Ore fluid, geochronology and tectonic setting of mesothermal gold metallogeny in southeastern Jilin Province, Northeast China: A case study of the Shajingou gold deposit

Abstract

Abstract The Shajingou gold deposit is a recently discovered medium-scale gold deposit in southeastern Jilin Province, Northeast China. The deposit is located in the easternmost segment of the northern margin of the North China Craton (NCC). This study investigated the mineralization age and tectonic setting of the Shajingou gold deposit based on geology, microthermometry and H–O isotopic analyses of fluid inclusions (FIs), pyrite Re–Os and zircon U–Pb dating, petro-geochemistry and zircon Hf isotopic analyses. The findings are placed in the wider context of mesothermal gold metallogeny in Northeast China. Metallogenesis of the gold deposit can be divided into three stages based on the ore mineral assemblage: quartz–K-feldspar–pyrite (stage I), quartz–polymetallic sulfide–native gold (stage II) and quartz–carbonate (stage III). Four types of primary FI were identified in quartz and calcite minerals: liquid-rich aqueous (L-type), CO2-bearing (C1-type), CO2-rich (C2-type) and CO2-pure (C3-type). From stage I to stage III, the fluid inclusion suite changes from L-type and C2-type in stage I, all four types in stage II and only L-type in stage III. Homogenization temperatures of stages I to III are 318–394\u202f°C, 264–356\u202f°C and 202–276\u202f°C, respectively. Salinities of stages I–III are 7.31–12.42, 2.02–8.94 and 0.53–3.53\u202fwt% NaCl equiv., respectively. The ore-forming fluid system evolved from a H2O–NaCl–CO2 system in stages I and II to a H2O–NaCl system in stage III. During the stage II, the addition of meteoric water led to decreases in temperature and pressure, resulting in an immiscible H2O–NaCl–CO2 fluid system. This immiscibility in turn led to the escape of CO2 and other volatiles, the increase of pH and the decrease in solubility and stability of Au–S clathrates, thus facilitating the deposition of gold and associated polymetallic sulfides. Laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) zircon U–Pb dating of the Shajingou diorite yielded a weighted mean 206Pb/238U age of 171.7\u202f±\u202f0.9\u202fMa (MSWD\u202f=\u202f3.0). Re–Os dating of the gold-bearing pyrite yielded an isochron age of 166\u202f±\u202f29\u202fMa (MSWD\u202f=\u202f5.2). The Shajingou diorite has high concentrations of SiO2, Al2O3, Na2O, Mg#, Cr and Ni, weakly positive Eu anomalies, and is LREE- and LILE-enriched, HREE- and HFSE-depleted, as well as has positive eHf (t) values (+3.0 to +7.1). This indicates the diorite was sourced from the partial melting of a depleted-mantle wedge that had been modified by the subducted- slab-derived fluids. The deposit geology, ore genesis, geochronology and tectonic setting of the Shajingou gold deposit differ from those of other gold deposits in the eastern segment of the XMOB, but are similar to those of gold deposits in the Jiapigou–Haigou gold belt (JHGB) of southeastern Jilin Province at the northern margin of the NCC. These findings suggest the JHGB extends southeastward to the Helong area of Yanbian, which indicates the need for further research on the gold mineralization potential of the southeastern JHGB and further gold prospecting in the region. Comparative studies of Shajingou and other gold deposits in the JHGB have confirmed that a significant tectonic–magmatic–hydrothermal event occurred during the Middle Jurassic due to the subduction of the Paleo-Pacific Plate beneath the Eurasian Plate.