Microbial carbon within and above exotic copper deposits in northern Chile : implications for ore genesis and exploration
Nelson, Mark Alan
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“Exotic-type” Cu silicate-oxide deposits hosted by Miocene pediment gravels represent an unusual, but characteristic, by-product of the supergene enrichment of Cenozoic porphyry Cu deposits in northern Chile. Carbon stable isotopic analysis is employed herein to clarify the environment of exotic ore formation and to provide guidelines for the exploration for non-outcropping mineralisation. Two main sample suites were examined: chrysocolla-rich ores from the Huinquintipa and Mina Sur deposits; and soils overlying a paleochannel in the Huinquintipa area known to be mineralised. The samples were processed using four different analytical techniques to determine their 13C values: (1) Elemental Analysis Isotope Ratio Mass Spectrometry (EA/IRMS) of the whole sample; (2) crushing in vacuo followed by IRMS to analyse fluid inclusions; (3) thermal extraction at 100˚C, followed by IRMS to analyse weakly bound carbon dioxide; and (4) ethylenediaminetetraacetic acid (EDTA) extraction followed by IRMS. EDTA-partial extraction favours the dissolution of minerals with divalent cations, releasing as carbon dioxide the carbon trapped within the crystal structure. All four of these analytical techniques have been used before, but this is the first time that they have all been used together on exotic copper silicate and oxide mineralisation. Three major carbon sources are identified: (1) atmosphere-derived carbon dioxide with a 13C value of around 0 ‰; (2) plant-derived carbon dioxide with a 13C of about -25 ‰; and (3) microbe-derived carbon dioxide with a 13C of approximately -50 ‰. The bulk of the carbon liberated by EA/IRMS was plant-derived. The thermally- and crushing-released carbon dioxide has the highest proportion of atmosphere-derived carbon, whereas EDTA-extraction preferentially liberated the lightest of carbon. On the conclusion that EDTA preferentially dissolved Cu-rich silicate mineraloids, it is concluded that microbial consortia, including methanogenic microbes, were hosted specifically by the high-grade Cu assemblages and plausibly played a critical role in their precipitation. The same microbial-carbon signature was obtained through the EDTA-extraction of soil samples above the paleochannel. Carbon isotopic analysis of CO2 sequestered through EDTA-extraction could therefore be used as an exploration tool for buried exotic mineralisation. Future exploration should exploit the presence of microbes in niche-specific environments.