Genesis and ore-forming process of the Benqu mesothermal gold deposit in the Jiapigou ore cluster, NE China: Constraints from geology, geochronology, fluid inclusions, and whole-rock and isotope geochemistry

Abstract

The Jiapigou ore cluster (JOC) located on the northeastern margin of the North China Craton (NCC) is a major producer of gold in China and is famous for its quartz vein-type gold deposits. It currently contains a dozen gold deposits with total proven reserves of >180 t Au, among which the Benqu deposit is the earliest discovered and most representative. The deposit is hosted within Neoarchean granitic gneisses and metamorphosed volcanic-sedimentary sequences. Gold orebodies mainly occur as gold-bearing quartz veins controlled by NE–NEE-trending ductile–brittle faults. Hydrothermal mineralization can be divided into early (quartz–pyrite), main (quartz–polymetallic sulfide–native gold), and late (quartz–carbonate) stages, with gold precipitation occurring in the main stage. Three types of primary fluid inclusions (FIs) are identified in quartz formed at different stages: CO2–H2O–NaCl (C-type), H2O–NaCl (W-type) and pure CO2 (PC-type). The early-stage quartz contains C- and W-type FIs, which have homogenization temperatures of 269–298 °C and salinities of 8.8–16.7 wt% NaCl equiv. The main-stage quartz contains all three types of FIs, with homogenization temperatures of 197–268 °C and salinities of 5.1–15.2 wt% NaCl equiv. The late-stage quartz contains only W-type FIs with homogenization temperatures of 145–198 °C and salinities of 3.2–12.4 wt% NaCl equiv. The ore-forming fluid system evolved from a CO2–H2O–NaCl system in the early stage to a H2O-dominated system in the late stage. The addition of meteoric water led to fluid immiscibility, facilitating the precipitation of gold and associated sulfides. The H–O–S–Pb–Sr isotope results indicate that the ore-forming fluids were derived from magmatic water and that the ore-forming materials were mainly extracted from a deep magmatic reservoir sourced mostly from the lower crust, with trace mantle components. Rb–Sr dating of the gold-bearing sulfides yields an isochron age of 175.8 ± 4.6 Ma (MSWD = 1.9). Zircon U–Pb dating of the Benqu diorite yields a weighted mean 206Pb/238U age of 176.5 ± 2.4 Ma (MSWD = 1.2). The Benqu diorite is high-K calc-alkaline, with enriched LREEs and LILEs (e.g., Rb, Pb, and K) and depleted HFSEs (e.g., Nb, Ta, and Ti), which is typical of the geochemical signature of arc magmas. The zircons have negative εHf(t) values of −11.3 to −8.9, indicating strong involvement of old crustal materials. These data suggest that the primary magma originated mainly from the partial melting of ancient lower crust that mixed with minor underplated mantle-derived magma. The above results systematically reveal that the Benqu is a mesothermal magmatic–hydrothermal lode gold deposit that formed in an active continental margin environment in the late Early Jurassic. Comparative studies of Benqu and other gold deposits in the JOC confirm that a significant magmatic–hydrothermal event occurred during the conversion from compression to extension that followed the first subduction of the Paleo-Pacific Plate beneath the Eurasian Plate.