotope evidence for acrustal origin of Platinum Group Elements in the Sudburynickel ore, Ontario, Canada. Geochimica et CosmochimicaActa 56 (9), 3531–3537.

Dietz, R. S., 1964. Sudbury Structure as an astrobleme. Journalof Geology 72 (4), 412.

Dressler, B., Sharpton, V., 1998. Comment on ”Isotopic ev-idence for distinct crustal sources of North and SouthRange ores, Sudbury Igneous Complex”. Geochimica etCosmochimica Acta 62 (2), 315–317.

Dressler, B. O., 1984. General geology of the Sudbury area. In:Pye, E., Naldrett, A., Giblin, P. (Eds.), The geology and oredeposits of the Sudbury Structure. Vol. Special Volume 1.Ontario Geological Survey, pp. 52–87.

Faggart, B. E., Basu, A. R., Tatsumoto, M., 1985. Origin ofthe Sudbury Complex by metoritic impact - neodymium iso-topic evidence. Science 230 (4724), 436–439.

Floran, R. J., Grieve, R. A. F., Phinney, W. C., Warner, J. L.,Simonds, C. H., Blanchard, D. P., Dence, M. R., 1978. Man-icouagan impact melt, quebec. 1; stratigraphy, petrologyand chemistry. Journal of Geophysical Research 83 (NB6),2737–2759.

Grieve, R. A. F., Cintala, M. J., 1992. An analysis of di"eren-tial impact-melt crater-scaling and implications for the ter-restrial impact record. Meteoritics 27 (5), 526–538.

Grieve, R. A. F., Sto$er, D., Deutsch, A., 1991. The SudburyStructure - controversial or misunderstood. Journal of Geo-physical Research - Planets 96 (E5), 22753–22764.

78

Hurst, R. W., Wetheril, G., 1974. Rb-Sr study of the SudburyNickel Irruptive. Eos, Transactions, American GeophysicalUnion 55 (4), 466–466.

Ivanov, B., Deutsch, A., 1999. Sudbury impact event: Cra-tering mechanics and thermal history. In: Dressler, B. O.,Sharpton, V. L. (Eds.), Large Meteorite Impacts and Plane-tary Evolution II. Vol. 339. Geological Society of America,Boulder, Colorado, pp. 389–398.

Keays, R. R., Lightfoot, P. C., 2004. Formation of Ni-Cu-Platinum Group Element sulfide mineralization in the sud-bury impact melt sheet. Mineralogy and Petrology 82 (3-4),217–258.

Kettrup, B., Deutsch, A., Masaitis, V., 2003. Homogeneous im-pact melts produced by a heterogeneous target? Sr-Nd iso-topic evidence from the Popigai crater, Russia. Geochimicaet Cosmochimica Acta 67 (4), 733–750.

Krogh, T. E., Davis, D., Corfu, F., 1984. Precise U-Pb zirconand baddeleyite ages for the Sudbury area. The Geology andOre Deposits of the Sudbury Structure Special Volume 1,431–446.

Krogh, T. E., McNutt, R. H., Davis, G., 1982. Two high preci-sion U-Pb ages for the Sudbury Nickel Irruptive. CanadianJournal of Earth Sciences 19, 723–728.

Li, C., Naldrett, A. J., 1993. Sulfide capacity of magma -a quantitative model and its application to the formationof sulfide ores at sudbury, ontario. Econ. Geol. Bull. Soc.88 (5), 1253–1260.

Lightfoot, P., Hawkesworth, C., Olshefsky, K., Green, T., Do-herty, W., Keays, R., 1997a. Geochemistry of tertiary tholei-ites and picrites from Qeqertarssuaq (Disko Island) and Nu-ussuaq, West Greenland with implications for the mineralpotential of comagmatic intrusions. Contributions To Min-eralogy and Petrology 128 (2-3), 139–163.

Lightfoot, P. C., Doherty, W., K.Farrell, Keays, R. R., Moore,M. L., Pekeski, D., 1997b. Geochemistry of the Main Mass,Sublayer, O"sets and inclusions from the Sudbury IgneousComplex, Ontario. Ontario Geological Survey, Open FileReport 5959.

Lightfoot, P. C., Farrow, C. E. G., 2002. Geology, geochem-istry, and mineralogy of the Worthington O"set Dike: Agenetic model for o"set dike mineralization in the SudburyIgneous Complex. Economic Geology 97 (7), 1419–1446.

Lightfoot, P. C., Keays, R. R., Doherty, W., 2001. Chemicalevolution and origin of nickel sulfide mineralization in theSudbury Igneous Complex, Ontario, Canada. Economic Ge-ology 96 (8), 1855–1875.

Lightfoot, P. C., Keays, R. R., Morrison, G. G., Bite, A., Far-rell, K. P., 1997c. Geochemical relationships in the Sud-bury Igneous Complex: Origin of the Main Mass and O"setDikes. Economic Geology 92 (3), 289–307.

Lightfoot, P. C., Keays, R. R., Morrison, G. G., Bite, A., Far-rell, K. P., 1997d. Geologic and geochemical relationshipsbetween the Contact Sublayer, inclusions, and the MainMass of the Sudbury Igneous Complex: A case study ofthe Whistle Mine embayment. Economic Geology 92 (6),647–673.

Lightfoot, P. C., Naldrett, A. J., Gorbachev, N. S., Federenko,V. A., Hawkesworth, C. J., Hergt, J., 1994. Source and evo-lution of the Siberian trap lavas, Noril’sk district, Russia:Implications for the evolution of sulfides. In: Ontario Geo-logical Survey. Spevial volume 5, pp. 283–312.

Lightfoot, P. C., Zotov, I. A., 2005. Geology and geochemistryof the Sudbury Igneous Complex, Ontario, Canada: Originof nickel sulfide mineralization associated with an impact-generated melt sheet. Geology of Ore Deposits 47 (5), 349–381.

McLennan, S. M., Fryer, B. J., Young, G. M., 1979. Rare-EarthElements in Huronian (lower Proterozoic) sedimentary-rocks - composition and evolution of the post-Kenoran up-per crust. Geochimica et Cosmochimica Acta 43 (3), 375–388.

Melosh, H. J., 1989. Impact cratering: A geological process.Oxford University Press, New York.

Melosh, H. J., Ivanov, B. A., 1999. Impact crater collapse. An-nual Review of Earth and Planetary Sciences 27 (385-415).

Morgan, J. W., Walker, R. J., Horan, M. F., Beary, E. S.,

Naldrett, A. J., 2002. Pt-190-Os-186 and Re-187-Os-187systematics of the Sudbury Igneous Complex, Ontario.Geochimica et Cosmochimica Acta 66 (2), 273–290.

Morrison, G., 1984. Morphological features of the SudburyStructure in relation to an impact origin. In: Pye, E., Nal-drett, A., Giblin, P. (Eds.), The geology and ore deposits ofthe Sudbury Structure. Vol. Special Volume 1. Ontario Ge-ological Survey, pp. 513–520.

Mungall, J. E., Ames, D. E., Hanley, J. J., 2004. Geochemicalevidence from the Sudbury Structure for crustal redistribu-tion by large bolide impacts. Nature 429 (6991), 546–548.

Mungall, J. E., Hanley, J. J., 2004. Origins of outliers of theHuronian Supergroup within the Sudbury Structure. Journalof Geology 112 (1), 59–70.

Naldrett, A., 1989. Magmatic sulfide deposits. Oxford Univer-sity Press.

Naldrett, A. J., Hewins, R. H., 1984. The Main Mass of theSudbury Igneous Complex. In: Pye, E., Naldrett, A., Giblin,P. (Eds.), The Geology and Ore Deposits of the SudburyStructure. Ontario Geological Survey, Ch. 10, pp. 236–251.

Phinney, W. C., Simonds, C. H., 1977. Dynamical implicationsof the petrology and distribution of impact melt rocks. In:Roddy, J., Pepin, O., Merrill, B. (Eds.), Impact and Explo-sion Cratering: Planetary and Terrestrial Implications. Perg-amon Press, Flagsta", Arizona.

Prevec, S. A., Lightfoot, P. C., Keays, R. R., 2000. Evolution ofthe Sublayer of the Sudbury Igneous Complex: geochemi-cal, Sm-Nd isotopic and petrologic evidence. Lithos 51 (4),271–292.

Pye, E. G. (Ed.), 1984. The geology and ore deposits of theSudbury Structure. Ontario Geological Survey.

Rao, B. V., Naldrett, A. J., Evensen, N. M., 1985. Crustal con-tamination of the sublayer, Sudbury Igneous Complex, andits relevance to the genesis of Ni-Cu sulfides. Canadian Min-eralogist 23, 329–330.

Rousell, D. H., Fedorowich, J. S., Dressler, B. O., 2003. Sud-bury Breccia (Canada): a product of the 1850 Ma Sud-bury Event and host to footwall Cu-Ni-PGE deposits. Earth-Science Reviews 60 (3-4), 147–174.

See, T. H., Wagsta", J., Yang, V., Horz, F., McKay, G. A.,1998. Compositional variation and mixing of impact meltson microscopic scales. Meteoritics and Planetary Science33 (4), 937–948.

Simonds, C. H., Kie"er, S. W., 1993. Impact and volcanism - amomentum scaling law for erosion. Journal of GeophysicalResearch - Solid Earth 98 (B8), 14321–14337.

Spray, J. G., Butler, H. R., Thompson, L. M., 2004. Tectonicinfluences on the morphometry of the Sudbury impact struc-ture: Implications for terrestrial cratering and modeling.Meteoritics and Planetary Science 39 (2), 287–301.

Spray, J. G., Thompson, L. M., Dec. 2008. Constraints on cen-tral uplift structure from the Manicouagan impact crater.Meteoritics and Planetary Science 43 (12), 2049–2057.

Stacey, J. S., Kramers, J. D., 1975. Approximation of terres-trial lead isotope evolution by a 2-stage model. Earth andPlanetary Science Letters 26 (2), 207–221.

Thirlwall, M., 2002. Multicollector ICP-MS analysis of Pb iso-topes using a (207)pb-(204)pb double spike demonstratesup to 400 ppm/amu systematic errors in Tl-normalization.Chemical Geology 184 (3-4), 255–279.

Tuchscherer, M. G., Spray, J. G., 2002. Geology, mineraliza-tion, and emplacement of the Foy O"set dike, Sudbury im-pact structure. Economic Geology 97 (7), 1377–1397.

Walker, R. J., Morgan, J. W., Naldrett, A. J., Li, C., Fassett,J. D., 1991. Re-Os isotope systematics of Ni-Cu sulfide ores,Sudbury Igneous Complex, Ontario - Evidence for a ma-jor crustal component. Earth and Planetary Science Letters105 (4), 416–429.

Young, G. M., Long, D. G. F., Fedo, C. M., Nesbitt, H. W.,2001. Paleoproterozoic Huronian basin: product of a Wil-son cycle punctuated by glaciations and a meteorite impact.Sedimentary Geology 141, 233–254.

Zieg, M. J., Marsh, B. D., 2005. The Sudbury Igneous Com-plex: Viscous emulsion di"erentiation of a superheated im-pact melt sheet. Bulletin of the Geological Society of Amer-ica 117 (11-12), 1427–1450.

79

4 Isotopic heterogeneity in the Sudbury impact melt sheet

4.3 Pb isotopes as a tool for sulphide ore exploration?

The recognition that sulphide ores at the base of the Main Mass reflect a protracted

segregation history (see Darling et al., 2010) leads to three principal hypotheses regarding

the formation of sulphide ore deposits:

1. Chemical heterogeneity in the Main Mass was a fundamental control on sulphide

segregation and the location of major ore deposits.

2. Early sulphide segregation led to the formation of the principal reserves of massive

sulphide at the base of the melt sheet.

3. Sulphides that segregate early will have access to the most Ni, Cu and PGE rich

melts, and will thus have the highest concentrations of these elements (tenor).

The correlation between the Pb isotope composition of basal sulphides (a proxy for

timing of segregation) and the extent of chalcophile element depletion in the Lower Unit

of the Main Mass, indicates that the timing of sulphide segregation and the volume of

sulphide removed are intrinsically linked. In order to further understand the controls

on this relationship, it is necessary to better understand the inputs into the melt sheet

(i.e. target rock contributions). For example, in addition to temperature and oxidation

state (see Naldrett, 1989) sulphur saturation is controlled by a range of compositional

variables, principally: sulphur and Fe content (Haughton et al., 1974) and the concentra-

tions of other metal cations that form sulphide phases (e.g. Ni, Cu, Mn; Evans, 2008).

An assessment of the main target rock contributions to the melt sheet is presented in the

following chapter. The rest of this section focuses upon the third of these hypotheses.

In order to test whether sulphides that segregated early have the highest metal tenors,

ore samples of varying mineralisation styles and sulphide contents were taken from the

Creighton embayment. The Pb isotope compositions of sulphide separates was deter-

mined and the model initial Pb isotope ratios (following Darling et al., 2010) compared

to whole rock major and minor element concentrations.

4.3.1 Sampling strategy

The basal contact of the Creighton embayment dips ∼ 50 to the north, and contains

a series of footwall and contact orebodies that have been mined to a depth of ∼ 3 km.

The sample set analysed in this investigation was selected from a much larger sampling

program of the Creighton ore-system undertaken by Vale Inco. Selected samples have

a range of mineralisation styles and sulphide contents similar to those sampled from

80

4.3. PB ISOTOPES AS A TOOL FOR SULPHIDE ORE EXPLORATION?

15.4!

15.5!

15.6!

15.7!

15.8!

15.9!

16.0!

15.5! 16.5! 17.5! 18.5! 19.5! 20.5!

20

7P

b/2

04P

b!

206Pb/204Pb!

34!

36!

38!

40!

42!

44!

46!

15.0! 16.0! 17.0! 18.0! 19.0! 20.0!

20

8P

b/2

04P

b!

206Pb/204Pb!

0 Ga!

1 Ga!

Figure 4.1: Plots of uranogenic and thorogenic Pb isotope ratios of ore samples from throughoutthe Creighton embayment. A regression line through all samples on the plot of 206Pb/204Pb versus207Pb/204Pb coincides with a 2-stage model Pb isotope evolution (Stacey and Kramers, 1975)reference isochron of 1.856 Ga. Regression line through all samples on the plot of 206Pb/204Pbversus 208Pb/204Pb intercepts the Stacey and Kramers (1975) growth curve at ∼1.9 Ga.

the 1206010 drill core (Darling et al., 2010, Figure 6) which is also from the Creighton

embayment, and range in depth from the surface (Gertrude Pit) to ∼ 1.5 km (402

orebody). The samples were also selected on the basis that whole rock geochemical data

was available from Vale inco.

4.3.2 Pb isotope results

The Pb isotope ratios of sulphide separates from the samples are presented in Table

4.1, along with the whole rock geochemistry data provided by Vale Inco. The samples

define linear arrays on plots of both uranogenic and thorogenic Pb, consistent with their

having been no significant disturbance of the Pb isotope system since formation at 1.85

Ga (Figure 4.1). However, there is considerable scatter around the defined reference

isochrons that suggests that Pb in the samples does not have a single source. In order

to further evaluate these variations, model initial 207Pb/204Pb ratios were calculated.

The total range of 207Pb/204Pbm (15.383 - 15.454) is slightly larger than measured in

the 1206010 drill core (15.388 - 15.431) and comparable to the total range of Lower Unit

samples from the overlying 144 Bypass traverse (15.365 - 15.467). Consistent with the

conclusions drawn from the 1206010 drill core data, this implies a protracted sulphide

segregation history for the formation of the ores in the Creighton embayment.

4.3.3 Comparison of Pb isotopes and whole rock geochemistry

Comparison of the 207Pb/204Pbm values of the sample sulphide separates with whole rock

geochemical data is presented in Figure 4.2. Given that sulphides will dominate the S

and Ni budget of the samples, the concentrations of these elements can be taken as a mea-

81


Sam

ple

loca

tion

Sulp

hid

eC

uw

r

wt.%

Niw

r

wt.%

Cow

r

wt.%

Sw

r

wt.%

Ni/

SC

o/S

206P

b/

204P

b2σ

207P

b/

204P

b2σ

208P

b/

204P

b2σ

207P

b/

204P

bm

2σ

Nip

o

wt.%

Copo

(ppm

)

RX

378909

402

BL

BY

-M

ASU

0.1

50.2

70.0

11.6

10.1

69

0.0

08

20.3

95

0.0

03

15.9

43

0.0

03

45.1

52

0.0

13

15.3

85

0.0

05

RX

378903

402

OB

BL

BY

0.4

80.5

20.0

23.1

10.1

66

0.0

05

18.7

42

0.0

03

15.7

79

0.0

03

42.2

23

0.0

12

15.4

08

0.0

04

2.0

0289

RX

378926

Up-D

ip402

OB

PT

CH

0.2

10.6

90.0

35.7

50.1

21

0.0

05

16.1

57

0.0

03

15.4

78

0.0

03

36.7

06

0.0

10

15.4

00

0.0

04

1.9

0447

RX

357177

Gertru

de

Pit

INSU

0.2

50.8

80.0

47.4

90.1

17

0.0

05

16.6

94

0.0

03

15.5

29

0.0

03

37.4

01

0.0

11

15.3

90

0.0

04

2.0

1615

RX

357177

Gertru

de

Pit

INSU

0.2

50.8

80.0

47.4

90.1

17

0.0

05

16.6

02

0.0

03

15.5

14

0.0

03

37.3

65

0.0

11

15.3

86

0.0

04

1.6

5492

RX

378910

402

OB

BL

BY

-M

ASU

1.5

21.0

80.0

47.9

50.1

35

0.0

04

16.5

13

0.0

03

15.5

01

0.0

03

36.6

49

0.0

10

15.3

83

0.0

04

2.2

5414

RX

378921

B.

Gertru

de

Pit

INM

S2.1

11.1

80.0

512.2

50.0

96

0.0

04

17.5

33

0.0

03

15.6

43

0.0

03

38.3

44

0.0

11

15.4

09

0.0

04

2.3

9651

RX

378907

402

OB

BL

BY

-M

ASU

0.3

21.2

30.0

47.1

90.1

71

0.0

05

18.7

05

0.0

03

15.7

81

0.0

03

41.5

06

0.0

12

15.4

15

0.0

04

1.5

8220

RX

378927

Up-D

ip402

OB

MA

SU

0.4

71.4

50.0

510.4

50.1

38

0.0

05

17.2

18

0.0

03

15.6

51

0.0

03

37.9

88

0.0

11

15.4

52

0.0

04

3.1

0616

RX

378913

402

OB

INM

S1.0

71.6

30.0

510.6

00.1

54

0.0

05

16.6

28

0.0

03

15.5

59

0.0

03

37.1

76

0.0

11

15.4

27

0.0

04

2.0

2381

RX

378934

Up-D

ip402

OB

INM

S0.1

81.8

70.0

612.0

00.1

56

0.0

05

17.5

66

0.0

03

15.6

88

0.0

03

38.4

73

0.0

11

15.4

51

0.0

04

2.5

5481

RX

378942

Up-D

ip402

OB

INM

S0.2

82.0

40.0

612.5

50.1

63

0.0

05

16.9

82

0.0

03

15.6

04

0.0

03

37.5

46

0.0

11

15.4

32

0.0

04

2.7

3556

RX

357179

Gertru

de

pit

INM

S3.4

32.4

60.0

820.1

00.1

22

0.0

04

20.3

08

0.0

03

15.9

50

0.0

03

41.5

38

0.0

12

15.4

02

0.0

05

RX

378911

402

OB

INM

S4.6

32.7

40.0

822.0

00.1

25

0.0

04

16.6

44

0.0

03

15.5

47

0.0

03

37.3

53

0.0

11

15.4

14

0.0

04

2.3

6420

RX

378922

B.

Gertru

de

Pit

BL

BY

0.1

72.8

70.1

123.5

00.1

22

0.0

05

16.7

89

0.0

03

15.6

01

0.0

03

37.6

26

0.0

11

15.4

51

0.0

04

2.3

9687

RX

378918

B.

Gertru

de

Pit

RG

DI

1.1

73.0

50.0

919.7

50.1

54

0.0

04

17.3

28

0.0

03

15.6

17

0.0

03

38.5

77

0.0

11

15.4

06

0.0

04

2.1

1643

RX

378929

Up-D

ip402

OB

RG

DI

0.3

64.6

90.1

430.5

00.1

54

0.0

04

16.7

58

0.0

03

15.5

49

0.0

03

37.5

64

0.0

11

15.4

03

0.0

04

2.3

7575

RX

357182

Gertru

de

Pit

MA

SU

0.0

74.0

90.1

834.5

00.1

19

0.0

05

16.8

18

0.0

03

15.5

38

0.0

03

37.2

30

0.0

11

15.3

85

0.0

04

1.7

4920

1033790-2

486

402

18.6

78

0.0

03

15.8

13

0.0

03

40.7

47

0.0

12

15.4

50

0.0

04

3.1

2676

1033790-2

430

402

16.9

44

0.0

03

15.5

72

0.0

03

38.3

39

0.0

11

15.4

04

0.0

04

1.8

8434

1033790-2

029

Up-D

ip402

OB

17.2

29

0.0

03

15.6

03

0.0

03

38.3

13

0.0

11

15.4

04

0.0

04

2.1

7509

1033790-1

083

Up-D

ip402

OB

16.5

44

0.0

03

15.5

22

0.0

03

37.4

23

0.0

11

15.4

00

0.0

04

1.3

9304

1033790-2

042

Up-D

ip402

OB

16.6

71

0.0

03

15.5

37

0.0

03

37.3

93

0.0

11

15.4

01

0.0

04

2.1

0535

Table

4.1:Sum

mary

ofP

bisotope

andm

ajorand

minor

element

datafor

sulphideore

bearingsam

plesfrom

theC

reightonem

bayment.

Whole

rockN

i,Cu,C

oand

Svalues

areshow

n(C

uw

retc.),together

with

Pb

isotopeand

Niand

Co

(Nip

oetc.)

measurem

entson

pyrrhotiterich

sulphideseparates.

Location

relatesto

theorebody

within

theC

reightonem

bayment

mine

system;from

surfaceto

depththese

are:G

ertrudeP

it,Below

(B.)

Gertrude

Pit,

Up-dip

402O

rebody(O

B)

andthe

402O

rebody.Sulphide

mineralisation

styles:B

LB

Y-

blebby,M

ASU

-m

assivesulphide,

PT

CH

-patchy,

INM

S-

inclusionbearing

massive

sulphide,R

GD

I-

ragged(inclusions

insulphide

matrix).

82


0!

5!

10!

15!

20!

25!

15.36! 15.38! 15.40! 15.42! 15.44! 15.46!

S (

wt %

)!

207Pb/204Pbm!

0!

0.5!

1!

1.5!

2!

2.5!

3!

3.5!

15.36! 15.38! 15.40! 15.42! 15.44! 15.46!

Ni (w

t %

)!

207Pb/204Pbm!

0.06!

0.08!

0.10!

0.12!

0.14!

0.16!

0.18!

0.20!

15.36! 15.38! 15.40! 15.42! 15.44! 15.46!

Ni/S!

207Pb/204Pbm!

0.002!

0.003!

0.004!

0.005!

0.006!

0.007!

0.008!

0.009!

15.36! 15.38! 15.40! 15.42! 15.44! 15.46!

Co/S!

207Pb/204Pbm!

(a)

0!

5!

10!

15!

20!

25!

15.36! 15.38! 15.40! 15.42! 15.44! 15.46!

S (

wt %

)!

207Pb/204Pbm!

0!

0.5!

1!

1.5!

2!

2.5!

3!

3.5!

15.36! 15.38! 15.40! 15.42! 15.44! 15.46!

Ni (w

t %

)!

207Pb/204Pbm!

0.06!

0.08!

0.10!

0.12!

0.14!

0.16!

0.18!

0.20!

15.36! 15.38! 15.40! 15.42! 15.44! 15.46!

Ni/S!

207Pb/204Pbm!

0.002!

0.003!

0.004!

0.005!

0.006!

0.007!

0.008!

0.009!

15.36! 15.38! 15.40! 15.42! 15.44! 15.46!

Co/S!

207Pb/204Pbm!

(b)

0!

5!

10!

15!

20!

25!

15.36! 15.38! 15.40! 15.42! 15.44! 15.46!

S (

wt %

)!

207Pb/204Pbm!

0!

0.5!

1!

1.5!

2!

2.5!

3!

3.5!

15.36! 15.38! 15.40! 15.42! 15.44! 15.46!

Ni (w

t %

)!

207Pb/204Pbm!

0.06!

0.08!

0.10!

0.12!

0.14!

0.16!

0.18!

0.20!

15.36! 15.38! 15.40! 15.42! 15.44! 15.46!

Ni/S!

207Pb/204Pbm!

0.002!

0.003!

0.004!

0.005!

0.006!

0.007!

0.008!

0.009!

15.36! 15.38! 15.40! 15.42! 15.44! 15.46!

Co

/S!

207Pb/204Pbm!

(c)

0!

5!

10!

15!

20!

25!

15.36! 15.38! 15.40! 15.42! 15.44! 15.46!

S (

wt %

)!

207Pb/204Pbm!

0!

0.5!

1!

1.5!

2!

2.5!

3!

3.5!

15.36! 15.38! 15.40! 15.42! 15.44! 15.46!

Ni (w

t %

)!

207Pb/204Pbm!

0.06!

0.08!

0.10!

0.12!

0.14!

0.16!

0.18!

0.20!

15.36! 15.38! 15.40! 15.42! 15.44! 15.46!

Ni/S!

207Pb/204Pbm!

0.002!

0.003!

0.004!

0.005!

0.006!

0.007!

0.008!

0.009!

15.36! 15.38! 15.40! 15.42! 15.44! 15.46!

Co

/S!

207Pb/204Pbm!

(d)

Figure 4.2: Model initial 207Pb/204Pb plotted against whole rock assay data (from Vale IncoExploration) for samples from the Creighton Mine. (a) and (b) whole rock S and Ni contents; (c) and (d) metal sulphur ratios. Model initial Pb isotope values calculated as Darling et al.(2010).

sure of sulphide abundance. Figures 4.2a and 4.2b show that, in general, samples with

higher sulphide content (i.e. massive sulphide ores) have higher 207Pb/ 204Pbm. Again

comparing the ore Pb isotope characteristics with the defined Lower Unit Pb isotope

stratigraphy, this strongly suggests that the majority of massive sulphides segregated

early in the history of the Main Mass.

Figures 4.2c and 4.2d compares 207Pb/ 204Pbm with the chalcophile element ratios

Ni/S and Co/S. These ratios are indicators of the metal content of the sulphide in the

rock sample. Clearly there is no systematic variation, which would suggest that there

is no link between the timing of sulphide segregation and the metal content (tenor) of

the sulphide. However, these metal ratios are particularly sensitive to sulphide frac-

tionation processes. As discussed in Section 2.2.3, upon cooling sulphide melts ex-solve

chalcopyrite rich intermediate solid solution (iss), which can be fractionated from the

pyrrhorite-pentlandite monosulphide solid solution (mss). As such the mineralogy of sul-

phides in the selected samples controls the Ni/S and Co/S, and is unlikely to represent

83


R! = 0.51"

1.0"

1.5"

2.0"

2.5"

3.0"

3.5"

15.36" 15.38" 15.40" 15.42" 15.44" 15.46"

Ni in

Pyrr

hotite

(w

t %

)"

207Pb/204Pbm"

Figure 4.3: Model initial 207Pb/204Pb plotted against the Ni content of pyrrhotite separates.Uncertainties on Ni content are 1 standard deviation.

the bulk sulphide melt that segregated from the Lower Unit.

