Research Topic: Gold in Porphyry Copper Systems


The Relationship Between Gold and Copper in The Bingham Canyon Porphyry Deposit

Patrick B. Redmond and Marco T. Einaudi,

Department of Geological and Environmental Sciences,

Stanford University, Stanford, CA 94305

Five separate porphyry intrusions have been mapped on the basis of crosscutting relations between dikes and veins. Each porphyry produced high-grade, copper-gold ore and potassically altered wall rock, although the amount of copper and gold introduced and the degree of alteration decreased significantly with each successive intrusion. Four of the five porphyries are latitic to quartz-latitic in composition whereas the third in the sequence of intrusion is more mafic in composition. The occurrence of this mineralized mafic dike within the intrusive sequence supports the hypothesis proposed by Deino and Keith (1997) and Waite and others (1997) that mafic (minette) magma mixed with latitic magma in the parent magma chamber below the porphyry system. Both copper and gold are associated with potassic alteration that varies in style from biotitization of hornblende to complete replacement of porphyry by quartz and K-feldspar with minor biotite. Zones of intense K-feldspar-quartz replacement are best developed within the earliest porphyry, have the highest grades of copper and gold, and generally lack quartz-sericite-pyrite veins. These high-grade zones (>1% Cu, > 2ppm Au) consist of the assemblage bornite3-chalcopyrite1-chalcocite1 (vol ratios) with no pyrite. Composite grains containing all three sulfide phases are common. Chalcocite and chalcopyrite rarely occur in contact, chalcocite most commonly occurs as exsolution laths within bornite, and bornite and chalcopyrite display mutual boundaries. Gold to copper ratios (Au ppm/Cu %) are typically greater than 2 in the high-grade zone containing chalcocite and systematically decrease outward to less than 0.2 at the 0.15 % Cu grade contour, where the sulfide assemblage consists of chalcopyrite + pyrite. The mineralogical/textural forms of gold include: gold grains enclosed within, or attached to, sulfide grains (most commonly bornite); gold grains enclosed in quartz or K-feldspar gangue minerals; and inclusions of gold-silver tellurides and bismuthides in sulfide grains. Copper and gold were introduced contemporaneously during potassic alteration at high temperatures. Sulfide textures and assemblages suggest that the Cu-Fe-sulfides were deposited at temperatures less than 500°C (upper limit for chalcopyrite as phase separate from iss) but greater than 300°C (lower limit for continuous bornite solid solution). Gold may have been preferentially deposited within bornite solid solution that later exolved on cooling to chalcocite-bornite intergrowths plus native gold.

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