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Geothermobarometry and PetrographicInterpretations of Christensen Ranch Meta-Banded Iron Formation from the Ruby Range,MontanaJacob HughesWestern Kentucky University, jacob.hughes163@topper.wku.edu

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Recommended CitationHughes, Jacob, "Geothermobarometry and Petrographic Interpretations of Christensen Ranch Meta- Banded Iron Formation from theRuby Range, Montana" (2015). Honors College Capstone Experience/Thesis Projects. Paper 580.http://digitalcommons.wku.edu/stu_hon_theses/580

GEOTHERMOBAROMETRY AND PETROGRAPHIC INTERPRETATIONS OF

CHRISTENSEN RANCH META- BANDED IRON FORMATION FROM THE RUBY

RANGE, MONTANA

A Capstone Experience/Thesis Project

Presented in Partial Fulfillment of the Requirements for

the Bachelor of Science Degree with

Honors College Graduate Distinction at Western Kentucky University

By

Jacob Hughes

*****

Western Kentucky University

2015

CE/T Committee: Approved by

Professor Andrew Wulff Ph. D, Advisor

Professor Fredrick Siewers Ph. D ________________________

Professor Clay Motley Ph. D Advisor

Department of Geography &

Geology

Copyright by

Jacob Hughes

2015

ii

ABSTRACT

The Ruby Range in southwestern Montana was tectonically uplifted during the

mountain-building event of the Big Sky orogeny. Contained within the Christensen

Ranch Formation, made up of sedimentary units metamorphosed during the Big Sky

orogeny, are a small number of metamorphosed banded iron formations (BIFs). It is the

presence and composition of these BIFs which is the focus of this research, principally

their depositional origin, relationship to surrounding sedimentary packages, elemental

and mineral compositions, and lastly, peak metamorphic conditions. Samples were

collected, cut into thin sections, and powdered for textural and compositional analyses

employing polarized light microscopy, Raman microscopy, scanning electron

microscopy, X-ray diffraction and electron microprobe analyses. Mineral phase

assemblages and compositional data suggest differences in protoliths between Stone

Creek and Iron mine metamorphosed BIFs in addition to greater activity of fluid phases

during the metamorphism of Iron Mine formations.

Keywords: Geology, Montana, Ruby Range, Big Sky orogeny, Metamorphosed Banded

Iron Formation, Geothermobarometry

iii

ACKNOWLEDGMENTS

The author would like to thank the Keck Geology Consortium for the incredible

opportunity, Julie Baldwin (University of Montana) and Tekla Harms (Amherst College)

for their guidance and patience throughout the course of the project, the other members of

the “Ruby Rangers” for their help and camaraderie, Andrew Wulff (Western Kentucky

University) for his tremendous instruction and continued support, as well as John

Andersland (Western Kentucky University) and Dave Moecher (University of Kentucky)

for their expertise and kindness.

iv

VITA

2015…………………………………….. Bachelor of Science, Western Kentucky

University, Bowling Green, Kentucky

2014…………………………………….. Research Experience for Undergraduates, Keck

Geology Consortium

2010…………………………………….. Greenwood High School, Bowling Green,

Kentucky

PUBLICATIONS AND PRESENTATIONS

Hughes, J.M., Wulff, A.H., 2015 Geothermobarometry and Petrographic Interpretations

of Christensen ranch Meta- Banded Iron Formation from the Ruby Range,

Montana: Keck Geology Consortium Spring Symposium Journal.

Hughes, J.M., Wulff, A.H., 2015 Geothermobarometry and Petrographic Interpretations

of Christensen ranch Meta- Banded Iron Formation from the Ruby Range,

Montana: WKU Student Research Conference, Bowling Green, KY,

Hughes, J.M., Wulff, A.H., Garmon, W. T., Higgins, B., 2014, A Comparison of

Kentucky Mississippi Valley Type Deposits: Geological Society of America

Annual Meeting, Vancouver, BC,

Hughes, J.M., Wulff, A.H., Garmon, W. T., Higgins, B., 2014, A Comparison of

Kentucky Mississippi Valley Type Deposits: WKU Student Research Conference,

Bowling Green, KY,

FIELD OF STUDY

Major Field: Geology

v

TABLE OF CONTENTS

Page

Abstract…………………………………………………………………………………... ii

Acknowledgments………………………………………………………………………. iii

Vita………………………………………………………………………………………. iv

List of Figures…………………………………………………………………………… vi

List of Tables………………………………………………………………………….... vii

Chapters:

