Fluid inclusion characteristics of mesothermal gold deposits in the Xiaoqinling district, Shaanxi and Henan Provinces, People's Republic of China

Fluid inclusions were studied in quartz samples from early (stage I) gold-p oor quartz veins and later (stage II) gold- and sulphide-rich quartz veins from the Wenyu, Dongchuang, Qiangma, and Guijiayu mesothermal gold deposits in the Xiaoqinling district, China. Fluid inclusion petrography, microther mometry, and bulk gas analyses show remarkably consistent fluid composition in all studied deposits. Primary inclusions in quartz samples are dominate d by mixed CO2-H2O inclusions, which have a wide range in CO2 content and c oexist with lesser primary CO2-rich and aqueous inclusions. In addition, a fen secondary aqueous inclusions are found along late-healed fractures. Mic rothermometry and bulk gas analyses suggest hydrothermal fluids with typica lly 15-30 mol% CO2 in stage I inclusions and 10-20 mol% CO2 in stage II inc lusions. Estimates of fluid salinity decrease from 7.4-9.2 equivalent wt.% NaCl to 5.7-7.4 equivalent wt.% NaCl between stage I and II. Primary aqueou s inclusions in both stages show consistent salinity with, but slightly low er Th-total than, their coexistent CO2-H2O inclusions. The coexisting CO2-r ich, CO2-H2O, and primary aqueous inclusions in both stage I and II quartz are interpreted to have been trapped during unmixing of a homogeneous CO2-H 2O parent fluid. The homogenisation temperatures of the primary aqueous inc lusions give an estimate of trapping temperature of the fluids. Trapping co nditions are typically 300-370 degrees C and 2.2 kbar for stage I fluids an d 250-320 degrees C and 1.6 kbar for stage II fluids. The CO2-H2O stage I a nd II fluids are probably from a magmatic source, most likely devolatilizin g Cretaceous Yanshanian granitoids. The study demonstrates that gold is lar gely deposited as pressures and temperatures fall accompanying fluid immisc ibility in stage II veins.