Rusk, B.G., Hofstra, A.H., Emsbo, P., Hunt, A.G., Landis, G.P., and Rye, R.O. (2008) Fluid inclusion insights into the origins of fluids and metals in porphyry copper deposits. In: Papers from PACRIM Congress 2008. pp. 289-293. From: PACRIM Congress 2008 - The Pacific Rim: Mineral Endowment, Discoveries & Exploration Frontiers, 24-26 November, 2008, Gold Coast, QLD, Australia.

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Abstract

Models of the evolution of the hydrothermal systems that form porphyry Cu (Mo-Au) deposits are compromised because aqueous magma-derived fluids in the ore zones of most deposits have changed from their original magmatic compositions as a result of cooling, depressurising, mineral precipitation, brine-vapour unmixing and fluid-rock reactions. However, in deep quartz-rich, sulfide-poor veins from numerous porphyry type deposits, we have identified parental fluids trapped in inclusions at near magmatic temperatures and pressures above the brine-vapour unmixing solvus. We have analysed these inclusions for bulk salinity, density, solute chemistry, helium isotopic ratios and elemental composition, These parental inclusions contain 35 - 70 volume per cent bubble, are low to moderate salinity, contain up to ten mol per cent CO2, and commonly contain a chalcopyrite daughter crystal. Our results indicate that these Cu-rich fluids transport Cu from a plutonic complex below upward into a hydrothermal system, where decompression, cooling, unmixing and water-rock reaction drive ore-mineral precipitation. Na/CI ratios greater than one indicate that in addition to chlorine, sulfur and/or carbonate must play a key role in Cu transportation. Helium isotope ratios indicate that between - 15 and 100 per cent of helium in these fluids is mantle-derived. We suggest that in addition to He, volatiles from mafic magmas in the mantle are also likely to supply CO2 Cu and S to the fluids that form porphyry copper deposits.

Item ID:	7690
Item Type:	Conference Item (Research - E1)
ISBN:	978-1-920806-93-4
Date Deposited:	26 Jul 2010 22:52
FoR Codes:	04 EARTH SCIENCES > 0402 Geochemistry > 040201 Exploration Geochemistry @ 100%
SEO Codes:	84 MINERAL RESOURCES (excl. Energy Resources) > 8401 Mineral Exploration > 840102 Copper Ore Exploration @ 100%
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