Contribution of metasomatized lithospheric mantle to Early Mesozoic gold mineralization in Jiangnan Orogen, South China: Insights from fluid inclusions and C−O−S−He−Ar isotopes

The metasomatized subcontinental lithospheric mantle (SCLM) is increasingly recognized as a key factor in the formation of gold deposits, yet its contribution to intracontinental systems remains poorly constrained. The Early Mesozoic Yanlinsi gold deposit in the Jiangnan Orogen provides an ideal case study, owing to the spatial and temporal association of diabase with orebodies. The deposit is hosted in low-grade metamorphic rocks of the Neoproterozoic Lengjiaxi Group, and mineralization is divided into three stages: (I) disseminated pyrite−arsenopyrite, (II) quartz−dolomite−polymetallic sulfides−native gold, and (III) quartz−calcite. Scanning electron microscope−cathodoluminescence imaging identifies three quartz generations from stage II, with fluid inclusions indicating an H2O−CO2−NaCl system comprising aqueous, H2O−CO2−NaCl, and pure CO2 types. Microthermometric data reveal low salinity (3.4−9.1 wt% NaCl equivalent) and medium- to low-temperature conditions (159−305 °C). δ18Ofluid values of quartz (4.91‰−8.75‰) suggest a deep-seated origin for early fluids (Qz-1, Qz-2), with later mixing of meteoric water (Qz-3). Dolomite displays δ13CPDB values of −8.17‰ to −2.79‰ and δ18OSMOW values of 13.51‰ to 15.75‰. Pyrite-hosted fluid inclusions exhibit 3He/4He ratios of 0.03−0.17 Ra and 40Ar/36Ar ratios of 312−1695, consistent with mantle-derived fluids metasomatized in a subduction setting. δ34S values of diabase (−1.30‰ to +1.98‰) overlap with those of auriferous pyrite, further indicating a subcrustal source of ore materials. Early Mesozoic diabase contains 1.6−4.2 ng/g Au, indicating that the metasomatized SCLM could have served as an important volatile and gold reservoir. Gold precipitation was primarily driven by fluid immiscibility and fluid mixing. Integrated geochronological and geochemical evidence suggests that the Yanlinsi deposit formed during the Early Mesozoic intracontinental orogeny, triggered by flat-slab subduction of the Paleo-Pacific Plate. This study establishes a genetic metallogenic model that highlights the critical role of the metasomatized SCLM in supplying fluids and metals to intracontinental orogenic gold systems.