One approach with the potential to avoid this problem is the characterisation of the

metal contents of pyrrhotite-pentlandite (Po-Pn) mixtures, which represent the mss. The

composition of Po has been shown to reflect variations in the original bulk composition

of sulphide melts (Vaughan et al., 1971) and, given that in Sudbury Pn occurs almost

exclusively as exsolution textures within Po, the combined Ni content of Po-Pn mixtures

can be considered representative of the bulk Ni content of the sulphide melt. Pyrrhotite

rich fractions were separated using the techniques described in Section 3.1 and analysed

for major and minor element concentrations by ICP-OES (inductively coupled plasma

optical emission spectroscopy), using a Jobin Yvon Ultima 2 Sequential Spectrometer,

operated by Chung Choi (University of Bristol). Ni, Cu, Fe and Co were analysed

and calibrated using a series of S-doped standards that were made from single element

certified concentration standards. Precision and accuracy were monitored by analysing

in-house S-doped standards. The measured Ni and Co contents are shown in Table 4.1.

A moderately strong positive correlation is seen between Ni, Co and Fe contents and207Pb/204Pbm (Figure 4.2). Ni contents in the Po and Pn mixtures range from 1.39

to 3.12 wt %. Samples with higher 207Pb/204Pbm generally have higher Ni, Co and Fe

contents in Po-Pn mixtures. This suggests that the early formed sulphides do indeed

have the highest metal tenor.

Preliminary results from this investigation therefore indicate that early (prior to

significant silicate crystallisation of the Main Mass) sulphide segregation is responsible

for the majority of massive sulphide ores in the Creighton embayment. Furthermore

these early sulphide melts appear to have the highest metal concentrations. However,

significant further work is required to test these relationships. In particular, systematic

84


trends in the metal contents of sulphide melts in different orebodies in Sudbury are often

only well defined in very large data compilations (many tens to hundreds of samples; P.

Lightfoot, 2008, pers. comms.). As such a larger sampling population is required to test

the statistical significance of the trends seen between metal content and 207Pb/204Pbm.

85


86


.

87

5

Pb isotope systematics of the Offset Dykes

5.1 Introduction

The results of Chapter 4 show that isotopic heterogeneity is present throughout the

Sudbury impact melt sheet. This investigation highlighted two main avenues for fur-

ther research regarding the formation and evolution of the melt sheet. Firstly, it was

noted that the high 207Pb/204Pbm values of samples from throughout the South Range

stratigraphy were not consistent with melt sources dominated by the Superior Province

(Grieve et al., 1991; Deutsch et al., 1995) or lower crust (Mungall et al., 2004) as pre-

viously suggested. Secondly, while the melt sheet was clearly heterogeneous around the

time of the onset of silicate crystallisation, the extent to which the initial superheated

products of impact melting were homogenised is unclear.

In order to further examine these issues, the Pb isotope systematics of the earliest

samples of the melt sheet have been investigated. These are to be found in the Offset

Dykes, which contain quartz diorite phases that were emplaced prior to differentiation

of the Main Mass (Lightfoot and Farrow, 2002; Lightfoot et al., 1997b,a; Tuchscherer

and Spray, 2002). The details and results of this study are presented in in the following

paper, which is included in this chapter:

Darling J.R., Hawkesworth C.J. and Storey C.D. Shallow impact: isotopic insights

into crustal contributions to the Sudbury impact melt sheet. in-review.

88

Shallow impact: Isotopic insights into crustal contributions to theSudbury impact melt sheet

Darling J.R.a,1, Hawkesworth C.J.a,2, Storey C.D.b, Lightfoot P.C.c

a Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UKb School of Earth and Environmental Sciences, University of Portsmouth, Portsmouth PO1 3QL, UK

c Vale Inco Exploration, Highway 17 West, Copper Cli!, Ontario, P0M 1N0, Canada

Abstract

The largest known terrestrial impact melt sheet occurs within the 1850 Ma Sudbury Structure, Ontario.In order to evaluate the relative contributions of di!erent target lithologies to the melt sheet, we haveinvestigated the Pb isotope compositions of feldspar separates from early formed quartz diorite magmas,within O!set Dykes from around the impact structure. The samples define a linear array on plots of agecorrected 206Pb/204Pb versus 207Pb/204Pb. Samples from O!set Dykes hosted by the Huronian Supergroup(South Range) have a range of 206Pb/204Pb1850 from 15.424 to 17.255 and 207Pb/204Pb1850 from 15.390 to15.801, whilst those hosted by Archean gneisses of the Superior Province (North Range) cluster around206Pb/204Pb1850 ! 14.8 and 206Pb/204Pb1850 ! 15.1. These values can be approximated by binary mixingbetween the two major groups of target lithologies. A mix of 60-70 % of Superior Province gneisses with30-40 % of Huronian metasedimentary material closely matches the Pb isotope compositions of NorthRange O!set Dyke samples, whereas in the South Range the required Huronian component is up to ca. 80%.

These mixing proportions are consistent with Sr, Nd and Os isotope and trace element constraints. Athird minor component, either locally exposed Paleproterozoic mafic rocks or the lower crust is also re-quired. However, all of the geochemical characteristics of the melt sheet can be accommodated without theinvolvement of an average lower crust component.

A major component of Huronian supracrustal material, which had a pre-impact thickness of up to 12km, is required to explain the chemical characteristics of the impact melts, which also have a strong uppercrustal a"nity (e.g. Eu/Sm = 0.22, Rb/Sr = 0.2-0.35). As such, a shallower level of melting is apparentthan that predicted by many previous impact models for the Sudbury event. This can be accommodatedby considering approach trajectories for the impactor oblique to the Earth’s surface. In addition, the vari-ability identified indicates that the melt sheet was heterogeneous at an early stage, and may not have beencompletely homogenised during crater formation. Our findings have significant implications for the natureof the Sudbury impact event, evolution of the melt sheet and the crustal sources of metals contained inSudbury’s world class Ni-Cu-PGE sulphide ores.

Key words: Sudbury, impact melt, Pb isotopes, sulphide, O!set Dyke

1. INTRODUCTION

1.1. The Sudbury Impact Melt Sheet - Some Out-standing Issues

There has been much debate regarding thesource of the Sudbury melt sheet. Isotopic char-acteristics of silicates and sulphide ores requirea crustal origin (Dickin et al., 1999, 1992, 1996;Faggart et al., 1985; Hurst and Wetheril, 1974;Walker et al., 1997, 1991; Darling et al., 2010),although the depth of melting is currently unclear.

1Correspondence: [email protected] address: O"ce of the Principal, University of St


Current impact models for the Sudbury event sug-gest that the upper crust would have been vapor-ised and ejected during excavation of the tran-sient cavity (Deutsch et al., 1995; Grieve et al.,1991). This has been supported by the recognitionthat the concentration of certain transition metals,in particular Ni and the platinum group elements(PGE), in the melt sheet have a lower crustal a"n-ity (Mungall et al., 2004). In addition, Grieve et al.(1991) show that the bulk major element compo-sition of the Sudbury Igneous Complex (SIC) canbe modelled by whole rock melting of 93 % lowercrustal granite-greenstone terrain and 7 % of uppercrustal metasedimentary rocks.

In contrast, isotopic investigations suggest amore significant involvement of the upper crust in

Submitted to Geochimica et Cosmochimica Acta February 12, 2010

Chelmsford FormationOnwatin FormationOnaping Formation

10 km

FS

46º4

5'46

º30'

81º00'81º30'

LakeWanapitei

Upper UnitMiddle UnitLower UnitQuartz diorite

SIC

Supe

rior

Prov

ince Cartier Batholith

Levack Gneiss Complex

Nipissing gabbrosGranitic plutonsHuronian SupergroupSo

uthe

rnPr

ovin

ce

Whi

tewa

ter

Gro

up

Sudbury

CopperCliffWorthington

Ministic

Hess

FoyParkin

Manchester

Grenville Front

Figure 1: Summary geological map of the Sudbury Igneous Complex (SIC). Locations of the sampled O!set Dykes areshown; FS; Frood-Stobie. Modified after Ames et al. (2005).

the source of the melt sheet. For example, theinitial Nd isotope composition of the SIC lies onthe upper continental crust Nd isotope evolutioncurve (Faggart et al., 1985), and a significant con-tribution of upper crustal material appears to berequired to explain the Pb isotopic characteristicsof the melt sheet (Darling et al., 2010). As such itis currently di"cult to reconcile predictions fromimpact models with the chemical characteristics ofthe largest impact melt sheet on Earth.

The processes that produce chemical hetero-geneity in impact melt sheets are also unclear. Ithas been recognised that the Sudbury melt sheetcontains isotopic heterogeneities inherited fromvaried target lithologies (Dickin et al., 1999, 1996;Darling et al., 2010). However, the initial prod-ucts of impact melting are expected to be ho-mogenised by violent mechanical mixing (Phinneyand Simonds, 1977; Simonds and Kie!er, 1993),within minutes to hours of impact. The assimi-lation of country rocks, entrained clasts and fall-back breccia by superheated impact melts providesa mechanism by which to produce heterogeneityover time (Darling et al., 2010), although whetherprimary heterogeneity resulting from impact melt-ing of di!erent target lithologies can be preserved

is not known. Understanding the timing and scaleof inherited variability is important not only forour understanding of the evolution of the Sudburymelt sheet, but also for scaling-laws used to pre-dict melt volumes in impact structures.

Resolving these issues is important for our un-derstanding of the mechanisms of crater and im-pact melt development, and the e!ects of planetarycollisions on the evolution of the crust. There aretwo predominant questions regarding the origin ofthe Sudbury melt sheet that this investigation seeksto address:

• At what depth in the crust did the majority ofimpact melt originate?

• To what extent were these early melts ho-mogenous?

1.2. Geology of the Sudbury Igneous Complex

The SIC straddles the contact of the ArcheanSuperior Province and Paleoproterozoic HuronianSupergroup (Figure 1), and formed as a result ofimpact melting of the crust at 1850 Ma (Dietz,1964; Krogh et al., 1982). The associated im-pact basin, known as the Sudbury Basin, is thought

90

to have had a diameter of ca. 250 km (Sprayet al., 2004), making it one of the largest and old-est known terrestrial impact structures. In addi-tion to containing one of the principal reserves ofNi-Cu-Platinum Group Element (PGE) sulphideson Earth, Sudbury is of great importance in thestudy of impact processes due to the preservationof multiple features of the impact structure. Theimpact melt sheet (the Main Mass), together withradial and concentric dykes, locally termed O!setDykes, comprise the Sudbury Igneous Complex(SIC). Overlying the SIC is a sequence of craterfilling breccias and clastic sediments of the White-water Group. Extensive pseudotachylitic brecciabelts, known as Sudbury Breccias, occur withinthe footwall. Detailed accounts of the local geol-ogy of the SIC are given in volumes edited by Pyeet al. (1984) and Lightfoot and Naldrett (1994).

The Main Mass is divisible into Lower, Middleand Upper Units (Figure 1), with the approximateproportions 30:10:60 respectively. The LowerUnit, often termed the Sudbury norite, typicallyconsists of quartz monzogabbro to quartz gabbro,the Middle Unit quartz gabbro to quartz monzo-gabbro and the Upper Unit is granitic. The discon-tinuous Sublayer occurs at the melt sheet-footwallcontact and consists of inclusion and sulphide-richnorite to monzogabbro, locally underlain by foot-wall breccias. Although highly variable, the MainMass is typically ca. 2.5 km thick (Keays andLightfoot, 2004).

The O!set Dykes have an estimated total vol-ume of 100 km3 when propagated to depth (Keaysand Lightfoot, 2004) and intrude up to 30 km ra-dially into the footwall rocks. Varying in thick-ness from hundreds to tens of meters, the dykesgenerally thin away from the melt sheet. O!setDykes also occur concentrically, focused aroundthe melt sheet, and are often associated with widezones of pseudotachylite. There are two principalphases of quartz diorite that can be distinguishedin many, but not all, O!set Dykes. Marginal,inclusion and sulphide free, quartz diorite (QD)was emplaced prior to di!erentiation of the meltsheet (e.g. Lightfoot et al., 1997b) and beforethe melt sheet reached sulphide saturation (Light-foot and Farrow, 2002), which likely occurred attemperatures in excess of the Lower Unit liquidus(Keays and Lightfoot, 2004). This phase may berepresentative in composition of the bulk crustalmelt (Lightfoot et al., 1997b; Lightfoot and Far-row, 2002; Mungall et al., 2004). A second quartzdiorite phase contains abundant inclusions and sul-phide (inclusion bearing quartz diorite; IQD), butalso has geochemical characteristics that suggestit was emplaced prior to di!erentiation of the meltsheet (Lightfoot and Farrow, 2002).

The exact timing of O!set Dyke emplacement isuncertain, due to ambiguity in geological relation-

ships. For example, there is evidence supporting aviolent, energetic mode of emplacement of someO!set Dykes (e.g. Murphy and Spray, 2002),whereas in other areas mingling between QD andIQD suggests a more passive mechanism (Riller,2005). It is most likely that the O!set Dykes wereemplaced after the collapse of the transient cav-ity (Tuchscherer and Spray, 2002), during forma-tion of the central uplift (Wood and Spray, 1998;Tuchscherer and Spray, 2002) or later readjust-ment of the crater floor (Hecht et al., 2008; Scottand Benn, 2001; Riller, 2005). The latest time foremplacement is constrained by thermal models forthe melt sheet. Cooling rates for the melt sheetdepend heavily upon the method of heat loss atthe upper contact. If the overlying fallback brec-cias (Onaping Formation) acted as a purely con-ductive medium, then vigorous thermal convec-tion in the melt sheet would have dissisipated su"-cient heat to reach liquidus temperatures in 10000-110000 years (Prevec and Cawthorn, 2002; Ziegand Marsh, 2005). However, there was an exten-sive hydrothermal system in the Onaping Forma-tion (Ames et al., 1998), which allowed for cool-ing to liquidus temperatures in hundreds of years(Zieg and Marsh, 2005).

The elliptical shape, dip of contacts at the sur-face and the assymetric deep structure of the SICare a function of post impact deformation, gener-ally attributed to the Paleoproterozoic Penokeanorogeny (see Riller, 2005). The two main ex-posed sections of the melt sheet, termed the Northand South Ranges respectively, are separated by amajor fault system. Underlying the South Rangeis the Paleoproterozoic Huronian Supergroup, aseries of predominantly metasedimentary rocksintruded by 2.3-2.4 Ga granites (Krogh et al.,1984) and voluminous 2.2 Ga gabbroic sills of theNipissing suite (Corfu and Andrews, 1986). Incontrast, the North range of the melt sheet liesmostly upon 2.71 Ga granulite facies metamorphicrocks of the Levack Gneiss Complex (Krogh et al.,1984), part of the Archean Superior Province.

Investigations of the Sr, Nd, Os and Pb isotopiccharacteristics of SIC lithologies have revealed astrong crustal signature for all units (Dickin et al.,1999, 1992, 1996; Faggart et al., 1985; Hurst andWetheril, 1974; Walker et al., 1991; Morgan et al.,2002). Lightfoot et al. (1997b) suggest that a man-tle contribution of 20 % can be accommodatedin the SIC, however it is not required to explainthe chemical variations throughout the Main Mass(Lightfoot et al., 2001). Indeed no unambiguousevidence for significant mantle input into the meltsheet has been reported, from either geochemical,field or petrological studies. The relatively con-stant incompatible element ratios throughout theLower Unit and Upper Unit stratigraphy have beentaken as evidence for a single parent magma for allSIC lithologies (Lightfoot et al., 1997a).

91

Trace element data show subtle variationsaround the Main Mass, whereas systematic di!er-ences in Pb isotope characteristics have been iden-tified between ore and Sublayer samples from theNorth and South Ranges (Dickin et al. 1996) andbetween Lower Unit samples from each side of thecomplex (Dickin et al. 1999). Furthermore Dar-ling et al. (2010) identified significant stratigraphicand lateral heterogeneity in Pb isotopes throughoutthe South Range Lower Unit. These findings indi-cate varying crustal sources for the melts aroundthe complex, with both broad (North versus SouthRange) and small spatial scale (intra-Range) vari-ations.

1.3. Impact Melting and Models of the SudburyEvent

During an impact event, the immense kineticenergy of the impacting body is transferred to thetarget via the propagation of shock waves. Resul-tant compression and heating leads to melting. Atpressures greater than ca. 50 GPa, shock waveswill deposit su"cient energy for total melting ofcrystalline targets (Melosh, 1989). Modelling ofpeak shock pressures has shown that the majorityof melted material comes from an approximatelyspherical volume below the maximum depth ofpenetration of the impacting body (Melosh, 1989).This depth is dependant upon impactor densityand velocity (Melosh, 1989; Pierazzo et al., 1997),constraints upon which rely on scaling relation-ships between impact melt volume and the sizeof the transient cavity (e.g. Grieve and Cintala,1992; Melosh, 1989). The transient cavity is theinitial product of the excavation phase of an im-pact event, and undergoes modification as a resultof gravitational instability and collapse to formthe final impact basin (Melosh and Ivanov, 1999;Melosh, 1989).

Based upon the distribution of shock defor-mation features, Huronian outliers and pseudo-tachylites, estimates of the Sudbury transient cav-ity diameter are 80 to 110 km (Deutsch et al.,1995; Grieve et al., 1991; Pope et al., 2004;Thompson and Spray, 1994). The total impactmelt volume in Sudbury has been of much de-bate. Based primarily on the calculated volumeof the melt sheet, suggested total impact melt vol-umes range from 8000 to 15000 km3 (Grieve et al.,1991; Wu et al., 1995). Taking into account meltin suevites (the Onaping Formation) Sto#er et al.(1994) estimate 12000 to 13000 km3. Using thescaling relationships of Grieve and Cintala (1992)and Melosh (1989), such values can be explainedby a vertical impact of a 10-15 km in diameterchondritic body, travelling at 20-25 kms"1. Mod-els of thermal, structural and geochemical pro-cesses during the Sudbury event have largely beenbased upon such parameters (Deutsch et al., 1995;

Grieve et al., 1991; Mungall et al., 2004; Ivanovand Deutsch, 1999; Zieg and Marsh, 2005).

The maximum depth of penetration of such animpactor is approximately equal to its diameter(Grieve and Cintala, 1992). The upper 10-15 kmof crust would have been excavated or vaporised,and the resultant transient cavity ca. 30 km deep(Deutsch et al., 1995). Given estimated total thick-nesses of the Huronian Supergroup of 8-12 km(Dressler, 1984; Young et al., 2001) the majority ofmelt is therefore predicted to have originated be-low this supracrustal succession, in the underlyingArchean basement (Deutsch et al., 1995; Grieveet al., 1991; Mungall et al., 2004). During collapseof the transient cavity, which likely took a fewminutes (Melosh and Ivanov, 1999), fragmentedtarget rocks would have been mixed into the super-heated impact melt (Ivanov and Deutsch, 1999).This process has been suggested to account for theincorporation of Huronian material into the SouthRange melt (e.g. Deutsch et al., 1995).

1.4. New Approach

Isotope geochemistry is established as a valu-able tool in assessing variability in impact melts,inherited from target lithologies (Dickin et al.,1999, 1996; Kettrup et al., 2003; Darling et al.,2010). In Sudbury, the Pb isotope system hasproven to be more sensitive to target rock vari-ations than Nd isotopes. This is because theHuronian metasedimentary sequence, which un-derlies the southern exposed limb of the melt sheet(the South Range), was derived from the SuperiorProvince (McLennan et al., 1979, 2000), whichunderlies the northern limb (the North Range; seeFigure 1). Due to the di!erences in chemicalproperties of parent and daughter isotopes, frac-tionation of U and Th from Pb occurs duringsedimentary processes. For example U is redoxsensitive, and in oxidised environments is trans-ported as dissolved uranyl complexes because U6+

is highly soluble in aqueous solutions (e.g. Lang-muir, 1978). This results in di!erent Pb iso-tope evolution over the time between sedimenta-tion (<2450 Ma; Bennet et al., 1991; Young et al.,2001) and the time of impact (1850 Ma; Kroghet al., 1982). Other isotopic systems, such as Lu-Hf and Sm-Nd, do not fractionate significantlyin this way (McCulloch and Wasserburg, 1978;Patchett et al., 1984; Vervoort et al., 1999), result-ing in similar isotopic ratios in each province (Pre-vec et al., 2000; Naldrett et al., 1986; McLennanet al., 2000).

Sampling of quartz diorites of the O!set Dykeso!ers the best available insight into early melt het-erogeneity and the relative contributions of targetlithologies. Therefore our strategy has been to in-vestigate the Pb isotope systematics of quartz dior-ites from O!set Dykes from around the SIC.

92

2. SAMPLING AND METHODS

2.1. Sampling Strategy

A variety of O!set Dyke environments weresampled. Both radial (e.g. Worthington, CopperCli!, Foy) and concentric (e.g. Hess, Manchester)O!set Dykes, including quartz diorites proximaland distal to the Main Mass were sampled (Fig-ure 1). Where possible, sampling focused uponQD, as this represents the earliest melt phase inthe Sudbury Structure. It has been shown that thetrace element geochemistry of IQD matrices hasnot been significantly a!ected by assimilation ofclasts (Lightfoot et al., 1997a; Lightfoot and Far-row, 2002) , therefore this unit reflects early meltcomposition and was also sampled for Pb isotopeanalysis.

Distinctions between the two quartz dioritephases is perhaps best defined in the Worthing-ton O!set Dyke. Multiple samples were taken inthe Totten Mine area (e.g. Lightfoot and Farrow,2002), in order to identify variability within indi-vidual O!sets, and test the degree of coherence be-tween QD and IQD Pb isotope systematics.

2.2. Methodology

2.2.1. Sample preperation and chemistry

Of those samples that contained inclusions, carewas taken to analyse only the quartz diorite ma-trices. Following careful cutting of each sampleand subsequent crushing and sieving (<500 µm),plagioclase feldspars were separated by a combi-nation of density and magnetic separation tech-niques. Feldspars are less susceptible to post crys-tallisation alteration than whole rock Pb isotopicmeasurements and have lower U/Pb ratios, result-ing in less in-growth of radiogenic Pb and thepreservation of near magmatic isotope ratios.

Aliquots for analysis (100-400 mg) were hand-picked under a binocular microscope. Sampleswere leached for 20 minutes in both 6 M HCl and7 M HNO3 to remove surface contamination andloosely bound Pb. Subsequently, samples weredissolved in 9 M HF and 7 M HNO3. Step disso-lution experiments similar to Housh and Bowring(1991) revealed constant U/Th/Pb ratios and iso-tope compositions after the initial leaching steps.The 6 M HCl and 7 M HNO3 leaches had radio-genic Pb isotope ratios and high U/Pb ratios, butremoved less than 10% of the total mass of Pb.The procedures for chemical separation of Pb weremodified from Strelow (1978) and were detailed inDarling et al. (2010). Total procedural blanks werebetween 15 and 40 pg and had no a!ect on the re-ported numbers within the quoted uncertainty.

2.2.2. Analytical procedures

The isotopic composition of Pb was determinedon a Thermo Scientific Neptune multi-collector(MC) ICP-MS at the Department of Earth Sci-ences, University of Bristol. Methods were de-scribed in detail by Darling et al. (2010) , andutilised a sample-standard bracketing technique tocorrect for instrumental mass bias. NIST SRM981 was used as the bracketing standard and thedouble-spike values of Baker et al. (2004) used fornormalisation. NIST SRM 982 was used as a con-sistency standard, which yielded averages, over a 9month period (n = 96), of 36.7484 ± 50 (137 ppm),17.1649 ± 31 and 36.7557 ± 88 for 206Pb/204Pb,207Pb/204Pb and 208Pb/204Pb respectively (uncer-tainties are 2 standard deviations; 2!). Concen-trations of analysed standard and sample solutionswere closely matched at 50 ppb. Both the BHVO-2 and BCR-2 basalt standards were also measured,and were within uncertainty of the values of Bakeret al. (2004). Multiple aliquots of BCR-2 (n =14) yielded averages of 18.754 ± 13, 15.628 ± 8and 38.739 ± 30 for 206Pb/204Pb, 207Pb/204Pb and208Pb/204Pb respectively.

U/Th/Pb ratios and concentrations were mea-sured on a Thermo Scientific Element II at theUniversity of Bristol. A tandem quartz glass spraychamber (Cyclonic + Scott double pass) and PFA100 µl/min nebuliser were used, as they provideexcellent signal stability. Employing an exter-nal calibration, precision of U/Pb ratios was <0.5% (2!) and accuracy was monitored using basaltstandards BCR-2, BHVO-2 and in-house calibra-tion standards, all of which were within uncer-tainty of reference values.

3. RESULTS

Lead isotope data, together with U/Pb ratios arepresented in Table 1. Plots of 206Pb/204Pb versus207Pb/204Pb and 208Pb/204Pb are shown in Figure2. Quartz diorite samples from North and SouthRange O!set Dykes respectively define di!eringarrays, although there is significant variation be-tween O!set Dykes from each side of the com-plex. In general, North range samples have lower206Pb/204Pb and 207Pb/204Pb than South Rangesamples. In plots of uranogenic Pb, Dickin et al.(1996) found that basal samples of the Lower Unitof the Main Mass and sulphide scattered around1850 Ma reference lines. Quartz diorites from in-dividual O!set Dykes do lie along 1850 Ma ref-erence lines, although the regressions through allNorth and South Range samples correspond to twostage model Pb (Stacey and Kramers, 1975) refer-ence lines of ca. 2.5 Ga and 2.2 Ga respectively.

It is clear that the Pb isotope compositionsof O!set Dykes do not share a common source.

93

Sa

mp

leO

ffse

t Dy

ke

Ra

ng

eE

as

t1

No

rth1

20

6Pb

/2

04P

b2!

20

7Pb

/2

04P

b2!

20

8Pb

/2

04P

b2!

23

8U/2

04P

b2!

20

6Pb

/ 2

04P

b1

85

0 2

2!

20

7Pb

/ 2

04P

b1

85

0 2

2!

20

7Pb

/ 2

04P

bm

32!

