Geological, fluid inclusion, and H–O–S–Pb isotopic constraints on the genesis of the Bajiazi mesothermal lode gold deposit in the Jiapigou ore cluster, northeast China

Abstract

The Bajiazi gold deposit, located in the northeast of the North China Craton, is a typical representative of the Jiapigou ore cluster in northeast China. The deposit is hosted within the Neoarchean–Paleoproterozoic supracrustal rocks (greenstone belts) and metamorphosed plutonic rocks. The orebodies occur along NE-trending ductile–brittle faults as auriferous quartz veins. The mineral assemblages reveal three stages of mineralization: quartz–pyrite (early), quartz–polymetallic sulfide (main), and quartz–carbonate (late). Three types of primary fluid inclusions (FIs), namely A-type (aqueous), C-type (aqueous–carbonic), and PC-type (pure carbonic) FIs, have been distinguished in different stages of quartz. The early- and main-stage quartz grains contain C-, A-, and a few PC-type FIs, while the late-stage quartz grains contain only A-type FIs. The early-, main-, and late-stage FIs homogenized at temperatures of 288–397 °C, 183–281 °C, and 120–190 °C, corresponding to salinities of 8.8–19.5, 3.9–18.8, and 3.2–11.3 wt% NaCl eqv., respectively. The ore fluid system evolved from H2O–NaCl–CO2 to H2O–NaCl during ore formation. The gold precipitation resulted from fluid immiscibility during the main mineralization stage. The HO isotope data reveal that the ore fluids originally came from magmatic water, and mixed with meteoric water during ore formation. The δ34S values (−0.2 ‰ to 8.3 ‰) of ore-related sulfides suggest that the sulfur in the ores was originally magmatic-sourced, and mixed with the sulfur in wall rocks during fluid migration. The Pb isotope ratios (206Pb/204Pb = 15.890–16.794, 207Pb/204Pb = 15.188–15.530, and 208Pb/204Pb = 36.589–37.710) of ore-related sulfides indicate the metals in the ores originated primarily from the lower crust, with a minor mantle contribution, and most likely came from a deep magmatic system. The Late Triassic gold mineralization has a clear genetic link to a quartz syenite porphyry. Based on the geological, FI, and isotopic constraints, the Bajiazi deposit is classified as a mesothermal lode gold deposit, which formed in an extensional setting after the closure of the Paleo-Asian Ocean.