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We collected groundwaters in and around a large (313 Mt at 1.08% Cu and 0.3% cutoff) undisturbed porphyry copper deposit (Spence) in the hyperarid Atacama Desert of northern Chile, which is buried beneath 30-180 m of Miocene piedmont gravels. Groundwaters within and down-flow of the Spence deposit have elevated Se (up to 800 mu g/l), Re (up to 31 mu g/l), Mo (up to 475 mu g/l) and As (up to 278 mu g/l) concentrations compared to up-flow waters (interpreted to represent regional groundwater flow). In contrast, Cu is only elevated (up to 2036 mu g/l) in groundwaters recovered from within the deposit; Cu concentrations are low down gradient of the deposit. The differential behavior of the metals/metalloids occurs because the former group dissolves as anions, enhancing their mobility, whereas the base metals dissolve as cations and are lost from solution most likely through adsorption to clay surface exchange sites and through formation of secondary copper chlorides, carbonates, and oxides. Most groundwaters within and down-flow of the deposit have Eh-pH values around the Fe-II/Fe-III phase boundary, limiting the impact of Fe-oxyhydroxides on oxyanions mobility. Se, Re, Mo, and As are all mobile (with filtered/unfiltered samples similar to 1) to the limit of sampling 2 km down gradient from the deposit. The increase in ore-related metals, metalloids, and sulfate and decrease in sulfate-S isotope ratios (from values similar to regional salars, +4 to +8%o delta(CDT)-C-34 to lower values closer to hypogene sulfides, +1 to +4 parts per thousand delta S-34(CDT)) is consistent with active water-rock reactions between saline groundwaters and the Spence deposit: It is likely that hypogene and/or supergene sulfides are being oxidized under the present groundwater conditions and mineral saturation calculations suggest that secondary copper minerals (antlerite, atacamite, malachite) may also be actively forming, suggesting that supergene and exotic copper mineralization is possible even under the present hyperarid climate of the Atacama Desert. (C) 2007 Elsevier B.V. All rights reserved.