JD

08

SIC

03

Foy

QD

North

489750

5176860

15.5

25

0.0

02

15.1

92

0.0

03

35.6

61

0.0

10

2.2

27

0.0

02

14.7

86

0.0

03

15.1

08

0.0

23

15.1

85

0.0

04

JD

08

SIC

01

Foy

IQD

North

481630

5178320

15.8

40

0.0

03

15.1

96

0.0

03

37.0

06

0.0

09

3.4

82

0.0

10

14.6

84

0.0

07

15.0

66

0.0

04

15.1

54

0.0

04

FO

QD

1F

oy

QD

North

489530

5176930

15.2

25

0.0

02

15.1

12

0.0

03

36.2

09

0.0

10

0.9

74

0.0

20

14.9

02

0.0

14

15.0

75

0.0

04

15.1

39

0.0

04

MO

IQD

1M

inis

ticIQ

DN

orth

459620

5154770

15.3

40

0.0

02

15.1

89

0.0

03

36.0

27

0.0

11

1.1

93

0.0

02

14.9

43

0.0

03

15.1

45

0.0

04

15.2

03

0.0

04

PO

IQD

1P

ark

inIQ

DN

orth

509980

5184520

16.8

96

0.0

03

15.3

94

0.0

03

37.5

80

0.0

09

6.5

58

0.0

02

14.7

17

0.0

03

15.1

47

0.0

04

15.2

32

0.0

04

PO

QD

1P

ark

inQ

DN

orth

509900

5184420

16.1

11

0.0

03

15.3

32

0.0

03

35.8

98

0.0

10

3.5

55

0.0

20

14.9

31

0.0

14

15.1

99

0.0

04

15.2

59

0.0

04

JD

08

SIC

11

Hess

IQD

North

464960

5178370

20.7

98

0.0

03

16.0

45

0.0

03

38.9

73

0.0

10

21.9

21

0.1

13

13.5

16

0.0

75

15.2

21

0.0

09

15.4

41

0.0

05

JD

08

SIC

11

bH

ess

IQD

North

464960

5178370

15.4

36

0.0

02

15.1

87

0.0

03

35.6

61

0.0

10

1.8

54

0.0

07

14.8

20

0.0

05

15.1

17

0.0

04

15.1

90

0.0

04

FS

OIQ

D1

Fro

od S

tobie

IQD

South

502020

5155300

15.9

65

0.0

03

15.4

57

0.0

03

35.7

42

0.0

10

1.4

49

0.0

05

15.4

84

0.0

04

15.4

02

0.0

04

15.4

00

0.0

04

WO

IQD

2W

orth

ingto

nIQ

DS

outh

464830

5136190

16.4

02

0.0

03

15.5

55

0.0

03

35.8

45

0.0

11

0.6

69

0.0

01

16.1

80

0.0

03

15.5

30

0.0

04

15.4

49

0.0

04

WO

IQD

1W

orth

ingto

nIQ

DS

outh

465670

5136940

16.7

57

0.0

03

15.6

34

0.0

03

36.1

36

0.0

10

1.6

32

0.0

06

16.2

15

0.0

05

15.5

73

0.0

04

15.4

88

0.0

04

WO

QD

2W

orth

ingto

nQ

DS

outh

464830

5136190

16.3

09

0.0

03

15.5

34

0.0

03

35.8

37

0.0

09

0.6

46

0.0

03

16.0

94

0.0

03

15.5

09

0.0

04

15.4

38

0.0

04

WO

QD

1W

orth

ingto

nQ

DS

outh

465680

5136930

16.7

43

0.0

03

15.6

28

0.0

03

36.2

50

0.0

10

1.4

57

0.0

08

16.2

59

0.0

06

15.5

73

0.0

04

15.4

83

0.0

04

WO

QD

3W

orth

ingto

nQ

DS

outh

464830

5136100

16.7

08

0.0

04

15.6

23

0.0

03

36.3

15

0.0

10

1.4

57

0.0

08

16.2

24

0.0

07

15.5

68

0.0

04

15.4

82

0.0

05

CC

OIQ

D1

Copper C

liffIQ

DS

outh

494200

5146420

29.0

15

0.0

05

16.8

35

0.0

03

37.7

94

0.0

11

47.5

84

0.1

87

13.2

08

0.1

25

15.0

48

0.0

15

15.3

02

0.0

06

JD

07

SIC

25

Copper C

liffQ

DS

outh

494820

5147740

17.6

47

0.0

05

15.6

20

0.0

03

37.4

32

0.0

10

6.1

35

0.0

04

15.6

08

0.0

06

15.3

90

0.0

04

15.3

73

0.0

06

JD

07

SIC

26

Copper C

liffQ

DS

outh

494720

5142300

18.3

33

0.0

03

15.6

79

0.0

03

37.7

42

0.0

09

8.0

19

0.0

04

15.6

70

0.0

04

15.3

78

0.0

04

15.3

55

0.0

04

CC

OQ

D1

Copper C

liffQ

DS

outh

494200

5146380

16.1

74

0.0

04

15.4

41

0.0

03

35.7

49

0.0

10

1.4

00

0.0

15

15.7

09

0.0

11

15.3

89

0.0

04

15.3

61

0.0

05

JD

08

SIC

14

Mancheste

rQ

DS

outh

521260

5156150

25.3

12

0.0

04

16.7

12

0.0

03

43.6

49

0.0

11

24.2

54

0.1

01

17.2

55

0.0

67

15.8

01

0.0

09

15.5

98

0.0

05

JD

08

SIC

15

Mancheste

rQ

DS

outh

521260

5156150

19.2

75

0.0

03

16.0

32

0.0

03

38.5

64

0.0

13

6.2

10

0.0

03

17.2

12

0.0

04

15.7

99

0.0

04

15.6

01

0.0

04

Table1:

Pbisotope

datafrom

O!setD

ykequartz

diorite(Q

D)sam

plesand

inclusionbearing

quartzdiorite

(IQD

).1

Grid

references(easting

andnorthing)are

givenin

UT

Mcoordinates

(zone17N

;NA

D27).

2InitialPb

isotoperatios

agecorrected

usingm

easuredµ.

3M

odelinitial207Pb

/ 204Pbcalculated

asby

Darling

etal.(2010)forreference.O!setD

ykelocations

areshow

nin

Figure1

94

Hess IQD

Parkin QD

Parkin IQD

Ministic IQD

Foy IQD

Foy QD

Manchester QD

Copper Cliff QD

Worthington QD

Worthington IQD

Frood-Stobie IQDNo

rth

Ra

ng

e

So

uth

Ra

ng

e

20

7P

b/2

04P

b2

08P

b/2

04P

b

206Pb/204Pb

14 16 18 20 22 24 26

15.0

15.5

16.0

16.5

36

38

40

42

44

Figure 2: Plots of Pb isotope data for O!set Dykequartz diorites (QD) and inclusion bearing quartz dior-ites (IQD). Two stage model Pb curves are shown from 0to 2.8 Ga (Stacey and Kramers, 1975). Regression linesthrough all North Range (black) and South Range (grey)samples.

There are significant di!erences between the sam-ples from the North and South Ranges, as well asminor isotopic variability between O!set Dykesfrom each side of the complex. In order to bestresolve these variations it is useful to calculateinitial ratios at the time of impact, by age cor-recting using measured U/Pb ratios. Values of238U/204Pb (µ) are generally well below 8. Sam-ples from O!set Dykes hosted by Huronian Su-pergroup metasedimentary rocks (South Range)have a range of 206Pb/204Pb1850 from 15.484 -17.255 and 207Pb/204Pb1850 from 15.378 - 15.801,while those hosted by Archean gneisses of theSuperior Province (North Range) cluster around206Pb/204Pb1850 ! 14.8 and 206Pb/204Pb1850 ! 15.1.Dickin et al (1996) found variable disturbance ofthe U/Th/Pb ratios in ores from around the com-plex. Three samples analysed in this investigationwere found to have high µ values (21-48), whichmay have been caused by Pb loss or U gain. Thesesamples also have the most radiogenic measuredPb isotope ratios, suggesting that U/Th/Pb distur-

bance was not recent. The age corrected Pb iso-tope ratios of these samples are distinct from allother samples, with very unradiogenic initial val-ues. Uncertainty in the initial ratios includes prop-agated uncertanty in the measured Pb isotope ra-tios, µ, decay constants (Ja!ey et al., 1971) andcrystallisation age (Krogh et al., 1984; Ostermannet al., 1996).

4. DISCUSSION

4.1. Source of Melt

4.1.1. Pb isotope composition of target lithologiesat 1850 Ma

In order to evaluate the contributions of tar-get rocks to the melt sheet, it is necessary toconstrain the isotopic composition of target rockgroups at the time of impact, 1850 Ma. Presentday Pb isotope compositions must be back pro-jected using either measured U/Pb, or modelledPb isotope evolution. Unfortunately, whilst thereare a number of Pb isotope studies of locally ex-posed target rocks, few contain U/Pb element ra-tio information. The modelling of Pb isotope ra-tios at the time of impact is represented in Figure3. The Huronian Supergroup was largely derivedfrom Superior Province gneisses (McLennan et al.,1979, 2000). Following McLennan et al. (2000),the µ required to evolve Pb isotope compositionsfrom initial Superior Province ratios, constrainedby least radiogenic feldspars (Gariepy and Allegre,1985; Hemming et al., 1996), to measured Huro-nian values, can be used to model Pb isotope evo-lution of the Huronian Supergroup. In contrast,values of µ for the Nipissing gabbro intrusionsare poorly constrained. Extensive datasets of thechemistry of these intrusions can be found in theliterature (Lightfoot, 2002; Lightfoot et al., 1993),however none report Pb concentrations. Thesedominantly tholeiitic intrusions show evidence forvariable crustal contamination (Lightfoot, 2002).As such a µ of around 9-10 seems reasonable tomodel their Pb isotope evolution.

Gneisses of the Superior Province have a com-plex Pb isotope history (Figure 3). Initial ra-tios at 2.71 Ga are provided by least radiogenicfeldspars (Gariepy and Allegre, 1985; Hemminget al., 1996) and galenas (Vervoort et al., 1994).Present day ratios of Levack Gneisses and Sud-bury Breccias (pseudotachylites) fall on a 2.7 to1.1 Ga paleoisochron, potentially indicating U lossoccured at 1.1 Ga. Neither ca. 1.85 Ga Penokean(e.g. Sims et al., 1989), 1.85 Ga Sudbury or ca.1.1 Ga Grenvillian metamorphism (e.g. Card et al.,1984) led to similar U loss in the Huronian Super-group. As such it is likely that gneisses evolvedwith variable U/Pb ratios prior to ca. 2.6 Ga par-tial U loss during granulite facies metamorphism

95

13.0 14.0 15.0 16.0 17.0 18.0 19.0

14.4

14.6

14.8

15.0

15.2

15.4

15.6

15.8

16.0

16.2

µ = 9.5

µ = 22

20

7P

b/2

04P

b 0

1.0

2.5

Nipissing gabbro

L. Huronian metased.

Levack Gneiss

Superior Province feldspar

NR Sudbury Breccia

Nipissing gabbro

Me

asu

red

Mo

de

l

(1.8

5 G

a)

206Pb/204Pb

U. Huronian metased.

Figure 3: Modelling of Pb isotope compositions of target rocks at 1850 Ma. North Range (NR) country rocks fallalong a paleoisochron between 2750 and 1000 Ma, anchored by least radiogenic detrital feldspars from the SuperiorProvince (Gariepy and Allegre, 1985; Hemming et al., 1996). Evolution of the Pb isotope composition of Huronianmetasedimentary rocks is represented by a µ = 22 growth curve from the North Range feldspar data. Gray shadingrepresents range of Huronian samples back projected from measured values with variable µ (data from Dickin et al.,1996 and McLennan et al., 2000). Age corrected Nipissing gabbro acquired by back projecting measured values withan assumed µ of 9.5. Also shown is the 2-stage model Pb isotope evolution, from 0 to 3000 Ma, of Stacey and Kramers(1975).

(see Easton, 2000). It is not possible to resolve theabsolute Pb isotope evolution of these gneisses, al-though it is evident that at 1850 Ma they wouldhave had Pb isotope ratios evolved little from theleast radiogenic feldspar data.

4.1.2. Isotopic constraints on melt sources

The age corrected Pb isotope ratios of O!setDyke samples define a linear array on plots of206Pb/204Pb versus 207Pb/204Pb. When comparedto modelled 1850 Ma Pb isotope ratios of targetrocks, the variations in initial Pb isotope ratioscan be approximated by binary mixing of SuperiorProvince gneisses and Huronian metasedimentaryrocks (Figure 4). The relative proportions of eachtarget group required to produce the Pb isotopecomposition of O!set Dyke samples varies signif-icantly. North Range O!set Dykes require mix-ing of Superior Province gneisses with a ca. 30%Huronian component. South Range O!set Dykesrequire a much higher Huronian component of ca.50% in Copper Cli! and Frood Stobie QD, ca.65% in Worthington QD and ca. 90% in Manch-ester QD. Deviation of the samples from the bi-nary mixing line, to higher 207Pb/204Pb and lower206Pb/204Pb, can be accommodated by the incor-poration of variable amounts of Nipissing gabbro.

A compilation of strontium and neodymium iso-tope data for the Main Mass and Sublayer ofthe melt sheet and locally exposed target rocks is

shown in Figure 5. The Main Mass and Sublayersamples have initial Sr and Nd isotope composi-tions that fall between those of Huronian metased-imentary rocks and the Levack Gneisses. Mixinglines between these two target rock groups satis-factorily model the range of Main Mass initial Srand Nd isotope compositions. Main Mass samplesrequire a 40-60 % Huronian melt component. In-terestingly, whilst Sublayer samples more closelyreflect their respective footwall rocks, they too re-quire a mixture of Huronian and Superior Provincematerial to explain their Sr isotope compositions.Binary mixing of Superior Province gneisses withHuronian metasedimentary rocks can also explainthe Re-Os isotope systematics of sulphide oresfrom around the complex (Morgan et al., 2002).Variations in initial 187Os/188Os suggest a 12 to 55% Huronian melt contribution to the source of theores. Morgan et al. (2002) also found suprachon-dritic 186Os/188Os, which likely reflects the incor-poration of tens of percent of mafic Huronian vol-canic rocks or Nipissing gabbro.

The Sr-Nd, Os and Pb isotope systematics ofSIC lithologies can all be accounted for by mix-ing of locally exposed target lithologies, and arebroadly consistent in their constraints on the rela-tive contributions of target lithologies to the meltsheet (Table 2). That the O!set Dyke samples ex-hibit a greater range of mixing proportions than theMain Mass is interesting, and presumably reflects

96

North Range Offset DykeSouth Range Offset DykeNipissing diabase

Superior Province feldsparUpper Huronian modelLower Huronian model

13. 0 14. 0 15. 0 16. 0 17. 0 18. 0 19. 014. 5

15. 0

15. 5

16. 0

16. 5

206Pb/204Pb

207 P

b/20

4 Pb

Figure 4: Pb isotope data from O!set Dyke samples corrected to 1850 Ma. Samples define a mixing array betweenthe modelled 1850 Ma composition of Superior Province gneisses (dary grey shading), and Huronian metasedimentaryrocks. A binary mixing line is shown with 10 % graduations. Deviations from this mixing line, to higher 207Pb/204Pb1850,suggests significant contributions of Nipissing gabbro (Dickin et al., 1996). Age corrected upper and lower Huronianmetasedimentary rocks are plotted in the light grey shaded field. Open squares are least radiogenic North Rangefeldspars (Gariepy and Allegre, 1985; Hemming et al., 1996). The three samples with low 206Pb/204Pb have high µ(20-40), indicative of relative Pb loss. Error bars are 2 standard deviations.

partial mixing of the melt sheet after O!set Dykeformation. It is clear that a significant to domi-nant Huronian melt source is required to explainthe isotopic characteristics of the early formed im-pact melts in Sudbury.

4.1.3. Trace element constraints

It is important to reconcile the isotopic con-straints upon the melt sources with limits providedby trace elements. Although major element mix-ing models have previously been utilised to modelthe bulk SIC composition (Grieve et al., 1991),considerable lithological variations occur withineach of the main target groups discussed. As suchit is di"cult to assign end member compositionsto be used in modelling. Given that the Huroniansediments were predominantly derived from theSuperior Province (McLennan et al., 1979, 2000)trace element ratios with distinctive fractionationbehaviour during sedimentary processes o!er par-ticularly discriminatory tools.

Representative trace element plots are shownin Figure 6. Ratios such as Eu/Sm, which areonly a!ected by igneous processes (see Kemp andHawkesworth, 2003), are relatively uniform for allunits. However, ratios that fractionate during sed-imentary processes (e.g. Th/U and Sr/Yb) showlarge di!erences between target rock groups. O!-set Dyke quartz diorites and average Main Mass

lie along mixing lines between Huronian metased-iments and Superior Province gneisses. End mem-ber compositions for the mixing lines, shownin Figure 6a-b, were estimated from availablepublished data. For the Huronian metasedimentend member a mix of 2/3 Elliot Lake mudstone(lower Huronian) and 1/3 Gogwanda argillite (up-per Huronian) was used, which is geologically rea-sonable given the estimated thicknesses of Huro-nian units (Dressler, 1984; Young et al., 2001).For the Superior Province, a mix of 2/3 Levackgneiss and 1/3 Cartier Batholith was used. Assum-ing bulk melting, such trace element mixing mod-els suggest a Huronian contribution of 30 to 70 %for the O!set Dykes, with the average Main Massgenerally requiring intermediate proportions. Re-assuringly these estimates are entirely consistentwith the isotopic constraints (Table 2).

The Pb isotope data, together with Os iso-tope investigations, suggested that Nipissing maficrocks might have been a significant contribut-ing target lithology to the melt sheet. This canbe further investigated by plotting trace elementratios that are not a!ected by sedimentary pro-cesses or hydrodynamic sorting, i.e. ratios that aregood provenance indicators. In Figure 6c Th/Scconfirms the intimate association of Huronianmetasediments and Superior Province gneisses.Deviation of O!set Dyke QD’s to higher Sc and

97

0 50 100 150 200 250

Main Mass

Huronian metased.

NR Sublayer

SR Sublayer

Levack Gneiss

!Sr1850

!Nd

18

50

-50

-15

-10

-5

0

5

Figure 5: Sr and Nd isotope compositions of the Main Mass, Sublayer and target rocks of the SIC. Measured valueswere recalculated to 1850 Ma. Example mixing curve between Huronian metasediments and Levack gneisses, with10% graduations, is shown. The bar indicates the total range of Huronian Nd isotope values. Data sources: Levackgneisses (Hurst and Wetheril, 1974; Naldrett et al., 1986); Huronian metasediments (Dickin et al., 1999; Fairbairnet al., 1967; McLennan et al., 2000); Sublayer and Main Mass (Dickin et al., 1999; Faggart et al., 1985; Gibbins andMcNutt, 1975; Hurst and Wetheril, 1974; Naldrett et al., 1986).

System SIC lithologyHuronian

metased. (%)Superior Province

gneisses (%)Notes

Pb isotopes SR Offset Dykes 50-90 10-50

NR Offset Dykes 25-35 65-75

Sr-Nd isotopes Main Mass 30-75 25-70

NR Sublayer 25-40 60-75

SR Sublayer ~60 ~40

Os isotopes Sulphide ores 12-55 45-88 Tens % of mafic rocks (Nipissing gabbro) required to explain 186Os/188Os systematics. Target rock end members poorly constrained.

Th/U vs. Eu/Sm NR Offset Dykes ~40 ~60

SR Offset Dykes 50-75 25-50

Av. Main Mass ~50 ~50

Th/U vs. Sr/Yb NR Offset Dykes 25-35 65-75

SR Offset Dykes 55-65 35-45

Av. Main Mass ~45 ~55

May require tens % of Nipissing gabbros

Table 2: Summary of the relative proportions of target rock end members indicate by various isotopic and trace elementmixing models. See text for discussion and data sources.

98

0.1

0.2

0.3

0.4

0 2 4 6 8 0.01 0.1 1 10

10

20

30

40

2 4 6 80

100

200

300

400

1 2 3 4

1

2

3

Eu/

Sm

Th/U

Th/Sc

Sc

(ppm

)

Th/U

Ce n

(Ce/Yb)n

Sr/Y

b

L

L

U

L

U

U

L

UL

Superior ProvinceHuronian metasediments

Nipissing diabase

Offset Dyke QDMain Mass

Huronian volcanics Lower crustUpper crust

A)

B)

C)

D)

Figure 6: Trace element constraints on the contributions of target lithologies. A) Th/U versus Eu/Sm. Mixing line isshown between estimated Huronian and Superior Province end member compositions (see text for discussion), with10% graduations. B) Th/U versus Sr/Yb. Mixing line calculated as A. C) Th/Sc versus Sc. D) Average continentalcrust normalised Ce and Ce/Yb. Data Sources: Huronian metasediments from average Gogwanda Formation argillites(Young, 2001), Elliot Lake Group mudstones (Fedo et al., 1997), Serpent Formation (Fedo et al., 1997; McLennanet al., 1979); Huronian volcanics from average rhyolites, mafic end members and intermediate fractionates (Jolly et al.,1992); Superior Province data from average North Range Sudbury Breccias (LaFrance et al., 2008), Levack Gneissesand Cartier Batholith (Meldrum et al., 1997); average chilled Nipissing gabbro (Lightfoot et al., 1997b); average MainMass (Lightfoot et al., 2001); averages from O!set Dyke quartz diorites (Lightfoot et al., 1997a); average continentalcrust, upper crust and lower crust (Rudnick and Gao, 2003).

lower Sc/Th could be explained by involvementof Nipissing gabbro, mafic end members of Huro-nian volcanics, or an otherwise unidentified lowercrustal source. The O!set Dyke samples are lightrare earth element (LREE) enriched (Figure 6d),and measures such as (Ce/Yb)n have previouslybeen used to discriminate between crustal sourceend member compositions in Sudbury (Lightfootet al., 1997c; Naldrett and Hewins, 1984; Prevecet al., 2000). Some South Range samples are lessLREE enriched than either the Superior Provinceor Huronian metasediments, suggesting a similarcontribution of Nipissing gabbro, mafic Huronianvolcanics or the lower crust.

The upper crustal a"nity of all SIC samples isclear in Figure 6. O!set Dykes and the averageMain Mass have highly fractionated Eu/Sm andRb/Sr (ca. 0.2 in North Range QD, 0.25-0.36 inSouth Range QD), which alone are strong indi-cators of an upper crustal melt. Indeed we find

it di"cult to reconcile the trace element concen-trations or ratios of O!set Dyke samples with asource dominated by the lower crust, as proposedby Mungall et al. (2004). Significant exceptionsare the concentrations of certain chalcophile ele-ments, in particular Ni, Pt, Pd, Ru and Rh whichhave concentrations significantly higher than thoseof the average upper crust (Rudnick and Gao,2003) .

4.1.4. Source of metals

The Sudbury melt sheet is host to a vast resourceof Ni-Cu-PGE sulphide ore. The historic resourceexceeds 1500 million tonnes with an average gradeof ca. 1.2 wt% Ni, 1.1 wt% Cu and 0.4 g/t of Ptand Pd (Keays and Lightfoot, 2004). Resolvingthe source of metals contained in sulphide ores re-mains a significant problem for models of ore for-mation in Sudbury. Important estimates of the ini-tial metal content of the melt sheet are provided

99

20 40 60 80 100 120 140 160

20

40

60

80

100

120

140

160

0

Av. Nipissing

Av. Huronian metased.

Huronian volcanics

Av. Superior Province

Av. Main Mass

Av. Quartz DioriteLower Unit

Middle Unit

Sublayer

Subla

yer

Ni/C

u =

0.5

1 10

1

10

L

MU

Pd

(p

pb

)

Pt (ppb)

Cu

(p

pm

)

Ni (ppm)

Ni/Cu = 2

L

UML

Av. Upper, Middle

& Lower crust

(A)

(B)

Figure 7: (A) Ni and Cu contents of target rocks andaverage SIC lithologies. Light grey shading is therange of fine grained Huronian metasedimentary rocks,with average upper and lower Huronian shown. Darkgrey shading is the range of Levack Gneiss and CartierBatholith samples, with the estimated Superior Provinceaverage calculated as per figure 6. Mixing line shown(with 20 % graduations) between an equal mix ofHuronian metasedimentary rocks and Superior Provincegneisses, with Nipissing mafic rocks. Error bars on av-erage Nipissing are 1 standard error. Also shown is thearray of Main Mass and Sublayer samples (Lightfootand Zotov, 2005). Data sources as Figure 6, with addi-tional data from Chai and Eckstrand (1994). (B) Pt andPd concentrations in average quartz diorite (Mungallet al., 2004), compared to Nipissing mafic rocks (Valain-court et al., 2003) and average crustal concentrations(Rudnick and Gao, 2003). Mixing line between aver-age Nipissing mafic rocks and upper crust, with 10 %graduations. Light grey circles are individual Nipiss-ing samples. Note that the average Nipissing Pd con-tent includes samples without corresponding Pt mea-surements.

by marginal, chilled quartz diorites. These unitscontain 60-70 ppm Ni, ca. 60 ppm Cu and 4 ppbof Pt and Pd (Keays and Lightfoot, 2004; Mungallet al., 2004). There is a general lack of publishedtarget rock data, however broad constraints uponwhether the mixing proportions identified in thisinvestigation can account for the bulk melt sheetmetal contents are possible. The Ni and Cu con-tents of the average Main Mass and O!set DykeQD’s are higher than Huronian metasedimentaryrocks or Superior Province gneisses. However,estimating average Huronian metasediment com-position as section 4.1.3, and using the averagegneiss data of Chai and Eckstrand (1994), it isshown in Figure 7 that addition of a ca. 20 %Nipissing component to an equal mix of Huronianand Superior Province melt can reproduce the Niand Cu contents, and Ni/Cu ratio, of the bulk SIC.The average lower crustal composition of Rudnickand Gao (2003) has a Ni/Cu ratio of 3.4, suggest-ing that addition of a significant component of av-erage lower crustal material cannot reproduce theCu content or Ni/Cu ratio of the bulk SIC.

Platinum group element abundances in the tar-get rocks are not well documented, making a ro-bust assessment of whether su"cient PGE’s arecontained in locally exposed lithologies very di"-cult. Our findings indicate that the Ni and Cu con-tents of the melt sheet can be explained by mixingof known target rocks, and as such we would pre-dict the same for the PGE. It is shown in Figure 7that a mix of ca. 80 % upper crustal material withca. 20 % of Nipissing gabbros closely matches thePt and Pd concentrations of average quartz diorite(3.11 and 2.51 ppb respectively), and the averagelower crust.

Given the current distribution of the Nipissingmafic suite, it is geologically reasonable that itwould have contributed tens of percent of the to-tal impact melt volume. The above observationssuggest that a large volume of lower crust is notnecessary, or indeed ideally suited, to explain themetal contents of the melt sheet. However, thereis clearly much scope for a systematic study of Ni,Cu and PGE concentrations of target rocks in theSudbury area, which would allow for further as-sessment of the su"ciency of metal contents in lo-cally exposed target rocks.

An important implication of our findings is thatthe contribution of Nipissing gabbros (and sim-ilar mafic rocks) may be fundamental to raisingmetal contents of the melt sheet. As previouslydiscussed, the Th/Sc ratio is useful in identifyinginvolvement of Nipissing gabbro in the source ofQD magmas. Figure 8 compares the Th/Sc ra-tio with initial Pb isotope composition. Deviationfrom the Huronian-Superior Province mixing line,to lower Th/Sc in South Range QD, requires up toca. 30 % Nipissing gabbro. Consistent with the

100

14.514.714.915.115.315.515.715.916.1

0 0.2 0.4 0.6 0.8Th/Sc

207 P

b/20

4 Pb 1

850

NipissingHuronian metased.Levack gneiss

South RangeNorth Range

Offset Dyke samples

Figure 8: Pb isotope and Th/Sc mixing of target rock endmembers.

spatial distribution of Nipissing gabbro at currentexposure levels, the contribution to North RangeQD is lower (<20 %). Such proportions also ex-plain the MgO contents of O!set Dyke quartz dior-ites (3.7-4.5 wt%), which are higher than SuperiorProvince gneisses (mean ca. 3.5 wt %) and finegrained Huronian metasedimentary rocks (1.1-3.5wt%) . It would therefore be predicted that theinitial Ni Cu, and PGE contents of North Rangeimpact melts were lower than the South Range.

4.2. Accomodating a Shallow Impact

It has been shown that the chemistry of themelt sheet can be derived from melting of varyingproportions of locally exposed target lithologies.A significant contribution of Huronian metasedi-ments is required throughout, and dominated thesource of South Range impact melts. The Huro-nian Supergroup was deposited in a rift to pas-sive margin environment on the subsiding marginof the Superior Craton between 2450 and 2219Ma (McLennan et al., 2000; Young et al., 2001;Mungall and Hanley, 2004). The resultant south-ward thickening sedimentary wedge, which hassince undergone multiple episodes of greenshist toamphibolite facies metamorphism (Riller, 2005),has a maximum total thickness of 8-12 km inthe Sudbury region (Dressler, 1984; Young et al.,2001; Corfu and Andrews, 1986). The O!set Dykequartz diorites therefore record a predominantlyupper crustal precursor. Clearly such requirementsare not in agreement with models that require neartotal vaporisation or excavation of the upper crustduring the impact event (Deutsch et al., 1995;Grieve et al., 1991; Mungall et al., 2004; Ivanovand Deutsch, 1999; Zieg and Marsh, 2005).

There are many factors that can influence thevolume and depth of melting in an impact event.As previously discussed, impact models for theSudbury event have relied upon scaling between

transient cavity diameter and melt volumes to es-timate the nature of the impactor. One critical pa-rameter that these models do not take into accountis the angle of impact. Whereas vertical impactscenarios are desirable for simplifying modelling,the probability of a vertical impact is negligible.Impact events are expected to occur at an angleto the planetary surface (see Pierazzo and Melosh2000 for review), with the most likely scenario be-ing 45#, irrespective of the planets gravitationalfield (Shoemaker, 1962; Gilbert, 1893).

