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Abstract

Skarn deposits are one of the more abundant ore types in the earth’s crust and form in rocks of almost all ages. Skarn is a relatively simple rock type defined by a mineralogy usually dominated by calcsilicate minerals such as garnet and pyroxene. Although the majority of skarns are found in lithologies containing at least some limestone, they can form in almost any rock type during regional or contact metamorphism and from a variety of metasomatic processes involving fluids of magmatic, metamorphic, meteoric, and/or marine origin. Although most are found adjacent to plutons, they also can occur along faults and major shear zones, in shallow geothermal systems, on the sea floor, and at lower crustal depths in deeply buried metamorphic terranes. Thus, neither a pluton nor limestone is necessarily required to form skarn. Most skarn deposits are zoned, and the general pattern is proximal garnet, distal pyroxene, and minerals like wollastonite, vesuvianite, or massive sulfides and/or oxides near the marble front. Recognition of distal alteration features such as bleaching, fluid escape structures, and isotopic halos can be critically important in exploration. Because most economic skarn deposits are related to magmatism, details of igneous petrogenesis and tectonic setting form a framework for exploration and classification.

For the seven major skarn types (Fe, Au, Cu, Zn, W, Mo, and Sn) a general correlation exists among igneous major and trace element composition and skarn type. Plutons associated with Fe and Au skarns contain significantly more MgO and less K2O or SiO2, Au and Sn skarn plutons are more reduced, and Cu, Zn, and Mo skarn plutons are more oxidized than average skarn plutons. In terms of geochemical evolution, there is a fairly linear array from relatively primitive calcic Fe skarn plutons through Au, Cu, Zn, to W, Mo, to relatively evolved Sn skarn plutons. Calcic Fe skarn plutons are metaluminous, high in compatible elements such as Ni, V, and Sc, and have Rb/Sr <1. Relative to Fe skarn plutons, Cu skarn plutons have higher Si, K, Ba, Sr, La, and Fe3+/Fe2+, and contain significantly less Mg, Sc, Ni, Cr, and V. Au skarn plutons are similar to Fe skarn plutons in their metaluminous nature and their Si, Mg, Cr, and Sc contents and are similar to Cu skarn plutons in their Ni, V, and Y content. In general, plutons associated with Sn, Mo, and Wskarns have a much stronger crustal signature than do plutons associated with other skarn types.

The seven major skarn classes have some distinctive differences. Iron skarns are mined for their magnetite content although minor amounts of Cu, Co, Ni, and Au may be present. Endoskarn may exceed exoskarn, including widespread albite, orthoclase, and/or scapolite that sometimes are of regional extent. Gold skarns span a range of geologic environments but most deposits are relatively reduced, are mined solely for their gold content, and have a distinctive Au-Bi-Te-As ± Co geochemical association. Most are associated with reduced, mafic plutons and dike/sill complexes with distal/early biotite ± K-feldspar hornfels aureoles; Fe-rich pyroxene is a characteristic mineral. Copper skarns are associated with I-type, magnetite series, calc-alkaline, porphyritic plutons, many of which have cogenetic volcanic rocks, stockwork veining, brittle fracturing and brecciation, and intense hydrothermal alteration; features collectively indicative of a relatively shallow environment of formation. Copper skarn mineralogy is dominated by andraditic garnet. Most Zn skarns occur distal to associated igneous rocks and are mined predominantly for Zn, although Cu, Pb, and Ag can be economically important. They can be distinguished from other skarn types by their Mn-and Fe-rich mineralogy. Most Zn skarn districts grade outward from skarn-rich mineralization to skarn-poor ores, veins, and massive sulfide bodies, which may contain few if any skarn minerals. Tungsten skarns are associated with coarse-grained, generally unaltered, equigranular batholiths (with pegmatite and aplite dikes) surrounded by large, high-temperature, metamorphic aureoles; subcalcic garnet is a characteristic mineral. Molybdenum skarns are associated with leucocratic granites and commonly contain accessory W, Cu, Zn, Pb, Bi, Sn, and U; some are truly polymetallic in that several metals need to be recovered together in order for the deposits to be economic. Hedenbergitic pyroxene is the most common calc-silicate mineral in Mo skarns and wollastonite, amphibole, and fluorite may be unusually abundant. Tin skarns are almost exclusively associated with high silica granites, typically generated by partial melting of continental crust, and have a characteristic suite of trace elements (Sn, F, B, Be, Li, W, Mo, and Rb). Many Sn skarn deposits develop a greisen alteration stage, characterized by high fluorine activities and the presence of minerals like fluorite, topaz, tourmaline, and muscovite, which is superimposed upon plutons, early skarn, and unaltered carbonate rocks.

Most large skarn deposits record a transition from early/distal metamorphism resulting in hornfels, reaction skarn, and skarnoid, to later/proximal metasomatism resulting in relatively coarse grained ore-bearing skarn. Fluid inclusion and isotopic evidence indicates early metasomatism is caused by high-temperature (≥500°C), high-salinity (>50 wt % total salts) fluids of magmatic origin, which typically are enriched in Si, K, Na, Al, Fe, and Mg, and depleted in Ca, 18O, and CO2 relative to protoliths. Documented daughter minerals in fluid inclusions in skarn minerals include NaCl, KCl, CaCl2, FeCl2, CaCO3, CaF2, C, NaAlCO3(OH)2, Fe2O3, Fe3O4, AsFeS, CuFeS2, and ZnS. In addition, melt inclusions have been found in some skarn minerals. These contain complex daughter mineral assemblages and evidence for both chloride and carbonate melts at the time of skarn formation. Analyses of skarn-hosted inclusion fluids, both liquid and vapor, show high concentrations of K, Ca, Mn, Fe, Cl, Br, Cu, Pb, Zn, and As. With time, skarn metasomatic alteration evolves to lower temperature (≤400°C), generally hydrous, and sulfide-rich assemblages, termed retrograde alteration. Such retrograde alteration usually is accompanied by brecciation and is caused by influx of cooler, lower salinity (≤20 wt % total salts) fluids that are still magmatic in origin. One important difference between prograde and retrograde alteration is thought to be the timing and extent of phase separation of hydrothermal fluids.

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