Geological Characteristics of Epithermal Precious and Base Metal Deposits

Abstract Epithermal deposits are important sources of gold and silver that form at <1.5-km depth and <300°C in high-temperature, mainly subaerial hydrothermal systems. Such hydrothermal systems commonly develop in association with calc-alkaline to alkaline magmatism, in volcanic arcs at convergent plate margins, as well as in intra-arc, back-arc, and postcollisional rift settings. Many important deposits are T ertiary and younger in age and are concentrated around the Pacific Rim and in the Mediterranean and Carpathian regions of Europe. Older deposits occur in the Tethyan arc from Europe to Asia and others are scattered in volcanic arcs of all ages with rare examples as old as Archean. Precious metal mineralization develops in zones of high paleopermeability, hosted within sequences of coeval volcanic and underlying basement rocks. V eins with steep dips are common and these tend to host highest grade ores. Precious metal mineralization also occurs in breccias, coarse clastic rocks, and intensely leached rocks; such disseminated ore is much lower in grade but greater in total tonnage and may be amenable to bulk mining methods. Deposits and districts, comprising one or more orebodies, cover areas from <10 to -200 km2. Epithermal deposits have been classified on the basis of alteration and gangue mineral assemblages, metal contents, sulfide contents, and sulfide mineral assemblages, and each classification scheme has its merits. Because ores are oxidized by weathering, we prefer a classification that utilizes gangue mineral assemblages. We describe two types of mineralization associated with quartz + calcite + adularia + illite and quartz + alunite + pyrophyllite + dickite + kaolinite assemblages, which reflect the pH of hydrothermal solutions. Epithermal deposits associated with quartz + calcite + adularia + illite contain Au-Ag, Ag-Au, or Ag-Pb-Zn ores. Electrum, acanthite, silver sulfosalts, silver selenides, and Au-Ag tellurides are the main gold- and silver -bearing minerals, with generally minor sphalerite, galena, and chalcopyrite; in some deposits base metals dominate the metal assemblage. Quartz is the principal gangue mineral accompanied by variable amounts of chalcedony, adularia, illite, pyrite, calcite, and/or rhodochrosite, the latter in more Ag- and base metal-rich deposits. Distinctively banded crustiform-colloform textures, and lattice textures comprising aggregates of platy calcite and their quartz pseudomorphs, are common. Hydrothermal alteration is zoned and comprises deep regional propylitic alteration, which gives way upward to increasing amounts of clay, carbonate, and zeolite minerals, whereas quartz, adularia, illite, and pyrite form proximal alteration zones enveloping orebodies. Ore-grade mineralization commonly terminates upward, and where there has been minimal erosion, it can be concealed beneath regionally extensive blankets of clay-carbonate-pyrite or kaolinite-alunite-opal +pyrite alteration. Fluid inclusion data indicate salinities are commonly <5 wt percent NaCl equiv for Au-Ag deposits and <10 to >20 wt percent NaCl equiv for Ag-Pb-Zn deposits. Stable isotope data indicate that hydrothermal solutions were composed mostly of deeply circulated meteoric water, with a nil to small and variable component of mag-matic water. Epithermal deposits associated with quartz + alunite + pyrophyllite + dickite + kaolinite assemblages contain Au + Ag + Cu ores. Native gold and electrum are the main ore-bearing minerals, with variable amounts of pyrite, Cu-bearing sulfides and sulfosalts such as enargite, luzonite, covellite, tetrahedrite, and tennantite, plus sphalerite and telluride minerals; enargite dominates the Cu sulfides and indicates a high-sulfidation state. Quartz (both massive and vuggy) and alunite are the main gangue minerals with kandite minerals (dickite and/or kaolinite) and/or pyrophyllite. Concentric patterns of hydrothermal alteration envelop thEzone of vuggy and massive quartz alteration, which hosts ore. Outward, these comprisEzones of quartz and alunite, dickite + kaolinite or pyrophyllite, and illite or smectite alteration, surrounded by regional propylitic alteration. Zones of illite or pyrophyllite alteration occur in the roots beneath some deposits. Fluid inclusion data indicate that salinities are typically <5 to 10 wt percent NaCl equiv but may be as high as >30 wt percent NaCl equiv. Stable isotope data indicate that the altering fluids are composed mostly of magmatic fluids with a minor to moderate component of meteoric water. Critical genetic factors include: (1) at several-kilometers depth, the development of oxidized and acidic versus reduced and near-neutral pH solutions, controlled by the proportions of magmatic and meteoric components in solution, and the amount of subsequent water -rock interaction during ascent to the epithermal environment; (2) at epithermal depths, the development of boiling and/or mixing conditions which create sharp physical and chemical gradients conducive to precious and base metal precipitation; and (3) at shallow level, the position of the water table, which controls the hydrostatic pressure-temperature gradients at depth where epithermal mineralization forms. Epithermal mineralization can occur in large areas, with orebodies that range in shape, size, and grade, and lie easily concealed beneath blankets of clay alteration or unaltered volcanic deposits. Efficient exploration requires integration of all geological, geochemical, and geophysical data, from regional to deposit scale. Vein mineralogy and texture, patterns of hydrothermal alteration, patterns of geochemical dispersion, and three-dimensional interpretation of related geophysical signatures are important guides. W illingness to drill is crucial, as surface features may not reliably indicate what is present at depth.