Modelling of oblique impacts demonstrates thatthe depth and volume of melted target rocks variessignificantly with impact angle. Pierazzo andMelosh (2000b) show that impact angle a!ectsthe strength and distribution of the shock wave,with the disposition of peak shock pressures be-coming asymmetric and concentrated in the down-range, shallower, portion of the target. As a re-sult, the volume of the target that experiences therequired shock pressures for melting (between 50and 100 GPa) has a maximum depth for verticalimpacts and shallows significantly for non-verticalimpacts. In addition, the volume of vaporisedmaterial (>100 GPa) decreases significantly (Pier-azzo and Melosh, 2000b). The modelled distribu-tion of peak shock pressures for a range of impactangles of a 10 km in diameter dunite body, travel-ling at 20 kms"1, is shown in Figure 9. Such an im-pactor is similar to that predicted for the Sudburyevent (Deutsch et al., 1995; Grieve et al., 1991;Mungall et al., 2004; Ivanov and Deutsch, 1999;Zieg and Marsh, 2005). At the most likely impactangle (45#) there is a significant shallowing of thevolume of melted material, accompanied by only aca. 20 % reduction in melted volume. The regionof total melting is constrained to the upper 20 kmof the crust. In the 30# scenario, impact melting isconstrained to the upper 16 km of the crust and thevolume of vaporised material is far less than in ver-tical impact, however a melt volume reduction of50 % also occurs. Oblique impacts can thereforeaccount for the upper crustal nature of the impactmelts.

Oblique impact events can be recognised byasymmetric distributions of ejecta, shock meta-morphism and deformation (Gault and Wedekind,1978). Erosion and deformation of the SudburyStructure unfortunately make it unlikely that suchevidence has been preserved.

4.2.1. A comet impactor?

The estimated impact melt volume for Sudburyof Sto#er et al. (1994) (12000 to 13000 km3) caneasily be accommodated by an oblique impact ofan asteroid with density ca. 3 gcm"3, given themodelling results of Pierazzo and Melosh (2000b).However, a re-evaluation of the melt volume inSudbury by Pope et al. (2004) suggests that current

101

90 deg. 60 deg.

45 deg. 30 deg. 15 deg.

Peak shock pressure (GPa)

>250>200>150>100

>50>30>18

Figure 9: Peak shock pressure contours in the plane of impact for varying impact angles (from Pierazzo and Melosh,2000b). Models for a 10 km in diameter dunite impactor, travelling at 20 kms"1. Axes units are kilometers. Alsoshown are the approximate maximum depths of impact melting for each scenario (taken as pressure >50 GPa).

estimates are not entirely satisfactory. The meltvolumes in suevites (the Onaping formation) wereunderestimated by Sto#er et al. (1994), and noprevious estimates account for melt in the annulartrough, or that was expelled from crater. Accord-ingly Pope et al. (2004) calculate a total impactmelt (+ vapour) volume of ca. 30000 km3. If thelargest possible transient cavity (110 km), highestreasonable impact velocity (30 kms"1), and impactangles >45# are taken, then an asteroid (density= 3 gcm"3) can produce such large melt volumes.However Pope et al. (2004) suggest that a cometimpactor (0.9 gcm"3, 40-50 kms"1) is more likely,given their calculated melt (+ vapour) volumes forvarious impact scenarios (Figure 10).

A comet impactor does appear to reconcile withestimated melt volumes better than an asteroid,particularly given an oblique impact. The identifi-cation of a meteoritic component can be achievedby assessing the refractory platinum group ele-ment (PGE) charactersitics of fallback materialand distal ejecta (Koeberl, 1998). Although PGEconcentrations will be higher in meteorites thancomets, the composition of comets is poorly con-strained. Given that comets may contain a signifi-cant amount of rocky material similar to CI chon-drites, iridium contents have been estimated to bebetween 113 - 338 ppb for ice contents of 25 -75%, a factor of 1 to 8 less than chondrites (Kringet al., 1996) . Iridium concentrations in the On-

aping Formation are enriched relative to the bulkcontinental crust, with up to 0.5 ppb Ir (Mungallet al, 2004) and distal ejecta, recently recognisedin Michigan, has up to 0.8 ppb Ir (Pufahl et al.,2007). It is not clear if these concentrations re-quire a meteoritic impactor, particularly given thattotal vaporisation of the impacting body may notoccur at impact angles of 45# or less Pierazzo andMelosh (2000a). There is much scope for a sys-tematic geochemical investigation into the natureof the Sudbury impactor.

4.3. Heterogeneity of Impact Melts

Variability in the Pb isotope composition ofNorth Range O!set Dyke samples is small, andnot easily resolved (Figure 4). Several SouthRange O!set Dykes have di!erent initial Pb iso-tope compositions (Figure 11). The variation inSouth Range samples could be a result of theirsource containing variable proportions of Supe-rior Province gneisses (up to 50%), and/or signif-icant contribution from Nipissing gabbros. In ei-ther case, it is clear that they do not share commonmixes of precursor target lithologies. Assumingall O!set Dykes were formed at the same time thismust reflect spatial variations in target rock con-tributions. In the North Range, the relative homo-geneity of O!set Dykes likely reflects a lack ofvariation of target rock Pb isotope compositions.

102

30 45 900

10000

20000

30000

40000

50000

Impact angle (degrees)

Mel

t + v

apou

r vol

ume

(km

3 )

AsteroidComet

Dtc = 100km

Dtc = 90km

Dtc = 80km

Dtc = 80km

Dtc = 90km

Dtc = 100km

Dtc = 110km

Figure 10: Modelled melt (+ vapour) volumes from a va-riety of possible impact scenarios. Grey band is the es-timated melt (+ vapour) volume for the Sudbury Struc-ture. From Pope et al. (2004).

Multiple samples of both QD and IQD wereanalysed from the Worthington O!set Dyke. Noresolvable di!erences in initial Pb isotopes are ap-parant between QD and IQD, indicating that, asconcluded by Lightfoot and Farrow (2002), as-similation of inclusion populations has not signifi-cantly a!ected the quartz diorite matrices of thesesamples. Samples of QD from di!erent locationson the O!set have initial ratios that are just aboutresolvable given the analytical uncertainty (Figure11). However it is clear that the isotopic variabil-ity on the scale of individual O!sets is small com-pared to inter-O!set variability.

Isotopic heterogeneity has been identifiedthroughout the Main Mass of the melt sheet (Dar-ling et al., 2010), whereby it has been suggestedto result from continued assimilation of footwallrocks, fallback material and entrained clasts af-ter the formation of the melt sheet. However, thegeochemical variability identified between O!setDykes shows that the melt sheet was heteroge-neous within a relatively short time of impact.

Theoretical considerations on the violent move-ment and mixing of superheated, low viscosityshock melt (Phinney and Simonds, 1977; Simondsand Kie!er, 1993) suggest that the initial meltproducts of impact melting would have been ho-mogenised during collapse of the transient cav-ity, within minutes of impact (Melosh and Ivanov,1999). The products of shock melting, i.e. thoseformed directly by high shock pressures, wouldhave been mixed with material entrained duringflow along the transient cavity walls. The largedi!erences between North and South Range quartzdiorites suggests that, on a broad scale, these meltsretain chemical variations inherited from di!er-

207 P

b/20

4 Pb 1

850

206Pb/204Pb1850Frood-Stobie IQDCopper Cliff QD Worthington QD

Worthington IQD

Manchester QD

14.5 15.0 15.5 16.0 16.5 17.0 17.5 18.0

15.2

15.4

15.6

15.8

16.0

16.2

80%

40%

60%

Figure 11: Age corrected Pb isotope compositions ofSouth Range O!set Dyke samples. Mixing line andHuronian end member as Figure 3. QD; quartz diorite:IQD; inclusion rich quartz diorite. Nipissing gabbrosare also shown (solid circles).

ent target rock precursors, despite extensive me-chanical mixing during crater formation. Fol-lowing collapse of the transient cavity, a meltsheet would have formed with an initial temper-ature of ca. 2000 K (Ivanov and Deutsch, 1999).Entrained clasts would have been completely di-gested and the melt sheet had potential to ther-mally erode hundreds of meters down into thefootwall (Prevec and Cawthorn, 2002), as well asdigest large volumes of fallback breccia. The iden-tification of isotopic and trace element variabilitybetween O!set Dykes from the South Range indi-cates that at no stage was the melt sheet homoge-neous. Indeed, continued assimilation maintainedgeochemical variabilities throughout the history ofthe melt sheet (Darling et al., 2010), despite vigor-ous convective mixing.

4.4. Implications for Crustal Evolution

Of the preserved terrestrial impact structures,Sudbury is second in size only to the deeply erodedVredefort impact structure (Grieve and Therriault,2000). As such, the findings of this investigationhave some interesting implications for the a!ectof large meteorite impacts upon the crust. Im-pact models invoking melting to depths that ap-proach the maximum transient cavity depth of ca.30 km (Deutsch et al., 1995; Grieve et al., 1991;Mungall et al., 2004), infer that Sudbury sized im-pact events would have profoundly disturbed thecompositionsal layering of the continental crust.If the melt sheet had a lower crustal source, asconcluded by Mungall et al. (2004), then completeinversion of the crust would have occured. Fur-thermore, given a present day crustal thickness of32-37 km in the southeastern Superior Province(Darbyshire et al., 2007), a 30 km deep transient

103

cavity would have extended towards the base ofthe crust. It has been di"cult to reconcile suchmodels with the fact that there is no record of de-compressive mantle melting in the Sudbury areaaround the time of impact, or significant mantlecontribution to the melt sheet. The recognition ofan upper to mid-crustal source for the melt sheet,and re-evaluation of the Sudbury event in termsof an oblique impact, indicates that the excavationand melting depths are significantly shallower thanpreviously considered. The compositional layer-ing of the crust would not have been inverted, andthe formation of the melt sheet without significantmantle contribution becomes conceptually easier.

5. CONCLUSIONS

The Pb isotope compositions of early formedquartz diorites in the SIC are readily explainedby mixing of variable proportions of locally ex-posed target lithologies. The range of age cor-rected Pb isotope ratios in these samples can beapproximated by binary mixing between SuperiorProvince gneisses and metasedimentary rocks ofthe Huronian Supergroup. Deviation of samplesfrom the binary mixing line can be explained bythe incorporation of Nipissing mafic rocks.

Such an origin for the impact melts is consistentwith previously published Sr, Nd and Os isotopedata, together with trace element considerations.A minor lower crustal contribution cannot be ex-cluded, although it is not required to explain thecomposition of the melt sheet. A Huronian com-ponent of up to 40 % is required in North Rangesamples, whereas in the South Range the Huroniancontribution may be up to 80 %. This supracrustalsuccession had a maximum pre-impact thicknessof ca. 12 km, requiring significantly shallowermelting than that predicted in previous impactmodels for the Sudbury event. This can be accom-modated by considering approach trajectories forthe impactor that are oblique to the Earth’s surface.

Consistent with shallower impact melting, andin contrast to Mungall et al. (2004), we find thatthe bulk composition of melt sheet has a strongupper to mid-crustal a"nity (e.g. Eu/Sm = 0.22,Rb/Sr = 0.2-0.35). Exceptions to this are the con-centration of MgO and certain transition metals, inparticular Ni, Cu and Platinum Group Elements.By compiling available target rock data, we findthat the incorporation of 10 to 30 % of Nipissinggabbro into the source of the melt sheet can ac-count for the concentrations of these elements. In-deed, the incorporation of Nipissing mafic rocksappears to better reproduce the bulk Ni, Cu andPGE composition of the melt sheet than does alower crustal component. An important implica-tion of this finding to the formation of sulphideores in Sudbury is that the metal budget of the

Main Mass may be controlled by the spatial dis-tribution of mafic target lithologies. A system-atic study of Ni, Cu and PGE abundances in targetrocks is required to test this hypothesis.

The variability in the Pb isotope compositionof O!set Dykes from around the complex revealsthat the melt sheet was heterogeneous at an earlystage. At the time of O!set Dyke formation,the melt sheet was superheated and had signifi-cant potential to generate chemical heterogeneityby assimilation of entrained clasts, footwall rocksand fallback material. However, given the largedi!erences in the relative proportions of targetrocks required to explain the observed variations,it seems unlikely that the impact melts were fullyhomogenised at any stage. The major geochemicaldi!erences between North and South Range meltsprobably survived mechanical mixing during col-lapse of the transient cavity.

References

Ames, D., Watkinson, D., Parrish, R., 1998. Dating of a re-gional hydrothermal system induced by the 1850 Ma Sud-bury impact event. Geology 26 (5), 447–450.

Ames, D. E., Davidson, A., Buckle, J. L., Card, K. D., 2005.Geology, Sudbury bedrock compilation, Ontario. Vol. OpenFile 4570. Geological Survey of Canada.

Baker, J., Peate, D., Waight, T., Meyzen, C., 2004. Pb isotopicanalysis of standards and samples using a Pb-207-Pb-204double spike and thallium to correct for mass bias with adouble-focusing MC-ICP-MS. Chemical Geology 211 (3-4), 275–303.

Bennet, G., Dressler, B., Robertson, J., 1991. The HuronianSupergroup and associated intrusive rocks. In: Thurston, P.,Williams, H., Sutcli!e, R., Stott, G. (Eds.), Geology of On-tario. Vol. Special Paper 4. Ontario Geological Survey, pp.549–592.

Card, K., Gupta, V., McGrath, P., Grant, F., 1984. The SudburyStructure: Its regional geological and geophysical setting.In: Pye, E., Naldrett, A., Giblin, P. (Eds.), The geology andore deposits of the Sudbury Structure. Vol. Special Volume1. Ontario Geological Survey, Ch. 2, pp. 25–45.

Chai, G., Eckstrand, R., 1994. Rare Earth Element character-istics and origin of the Sudbury Igneous Complex, Ontario,Canada. Chemical Geology 113 (3-4), 221–244.

Corfu, F., Andrews, A. J., 1986. A U-Pb Age for MineralizedNipissing Diabase, Gowganda, Ontario. Canadian Journalof Earth Sciences 23 (1), 107–109.

Darbyshire, F. A., Eaton, D. W., Frederiksen, A. W., Erto-lahti, L., 2007. New insights into the lithosphere beneath theSuperior Province from Rayleigh wave dispersion and re-ceiver function analysis. Geophysical Journal International169 (3), 1043–1068.

Darling, J. R., Hawkesworth, C. J., Lightfoot, P. C., Storey,C. D., Tremblay, E., 2010. Isotopic heterogeneity in theSudbury impact melt sheet. Earth and Planetary ScienceLetters 289 (3-4), 347–356.

Deutsch, A., Grieve, R., Avermann, M., Bischo!, L., Brock-meyer, P., Buhl, D., Lakomy, R., MullerMohr, V., Oster-mann, M., Sto#er, D., 1995. The Sudbury Structure (On-tario, Canada): A tectonically deformed multi-ring impactbasin. Geol Rundsch 84 (4), 697–709.

Dickin, A. P., Artan, M. A., Crocket, J. H., 1996. Isotopicevidence for distinct crustal sources of North and SouthRange ores, Sudbury Igneous Complex. Geochimica et Cos-mochimica Acta 60 (9), 1605–1613.

Dickin, A. P., Nguyen, T., Crocket, J. H., 1999. Isotopic evi-dence for a single impact melting origin of the Sudbury Ig-neous Complex. In: Dressler, B., Sharpton, V. (Eds.), Large

104

Meteorite Impacts and Planetary Evolution II. Vol. SpecialPaper 339. Geological Society of America, Boulder, Col-orado, pp. 361–371.

Dickin, A. P., Richardson, J. M., Crocket, J. H., Mcnutt, R. H.,Peredery, W. V., 1992. Osmium isotope evidence for acrustal origin of Platinum Group Elements in the Sudburynickel ore, Ontario, Canada. Geochimica et CosmochimicaActa 56 (9), 3531–3537.

Dietz, R. S., 1964. Sudbury Structure as an astrobleme. Journalof Geology 72 (4), 412.

Dressler, B. O., 1984. General geology of the Sudbury area. In:Pye, E., Naldrett, A., Giblin, P. (Eds.), The geology and oredeposits of the Sudbury Structure. Vol. Special Volume 1.Ontario Geological Survey, pp. 52–87.

Easton, R. M., 2000. Metamorphism of the Canadian Shield,Ontario, Canada. I. The Superior Province. Canadian Min-eralogist 38, 287–317.

Faggart, B. E., Basu, A. R., Tatsumoto, M., 1985. Origin ofthe Sudbury Complex by metoritic impact - neodymium iso-topic evidence. Science 230 (4724), 436–439.

Fairbairn, H., Knight, C., Card, C., Pinson, W., Hurley, P.,1967. Rb-sr age and initial Sr87/Sr86 of the Huronian sec-tion south-west of Sudbury, Ontario. In: 15th Annual Re-port. Vol. Atomic Energy Commission contract 1381-15.Massachusetts Institute of Technology, Cambridge, Mas-sachusetts, pp. 53–60.

Fedo, C., Grant, G., Nesbitt, H., 1997. Paleoclimatic control onthe composition of the Paleoproterozoic Serpent Formation,Huronian Supergroup, Canada: a greenhouse to icehousetransition. Precambrian Research 86 (3-4), 201–223.

Gariepy, C., Allegre, C., 1985. The lead isotope geochem-istry and geochronology of late-kinematic intrusives fromthe Abitibi greenstone-belt, and the implications for lateArchean crustal evolution. Geochimica et CosmochimicaActa 49 (11), 2371–2383.

Gibbins, W., McNutt, R., 1975. The age of the Sudbury NickelIrruptive and the Murray Granite. Canadian Journal of EarthSciences 12, 1970–1989.

Gilbert, G. K., 1893. The Moon’s face, a study of the origin ofits features. Bulletin of the Philosophical Society of Wash-ington 12, 241–293.

Grieve, R. A. F., Cintala, M. J., 1992. An analysis of di!eren-tial impact-melt crater-scaling and implications for the ter-restrial impact record. Meteoritics 27 (5), 526–538.

Grieve, R. A. F., Sto#er, D., Deutsch, A., 1991. The SudburyStructure - controversial or misunderstood. Journal of Geo-physical Research - Planets 96 (E5), 22753–22764.

Grieve, R. A. F., Therriault, A., 2000. Vredefort, Sudbury,Chicxulub: Three of a kind? Annual Review of Earth andPlanetary Sciences 28, 305–338.

Hecht, L., Wittek, A., Riller, U., Mohr, T., Schmitt, R. T.,Grieve, R. A. F., 2008. Di!erentiation and emplacement ofthe Worthington O!set Dike of the Sudbury impact struc-ture, Ontario. Meteorit Planet Sci 43 (10), 1659–1679.

Hemming, S., McDaniel, D., McLennan, S., Hanson, G., 1996.Pb isotope constraints on the provenance and diagenesis ofdetrital feldspars from the Sudbury Basin, Canada. Earthand Planetary Science Letters 142 (3-4), 501–512.

Housh, T., Bowring, S., 1991. Lead isotopic heterogeneitieswithin alkali feldspars - implications for the determinationof initial lead isotopic compositions. Geochimica et Cos-mochimica Acta 55 (8), 2309–2316.

Hurst, R. W., Wetheril, G., 1974. Rb-Sr study of the SudburyNickel Irruptive. Eos, Transactions, American GeophysicalUnion 55 (4), 466–466.

Ivanov, B., Deutsch, A., 1999. Sudbury impact event: Cra-tering mechanics and thermal history. In: Dressler, B. O.,Sharpton, V. L. (Eds.), Large Meteorite Impacts and Plane-tary Evolution II. Vol. 339. Geological Society of America,Boulder, Colorado, pp. 389–398.

Ja!ey, A., Flynn, K., Glendeni, L., Bentley, W., Essling, A.,1971. Precision measurement of half-lives and specific ac-tivities of U-235 and U-238. Physical Review C 4 (5), 1889–1891.

Jolly, W., Dickin, A., WU, T., 1992. Geochemical stratigraphyof the Huronian continental volcanics at Thessalon, Ontario

- contributions of 2-stage crustal fusion. Contrib MineralPetrol 110 (4), 411–428.

Keays, R. R., Lightfoot, P. C., 2004. Formation of Ni-Cu-Platinum Group Element sulfide mineralization in the sud-bury impact melt sheet. Mineralogy and Petrology 82 (3-4),217–258.

Kemp, A. I. S., Hawkesworth, C. J., 2003. Granitic perspectiveson the generation and secular evolution of the continentalcrust. In: H.D., H., K.K., T. (Eds.), The Crust. Vol. 3 ofTreatise on Geochemistry. Elsevier-Pergamon, pp. 349–410.

Kettrup, B., Deutsch, A., Masaitis, V., 2003. Homogeneous im-pact melts produced by a heterogeneous target? Sr-Nd iso-topic evidence from the Popigai crater, Russia. Geochimicaet Cosmochimica Acta 67 (4), 733–750.

Koeberl, C., 1998. Identification of meteoritic components inimpactites. Geological Society, London, Special Publica-tions 140 (1), 133–153.

Kring, D., Melosh, H., Hunten, D., 1996. Impact-induced per-turbations of atmospheric sulfur. Earth and Planetary Sci-ence Letters 140 (1-4), 201–212.



LaFrance, B., Legault, D., Ames, D. E., 2008. The formation ofthe Sudbury Breccia in the North Range of the Sudbury im-pact structure. Precambrian Research 165 (3-4), 107–119.

Langmuir, D., 1978. Uranium solution-mineral equilibria atlow-temperatures with applications to sedimentary ore-deposits. Geochimica et Cosmochimica Acta 42 (6), 547–569.

Lightfoot, P. C., 2002. Petrology and geochemistry of theNipissing gabbro: exploration strategies for nickel, copperand platinum group elements in a large igneous province.Ontario Geological Survey, Special Volume 58, 81.

Lightfoot, P. C., Desouza, H., Doherty, W., 1993. Di!erentia-tion and source of the Nipissing diabase intrusions, Ontario,Canada. Canadian Journal of Earth Sciences 30 (6), 1123–1140.

Lightfoot, P. C., Doherty, W., K.Farrell, Keays, R. R., Moore,M. L., Pekeski, D., 1997a. Geochemistry of the Main Mass,Sublayer, O!sets and inclusions from the Sudbury IgneousComplex, Ontario. Ontario Geological Survey, Open FileReport 5959.

Lightfoot, P. C., Farrow, C. E. G., 2002. Geology, geochem-istry, and mineralogy of the Worthington O!set Dike: Agenetic model for o!set dike mineralization in the SudburyIgneous Complex. Economic Geology 97 (7), 1419–1446.

Lightfoot, P. C., Keays, R. R., Doherty, W., 2001. Chemicalevolution and origin of nickel sulfide mineralization in theSudbury Igneous Complex, Ontario, Canada. Economic Ge-ology 96 (8), 1855–1875.

Lightfoot, P. C., Keays, R. R., Morrison, G. G., Bite, A., Far-rell, K. P., 1997b. Geochemical relationships in the Sud-bury Igneous Complex: Origin of the Main Mass and O!setDikes. Economic Geology 92 (3), 289–307.

Lightfoot, P. C., Keays, R. R., Morrison, G. G., Bite, A., Far-rell, K. P., 1997c. Geologic and geochemical relationshipsbetween the Contact Sublayer, inclusions, and the MainMass of the Sudbury Igneous Complex: A case study ofthe Whistle Mine embayment. Economic Geology 92 (6),647–673.

Lightfoot, P. C., Naldrett, A. J. (Eds.), 1994. Proceedings ofthe Sudbury-Noril’sk Symposium. Vol. Special Volume 5.Ontario Geological Survey.


McCulloch, M., Wasserburg, G. J., 1978. Sm-Nd and Rb-Sr chronology of continental crust formation. Science200 (4345), 1003–1011.

105

McLennan, S. M., Fryer, B. J., Young, G. M., 1979. Rare-EarthElements in Huronian (lower Proterozoic) sedimentary-rocks - composition and evolution of the post-Kenoran up-per crust. Geochimica et Cosmochimica Acta 43 (3), 375–388.

McLennan, S. M., Simonetti, A., Goldstein, S. L., 2000.Nd and Pb isotopic evidence for provenance and post-depositional alteration of the Paleoproterozoic Huronian Su-pergroup, Canada. Precambrian Research 102 (3-4), 263–278.

Meldrum, A., AbdelRahman, A. F. M., Martin, R. F., Wodicka,N., 1997. The nature, age and petrogenesis of the Cartierbatholith, northern flank of the Sudbury Structure, Ontario,Canada. Precambrian Research 82 (3-4), 265–285.

Melosh, H. J., 1989. Impact cratering: A geological process.Oxford University Press, New York.

Melosh, H. J., Ivanov, B. A., 1999. Impact crater collapse. An-nual Review of Earth and Planetary Sciences 27 (385-415).

Morgan, J. W., Walker, R. J., Horan, M. F., Beary, E. S.,Naldrett, A. J., 2002. Pt-190-Os-186 and Re-187-Os-187systematics of the Sudbury Igneous Complex, Ontario.Geochimica et Cosmochimica Acta 66 (2), 273–290.


Mungall, J. E., Hanley, J. J., 2004. Origins of outliers of theHuronian Supergroup within the Sudbury Structure. Journalof Geology 112 (1), 59–70.

Murphy, A. J., Spray, J. G., 2002. Geology, mineralization, andemplacement of the Whistle-Parkin O!set Dike, Sudbury.Economic Geology 97 (7), 1399–1418.

Naldrett, A. J., Hewins, R. H., 1984. The Main Mass of theSudbury Igneous Complex. In: Pye, E., Naldrett, A., Giblin,P. (Eds.), The Geology and Ore Deposits of the SudburyStructure. Ontario Geological Survey, Ch. 10, pp. 236–251.

Naldrett, A. J., Rao, B. V., Evensen, N. M., 1986. Contami-nation at Sudbury and its role in ore formation. In: Metal-logeny of basic and ultrabasic rocks. The Institute of Miningand Metallurgy, London, pp. 75–91.

Ostermann, M., Scharer, U., Deutsch, A., 1996. Impact meltdikes in the sudbury multi-ring basin (canada): Implicationsfrom uranium-lead geochronology on the foy o!set dike.Meteorit Planet Sci 31 (4), 494–501.

Patchett, P. J., White, W. M., Feldmann, H., Kielinczuk, S.,Hofmann, A. W., 1984. Hafnium rare-earth element frac-tionation in the sedimentary system and crustal recyclinginto the Earths mantle. Earth and Planetary Science Letters69 (2), 365–378.

Phinney, W. C., Simonds, C. H., 1977. Dynamical implicationsof the petrology and distribution of impact melt rocks. In:Roddy, J., Pepin, O., Merrill, B. (Eds.), Impact and Explo-sion Cratering: Planetary and Terrestrial Implications. Perg-amon Press, Flagsta!, Arizona.

Pierazzo, E., Melosh, H. J., 2000a. Hydrocode modeling ofoblique impacts: The fate of the projectile. Meteoritics andPlanetary Science 35 (1), 117–130.

Pierazzo, E., Melosh, H. J., 2000b. Melt production in obliqueimpacts. Icarus 145 (1), 252–261.

Pierazzo, E., Vickery, A. M., Melosh, H. L., 1997. A reevalua-tion of impact melt production. Icarus 127 (2), 408–423.

Pope, K. O., Kie!er, S. W., Ames, D. E., 2004. Empirical andtheoretical comparisons of the Chicxulub and Sudbury im-pact structures. Meteoritics and Planetary Science 39 (1),97–116.

Prevec, S., Cawthorn, R., 2002. Thermal evolution and in-teraction between impact melt sheet and footwall: A ge-netic model for the contact Sublayer of the Sudbury IgneousComplex, Canada. Journal of Geophysical Research - SolidEarth 107 (B8), 2176.

