Genesis of Zixi gold deposit in Xuefengshan, Jiangnan Orogen (South China): Age, geology and isotopic constraints

Abstract

The medium-sized Zixi gold deposit is hosted in a meta-gabbro–diabase dike, and located geologically in the Xuefengshan belt of the Jiangnan Orogen in South China. Gold-bearing quartz–carbonate–sulfide veins and the ore-hosting meta-gabbro–diabase dike are structurally controlled by the NE-trending first stage faults, which are secondary faults to the regional Xupu–Jingzhou Fault. Metallic minerals at Zixi include arsenopyrite and pyrite, along with minor chalcopyrite, sphalerite, galena, bournonite and native gold. Ore-related alteration includes silicic, carbonate, sericite, and chlorite. Based on vein crosscutting relationships and mineral paragenesis, the alteration/mineralization is divided into three stages: (1) arsenopyrite–pyrite–native gold–quartz mineralization, (2) polymetallic sulfides–native gold–chlorite–carbonate–quartz mineralization, and (3) post-ore carbonate–quartz alteration. To constrain any temporal relationship between the gold mineralization and the ore-hosting meta-gabbro–diabase dike, we obtained a Re–Os isochron age for the gold-bearing arsenopyrite (425 ± 28 Ma, n = 5, MSWD = 0.47), which is 350 Myr younger the meta-gabbro–diabase (zircon U–Pb age: 783.08 ± 0.97 Ma, n = 30, MSWD = 0.39). This precludes the dike from any direct metallogenetic link with the gold deposit. The ore-forming fluids have calculated δ18OH2O and δD values of + 3.5‰ to + 9.1‰ and − 68.1‰ to –55.5‰, respectively, indicating a metamorphic source, probably through dehydration breakdown of chlorite. Sulfur isotope compositions of the gold-bearing pyrite (–5.8‰ to –3.6‰) are distinct from those of the Lengjiaxi Group metamorphic rocks, but marginal overlap with those from the Banxi Group metamorphic rocks. Lead isotope compositions of the gold-bearing pyrite (206Pb/204Pb: 17.08 to 17.63; 207Pb/204Pb: 15.48–15.54; 208Pb/204Pb: 37.64–37.94) suggest that the lead was sourced from metamorphic rocks. The H–O–S–Pb isotopic data indicate that ore-forming materials were derived from a metamorphic source. However, the precise nature of that source is unclear. The Zixi gold mineralization displays many typical features of orogenic gold deposits, such as being structurally controlled, and have similar alteration and mineralization styles, ore mineral assemblages, and H–O–S stable isotopic features. The Zixi gold deposit was likely formed during the early Paleozoic intracontinental orogeny of the Jiangnan Orogen.