Physiographic and tectonic settings of high-sulfidation epithermal gold–silver deposits of the Andes and their controls on mineralizing processes

Gold and silver ores in the vast majority of Andean high-sulfidation epithermal Au–Ag deposits occur at high present day elevations and typically 200–500 m below low relief landforms situated at 3500 to 5200 m a.s.l. Most deposits are middle Miocene and younger and include, El Indio, Tambo, Pascua–Lama, Veladero (El Indio belt, Chile/Argentina), Cerro de Pasco (Central Peru), Pierina, Lagunas Norte, Yanacocha (northern Peru), Quimsacocha (Ecuador), and the California–Vetas mining district (Santander, Colombia), jointly accounting for > 130 Moz Au resources. Slightly older examples are only preserved in the Atacama Desert and include the middle Eocene El Guanaco and El Hueso and the late Oligocene/early Miocene La Coipa deposits. The absence of Paleocene and older high-sulfidation epithermal deposits can be explained by limited preservation potential imposed by transpressional tectonics within overall contractile episodes and surface uplift. These conditions prevailed predominantly in segments of shallow-angle subduction of the Nazca or Caribbean plate below the South American continent, a tectonic setting also common for porphyry-style Cu (–Au, Mo) deposits. Stratovolcanoes are uncommon ore hosts and volcanic rocks coincident with mineralization are in most cases volumetrically restricted or absent, recording the terminal stages of local arc magmatism. However, dacitic domes are important at, e.g., Yanacocha and La Coipa. At Lagunas Norte, a small stratovolcano largely pre-dating but temporally overlapping with mineralization occurs immediately east of the deposit and volcanic sector collapse may have occurred during hydrothermal activity.Mineralization is typically located near the backscarp of pediments or the heads of valleys incising now high-elevation, low-relief surfaces. In the California–Vetas Mining District and El Indio belt, hydrothermal alunite ages become generally younger upstream along the incising valleys, indicating that hydrothermal activity and, by inference, ore deposition were facilitated by erosion. The lowering of the water table and reduction of hydrostatic and lithostatic pressure at these sites of high local relief are believed to have enhanced both boiling and mixing of magmatic with meteoric fluids, ultimately enhancing ore deposition.The host rock composition, permeability and location of the water table control the distribution of alteration zones and ore. Intermediate volcanic rocks are the most common ore-hosts but they typically pre-date mineralization by several Ma. However, high-sulfidation epithermal mineralization can be hosted in any conceivable rock type including high grade metamorphic rocks (California–Vetas mining district), significantly older plutonic rocks (Pascua–Lama) or quartzites (Lagunas Norte). Large vuggy quartz alteration zones and commonly oxidized low-grade large-tonnage mineralization are best developed in relatively permeable volcaniclastic rocks or hydrothermal breccia bodies, whereas coherent volcanic, plutonic, or metamorphic rocks may host fault- and breccia-controlled ores. The near-surface steam-heated zone can attain a thickness of several hundred meters in dry climates (e.g. Veladero, Pascua–Lama, Tambo) but is typically poorly developed and less than 20 m thick in humid climatic zones.The physiographic and tectonic settings of high-sulfidation epithermal deposits are distinct from low-sulfidation epithermal districts such as those of Patagonia, El Peñón (Chile) or Fruta del Norte (Ecuador). The latter range to significantly older ages (Jurassic to early Eocene) occur at mainly lower elevations and were emplaced in extensional settings. A temporal coincidence between uplift, erosion and mineralizing processes as well as a spatial and temporal association with porphyry style mineralization is not evident for these low-sulfidation districts.