Prevec, S. A., Lightfoot, P. C., Keays, R. R., 2000. Evolution ofthe Sublayer of the Sudbury Igneous Complex: geochemi-cal, Sm-Nd isotopic and petrologic evidence. Lithos 51 (4),271–292.

Pufahl, P. K., Hiatt, E. E., Stanley, C. R., Morrow, J. R., Nelson,G. J., Edwards, C. T., 2007. Physical and chemical evidenceof the 1850 Ma Sudbury impact event in the Baraga Group,

Michigan. Geology 35 (9), 827–830.Pye, E. G., Naldrett, A. J., Giblin, P. E. (Eds.), 1984. The geol-

ogy and ore deposits of the Sudbury Structure. Vol. SpecialVolume 1. Ontario Geological Survey.

Riller, U., 2005. Structural characteristics of the Sudbury im-pact structure, Canada: Impact-induced versus orogenic de-formation - A review. Meteoritics and Planetary Science40 (11), 1723–1740.

Rudnick, R. L., Gao, S., 2003. Composition of the continentalcrust. In: Rudnick, R. (Ed.), The Crust. Vol. 3. Elsevier-Pergamon.

Scott, R. G., Benn, K., 2001. Peak-ring rim collapse accommo-dated by impact melt-filled transfer faults, Sudbury impactstructure, Canada. Geol 29 (8), 747–750.

Shoemaker, E. M., 1962. Interpretation of Lunar craters. In:Kopal, Z. (Ed.), Physics and Astronomy of the Moon. Aca-demic Press, New York/London, pp. 283–359.

Simonds, C. H., Kie!er, S. W., 1993. Impact and volcanism - amomentum scaling law for erosion. Journal of GeophysicalResearch - Solid Earth 98 (B8), 14321–14337.

Sims, P. K., Van Schmus, W. R., Schultz, K. J., Petermann,Z. E., 1989. Tectono-stratigraphic evolution of the early Pro-terozoic Wisconsin magmatic terranes of the Penokean oro-gen. Canadian Journal of Earth Sciences 26, 2145–2158.


Stacey, J. S., Kramers, J. D., 1975. Approximation of terres-trial lead isotope evolution by a 2-stage model. Earth andPlanetary Science Letters 26 (2), 207–221.

Sto#er, D., Deutsch, A., Avermann, M., Bischo!, L., Brock-meyer, P., Buhl, D., Lakomy, R., Muller-Mohr, V., 1994.The formation of the Sudbury Structure, Canada: Towardsa unified model. In: Dressler, B., Grieve, R., Sharpton,V. (Eds.), Large meteorite impacts and planetary evolution.Vol. 293 of Special Paper. Geological Society of America,Boulder, Colorado, pp. 303–318.

Strelow, F. W. E., 1978. Distribution coe"cients and anion-exchange behaviour of some elements in hydrobromic nitricacid mixtures. Analytical Chemistry 50 (9), 1359–1361.

Thompson, L. M., Spray, J. G., 1994. Pseudotachylytic rockdistributions and genesis within the Sudbury impact struc-ture. In: Dressler, B., Grieve, R., Sharpton, V. (Eds.), LargeMeteorite Impacts and Planetary Evolution. Special Paper293. Geological Society of America, pp. 275–287.

Tuchscherer, M. G., Spray, J. G., 2002. Geology, mineraliza-tion, and emplacement of the Foy O!set dike, Sudbury im-pact structure. Economic Geology 97 (7), 1377–1397.

Valaincourt, C., Sproule, C. A., MacDonald, C. A., Lesher,C. M., 2003. Investigation of Mafic-Ultramafic Instrusionsin Ontario and implications for Platinum Group Elementmineralization: Operation Treasure Hunt. No. 6102 in OpenFile Report. Ontario Geological Survey.

Vervoort, J., Patchett, P., Blichert-Toft, J., Albarede, F., 1999.Relationships between Lu-Hf and Sm-Nd isotopic systemsin the global sedimentary system. Earth and Planetary Sci-ence Letters 168 (1-2), 79–99.

Vervoort, J. D., White, W. M., Thorpe, R. I., 1994. Nd and Pbisotope ratios of the Abitibi greenstone-belt - new evidencefor very early di!erentiation of the Earth. Earth and Plane-tary Science Letters 128 (3-4), 215–229.

Walker, R. J., Morgan, J. W., Beary, E. S., Smoliar, M. I.,Czamanske, G. K., Horan, M. F., 1997. Applications ofthe pt-190-os-186 isotope system to geochemistry and cos-mochemistry. Geochimica et Cosmochimica Acta 61 (22),4799–4807.

Walker, R. J., Morgan, J. W., Naldrett, A. J., Li, C., Fassett,J. D., 1991. Re-Os isotope systematics of Ni-Cu sulfide ores,Sudbury Igneous Complex, Ontario - Evidence for a ma-jor crustal component. Earth and Planetary Science Letters105 (4), 416–429.

Wood, C. R., Spray, J. G., 1998. Origin and emplacement ofO!set Dykes in the Sudbury impact structure: Constraintsfrom Hess. Meteoritics and Planetary Science 33 (2), 337–347.

106

Wu, J. J., Milkereit, B., Boerner, D. E., 1995. Seismic imagingof the enigmatic Sudbury Structure. Journal of GeophysicalResearch - Solid Earth 100 (B3), 4117–4130.

Young, G. M., 2001. Comparative geochemistry of Pleistoceneand Paleoproterozoic (Huronian) glaciogenic laminated de-posits: Relevance to crustal and atmospheric composition inthe last 2.3 Ga. Journal of Geology 109 (4), 463–477.

Young, G. M., Long, D. G. F., Fedo, C. M., Nesbitt, H. W.,2001. Paleoproterozoic Huronian basin: product of a Wil-son cycle punctuated by glaciations and a meteorite impact.Sedimentary Geology 141, 233–254.

Zieg, M. J., Marsh, B. D., 2005. The Sudbury Igneous Com-plex: Viscous emulsion di!erentiation of a superheated im-pact melt sheet. Bulletin of the Geological Society of Amer-ica 117 (11-12), 1427–1450.

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5 Pb isotope systematics of the Offset Dykes

108

6

Impact melt sheet zircons

6.1 Introduction

The preceding chapters have focused upon the characterisation of isotopic variations

throughout various melt sheet lithologies. The scientific objectives of this chapter are

somewhat different, centering on the analysis of zircons from throughout the melt sheet

stratigraphy. The importance of zircon (ZrSiO4) to a wide range of geological problems

is well documented (see volume edited by Hanchar, 2003), and a vast literature exists

detailing the characteristics and compositions of zircons from a wide range of magmatic

and metamorphic environments. However, as yet there is almost no data relating to

zircons that have crystallised from impact melt sheets.

108 10201010 1012 1014 1016 1018

Late Heavy Bombardment

?

Accretion of the moon4.5

4.0

3.5

3.0

2.5

2.0Sudbury Igneous Complex (1.85 Ga)7Oldest terrestrial melt sheet.

Vredefort impact (2.02 Ga)6

Oldest zircon (4.4 Ga)1

Oldest Impact spherule layers (3.48 Ga)5

Oldest rock - Acasta gneisses3

Isua supracrustal belt, evidence of oceans4

Oldest diamond bearing zircon (4.25 Ga)2

Mass (g/years)

Nec

taris

(3.9

Ga)

Imbr

ium

(3.8

5Ga)

Orie

ntal

e (3

.82G

a?)

Age

in 1

09 y

ears

50 100 150

Figure 6.1: Selected features from the record of early Earth processes (left), along with ahistogram of >3.94 Ga detrital zircon ages from the Yilgarn Craton (Holden et al., 2009). Right:interpretations of the mass flux to the Moon (simplified after Koeberl, 2003). 1 - Wilde et al.(2001); 2 - Menneken et al. (2007); 3 -Bowring et al. (1989); 4- Fedo et al. (2001); 5 - Byerlyet al. (2002); 6 - Kamo et al. (1996); 7 - Krogh et al. (1982).

The importance of such information is particularly related to our understanding of

processes on the early Earth (Figure 6.1), given that intense post-accretionary bom-

bardment occurred between 4.5 and 3.9 Ga (e.g. Kring and Cohen, 2002), and that the

crustal record of this period is almost exclusively limited to detrital zircons with ages up

109

6 Impact melt sheet zircons

to ∼ 4.4 Ga (e.g. Wilde et al., 2001).

As the largest and oldest terrestrial melt sheet, Sudbury offers a unique analogue for

such features on the early Earth. Zircons from throughout the main units of the SIC

have been characterised, with particular emphasis upon trace element concentrations and

inclusion populations. The findings, together with comparisons between the Sudbury

data and the >3.9 Ga detrital zircon population, are presented in the following paper,

which is included in this chapter:

Darling J., Storey C. and Hawkesworth C. (2009) Impact melt sheet zircons and

their implications for the Hadean crust; Geology, v.37, no. 10, p. 927-930; doi:

10.1130/G30251A.1

110

Impact melt sheet zircons and their implications for the Hadean crust

Darling J.R.a,1, Storey C.D.b, Hawkesworth C.J.a,2

a Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UKb School of Earth and Environmental Sciences, University of Portsmouth, Portsmouth PO1 3QL, UK

Abstract

Impacts may have been important mechanisms of crustal redistribution and di!erentiation, particularly dur-ing intense postaccretionary bombardment between 4.5 Ga and 3.9 Ga ago. Evidence of crustal processesduring this period is largely provided by detrital zircons from the Yilgarn craton, Australia. Trace elementcompositions, crystallization temperatures, and inclusion populations of these ancient zircons have beentaken as evidence for predominantly granitic source magmas, implying widespread felsic continental cruston the early Earth. However, there is little knowledge of zircons formed in impact melt sheets, a potentialsource for the Hadean zircons. Here we present Ti thermometry, trace elements, and inclusion populationsof zircons from the 1.85 Ga Sudbury impact melt sheet (Ontario, Canada). Our results demonstrate thatlarge variations in zircon crystallization temperature and composition will be an inevitable consequence ofthe evolution of such magmatic systems. We also show that zircons in mafic rocks crystallize in residualliquids of granitic composition, producing inclusion assemblages that are remarkably similar to those re-ported for the ancient Yilgarn grains. Thus, we conclude that the trace element compositions and inclusionpopulations of the Hadean zircons are consistent with crystallization from more mafic melts than previouslyrecognized, although high crystallization temperature distributions of Sudbury zircons indicate that impactmelt sheets were not a dominant source for the grains older than 3.9 Ga.

Key words: Sudbury, impact melt, Pb isotopes, sulphide

1. Introduction

Given limited exposures of terrestrial rocksolder than 3.7 Ga, knowledge of Hadean andEoarchean crustal processes relies predominantlyon studies of detrital zircons, with ages as old asca. 4.4 Ga (Wilde et al., 2001), from the Jack Hillsand Mount Narryer regions in the Yilgarn craton.Hafnium isotope studies of these zircons provideevidence for di!erentiation of the silicate Earthfrom very early in Earths history (Amelin et al.,1999; Harrison et al., 2008), yet the mechanismsof this di!erentiation are poorly understood. Sig-nificant emphasis has been placed upon constrain-ing the source magmas from which these zirconscrystallized. Studies have highlighted inclusionassemblages dominated by quartz, alkali-feldspar,apatite, plagioclase, biotite and muscovite (Hop-kins et al., 2008; Maas et al., 1992; Mennekenet al., 2007), while application of recently devel-oped Ti in zircon thermometry revealed a domi-nant peak in crystallization temperatures at !680"C, considered indicative of prograde minimumwet melting conditions (Watson and Harrison,2005, 2006). The interpretation of predominantly

1Correspondence: [email protected] address: O"ce of the Principal, University of St


granitic source rocks, based on these findings,implies widespread felsic continental crust for-mation before 3.9 Ga, in contrast to predictionsof long lived mafic-ultramafic early crust follow-ing magma ocean crystallization (Kamber, 2007;Kramers, 2007).

Interpretation of detrital zircon populations re-lies upon understanding of zircons from well con-strained magmatic and metamorphic rocks. Thereis currently little knowledge of zircons crystallizedfrom impact melt sheets, an as yet unexploredorigin for the ancient Yilgarn zircons and poten-tially an important mechanism of crustal redistri-bution and di!erentiation during intense post ac-cretionary bombardment of the early Earth (Grieveet al., 2006; Mungall et al., 2004). Accordingly,we have characterized the morphology, internalstructure, inclusion assemblages and trace elementcomposition of zircons from the Sudbury impactmelt sheet. Sudbury is one of the largest and old-est terrestrial impact structures, it formed at 1.85Ga (Krogh et al., 1982), with an original basindiameter of ! 250 km (Spray et al., 2004) and amelt volume of at least 8000 km3 (Grieve and Cin-tala, 1992). The associated melt sheet is a uniqueterrestrial analogue for such environments on theearly Earth and terrestrial planets, and it di!eren-tiated into !1-2 km thick silica saturated, hydrousgranitic and gabbroic melts.

Geology, 2010, vol. 37, no. 10, 927-930 February 12, 2010

2. Sample suite

Zircons were separated from seven Lower Unitand three Upper Unit samples, along a transect ofthe southern exposed limb of the melt sheet previ-ously studied by (Lightfoot and Zotov, 2005). Zir-cons from a quartz diorite from the WorthingtonO!set Dyke, representing an early melt phase in-jected into fractures created by re-adjustment ofthe crater floor (Lightfoot and Farrow, 2002), werealso analyzed. Upper Unit samples are granitic,and consist predominantly of quartz and alkalifeldspar intergrowths, plagioclase and biotite. TheLower Unit rocks vary from quartz monzogabbroto quartz gabbro, and consist of hypersthene andaugite (! 2:1), plagioclase, quartz, biotite and Fe-Ti oxides. Toward the base of the Lower Unit,quartz content increases to a maximum of !20 %.

Zircons from all units occupy late interstitialsites, often in association with quartz and biotite.Generally equant, with long axes <300 µm andvarying definition of crystal faces, the zircons dis-play oscillatory zonation of varying complexity(see Supplementary materials in Appendix, Fig-ure 8.1). Scattered, randomly orientated inclu-sions up to 20 µm and rare polyphase inclusionsup to 40 µm are apparent. With the exception ofsome skeletal zircons in samples from the middleof the Lower Unit, the internal structures are typi-cal of magmatic zircons from varying lithologies.Consistent with extreme initial melt temperatures,there is no evidence of inherited zircon in the Sud-bury melt sheet, either from this study, or from pre-vious U-Pb investigations (Corfu and Lightfoot,1996; Davis, 2008; Krogh et al., 1982, 1984; Os-termann et al., 1996).

3. Analytical methods

Trace element analyses were undertaken by LA-ICPMS at the University of Bristol, utilizing a 193nm excimer laser and Thermo Fisher Scientific El-ement 2. Hafnium, determined by electron micro-probe, was used as an internal standard and analy-ses were normalized against NIST 612. Laser spotsizes were between 20 and 30 µm. Titanium, mea-sured on the 49Ti peak due to 96Zr++ interferenceson 48Ti, was measured together with U and Th.Accuracy of Ti measurements was monitored us-ing NIST 614 and zircon standard Temora 2, yield-ing averages of 3.80 ppm ± 0.34 (2sd, n = 42) and9.05 ppm ± 0.44 (2sd, n = 12) respectively. TheREE were measured simultaneously in around athird of the analyses, using similar operating con-ditions. Forty additional REE analyses were un-dertaken without simultaneous Ti measurement.

600 650 700 750 800 850

0

1000

2000

3000

4000

5000

Rel

ativ

e st

atig

rap

hic

pos

ition

(m)

900

QD

LU

MU

UU

T (ºC)

Figure 1: Titanium thermometry results throughout the Sud-bury melt sheet stratigraphy. Closed circles are individual anal-yses, open circles are sample means. Error bars are 2sd. Calcu-lated whole rock Zr saturation temperatures are shown in grey(data from Lightfoot and Zotov, 2005). UU (Upper Unit), MU(Middle Unit; quartz gabbro) and LU (Lower Unit). Quartzdiorite (QD) given arbitrary height of -500 m, and an average Zrsaturation of Worthington O!set samples is shown (data fromLightfoot and Farrow, 2002).

4. Results

4.1. Titanium-in-zircon thermometry

Measured titanium concentrations were used tocalculate Ti-in-zircon thermometry temperatures(T Ti

zir), following Watson and Harrison (2005).Given that the thermometer applies strictly to zir-con grown in equilibrium with rutile, i.e., tita-nium activity (!TiO2) equals one, all tempera-tures reported here are minimum estimates. How-ever, there is an abundance of titanite and ilmenitethroughout the stratigraphy, which Watson andHarrison (2005) argue constrains !TiO2 to >0.6,resulting in a possible underestimation of T Ti

zir byaround 50 "C.

Mean measured temperatures for zircon in theLower Unit samples vary from 850 "C to 750 "C,Upper Unit from 760 "C to 715 "C and the quartzdiorite mean is 774 "C. The results show a system-atic trend in crystallization temperature through-out the stratigraphy of the melt sheet (Fig. 1; Sup-plementary Table 8.1 in Appendix), and there aresignificant within sample ranges of up to 112 "C,and within grain variations of up to 36 "C. In-tracrystalline variations show systematic changesfrom higher Ti contents in the center of grains tolower in the rims. Interestingly, the spread of tem-peratures increases upwards through the LowerUnit and, to a lesser extent, the Upper Unit stratig-raphy.

112

Zircon crystallization temperatures of theLower Unit samples and quartz diorites are consis-tent with the findings of previous studies, in thatbulk rock Zr saturation temperatures tend to sig-nificantly underestimate the onset of zircon crys-tallization in an initially Zr under-saturated melt(Harrison et al., 2007). Modelling of liquid com-positions during equilibrium crystallization of theLower Unit magmas using the MELTS algorithm(Ghiorso and Sack, 1995), indicates that Zr sat-uration temperatures come into the range of up-permost T Ti

zir (with an estimated correction for sub-unity !TiO2 of 50 "C) at !85 % crystallization forboth the basal and upper samples. Zirconium sat-uration in the model occurs at very similar degreesof crystallization, and the residual liquid composi-tions at this stage are granitic (!75 % SiO2) andperaluminous. The similar trend in bulk rock Zrsaturation temperatures to that of T Ti

zir throughoutthe Lower Unit reflects the influence of major ele-ment chemistry and Zr content on the temperatureat which zircons will crystallize in residual melts.Calculated Zr saturation temperatures in the Up-per Unit are significantly higher than T Ti

zir, even af-ter correction for !TiO2 <1. It is possible that thisdi!erence is related to the composition being out-side the experimentally calibrated range of Zr sat-uration studies (Watson and Harrison, 1983).

4.2. Zircon composition

The composition of igneous zircon is sensitiveto magmatic di!erentiation (e.g., Wark and Miller,1993), given that typically incompatible elementsin magmatic rocks (e.g., Hf, Th, U) are compatiblein zircon. Figure 2 shows relationships betweenzircon crystallization temperature (T Ti

zir), (HfO2)and U. At high temperatures the data cluster, witha narrow range of U and Hf, but with decreas-ing temperature the data fan out. Similar trendsare also seen for Th and the rare earth elements(REE). Zircons from each lithology display typ-ical magmatic REE profiles (see SupplementaryMaterials in Appendix, Figure 8.2), with positiveCe anomalies and negative Eu anomalies of vary-ing magnitude, as expected given the significantrole of plagioclase in the evolution of the Sudburymelts. However there is a large range within eachsample, and a general increase in the REE contentwith decreasing T Ti

zir suggests that a wide spectrumof REE profiles is an inherent feature of zirconscrystallizing in late interstitial liquids. The use ofREE compositions as a tool for discrimination ofigneous zircon sources has been questioned in pre-vious studies (Coogan and Hinton, 2006; Hoskinand Ireland, 2000). Although the REE concentra-tions of Sudbury zircons are somewhat high rela-tive to other mafic and intermediate suites (Sup-plementary Table 8.2 in Appendix), we also findthat REE profiles and ratios cannot unambiguously

700 750 800 850 900

0.7

0.8

0.9

1.0

1.1

1.2

1000

2000

3000

4000

5000

T (ºC)

HfO

2 (w

t%)

U (p

pm

)

0

2sd

2sd

Figure 2: Titanium thermometry temperature versus zirconcomposition. Open symbols are Lower Unit samples, closedsymbols represent Upper Unit samples.

distinguish between impact melt sheet zircons, nu-merous well constrained magmatic zircon popula-tions or detrital >3.9 Ga zircons from the YilgarnCraton.

4.3. Inclusion populations

Inclusion populations of zircons from a quartzdiorite (QD), basal Lower Unit (BLU), middleLower Unit (MLU) and Upper Unit (UU) samplewere characterized by energy and wavelength dis-persive X-ray spectroscopy. The inclusion popula-tions from all samples are dominated by quartz,plagioclase, k-feldspar, biotite and hornblende,with apatite a major constituent of Lower Unitinclusion assemblages. Six polyphase inclusionswere identified within basal Lower Unit sample144IBNR, consisting of quartz, plagioclase and k-feldspar ± hornblende ± biotite. Additional minorphases (<5 %) include muscovite (UU), monazite(MLU, UU) titanite (BLU, MLU), ilmenite (MLU,UU), chlorite (BLU, MLU, UU), Fe-oxide (UU),flourite (UU) and FeS (QD, BLU). Only 1 chlo-rite bearing grain shows evidence for secondaryalteration in CL. Figure 3 shows that the propor-tion of major inclusion phases does vary betweensamples, but this distinction can only be made bystatistical analysis of relatively large data sets (>40inclusions from each sample). It is also shown thatinclusion assemblages cannot easily be related towhole rock mineralogy.

113

0

10

20

30

40

50

ApatiteHblBiotiteK-FeldPlagQuartz

Perce

ntag

e

Quartz

Alkali-Feldspar Plagioclase

Middle Lower Unit

Basal Lower Unit

Quartz Diorite

Upper Unit

Figure 3: Zircon inclusion populations. Left: Relative frequency of the major inclusion phases in each lithology. Right: Streckeisendiagram showing the rock and inclusion mineralogy for each sample. The arrows may not be representative of the exact path ofresidual melt, but indicate the change in mineralogy from whole rock to zircon inclusions.

5. Discussion

The Sudbury impact melt sheet likely had ex-treme initial temperatures, several hundred de-grees in excess of the dry liquidus of both theLower Unit and Upper Unit (! 1200 "C). The re-sults of our investigation reveal that zircons fromboth units crystallized at temperatures comparableto other mafic suites (figure 4). The temperaturesare distinctly higher than the ! 680 "C peak in>3.9 Ga zircons, although it should be noted that! 43 % of the >3.9 Ga T Ti

zir data set falls withinthe temperature range 700-900 "C (Fu et al., 2008;Watson and Harrison, 2006). Corrections for sub-unity !TiO2 in Sudbury samples would only in-crease calculated temperatures, and given that ru-tile saturation studies indicate that !TiO2 is gener-ally >0.5 in magmatic rocks (Hayden and Watson,2007; Ryerson and Watson, 1987), the correctionof the >3.9 Ga zircons would be too small to over-lap the peaks in the data sets.

It has previously been argued that the ! 680 "Cpeak for >3.9 Ga grains is significantly lower thanthat of mafic rocks, indicating a felsic source (Har-rison et al., 2007; Watson and Harrison, 2005).Comparison with compilations of granitic andmafic rocks indicates significant overlap betweenthe data sets, with the >3.9Ga peak seemingly ly-ing between the mafic and felsic peaks (Fig. 4).Tentatively, an intermediate composition sourcecould be invoked. For example tonalites have anear identical peak in zircon crystallisation tem-peratures at ! 685 "C (Fu et al., 2008; Hiess et al.,2006).

The Sudbury melt sheet crystallized from thebottom upwards and top downwards, via the in-ward propagation of solidification fronts (Zieg andMarsh, 2005). Increasing ranges of Hf, Th, Uand REE concentrations with decreasing crystalli-sation temperatures indicate that the main control-ling factor on the abundance of these elements isthe composition of the melt in equilibrium withzircon. As such these trends represent an increasein the potential heterogeneity preserved in resid-ual melts, presumably reflecting variable isolation

of interstitial melt, resulting in large variations inT Ti

zir and zircon composition. Accordingly, zir-cons from the most di!erentiated samples yield awide range of temperatures (>100 "C). The overallrange of T Ti

zir in the Lower Unit is 166 "C, indicat-ing that individual magmatic bodies can producea very wide range of T Ti

zir. This may have signif-icant implications for studies using zircon crys-tallization temperatures to calculate cooling rates(e.g. Lissenberg et al., 2009) and raises further dif-ficulty in interpreting the source of detrital zirconassemblages based on Ti crystallization tempera-tures. For example, it has been argued that neitherREE, nor T Ti

zir can unambiguously distinguish the>3.9 Ga zircons from those formed by basalt dif-ferentiation at mid ocean ridges (Coogan and Hin-ton, 2006).

Mafic magmas in Sudbury have crystallised zir-con inclusion populations of granitic mineralogy.This is consistent with the results of crystallizationmodeling, which indicates that zircons grew in lateresidual liquids that are highly evolved relative tothe bulk rock composition. The inclusion popula-tions are remarkably similar to those reported forthe >3.9 Ga Yilgarn zircons (Cavosie et al., 2004;Hopkins et al., 2008; Maas et al., 1992; Mennekenet al., 2007), with the exception of a significantlyhigher proportion of muscovite found by Hopkinset al. (2008). Distinguishing the sources of theseinclusion populations based upon the major min-eralogy of quartz, alkali-feldspar, plagioclase, bi-otite, hornblende and apatite is only possible viaanalysis of large data sets. Clearly this is not pos-sible for detrital grains, unless there is certaintythat multiple grains are derived from a single melt.

Impact melt compositions will vary dependingon target rock lithologies. The bulk compositionof impact melt sheets in the Hadean may have dif-fered significantly from the dioritic bulk compo-sition of the Sudbury melt sheet (Lightfoot et al.,1997), particularly if the Hadean crust was basaltic(e.g. Kramers, 2007). However, di!erentiation ofbasaltic melt sheets would produce significant vol-umes of melt of intermediate to felsic composition(Grieve et al., 2006) and inevitably, according to

114

600 700 800 900 1000048

1216

4

8

12

4

8

12

48

1216

26

10141822 A) Sudbury norite (n = 90)

B) Sudbury granophyre (n = 46)

C) Mafic (n = 279)

D) Granitic (n = 81)

E) >3.9Ga (n = 191)

Temp (°C)

Rel

ativ

e fr

eque

ncy

(%)

Figure 4: Titanium in zircon temperature probability distribu-tions for various suites of rocks. a) and b) are data from themain Sudbury lithologies. c) Complilation of anorthosites, gab-bros, norites and diorites from Fu et al. (2008) and Coogan andHinton (2006). d) compilation of granites and granodioritesfrom Fu et al. (2008). e) Compilation of >3.9 Ga Yilgarn zir-cons from Watson and Harrison (2006) and Fu et al. (2008).The bimodal temperature distribution of Lower Unit zirconslikely represents a sampling artifact, reflecting higher zirconcrystallization temperatures of basal Lower Unit samples.

our findings, a wide range of zircon crystalliza-tion temperatures and compositions. As such thehighly di!erentiated nature of the Sudbury meltsheet is considered to provide broad constraintson the range of zircon crystallization temperaturesand compositions likely in such settings.

We conclude that titanium in zircon crystal-lization temperatures broadly correlate with mag-matic di!erentiation, which in Sudbury has re-sulted in systematic variations in T Ti

zir throughoutthe melt sheet stratigraphy. Zircons in variousmafic lithologies from the melt sheet crystallizedin residual liquids of granitic composition, pro-ducing inclusion assemblages that are remarkablysimilar to those reported for the ancient Yilgarnzircons. This indicates that it remains di"cultto use inclusion assemblages and REE as a dis-criminator of the source of detrital zircons. In-terpretation of source rock characteristics of the>3.9 Ga zircons based upon Ti thermometry is alsosomewhat ambiguous. However, assuming that thehighly di!erentiated Sudbury melt sheet o!ers arepresentative spectrum of T Ti

zir in such settings, the

! 680 "C titanium thermometry peak of >3.9 Gazircons cannot be explained by zircons crystallizedin large impact melt sheets.

