Centre for Global Metallogeny School of Earth and Geographical Sciences The University of Western...

Centre for Global MetallogenySchool of Earth and Geographical Sciences

The University of Western Australia

A COMPARISON BETWEEN THE WITWATERSRAND

AND OROGENIC GOLD SYSTEMS: A TEST OF THE

HYDROTHERMAL WITWATERSRAND MODEL

David I. Groves*, Gavin England,Birger Rasmussen and Bryan Krapez

Gold Deposits >100t Au: Gold ProducedGold Deposits >100t Au: Gold Produced

Foster (2002)

OUTLINE OF TALK

• Statement of Conflicting Models

• Importance of Scale of Observation

• Current Knowledge of Origin of Critical Components (excl. Gold) Rounded Pyrite Uraninite and Bitumen

• Similarities and Contrasts with Orogenic Gold Deposits

• Other Confirmatory Evidence

• A Potential Compromise Model

• Exploration Implications

WITWATERSRAND : A MAJOR ENIGMA

SUPERGIANT GOLD DEPOSITS WITH EQUIVOCAL ORIGIN

• Palaeoplacer Model For All Components ?

• Hydrothermal Model For All Major Components ?

• Modified Palaeoplacer For All Components ?

• Hydrothermal Model For Gold : All Other Components Modified Palaeoplacer ?

MODELS APPEAR VERY DEPENDENT ON

SCALE OF OBSERVATION

TECTONIC TO BASIN SCALE

Phillips and Law (2000)

OUTCROP/FACE TO MACROSCOPICTO MICROSCOPIC SCALE

Photos courtesy of L. Minter and G.N. Phillips

Most gold now hydrothermal in textureBUT what about pyrite, uraninite and carbon (bitumen) ?

MICROSCOPIC SCALE

ROUNDED PYRITE : DETRITAL OR REPLACEMENT(?)

England et al. (2002)



c

b

d

a

ROUNDED PYRITE : DETRITAL OR REPLACEMENT(?)


c

b

d

a

Cross section through the Damang gold mine Flat-lying, gold-rich quartz veins in Damang pit wall

DAMANG GOLD MINE - ASHANTI BELT, GHANA

Example of sulphidised detrital iron oxides in Banket Sandstone/ Conglomerate

Similar host rocks to Witwatersrand

Pigois (2003)

DAMANG GOLD MINE, GHANA

quartz veinS

0

SulphidisedCross-beds

sample JP023, sandstone

sample JP070, sandstone

Pigois (2003)

ROUNDED PYRITE : SULPHUR-ISOTOPE EVIDENCE


b

c

DETRITAL MONAZITE AND SURROUNDING BITUMEN AFTER OIL

SANDSTONE FROM MESOZOIC PERTH BASIN, W.A.


c

b

d

a

DETRITAL? URANINITE AND SURROUNDING BITUMEN AFTER OIL

CONGLOMERATE FROM ARCHAEAN WITWATERSRAND BASIN


c

b

d

a

Phillips and Law (2000)McCarthy (1992)

COMPARISON WITH OROGENIC DEPOSITS (1)


WITWATERSRANDOROGENIC DEPOSITS


COMPARISON WITH OROGENIC DEPOSITS (2)

Phillips and Law (2000)McCarthy (1992)


Newton (2001)

From Ridley (2003)

Un

ion

Ree

fsM

t C

har

lott

e

Wit

wat

ersr

and

Wit

wat

ersr

and

SIMILARITIES:OROGENIC GOLD vs WITWATERSRAND

MetamorphicSetting

Both generally in greenschist facies rocks

MineralAssociation

Gold-pyrite-carbon association a common factor

Ore Textures Gold is late paragenetically and associated withcarbon and hydrothermal sulphides

CONTRASTS:OROGENIC GOLD vs WITWATERSRAND

Age Range

Orogenic gold all agesEconomic Witwatersrandstyle restricted to Archaean(Wits) and Palaeoproterozoic(Tarkwa)

Tectonic Setting

Fore-arc to back-arcsetting vs forelandbasin setting Stage 1:

Convergence

Limpopo oceanLimpopo Orogen

Kaapvaal Craton

Witwatersrand Basin

>3.1 Ga granitoid-greenstone basement

ZimbabweCraton

WitwatersrandBasin

Stage 2: Collision

Witwatersrand

Kositcin (2003)

CONTRASTS:OROGENIC GOLD vs WITWATERSRAND

Host Rock Volcanic rocks or greywackes vs clastic conglomeratesor sandstones

Mineral Complex brittle-ductile structural controls, commonlyStyles in anticlines vs thin, laterally extensive horizons

(reefs) with discontinuous brittle structures in broadly synclinal (or monoclinal) structure

Veins Abundant, multi-generation veins, commonly withhigh gold grades vs few veins, commonly with noessential relationship to gold ores and no grade,despite SiO2-rich host rocks

Mine Abundant drilling required to define extent and gradeDevelopment of ore vs deep shafts based on a few deep drill holes:

reflects total contrast in complexity

WITWATERSRAND

SIMPLICITY ?

OROGENIC GOLD(Victory Defiance)

COMPLEXITY ?

CONTRASTS: OROGENIC GOLD vs WITWATERSRAND


Witwatersrand Ore Surface:Simplicity ?


Goldfarb (2003)

Nome Beach Placers:Simplicity !

CONTRASTS: OROGENIC GOLD vs WITWATERSRAND

MetalAssociations

Au, Ag, As, Bi, Sb, Te, W vs Au, U, As, Co, Hg, Ni

Textures Euhedral to subhedral pyrite vs rounded pyrite(magnetite and hematite at Tarkwa)

Carbon textures also show marked contrasts

Alteration Sericite – carbonate – chlorite vs pyrophyllite – chloritoid – muscovite – chlorite

Major contrasts in scale / extent of alteration

ALTERATION ZONES ON REGIONAL SCALE

From Barnicoat et al. (1997) after Phillips and Law (2000)Phillips and Law (2000)After Barnicoat et al. (1997)

SCALE OF ALTERATIONZONES INWITWATERSRAND

Photo courtesy of G.N. Phillips

SCALE OF ALTERATION IN OROGENIC GOLD DEPOSITS

VeinVein

Photo: C. Mathison

IntermediateIntermediate ProximalProximal

Eilu et al. (2001)

Alteration around quartz vein:Mt Charlotte, Kalgoorlie, W.A.

Alteration zones surrounding orezones: Bronzewing, Yandal Belt, W.A.

OTHER CONFIRMATORY EVIDENCE

Distinctive Shape of Rare RelictDetrital Gold Grains With HighlyAnomalous Shape Only MatchedBy Detrital Gold from ModernGold Placers

[Gold toroids from Baaga stream in Yakutia (a,c)and Basal Reef in Witwatersrand (b,d)]

Re-Os Age of Rounded Pyrites of 2.99 ± 0.11 Ga (Kirk et al., 2001)- Significantly Older Than Sedimentation – and Re-Os Age of GoldGrains of 3033 ± 21 Ma (Kirk et al., 2002).Other Re-Os Evidence of Post-Ventersdorp Remobilizations ofNoble Metals from a 3.1 - 3.0 Ga Terrane (Schaefer et al., submitted)

Minter (1999)

a b

c d

MOST PROBABLE GENETIC MODELS FOR WITWATERSRAND

1. Detrital pyrite and uraninite (plus gold?) deposited in a

foreland basin: arc-type source and very effective

sorting/concentration in tectonically-active basin (cf

modern placers)

2. Oil migration from lower Wits shales with bitumen

trapped on or near radioactive minerals such as

uraninite

3a. Gold-bearing basinal or external fluid infiltrates basin

along thrusts, causing widespread alteration. Gold fluid

infiltrates fracture network and deposits Au in

conglomerates as hydrothermal phase

OR 3b. Basinal or external fluid infiltrates basin, causing

heating of detrital components, release of HC from

oil/bitumen, and gold remobilisation in non-H2O fluid

(hence no quartz veins) in a closed system with no

external connectivity - - PREFERRED MODELPREFERRED MODEL

WHAT MAKES THE WITWATERSRANDPALAEOPLACERS UNIQUE ? (1)

