Geology and Mineralization of the Undeveloped Cochise Supergene Porphyry Copper Deposit, Warren (Bisbee) Mining District, Cochise County, Arizona
AdvisorSeedorff, Charles E.
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PublisherThe University of Arizona.
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AbstractThe Cochise porphyry copper deposit, owned by Freeport-McMoRan, is located outside of Bisbee, Arizona, immediately northeast of the historic Lavender pit. This supergene deposit is one of the few known unmined chalcocite blankets in North America and thus is a valuable natural geologic laboratory. This study synthesizes information from existing publications and unpublished feasibility studies with new mapping, petrography, and SEM data to provide a more comprehensive account of the geology and exploration history of the deposit. Base and precious metal mineralization in the Warren district is related to the ~200-Ma Sacramento Hill Intrusive Complex (SHIC), consisting of quartz monzonitic to granodioritic intrusions and hydrothermal breccias. The post-mineral Dividend fault down-dropped the southwestern portion of the SHIC and extensive networks of surrounding carbonate replacement deposits in Paleozoic rocks. The footwall contains the Cochise deposit, hosted by the northeastern portion of the SHIC and Proterozoic Pinal Schist. The Cochise chalcocite resource of 262 Mt averaging 0.46% Cu overlies hundreds of millions of tons of low-grade (0.1-0.2% Cu) hypogene mineralization centered on the Cochise feldspar-quartz-biotite porphyry stock and crosscutting heterolithic hydrothermal (phreatic) breccias. Shallow sericite-pyrite ± chlorite and intense sericite-silica-pyrite alteration contains weak chalcopyrite and trace bornite. At depth, a phaneritic biotite granodiorite stock hosts weak chalcopyrite-molybdenite-pyrite mineralization associated with quartz-orthoclase ± tourmaline-siderite veins and pervasive chlorite with a variable white to green sericite-pyrite-chalcopyrite ± bornite overprint. Metasomatism extends into the Pinal Schist but its intensity wanes rapidly. The supergene blanket covers an area of ~1,000 x 800 m and is 15-180 m thick. Chalcocite and covellite coat and replace pyrite and chalcopyrite. Abundant pyrite (5-15%) and hypogene sericitization of feldspars and mafic minerals lowered the acid-buffering capacity of the rock and allowed efficient leaching and transport of Cu from the low-grade protore and reprecipitation in the supergene blanket during weathering. The overlying leached cap contains supergene alunite and kaolinite, is virtually barren (100-400 ppm Cu) with almost no Cu oxides or carbonates, and changes from hematite- to jarosite-dominated limonite near the enrichment interface. An estimated 2 million m3 of protore material was leached and accumulated into the current resource with an enrichment factor of 3X that of hypogene tenor. Restoring normal movement on the Dividend fault places the hanging wall block 1-1.5 km above and ~1 km laterally offset from the Cochise deposit, suggesting that the Cochise deposit corresponds, prior to faulting, to an original deep flank of the magmatic-hydrothermal system. A comparison to other supergene deposits within the North and South American Cordillera emphasizes the importance of suitable pyrite:chalcopyrite ratios and hydrolytic alteration of a porphyry deposit to enable efficient leaching and appreciable enrichment. A paucity of Jurassic porphyry deposits in southwestern North America despite preservation of oxidized and hydrous intermediate to alkaline plutons suggests a possible tectonic impediment.
Degree ProgramGraduate College