ACKNOWLEDGMENTS

We thank P. Lightfoot and E. Tremblay for as-sistance with fieldwork in Sudbury, and S. Kearnsfor help with electron microbeam techniques. Thisstudy was supported by a NERC Studentshipto J. Darling, the Geological Society of Amer-ica Eugene M. Shoemaker Award, and by ValeInco. C. Storey acknowledges NERC FellowshipNE/D008891/1. The manuscript benefited fromreviews by L.A. Coogan and two anonymous re-viewers and discussions with H. Marschall.

References

Amelin, Y., Lee, D., Halliday, A., Pidgeon, R., 1999. Natureof the Earth’s earliest crust from hafnium isotopes in singledetrital zircons. Nature 399 (6733), 252–255.

Cavosie, A., Wilde, S., Liu, D., Weiblen, P., Valley, J., 2004.Internal zoning and U-Th-Pb chemistry of Jack Hills de-trital zircons: a mineral record of early Archean to Meso-proterozoic (4348-1576 Ma) magmatism. Precambrian Re-search 135 (4), 251–279.

Coogan, L. A., Hinton, R. W., 2006. Do the trace elementcompositions of detrital zircons require Hadean continentalcrust? Geology 34 (8), 633.

Corfu, F., Lightfoot, P. C., 1996. U-Pb geochronology of theSublayer environment, Sudbury Igneous Complex, Ontario.Economic Geology 91 (7), 1263–1269.

Davis, D. W., 2008. Sub-million-year age resolution of Pre-cambrian igneous events by thermal extraction-thermal ion-ization mass spectrometer Pb dating of zircon: Applicationto crystallization of the Sudbury impact melt sheet. Geology36 (5), 383–386.

Fu, B., Page, F. Z., Cavosie, A. J., Fournelle, J., Kita, N. T.,Lackey, J. S., Wilde, S. A., Valley, J. W., 2008. Ti-in-zirconthermometry: applications and limitations. Contrib MineralPetrol 156 (2), 197–215.

Ghiorso, M., Sack, R. O., 1995. Chemical mass-transfer inmagmatic processes .4. a revised and internally consis-tent thermodynamic model for the interpolation and extrap-olation of liquid-solid equilibria in magmatic systems atelevated-temperatures and pressures. Contributions To Min-eralogy and Petrology 119 (2-3), 197–212.

Grieve, R., Cintala, M. J., Therriault, A., 2006. Large-scaleimpacts and the evolution of the Earth’s crust: The earlyyears. In: Reimold, W., Gibson, R. (Eds.), Processes on theEarly Earth. Geological Society of America, pp. 23–31.

Grieve, R. A. F., Cintala, M. J., 1992. An analysis of di!eren-tial impact-melt crater-scaling and implications for the ter-restrial impact record. Meteoritics 27 (5), 526–538.

Harrison, T., Schmitt, A., McCulloch, M., Lovera, O., 2008.Early ( 4.5 Ga) formation of terrestrial crust: Lu-Hf, 18O,and Ti thermometry results for Hadean zircons. Earth andPlanetary Science Letters 268, 476–486.

Harrison, T., Watson, E., Aikman, A. B., 2007. Temperaturespectra of zircon crystallization in plutonic rocks. Geology35 (7), 635–638.

Hayden, L. A., Watson, E. B., 2007. Rutile saturation in hy-drous siliceous melts and its bearing on Ti-thermometryof quartz and zircon. Earth and Planetary Science Letters258 (3-4), 561–568.

Hopkins, M., Harrison, T. M., Manning, C., 2008. Low heatflow inferred from greater than 4 Gyr zircons suggestsHadean plate boundary interactions. Nature 456 (7221),493–496.

Hoskin, P., Ireland, T., 2000. Rare earth element chemistry ofzircon and its use as a provenance indicator. Geology 28 (7),627–630.

115

Kamber, B., 2007. The enigma of the terrestrial protocrust: Ev-idence for its former existence and the importance of itscomplete dissapearance. Developments in Precambrian Ge-ology 15, 75–89.

Kramers, J. D., 2007. Hierarchical Earth accretion and theHadean eon. J Geol Soc London 164, 3–17.



Lightfoot, P. C., Doherty, W., K.Farrell, Keays, R. R., Moore,M. L., Pekeski, D., 1997. Geochemistry of the Main Mass,Sublayer, O!sets and inclusions from the Sudbury IgneousComplex, Ontario. Ontario Geological Survey, Open FileReport 5959.

Lightfoot, P. C., Farrow, C. E. G., 2002. Geology, geochem-istry, and mineralogy of the Worthington O!set Dike: Agenetic model for o!set dike mineralization in the SudburyIgneous Complex. Economic Geology 97 (7), 1419–1446.


Lissenberg, C. J., Rioux, M., Shimizu, N., Bowring, S. A.,Mevel, C., 2009. Zircon dating of oceanic crustal accretion.Science 323 (5917), 1048–1050.

Maas, R., Kinny, P. D., Williams, I. S., Froude, D. O.,Compston, W., 1992. The Earths oldest known crust - ageochronological and geochemical study of 3900-4200Maold detrital zircons from Mt Narryer and Jack Hills, West-ern Australia. Geochimica et Cosmochimica Acta 56 (3),1281–1300.

Menneken, M., Nemchin, A. A., Geisler, T., Pidgeon, R. T.,Wilde, S. A., 2007. Hadean diamonds in zircon from JackHills, Western Australia. Nature 448 (7156), 917–925.


Ostermann, M., Scharer, U., Deutsch, A., 1996. Impact meltdikes in the sudbury multi-ring basin (canada): Implicationsfrom uranium-lead geochronology on the foy o!set dike.Meteorit Planet Sci 31 (4), 494–501.

Ryerson, F., Watson, E., 1987. Rutile saturation in magmas -implications for Ti-Nb-Ta depletion in island-arc basalts.Earth and Planetary Science Letters 86 (2-4), 225–239.


Wark, D. A., Miller, C. F., 1993. Accessory mineral behaviorduring di!erentiation of a granite suite - monazite, xeno-time and zircon in the Sweetwater Wash Pluton, Southeast-ern California, USA. Chemical Geology 110 (1-3), 49–67.

Watson, E. B., Harrison, T., 2005. Zircon thermometer re-veals minimum melting conditions on earliest Earth. Sci-ence 308 (5723), 841–844.

Watson, E. B., Harrison, T. M., 1983. Zircon saturation revis-ited - temperature and composition e!ects in a variaty ofcrustal magma types. Earth and Planetary Science Letters64 (2), 295–304.

Watson, E. B., Harrison, T. M., 2006. Response to commentson ”Zircon thermometer reveals minimum melting condi-tions on earliest Earth”. Science 311 (5762).

Wilde, S. A., Valley, J. W., Peck, W. H., Graham, C. M., 2001.Evidence from detrital zircons for the existence of conti-nental crust and oceans on the Earth 4.4 Gyr ago. Nature409 (6817), 175–178.

Zieg, M. J., Marsh, B. D., 2005. The Sudbury Igneous Com-plex: Viscous emulsion di!erentiation of a superheated im-pact melt sheet. Bulletin of the Geological Society of Amer-ica 117 (11-12), 1427–1450.

116

7

Summary and future directions

A number of significant findings have been made in this investigation that reveal new

insights into the formation and evolution of the Sudbury impact melt sheet, as well as

the processes of zircon crystallisation. The aims of this chapter are to summarise the

key findings made and highlight directions for future research.

7.1 Melt sheet evolution

Pb isotopes have proven to be a useful tool in identifying chemical variability within

the Main Mass and Offset Dykes. The results presented in Chapters 4 and 5 emphasise

that the melt sheet is heterogeneous at a range of scales. Of particular importance is

the observation that not only are isotopic differences preserved between the North and

South Ranges (Cooper, 2000; Dickin et al., 1999, 1996), but that significant variations

occur both laterally and vertically throughout the Main Mass, as well as between Offset

Dykes, from the same side of the Complex.

It is clear that the various units of the melt sheet have not been efficiently ho-

mogenised by convection over time, as has previously been suggested (Dickin et al.,

1999). Instead a dynamic system is apparent, with heterogeneity present early in the

history of the melt sheet, as revealed by the Offset Dykes, through to the final products

of silicate crystallisation. Importantly, the Pb isotope systematics of Offset Dyke quartz

diorites, together with considerations of previously published trace element and Sr-Nd

isotope data, demonstrate that such heterogeneity can be accounted for by mixing of

locally exposed target lithologies.

Particularly given the initially superheated nature of the melt sheet, the assimilation

of fallback breccias, entrained clasts and footwall rocks provide mechanisms to maintain

or develop heterogeneity despite probable vigorous thermal convection (Zieg and Marsh,

2005). However, a key question remains to what extent the products of impact melting

were homogenised during crater formation. Theoretical considerations on the violent

movement and shearing of superheated, low viscosity, shock melt suggest that the initial

products of impact melting would have been homogenised during collapse of the transient

cavity (Phinney and Simonds, 1977; Simonds and Kieffer, 1993). The heterogeneities

identified in early formed Offset Dyke lithologies, together with the distinct isotopic

compositions of North and South Range Main Mass rocks, indicates that this may not

be the case. Without better constraints upon the timing of Offset Dyke emplacement, it is

not clear whether further resolution of this issue will be possible in Sudbury. Investigation

117

7 Summary and future directions

is required of chemical variability in coherent impact melt bodies from smaller craters,

where there is less potential for post crater-formation melt interaction and assimilation

of target rocks. From the terrestrial crater record, the Morokweng (Koeberl et al.,

1997a), Chesapeake Bay (Wittmann et al., 2009) and Chixculub (Schuraytz et al., 1994)

structures are known to contain such units of interest, and also have variable target

lithologies.

It is clear from the findings of this study that compositionally variable target rocks

play a key role in controlling the geochemistry of the melt sheet, with both spatial and

temporal variations common. In Chapter 4 it was shown that significant lateral variations

in Pb isotopes occur over distances of a few kilometers. Preliminary investigations of

major and trace elements in the same region suggest that similar variations occur (e.g.

SiO2, La/Y), although they are more difficult to resolve given analytical uncertainties.

Significant geochemical differences between Main Mass and Offset Dyke samples from the

North and South Ranges (e.g. Sr, Sc) are also well documented (Lightfoot et al., 1997a).

Collectively, these observations indicate that impact melt sheets are not as efficient at

mixing as previously suggested. This may have significant implications for the effects of

intense post-accretionary bombardment of early planetary crusts.

7.2 Origin of the melt sheet

Reconciling the geochemical characteristics of the melt sheet in Sudbury with mathe-

matical models of impact processes has revealed new insights into the Sudbury event.

Chapter 5 shows that the main geochemical characteristics of the melt sheet can be ex-

plained by mixing of locally exposed target lithologies. An upper-mid crustal source for

the melt sheet is evident, a finding that conflicts with previous predictions of deep seated

crustal melting from impact modelling. An oblique impact can accommodate shallower

melting, however further research is required in order to consolidate an oblique impact

model with the melt volume in Sudbury. Firstly, there are considerable discrepancies be-

tween estimates of impact melt volume, and the various approaches should be reviewed

in light of ongoing developments in impact research. Secondly, it is possible that given

the Sudbury impact melt volume estimate of Pope et al. (2004), a high velocity and low

density impactor (comet) may be required. A systematic study of refractory platinum

group element abundances in the Onaping Formation, as well as distal ejecta (Pufahl

et al., 2007) is required to test whether such an impactor was likely.

Previous impact models invoking melting to depths that approached the maximum

transient cavity depth of ∼ 30 km (Deutsch et al., 1995; Grieve et al., 1991; Mungall

et al., 2004), infer that Sudbury sized impact events would have profoundly disturbed

118

7.3. IMPLICATIONS FOR SULPHIDE ORE FORMATION

the compositional layering of the continental crust. It has been difficult to reconcile

such models with the fact that there is no record of decompressive mantle melting in the

Sudbury area around the time of impact, or significant mantle contribution to the melt

sheet (e.g. Lightfoot et al., 2001; this study) . The recognition of an upper to mid-crustal

source for the melt sheet, and re-evaluating the Sudbury event in terms of an oblique

impact, it is evident that the excavation and melting depths are significantly shallower

than previously considered. The compositional layering of the crust would not have

been inverted, and formation of the melt sheet without significant mantle contribution

becomes conceptually easier.

7.3 Implications for sulphide ore formation

The findings of this study have potentially significant implications for the formation

of sulphide ore deposits in Sudbury. It has previously been recognised that sulphides

segregated from the Main Mass and accumulated at the base of the melt sheet (Keays and

Lightfoot, 2004), and that the location of ore deposits was controlled by: (a) gravitational

and convective accumulation of sulphide in depressions in the basal contact and (b) the

thickness of the Lower unit of the Main Mass (Keays and Lightfoot, 2004). In Chapter

4 it is shown that the formation of melt cells of differing composition may have also

had a significant affect on the sulphide segregation history of the Main Mass. Timing of

sulphide segregation was estimated by comparing the isotopic composition of ores with

that of the overlying Lower Unit stratigraphy, and was found to be highly variable.

Early sulphide saturation, at temperatures likely in excess of the Lower Unit liquidus,

formed much of the massive Offset Dyke and Contact Sublayer mineralisation, as high-

lighted by data from the Creighton 402 orebody in the Creighton embayment. However,

given that the range of initial Pb isotopes in ores have a similar range to the Lower

Unit stratigraphy, it is apparent that sulphide segregation occurred over a protracted

period. Those segments in which accumulated basal sulphide has high Pb isotope values,

consistent with early segregation, also have the strongest chalcophile element depletion

signatures. Given that those sulphides that segregate early will have access to the most

chalcophile element enriched melt sheet, it is predicted that these ores will also have the

highest metal concentrations (tenor). Preliminary results suggest that this is the case,

although further research is required to test this relationship. In particular, the effects of

sulphide fractionation on the Ni, Cu and PGE concentrations of samples analysed must

be carefully considered.

That the geochemical characteristics of the melt sheet can be accommodated by

melting of locally exposed footwall rocks also provides constraints upon sulphide ore

119


WorthingtonOffset Dyke

Copper CliffOffset Dyke

Main Mass

Creighton pluton

Elliot Lake Gp.Hough Lake Gp.

Nipissing mafic suite

10 km

N

Figure 7.1: Simplified geological map and sampling localities of sulphide ores from the CopperCliff and Worthington Offset Dykes

formation. In Chapter 5 it is shown that mafic target rocks may be fundamental to

raising the metal budget of the melt sheet. Accordingly, the spatial distribution of such

mafic bodies may influence the sulphide ore potential around the melt sheet. In order

to further investigate this concept, it is necessary to better constrain the isotopic and

Ni, Cu and PGE compositions of the various target rocks. This would allow for a more

detailed and accurate assessment of the relative contributions of differing target rock

groups around the melt sheet. Furthermore, the potential affects of varying target rock

contributions to the melt sheet on sulphur saturation (e.g. Fe and S content) require

investigation.

7.3.1 Distinct sources for ores in different Offset Dykes?

If the distribution of target rocks is a fundemental control on the Ni-Cu-PGE contents of

the melt sheet, it would be expected that ores from similar environments, but different

locations (for example within the South Range) would have distinct crustal sources.

Accordingly, inclusion and sulphide bearing quartz diorites were sampled along both the

Worthington and Copper Cliff Offset Dykes (Figure 7.1).

Preliminary data clearly show different model initial Pb isotopic compositions from

the respective Offset Dykes (Figure 7.2; data in Appendix Table 8.3), consistent with

distinct sources. Such studies are required from different environments in the melt

sheet and should focus upon producing age corrected Pb isotope data (i.e. corrected

with measured U-Th-Pb ratios) in order to resolve potential differences in target rock

precursors.

120

7.4. ZIRCONS IN IMPACT MELTS

0!

1!

2!

3!

4!

5!

6!

15.32!15.34!15.36!15.38! 15.4! 15.42!15.44!15.46!15.48! 15.5! 15.52!

n!

207Pb/204Pbm!

Worthington! Copper Cliff!

Figure 7.2: Histogram of model initial Pb isotope ratios (calculated as Chapter 4) for sulphideore samples from inclusion bearing quartz diorites of the Worthington and Copper Cliff OffsetDykes. Uncertainty is within the width of the columns.

7.4 Zircons in impact melts

The characterisation of zircons from throughout the Sudbury melt sheet revealed a num-

ber of significant findings:

1. Titanium in zircon crystallisation temperatures correlate with indexes of magmatic

differentiation.

2. The rare earth element concentrations of zircons from felsic and mafic impact melts

cannot easily be distinguished.

3. Inclusion populations in zircons from mafic rocks in Sudbury are of typically granitic

mineralogies.

These findings are particularly important for the interpretation of the source of detrital

zircons. In particular, the REE compositions and inclusion populations of Sudbury’s

zircons are very similar to those described in Hadean zircons from the Yilgarn Craton.

As such, the characteristics of the Hadean zircons are consistent with crystallization from

more mafic melts than previously recognized, although high crystallization temperature

distributions of Sudbury zircons indicate that impact melt sheets were not a dominant

source for the grains older than 3.9 Ga.

Further work is required to characterise inclusion populations from the impact melt

sheet zircons, in order to test whether characteristic phases are present. For example,

diamonds are known from a number of terrestrial impact structures, including Sudbury

(Hough et al., 1995; Koeberl et al., 1997b; Langenhorst et al., 1998, 1999; Masaitis et al.,

1999), and are likely to be stable in igneous rocks for sufficient time to be incorporated in

crystallising zircon (Collerson et al., 2000; Haggerty, 1999; Korsakov et al., 2004; Wirth

121


and Rocholl, 2003; De Corte et al., 2000). Thus far, electron microbeam techniques

have principally been used for inclusion characterisation. Further developments and

more detailed investigations will require the application of spectroscopic methods such

as micro-Raman spectroscopy.

122

References

Agrawal, R. M., Jha, S. N., Kaimal, R., Malhotra, S. K., Jangida, B. L., 1994. Determination of small con-centrations of hafnia in zirconia by selective excitation-energy dispersive-x-ray emission-spectrometry.Fresenius Journal of Analytical Chemistry 349 (6), 434–437.

Albarede, F., Telouk, P., Blichert-Toft, J., Boyet, M., Agranier, A., Nelson, B., 2004. Precise and accurateisotopic measurements using multiple-collector ICPMS. Geochimica Et Cosmochimica Acta 68 (12),2725–2744.

Ames, D., Watkinson, D., Parrish, R., 1998. Dating of a regional hydrothermal system induced by the1850 Ma Sudbury impact event. Geology 26 (5), 447–450.

Ames, D. E., Davidson, A., Buckle, J. L., Card, K. D., 2005. Geology, Sudbury bedrock compilation,Ontario. Vol. Open File 4570. Geological Survey of Canada.

Ames, D. E., Farrow, C. E. G., 2007. Metallogeny of the Sudbury mining camp, Ontario. In: MineralDeposits of Canada: A Synthesis of Major Deposit-Types, District Metallogeny, the Evolution ofGeological Provinces, and Exploration Methods. No. Special Publication No.5. Geological Associationof Canada, pp. 329–350.

Ames, D. E., Golightly, J. P., Lightfoot, P. C., Gibson, H. L., 2002. Vitric compositions in the OnapingFormation and their relationship to the Sudbury Igneous Complex, Sudbury Structure. EconomicGeology 97 (7), 1541–1562.

Armstrong, J. T., 1993. Effects of carbon coat thickness and contamination on quantitative analysis: anew look at an old problem. In: Proceedings of the 27th Annuam MAS Meeting. No. S13-14.

Bailey, J., Lafrance, B., McDonald, A., Fedorowich, J., Kamo, S., Archibald, D., 2004. Mazatzal-Labradorian-age (1.7-1.6 Ga) ductile deformation of the South Range Sudbury impact structure atthe Thayer Lindsley mine, Ontario. Canadian Journal of Earth Sciences 41 (12), 1491–1505.

Baker, J., Bizzarro, M., Wittig, N., Connelly, J., Haack, H., 2005. Early planetesimal melting from anage of 4.5662 Gyr for differentiated meteorites. Nature 436 (7054), 1127–1131.

Baker, J., Peate, D., Waight, T., Meyzen, C., 2004. Pb isotopic analysis of standards and samples using aPb-207-Pb-204 double spike and thallium to correct for mass bias with a double-focusing MC-ICP-MS.Chemical Geology 211 (3-4), 275–303.

Barling, J., Weis, D., 2008. Influence of non-spectral matrix effects on the accuracy of Pb isotope ratiomeasurement by MC-ICP-MS: implications for the external normalization method of instrumentalmass bias correction. Journal of Analytical Atomic Spectrometry 23, 1017–1025.

Belousova, E., Griffin, W., O’Reilly, S., Fisher, N., 2002. Igneous zircon: trace element composition asan indicator of source rock type. Contrib Mineral Petrol 143 (5), 602–622.

Bennet, G., Dressler, B., Robertson, J., 1991. The Huronian Supergroup and associated intrusive rocks.In: Thurston, P., Williams, H., Sutcliffe, R., Stott, G. (Eds.), Geology of Ontario. Vol. Special Paper4. Ontario Geological Survey, pp. 549–592.

Black, L., Kamo, S., Allen, C., Davis, D., Aleinikoff, J., Valley, J., Mundil, R., Campbell, I., Korsch, R.,Williams, I., Foudoulis, C., 2004. Improved Pb-206/U-238 microprobe geochronology by the monitor-ing of a trace-element-related matrix effect; SHRIMP, ID-TIMS, ELA-ICP-MS and oxygen isotopedocumentation for a series of zircon standards. Chemical Geology 205 (1-2), 115–140.

Bolhar, R., Kamber, B. S., Collerson, K. D., 2007. U-Th-Pb fractionation in Archaean lower continentalcrust: Implications for terrestrial Pb isotope systematics. Earth and Planetary Science Letters 254 (1-2), 127–145.

Bowring, S., Williams, I., Compston, W., Nov. 1989. 3.96 Ga gneisses from the Slave Province, Northwest-Territories, Canada. Geology 17 (11), 971–975.

Byerly, G., Lowe, D., Wooden, J., Xie, X., 2002. An Archean impact layer from the Pilbara and Kaapvaalcratons. Science 297 (5585), 1325–1327.

123

REFERENCES

Card, K., Gupta, V., McGrath, P., Grant, F., 1984. The Sudbury Structure: Its regional geological andgeophysical setting. In: Pye, E., Naldrett, A., Giblin, P. (Eds.), The geology and ore deposits of theSudbury Structure. Vol. Special Volume 1. Ontario Geological Survey, Ch. 2, pp. 25–45.

Card, K. D., Church, W. R., Franklin, J. M., Frarey, M. J., Robertson, J. A., West, G. F., Young, G. M.,1972. The Southern Province. In: Price, R. A., Douglas, R. J. W. (Eds.), Variations in tectonic stylesin Canada. Vol. Special Paper. Geological Association of Canada, pp. 335–380.

Cavosie, A. J., Valley, J. W., Wilde, S. A., 2006. Correlated microanalysis of zircon: Trace element, deltaO-18, and U-Th-Pb isotopic constraints on the igneous origin of complex >3900 Ma detrital grains.Geochimica et Cosmochimica Acta 70 (22), 5601–5616.

Chai, G., Eckstrand, R., 1994. Rare Earth Element characteristics and origin of the Sudbury IgneousComplex, Ontario, Canada. Chemical Geology 113 (3-4), 221–244.

Collerson, K., Hapugoda, S., Kamber, B., Williams, Q., 2000. Rocks from the mantle transition zone:Majorite-bearing xenoliths from Malaita, southwest Pacific. Science 288 (5469), 1215–1223.

Collerson, K., Kamber, B., Schoenberg, R., 2002. Applications of accurate, high-precision Pb isotoperatio measurement by multi-collector ICP-MS. Chemical Geology 188 (1-2), 65–83.

Collins, W., 1934. Life history of the Sudbury nickel irruptive. i. Petrogenesis. Royal Society of CanadaTransactions, Third Series 28, 123–177.

Cooper, M., 2000. The Sudbury Igneous Complex: Insights into melt sheet evolution and ore genesis.Ph.D. thesis, The Open University.

Corfu, F., Andrews, A. J., 1986. A U-Pb Age for Mineralized Nipissing Diabase, Gowganda, Ontario.Canadian Journal of Earth Sciences 23 (1), 107–109.

Corfu, F., Lightfoot, P. C., 1996. U-Pb geochronology of the Sublayer environment, Sudbury IgneousComplex, Ontario. Economic Geology 91 (7), 1263–1269.

Cowan, E., 1999. Magnetic fabric constraints on the initial geometry of the Sudbury Igneous Complex:a folded sheet or a basin-shaped igneous body? Tectonophysics 307 (1-2), 135–162.

Darling, J. R., Hawkesworth, C. J., Lightfoot, P. C., Storey, C. D., Tremblay, E., 2010. Isotopic hetero-geneity in the Sudbury impact melt sheet. Earth and Planetary Science Letters 289 (3-4), 347–356.

Davis, D. W., 2008. Sub-million-year age resolution of Precambrian igneous events by thermal extraction-thermal ionization mass spectrometer Pb dating of zircon: Application to crystallization of the Sud-bury impact melt sheet. Geology 36 (5), 383–386.

De Corte, K., Korsakov, A., Taylor, W., Cartigny, P., Ader, M., De Paepe, P., 2000. Diamond growthduring ultrahigh-pressure metamorphism of the Kokchetav Massif, northern Kazakhstan. Island Arc9 (3), 428–438.

Deutsch, A., 1994. Isotope systematics support the impact origin of the Sudbury Structure (Ontario,Canada). In: Dressler, B. O., Grieve, R. A. F., Sharpton, V. L. (Eds.), Large Meteorite Impacts andPlanetary Evolution. No. Special Paper 293. Geological Society of America, pp. 289–301.

Deutsch, A., Grieve, R., Avermann, M., Bischoff, L., Brockmeyer, P., Buhl, D., Lakomy, R., MullerMohr,V., Ostermann, M., Stoffler, D., 1995. The Sudbury Structure (Ontario, Canada): A tectonicallydeformed multi-ring impact basin. Geol Rundsch 84 (4), 697–709.

Dickin, A. P., Artan, M. A., Crocket, J. H., 1996. Isotopic evidence for distinct crustal sources of Northand South Range ores, Sudbury Igneous Complex. Geochimica et Cosmochimica Acta 60 (9), 1605–1613.

Dickin, A. P., Nguyen, T., Crocket, J. H., 1999. Isotopic evidence for a single impact melting origin ofthe Sudbury Igneous Complex. In: Dressler, B., Sharpton, V. (Eds.), Large Meteorite Impacts andPlanetary Evolution II. Vol. Special Paper 339. Geological Society of America, Boulder, Colorado, pp.361–371.

124

REFERENCES

Dickin, A. P., Richardson, J. M., Crocket, J. H., Mcnutt, R. H., Peredery, W. V., 1992. Osmium isotopeevidence for a crustal origin of Platinum Group Elements in the Sudbury nickel ore, Ontario, Canada.Geochimica et Cosmochimica Acta 56 (9), 3531–3537.

Dietz, R. S., Butler, L. W., 1964. Shatter-cone orientation at Sudbury, Canada. Nature 204 (495), 280–284.

Ding, T., Schwarcz, H., 1984. Oxygen isotopic and chemical compositions of rocks of the Sudbury Basin,Ontario. Canadian Journal of Earth Sciences 21 (3), 305–318.

Doucelance, R., Manhes, G., 2001. Reevaluation of precise lead isotope measurements by thermal ion-ization mass spectrometry: comparison with determinations by plasma source mass spectrometry.Chemical Geology 176 (1-4), 361–377.

