Genesis of the Yezhutang gold deposit, northern margin of the Yangtze Block: Constraints from fluid inclusions, H–O–S isotopes, and pyrite trace element compositions

The Yezhutang gold deposit, a recently discovered medium-sized gold deposit, is located in the southeastern section of the Micangshan terrane, northern margin of the Yangtze Block. Gold orebodies at Yezhutang are mainly distributed in the marble and phyllite of the Mesoproterozoic Mawozi Formation. Two mineralization periods (i.e., hydrothermal and supergene) have been identified and the hydrothermal period is divided into two stages: quartz-pyrite and quartz-muscovite-sulfide. In this study, fluid inclusion analyses, H–O–S isotopes, trace element mapping, and pyrite and limonite concentration measurements were performed to better clarify the genesis of the Yezhutang deposit. The fluid inclusion microthermometry and laser Raman analyses indicate that the ore-forming fluids belonged to a medium- to low-temperature, medium- to low-salinity H 2 O–NaCl system. The H–O isotopic results suggest that the ore-forming fluid is mainly composed of magmatic and metamorphic water, with progressive meteoric water involvement during fluid evolution. LA–ICP–MS elemental mapping results indicate that Fe in pyrite can be substituted by isomorphic substitution of Co 2+ +Ni 2+ ↔2Fe 2+ or direct substitution of Fe 2+ by Co 2+ and Ni 2+ . Titanium, Cu, Au, and As are primarily hosted in pyrite through isomorphic substitution or as micron-scale mineral inclusions. Zinc is proposed to exist predominantly as sulfide inclusions in the pyrite. In situ S isotopic results of pyrite from the two stages of the hydrothermal period indicate that sulfur was derived primarily from magmatic and associated hydrothermal fluids, with a small quantity probably originating from the strata. Combined with available geochronological constraints, the Yezhutang deposit is interpreted to have formed in a subduction-related setting during the Early Neoproterozoic. Under the subduction setting, deep-seated magma and ore-bearing fluids migrated upward along faults. During the fluid evolution, magmatic fluids mixed with metamorphic and meteoric waters. As physicochemical conditions changed, the ore minerals precipitated and gold orebodies formed along secondary faults in the Mawozi Formation. During the subsequent crustal uplift, the orebodies were exposed at the surface, which led to different degrees of limonitization of the ores and caused further enrichment of gold.