Genesis of the Hatu gold deposit: Constrained by in situ S[sbnd]Pb isotope geochemistry of pyrite

Abstract

As the largest gold deposit in west Junggar, the Hatu gold deposit is located between the Anqi and Hatu faults in the Hatu-Sartohay district. The orebodies, composed of gold-bearing quartz veins and auriferous altered wall-rocks, are controlled by NE-, NW-, and EW-trending faults in Early Carboniferous volcano-sedimentary rocks. In this study, integrated textural, in situ sulfur and lead isotope analyses of pyrite grains in the host rocks and ores reveal new information on the sources of the ore-forming fluids and metal, as well as the mechanism of gold deposition. Framboidal pyrite in tuffaceous siltstone has a large δ34S range of −26.7 to +54.0 ‰, which is attributed to sulfur-reducing bacterial (SRB) activity during sedimentary-diagenetic process. Fine-grained subhedral pyrite in meta-basalt shows negative δ34S values from −26.7 to −12.1 ‰, which may be generated by reduction of seawater sulfate by ferrous iron bearing minerals during metamorphism. The δ34S values of four generations of hydrothermal pyrite (Py1–Py4) in the orebodies vary in a narrow range from +1.4 to −1.9 ‰. The sulfur isotopic characteristics of the hydrothermal pyrite are similar to most magmatic hydrothermal deposits, but differ from the metamorphic pyrite in meta-basalt and the framboidal pyrite in tuffaceous siltstone. In addition, the uniform in situ lead isotope ratios of hydrothermal pyrite resemble the lead isotopic compositions of the granitic plutons in the west Junggar, but differ from those of pyrite in meta-basalt. The progressive decrease of δ34S values from early-stage pyrite (+0.7 ‰ to +1.4 ‰) to late-stage pyrite (−1.9 ‰ to +0.4 ‰) corresponds to the major gold precipitation processes, which can be explained by gradual decrease of oxygen fugacity and increase of pH with decrease of temperature of a gold-bearing magmatic-hydrothermal system. A relatively stable environment for Au precipitation probably is a key factor controlling the scale of gold deposit, as the ore-forming fluid has enough time to form large gold deposit such the Hatu gold deposit.