Geology and Rock-Water Geochemistry of Exotic Copper Deposits, Ferricretes, and Manganocretes Associated with Porphyry Copper Deposits in Arizona, New Mexico, and Sonora (Extended Abstract)
Timothy S. Hayes, James G. Brown, Stephen J. Sutley, Joseph F. Whelan, 2009. "Geology and Rock-Water Geochemistry of Exotic Copper Deposits, Ferricretes, and Manganocretes Associated with Porphyry Copper Deposits in Arizona, New Mexico, and Sonora (Extended Abstract)", Supergene Environments, Processes, and Products, Spencer R. Titley
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Exotic copper deposits, ferricretes, and manganocretes in Arizona, New Mexico, and Sonora are metal deposits in conglomerates that are spatially and genetically associated with porphyry Cu deposits. Deposits of this exotic class are typically zoned from ferricrete upgradient and extending into the bedrock leached capping, to black manganese oxides or Cu wad next downgradient, and to chrysocolla farther downgradient in the Cu-rich systems. Small deposits may lack zonation. Entire zoned systems vary from as little as 500 m long up to 6 km long (in Chile). In the Sonoran province, the El Pilar exotic Cu deposit exceeds 160 Mt at 0.4 percent Cu, but only one other known deposit was more than 10 Mt. Small tonnages of several percent Cu were selectively mined, between about 1880 and 1920, from chrysocolla zones. Average grades are typically 0.7 to 2 percent Cu in chrysocolla zones but mostly 0.5 percent or less in Cu wad zones. Solvent extraction-electrowinning (SXEW) will treat most exotic Cu ores. Ferricretes and manganocretes are not economic at present, although Mn deposits that may be of this class were mined in the past.
Exotic Cu deposits form when a headward-eroding stream intersects a forming supergene enrichment profile of a bedrock porphyry Cu deposit (Münchmeyer, 1996). Acidic metals-charged groundwater then discharges laterally into the alluvial aquifer and neutralizes and oxidizes while moving downgradient. It deposits metals in the conglomerates in the following order: first, hematite, then copper wad, and, finally, chrysocolla. Where the porphyry system is merely pyritic, upgradient goethitic
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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.