Dressler, B., 1984a. The effects of the Sudbury event on and the intrusion of the Sudbury IgneousComplex on the footwall rocks of the Sudbury Structure. In: Pye, E., Naldrett, A., Giblin, P. (Eds.),The geology and ore deposits of the Sudbury Structure. Vol. Special Volume 1. Ontario GeologicalSurvey, pp. 97–136.

Dressler, B., 2004. Order or chaos? Origin and mode of emplacement of breccias in floors of large impactstructures. Earth-Science Reviews 67 (1-2), 1–54.

Dressler, B., Weiser, T., Brockmeyer, P., 1996. Recrystallized impact glasses of the Onaping Formationand the Sudbury Igneous Complex, Sudbury Structure, Ontario, Canada. Geochimica et Cosmochim-ica Acta 60 (11), 2019–2036.

Dressler, B. O., 1984b. General geology of the Sudbury area. In: Pye, E., Naldrett, A., Giblin, P. (Eds.),The geology and ore deposits of the Sudbury Structure. Vol. Special Volume 1. Ontario GeologicalSurvey, pp. 52–87.

Dressler, B. O., Gupta, V. K., Muir, T. L., 1991. The Sudbury Structure. In: Thurston, P. C., Williams,H., Sutcliffe, R., Stott, G. (Eds.), Geology of Ontario. Vol. 4. Ontario Geological Survey, pp. 593–626.

Easton, R. M., 2000a. Metamorphism of the Canadian Shield, Ontario, Canada. I. The Superior Province.Canadian Mineralogist 38, 287–317.

Easton, R. M., 2000b. Metamorphism of the Canadian Shield, Ontario, Canada. II. Proterozoic meta-morphic history. Canadian Mineralogist 38, 319–344.

Elburg, M., Vroon, P., van der Wagt, B., Tchalikian, A., 2005. Sr and Pb isotopic composition of fiveUSGS glasses (BHVO-2G, BIR-1G, BCR-2G, TB-1G, NKT-1G). Chemical Geology 223 (4), 196–207.

Evans, K., 2008. Sulphur solubility and sulphide immiscibility in silicate melts as a function of theconcentration of manganese, nickel, tungsten and copper at 1 atm and 1400 c. Chemical Geology255 (1-2), 236–249.

Faggart, B. E., Basu, A. R., Tatsumoto, M., 1985. Origin of the Sudbury Complex by metoritic impact- neodymium isotopic evidence. Science 230 (4724), 436–439.

Farrow, C., Watkinson, D., 1992. Alteration and the role of fluids in Ni, Cu and platinum-group ele-ment deposition, Sudbury Igneous Complex contact, Onaping-Levack area, Ontario. Mineralogy andPetrology 46 (1), 67–83.

Farrow, C., Watkinson, D., 1996. Geochemical evolution of the Epidote Zone, Fraser Mine, Sudbury,Ontario: Ni-Cu-PGE remobilization by saline fluids. Exploration and Mining Geology 5 (1), 17–31.

Fedo, C., Grant, G., Nesbitt, H., 1997. Paleoclimatic control on the composition of the PaleoproterozoicSerpent Formation, Huronian Supergroup, Canada: a greenhouse to icehouse transition. PrecambrianResearch 86 (3-4), 201–223.

Fedo, C., Myers, J., Appel, P., 2001. Depositional setting and paleogeographic implications of earth’soldest supracrustal rocks, the >3.7 Ga Isua Greenstone belt, West Greenland. Sedimentary Geology141, 61–77.

125

REFERENCES

Fleet, M. E., 2006. Phase equilibria at high temperatures. In: Vaughan, D. (Ed.), Sulphide Mineralogyand Geochemistry. Vol. 61. Mineralogical Society of America, Ch. 7, pp. 365–419.

Fleet, M. E., Barnett, R. L., Morris, W. A., 1987. Prograde metamorphism of the Sudbury IgneousComplex. Canadian Mineralogist 25, 499–514.

Frei, R., Kamber, B. S., 1995. Single mineral Pb-Pb dating. Earth and Planetary Science Letters 129 (1-4), 261–268.

Gao, S., Liu, X., Yuan, H., Hattendorf, B., Gunther, D., Chen, L., Hu, S., 2002. Determination of fortytwo major and trace elements in USGS and NIST SRM glasses by laser ablation-inductively coupledplasma-mass spectrometry. Geostandards Newsletter-The Journal of Geostandards and Geoanalysis26 (2), 181–196.

Gibbins, S. F. M., Gibson, H. L., Ames, D. E., Jonasson, I. R., 1997. The Onaping Formation: Stratigra-phy, fragmentation, and mechanisms of emplacement. In: Conference on large meteorite impacts andplanetary evolution, Sudbury, 1997. Lunar and Planetary Institute Contribution 922, p. 16.

Giblin, P. E., 1984. History of exploration and development, of geological studies and development ofgeological concepts. In: Pye, E., Naldrett, A., Giblin, P. (Eds.), The geology and ore deposits of theSudbury Structure. Vol. Special Volume 1. Ontario Geological Survey, Ch. 1, pp. 3–24.

Golightly, J. P., 1994. The Sudbury Igneous Complex as an impact melt: evolution and ore genesis.In: Lightfoot, P., Naldrett, A. (Eds.), Proceedings of the Sudbury-Norils’k Symposium. Vol. SpecialPublication Vol. 1. Ontario Geological Survey, pp. 45–56.

Grant, R. W., Bite, A., 1984. Sudbury quartz diorite Offset Dykes. In: Pye, E., Naldrett, A., Giblin,P. (Eds.), The geology and ore deposits of the Sudbury Structure. Vol. Special Volume 1. OntarioGeological Survey, Ch. 12, pp. 275–300.

Grieve, R., Cintala, M. J., Therriault, A., 2006. Large-scale impacts and the evolution of the Earth’scrust: The early years. In: Reimold, W., Gibson, R. (Eds.), Processes on the Early Earth. GeologicalSociety of America, pp. 23–31.

Grieve, R. A. F., Cintala, M. J., 1992. An analysis of differential impact-melt crater-scaling and impli-cations for the terrestrial impact record. Meteoritics 27 (5), 526–538.

Grieve, R. A. F., Stoffler, D., Deutsch, A., 1991. The Sudbury Structure - controversial or misunderstood.Journal of Geophysical Research - Planets 96 (E5), 22753–22764.

Grieve, R. A. F., Therriault, A., 2000. Vredefort, Sudbury, Chicxulub: Three of a kind? Annual Reviewof Earth and Planetary Sciences 28, 305–338.

Haggerty, S., 1999. Earth and planetary sciences - A diamond trilogy: Superplumes, supercontinents,and supernovae. Science 285 (5429), 851–860.

Hanchar, J. M. (Ed.), Jan 2003. Zircon. Vol. 53. Mineralogical Society of America.

Hanley, J., Mungall, J., 2003. Chlorine enrichment and hydrous alteration of the Sudbury Breccia host-ing footwall Cu-Ni-PGE mineralization at the Fraser mine, Sudbury, Ontario, Canada. CanadianMineralogist 41, 857–881.

Hanley, J., Mungall, J., Pettke, T., Spooner, E., Bray, C., 2005. Ore metal transport by hydrocarbonvapour in the footwall of the Sudbury Igneous Complex, Canada. Geochimica et Cosmochimica Acta69 (10), A738–A738.

Haughton, D. R., Roeder, P. L., Skinner, B. J., 1974. Solubility of sulphur in mafic magmas. EconomicGeology 69 (4), 451–467.

Hawkesworth, C. J., Lightfoot, P. C., Cohen, A. S., 1999. Origin of the Sudbury melt sheet and ores:New insights from old isotopes. Abstracts of papers, GAC-MAC Joint Anuual Meeting, Sudbury, 199924, 51.

126

REFERENCES

Heumann, K., Gallus, S., Radlinger, G., Vogl, J., 1998. Precision and accuracy in isotope ratio mea-surements by plasma source mass spectrometry. Journal of Analytical Atomic Spectrometry 13 (9),1001–1008.

Hiess, J., Nutman, A., Bennett, V., Holden, P., 2008. Ti-in-zircon thermometry applied to contrastingArchean metamorphic and igneous systems. Chemical Geology 247, 323–338.

Holden, P., Lanc, P., Ireland, T. R., Harrison, T. M., Foster, J. J., Bruce, Z., 2009. Mass-spectrometricmining of Hadean zircons by automated SHRIMP multi-collector and single-col lector U/Pb zirconage dating: The first 100,000 grains. International Journal of Mass Spectrometry 286 (2-3), 53–63.

Hopkins, M., Harrison, T. M., Manning, C., 2008. Low heat flow inferred from greater than 4 Gyr zirconssuggests Hadean plate boundary interactions. Nature 456 (7221), 493–496.

Hoskin, P., Schaltegger, U., 2003. The composition of zircon and igneous and metamorphic petrogenesis.Zircon 53, 27–62.

Hough, R., Gilmour, I., Pillinger, C., Arden, J., Gilkes, K., Yuan, J., Milledge, H., 1995. Diamond andsilicon-carbide in impact melt rock from the Ries impact crater. Nature 378 (6552), 41–44.

Housh, T., Bowring, S., 1991. Lead isotopic heterogeneities within alkali feldspars - implications forthe determination of initial lead isotopic compositions. Geochimica et Cosmochimica Acta 55 (8),2309–2316.

Hunt, P. C., Moskowitz, B. M., Banjeree, S. K., 1995. Magnetic properties of rocks and minerals. In:Ahrens, T. J. (Ed.), Rock Physics and Phase Relations: A Handbook of Physical Constants. AmericanGeophysical Union, Ch. 14, pp. 189–204.

Hurst, R. W., Wetheril, G., 1974. Rb-Sr study of the Sudbury Nickel Irruptive. Eos, Transactions,American Geophysical Union 55 (4), 466–466.

Hyodo, H., Dunlop, D. J., McWilliams, M. O., 1986. Timing and extent of the Grenvillian magneticoverprint near Temagami, Ontario. In: Moor, J. M., Davidson, A., Baer, A. J. (Eds.), The GrenvilleProvince. Vol. 31. Geological Association of Canada, pp. 119–126.

Jaffey, A., Flynn, K., Glendeni, L., Bentley, W., Essling, A., 1971. Precision measurement of half-livesand specific activities of U-235 and U-238. Physical Review C 4 (5), 1889–1891.

James, R. S., Sweeny, J. M., Peredery, W. V., 1992. Thermobarometry of the Levack gneisses - footwallrocks to the Sudbury Igneous Complex (SIC). Lithoprobe Report 25, 179–182.

Kamber, B., 2007. The enigma of the terrestrial protocrust: Evidence for its former existence and theimportance of its complete dissapearance. Developments in Precambrian Geology 15, 75–89.

Kamber, B. S., Gladu, A. H., 2009. Comparison of Pb Purification by Anion-Exchange Resin Methodsand Assessment of Long-Term Reproducibility of Th/U/Pb Ratio Measurements by Quadrupole ICP-MS. Geostandards and Geoanalytical Research 33 (2), 169–181.

Kamenov, G., Mueller, P., Perfit, M., 2004. Optimization of mixed Pb-Tl solutions for high precisionisotopic analyses by MC-ICP-MS. Journal of Analytical Atomic Spectrometry 19 (9), 1262–1267.

Kamo, S., Reimold, W., Krogh, T., Colliston, W., 1996. A 2.023 Ga age for the Vredefort impact eventand a first report of shock metamorphosed zircons in pseudotachylitic breccias and Granophyre. Earthand Planetary Science Letters 144 (3-4), 369–387.

Keays, R. R., Lightfoot, P. C., 1999. The role of meteorite impact, source rocks, proto-ores and maficmagmas in the genesis of the Sudbury Ni-Cu-PGE sulphide ore deposits. In: Keays, R. R., Lesher,C. M., Lightfoot, P. C., Farrow, C. E. G. (Eds.), Dynamic processes in magmatic ore deposits andtheir application in mineral exploration. Vol. 13 of Short Course. Geological Association of Canada.

Keays, R. R., Lightfoot, P. C., 2004. Formation of Ni-Cu-Platinum Group Element sulfide mineralizationin the sudbury impact melt sheet. Mineralogy and Petrology 82 (3-4), 217–258.

127

REFERENCES

Kent, A., Jacobsen, B., Peate, D., Waight, T., Baker, J., 2004. Isotope dilution MC-ICP-MS rareearth element analysis of geochemical reference materials NIST SRM 610, NIST SRM 612, NISTSRM 614, BHVO-2G, BHVO-2, BCR-2G, JB-2, WS-E, W-2, AGV-1 and AGV-2. Geostandards andGeoanalytical Research 28 (3), 417–429.

Klein, C., Hurlburt, C. S., 1999. Manual of Mineralogy. John Wiley and Sons.

Klimczak, C., Wittek, A., Doman, D., Riller, U., 2007. Fold origin of the NE-lobe of the Sudbury Basin,Canada: Evidence from heterogeneous fabric development in the Onaping Formation and the SudburyIgneous Complex. Journal of Structural Geology, 13.

Koeberl, C., 2003. The Late Heavy Bombardment in the inner solar system: Is there any connection toKuiper belt objects? Earth Moon and Planets 92 (1-4), 79–87.

Koeberl, C., Armstrong, R., Reimold, W., 1997a. Morokweng, South Africa: A large impact structureof Jurassic-Cretaceous boundary age. Geology 25 (8), 731–734.

Koeberl, C., Masaitis, V., Shafranovsky, G., Gilmour, I., Langenhorst, F., Schrauder, M., 1997b. Dia-monds from the Popigai impact structure, Russia. Geology 25 (11), 967–970.

Korsakov, A., Theunissen, K., Smirnova, L., 2004. Intergranular diamonds derived from partial meltingof crustal rocks at ultrahigh-pressure metamorphic conditions. Terra Nova 16 (3), 146–151.

Kramers, J. D., 2007. Hierarchical Earth accretion and the Hadean eon. Journal of the Geolical Societyof London 164, 3–17.

Kring, D., Cohen, B., 2002. Cataclysmic bombardment throughout the inner solar system 3.9-4.0 Ga.Journal of Geophysical Research-Planets 107 (E2).

Krogh, T. E., Davis, D., Corfu, F., 1984. Precise U-Pb zircon and baddeleyite ages for the Sudbury area.The Geology and Ore Deposits of the Sudbury Structure Special Volume 1, 431–446.

Krogh, T. E., McNutt, R. H., Davis, G., 1982. Two high precision U-Pb ages for the Sudbury NickelIrruptive. Canadian Journal of Earth Sciences 19, 723–728.

LaFrance, B., Legault, D., Ames, D. E., 2008. The formation of the Sudbury Breccia in the North Rangeof the Sudbury impact structure. Precambrian Research 165 (3-4), 107–119.

Lakomy, R., 1990. Implications for cratering mechanics from a study of the Footwall Breccia of theSudbury impact structure, Canada. Meteoritics 25 (3), 195–207.

Langenhorst, F., Shafranovsky, G., Masaitis, V., 1998. A comparative study of impact diamonds from thePopigai, Ries, Sudbury, and Lappajarvi craters. Meteoritics and Planetary Science 33 (4), A90–A91.

Langenhorst, F., Shafranovsky, G., Masaitis, V., Koivisto, M., 1999. Discovery of impact diamonds ina Fennoscandian crater and evidence for their genesis by solid-state transformation. Geology 27 (8),747–750.

Langford, F. F., 1960. Geology of Levack Township and the northern part of Dowling Township, districtof Sudbury. Ontario. Preliminary Report 1960-5, Department of Mines.

Langmuir, D., 1978. Uranium solution-mineral equilibria at low-temperatures with applications to sedi-mentary ore-deposits. Geochimica et Cosmochimica Acta 42 (6), 547–569.

Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P. (Eds.), 2005.Igneous rocks: A classification and glossary of terms. Cambridge University Press, Cambridge, UK.

Li, C., Naldrett, A., Coats, C., Johannessen, P., 1992. Platinum, palladium, gold, and copper-richstringers at the Strathcona Mine, Sudbury - their enrichment by fractionation of a sulfide liquid.Economic Geology 87 (6), 1584–1598.

Lightfoot, P. C., 2002. Petrology and geochemistry of the Nipissing gabbro: exploration strategies fornickel, copper and platinum group elements in a large igneous province. Ontario Geological Survey,Special Volume 58, 81.

128

REFERENCES

Lightfoot, P. C., Desouza, H., Doherty, W., 1993. Differentiation and source of the Nipissing diabaseintrusions, Ontario, Canada. Canadian Journal of Earth Sciences 30 (6), 1123–1140.

Lightfoot, P. C., Doherty, W., K.Farrell, Keays, R. R., Moore, M. L., Pekeski, D., 1997a. Geochemistry ofthe Main Mass, Sublayer, Offsets and inclusions from the Sudbury Igneous Complex, Ontario. OntarioGeological Survey, Open File Report 5959.

Lightfoot, P. C., Farrow, C. E. G., 2002. Geology, geochemistry, and mineralogy of the WorthingtonOffset Dike: A genetic model for offset dike mineralization in the Sudbury Igneous Complex. EconomicGeology 97 (7), 1419–1446.

Lightfoot, P. C., Keays, R. R., Doherty, W., 2001. Chemical evolution and origin of nickel sulfide min-eralization in the Sudbury Igneous Complex, Ontario, Canada. Economic Geology 96 (8), 1855–1875.

Lightfoot, P. C., Keays, R. R., Morrison, G. G., Bite, A., Farrell, K. P., 1997b. Geochemical relationshipsin the Sudbury Igneous Complex: Origin of the Main Mass and Offset Dikes. Economic Geology 92 (3),289–307.

Lightfoot, P. C., Keays, R. R., Morrison, G. G., Bite, A., Farrell, K. P., 1997c. Geologic and geochemicalrelationships between the Contact Sublayer, inclusions, and the Main Mass of the Sudbury IgneousComplex: A case study of the Whistle Mine embayment. Economic Geology 92 (6), 647–673.

Lightfoot, P. C., Naldrett, A. J. (Eds.), 1994. Proceedings of the Sudbury-Noril’sk Symposium. Vol.Special Volume 5. Ontario Geological Survey.

Lightfoot, P. C., Naldrett, A. J., 1997. Sublayer and Offset Dikes of the Sudbury Igneous Complex - anIntroduction and Field Guide. Open File Report 5965, 50.

Lightfoot, P. C., Zotov, I. A., 2005. Geology and geochemistry of the Sudbury Igneous Complex, On-tario, Canada: Origin of nickel sulfide mineralization associated with an impact-generated melt sheet.Geology of Ore Deposits 47 (5), 349–381.

Maas, R., Kinny, P. D., Williams, I. S., Froude, D. O., Compston, W., 1992. The Earths oldest knowncrust - a geochronological and geochemical study of 3900-4200Ma old detrital zircons from Mt Narryerand Jack Hills, Western Australia. Geochimica et Cosmochimica Acta 56 (3), 1281–1300.

Masaitis, V., Grieve, R., Langenhorst, F., Peredery, W. V., Therriault, A., Balmasov, E., Federova, I.,1999. Impact diamonds in the suevitic breccias of the Black Member of the Onaping Formation, Sud-bury Structure, Ontario, Canada. In: Dressler, B. O., Sharpton, V. (Eds.), Large Meteorite Impactsand Planetary Evolution II. Vol. Special Paper 339. Geological Society of America, Boulder, Colorado,pp. 317–321.

McCormick, K., Fedorowich, J., McDonald, A., James, R., 2002. A textural, mineralogical, and statisticalstudy of the footwall breccia within the Strathcona embayment of the Sudbury Structure. EconomicGeology 97 (1), 125–143.

McCulloch, M., Wasserburg, G. J., 1978. Sm-Nd and Rb-Sr chronology of continental crust formation.Science 200 (4345), 1003–1011.

McDonough, W. F., Sun, S. S., 1995. The composition of the Earth. Chemical Geology 120 (3-4), 223–253.

McLennan, S. M., Fryer, B. J., Young, G. M., 1979. Rare-Earth Elements in Huronian (lower Protero-zoic) sedimentary-rocks - composition and evolution of the post-Kenoran upper crust. Geochimica etCosmochimica Acta 43 (3), 375–388.

McLennan, S. M., Simonetti, A., Goldstein, S. L., 2000. Nd and Pb isotopic evidence for provenanceand post-depositional alteration of the Paleoproterozoic Huronian Supergroup, Canada. PrecambrianResearch 102 (3-4), 263–278.

Meldrum, A., AbdelRahman, A. F. M., Martin, R. F., Wodicka, N., 1997. The nature, age and petrogen-esis of the Cartier batholith, northern flank of the Sudbury Structure, Ontario, Canada. PrecambrianResearch 82 (3-4), 265–285.

129

REFERENCES

Melosh, H. J., Ivanov, B. A., 1999. Impact crater collapse. Annual Review of Earth and PlanetarySciences 27 (385-415).

Menneken, M., Nemchin, A. A., Geisler, T., Pidgeon, R. T., Wilde, S. A., 2007. Hadean diamonds inzircon from Jack Hills, Western Australia. Nature 448 (7156), 917–925.

Milkereit, B., Green, A., Berrer, E., Boerner, D., Broome, J., Cosec, M., Cowan, J., Davidson, A.,Dressler, B., Fueten, F., Grieve, R., James, R., Krause, B., McGrath, P., Meyer, W., Moon, W.,Morris, W., Morrison, G., Naldrett, A., Peredery, W., Rousell, D., Salisbury, M., Schwerdtner, W.,Snajdr, P., Thomas, M., Watts, A., 1992. Deep geometry of the Sudbury Structure from seismic-reflection profiling. Geology 20 (9), 807–811.

Milton, D., 1977. Shatter cones - an outstanding problem in shock mechanics. In: Roddy, J., Pepin, O.,Merrill, B. (Eds.), Impact and Explosion Cratering: Planetary and Terrestrial Implications. PergamonPress, New York, pp. 703–714.

Molnar, F., Watkinson, D., Jones, P., 2001. Multiple hydrothermal processes in footwall units of theNorth Range, Sudbury Igneous Complex, Canada, and implications for the genesis of vein-type Cu-Ni-PGE deposits. Economic Geology 96 (7), 1645–1670.

Molnar, F., Watkinson, D., Jones, P., Gatter, I., 1997. Fluid inclusion evidence for hydrothermal enrich-ment of magmatic ore at the contact zone of the Ni-Cu-platinum-group element 4b deposit, Lindsleymine, Sudbury, Canada. Economic Geology 92 (6), 674–685.

Morgan, J. W., Walker, R. J., Horan, M. F., Beary, E. S., Naldrett, A. J., 2002. Pt-190-Os-186 andRe-187-Os-187 systematics of the Sudbury Igneous Complex, Ontario. Geochimica et CosmochimicaActa 66 (2), 273–290.

Morrison, G., 1984. Morphological features of the Sudbury Structure in relation to an impact origin. In:Pye, E., Naldrett, A., Giblin, P. (Eds.), The geology and ore deposits of the Sudbury Structure. Vol.Special Volume 1. Ontario Geological Survey, pp. 513–520.

Muir, T., Peredery, W. V., 1984. The Onaping Formation. In: Pye, E., Naldrett, A., Giblin, P. (Eds.),The geology and ore deposits of the Sudbury Structure. Vol. Special Volume 1. Ontario GeologicalSurvey, Ch. 7, pp. 139–210.

Muller-Mohr, V., 1992. Breccias in the basement of a deeply eroded impact structure, Sudbury, Canada.Tectonophysics 216, 219–226.

Mungall, J. E., Ames, D. E., Hanley, J. J., 2004. Geochemical evidence from the Sudbury Structure forcrustal redistribution by large bolide impacts. Nature 429 (6991), 546–548.

Murphy, A. J., Spray, J. G., 2002. Geology, mineralization, and emplacement of the Whistle-ParkinOffset Dike, Sudbury. Economic Geology 97 (7), 1399–1418.

Naldrett, A., 1989. Magmatic sulfide deposits. Oxford University Press.

Naldrett, A. J., 1984a. Ni-Cu ores of the Sudbury Igneous Complex - Introduction. In: Pye, E., Naldrett,A., Giblin, P. (Eds.), The Geology and Ore Deposits of the Sudbury Structure. Ontario GeologicalSurvey, Ch. 13, pp. 302–325.

Naldrett, A. J., 1984b. Summary, discussion and synthesis. In: Pye, E., Naldrett, A., Giblin, P. (Eds.),The Geology and Ore Deposits of the Sudbury Structure. Ontario Geological Survey, Ch. 25, pp.236–251.

Naldrett, A. J., 1999. Summary: Development of ideas on Sudbury geology, 1992-1998. In: Dressler,B. O., Sharpton, V. L. (Eds.), Large Meteorite Impacts and Planetary Evolution II. Vol. SpecialPaper 339. Geological Society of America, Boulder, Colorado, pp. 431–442.

Naldrett, A. J., Bray, J. G., Gasparri, E., Podolsky, T., Rucklidge, J., 1970. Cryptic Variation andPetrology of Sudbury Nickel Irruptive. Economic Geology 65 (2), 122.

130

REFERENCES

Naldrett, A. J., Hewins, R. H., 1984. The Main Mass of the Sudbury Igneous Complex. In: Pye, E.,Naldrett, A., Giblin, P. (Eds.), The Geology and Ore Deposits of the Sudbury Structure. OntarioGeological Survey, Ch. 10, pp. 236–251.

Naldrett, A. J., Rao, B. V., Evensen, N. M., 1986. Contamination at Sudbury and its role in ore formation.In: Metallogeny of basic and ultrabasic rocks. The Institute of Mining and Metallurgy, London, pp.75–91.

Patchett, P. J., White, W. M., Feldmann, H., Kielinczuk, S., Hofmann, A. W., 1984. Hafnium rare-earthelement fractionation in the sedimentary system and crustal recycling into the Earths mantle. Earthand Planetary Science Letters 69 (2), 365–378.

Pearce, N., Perkins, W., Westgate, J., Gorton, M., Jackson, S., Neal, C., Chenery, S., 1997. A compilationof new and published major and trace element data for NIST SRM 610 and NIST SRM 612 glassreference materials. Geostandards Newsletter-The Journal of Geostandards and Geoanalysis 21 (1),115–144.

Peredery, W. V., Morrison, G. G., 1984. Discussion of the origin of the Sudbury Structure. In: Pye, E.,Naldrett, A., Giblin, P. (Eds.), The geology and ore deposits of the Sudbury Structure. Vol. SpecialVolume 1. Ontario Geological Survey, Ch. 22, pp. 491–511.

Phinney, W. C., Simonds, C. H., 1977. Dynamical implications of the petrology and distribution ofimpact melt rocks. In: Roddy, J., Pepin, O., Merrill, B. (Eds.), Impact and Explosion Cratering:Planetary and Terrestrial Implications. Pergamon Press, Flagstaff, Arizona.

Pope, K. O., Kieffer, S. W., Ames, D. E., 2004. Empirical and theoretical comparisons of the Chicxuluband Sudbury impact structures. Meteoritics and Planetary Science 39 (1), 97–116.

Pouchou, J. L., Pichoir, F., 1985. ”PAP” procedure for improved quantitative analysis. MicrobeamAnalysis 20, 104–105.

Prasad, N., Roscoe, S., 1996. ”evidence of anoxic to oxic atmospheric change during 2.45-2.22 ga fromlower and upper sub-huronian paleosols, canada”. Catena 27 (2), 105–121.

Prevec, S., Baadsgaard, H., 2005. Evolution of Palaeoproterozoic mafic intrusions located within thethermal aureole of the Sudbury Igneous Complex, Canada: Isotopic, geochronological and geochemicalevidence. Geochimica et Cosmochimica Acta 69 (14), 3653–3669.

Prevec, S. A., Lightfoot, P. C., Keays, R. R., 2000. Evolution of the Sublayer of the Sudbury IgneousComplex: geochemical, Sm-Nd isotopic and petrologic evidence. Lithos 51 (4), 271–292.