1. Introduction……………………………………………………………………….. 1

2. Literature Review…………………………………………………………………. 6

3. Methods………………………………………………………………………….. 15

4. Results…………………………………………………………………………… 20

5. Conclusion………………………………………………………………………. 32

References………………………………………………………………………………. 35

vi

LIST OF FIGURES

Figure Page

Figure 1. Map of the Ruby Range and Surrounding Ranges…………………………….. 2

Figure 2. MMT and BBMT within the Wyoming Craton……………………………….. 7

Figure 3. Tectonic Evolution Model for the MMT……………………………………... 10

Figure 4. General Stratigraphy and Structure of the Ruby Range……………………… 12

Figure 5. Reference map of sample 14-JH-14………………………………………….. 18

Figure 6. SC BIF field photo…………………………………………………………… 21

Figure 7. Actinolite exsolution/intergrowth lamellae…………………………………... 23

Figure 8. Grunerite exsolution/intergrowth lamellae…………………………………… 23

Figure 9. SEM traverse of amphibole exsolution from sample 14-JH-6……………….. 24

Figure 10. SEM back-scatter image of sample 14-JH-6………………………………... 24

Figure 11. IM BIF field photo #1……………………………………………………….. 27

Figure 12. IM BIF field photo #2……………………………………………………….. 28

Figure 13. Photomicrographs of representative mineral species……………………….. 30

Figure 14. EDS spectrum from 14-JH-6 magnetite…………………………………….. 31

Figure 15. EDS spectrum from 14-JH-13d pyroxene…………………………………... 31

Figure 16. EDS spectrum from 14-JH-18b garnet……………………………………… 31

vii

LIST OF TABLES

Table Page

Table 1. XRD results of select samples..……………………………………………….. 25

Table 2. Estimated modal percents from sample thin sections.………………………… 30

1

CHAPTER 1

INTRODUCTION

Iron formation is defined as: a chemical sediment, typically thin bedded or

laminated, whose principal chemical characteristic is an anomalously high content of

iron, commonly but not necessarily containing layers of chert (Klein, 2005; James, 1954;

Trendall, 1983). The metamorphosed Banded Iron Formations (BIFs) found in the Ruby

Range and the surrounding Tobacco Root and Highland Mountains account for a

volumetrically small portion of a metasupracrustal (metamorphosed rocks deposited

directly on basement rocks) package (herein referred to as the Christensen Ranch suite) in

the Ruby Range. The presence and composition of these BIFs is the focus of this

research, principally their depositional origin, relationship to surrounding sedimentary

packages, elemental and mineral compositions, and lastly, peak metamorphic conditions.

Located in southwestern Montana near the town of Dillon (Figure 1), the Ruby

Range was formed by a mountain-building event in the Proterozoic which has been

termed the Big Sky or Trans-Montana orogeny (Brady et al., 2004; Cheney et al., 2004;

Alcock and Muller, 2012; Harms, et al., 2004; Roberts et al., 2002). The Ruby Range and

surrounding ranges comprise the Montana Metasedimentary Terrane (MMT) in the

northwestern portion of the Wyoming Province (Mogk et al., 1992, Mueller et al., 1993)

and are associated with Proterozoic orogenic activity. Farther to the east is the Beartooth-

Bighorn Magmatic Terrane (BBMT) believed to be arc-granitoids intruded

2

Figure 1. Map of the Ruby Range and Surrounding Ranges. Taken from Immega and

Klein, 1976.

3

into Archean crust (Roberts et al., 2002; Wooden and Muller, 1988). Three

metasedimentary units have been identified within the MMT (James, 1990). The criteria

for this categorization is largely predicated upon the presence of marble and iron

formation, as exhibited by the younger Christensen Ranch suite, or the absence thereof,

identified in regional basement as the Pre-Cherry Creek suite (Alcock and Muller, 2012).

The third group found between the two is the Dillon Gneiss which, while poorly

characterized, is known to include abundant biotite gneiss, quartzofeldspathic gneiss,

metagranite, as well as localized marble and amphibolite (James, 1990). These general

unit definitions were established based on structural relationships in the Madison and

Gravelly ranges where they were first recorded (Alcock and Muller, 2012; Erslev, 1983;

Erslev and Sutter, 1990; Hadley, 1969), but have been adapted and correlated throughout

the region.

Metamorphosed Banded Iron Formations (meta-BIFs) in the region are found

interspersed throughout inferred metasupracrustal sequences containing amphibolite,

metapelite, calc-silicate, marble, and quartzite. This Christensen Ranch metasupracrustal

suite has been identified as a part of the Cherry Creek sequence (Dahl, 1979; Garihan,

1973; James, 1990; Karasevich et al., 1981), while the Pony Group of the Tobacco Root

Mountains is roughly correlative with the Pre-Cherry Creek Group in the Ruby Range

(Wilson and Hyndman, 1990). The exact paleo-environmental setting has been contested

within the literature for some time. Disagreement between an accretionary wedge, fore-

arc depositional model and a passive margin, back-arc or intra-arc basin model has fueled

debate on the subject (Wilson and Hyndman, 1990), though the depositional and tectonic

history of southwestern Montana is likely even more complex.

