Copper Isotope Fractionation Used to Identify Supergene Processes
Ryan Mathur, Spencer Titley, Fernando Barra, Susan Brantley, Marc Wilson, Allison Phillips, Francisco Munizaga, Victor Makseav, Jeff Vervoort, Garret Hart, 2009. "Copper Isotope Fractionation Used to Identify Supergene Processes", Supergene Environments, Processes, and Products, Spencer R. Titley
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We examined the copper isotope ratio of primary and secondary copper mineralization of porphyry copper deposits. Distinct Cu isotope reservoirs exist for high-temperature hypogene, enrichment, and leach cap minerals. Chalcopyrite from high-temperature primary mineralization forms a relatively tight cluster of δ65Cu values of +1 to –1 per mil, whereas secondary minerals formed by low-temperature reveal a range of δ65Cu values from –16.96 to +9.98 per mil. Secondary chalcocite is relatively heavy, with δ65Cu varying from –0.3 to +6.5 per mil. Leach cap minerals dominated by Fe oxides (jarosite, hematite, and goethite) are relatively light, ranging from –9.9 to +0.14 per mil. A distinct pattern of heavier copper isotopes in supergene samples and a lighter isotopic signature exists in the leach cap and oxidation zone minerals. The pattern presents an excellent tool for using Cu isotopes for exploration through providing the following information: (1) identification of highly fractionated copper isotope ratios in copper sulfide and Fe oxide samples that indicate supergene processes and the extent of leaching and enrichment of copper, and (2) identification of highly fractionated copper isotope ratios in surface and/or groundwaters that indicate the active weathering of copper sulfides that experienced significant enrichment.
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Supergene Environments, Processes, and Products
At least five altered and mineralized porphyry centers related to the cooling of a polyphase Eocene intrusion occur within a 25-km2 "pampa"-type area in the southwestern sector of the Chuquicamata district in northern Chile. These deposits take place 1 to 2 km apart as discrete porphyry "columns" covered by postmineral, poorly consolidated Miocene sedimentary rocks. Such copper oxide and sulfide deposits were discovered and evaluated by drilling done by Codelco from 1996 through 2007 during a brownfield exploration program, driven by the necessity to replace and increase leacheable ore consumed by the Chuquicamata and Radomiro Tomic operations. During this program a resource of more than 20 million metric tons (Mt) Cu was discovered, including 6 Mt Cu of oxide, mixed and secondary sulfide ore, representing one of the largest supergene copper resources discovered worldwide during the last 10 years.
Despite their close location and their genetic relationship to a single, polyphase intrusion mineralization event, the five porphyry centers display contrasting host-rock and structural framework as well as different hypogene alteration and ore mineral assemblages. This picture reaches high levels of complexity because of the different levels of exposure of the mineral systems, resulting from primary emplacement processes and post-mineral faulting. These hypogene features and the effect of landscape and climate evolution controlled supergene alteration, thus generating different profiles in each specific porphyry center. The key controlling factors in the supergene overprint are discussed on the basis of their relationship to ore and gangue mineralogical abundance and occurrence, assemblage distribution, geochemical response, and the broad geologic setting.
As exploration for covered porphyry copper deposits in the southwestern sector of the Chuquicamata district progressed, numerous lessons were learned about the origin of supergene profiles and the analysis and use of supergene effects and their products as a guide for exploration. These lessons, which include geological and geochemical criteria among others, are discussed in the context of the appraisal of the mineral potential of copper oxide-mixed-secondary sulfide blankets and underlying sulfide protore.