Multiple element mapping and in-situ S isotopes of Au-carrying pyrite of Shuiyindong gold deposit, southwestern China using NanoSIMS: Constraints on Au sources, ore fluids, and mineralization processes

Abstract This study employed high-sensitivity and high-resolution secondary ionizing mass spectrometry (NanoSIMS) to determine the distribution of Au, As, Cu, Se, and S isotopes of pyrite from the Shuiyindong Au deposit, southwestern China. NanoSIMS maps show that Au occurs as solid solution, and heterogeneously distributes at micron scale in the ore pyrite, which is the overgrowth of the preexisting synsedimentary or hydrothermal pyrite core, commonly rimmed again by the As- and Au-poor outermost zone. There is no detectable Au in the synsedimentary pyrite. Invisible Au was observed to fill in the submicron-scale cracks of the ore pyrite. Meanwhile, element line profile analysis shows that Au occurs in three discrete climaxes in the ore pyrite. In-situ S isotopic analyses identified two types of S sources. The synsedimentary pyrites have large negative δ34S values ranging from −38.3‰ to −18.8‰, which is a typical biogenetic S generated by bacterial reduction of sea water sulfate. The hydrothermal pyrites with zoned texture have a narrow δ34S range of −3.5‰～−7.4‰ with a median of 2.3‰, similar to the recently published sulfur isotope data of the Southwest China’s Carlin-type gold deposits except for the Lannigou Mine. Combining the distribution of Au and S isotopes in pyrite indicates that the Au-carrying zones have lower δ34S values than that of the ore-unrelated zones. These NanoSIMS observations indicate that the Au mineralization was closely associated with strong deformation of the host rock. The Au forming the deposit was exotic, instead of the reactivated Au in the preexisting pyrite formed in the synsedimentary basin. The mineralization progress was multistage. The distribution of Au and S isotopes is hard to interpret by the viewpoint of metamorphic fluid or fluid pressure fluctuation. Instead, it is more consistent with magmatic fluid, likely contaminated by sedimentary sulfur during its ascending to the surface. In addition, the multistage ore formation pattern discovered in this study poses new challenges to determine the mineralization age of the Au deposits in southwestern China.