Age of supergene oxidation and enrichment in the Chilean porphyry copper province

Twenty-five samples of supergene alunite collected from deeply developed supergene profiles in porphyry copper deposits and prospects between latitudes 20 ø and 27 ø S in northern Chile yield K/Ar ages ranging from about 34 to 14 Ma. Therefore supergene oxidation and enrichment processes were active from the early Oligocene to the middle Miocene, aminimum of 20 m. y. Supergene activity at individual deposits lasted for at least 0. 4 to 6. 2 m. y. The early Oligocene supergene activity affected eposits in the Paleocene porphyry copper belt, whereas early and middle Miocene supergene processes are documented in the Early Cretaceous, Paleocene, and late Eocene to early Oligocene porphyo, copper belts. Middle Miocene oxidation also affected the oldest epithermal gold-silver deposits in the Maricunga belt farther east. Supergene activity commenced no less than 11 m. y. after generation of each porphyry copper deposit because of the time required to unroof the copper-bearing parts of the system. Supergene activity throughout northern Chile ceased at-14 Ma. The geologic features of deposits and prospects and their morphotectonic positions, present latitudes, and present elevations display no obvious correlations with the supergene chronology. Exploration for major cumulative nrichment blankets hould not be carried out either beneath thick sequences of piedmont gravels (_ ignimbrites) ofOligocene through middle Miocene age unless their accumu-