Linking Hydrothermal Processes to Trace Metal Variations in Pyrite from Geothermal Systems in Iceland

Pyrite is the most widespread sulfide mineral in hydrothermal and geothermal systems. Its geochemistry records changes in fluid and precipitation conditions and therefore provides a valuable tool for investigating hydrothermal processes. Sulfide mineralization and sulfide trace element compositions are controlled by fluid composition, temperature, pressure, pH and redox state. Thus, pyrite compositions can provide constraints on fluid origin and geochemical trends in pyrite help identify geothermal processes such as boiling, mixing, and cooling. However, different processes can produce similar geochemical trends and may overprint one another, making it difficult to attribute specific trends to individual processes. Here, we combine geochemical numerical modelling with natural trace metal datasets of pyrite from hydrothermal systems along the Iceland rift to decipher hydrothermal processes leading to metal enrichment in pyrite. 
Pyrite from boreholes, from seawater-fed and meteoric water-fed high-temperature geothermal systems located along the active Iceland rift, was sampled at regular depth intervals. Trace metal concentrations and δ³⁴S compositions were measured. The δ³⁴S values (+3.4 to +19.7 ‰) of pyrite from seawater-fed geothermal systems were systematically elevated compared to δ³⁴S values of pyrite from meteoric water fed systems (-13.1 to +2.4 ‰). Concentrations of Ni, Co, Te, Se, Ge, and Bi, along with Te/As, Co/Mo, and Se/Tl ratios in pyrite decreased with decreasing depth and temperature. Thallium, Sn, Mo, and In concentrations, along with Sb/Pb, Se/Te, and Tl/Pb ratios, increased toward the surface and cooler conditions.
Geochemical numerical modelling was used to evaluate trace metal behavior during pyrite formation under different hydrothermal processes, including progressive alteration, boiling, fluid mixing, and cooling. To achieve this, the thermodynamic databases implemented in PHREEQC were substantially expanded to include internally consistent thermodynamic data for a wide range of trace metal fluid species as well as numerous sulfide endmembers. The integration of modeling results with the observed trace metal systematics indicates that pyrite formation along the Iceland rift is dominantly associated with boiling of ascending hydrothermal fluids. Furthermore, the modelling suggests that direct magmatic contributions to either sulfur sources or trace metal budgets in pyrite are negligible, with host rock leaching and seawater (in coastal systems) representing the dominant sources of sulfur and metals.