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January 01, 2009


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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Special Publications of the Society of Economic Geologists

Supergene Environments, Processes, and Products

Spencer R. Titley
Spencer R. Titley
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Society of Economic Geologists
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January 01, 2009




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