Fingerprinting fluid evolution by trace elements in epithermal pyrite, Vatukoula Au-Te deposit, Fiji

Abstract

Tellurium (Te) is an important element for green technologies, including thin film photovoltaic panels. Stagnating supply caused by its by-product dependency together with an increasing demand will likely lead to a shortage in the near future. Pyrite may represent a potential target for the direct recovery of Te, since the concentrations of Te can reach weight percent levels under specific, but poorly constrained hydrothermal conditions. The Vatukoula (formerly Emperor) epithermal Au-Te deposit is currently mined for Au and ~ 75% are recovered from Au-tellurides and pyrite. Tellurium has been processed in the past, but is now sent to tailings despite of concentrations reaching 1.4 wt. % in pyrite, which makes this system a natural laboratory to investigate key Te enrichment processes.The epithermal mineralisation at Vatukoula consists of an early low-grade disseminated host-rock and a later high-grade Ag-Au-Te vein-type stage. Pyrite occurs in both ore-types with diverse As contents including concentrations < 0.5 wt. % in pure pyrite, 0.5 to 13 wt. % in low-As and 23 to 43 wt. % in high-As pyrite, reflecting increasing fluid As with system evolution towards the high-grade mineralisation. Tellurium contents in pyrite increase from pure pyrite (up to 220 ppm) to low-As pyrite (up to 1.4 wt. %), but with a decrease towards high-As pyrite (up to 185 ppm). This is a yet unknown relation, since trace element incorporation in pyrite is usually facilitated by increasing As. Instead, the presented results suggest a threshold between 13 and 23 wt. % As, above which the incorporation of Te and related elements (e.g., Ag, Au) may be less favourable. Mineral stabilities, trace element ratios (e.g., Sb/Pb, Tl/Pb) and LA-ICP-MS mapping of texturally distinct pyrite indicate that fluid boiling was a critical process for the Ag, Au and Te enrichment in the Vatukoula epithermal system. Fluid sulphidation states decreased with proceeding evolution of the epithermal system from initially intermediate- to late low-sulphidation conditions, which led to the diverse chemical character of pyrite. Early pure pyrite primarily formed as a replacement product of magnetite in the altered host rocks, whereas low- and high-As pyrite precipitated in the direct vicinity of Ag-Au-telluride veins, from which most of the As, Ag, Te and Au hosted in pyrite was derived from. Consequently, the trace element composition of pyrite allows the fingerprinting of the fluid evolution of Te-rich ore-forming systems like Vatukoula, Fiji.