We recovered groundwaters around Spence, a large (400 MT) porphyry copper deposit in the Atacama Desert, northern Chile. The deposit is completely covered by thick piedmont gravels. Groundwaters were collected to test hypotheses regarding transport of metals through the cover and to better understand water-porphyry reactions in hyper-arid environments. Groundwaters are variable, with lower salinity upflow of the deposit (<10000 mg/L) than within and downflow of the deposit (10000 to >100000 mg/L). The pH of most waters is in the range pH 6 - 8. Waters within and downflow of the deposit have substantially higher SO4, Re, Se, As, and Mo contents, relative to upflow waters. Copper, Co, Ni, Pb and Zn are also elevated in groundwaters within the deposit, but this increase is not maintained downflow. The difference in behavior for the latter group of elements we attribute to cation formation on dissolution, with strong sorption to colloids, e.g., Fe-oxyhydroxides, and removal from solution. In contrast, Re, Se, As and Mo dissolve as oxyanions, which are not adsorbed and fixed. The oxyanions can travel far in groundwater, thus serving as distal indicators of porphyry mineralization, whereas Cu and other metals that dissolve as cations show elevated contents only in proximity to mineralization.

Rhenium and Se are among the most characteristic elements in groundwaters associated with porphyry deposits. Re occurs within, which readily dissolves upon oxidation. Groundwaters within the deposit, and downflow of the deposit, have elevated Re contents, up to 16 µg/L. Re shows a general increase with increasing TDS and with increasing Se, but shows no consistent relationship with Cu or Mo. The lack of correlation with Cu reflects the precipitation of Cu after oxidation of sulfide minerals and thus its separation from the more mobile Re and Se. Se was present in the magmatic sulfide minerals. In groundwaters its contents range up to 800 µg/L. Consonant with the oxidation of magmatic sulfide minerals, the sulfur isotopic composition, which is >0 per mil upflow of the deposit, approaches 0 per mil within the deposit. The Se contents of the waters increase as the sulfur isotope composition approaches 0 per mil. Elemental and isotopic modeling indicates that in addition to water-porphyry reactions, there is mixing with deeper saline water.