Leybourne and Cameron reported in 2008 on the groundwater in and around a large (313 Mt at 1.08% Cu and 0.3% cutoff) undisturbed porphyry copper deposit (Spence) in the hyper-arid Atacama Desert of northern Chile, which is buried beneath 30–180 m of Miocene piedmont gravels. Groundwater within and down-flow of the Spence deposit have elevated Se (up to 800 μg/l), Re (up to 31 μg/l), Mo (up to 475 μg/l) and As (up to 278 μg/l) concentrations compared to up-flow waters (interpreted to represent regional groundwater flow).
In contrast, Cu is only elevated (up to 2,036 μg/l) in groundwater 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 groundwater within and down-flow of the deposit have Eh–pH values around the Fe+2/Fe+3 phase boundary, limiting the impact of Fe-oxyhydroxides on oxyanions mobility.
Se, Re, Mo, and As are all mobile (with filtered/unfiltered samples ∼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‰ δ34SCDT to lower values closer to hypogene sulfides, + 1 to + 4‰ δ34SCDT) is consistent with active water–rock reactions between saline groundwater and the Spence deposit.
The authors concluded 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 hyper-arid climate of the Atacama Desert.
Groundwater at the Spence porphyry copper deposit, Atacama Desert, northern Chile are characterized by elevated concentrations of metals and metalloids characteristic of this style of ore deposit. Groundwater up-flow (discharge) from the deposit represent the regional groundwater and are characterized by relatively low concentrations of metals and metalloids characteristics of porphyry copper ores in northern Chile, i.e., Cu, Mo, Se, Re, As. In contrast, most ore-related metals, metalloids and SO4 are elevated (Se up to 800 μg/l, Re up to 31 μg/l, Mo up to 475 μg/l and As up to 300 μg/l) in groundwater within and down-flow of the deposit. Similarly, copper (up to 1,300 μg/l; decline water = 29,000 mg/l) and some other metal cations (e.g., Pb, Zn, Ni, and Co) have elevated concentrations in groundwater within the deposit, but lower concentrations down-flow of the deposit.
Within the deposit, regional meteoric (recharge) waters from the east mix with saline waters of deep origin. Elevated salinities down-flow from the deposit indicates that inflow of saline waters dominates. The concentrations of ore associated metals and metalloids are elevated in these groundwater after accounting for changes in salinity indicating that these groundwater have interacted with the porphyry copper ore. Coincident with increasing concentrations of ore-associated metals and metalloids, groundwater δ34SCTD ratios decrease from regional values (+ 5 to + 8‰) towards ore-values (around 0‰), consistent with groundwater–deposit interaction.
The different behavior of Se, Re, As and Mo compared to Cu in the Spence groundwater is probably a function of speciation of the metals and metalloids and Eh–pH conditions of the groundwater; the former group behave as anions or neutral species in these groundwater, enhancing their mobility, whereas the base metals are present as cations and are lost from solution most likely through adsorption to colloidal material, consistent with differences in filtered/unfiltered ratios.
The authors also concluded that their groundwater geochemical data from the Spence deposit indicate that sulfides (in the hypogene and/or supergene zones) are being oxidized under the present conditions and mineral saturation calculations suggest that secondary copper minerals (antlerite, atacamite, malachite) may also be actively forming. Thus, supergene mineralization processes may be ongoing, despite the hyper-arid conditions and the assumption that these processes ceased in the mid-Miocene or towards the end of the Miocene.
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