4

Harms et al. (2004) present a cogent model for the region’s tectonic evolution on

the of basis exhaustive field and analytical work involving samples from the near-by

Tobacco Root Mountains. They describe the tectonic evolution as the product of partial

cratonic break-up followed by two successive mountain building events separated by

approximately 700 million years. The evidence put forward in support of this model is

some of the most compelling to date, encouraging the adoption of this early back-arc or

supra-arc deposition followed by a later arc collision model for the purposes of this

research (See Harms et al., 2004).

Previous work in the region (Immega and Klein, 1976) found iron formation

metamorphism ranging from 650-750°C and 4-6 kbar in the Tobacco Root Mountains,

temperatures from 500-600°C in the Carter Creek area of the Ruby Range and <400°C

and 2-4 kbar in the Ruby Creek area of the Gravelly Range. Additional work involving

samples taken predominately from the Kelly and Carter Creek iron formations of the

Ruby Range indicate that peak metamorphic conditions were 7.2 +/- 1.2 kbar, 745 +/-

50°C and 6.2 +/- 1.2 kbar, 675 +/- 45°C, respectively (Dahl, 1980).

This thesis is the product of fieldwork conducted during the summer of 2014 as

part of a Keck Geology Consortium Research Experience for Undergraduates. Samples

were collected from various meta-BIF locales within the Ruby Range for analysis using

polarized light microscopy (PLM), Raman microscopy, X-ray diffraction, (XRD),

scanning electron microscopy (SEM), and electron microprobe (EMP). Results from field

observations and various analytical methodologies named above have been collected with

the principal goals of textural and mineralogical characterization as well as constraining

peak metamorphic conditions experienced by meta-BIFs observed in both the previously

5

unstudied Stone Creek and well-documented Iron Mine (often referred to as “Carter

Creek”) localities within the Christensen Ranch. The data and conclusions presented in

this research aim to corroborate and augment existing work conducted on

metamorphosed iron formations in the region. The Ruby Range and the Big Sky orogeny

are of particular tectonic interest because the subsequent uplift of basement rocks in the

region offers a unique glimpse into the processes at work behind the amalgamation of

what went on to become the North American craton, the basement of the continent. The

area in and around the Ruby Range serves as an excellent natural laboratory for

modelling Archean and Proterozoic tectonism while further elucidating the conditions of

the Big Sky orogeny for the benefit of a greater scientific understanding of regional and

meso-scale metamorphism.

6

CHAPTER 2

LITERATURE REVIEW

Geologic Setting

The Ruby Range owes its name to localized occurrences of alluvial garnets

mistaken by early prospectors as rubies (Sisson, 2007). The Ruby Range and surrounding

Tobacco Root, Gravelly, Highland, Madison, and Greenhorn Mountains of southwestern

Montana lie in a region known as the Montana Metasedimentary Terrane (MMT). The

MMT is located on the northwestern flank of the Wyoming Province, an Archean craton

stabilized approximately 2.6-2.7 Ga (Harms et al., 2004, Houston et al., 1993; Frost and

Frost, 1993; Frost, 1993; Dutch and Nielson, 1990; Hoffman, 1988). The Beartooth-

Bighorn Magmatic Terrane (BBMT), named after the tonalite–trondhjemite–granodiorite

assemblages exposed in the Beartooth and Bighorn Mountains, lies to the east of the

MMT toward the center of the of the craton (Roberts et al., 2002) and contains zircons

dated to 2.9 Ga and greater (Foster et al., 2006). These spatial relations are shown in

Figure 2. The Wyoming Province underlies parts of Wyoming and Montana and is bound

by the Great Falls tectonic zone to the northwest (O’Neill and Lopez, 1985), the

Medicine Hat block to the north (Roberts et al., 2002; Harms et al., 2004), and the

Archean Wyoming Greenstone province (Hoffman, 1988; Sisson, 2007) to the south. The

western boundary is poorly defined due to Laramide and Tertiary events (Sisson, 2007).

7

Figure 2. MMT and BBMT within the Wyoming Craton. Taken from Harms, et al., 2004.

8

Giletti (1966) collected samples throughout the region for whole rock, biotite,

muscovite and K-feldspar K-Ar age-dating (Roberts et al., 2002) and arrived at the

conclusion that there were two distinct domains present within the MMT: one in which

no age greater than 1.8 Ga was identified and the other wherein ages were found to be

consistent with established Archean (approximately 2.6 Ga) ages for the area. Giletti’s

work shed new light on the uplift of the MMT which was previously thought to have

been uplifted solely by an identified 2.45 Ga orogenic event known as the Tendoy or

Beaverhead orogeny (Cheney et al., 2004; Baldwin et al., 2014; Jones, 2008). The two

domains are separated by a northeast-trending line now referred to as “Giletti’s Line”

(Harms et al., 2004). This geochronological discrepancy was interpreted to be the result

of a previously unrecognized tectono-thermal event characteristic of an orogeny which

reset K-Ar “clocks” on one side of the line, but left a hypothesized foreland to the

southeast unaffected (Erslev and Sutter, 1990; O’Neill, 1998; Harms et al., 2004). The

MMT is now interpreted to represent two separate orogenic events: the older

Tendoy/Beaverhead orogeny and the younger Big Sky or Trans-Montana orogeny (Brady

et al., 2004; Cheney et al., 2004; Alcock and Muller, 2012; Roberts et al., 2002) separated

by approximately 600 to 700 million years.