13 2 00.5Ga

Ga

Source• Late-Archaean peak time for orogenic gold globally• Most modern giant placers (e.g. Alaska, California, Ethiopia, New Zealand) not sourced from world-class gold deposits

Gol

d R

esou

rces

Per

cent

age

Cru

stal

Gro

wth

~ 50 %

~ 25 % Onset of modernplate tectonics

Groves (2003)

Processes• Strong chemical erosion in source (acid rain)• Sedimentary reworking in braided streams (no vegetation, rare organisms)• Increased wind sorting on exposed bars (no vegetation, rare organisms)• Faster spreading rates (?) with greater uplift at arc stage (?) and greater sedimentary reworking in constantly reactivated braided rivers (?)

2.71.9

1.2

WHAT MAKES THE WITWATERSRANDPALAEOPLACERS UNIQUE ? (2)

Preservation• Time of plate-plume interactions producing unique Archaean lithosphere

• Kaapvaal Craton represents oldest continental lithosphere (?)

• Oldest preserved clastic sedimentary basins and unique Precambrian stratigraphic section preserved

• Extension and Ventersdorp lavas preserved Witwatersrand from early erosion: cf. placers from Palaeozoic Ballarat deposits preserved under basalt: most other large placers very recent around Pacific margin

O’Reilly (2003)

GIANT PRECAMBRIAN PALAEOPLACERS: CONFIRMATION OF A PRESERVATIONAL PATTERN

• Giant Precambrian gold palaeoplacers broadly match temporal peak of orogenic gold formation

• No other preserved placers until Tertiary to Recent

• Again supports preservation in anomalously bouyant sub- continental mantle lithosphere

• Confirmed by almost complete stratigraphic record from ca 3500 Ma to 1600 Ma in South Africa

TRANSVAAL SUPERGROUP

IMPLICATIONS OF GENETIC MODELS IN EXPLORATION ?

I. PALAEOPLACER OR MODIFIED PALAEOPLACER MODEL (ALL components - pyrite (Fe-oxides at Tarkwa) - uraninite - gold are originally detrital)

• Extensive pyrite or magnetite - uraninite concentrations, in extensively reworked and exposed sedimentary environments likely to produce high gold grades

• Most probably Archaean or Palaeoproterozoic

• Based on equivocal evidence for change in atmosphere/hydrosphere and empirical evidence, including progressive change in uranium deposits with time

• Hence, model tends to restrict potential exploration targets to Archaean to Palaeoproterozoic foreland-basin clastic sequences

• Probably also need evidence for thick continental lithosphere to preserve sequences hosting the deposits

• Cannot use same structural criteria as those used to explore for orogenic gold deposits

IMPLICATIONS OF GENETIC MODELS IN EXPLORATION ?

II. HYDROTHERMAL GOLD OVERPRINT ON MODIFIED PALAEOPLACER MODEL [pyrite (magnetite)-uraninite detrital, gold hydrothermal]

• Similar to I in many respects

• If high gold grades and extensive "reef" development is related to bitumen occurrence, need detrital uraninite to fix large volumes of bitumens during oil migration

• Major differences to orogenic gold deposits explained by extremely effective Au depositional mechanism and/or unusual, high gold-solubility ore fluids

• Model still tends to restrict potential exploration targets to Archaean to Palaeoproterozoic foreland-basin clastic sequences

CONCLUSIONS

• Genetic Models for Witwatersrand Gold are Scale Dependent

• Microscopic to Mesoscopic Scales Features Favour Hydrothermal Origin

• Macroscopic to Tectonic Scale Features Favour Modified Palaeoplacer

• Rounded Pyrites and Uraninite are Clearly Detrital (or Early Diagenetic?)and Bitumen is Trapped Oil

• Comparisons with Known Orogenic Gold Deposits Indicate FewSimilarities to Witwatersrand Deposits

• In Particular, the Witwatersrand Deposits Lack the Local-ScaleComplexities that Typify Hydrothermal Systems and, in Fact,Make Them Operate Effectively

• Witwatersrand Gold is Best Explained as Placer Gold Remobilisedin a HC-rich and H2O-poor Closed System by Regional-ScaleFluid Infiltration in the Basin

Centre for Global Metallogeny School of Earth and Geographical Sciences The University of Western...

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