Geochemical and isotopic composition of auriferous pyrite from the Yongxin gold deposit, Central Asian Orogenic Belt: Implication for ore genesis

Abstract

The Yongxin gold deposit is a large, fracture-controlled ore deposit in the Duobaoshan gold-copper metallogenic belt within the eastern domain of the Central Asian Orogenic Belt (CAOB). Silicification, sericitization and potassic alteration are the main hydrothermal alteration types which display successive zoning surrounding the ore-bodies. Pyrite, galena, sphalerite and chalcopyrite are the major metallic minerals in the deposit, and gold is mainly distributed in pyrite. Here we investigate the trace element composition and isotope (Pb and S) characteristics of pyrite from this deposit. The auriferous pyrite is mainly allotriomorphic and homogeneous in chemistry with a homogenous distribution of As, Ni, Mo and Fe from the core to rim of grains. These features are consistent with the 207Pb/206Pb-206Pb/204Pb, and 208Pb/204Pb-206Pb/204Pb of the grains that suggest a single source with no evidence for involvement of late fluids. The δ34S values range from +3.5‰ to +5.4‰ with an average of +4.48‰, suggesting a mantle-derived magmatic provenance for the sulfur. The high Co content and Co/Ni ratios, low Ni and Mo/Ni ratios suggest a mafic/ultramafic source for the Yongxin gold deposit. The Co-Ni-As relation indicates a magmatic hydrothermal affinity with a classification as epithermal gold deposit. High arsenic and gold contents on the rims of pyrite grains indicate that Au and As were simultaneously precipitated in anoxic, low temperature and near-surface conditions.