A review of intermediate sulfidation epithermal deposits and subclassification

Abstract

Intermediate sulfidation (IS) veins is one of the subtypes of epithermal deposits formed in subduction-related arc settings or post-collisional orogenic belts. The economic and scientific significance of IS deposits has been highlighting importance in Ag-Au-Pb-Zn exploration and study of porphyry-epithermal systems. This epithermal clan of deposits typically have a close relationship with andesitic-dacitic volcanic-subvolcanic rocks, and formed at a depth of ∼0.3 to as much as 1 + km. IS deposits are typically related to oxidized calcic to calc-alkaline magmatism. Fluid homogenization temperatures and salinities range between 150 and 350 °C, and 0 and 23 wt% NaCl equivalent, respectively. The O and H isotope compositions are consistent with a mixture of magmatic and meteoric water, with an increase in meteoric diluent as the hydrothermal system wanes. Most of the IS deposits in the world, particularly those in Circum-Pacific metallogenic belts, formed during Cenozoic time. Several Mesozoic and Paleozoic IS deposits in the Central Asian Orogenic Belt imply great exploration potential for pre-Cenozoic IS deposits in this area.The presence of Mn-carbonate such as rhodochrosite and manganocalcite (locally Mn-silicate, e.g., rhodonite, helvite) typically in mid to late hydrothermal stages is a common diagnostic feature to discern IS from low-sulfidation (LS) deposits. In addition, the occurrence of intermediate-sulfidation state sulfides such as pyrite, chalcopyrite, sphalerite, galena, and tetrahedrite/tennantite associations are another indicator of the IS type; light-colored (Fe-poor) sphalerite is typical of IS deposits, consistent with relatively oxidized fluids. Elevated fluid salinity is another characteristic, with maximum salinity values of base metal-rich IS veins usually >5 wt% NaCl equiv.The reported IS deposits worldwide show that they develop in compressional volcanic arcs as well as in some extensional settings. In this review, IS deposits are subdivided into “NC (Neutral-Compressional)-type IS” with a low Ag/Au ratio (<60), formed in neutral to compressive stress state volcanic arcs, and “E (Extensional)-type IS”, on the contrary, with a high Ag/Au ratio (>60), formed in extensional settings such as extensional intra-arc, post-collisional orogenic belts, and back-arc settings. Another notable feature of E-type IS deposits is their large Ag endowment compared to NC-type IS. NC-type IS (Au ± Ag) deposits can be associated with porphyry Cu-Au and/or high sulfidation (HS) Au-Cu deposits, and their economic metals are mainly gold and/or silver. By contrast, some E-type IS deposits can occur on the flanks of porphyry molybdenum deposits; E-type IS veins can also occur together with LS precious metal veins in back arcs or extensional continental margins, the most representative examples occurring in Mexico. The occurrence of the two subtypes of IS are largely controlled by the parent magma, with parent magma of NC-type IS primarily derived from depleted mantle or juvenile crust, while parent magma of E-type IS mainly from (ancient?) continental crust.The occurrence of IS deposits is presumably controlled by tectono-magmatic settings and fluid evolution paths. Neutral to compressive stress regime, relatively great depth to an exsolving magma (>4 km) and low exsolution rate of magmatic fluids, plus the presence of syn-ore dikes in conjunction with the development of interconnected fracture networks above the porphyry stock could be conducive for the occurrence of IS (and also HS) Au veins upon porphyry copper deposits (PCDs). Confirmation of sub-types and variations of IS veins can aid in exploration for spatially and genetic-related mineralization types, such as porphyry and HS deposits.