Wilson and Hyndman (1990) favor a fore-arc accretionary wedge model to

explain the tectonic setting of the Montana Archean assemblage. They point to the

Mesozoic Franciscan Formation of California as an analog for the assemblages observed

within southwestern Montana. The Franciscan Formation, characterized as a large marine

accretionary wedge, is comprised of sediments which are compositionally and

volumetrically comparable to similar rocks exposed within the MMT. Wilson and

9

Hyndman (1990) detail the lithologies of the Franciscan Formation as dominantly

greywackes with various pillow basalts, cherty limestones, and black shale. They contend

that the relative uniformity of lithologic and structural features, as well as the notable

absence of ophiolitic sequences of the MMT basement cannot be the product of a back-

arc setting. They posit that thick deposits of terrestrial clastics, pelagic sediments and

marine turbidites characteristic of a fore-arc setting conform well to the observations

made concerning the MMT. The discrepancy between the typical blue-schist grade

metamorphism common to an accretionary wedge setting and the upper-amphibolite

grade metamorphism of southwestern Montana in this model is attributed to higher

thermal gradients during the Archean. While Wilson and Hyndman’s proposal offers

unique insight into disentangling vexing paleo-environmental questions, it falls short in

explaining the multiple tectonic episodes experienced by the region and the events

leading up to them.

In an attempt to reconcile the complexities of the paleo-environmental setting and

tectonics of the northwestern Wyoming Province, Harms and colleagues (2004) have

proposed a new model for the tectonic history of the region (Figure 3). An initial paleo-

environmental setting of inferred Pre-Cherry Creek units is postulated as a back-arc or

supra-arc basin within the Wyoming Province from which arc protoliths are generated

from 3.55 to 3.2 Ga and deposited between 3.13 and 2.85 Ga. After this phase the

Tendoy/Beaverhead orogeny occurs during a collisional event at approximately 2.54 Ga.

Rifting to the northwest at around 2.06 Ga causes the subsequent injection of

metamorphosed mafic dikes and sills (MMDS). The resultant ocean basin was later

10

Figure 3. Tectonic Evolution Model for the MMT. Model proposed based on

observations and data from the Tobacco Root Mountains taken from Harms, et

al., 2004.

11

closed between 2.0 to 1.8 Ga. Arc sediments from a terrane exotic to the Wyoming

Province were then accreted to the craton between 1.78 and 1.72 Ga. Harms et al. (2004)

substantiate their claims by citing extensive zircon and monazite age dating (Cheney et

al., 2004), to accurately constrain the timing of events, REE profiles of MMDS samples

consistent with modern continental rifting (Brady et al., 2004), and thorough examination

of unit contacts and structural features.

Although structural complexity, discontinuous and poor exposure, and locally

defined lithologies (Wilson and Hyndman, 1990) have hampered stratigraphic correlation

within the ranges of the MMT, three general subdivisions have been widely accepted

within the literature although some contentions have been raised about established

naming conventions (see Wilson and Hyndman, 1990; James, 1990). These are, moving

up stratigraphic sequence, the Pre-Cherry Creek Group, the Dillon Gneiss, and the

Christensen Ranch suite (James, 1990) all of which comprise three northeast trending

belts (Figure 4) within the Ruby Range (Sisson, 2007). The Pre-Cherry Creek suite

consists largely of various gneisses and schists interlayered with minor quartzite (James,

1990) and a lack of marble (Alcock and Muller, 2012). Dillon Gneiss is comprised of

biotite and quartzofeldspathic gneiss, metagranite, and localized occurrences of

amphibolite and marble (James, 1990). James (1990) also notes that characterization of

the Dillon Gneiss is lacking to refine a unit definition. James also asserts that a

sedimentary or volcanic origin is, as of yet, unclear. Lastly, the Christensen Ranch suite

contains interbedded metapelites, quartzites, amphibolites, meta-BIFs, marbles, calc-

silicates and small ultramafic pods.

12

Figure 4. General Stratigraphy and Structure of the Ruby Range. Taken from Harms

(Proc. Keck Symp., 2015).

