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 Lihir gold deposit, Papua New Guinea, is the world’s largest alkalic low-sulfidation epithermal gold deposit in terms of contained gold (50 Moz). The deposit formed over the past million years and records a progression from porphyry- to epithermal-style hydrothermal activity. The early porphyry stage was characterized by biotite-anhydrite-pyrite ± K-feldspar ± magnetite alteration and weak gold ± copper mineralization and produced abundant anhydrite ± carbonate veins and anhydrite ± biotite-cemented breccias. These features collectively characterize the deep-seated anhydrite zone at Lihir. Several hundred thousand years ago, one or more catastrophic mass-wasting events unroofed the porphyry system after porphyry-stage hydrothermal activity ceased. Mass wasting may have been facilitated in part by dissolution of porphyry-stage anhydrite veins. Epithermal mineralization occurred after sector collapse, resulting in phreatic and hydraulic brecciation and veining, widespread adularia-pyrite ± carbonate alteration, and formation of mineralized zones at Lienetz, Minifie, Kapit, Kapit NE, Coastal, and Borefields. A NE- to ENE-striking fault array localized several of these orebodies. The pyrite-rich veins and pyrite-cemented breccias that formed during epithermal-stage hydrothermal activity define the sulfide zone at Lihir. This zone mostly contains refractory gold in pyrite, with minor free gold and precious metal tellurides hosted in late-stage quartz veins. A period of diatreme volcanism disrupted the Luise amphitheater during the latter stages of epithermal mineralization. The diatreme breccia complex truncated several of the epithermal ore zones and was crosscut locally by late-stage epithermal veins. Recent geothermal activity produced a steam-heated clay alteration blanket that has overprinted the refractory sulfide-rich epithermal assemblage near the present-day land surface. Gold was remobilized downward from the steam-heated zone into the sulfide zone during argillic and advanced argillic alteration, producing thin gold-rich rims around pyrite grains. This process produced a high-grade tabular enrichment zone immediately beneath the base of the clay blanket.