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Centre for Global Metallogeny School of Earth and Geographical Sciences The University of Western Australia A C OMPARISON B ETWEEN T HE W ITWATERSRAND.

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Presentation on theme: "Centre for Global Metallogeny School of Earth and Geographical Sciences The University of Western Australia A C OMPARISON B ETWEEN T HE W ITWATERSRAND."— Presentation transcript:

1 Centre for Global Metallogeny School of Earth and Geographical Sciences The University of Western Australia A C OMPARISON B ETWEEN T HE W ITWATERSRAND A ND O ROGENIC G OLD S YSTEMS: A T EST O F T HE H YDROTHERMAL W ITWATERSRAND M ODEL David I. Groves*, Gavin England, Birger Rasmussen and Bryan Krapez Gold Deposits >100t Au: Gold Produced Foster (2002)

2 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

3 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

4 TECTONIC TO BASIN SCALE Phillips and Law (2000)

5 OUTCROP/FACE TO MACROSCOPIC TO MICROSCOPIC SCALE Photos courtesy of L. Minter and G.N. Phillips

6 Most gold now hydrothermal in texture BUT what about pyrite, uraninite and carbon (bitumen) ? MICROSCOPIC SCALE

7 ROUNDED PYRITE : DETRITAL OR REPLACEMENT(?) England et al. (2002) Phillips and Law (2000) c b d a

8 ROUNDED PYRITE : DETRITAL OR REPLACEMENT(?) England et al. (2002) c b d a

9 ROUNDED PYRITE : DETRITAL OR REPLACEMENT(?) England et al. (2002) c b d a

10 Cross section through the Damang gold mineFlat-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)

11 DAMANG GOLD MINE, GHANA quartz vein S0S0 Sulphidised Cross-beds sample JP023, sandstone sample JP070, sandstone Pigois (2003)

12 ROUNDED PYRITE : SULPHUR-ISOTOPE EVIDENCE England et al. (2002) b c

13 DETRITAL MONAZITE AND SURROUNDING BITUMEN AFTER OIL SANDSTONE FROM MESOZOIC PERTH BASIN, W.A. England et al. (2002) c b d a

14 DETRITAL? URANINITE AND SURROUNDING BITUMEN AFTER OIL CONGLOMERATE FROM ARCHAEAN WITWATERSRAND BASIN England et al. (2002) c b d a

15 COMPARISON WITH OROGENIC DEPOSITS (1) Phillips and Law (2000) WITWATERSRAND OROGENIC DEPOSITS

16 Phillips and Law (2000) COMPARISON WITH OROGENIC DEPOSITS (2) Newton (2001) From Ridley (2003) Union Reefs Mt Charlotte Witwatersrand

17 SIMILARITIES: OROGENIC GOLD vs WITWATERSRAND Metamorphic Setting Both generally in greenschist facies rocks Mineral Association Gold-pyrite-carbon association a common factor Ore Textures Gold is late paragenetically and associated with carbon and hydrothermal sulphides

18 CONTRASTS: OROGENIC GOLD vs WITWATERSRAND Age Range Orogenic gold all ages Economic Witwatersrand style restricted to Archaean (Wits) and Palaeoproterozoic (Tarkwa) Tectonic Setting Fore-arc to back-arc setting vs foreland basin setting Stage 1: Convergence Limpopo ocean Limpopo Orogen Kaapvaal Craton Witwatersrand Basin >3.1 Ga granitoid- greenstone basement Zimbabwe Craton Witwatersrand Basin Stage 2: Collision Witwatersrand Kositcin (2003)

19 CONTRASTS: OROGENIC GOLD vs WITWATERSRAND Host RockVolcanic rocks or greywackes vs clastic conglomerates or sandstones MineralComplex brittle-ductile structural controls, commonly Stylesin anticlines vs thin, laterally extensive horizons (reefs) with discontinuous brittle structures in broadly synclinal (or monoclinal) structure VeinsAbundant, multi-generation veins, commonly with high gold grades vs few veins, commonly with no essential relationship to gold ores and no grade, despite SiO 2 -rich host rocks MineAbundant drilling required to define extent and grade Developmentof ore vs deep shafts based on a few deep drill holes: reflects total contrast in complexity

20 WITWATERSRAND SIMPLICITY ? OROGENIC GOLD (Victory Defiance) COMPLEXITY ? CONTRASTS: OROGENIC GOLD vs WITWATERSRAND

21 Witwatersrand Ore Surface: Simplicity ? Phillips and Law (2000) Goldfarb (2003) Nome Beach Placers: Simplicity !

22 CONTRASTS: OROGENIC GOLD vs WITWATERSRAND Metal Associations 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

23 ALTERATION ZONES ON REGIONAL SCALE From Barnicoat et al. (1997) after Phillips and Law (2000)

24 SCALE OF ALTERATION ZONES IN WITWATERSRAND Photo courtesy of G.N. Phillips

25 SCALE OF ALTERATION IN OROGENIC GOLD DEPOSITS Vein Photo: C. Mathison IntermediateProximal Eilu et al. (2001) Alteration around quartz vein: Mt Charlotte, Kalgoorlie, W.A. Alteration zones surrounding ore zones: Bronzewing, Yandal Belt, W.A.

26 OTHER CONFIRMATORY EVIDENCE Distinctive Shape of Rare Relict Detrital Gold Grains With Highly Anomalous Shape Only Matched By Detrital Gold from Modern Gold 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 Gold Grains of 3033 ± 21 Ma (Kirk et al., 2002). Other Re-Os Evidence of Post-Ventersdorp Remobilizations of Noble Metals from a 3.1 - 3.0 Ga Terrane (Schaefer et al., submitted ) Minter (1999) a b c d

27 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-H 2 O fluid (hence no quartz veins) in a closed system with no external connectivity - PREFERRED MODEL

28 WHAT MAKES THE WITWATERSRAND PALAEOPLACERS UNIQUE ? (1) 1 3 2 0 0.5 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 Gold Resources Percentage Crustal Growth ~ 50 % ~ 25 % Onset of modern plate 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 (?) 2.7 1.9 1.2

29 WHAT MAKES THE WITWATERSRAND PALAEOPLACERS 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 OReilly (2003)

30 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

31 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

32 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

33 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 Few Similarities to Witwatersrand Deposits In Particular, the Witwatersrand Deposits Lack the Local-Scale Complexities that Typify Hydrothermal Systems and, in Fact, Make Them Operate Effectively Witwatersrand Gold is Best Explained as Placer Gold Remobilised in a HC-rich and H 2 O-poor Closed System by Regional-Scale Fluid Infiltration in the Basin


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