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Geochemical sampling strategies for discovering buried copper porphyries, Atacama Desert, Chile Brown, Alexandra E.

Abstract

Several of the world’s largest porphyry copper deposits are found in the hyper-arid Atacama Desert of Chile, yet discovery of new deposits has become increasingly challenging due to the thick gravel cover. The Atlántida porphyry copper deposit, Atacama Region, Chile, is buried under 25-80m of Neogene-Quaternary gravel and 200m of barren to low-grade bedrock. Geochemical sampling occurred over two phases to develop sampling techniques to detect buried mineralization through the gravels. Over the two phases, physicochemical measurements were recorded at each site, and AGI GoresorberTM hydrocarbon collectors were installed. Sample lines extended into background and over mineralization, staying within consistent background chemistry. Phase I samples were collected with 50m spacing. Soil samples were analyzed by Aqua Regia (Phase I) and deionized water extraction with ICP-MS (Phase I and II). Phase I deionized water extraction results indicated elevated concentrations of Mo, Re, Se, As with increasing salinity. Anomalous responses occur spatially correlated with structures on the surface, and in close proximity to highly saline pockets of material (median 2.2% salt) identified through remote sensing used in Phase II regolith mapping. Saline pocket material was formed by seismically induced surface flooding of groundwater along faults extending to surface. Phase II sampled only saline pockets oriented along structural extent at 250m spacing. Samples (n=9) were analyzed for copper isotope composition over buried mineralization by partial digestion. Isotopic results indicate copper concentrations by partial extraction are dominantly a reflection of endogenic material. Both hydrocarbon and soil extraction results show strong structural control on anomaly formation at the surface. Strong correlations of increasing salinity and metals/metalloids (Mo, Re, Se, and Te) concentrations are observed. Elevated responses of these metals normalized to groundwater volume proxy occur directly over the deposit. The slow rate of erosion and deposition in the Atacama Desert preserves surficial anomalies as saline pockets. Targeted geochemical sampling of saline pockets along structures is a new methodology for the discovery of buried copper deposits in the Atacama Desert of Chile.

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