13

Extensive characterization and analysis of meta-BIF in the region has been carried

out by the likes of Immega and Klein (1976) and Dahl (1980). Immega and Klein (1976)

describe typical iron formation occurrences throughout the Tobacco Root, Gravelly, and

Ruby Ranges as thin, discontinuous, banded, lenticular bodies. Their work was derived

from samples collected during the summer of 1973 from which over 70 thin sections

were created for extensive petrographic characterization, as well as microprobe, and bulk

chemistry analyses. Samples were taken near Copper Mountain in the Tobacco Root

Mountains, from within the Ruby Creek area of the Gravelly Range and from the Carter

Creek area in the Ruby Range. Electron Microprobe data combined with computer

modelling of Fe2+

/Fe3+

in amphiboles, pyroxenes, and garnets, as well various

geothermobarometers (see Immega and Klein, 1976 for specific models used) were used

to arrive at peak pressure and temperature conditions. Through these methods they report

that likely metamorphic conditions were approximately 650-750°C and 4-6 kbar for

Tobacco Root BIFs, temperatures of about 600-650°C in the Carter Creek, Ruby Range

BIFs, and around 400°C and 3-4 kbar in the Ruby Creek area of the Gravelly Range.

Immega and Klein cite the potential effects of PH2O on calculated temperature differences

between the Tobacco and Ruby Ranges.

Dahl (1980) examined six metamorphic lithologies including meta-BIFs from two

locales within the Ruby Range: the Kelly region in the northeastern part of the range as

well as samples taken from the aforementioned Carter Creek locality. Dahl’s work was

centered on the distribution of iron and magnesium between garnet and pyroxene pairs.

Electron microprobe data from 13 mineral pairs were collected. This compositional data

was combined with known distribution coefficients of iron and magnesium into various

14

mineral structures, often expressed as an elemental KD, for calculations based on

established garnet-clinopyroxene geothermometers developed by Saxena (Saxena, 1976),

Ganguly (Ganguly, 1979) and others. Dahl reports that calculations predicated on these

geothermometers yield anomalously high metamorphic conditions for the study areas.

Refinement of existing work by Dahl’s incorporation of thermodynamic equations

designed to account for the effects of calcium and manganese substitution in garnets

yielded metamorphic conditions of 745 +/- 50°C, 7.2 +/- 1.2 kbar in the Kelly area and

675 +/- 45°C, 6.2 +/- 1.2 kbar for Carter Creek samples. Linear regression models served

to corroborate these new results and their use as a relative geothermometer that conforms

more closely to known KD temperature dependences. Dahl explains the importance of

larger data sets within the region and the need to account for the possible effects that

other elemental substitutions within pyroxenes may have on KD.

15

CHAPTER 3

METHODS

Sample Collection

Sample collection occurred during the summer of 2014 as part of a Keck Geology

Research Experience for Undergraduates (REU). The procedure for sampling involved

the division of the seven REU participants into working groups (typically 2 to 4 students)

supervised by one of two Keck project leaders. First, exploratory traverses were

conducted perpendicular to strike in order to locate units of interest as shown on local

quadrangle maps. Once target units were located, traverses were then conducted along

strike of units of interest and samples were collected from well-exposed areas and/or

areas characterized by textural or compositional differences. Sample positions were

recorded on a geologic map along with corresponding GPS coordinates for each sample.

Each sample was assigned a number and a sequential alphanumeric digit as necessary to

denote multiple samples collected from an individual locale. Detailed notes concerning

bed thickness in outcrop, textural observations, and adjacent stratigraphic units were

compiled for each sample. Oriented sampling was not conducted due to difficulties

induced by BIF magnetism. Sample collection was accomplished with chisels and

sledgehammers, which were used to field trim samples to remove heavily weathered

surfaces and moss. Edges were rounded before samples were wrapped with masking tape,

labelled, and placed into appropriately labelled Ziploc bags.

16

Sample Preparation

Upon returning to the University of Montana campus slabs were cut perpendicular to

foliation and perpendicular to any fold hinges. Select slabs were also cut parallel to

foliation. Once samples had been transported to Western Kentucky University slabs were

cut into billets and then ground using 400 grit followed by 600 grit to remove saw marks.

Slides were then frosted using 400 grit to better facilitate the epoxy hold between the

billet and the slide. A mix of three parts Buehler epoxy to one part Buehler epoxy

hardener was applied to affix the billet to the slide. The slab and slide were then left on a

hot plate overnight to cure. Once cured the slab was placed on a Microtek

Microsectioneer to cut the billet off of the slide leaving a thick section. Selected thin

sections were then made from samples of interest. Optical thin sections were prepared by

grinding down to a standardized 30 micron thickness, in addition to a number of “thick

sections” (thicknesses greater than 30 microns). These samples were then readied for

polarized light microscopy, scanning electron microscopy and electron microprobe

analyses as necessary. Thin sections were polished using Buehler .1 micron and .05

micron diamond paste on a lapidary wheel to produce an even polish.

Samples were readied for X-ray diffraction first by powdering sample using a ceramic

mortar and pestle and an acetone slurry. Once samples were sufficiently powdered the

sample powders were packed into sample disks for XRD analysis. High resolution scans

of each slide used for PLM, Raman, SEM, and EMP analyses were created using an

Epson scanner and Microsoft Silverlight photo editing software to produce useful thin

section reference maps. Zones or grains of interest identified using PLM or Raman

microscopy were noted accordingly on these reference maps to facilitate efficient

17

relocation for complimentary analyses. An example reference map produced from this

method is presented in Figure 5.

