Abstract

Northern Chile is a major producer of copper from porphyry deposits. Most ores worked to date were exposed near the surface. Exploration is now focused on deposits along a basement unconformity buried under piedmont gravels deposited by ca. 10 Ma. Two recent discoveries, the Spence and Gaby Sur deposits, are covered by gravel. Soils above these deposits have anomalies for elements present in saline, mineralized groundwaters. The anomalies occur above fracture zones in the gravels, believed to represent the upward propagation of basement faults that guided emplacement of the porphyries. We propose that during earthquakes in this seismically active region there was pumping of saline basement waters up the faults and fracture zones, entraining mineralized groundwaters from the deposits. After surface flooding and evaporation, elements were redistributed by infrequent rains. Sampling shows that surface-active cations, such as Cu2+, were adsorbed and retained in the top few centimeters of soil, whereas NaCl and porphyry indicator elements that dissolve as anions (Se, Re, and As) are not adsorbed and are removed to >40 cm depth. Given the 10 m.y. interval since the gravels were deposited, and the hyperarid climate of the Atacama Desert, anomalies may have formed by repeated episodes of seismically induced flooding. Company geochemical surveys show that such features are numerous in northern Chile, producing linear zones of saline soils plus other constituents of local groundwaters (e.g., B and I). Where faults controlling these features intersect mineralization, elements specific to the mineralization are also present. Saline zones may be outlined in the field by measuring the conductivity of soil plus water slurries; sampling for full geochemical analyses can be limited to areas so defined. Anomalies may be generated along fault lines in other seismically active, wetter climates, but only for elements that are adsorbed on soil minerals. Although the context of the study is mineral exploration, the process may be relevant to the formation of geochemical features of environmental interest above major faults.

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