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7/27/2019 AG5 Deposit Models 2-To-page Colour

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Archean Au DepositsDeposit models

T. Campbell McCuaig

Changes in last decade Resurgence of multiple models for

mineralisation:

yngene c a era secre on arge e a . ,

Metamorphosed deposits in high T domains, not

crustal continuum (Phillips and Powell 2009; Tomkins

and Grundy 2009) (Hemlo, Big Bell, Griffins Find)

Magmatic fluid source (Hall et al 2001; Mueller et al

2008)

Mantle fluid source (Cleverley et al. 2008; Kendrick etal. 2008)

Fluid mixing (Neumayr et al. 2008)

How do we use these to target?


2/23

Steps in Exploration Targeting

Hronsky andGroves (2008)

Deposit models

Built from deposit-scale observations

Focus on type deposits

Carlin style Au

Kambalda style NiS

Bushveld style PGE

swa ersran s y e u Kuroko style VHMS

Olympic Dam style IOCG


3/23

Position of PGE

Reefs and Ni deposits

in layered intrusions

Ni mineralisation associated with

ultramafic rocks (komatiites)

Gresham and Loftus-Hills (1981);

Stone and Masterman (1998)


4/23

Deposit models

Often focus on specific aspects of ore genesis

Usually focussed on deposition (where we study them)

ocusse on spec c geosc ence su sc p nes o ten n

isolation!

Natural consequence of academic system specialist

research championed over generalist (Groves, 2009)

Structural geology

Geochemistry

P-T-X conditions (FLINC, phase equilibria, mineral

thermometers, isotope fractionation, chemical process

modelling)

Fluid tracing (stable and radiogenic isotopes, with above)

Geochronology Alteration/gangue mineralogy/petrology

Igneous/metamorphic petrology

Conflicting and often mutually exclusive

interpretation

Alteration envelope around

oro enic old mineralisation

Orogenic Au - Structurally

Controlled

Distal alteration Proximal alteration Ore

Based on Colvine et al. (1988)Based on Colvine et al. (1988)Pasi Eilu 2001

Au:Ag>5;

As,+/- Sb, Te, W, Mo, Hg, W; generally low base metal abundances


5/23

Orogenic Au - control of host rock

and temperatureVariation in alteration mineralogy with host

rock and temperature

Cassidy, 1992

Deposit models More comprehensive syntheses encompass

multiple deposit styles

g su p at on versus ow su p at on ep t erma

Epithermal-Porphyry transition

Continuum model for Orogenic Au

Unconformity uranium

SEDEX-MVT

Komatiite-hosted NiS

styles

Can be excellent summaries of deposit scale

processes


6/23

Epithermal Au

Corbett 2004

Porphyry to Epithermal Cu-AuDeposit Model

Corbett 2004


7/23

Archaean Orogenic Gold Deposit

Model - Crustal Continuum for Au

Timing late tectonic= ca. 2640-2630 Ma in

Yilgarn

Fluid Sources

metamorphic-

dominated

Groves et al.,1993

Deposit models - limitations Often focus on one aspect of system, not holistic

Often too many variations on a theme for

practical application

IOCG

Porphyry model variants

Uranium 14 models, 22 submodels

Struggle to be predictive

Where predictive = local scale to terrane scale

Finds analogues of what you have already found

Show that giant deposits and small showings

often have similar fluids and deposit scale

features (Groves, 2009)

E.g. fertile magmas hard to differentiate from infertile

on deposit scale (Cooke et al, 2009)


8/23

Deposit models struggle to be

predictive

We have a much better (albeit very incomplete)

mineralisation than 40 years ago

So our targeting must be more effective, yes?

No

Find new deposits in brownfields, but struggle to find

new ore systems in new terranes or greenfields

terranes

DEPOSIT MODELS ARE AT WRONG SCALE

for regional exploration decisions!

McKeith (2009)


9/23

Prediction-detection tradeoff

HIGH Camp scale

decision

PREDICTIONRELATIVE

FECTIVENESS DETECTION

after McCuaig and Hronsky 2000

LOW

BROAD REGIONAL PROSPECT SCALE

SCALE

E

Models = belief systems

Timing late tectonic

= ca. 2640-2630 Ma in

Yilgarn

Fluid Sources

metamorphic-

dominated

Groves et al.,

1993


10/23

But, its hard to find what you are not

looking for

picture of

nine

dolphins

Use of Models

Exploration Process

at appropriate scales

generate targets

prioritize and rank targets

evaluate targets

EVERYONE does this whether they th ink

they do or not!


11/23

Yilgarn Example

Robert et al., 2006

Robert et al., 2005

Dolerite as an exploration

criteria in Yilgarn Au

Brown, 2002


12/23

Dolerite model good, but

Excludes majordeposits - would not

~Au), Sunrise Dam (~10Moz Au) and KanownaBelle (~5 Moz Au) +many others

Terrane Specific -would not be as

Robert et al., 2005

success u n ot erArchaean Cratons

Boddington Deposit a case of domestic

blindness

Robert et al., 2006

Perth

Boddington


13/23

The Boddington Gold Mine

>26Moz Au (pastproduction + resource)

>800Kt Cu

appreciable amounts of

Mo

Wandoo deposit

(basement resource) is

the largest undeveloped

Au resource in Australia

The Boddington Gold Mine -previous models Archaean porphyry Au-Cu-Mo deposit

synchronous with intrusion of host diorites

pre-metamorphic

Roth, 1992 - unpublished PhD.

