Fluid inclusion insights into the origins of fluids and metals in porphyry copper deposits

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 (Refereed Research Paper - 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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