Abstract

Exotic copper silicate and oxide mineralization, a peripheral facies of several Cenozoic porphyry Cu(-Mo) systems in northern Chile, is traditionally ascribed to the lateral migration of Cu-rich, initially highly acidic waters away from active zones of supergene leaching and sulfide enrichment. Copper precipitation as Cu(-Fe) silicates and Cu(-Mn, Fe) oxides in permeable, accumulating pediment gravels or underlying fractured bedrock resulted in part from fluid neutralization attending hydrolytic alteration. Supergene activity was most intense in the late Oligocene-early Miocene, during a protracted middle Cenozoic semiarid interval along the lower Pacific slope of the central Andean Cordillera Occidental. Samples from the Huinquintipa and Mina Sur deposits, contiguous with, respectively, the 32.7 to 34.4 Ma Rosario (Collahuasi) and 31.1 to 34.6 Ma Chuquicamata porphyry centers, were examined for carbon isotope evidence of the conditions of ore formation. Carbon was extracted as CO2 from representative high-grade, chrysocolla-rich samples using four different techniques (combustion, pyrolysis, crushing, and EDTA leaching) and has a widely varying isotopic composition consistent with mixing of atmosphere-derived carbon with a δ13C value of 0 per mil, plant-derived carbon with δ13C values between −25 and −15 per mil, and carbon with exceptionally low δ13C values of ≤ −47 per mil. The extremely light carbon, sampled primarily by leaching with a 10 percent EDTA solution, is most plausibly ascribed to a microbial consortium including anaerobic methanogenic microbes. The presence of mineralized carbon having such low δ13C values provides evidence that microbial metabolism may have contributed directly to copper silicate precipitation in these deposits. Lower δ13C values at Huinquintipa than at Mina Sur could be attributed to higher water tables and higher toxic metal contents in the mineralizing fluids. The new data reinforce the role played by microbes in the economically critical supergene modification of porphyry and epithermal ore deposits in the semiarid environment.

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