Abstract Peñasquito is an Au-Ag-Zn-Pb deposit and currently the principal Au-producing mine in Mexico. It is the most recent major discovery in the historically important Concepción del Oro mining district. Current Au reserves plus historic production at Peñasquito stand at 12.67 Moz, in addition to 527 Moz Ag, 3,600 lb Pb, and 8,000 lb Zn in remaining proven and probable reserves. Mineralization is centered on the Peñasco and Brecha Azul diatreme breccias, which cut an Upper Jurassic to Upper Cretaceous marine carbonate-dominated sedimentary sequence, which underwent folding during the Laramide orogeny. The diatreme breccias and associated mineralization are associated with early Oligocene quartz-feldspar porphyries dated at 34.4 ± 0.4 to 33.7 ± 0.4 Ma and thus 3 to 10 m.y. younger than the other skarn and polymetallic deposits known in the district. The Peñasco diatreme is about 1 km in diameter and hosts epithermal-style disseminated mineralization, whereas the contiguous Cretaceous carbonaceous and calcareous siltstone and interbedded sandstone of the Caracol Formation is the principal host for stockwork and manto-type, massive base metal sulfide mineralization. Skarn-type mineralization is Cu-Zn rich, extends to the current depth of drilling some 2 km below the premine surface, and is hosted by the Jurassic-Cretaceous sequence beneath the Caracol Formation. In addition, weakly developed stockwork Mo (±Cu) mineralization has also been intersected by drilling at depths of nearly 2 km.

Abstract The landscape from the Central Depression to the eastern Precordillera in the southern Atacama Desert, northern Chile (lat. 26°–27° S), is dominated by multiple pediplains which formed from the late Oligocene through the middle Miocene, followed by canyon incision in the late Miocene. The oldest such planer surfaces are observed in the eastern Precordillera and are represented by the probably late Oligocene Sierra Checos del Cobre and the ≥17.5 Ma Asientos surface. The El Hueso disseminated epithermal Au deposit and La Coya porphyry Au-Cu prospect are hosted beneath the Sierra Checos del Cobre surface. In the western Precordillera and adjacent Central Depression, planar landforms formed throughout the middle Miocene in several stages and gave rise to the Atacama pediplain, which is composite in nature. The El Salvador porphyry Cu deposit and associated exotic mineralization is situated at the back scarp of the Atacama pediplain. Supergene activity at El Salvador occurred from the late Eocene to late middle Miocene, but, based on published ages, was most important from ~25 to ~19 and ~15 to ~12.5 Ma. The multistage landscape evolution and related supergene activity at El Salvador during the late Oligocene and middle Miocene was favorable for the efficiency of supergene enrichment and exotic mineralization but climate desiccation after 13 Ma led to cessation of supergene processes. In contrast, El Hueso, La Coya, and Potrerillos were situated below a relatively stable landscape throughout most of the Miocene. Published alunite ages from El Hueso indicate that supergene activity took place in the late Oligocene and late Miocene but was absent through most of the early and middle Miocene. Late Miocene supergene processes coincided with a time when the Precordillera was uplifted and attained an elevation sufficient to capture increased orographically controlled precipitation. This increase in rainfall led to the incision of the El Salado and Asientos canyons, which, combined with the increased availability of meteoric water, exposed new hypogene mineralization to oxidation in the late Miocene, at a time when at El Salvador supergene activity had ceased.