Hydrothermal metallogeny of the Shanggong gold deposit, east Qinling: Studies on ore geology and fluid inclusion geochemistry

Abstract

The Shanggong Au deposit, with reserves of about 30 t Au, in the Xiong'er Group volcanic rocks of the Xiong'er Terrane, East Qinling, is one of the fault-controlled orogenic-type lode gold deposits in China. Ore-hosting structures are recognized to have evolved from compressional shear to tensional shear. The hydrothermal ore-forming process is divided into early, middle and late stages, characterised by pyrite-ankerite-quartz, polymetallic sulfides and carbonate-quartz respectively. The early stage veins and minerals are structurally deformed and broken, suggesting they have formed in a compression or compressional shear setting. The middle stage poly-metallic sulfide stockworks are intruded into the early stage veins and conjugate joints or cracks, and show no clear deformation, which suggests a tensional shear setting. The late stage quartz-carbonate veinlets, with comb structure, are likely formed in an extensional tectonic environment. The composition of fluid inclusions changed from CO2-rich in the early stage, to CO2-rich, pure CO2 and CO 2-poor in the middle stage, and to CO2 poor in the late stage. Homogenization temperatures of fluid inclusions are between 380°C-320°C for the early stage, 300°C-220°C for the middle stage and 120°C-200°C for the late stage. The trapping pressures for the early and middle stage fulid inclusions are estimated at 285 MPa-200 MPa and 160 MPa-100 MPa respectively. The middle stage fluid has the highest contents of cations and anions (e. g. SO4 2-, Cl-, K+), the highest (K+Na)/(Mg+Ca) ratios, the highest pH and lowest Eh values, which probably resulted from fluid-boiling charaterised by CO2 phase separation indicated by the lowest CO2/H2O ratios and divergent homogenization to gas and liquid respectively. The alkaline, supersaturated fluid in the middle stage is most conducive to the rapid deposition of polymetallic sulfides, tellerides and native elements such as Au, Ag and Te. In the east Qinling area, the collision between Yangtze and North China continents progressed from crustal thickening, shortening and uplift during Triassic-Early Jurassic, to compression-extension transition and uplift during Middle Jurassic-Early Cretaceous, to orogenic extension and fault-controlled depression during Cretaceous. These three evolutionary stages correspond to the three-stages of ore-hosting structures, ore textures, ore-forming processes and fluid inclusion populations of the Shanggong Au deposit. This shows that the Shanggong gold deposit formed during the Mesozoic continental collision and can be well interpreted by the tectonic model of collisional orogeny, metallogeny and fluid flow (CMF). During the collision, ore-froming fluids were generated from metamorphic dehydration of the underthrust slab which downwent beneath the Xiong'er Terrane along the Machaoying fault, a north-dipping A-type subduction zone. The metamorphic fluids migrated upward along the ore-hosting faults of the Shanggong gold deposit, boiled at the brittle-ductile transiton level, and then mixed with downgoing meteoric water, resulting in rapid deposition of ore-metalls.