Raman microscopy was utilized to corroborate and augment PLM mineral

observations. This instrument employs a specialized optical microscope fitted with an

excitation laser and a number of filters designed to take advantage of the Raman effect

for mineral phase identification and quantification. The excitation of the molecules within

the sample produces characteristic frequency and pattern shifts in the scattered beam

which are unique to particular molecular species and the intensity is directly proportional

to the number of molecules in the path of the beam. The spectrum that is produced from

this process is compared to spectra in a user-generated RRUF database for identification

based on percent correlation. Inferences can be made from this data concerning not only

the identity of a given mineral phase but also its relative abundance.

Geothermobarometric assertions rely not only on the compositions of the mineral

phases in question, but also on its crystallographic structure and site occupancy.

Goldschmidt’s rules state that given ions of equal size and charge, there should be free

substitution in and out of appropriate structural sites in a given crystal lattice. In reality,

minute differences in atomic radii or charge will cause one ion to be more “preferred”

since its smaller size results in less distortion of the crystal lattice. Furthermore, different

mineral species possess a number of different structural sites, each with different criterion

for the acceptance of ionic species. The pyroxenes, for example, possess two dominant

cation accepting sites referred to as M1 and M2, as well as a tetrahedral (T) site into

which certain elements may substitute. Certain elements are known to only substitute into

particular sites, while others substitute between different sites as a consequence of site

18

Figure 5. Reference map of sample 14-JH-14.

19

occupancy, and/or pressure and temperature conditions, or because their atomic radius

allows them to satisfy radius ratio requirements for multiple structural sites. Therefore,

compositional data acquired from a microprobe is simply not enough to indicate likely

pressure and temperatures of formation. Elemental partitioning between structural sites

must be taken into account to produce more detailed chemical formulae depicting not

only elemental composition, but the locations of these elements within the crystal lattice.

20

CHAPTER 4

RESULTS

Stone Creek Iron Formations

Stone Creek (SC) meta-BIFs, while often marked on geologic maps of the area,

are in many cases only thin, laterally continuous beds typically less than one or two

meters in width. Where exposed, these iron-formations are commonly weathered to a dull

red-brown and accompanied by distinctive red soil with ant colonies in some places. In

outcrop SC meta-BIFs exhibit alternating magnetite/hematite and quartz bands ranging

from less than .5 cm to 2.5 cm in width. Banding is continuous, though quartz lenses are

not uncommon. Magnetite/hematite bands (containing additional minor silicate

mineralization) are dominant, constituting 55% to 80% of the rock by composition with

quartz largely responsible for the remainder. Figure 6 provides a field photo of SC BIF

for reference. Trendall and Blockley (1970) have suggested terminology for the scale of

banding in BIF. According to this scheme, banding in the SC meta-BIFs ranges from

mesobands (average thickness of less than one inch) to microbands (0.3-1.7 mm). Grain

size of the magnetite/hematite is typically sub-millimeter in scale. Non-quartz and non-

iron oxide mineralization is sparse (accounting for two percent or less of total

composition), and, where present, generally occurs between quartz and iron oxide bands.

21

Figure 6. SC BIF field photo. Rock hammer for scale. Photo taken during the summer of

2014.

22

Meta-BIF occurrences are interlayered with a number of different lithologies

including amphibolite, marble, and metapelite. Narrow (between .5 to 2 meter) sections

of SC meta-BIF are bounded on both sides by a single lithology (dominantly metapelite)

or formed a boundary between disparate lithologies (commonly between metapelite and

either marble or amphibolite). The complex folding and faulting of the region makes it

difficult to resolve with certainty whether the observed iron formations in the study area

are all stratigraphically distinct or if many are simply the expression of the same unit

(James, 1990).

Banding variations include highly folded, two-centimeter width monomineralic

bands, and sub-centimeter scale, planar bedding. SC meta-BIFs (samples 14-JH-1a to 14-

JH-10b as well as 14-JH-20) tend to exhibit more well-defined, thin bedded layers. These

samples exhibit generally equal modal percentages of both quartz as well as various

oxides such as magnetite and hematite (40% to 50%, respectively). PLM inspection also

showed 5% to 15% amphibole as well as up to 5% pyroxene within SC meta-BIF.

Pyroxenes identified employing Raman microscopy on SC samples 1c and 6 were found

to be diopside with minor augite and hedenbergite. Most amphiboles from SC sample

numbers 3B, 5, and 6, were identified as clinoamphiboles on the basis of polarized light

microscopy techniques, and vary from tremolite to actinolite. Some observed grains have

exsolution/intergrowth features (crystallization of two mineral phases) showing both

actinolite and grunerite (Figures 7 and 8). Figure 9 shows a SEM transect highlighting

this phenomenon and Figure 10 demonstrates its relative abundance. Sample 6 also

contains sodium-rich phases: katophorite and winchite. Table 1 presents various non-

quartz phases identified in XRD analyses of selected samples.

