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Carbon Isotope Evidence for Microbial Involvement in Exotic Copper Silicate Mineralization, Huinquintipa and Mina Sur, Northern Chile

Mark Nelson, Kurt Kyser and Alan Clark

Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, Ontario K7L 3N6, Canada

Chris Oates

Anglo American plc, 20 Carlton House Terrace, London SW1Y5AN, United Kingdom

Corresponding author: e-mail: kyser{at}geol.queensu.ca

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 ¹³C value of 0 per mil, plant-derived carbon with ¹³C values between –25 and –15 per mil, and carbon with exceptionally low ¹³C 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 ¹³C values provides evidence that microbial metabolism may have contributed directly to copper silicate precipitation in these deposits. Lower ¹³C 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.

This article has been cited by other articles:

				M. A. Nelson, T. K. Kyser, A. H. Clark, and C. Oates Application of carbon isotope ratios in regolith to the exploration for buried exotic-type copper ore deposits, Collahuasi district, northern Chile Geochemistry: Exploration, Environment, Analysis, February 1, 2009; 9(1): 3 - 8. [Abstract] [Full Text] [PDF]