Geological, fluid inclusion, H–O–S–Pb isotope, and Ar–Ar geochronology constraints on the genesis of the Nancha gold deposit, southern Jilin Province, northeast China

Abstract

The Nancha gold deposit, located in the central part of southern Jilin Province in the northeastern portion of the North China Craton, contains orebodies hosted in a Proterozoic metamorphosed volcanic–sedimentary sequence. The distribution of the orebodies is controlled by NNE-trending brittle–ductile shear zones and faults. The gold orebodies consist of auriferous quartz veins and auriferous altered rocks. Mineralization can be divided into three stages: (1) quartz–pyrite, (2) quartz–gold–polymetallic sulfide, and (3) quartz–carbonate, with gold being introduced mainly in the second stage. Three types of fluid inclusions were identified in the vein mineral assemblages based on petrography and laser Raman spectroscopy: NaCl–H2O (W-type), CO2–H2O (C-type), and pure CO2 (PC-type). Crystals in the early quartz–pyrite stage veins mainly contain C-type primary fluid inclusions and rare PC-type inclusions. The fluid inclusions in these veins completely homogenize at temperatures of 273–432 °C and show low salinities of 0.63–7.78 wt.% NaCl equiv. In the middle quartz–gold–polymetallic sulfide stage, all three types of fluid inclusions were observed. These fluid inclusions homogenize at temperatures of 132 to 255 °C and show salinities of 0.83–11.72 wt.% NaCl equiv. In contrast, crystals in the late quartz–carbonate stage contain only W-type fluid inclusions that show homogenization temperatures of 132–255 °C and salinities of 0.35–7.86 wt.% NaCl equiv. These data indicate that the metallogenic system evolved from a CO2-rich metamorphogenic fluid to a CO2-poor fluid due to inputs of meteoric waters. Fluid boiling and mixing caused the rapid precipitation of sulfides and gold. Trapping pressures estimated from boiling C-type fluid inclusions were 152–367 MPa in the ore-forming stage. This suggests an alternating lithostatic–hydrostatic fluid system controlled by a fault-valve activity at a depth of 13.8–15.2 km. One hydrothermal sericite sample from an auriferous quartz vein yielded an 40Ar/39Ar isotopic plateau age of 170.1 ± 1.9 Ma, indicating that mineralization occurred in the Middle Jurassic and was unrelated to the volcanics and intrusions in the mineralized area. The characteristics of H–O–S–Pb isotopes and fluid inclusions suggest that the ore-forming fluids were of metamorphic origin, with the S originating from Proterozoic crustal components and the Pb originating from a mixture of ore-forming metamorphogenic fluids and host rocks. The results, combined with existing data on the regional geology, ore geology, fluid inclusions, H–O–S–Pb isotope geochemistry, age of mineralization, and tectonic setting, indicate that the Nancha gold deposit was an orogenic-type system that formed in a Middle Jurassic accretionary orogenic regime, following subduction of the Paleo-Pacific Plate beneath the Eurasian continent.

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