23

Figure 7. Actinolite exsolution/intergrowth lamellae. Crystal denoted by red cross-hair.

Figure 8. Grunerite exsolution/intergrowth lamellae. Crystal denoted by red cross-hair.

24

Figure 9. SEM traverse of amphibole lamellae from sample 14-JH-6.

Figure 10. SEM back-scatter image of sample 14-JH-6. Amphibole

exsolution/intergrowth prolific and marked by alternating light and dark layers.

Ca

Ca

Fe

Al

Ca

Intensity (CPS)

µm

25

XRD Results

Sample Mineral Formula

14-JH-1b petedunnite CaZnSi2O6

spinel Cu.6Zn.4Al2O4

diopside CaMgSi2O6

14-JH-4 ferriwinchite NaCaMg4Fe(Si8O22)(OH,F)2

cummingtonite (Fe2+

Mg)7Si8O22(OH)2

tremolite Ca2Mg5Si8O22(OH)2

magnetite Fe(Fe.04Ti.67)O4

ferripedrizite NaLi2(Fe3+

2Mg2Li)Si8O22(OH)2

14-JH-6 magnesioferrite MgFe3+

2O4

feriwinchite NaCaMg4Fe(Si8O22)(OH,F)2

cummingtonite (Fe2+

Mg)7Si8O22(OH)2

14-JH-13d diopside CaMgSi2O6

sodic-amphibole Na(Na1.97Mg.03)Mg3(Mg1.03Cr.97)Si8O22F2

14-JH-17a magnetite Fe.89Co.19(Fe1.02Co.94)O4

fluoro-edenite NaCa2Mg5(Si7Al)O22F2

hematite Fe2O3

14-JH-18b hematite Fe1.98H.06O3

fluor-boron edenite NaCa2Mg5Si7BO22F2

magnetite Fe.88Co.19(Fe.02Co.94)O4

Table 1. XRD results of select samples. All samples contain quartz (SiO2).

26

Iron Mine Iron Formations

Unlike its Stone Creek counterparts the meta-BIF of the Iron Mine (IM) is well-exposed

in almost continuous outcrop. Located near the abandoned Christensen Ranch homestead

off of Sweetwater Road the IM meta-BIF forms a prominent ridge adjacent to recent

unconsolidated sediments. IM meta-BIFs are often highly-folded, with a thickness in

outcrop greater than six meters, and form the crest of a ridge spanning over 100 meters.

Folded IM meta-BIFs feature thickened, parallel folded hinges coupled with elliptical

folds. The intense folding suggests that IM BIFs may have undergone higher grade

metamorphism than that of SC BIFs. The texture of the iron formation varies from wavy

bands to fine, well-laminated bands (up to .25 cm), discontinuous quartz lenses, locally

folded, quartz-rich bands, pyroxene-rich and quartz-poor bands, well-layered bands of

equal proportions quartz and other phases up to 1.5 cm thick, and poorly developed

fabrics with coarse grain sizes of .25 cm. Figures 11 and 12 showcase some of the highly

folded sections of IM meta-BIFs. This variability in texture parallels a pronounced

variability in composition as respective portions of the outcrop contain green, pyroxene-

rich bands up to .5 cm, abundant specular hematite, and garnet-rich zones. Grain-size is

typically fine to microscopic. Pyroxene and garnet phases are largely concentrated at the

contact between magnetite/hematite and quartz layers. Pyroxenes from sample 13d range

from diopside to hedenbergite, but are dominated by augite. Raman spectra of garnets

found in sample 18b are strong matches to andradite. Amphiboles in IM samples 13d, and

14 vary from tremolite to actinolite. Modal percentages of major mineral phases are also

highly variable with quartz ranging between 30% and 70%, oxides from 20% to 65%,

pyroxene from

27

Figure 11. IM BIF field photo #1. Rock hammer for scale. Photo taken during the

summer of 2014.

28

Figure 12. IM BIF field photo #2. Rock hammer for scale. Highly contorted meta-BIF

banding. Photo taken during the summer of 2014.

29

0% to 30%, amphibole from 0% to 15%, and garnet up to 10%. In addition to this

mineralogical variability, many of the observed IM thin sections contain pyroxene and

amphibole phases with distinct exsolution lamellae. Table 2 provides a more complete

representation of major mineral phases observed in select thin sections and Figure 13

provides photomicrographs taken of both SC and IM samples. Additionally SEM back-

scatter images of energy dispersive spectroscopy (EDS) analyses included in Figures 14,

15, and 16 were conducted to obtain additional compositional data from a variety of

mineral phases.

