Fingerprinting fluctuations in auriferous fluid chemistry in pyrite zonation: Implications for ore-forming processes

Gold can be hosted by pyrite in a multiplicity of hydrothermal ore systems, but the processes controlling the precipitation of Au and its incorporation into pyrite remain unclear. Here, we present a detailed petrographic, geochemical, and theoretical study of pyrite from the Liba gold deposit, China, using both trace element chemistry and sulfur isotopes. Gold-bearing pyrite grains from Liba record a compositional zonation including a Co-Ni−rich, an As-Au−poor core (Py-1), an As-rich mantle (Py-2), and finally an outmost rim (Py-3) enriched only in Co and Ni. An As-Au-Cu−rich thin zone (Py-2a) with numerous fine pores and arsenopyrite inclusions exists between Py-2 and Py-3. The range of sulfur isotopes is narrow, showing a gradually decreasing trend from Py-1 (mean = +9.57‰) to Py-1a (mean = +8.45‰), Py-2 (mean = +7.82‰), and Py-2a (mean = +6.82‰) and an increase in Py-3 (mean = +9.38‰), which is similar to Py-1. We interpret the periodic cycling of trace elements and sulfur isotopes as indicative of multiple, consecutive fluid-release events that tapped one single reservoir, which led to the formation of zoned pyrite. The elevated As and depleted S contents in Py-2 potentially indicate a change in sulfur fugacity, which would have led to a decreased solubility of Au in the hydrothermal fluid. Elemental variations from Py-2 to Py-2a (i.e., Au/Cu ratios ranging from 0.01 to 1) and the presence of pyrrhotite in Py-2a suggest that mineralization occurred under reducing conditions. Lower fO2 and fS2 would lead to gold supersaturation in the fluid, triggering Au precipitation in Py-2a. Additionally, coupled dissolution and reprecipitation reactions are found to play a role during the formation of key features observed in Liba pyrites (e.g., formation of porosity and inclusions), which promote fluid evolution. In addition, theoretical evaluation reveals that Cu incorporation may create conditions favorable for Au uptake into the pyrite structure, triggering As-Au-Cu enrichment in the rims. Our use of multiple scales of observation provides significant insights into the complex formation processes of mineralized hydrothermal systems.