Geological and geochemical constraints on genesis of the Liziyuan gold-dominated polymetal deposit, western Qinling orogen, central China

Abstract

The Liziyuan gold deposit, located on the northern margin of the western Qinling orogen (WQO), consists of five mineralized sites hosted by metavolcanic rocks, and one hosted by the Tianzishan monzogranite. Orebodies mainly occur as lenticular veins along NW-striking dextral ductile strike–slip shear zones. Major wall rock alteration includes silicification, pyritization, and carbonation, progressively increasing in intensity towards the orebodies. Ore minerals are dominated by pyrite, galena, and chalcopyrite; the major gangue minerals are quartz and calcite. Native gold is present chiefly as separate phases in sulphide and quartz microfractures. We recognize four stages of mineralization: (I) pyrite-quartz, (II) native gold-chalcopyrite-pyrite-quartz, (III) pyrite-freibergite-galena-quartz-carbonate, and (IV) carbonate stages. Fluid inclusion petrography and microthermometric results suggest that three types of primary fluid inclusions (carbonic, CO2–H2O, and aqueous) are present in the deposit. Microthermometric data and Laser Raman analyses indicate that the ores were deposited from moderate temperature (240–280°C), low salinity (0.5–9.1 wt.% NaCl equiv.) H2O–NaCl–CO2–(CH4) fluids. The common coexistence of aqueous, CO2–H2O, and carbonic inclusions indicates the trapping of heterogeneous immiscible fluids. Trace elements in mineralized quartz and carbonate veins suggest that the ore-forming fluids were mainly produced by prograde metamorphism of the upper crust. The calculated and δD values are 1.8–3.8‰ and –77‰ to –75‰, respectively, for quartz from stage II and –8.5‰ to –6.6‰ and –72‰ to –71‰ for calcite from stage III, implying that the ore-forming fluids were metamorphic in origin and evolved by late-stage mixing with meteoric water. Pyrite and galena have δ34S values of 3.9–8.5‰ with a pronounced mode at 5–8‰. Such relatively homogeneous sulphur isotopic compositions are consistent with orogenic gold deposits throughout the world, indicating that the sulphur was mainly sourced from reduced metamorphic fluids. The Liziyuan gold deposit can be grouped with other orogenic gold deposits that have similar characteristics. Geological observations and available isotopic ages suggest that the Liziyuan lode mineralization was related to the Indosinian orogenesis of the Qinling belt. Triassic continental collision caused crustal thickening, dextral ductile shearing, and greenschist facies metamorphism throughout the orogen. Due to the intensive deformation and metamorphism, crustal rocks released copious fluids that evidently mobilized and extracted ore elements along their flow pathways. The ductile shear zone in the Liziyuan mining area provided regions of low mean stress and high permeability, and thus appeared to be preferential channels for precipitation of the ore-forming fluids. As highly charged fluids migrated into the high permeability shear zones, rapid changes in physicochemical conditions resulted in sulphide precipitation and gold mineralization.