30

Table 2. Estimated modal percents from sample thin sections. Chart created from PLM

and Raman microscope observations

Figure 13. Photomicrographs of representative mineral species. A)

Exsolution/intergrowth lamellae showing alternating grunerite and actinolite in

sample 14-JH-6. B) Clinopyroxene layer cut parallel to banding in sample 14-

JH-13d. C) Discontinuous bands of andradite garnet in sample 14-JH-18b. D)

Cross-section of thickened fold hinge with disseminated alkali-amphiboles in

sample 14-JH-14. All images in plane polarized light.

31

Figure 14. EDS spectrum from 14-JH-6 magnetite.

Figure 15. EDS spectrum from 14-JH-13d pyroxene.

Figure 16. EDS spectrum from 14-JH-18b garnet.

32

CHAPTER 5

CONCLUSIONS

In the nearby Tobacco Root and Gravelly Mountains two-pyroxene and garnet-

pyroxene assemblages indicated metamorphic conditions of 650-700°C, 4-6 kbar and

<400°C , 2-4 kbar respectively (Immega and Klein, 1976). Peak pressure and temperature

constraints derived from Christensen Ranch metapelites by Hamelin (Proc. Keck Symp.,

2015) give temperatures of 700-800 °C and 7-9 kbar pressures. This agrees with other

temperatures calculated from other lithologies within the Christensen Ranch. Bland

(Proc. Keck Symp., 2015) found temperatures of 711-735 °C from Christensen Ranch

marble while Berg (Proc. Keck Symp., 2015) obtained temperatures and pressures of

680-710°C and 7.5-8.5 kbar from Christensen Ranch amphibolite. It seems likely,

therefore, that Christensen Ranch meta-BIFs were subjected to equivalent pressures and

temperatures. Forthcoming electron microprobe analyses and two-pyroxene

geothermobarometry techniques outlined by Saxena (1976) as well as Lindsley (1983)

should serve to constrain the temperatures of BIF metamorphism in the Ruby Range and

provide an important comparison.

X-ray diffraction and Raman microscope results suggest that the SC and IM meta-

BIFs represent two separate occurrences of iron formation within the Christensen Ranch

suite. This has been inferred on the basis of a number of relationships

33

including compositional and textural differences between SC and IM BIF as well as

stratigraphic relationships as noted in the field. Mineral assemblages within SC and IM

meta-BIF point to differences in protolith compositions. SC BIFs are well-layered and are

characterized by narrower bands than the irregular, folded bands present in the Iron Mine.

While this may be partially explained by layer thickening during deformation and

metamorphism, this same relationship may be observed in areas of IM BIFs that appear

to have experienced little to no folding suggesting that layer thicknesses of IM BIFs are a

primary feature. SC and IM BIFs are generally observed in contact with lithologies that

differ between the two locations. SC BIFs are commonly found adjacent to metapelites

and marbles whereas IM BIFs are generally in contact with metapelites and amphibolites.

This may also imply slightly different lithofacies.

The presence of specular hematite, various alkali and sodic-amphibole

mineralization, as well as andradite garnet within IM BIFs seems to argue for the

presence of a significant quantity of metamorphic water and/or decarbonation. This

agrees well with the postulate of relatively high PH2O within the Ruby Range put forward

by Immega and Klein (1976). Dewatering of hydrous phases from amphibolites and

pelites (amphiboles and micas) adjacent to IM BIFs or some other source of metamorphic

fluid may have been sufficient to mobilize sodium and calcium ions from nearby marble

lenses. This would greatly enhance ion diffusion between units during metamorphism and

permit the growth of andradite garnet and explain sodic and alkali amphibole

mineralization. An influx of metamorphic fluid during metamorphism of the IM region

may also partially explain the high degree of textural deformation (seemingly indicative

of higher-grade metamorphism). Introduction of volatiles such as metamorphic fluids and

34

CO2 can act to lower melting temperatures and provide a higher degree of deformation

and metamorphism. It is quite likely that SC BIFs were also exposed to metamorphic

fluids as well, given prolific sodic amphibole mineralization observed in samples

throughout the area, though the degree of saturation and/or the conditions of

metamorphism in the area can be assumed to be less.

FUTURE WORK

In conducting future analyses, careful attention must be paid to mineral

assemblages due to reactions occurring under set Eh-pH conditions as outlined by Klein

(Klein, 2005). Examples of such reactions include the formation of amphibole by the

reaction between iron-rich carbonates such as siderite and quartz during metamorphism

(Klein, 2005). Future work could also strive to assess the oxidation states of iron-rich

phases as a proxy for oxygen fugacity and the availability of metamorphic fluids to

provide empirical evidence to substantiate hypotheses made regarding the availability of

fluid during metamorphism of IM BIFs. Whole-rock geochemistry implementing X-ray

fluorescence can provide greater composition resolution and serve to better constrain

diffusion of trace elements, particularly Large Ion Lithophile Elements, during

metamorphism. This could then be used to see if the assumed differences in metamorphic

conditions between SC and IM BIFs can be substantiated on the grounds of differential

fluid saturation, pressures and temperatures, or both.

35

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