Geological And Geochemical Constraints On The Genesis And Tectonic Setting Of The Liziyuan Orogenic Gold Deposit In West Qinling Orogen, China

The Liziyuan goldfield is located along the northern margin of the western part of the Qinling Orogen. Magmatism is active in the mining area, including the Tianzishan Monzogranite to the southwest and the Jiancaowan Porphyry in the central part, so the study of the deposit provides a crucial insight into the relationship between tectonic magmatic events and gold metallogenesis in the western part of the Qinling Orogen. In this paper, we provides the first systematic description of the ore geology, fluid inclusion microthermometry and Raman microprobe analyses, trace elements of mineralized quartz and carbonate veins, hydrogen and oxygen isotopes for quartz and calcite, and sulfur isotopes for pyrite and galena, which constrain the genesis and tectonic setting of the Liziyuan gold deposit. Moreover, we present whole-rock major and trace element geochemistry, and in-situ zircon U-Pb and Lu-Hf isotopic data from the Tianzishan Monzogranite and Jiancaowan Porphyry, which determine the petrogenesis of these two granitic plutons and constrain the relationship between granitic rokcs and gold metallogenesis of the Liziyuan gold deposit.The Liziyuan goldfield consists of five gold-only deposits hosted by metavolcanic rocks, and one polymetallic (Au-Ag-Pb) deposit hosted by the Tianzishan Monzogranite. Orebodies mainly occurred as lenticular discrete veins along NW-striking dextral ductile strike-slip shear zone. Ore minerals are dominated by pyrite, galena and chalcopyrite; major gangue minerals are quartz and calcite. Gold is present chiefly as independent minerals in the microfractures of sulfides and quartz. Major wallrock alteration includes silicification, pyritization, and carbonation, and its intensity progressively increases towards the orebodies. The mineralization can be divided into four stages:pyrite-quartz stage (Ⅰ), native gold-chalcopyrite-pyrite-quartz stage (Ⅱ), pyrite-freibergite-galena-quartz-carbonate stage (Ⅲ), and carbonate stage (Ⅳ). Fluid inclusion petrography and microthermometric results suggest that three types of primary fluid inclusions (carbonic, CO2-H2O, and aqueous inclusions) are present at the deposit. Microthermometric data and Laser Raman analyses indicate that the ores were deposited from H2O-NaCl-CO2-(CH4) fluids with medium temperature (240to280℃) and low salinity (2.2to9.1wt.%NaCl equiv.). The common coexistence of aqueous, CO2-H2O and carbonic inclusions is indicative of them are heterogeneous trapped of immiscible fluids. Trace elements of mineralized quartz and carbonate veins suggest that the ore-forming fluids are mainly produced by the prograding metamorphism of upper crust. The calculated δ18OH2O and δD values are1.8to3.8‰and-77to-75‰for quartz from stage Ⅱ, and-8.5to-6.6‰and-72to-71‰for calcite from stage Ⅲ, implying that the ore-forming fluids are magmatic-hydrothermal and/or metamorphic in origin and have evolved by mixing with meteoric water in later stage. The pyrite and galena have δ34S values of3.90to8.50%o with a pronounced mode at5to8‰. The relatively homogeneous sulfur isotopic compositions are consistent with other orogenic gold deposits widespread in the world, indicating the sulfur mainly sourced from reduced metamorphic fluids, but a contribution from magmatic source cannot be ruled out.The Tianzishan Monzogranite and Jiancaowan Porphyry are enriched in LILE and LREE, deplete in HFSE, and have zircon εHf(t) values between-5.1and-14.1for the Tianzishan Monzogranite and between-21.0and-8.4for the Jiancaowan Porphyry. These characteristics indicate that the granites are derived from the crust. The Tianzishan Monzogranite has LA-ICP-MS zircon U-Pb ages of256.1±3.7to260.0±2.1Ma, which suggests that it was emplaced in the WQO during the convergence of the North and South (Yangtze) China Cratons in the early stage of the Qinling Orogeny. In contrast, the porphyry has a LA-ICP-MS zircon U-Pb age of229.2±1.2Ma, which is younger than the peak collision age, but corresponding to the widespread Late Triassic post-collisional granitic plutons in the WQO. The Tianzishan Monzogranite has somewhat higher Sr contents (196-631ppm), lower Y (2.23-19.6ppm) and Yb (0.20-2.01ppm) contents, and a positive Eu/Eu*averaging1.15. These characteristics suggest the pluton was derived from partial melting of the thickened crust. In contrast, the relatively higher MgO (0.85-2.08wt%) and Mg#(43.4-58.2) of the Jiancaowan Porphyry indicates that insignificant amounts of subcontinental lithospheric mantle-derived mafic melts were involved in the generation of the magma.In terms of the Liziyuan gold deposit that has similar geological and geochemical characteristics with orogenic gold deposits throughout the world, it can be grouped into the class of orogenic gold deposits. For the orogenic-type model, geological observations and available isotopic ages suggest that the orogenic lode mineralization was related to the Indosinian orogenesis of the Qinling orogen. Triassic collisional orogenesis caused crust thickening, dextral ductile shear zone, and greenschist facies metamorphism throughout the orogen. Due to the intensive deformation and metamorphism, the crustal rocks released massive fluids (e.g. water, CO2and silica) that could have 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 higher permeability, and thus appeared to be preferential channels and precipitated area for the ore-forming fluids. When fluids migrated into the high-permeability shear zone, the rapid change of physicochemical condition resulted in the sulfides precipitation and gold mineralization.