Role of As in the formation of giant Au deposits: Insights from sulfur isotope and geochemistry of pyrite from the Shuangwang Au deposit, West Qinling, central China

Abstract

The Shuangwang Au deposit (SWGD), located in the Feng-Tai ore district, is one of the largest Au deposits in the West Qinling orogen, central China. Au orebodies in this deposit are mainly hosted in the Upper Devonian Xinghongpu Formation. The source of the fluids and metals ore-forming processes and the enrichment mechanism of Au in this ore deposit remain equivocal. Here we present results from geochemical studies of pyrite from the SWGD to address these issues through the application of Scanning electron microscopy (SEM), electron probe microanalysis (EPMA), in-situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and LA-MC-ICP-MS analyses. Based on detailed petrographic studies of the mineralization and host rocks, we identify seven generations of pyrite. Among these, the pre-ore syn-sedimentary pyrite (Py0) is characterized by relatively elevated concentrations of As, Co, Ni and Au compared to the Py1 in the albite-ankerite veins and Py2 in the quartz-ankerite-pyrite veins, and has positive δ34S values of + 6.00 to + 10.48 ‰, whereas Py3 forms as veinlets or aggregates in the pyrite-tourmaline veins and is enriched in As, Co, Cu, Se and Au. Py4 occur as coarse-grained subhedral to euhedral crystals or aggregates in the ankerite-pyrite veins and exhibit oscillatory zones. Py5 comprises fined-grained subhedral to anhedral grains in the fluorite-ankerite-pyrite cements, and is relatively depleted in trace elements when compared to the other pyrite types. The δ34S values of the different types of pyrite from the SWGD have significant differences, indicating a multiple and mixed source for sulfur and, by inference, for the ore-forming fluid(s). Two periods of Au mineralization are identified in the SWGD: the early NW-trending mineralization sourced from the metamorphism of the metasedimentary sequences in a compressional environment and the late NE-trending mineralization probably exsolved from an unexposed and coeval granitoid pluton at depths in a post-collisional extensional setting. The coupled geochemical behavior of Au and As can be explained by mixing-induced cooling. Our results also indicate that arsenic might have played an important role in the enrichment of Au and the formation of this hydrothermal Au deposit.