Partitioning of gold in young calc-alkalic volcanic rocks from Guatemala

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Gold analyses of whole rock, mineral, and groundmass of seven calc-alkalic volcanic rocks (basalt to rhyolite) show that gold is less abundant in the dominant phenocrystic phases than in the whole rock; groundmass concentrations are also lower than whole rock. Crystal-liquid partition coefficients are <1 for plagioclase and hornblende, and about 1 for olivine, pyroxene, magnetite, and biotite. If only the principal phases are considered, the bulk gold partition coefficient is typically <1, yet gold seems to be depleted rather than enriched in many evolved magmas. A significant proportion of gold in these rocks is in neither the groundmass nor the dominant phenocrysts but instead is contained in tiny sulflde blebs that are found in these rocks. During crystallization the continual separation of a sulflde phase that concentrates gold is consistent with compatible gold behavior and petrography. Further data is needed to evaluate this hypothesis.

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Publisher's version of record: https://doi.org/10.1086/629039

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The Journal of Geology