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Mark Barley, The University of Western Australia (Australia)
Holly Stein, Colorado State University (United States)
Brendan Cummins, Moly Mines Ltd (Australia)
Andrey Bekker, University of Manitoba (Canada)
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The Spinifex Ridge (Coppins Gap) Mo-Cu deposit is north of the Mount Edgar batholith in the 3.52-2.85 Ga East Pilbara granite-greenstone terrane. The deposit (470 million tons at 0.06% Mo and 0.09% Cu) is associated with a suite of magnetite-bearing, high-level quartz-plagioclase porphyries and porphyritic granodiorites intruded into basalts and rhyolites. The mineralization consists of a series of multiphase stockwork veins. Quartz-potasium feldspar-carbonate veins with molybdenite and chalcopyrite are most abundant where both granodiorite and quartz-plagioclase porphyries are present. Although regional and contact metamprphism partly overprint alteration, potasic alteration is preserved as potasium feldspar veins and replacement of plagioclase by potasium feldspar within the core of the deposit, and as biotite alteration of both porphyries and basalts at the periphery of deposit. Both phyllic and propylitic alteration are also associated with the mineralization. Re-Os dating of molybdenite confirms the timing of mineralization. Molybdenite ages of 3298 ± 11 and 3284 ± 11 Ma are indistinguishable within their 2-sigma uncertainties, and agree with SHRIMP U-Pb zircon ages for granodiorites in the batholith (3315 Ma to 3290 Ma). Multiple S isotope analyses of molybdenite also confirm a magmatic-hydrothermal source for sulphur. Together these observations confirm that Spinifex Ridge is a porphyry-style Mo-Cu deposit and the world's oldest world-class ore deosit.
Phanerozoic porphyry-style deposits are typically associated with hydrous, volatile- and metal-rich inermediate to silicic magmas at convergent plate margins with porphyry Cu deposits showing the strongest link to subduction. Mineral deposits formed at high levels in active convergent margins have limited chances of preservation. Modern porphyry-style Mo-Cu deposits are associated with crustally derived silicic magmas in weakly extensional settings at either convergent or rifted margins and have a higher chance of preservation. These magmatic-hydrothermal systems are also characterised by lower oxygen fugacities than those associated with Cu-Au deposits. The 3.3 Ga granodiorites and rhyolites in the East Pilbara formed by crustal melting of an older intermediate TTG suite during the late stage of a major period of magmatism. Ore zone geometry suggests that the mineralization system was emplaced during an episode of extensional deformation and subsequently tilted during the deformation that accompanied the uplift of the batholith into its present position. The regional geology supports formation of the Spinifex Ridge Mo-Cu deposit in a weakly extending regime at ∼3.3 Ga, similar to magmatic-tectonic conditions that produce Mo-Cu porphyry-style deposits today. Regardless of the interpreted tectonic setting for the East Pilbara 3.3 billion years ago, it is not surprising that the oldest world-class, high-level, magmatic-hydrothermal ore deposit is a Mo-Cu porphyry-style deposit.
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