Geological, fluid inclusion, H–O isotopic, and U–Pb geochronological constraints on the genesis of the Xiahuolong gold deposit, southern Jilin Province, NE China

Abstract

The Xiahuolong gold deposit (3.05 t Au, average grade: 6.7 g/t), located in the Ji'an district of southern Jilin Province in the northeastern part of the North China Craton, contains orebodies in a Proterozoic metamorphosed sedimentary sequence. The distribution of the orebodies is controlled by NW- and NE-trending shear zones and faults, and the orebodies consist of auriferous quartz veins and auriferous altered rocks. The mineralization can be divided into four stages, namely (I) quartz-sericite, (II) quartz-pyrite-copper-gold, (III) quartz-pyrite-sphalerite-galena, and (IV) quartz-carbonate, with gold being introduced mainly during the second stage. Systematic analysis of the fluid inclusions suggests that two-phase vapor-rich (WV-type) and liquid-rich (WL-type) are the dominant types in the quartz veins. The early (stage I), main (stages II and III), and late (stage IV) fluid inclusions homogenize at temperatures of 273–367 °C, 201–346 °C, and 120–196 °C, corresponding to salinities of 0.68–0.79, 0.69–0.88, and 0.90–0.96 wt% NaCl equiv., respectively. The ore-forming fluid system evolved from CO2-H2O-NaCl ± CH4 to H2O-NaCl during ore formation, and fluid immiscibility and mixing caused the precipitation of sulfides and gold in the main stage. The H–O isotopes indicate that the ore-forming fluids were of magmatic origin, with a continuous input of meteoric water during ore formation. Two types of diorite porphyry have been identified, one cutting orebodies and the other filled with auriferous quartz veins. The diorite porphyry associated with the auriferous quartz vein yielded a zircon U–Pb isotopic age of 124.1 ± 1.9 Ma, indicating that Xiahuolong mineralization occurred in the Early Cretaceous, and that the high water content and low oxygen fugacity (ΔFMQ = -2.53 to +0.23; logƒO2 = -17.56 to -20.61) of the dioritic magma provided an important basis for mineralization. The results, combined with the deposit geology, fluid geology, H-O isotope geochemistry, timing of mineralization, and tectonic setting, suggest that Xiahuolong is a mesothermal gold deposit formed in an extensional setting characterized by lithospheric thinning related to the rollback of the subducting Paleo-Pacific Plate.