Based on alteration and geochemical studies

Au postdates host intrusions by 30-80Ma

associated with regional transcurrent shearing event and brittle

faulting

post-pea metamorp c Allibone et al., 1998

based on limited geochronology and structural paragenesis


14/23

Geology

Hotham

metasediments,

metavolcanics

Wells Formation

dacite-andesite-

diorites

Marradong

Formation basalt

and dolerite. 2Km

McCuaig et al. (2001)

Intrusives Eastern Diorite

ca. 2675 Ma

Wouhraming

Monzogranite

ca. 2612 Ma

BGM diorite

suite ca. 2700

2Km

Ma Several

generations of

post-Archaean

dolerite intrusion McCuaig et al. (2001)


15/23

Size of Hydrothermal System

2 km

Mineralized AreaMineralized Area

Open pit minesOpen pit minesAuAu--CuCu--MoMo--BiBi100*100m grid @ BOX


Stage I: quartz-sulphide veins

2707+/2707+/--17 Ma17 Ma (Re(Re--Os)Os)

Diorite SuiteDiorite Suite

low metal values,low metal values,

dominantly Modominantly Mo

Contain AuContain Au

McCuaig et al. (2001); Stein et al (2001)


16/23

Post regional cleavage: Quartz-albite-sulphide veins

Complex mineralogy

postdate regional cleavage

locall stron l deformed near D4

shear zones

Dominant control on molybdenite

distribution in the deposit

crosscut D3 shear zones and 2675

Ma host rocks

2616+/2616+/--12 Ma12 Ma (Re(Re--Os)Os) McCuaig et al. (2001); Stein et al (2001)

Stage II: Clinozoisite-sulphide-quartz

veins and actinoli te-sulphide veins

(1) associated with late brittle-ductilefaults

(2) Cz-sx - control the bulk of low-grade Au-Cu mineralisation

(3) Act-Sx control high grade Au

(4) Cut regional foliation



17/23

Roth, 1992

Allibone et

al., 1988

McCuaig et

al., 2001


Camp-Scale Metal Zonation

Late monzograniteLate monzogranite

at surfaceat surface

2 km 2 km

PbPb--ZnZn AuAu--CuCu--MoMo--BiBi



18/23

P-T-X Hydrothermal Fluids

Exsolution of volatile phase from

magma (>600oC, 60 eq. wt%

NaCl, 0.6 to 0.15 kbars )

Cooling and transient de-

pressurisation; boiling over large

P-T range (300-550oC, 0.16 to0.15 kbars) synchronous with Au-

Cu-Mo mineralization

Incursion of surface waters (150-

300oC,


19/23

LateLate MonzogranitesMonzogranites::

Oxidized, fractionatedOxidized, fractionated

Overlap with IntrusionOverlap with Intrusion--related suites in Easternrelated suites in Eastern

AustraliaAustralia (e.g.,(e.g., TimbarraTimbarra,,

KidstonKidston)) ((BlevinBlevin))

BoddingtonBoddington: Host Rocks: Host Rocks

IntrusionIntrusion--relatedrelated(Blevin)(Blevin)

Porphyry CuPorphyry Cu--AuAu

Fe2O3/FeO

Early Diorites:Early Diorites:

Less oxidized, lessLess oxidized, lessfractionatedfractionated

Overlap with AuOverlap with Au--BiBi--typetype

IntrusionIntrusion--related suites o frelated suites of

Alaska/ Yukon (Bak er)Alaska/ Yukon (Bak er)

Cu-Au

Rb/Sr

AuAu--BiBi (Baker)(Baker)


Laser ICP-MS analyses

Melt Inclusions late monzogranite

Melt inclusion

20 microns

4000

5000

6000

7000

8000

9000

10000Cu65

Zn66

Mo98

Ag109

W184

Au197

Pb208

--

CuCuZnZn

PbPbUU

0

1000

2000

3000

0.25 4.65 9.04 13.4 17.8 22.2 26.6 31 35.4 39.8 44.2 48.6 53 57.4 61.7 66.1 70.5 74.9 79.3 83.7 88.1 92.5 96.9 101 106 110 114 119

U238

AuAu

Hagemann et al. (2007)


20/23


Boddington Summary

Boddington is a result of two superimposed

ma matic h drothermal s s tems

ca. 2700 Ma, synchronous with emplacement of

host d iorites (Stage I: ?Au-Bi intrusion-related?

or porphyry Cu-Au MINOR)

ca. 2612 Ma, coeval with intraplate monzogranites

(Stage II: intrusion-related style MAJOR)

Kilometre-scale metal footprint

Melt inclusions of 2612 Ma monzogranites contain

Au-Cu-Mo and Pb-Zn


21/23

Analogous

deposits

elsewhere inArchean?

Superior

Province?

Perth

Boddington

So we know many styles possible!Therefore, how do we overcome limitations of models?

Goldfarb et al. (2005)


22/23

Deposit model versus Mineral

System approaches Deposit models based on finding geological

mineralisation in known deposits E.g. Differentiated dolerite in Yilgarn

Mineral Systems based on finding evidence forinterpreted critical mineralising processes E.g. Olympic Dam

Olympic Dam a mineral system

approachWMC model focused on :

Underlying oxidized and altered basaltst at wou re ease u ource

Major lineaments that could have tappedcopper-bearing fluids (Pathway)

Magnetic and gravity highs potentiallyindicative of alteration and metalaccumulation (Physical throttle)

Reduced sedimentary rock packages(Scrubber)


23/23

Take home message

people often focus on differences between styles of

.

In exploration, you want to focus on the common

features between mineral systems!

Be permissive, allow for multiple possible models

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