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Geochemistry of stream sediments and surficial deposits at Pascua-Lama high sulfidation epithermal gold deposit, Chile-Argentina Chastain, Emily Ann

Abstract

Pascua-Lama is a 17.1 moz Au and 560 moz Ag high sulfidation epithermal gold deposit on the crestline of the High Andes of Argentina and Chile. In this arid, mountainous terrain, talus cones and alluvial fans flank deeply incised valleys and debris flows infill valley floors decoupling streams from valley walls. Glacial till is preserved locally. Surficial deposits, stream sediments and waters were sampled along valleys draining the deposit. Samples were wet sieved and fractions finer than -150µm analyzed for gold by FA-AAS. The -53µm fraction of medium energy sediments was also analyzed by ICP-MS after total, aqua regia, and a weak hydroxylamine leach. Heavy mineral concentrates (HMCs) were analyzed by NAA and SEM-EDS. Water samples were analyzed by ICP-MS. Geochemical patterns at Lama-Pascua developed as a result of both mechanical and chemical processes. The occurrence of gold and associated elements (e.g., Ag, As, Pb, Hg, Sb, Bi) in HMCs allows their progressive enrichment as light minerals are selectively removed during erosion and transport of surficial materials. Thus, till, with the greatest abundance of fines, has the lowest geochemical contrast. Conversely, sediments have the highest concentration of HMC associated elements. More mobile elements such as Cu have similar concentrations in surficial deposits and stream sediments, but under extremely acidic conditions are depleted close to the deposit. In stream sediments, gold provides the best anomaly contrast with the longest and most consistent anomalies in the fine fractions (-75µm or finer). Based on sediment texture, ~2 kg -2 mm field samples should provide 50 g of -75µm material for analysis. Stream water pH should be measured and analysis of stream waters could be a valuable adjunct to sediment data, particularly in areas where debris flows decouple streams from their valley sides or headwalls. Follow-up to drainage anomalies would involve base-of-slope sampling of talus and alluvial fans and should consider the possible depletion of more mobile metals close to a deposit.

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