Pufahl, P. K., Hiatt, E. E., Stanley, C. R., Morrow, J. R., Nelson, G. J., Edwards, C. T., 2007. Physicaland chemical evidence of the 1850 Ma Sudbury impact event in the Baraga Group, Michigan. Geology35 (9), 827–830.

Pye, E. G. (Ed.), 1984. The geology and ore deposits of the Sudbury Structure. Ontario GeologicalSurvey.

Rae, D. R., 1975. Inclusions in the Sublayer from Strathcona Mine, Sudbury, and their significance.Ph.D. thesis, University of Toronto.

Rao, B. V., Naldrett, A. J., Evensen, N. M., 1985. Crustal contamination of the Sublayer, SudburyIgneous Complex, and its relevance to the genesis of Ni-Cu sulfides. Canadian Mineralogist 23, 329–330.

Rehkamper, M., Halliday, A., 1998. Accuracy and long-term reproducibility of lead isotopic measurementsby multiple-collector inductively coupled plasma mass spectrometry using an external method forcorrection of mass discrimination. International Journal of Mass Spectrometry 181, 123–133.

Rehkamper, M., Mezger, K., 2000. Investigation of matrix effects for Pb isotope ratio measurements bymultiple collector ICP-MS: verification and application of optimized analytical protocols. Journal ofAnalytical Atomic Spectrometry 15 (11), 1451–1460.

131

REFERENCES

Riller, U., 2005. Structural characteristics of the Sudbury impact structure, Canada: Impact-inducedversus orogenic deformation - A review. Meteoritics and Planetary Science 40 (11), 1723–1740.

Roddy, J., Davis, L., 1977. Shatter cones formed in large scale experimental explosion craters. In: Roddy,J., Pepin, O., Merrill, B. (Eds.), Impact and Explosion Cratering: Planetary and Terrestrial Implica-tions. Pergamon Press, New York, pp. 715–750.

Roscoe, S., Card, K., 1992. Early Proterozoic tectonics and metallogeny of the Lake Huron region of theCanadian Shield. Precambrian Research 58 (1-4), 99–119.

Rousell, D. H., 1972. The Chelmsford Formation of the Sudbury Basin - A Precambrian turbidite.In: Guy-Bray, J. V. (Ed.), New developments in Sudbury geology. No. Special Paper number 10.Geological Association of Canada, pp. 79–91.

Rousell, D. H., 1975. Origin of foliation and lineation in the Onaping Formation and deformation of theSudbury Basin. Canadian Journal of Earth Sciences 12, 1379–1395.

Rousell, D. H., 1984a. Onwatin and Chelmsford Formations. In: Pye, E., Naldrett, A., Giblin, P. (Eds.),The geology and ore deposits of the Sudbury Structure. Vol. Special Volume 1. Ontario GeologicalSurvey, Ch. 8, pp. 211–232.

Rousell, D. H., 1984b. Structural geology of the Sudbury Basin. In: Pye, E., Naldrett, A., Giblin,P. (Eds.), The geology and ore deposits of the Sudbury Structure. Vol. Special Volume 1. OntarioGeological Survey, Ch. 5, pp. 83–96.

Rousell, D. H., Fedorowich, J. S., Dressler, B. O., 2003. Sudbury Breccia (Canada): a product of the1850 Ma Sudbury Event and host to footwall Cu-Ni-PGE deposits. Earth-Science Reviews 60 (3-4),147–174.

Schuraytz, B., Sharpton, V., Marin, L., 1994. Petrology of impact-melt rocks at the Chicxulub multiringbasin, Yucatan, Mexico. Geology 22 (10), 868–872.

Scott, R., Spray, J., 2000. The South Range Breccia Belt of the Sudbury Impact Structure: A possibleterrace collapse feature. Meteoritics and Planetary Science 35 (3), 505–520.

Shand, S. J., 1916. The pseudotachylyte of Parijs (Orange Free State) and its relation to ”trap-shottengneiss” and ”flinty crush-rock”. Quarterly Journal of the Geological Society of London 72, 198–221.

Shanks, W. S., Schwerdtner, W. M., 1991. Structural analysis of the central and southwestern SudburyStructure, Southern Province, Canadian Shield. Canadian Journal of Earth Sciences 28 (3), 411–430.

Simonds, C. H., Kieffer, S. W., 1993. Impact and volcanism - a momentum scaling law for erosion.Journal of Geophysical Research - Solid Earth 98 (B8), 14321–14337.

Sims, P. K., Van Schmus, W. R., Schultz, K. J., Petermann, Z. E., 1989. Tectono-stratigraphic evolutionof the early Proterozoic Wisconsin magmatic terranes of the Penokean orogen. Canadian Journal ofEarth Sciences 26, 2145–2158.

Sinha, A. K., 1969. Removal of radiogenic Pb from potassium feldspars by volatilization. Earth andPlanetary Science Letters 7, 109–115.

Speers, E. C., 1957. The age relation and origin of common Sudbury breccia. Journal of Geology 65,497–514.

Spray, J. G., Butler, H. R., Thompson, L. M., 2004. Tectonic influences on the morphometry of the Sud-bury impact structure: Implications for terrestrial cratering and modeling. Meteoritics and PlanetaryScience 39 (2), 287–301.

Stacey, J. S., Kramers, J. D., 1975. Approximation of terrestrial lead isotope evolution by a 2-stagemodel. Earth and Planetary Science Letters 26 (2), 207–221.

Strelow, F. W. E., 1978. Distribution coefficients and anion-exchange behaviour of some elements inhydrobromic nitric acid mixtures. Analytical Chemistry 50 (9), 1359–1361.

132

REFERENCES

Therriault, A., Fowler, A., Grieve, R., 2002. The Sudbury Igneous Complex: A differentiated impactmelt sheet. Economic Geology 97 (7), 1521–1540.

Thirlwall, M., 2002. Multicollector ICP-MS analysis of Pb isotopes using a (207)Pb-(204)Pb double spikedemonstrates up to 400 ppm/amu systematic errors in Tl-normalization. Chemical Geology 184 (3-4),255–279.

Thirlwall, M. F., 2000. Inter-laboratory and other errors in Pb isotope analyses investigated using a207Pb-204Pb double spike. Chemical Geology 163, 299–322.

Thompson, L. M., Spray, J. G., 1994. Pseudotachylytic rock distributions and genesis within the Sudburyimpact structure. In: Dressler, B., Grieve, R., Sharpton, V. (Eds.), Large Meteorite Impacts andPlanetary Evolution. Special Paper 293. Geological Society of America, pp. 275–287.

Thompson, M. L., Barnett, R. L., Fleet, M. E., Kerrich, R., 1985. Metamorphic assemblages in theSouth Range norite and footwall mafic rocks near the Kirkwood mine, Sudbury, Ontario. CanadianMineralogist 23, 173–186.

Tuchscherer, M. G., Spray, J. G., 2002. Geology, mineralization, and emplacement of the Foy Offset dike,Sudbury impact structure. Economic Geology 97 (7), 1377–1397.

Vaughan, D. J., Schwarz, E. J., Owens, D. R., 1971. Pyrrhotites from the Strathcona Mine, Sudbury,Canada; A thermomagnetic and minerlaogical study. Economic Geology 66, 1131–1144.

Vervoort, J., Patchett, P., Blichert-Toft, J., Albarede, F., 1999. Relationships between Lu-Hf and Sm-Ndisotopic systems in the global sedimentary system. Earth and Planetary Science Letters 168 (1-2),79–99.

Walder, A. J., Freedman, P. A., 1992. Isotopic ratio measurement using a double focusing magnetic-sector mass analyzer with an inductively coupled plasma as an ion-source. Journal of AnalyticalAtomic Spectrometry 7 (3), 571–575.

Walker, R. J., Morgan, J. W., Naldrett, A. J., Li, C., Fassett, J. D., 1991. Re-Os isotope systematicsof Ni-Cu sulfide ores, Sudbury Igneous Complex, Ontario - Evidence for a major crustal component.Earth and Planetary Science Letters 105 (4), 416–429.

Watson, E. B., Harrison, T., 2005. Zircon thermometer reveals minimum melting conditions on earliestEarth. Science 308 (5723), 841–844.

Watson, E. B., Harrison, T. M., 2006. Response to comments on ”Zircon thermometer reveals minimummelting conditions on earliest Earth”. Science 311 (5762).

Wiedenbeck, M., Hanchar, J., Peck, W., Sylvester, P., Valley, J., Whitehouse, M., Kronz, A., Morishita,Y., Nasdala, L., Fiebig, J., Franchi, I., Girard, J., Greenwood, R., Hinton, R., Kita, N., Mason,P., Norman, M., Ogasawara, M., Piccoli, R., Rhede, D., Satoh, H., Schulz-Dobrick, B., Skar, O.,Spicuzza, M., Terada, K., Tindle, A., Togashi, S., Vennemann, T., Xie, Q., Zheng, Y., 2004. Furthercharacterisation of the 91500 zircon crystal. Geostandards and Geoanalytical Research 28 (1), 9–39.

Wilde, S. A., Valley, J. W., Peck, W. H., Graham, C. M., 2001. Evidence from detrital zircons for theexistence of continental crust and oceans on the Earth 4.4 Gyr ago. Nature 409 (6817), 175–178.

Williams, G. H., 1891. Notes on the microscopial character of rocks from the Sudbury mining district.In: Annual Report. Vol. 5. Geological Survey of Canada, pp. 55F–82F.

Wirth, R., Rocholl, A., 2003. Nanocrystalline diamond from the Earth’s mantle underneath Hawaii.Earth and Planetary Science Letters 211 (3-4), 357–369.

Wittmann, A. amd Schmitt, R. T., Hecht, L., Kring, D. A., Reimold, W. U., Povenmire, H., 2009.Petrology of impact melt rocks from the Chesapeake Bay crater, USA. Geological Society of AmericaSpecial Papers 458, 377–396.

Woodhead, J., 2002. A simple method for obtaining highly accurate Pb isotope data by MC-ICP-MS.Journal of Analytical Atomic Spectrometry 17 (10), 1381–1385.

133

REFERENCES

Woodhead, J., Hergt, J., 2000. Pb-isotope analyses of USGS reference materials. GeostandardsNewsletter-The Journal of Geostandards and Geoanalysis 24 (1), 33–38.

Young, G. M., Long, D. G. F., Fedo, C. M., Nesbitt, H. W., 2001. Paleoproterozoic Huronian basin:product of a Wilson cycle punctuated by glaciations and a meteorite impact. Sedimentary Geology141, 233–254.

Zartmann, R. E., Wasserburg, G. L., 1969. The isotopic composition of lead in potassium feldspars fromsome 1.0 By-old North American igneous rocks. Geochimica et Cosmochimica Acta 33, 901–942.

Zieg, M. J., Marsh, B. D., 2005. The Sudbury Igneous Complex: Viscous emulsion differentiation of asuperheated impact melt sheet. Bulletin of the Geological Society of America 117 (11-12), 1427–1450.

Note that this list of references does not include those within the three published and

submitted papers. Please refer to each individual paper.

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8

Appendices

8.0.1 Supplementary materials for Darling et al. (2009) Impact melt

sheet zircons and their implications for Hadean crustal processes;

Geology; v. 37; no.10; p. 927-930

135

8 Appendices

RSP (m) Sample Grain/Spot

HfO2 Th(ppm)

U(ppm)

Th/U Ti(ppm)

2sd T(C)

2sd

Basal norite

0 144IBNR 16 0.795 381 465 0.82 30.0 3.4 848 1819 0.842 479 502 0.95 41.6 3.6 884 1620 0.785 276 291 0.95 36.7 3.7 870 1728 0.912 344 366 0.94 28.4 3.6 842 1931 0.828 226 277 0.82 30.8 4.4 850 2133 0.781 316 384 0.82 32.8 3.7 857 1835 0.800 458 469 0.98 30.2 3.5 848 1838 0.825 460 462 1.00 36.5 4.0 869 1839 0.806 426 433 0.98 31.1 3.6 852 1942 0.797 441 485 0.91 32.1 1.6 855 1245 0.839 355 364 0.98 35.3 1.8 865 12

98 144BNR 19 0.906 287 224 1.28 33.7 1.4 860 1120 0.888 263 190 1.38 31.8 1.4 854 1124 0.911 548 536 1.02 37.5 1.5 872 1130 0.851 765 562 1.36 35.8 1.7 867 1230B 0.884 330 273 1.21 35.5 1.9 866 1231 0.901 357 263 1.36 35.9 1.5 867 1134A 0.901 735 705 1.04 30.0 1.2 848 1134B 0.808 489 502 0.97 38.6 1.6 876 1134C 0.901 347 259 1.34 36.5 2.0 869 1345 0.900 416 329 1.26 38.3 1.5 875 1145 0.890 516 498 1.04 39.8 2.4 879 1346 0.890 254 156 1.63 33.0 1.7 858 1246B 0.808 308 301 1.02 29.4 1.7 846 1347 0.911 544 490 1.11 36.4 1.9 869 1249A 0.904 372 277 1.34 39.8 2.2 879 1349B 0.808 721 620 1.16 33.0 1.8 858 1349C 0.841 549 563 0.98 38.7 2.2 876 1350A 0.900 628 619 1.01 40.3 1.6 880 1150B 0.841 550 526 1.05 37.7 1.6 873 1152 0.884 791 787 1.01 36.9 1.4 870 1160 0.860 764 590 1.29 33.2 1.9 859 1361 0.832 423 400 1.06 31.2 1.7 852 1261B 0.832 254 211 1.20 29.2 1.7 845 13

304 JDSIC0702 63 0.828 470 475 0.99 36.0 2.0 868 131 0.921 257 334 0.77 30.7 2.1 850 1418 0.849 364 620 0.59 31.6 2.2 853 142 0.748 179 275 0.65 32.9 2.9 858 1620 0.751 422 761 0.55 32.9 2.2 858 1423 0.884 303 383 0.79 35.8 2.5 867 1424 0.806 398 733 0.54 32.4 2.2 856 1426 0.853 495 915 0.54 38.6 2.7 875 1527 0.851 217 361 0.60 29.2 2.0 845 1429 0.881 349 600 0.58 32.8 2.3 857 143 0.906 337 484 0.70 32.1 2.2 855 147 0.883 141 150 0.94 32.0 2.2 855 14

874 JDSIC0701 27 0.957 861 1364 0.63 19.8 1.2 805 1228 0.952 951 1451 0.66 16.5 1.0 787 1229 0.957 1101 1475 0.75 23.3 1.5 821 1333 0.980 909 1199 0.76 21.0 1.6 811 1449 0.965 1284 1744 0.74 20.1 1.1 806 1256 0.814 1035 2352 0.44 27.8 1.6 839 1274 0.840 2263 3831 0.59 15.6 0.9 782 12

1070 144NR 39 0.864 751 1349 0.56 26.5 1.2 834 1145 0.895 841 1751 0.48 20.7 1.1 809 1247 0.893 964 2394 0.40 22.2 1.3 816 125 0.874 1093 2340 0.47 33.7 2.7 860 15

136


HfO2 Th(ppm)

U(ppm)

Th/U Ti(ppm)

2sd T(C)

2sd

Norite

1070 144NR 50 0.940 727 1197 0.61 20.1 1.2 806 1251 0.936 780 1543 0.51 15.6 1.0 781 1255A 0.862 610 1122 0.54 16.9 1.0 789 1255B 0.862 568 1034 0.55 15.2 0.9 779 1155C 0.873 673 1268 0.53 15.0 0.9 778 1161 0.985 621 992 0.63 17.0 1.0 790 1265 0.901 589 1033 0.57 19.1 1.1 801 12

1384 144MLNR 19 0.863 564 551 1.02 19.7 2.0 804 1622 0.851 686 773 0.89 21.6 2.1 814 1628 0.812 969 2052 0.47 25.5 1.4 830 1239 1.063 639 975 0.66 14.0 0.9 772 1246 1.028 669 906 0.74 12.3 0.8 759 1168A 1.001 725 1203 0.60 20.9 1.2 810 1268B 0.971 650 1147 0.57 19.4 1.2 803 1268C 1.017 490 528 0.93 14.5 0.9 775 1169A 0.955 773 971 0.80 19.9 1.1 805 1269B 0.979 767 1027 0.75 19.8 1.1 805 11

1614 JDSIC0708 1 0.927 1080 2069 0.52 16.7 1.6 788 157 1.153 550 852 0.65 9.4 0.8 736 1310 1.145 1103 1623 0.68 12.3 1.1 760 1418A 1.169 794 781 1.02 12.4 1.4 760 1518B 1.169 940 1177 0.80 11.7 1.1 755 1431 0.882 1143 2245 0.51 18.9 1.2 800 1235 0.848 1020 1920 0.53 17.5 1.6 793 1537 0.940 965 1840 0.52 16.6 1.5 788 1448A 0.940 1090 2124 0.51 16.1 1.3 785 1348B 0.940 888 1297 0.68 16.2 1.2 785 1349 0.940 1226 2404 0.51 16.2 1.2 786 1350 0.940 1396 2355 0.59 15.4 1.9 781 17

Granophyre

2942 144GRAN1 42 0.694 162 182 0.89 13.3 1.5 766 1644 0.764 292 407 0.72 12.0 2.3 757 2245 0.750 71 113 0.63 13.5 1.7 768 1749 0.739 188 221 0.85 12.1 1.7 758 1850A 0.739 254 268 0.95 13.5 1.6 768 1750B 0.745 128 184 0.70 17.4 1.6 792 1451 0.741 109 163 0.67 19.5 1.7 803 15

2942 144GRAN1 53 0.736 166 201 0.83 16.1 1.7 784 1654 0.754 116 147 0.79 13.4 1.7 767 1756 0.734 184 218 0.84 14.1 1.8 772 1758 0.735 187 228 0.82 16.2 1.6 786 1563 0.733 165 189 0.87 15.2 1.7 779 1676 0.744 164 130 1.26 19.4 1.3 803 1280 0.746 195 142 1.37 17.2 1.5 791 1485 0.761 168 121 1.39 18.7 1.4 799 1387 0.778 175 160 1.09 16.3 2.0 786 1797 0.740 144 111 1.30 16.4 1.2 786 1398 0.711 173 146 1.18 16.5 1.4 787 1499A 0.710 200 187 1.07 13.7 1.8 770 18

3300 JDSIC0704 10 1.082 1070 1459 0.73 8.6 1.2 727 1713 1.006 4700 4235 1.11 10.5 1.3 746 1615 1.058 1787 2151 0.83 7.9 0.7 721 1318 0.990 2851 3022 0.94 7.0 0.9 710 1520 1.236 507 938 0.54 8.6 0.8 728 1328 0.957 1893 2306 0.82 9.3 0.9 735 1429 0.966 1168 1634 0.71 7.0 0.8 710 14

137

8 Appendices


HfO2 Th(ppm)

U(ppm)

Th/U Ti(ppm)

2sd T(C)

2sd

Granophyre

3300 JDSIC0704 29 0.966 941 1072 0.88 11.3 2.2 752 2230 0.777 2968 2804 1.06 9.6 1.2 737 1636 0.784 1689 2013 0.84 7.9 0.9 720 1536 0.784 2089 2216 0.94 10.7 1.3 747 1637 0.821 2564 2829 0.91 11.0 1.2 750 1540 0.921 3351 3375 0.99 7.2 1.9 712 265 0.763 3133 2549 1.23 9.6 1.8 737 2166A 0.768 1857 1775 1.05 6.9 1.1 709 1866B 0.722 1431 1298 1.10 9.4 0.9 735 1467A 0.951 1339 983 1.36 7.7 1.1 719 1767B 0.951 2502 2353 1.06 7.5 1.1 717 178 0.887 1649 1493 1.10 12.7 1.6 762 179 0.812 2374 2944 0.81 6.5 0.7 704 149 0.721 2817 3883 0.73 11.0 1.0 749 14

3800 JDSIC0705 13A 0.806 312 426 0.73 14.8 1.4 777 1513B 0.772 311 352 0.88 15.7 1.2 782 1313C 0.814 153 112 1.37 15.6 1.2 782 131A 0.763 2381 3823 0.62 8.8 1.2 730 161B 0.763 3222 3964 0.81 8.4 1.6 726 204 0.763 1080 954 1.13 7.2 1.4 713 21

Quartz diorite

WOQD1 A1a 0.981 1500 662 2.27 10.6 1.2 746 15A1b 0.958 2055 826 2.49 12.0 1.1 757 14A1c 0.930 3640 1202 3.03 15.1 1.3 778 14A7a 0.871 2961 1095 2.70 13.4 0.8 767 11A7b 0.871 2103 821 2.56 12.9 1.3 763 15B11 1.070 2831 1749 1.62 13.3 1.4 767 15B12 0.852 1437 980 1.47 21.5 1.4 813 13E8 0.952 1699 727 2.34 16.6 1.4 788 14F1 0.820 208 179 1.16 19.8 1.7 805 15F2 0.967 452 334 1.35 17.7 1.5 794 14E9 0.952 1472 683 2.16 10.9 1.2 749 16E5 0.862 2523 1015 2.49 12.5 1.4 761 16

Table 8.1: Summary of HfO2, Th, U and Ti concentrations for zircons from throughoutthe Sudbury impact melt sheet stratigraphy.

138

Figure 8.1: Cathodoluminescence images of representative zircons from each studied unit.Laser spot positions and corresponding measured T T izir are shown, along with identifiedinclusions. K-feld - K-feldspar; Qtz - quartz; plag - plagioclase; bt - biotite; ap - apatite.

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf nBasal Lower UnitIBNR 0.6 28 1.2 14 17 1.6 67 19 224 75 316 67 596 86 9668 141! 0.5 2 0.4 3 3 0.3 12 3 43 14 60 11 92 15 477BNR 2.0 33 1.4 14 16 0.9 50 18 217 72 302 73 762 80 8951 121! 1.4 6 1.1 9 7 0.6 15 4 48 14 55 12 113 12 386

Lower UnitJDSIC0701 3.7 103 3.1 31 36 11.6 106 35 393 121 470 107 1081 99 5862 51! 3.3 22 2.1 14 11 9.6 37 12 130 39 148 33 313 29 596144NR 12.4 175 9.3 81 71 22.3 174 47 487 143 532 108 959 108 10166 101! 4.2 60 4.9 39 28 12.7 75 22 241 71 257 51 434 45 1354144MLNR 2.0 145 2.1 27 43 4.2 167 51 554 168 634 134 1261 132 10711 81! 3.0 79 1.3 11 22 2.2 130 36 371 110 399 68 536 76 1934JDSIC0708 0.3 129 1.4 25 46 2.1 169 58 667 204 785 172 1585 160 10090 51! 0.2 81 0.8 14 29 1.3 100 32 350 100 364 76 683 65 1676

Upper Unit144GRAN1 0.9 13 0.7 8 10 0.8 48 14 163 56 234 51 458 65 8689 121! 1.7 4 0.6 4 3 0.3 16 4 50 17 66 14 119 15 181JDSIC0704 0.7 115 1.5 24 52 1.6 280 94 1159 382 1543 331 2825 325 11129 121! 0.8 64 0.7 8 20 0.7 108 36 426 137 530 107 871 100 1708

Quartz DioriteWOIQD 5.8 72 2.2 18 17 3.2 76 23 271 91 378 83 759 108 10636 71! 5.3 46 1.6 10 12 1.3 68 20 236 74 290 58 473 54 668

Table 8.2: Mean rare earth element concentrations (ppm) for zircons from studies sam-ples.

139

8 Appendices

0.1!

1.0!

10.0!

100.0!

1000.0!

10000.0!

100000.0!

0.1!

1.0!

10.0!

100.0!

1000.0!

10000.0!

100000.0!

0.1!

1.0!

10.0!

100.0!

1000.0!

10000.0!

100000.0!

0.1!

1.0!

10.0!

100.0!

1000.0!

10000.0!

100000.0!

!"# $%# &'# ()# *+# ,-# .)# /0# 12# 34# ,'# /+# 50# !-#

Basal Lower Unit!

Lower Unit!

Upper Unit!

Quartz diorite!

Figure 8.2: Chondrite normalised rare earth element profiles for zircons analysed in thisstudy. Chondrite values are from (McDonough and Sun, 1995). Zircons from each unitdisplay a wide range of REE concentrations and magnitudes of Ce and Eu anomalies.The Lower Unit zircons with small to no Eu anomalies are from the top of this unit andhave similar morphologies and zonation patterns to other Lower Unit zircons. The threeQD zircons that have no Eu anomalies show signs of alteration such as spongy texturesand no zonation in CL images.

140

8.0.2 Supplementary material to Chapter 7

141

8 Appendices

Offset

Sample

IDSulph.

206P

b/204P

b2σ

207P

b/204P

b2σ

208P

b/204P

b2σ

208P

b/206P

b2σ

207P

b/204P

bm

2σ

WO

JD07SIC

19Apo

21.3700.003

16.1760.003

44.7660.013

2.0950.000

15.5080.005

WO

JD07SIC

0719Bcpy

19.3300.003

15.9020.003

40.7790.012

2.1100.000

15.4650.005

WO

JD07SIC

0720po

18.6700.003

15.8270.003

39.3670.011

2.1090.000

15.4650.004

WO

JD07SIC

0721Acpy

18.3710.003

15.7700.003

39.5310.011

2.1520.000

15.4420.004

WO

JD07SIC

0721BP

o19.872

0.00315.951

0.00342.176

0.0122.122

0.00015.452

0.005W

OJD

07SIC0722A

Po

19.3640.003

15.8600.003

41.7340.012

2.1550.000

15.4190.005

WO

JD07SIC

0722Bcpy

16.9900.003

15.6010.003

37.7220.011

2.2200.001

15.4280.004

WO

JD07SIC

0723m

ix22.000

0.00316.238

0.00344.059

0.0132.003

0.00015.499

0.005W

OJD

07SIC0727

po19.442

0.00315.859

0.00343.324

0.0122.228

0.00115.409

0.005W

OR

X182907

po19.091

0.00315.873

0.00339.321

0.0112.060

0.00015.463

0.005W

OW

OIQ

D1

po24.784

0.00416.537

0.00446.435

0.0131.874

0.00015.483

0.005W

OW

OIQ

D2A

cpy19.720

0.00315.944

0.00340.110

0.0112.034

0.00015.463

0.005W

OW

OIQ

D2B

po20.600

0.00316.029

0.00341.315

0.0122.006

0.00015.448

0.005W

OW

OM

S1po

19.6300.003

15.9370.003

39.7060.011

2.0230.000

15.4660.005

CC

OJD

09SIC01

mix

16.7320.003

15.5530.003

36.9340.011

2.2070.001

15.4090.006

CC

OJD

09SIC02

mix

16.0080.003

15.4410.003

35.9170.010

2.2440.001

15.3800.005

CC

OJD

09SIC03

mix

16.6840.003

15.5180.003

36.7630.011

2.2030.001

15.3800.004

CC

OJD

09SIC04

mix

15.7100.002

15.3880.003

35.6350.010

2.2680.001

15.3600.005

CC

OJD

09SIC05

mix

15.7970.002

15.3790.003

35.8430.010

2.2690.001

15.3410.003

CC

OJD

09SIC06

mix

15.8390.003

15.3710.003

36.1470.010

2.2820.001

15.3290.004

CC

OJD

09SIC07

mix

16.2470.003

15.4520.003

36.4160.010

2.2410.001

15.3640.003

CC

O865O

Bm

ix17.274

0.00315.585

0.00337.612

0.0112.177

0.00115.381

0.005

Table

8.3:P

bisotope

datafor

sulphideseperates

frominclusion

bearingquartz

dioritephases

oftheW

orthington(W

O)

andC

opperC

liff(C

CO

)O

ffsetD

ykes.T

heanalysed

sulphide(Sulph.)

phasesare

specified.M

odelinitial

Pb

isotoperatios

(207P

b/204P

bm

)w

erecalculated

following

them

ethodof

Darling

etal.

(2010)

142

jd_thesis

Documents

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melt sheet stratigraphy

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