Genesis of Xinjiazui Gold Deposit: In Situ Geochemical Constraints from Arsenopyrite

Abstract

The Xinjiazui gold deposit marks a notable significance in prospecting within the Back-Longmenshan tectonic belt, located on the northwest margin of the Yangtze Block, China. Despite the extensive studies conducted on this deposit, the source of the ore-forming materials remains unclear, leading to ongoing debates regarding the genesis of this deposit. This study analyzed in situ (EPMA and LA-ICP-MS) trace elements and S-Pb isotopes of arsenopyrite, solely from the principal metallogenic stage and paragenetic with native gold. The results show that the gold in arsenopyrite occurs as invisible gold (Au3+), with an average concentration of 9.38 ppm, whereas the concentrations of magma-related elements, such as W, Sn, Mo, and Bi, are very low. The sulfur isotopes (34S) of arsenopyrite range from 8.32‰ to 10.16‰, aligning closely with the deep metamorphic basement (Pt3l). Meanwhile, the lead isotopes in arsenopyrite display characteristics typical of those found in orogenic belts. A comprehensive analysis of the abundance of gold indicated that the metallogenic materials (sulfur and gold) primarily originated from Pt3l. Additionally, the arsenopyrite thermobarometer indicated that the Xinjiazui gold deposit formed in a medium–low-temperature, medium metallogenic environment (5.57–8.69 km), with a sulfur fugacity (log f (S2)) below −8.4. Combined with previous research results, this study proposes that the Xinjiazui gold deposit is a subduction-related mesozonal orogenic gold deposit. In gold prospecting and exploration in the Back-Longmenshan tectonic belt, it is essential to focus on the distribution of brittle-ductile shear zones and location of the quartz veins associated with pyrite and arsenopyrite mineralization.