Alluvial fan sediments of the Lower Cretaceous Coloso Formation of northern Chile were deposited during the evolution of a half-graben, arid, intra-arc basin. The sediments were derived from an andesitic volcanic arc and show compositional variation which can be interpreted as an unroofing sequence. Copper mineralization is stratigraphically restricted to the upper 150 m of the sequence in conglomerates of predominantly granodioritic composition. The presence of mineralized clasts at this level indicates the erosion of bedrock mineralization. Lead isotopic signatures of ore sulphide support the sedimentological evidence for derivation of the sediments and metals from an island arc volcano-plutonic complex to the west of the basin.

A genetic model is proposed in which uneconomic metal concentrations were transported into the basin as detrital phases and released by diagenetic reactions. The precipitation of copper minerals followed authigenic formation of quartz, feldspar, analcime and calcite and also some secondary parosity generation. Copper minerals include the oxychloride atacamite, and the suphides bornite, digenite, djurleite and covellite. Sulphur isotope values for the sulphides are relatively heavy isotope-enriched (δ34S = −3.6 to −12.8‰). Although these results could be interpreted in terms of closed-system bacterial sulphide reduction there is no known occurrence of organic material within the Coloso basin. An alternative explanation involving the mixing of diagenetically derived copper-rich brines and sulphur-rich hydrothermal fluids is proposed to account for the Coloso mineralization.