Textural, compositional and isotopic characteristics of pyrite from the Zaozigou gold deposit in West Qinling, China: Implications for gold metallogeny

Abstract

The Zaozigou gold deposit (ZZGD) located within the West Qinling Orogen in China, with 142 t of Au at an average grade of 2.69 g/t, is hosted in a sequence of metasediments and altered dacitic to granodioritic rocks. The majority of gold is present as invisible gold in disseminated arsenian pyrite. Here we investigate the morphological and compositional evolution of the pyrite grains and constrain the ore-forming fluid source(s) and genetic models related to the ore-forming processes through the application of Scanning electron microscopy, electron probe microanalysis, in-situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and fsLA-MC-ICP-MS analyses. Based on texture and trace element concentrations, we identify five generations of pyrite (namely, Py1, Py2, Py3, Py4, and Py5). Among these, the pre-ore framboidal pyrite (Py1), which is characterized by relatively low concentrations of As, Au, Co, Ni, and Sb, has negative δ34S values of −31.07 to −20.79‰, which we correlate to bacterial reduction of marine sulfate during sedimentation and diagenesis. The auriferous Py2 and Py5, which occur as either porous aggregates or epitaxial overgrowth on earlier pristine pyrite, contain relatively high concentrations of As, Sb, Tl, and Au compared with pyrite in the other generations. Both (Py2 and Py5) display a relatively narrow range of negative δ34S values, ranging from −5.66 to −1.16‰, which are interpreted to result from a magmatic source. Gold-free Py3 and Py4 exhibit relatively variable δ34S values of −17.1 to −6.02‰, indicating that a small amount of sulfur in the sedimentary strata was added to the magmatic hydrothermal systems during fluid-rock interaction. Sulfur isotopes and LA-ICP-MS mapping of pyrite grains revealed that the gold endowment of the ZZGD is associated with multiple mineralizing events and multiple magmatic and hydrothermal activities. We propose that gold in the arsenian pyrite from the ZZGD was scavenged in ore-forming fluids by chemical adsorption onto the As-rich, Fe-deficient, and actively growing surfaces of pyrite. Our results indicate that the primary source of the sulfur, gold, and mineralizing fluids was likely fluids from magmatic hydrothermal systems in the West Qinling orogen.