Evidence for SEDEX-style mineralization in the 1.7 Ga Tawallah Group,McArthur Basin,Australia |
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| Institution: | 1. CSIRO Mineral Resources, Australian Resources Research Centre, 26 Dick Perry Avenue, Kensington, WA 6151, Australia;2. Armour Energy Limited, Level 27, 111 Eagle Street, Brisbane, QLD 4000, Australia;1. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China;2. Open Laboratory of Orogenic and Crustal Evolution, Peking University, Beijing 100871, China;3. Henan Provincial Non-ferrous Metals Geological and Mineral Resources Bureau, Zhengzhou 450016, China;1. Geoscience Australia, Canberra, ACT 2601, Australia;2. Research School of Earth Sciences, Australian National University, Canberra 2601, Australia;3. Geological Survey of Queensland, Brisbane, Australia;1. Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia;2. Université de Lorraine, CNRS, CREGU, GeoRessources, UMR 7359, Boulevard des Aiguillettes, B.P. 239, F-54506 Vandoeuvre-lès-Nancy, France;3. PRISE, Australian National University, ACT 2600, Australia;4. Centre de Recherches Pétrographiques et Géochimiques, UMR 7358, CNRS, Université de Lorraine, 15 rue Notre Dame des Pauvres, BP 20, 54501 Vandoeuvre-lès-Nancy, France;1. Department of Earth Science, Utrecht University, 3584 CD Utrecht, Netherlands;2. Faculty of Earth and Life Sciences, VU University, 1081 HV Amsterdam, Netherlands |
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| Abstract: | The Paleoproterozoic McArthur Basin (McArthur Group) of northern Australia hosts world-class sedimentary ‘exhalative’ (SEDEX) McArthur type Zn–Pb deposits, which are largely hosted within a sequence of 1.64 Ga pyritic carbonaceous shales deposited in an extensional rift setting. A well-known example of these is McArthur River (or Here's Your Chance HYC] Zn–Pb–Ag deposit). The ~ 1.78 Ga McDermott and ~ 1.73 Ga Wollogorang formations (Tawallah Group) both contain carbonaceous shales deposited in similar environments. Our observations suggest the carbonaceous facies of the Wollogorang Formation were deposited under mostly euxinic conditions, with periodically-high concentrations of sedimentary pyrite deposition. The carbonaceous shales in the older McDermott Formation contain considerably less early pyrite, reflecting a mostly sulfide-poor, anoxic depositional environment. Localized fault-bound sub-basins likely facilitated lateral facies variations, which is evident from synsedimentary breccias.The presence of evaporitic oxidized facies within the McDermott and Wollogorang formations, alongside evidence for synsedimentary brecciation in reduced shales are favourable criteria for SEDEX-style base metal deposition. Both formations overlie volcanic units, which could have been sources of base metals. Detailed X-ray petrography, new geochemical data and sulfur isotope data from historical drill cores indicate multiple horizons of stratiform and sediment breccia-hosted base metal sulfide within carbonaceous shale units, with high-grade Zn concentrations. A close association between sphalerite and ferromanganean dolomite alteration draws comparisons with younger SEDEX mineralization at HYC. Additionally, SEDEX alteration indices, used demonstrably as a vector to the younger orebodies, indicate the sedimentary rocks analyzed in this study are marginally below the ore window when compared to the overlying mineralized stratigraphy.Our data imply that localized active circulation of metalliferous brines occurred in the Tawallah Group basin. High-grade sulfide deposition in reduced facies alteration may represent distal expressions of larger SEDEX-style deposits. Furthermore, abundant pyrite and high molybdenum in the Wollogorang Formation suggest the global oceanic sulfate concentration was sufficient by ~ 1.73 Ga to engender intermittent but strong bottom-water euxinia during shale deposition, thus providing a robust chemical trap for base metal sulfide mineralization. |
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