Metallogenic mechanism of the Xiaoxinancha Au-rich Cu deposit in Yanbian area, Jilin Province: Constrains from fluid inclusions and isotope geochemistry

The Xiaoxinancha Au-rich Cu deposit is an important hydrothermal deposit in eastern China. The deposit includes two mines, known as the North mine (veinlet-dissemination type) and the South mine (vein type), which show different ore grades and orebody characteristics. The total homogenization temperatures (T_{h, total}) range from 120 to 470 ° C, and the salinities in the Ⅰ mineralization stage and Ⅱ-Ⅲ mineralization stage are 10.1%-20.0% NaCl eqv and 0.4%-45.5% NaCl eqv, respectively, with the gas composition consisting mainly of H₂O, CO₂, CH₄ and N₂ in the North mine. The South mine fluids yield T_{h, total} of 150 to 450℃, salinities of 4.0%-11.1% NaCl eqv in the Ⅰmineralization stage, and the salinities in theⅡ-Ⅲ mineralization stage decrease with the decreasing temperatures, and the main gas composition consists of H₂O, CO₂, and CH₄. The quartz-hosted fluid inclusions in the hornblende-granodiorite have homogenization temperatures of 150 to 510℃, and salinities of 4.9%-11.5% NaCl eqv, being similar to those in the North and South mine. The authors hold that the North mine was formed by replacement of fluids which experienced mixing with crustal fluids in the Wudaogou Group after the boiling, whereas the South mine was mainly formed through filling-crystallization of the initial ore-forming fluids with the decreasing temperatures and pressures, and the ore-forming fluids that involved the meteoric water in the mid-late stage produced melnikovite-quartz veins in the North mine and pure sulfide veins in the South mine. The fluid inclusions in the hornblende-granodiorite possibly reveal initial ore-forming fluid signatures characterized by low-moderate salinity and mantle-derived magmatic fluids.