Sulfur isotope and trace element systematics of zoned pyrite crystals from the El Indio Au–Cu–Ag deposit, Chile

Abstract

crystals in high-temperature feeder zones range from −3.19 to 1.88 ‰ (±0.5 ‰), consistent with sublimation directly from a high-temperature magmatic vapor phase. Late pyrite crystals are distinctly more enriched in δ 34 S than early pyrite (δ 34 S = 0.05-4.77 ‰, ±0.5 ‰), as a consequence of deposition from a liquid phase at lower temperatures. It is unclear whether the late pyrite was deposited from a small volume of liquid condensate, or a larger volume of hydrothermal fluid. Both types of pyrite exhibit intracrystalline δ 34 S variation, with a range of up to 3.31 ‰ recorded in an early pyrite crystal and up to 4.48 ‰ in a late pyrite crystal. Variations in δ 34 S pyrite at El Indio did not correspond with changes in trace element geochemistry. The lack of correlation between trace elements and δ 34 S, as well as the abundance of microscale mineral inclusions and vugs in El Indio pyrite indicate that the trace element content of pyrite at El Indio is largely controlled by nanoscale, syndepositional mineral inclusions. Co and Ni were the only elements partitioned within the crystal structure of pyrite. Cu-rich oscillatory zones in early pyrite likely formed by nanoscale inclusions of Cu-rich sulfosalts or chalcopyrite, evidence of deposition from a fluid cyclically saturated in ore metals. This process may be restricted to polymetallic high-sulfidation-like deposits.

Geochemical and micro-textural fingerprints of boiling in pyrite

“…Studies in geothermal systems and epithermal Au-Ag deposits have shown that pyrite can display a large variety of textures, including brecciated, colloform, porous, fibrous and inclusionrich textures (e.g., Deditius et al, 2009a, b;Franchini et al, 2015;Tanner et al, 2016;Kouhestani et al, 2017). In the CPGS, pyrite grains show several of these textures and morphologies, including porous and inclusion-rich zones, and brecciated subhedral to euhedral pyrite grains (Figs.…”

“…Pyrite is a ubiquitous and abundant sulfide in ore deposits, and several geochemical studies have highlighted its role as a major Au-bearing phase and scavenger of metals and metalloids. Most notably, pyrite has been used as a geochemical tracer in a wide variety of hydrothermal ore deposits including orogenic, sediment-hosted Carlin-type, epithermal Au deposits, volcanic-massive sulfide (VMS), porphyry Cu and iron-oxide apatite (IOA) deposits (Cook and Chryssoulis, 1990;Fleet et al, 1993;Huston et al, 1995;Simon et al, 1999;Vaughan and Kyin, 2004;Reich et al, 2005Reich et al, , 2006Reich et al, , 2013Reich et al, , 2016Large et al, 2009Large et al, , 2014Cook et al, 2009a;Deditius et al, 2009aDeditius et al, ,b, 2011Deditius et al, , 2014Koglin et al, 2010;Franchini et al, 2015;Gregory et al, 2015a;Deditius and Reich, 2016;Tanner et al, 2016;Keith et al, 2018). These studies have provided not only a better understanding of metal speciation and partitioning during mineral precipitation, but also have illustrated how physico-chemical processes drive changes in trace element distributions during superimposed events, including hydrothermal alteration, metamorphism and/or associated deformation (Large et al, 2007;Cook et al, 2009a;Thomas et al, 2011;Reich et al, 2013;Deditius et al, 2014;Steadman et al, 2015;Meffre et al, 2016).…”

“…Furthermore, the significant diversity of textural features in pyrite has been investigated in detail. A large amount of our current knowledge derives predominantly from textural interpretations in orogenic Au systems (Large et al, 2007(Large et al, , 2009Cook et al, 2013;Gregory et al, 2016), sediment-hosted Au deposits (Large et al, 2014(Large et al, , 2015, VMS (Maslennikov et al, 2009(Maslennikov et al, , 2017Genna and Gaboury, 2015;Wohlgemuth-Ueberwasser et al, 2015;Keith et al, 2016;Melekestseva et al, 2017;Soltani Dehnavi et al, 2018), porphyry-Cu (e.g., Reich et al, 2013;Franchini et al, 2015) and epithermal Au-Ag deposits (e.g., Franchini et al, 2015;Tanner et al, 2016;Kouhestani et al, 2017;Sykora et al, 2018). These studies have shown that pyrite textures can record physico-chemical conditions during precipitation, such as supersaturation or near-equilibrium crystallization, as well as post-formational events, i.e., deformation, dissolution-reprecipitation and recrystallization processes (Cook et al, 2009a).…”

“…Recently, the well documented variations in chemistry and texture of pyrite have been linked to changes in fluid composition related to abrupt changes in P-T-X conditions (e.g., Peterson and Mavrogenes, 2014;Tanner et al, 2016;Sánchez-Alfaro et al, 2016;Tardani et al, 2017).…”

“…Hence, detailed information about textural features of pyrite from active continental geothermal systems is limited, and links to its geochemical signature have not been explored. This is of utmost importance because pyrite from young and active geothermal systems shows fewer growth zones and/or chemical oscillations associated with discrete boiling and fluid mixing events (Tardani et al, 2017), which provide crucial information to interpret the reported complex zoning of pyrite in ore deposits (e.g., Reich et al, 2013;Tanner et al, 2016). This is mostly due to the fact that pyrite in ore deposits records a time-integrated sequence of fluid flow episodes, hindering the accurate identification of major precipitation events.…”