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Surface lithogeochemistry of the Relincho porphyry copper-molybdenum deposit, Atacama region, Chile Greenlaw, Lauren


Porphyry copper deposits (PCDs) typically have large alteration haloes that extend several kilometers from economic mineralization providing a geochemical footprint potentially an order of magnitude larger than the deposit. The Paleocene (64±2 Ma) Los Morteros batholith comprises four granodiorite units and hosts four syn-mineralization porphyry units. These units are interpreted as the product of four magmatic differentiation cycles with three magmatic recharges. Alteration assemblages observed within the system include potassic, propylitic and phyllic, with intensities varying between weak to moderate for potassic and phyllic alteration, and weak to strong for propylitic. Lithogeochemical characterization and quantification of alteration is an important exploration tool that has the potential to lead to exploration success. Two hundred and ninety-six surface rock samples were collected in a grid covering 65 km² centered over the Relincho PCD in the Atacama region, Chile to assess the suitability of surface rock lithogeochemistry as a medium for lithological and alteration characterization. Aqua regia ICP-MS, pressed pellet XRF, and fusion-ICP results, combined with shortwave infrared (SWIR) spectra, alkali feldspar staining, petrography and field observations were used to classify lithological units and identify and quantify alteration. Data evaluation and modeling is completed through the use of exploratory data analysis, simple mass balances and molar element ratios (MER) complimented by hand and thin-section observations and SWIR analyses. Gain-loss variations are consistent with spatial element distributions indicating: the addition of SiO₂, K₂O, Ag, Cu and Mo and loss of CaO, Na₂O during potassic alteration; and the addition of Na₂O and loss of SiO₂ during propyltic alteration. Wavelengths of SWIR chlorite features indicate that chlorite is more Fe-rich proximal to mineralization and Mg-rich distally. Simple and molar element ratios are used as proxies for the potassic, propylitic and phyllic alteration assemblages. From these ratios, alteration indices are calculated. The potassic index (K₂O/Th) and propylitic-phyllic index ((18Ca + 14Na + 25K)/(2Si + 7Al + 4(Fe + Mg)) identify and quantify potassic, phyllic and propylitic alteration. Alteration thresholds derived from probability plots indicate that these indices would identify the Relincho deposit as a potential PCD exploration target at a sample spacing of up to 2000 m. Supplementary materials: http://hdl.handle.net/2429/50971

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