Structural deformation and fluid evolution associated with the formation of the Sawayardun gold deposit in Southwestern Tianshan Orogen

This paper is the result of mineral exploration engineering.

Objective The relationship between structure-fluid and mineralization is a frontier problem in modern study of mineral deposit. The orogenic-type gold deposits are hosted in fault zone, and are representative target to study structural deformation and fluid evolution responsible for mineralization process.

Methods The Sawayardun orogenic-type gold lode system is the largest gold deposit in the Southwestern Tianshan Orogen, northwestern China. Basing on results of structural analysis, fluid inclusions and C-H-O-S isotope, natures of ore-controlling structure and ore-fluid, and metal source were studied.

Results The structural deformation at the Sawayardun mine can be divided into the early ductile, middle ductile-brittle, and late brittle stages, according to the crosscutting relationships of veins, and macro and micro structures. The three-stage deformation process associated with three kinds of quartz veins (Qz1, Qz2, Qz3) from early to late, with gold mineralization being mainly introduced in middle stage. Quartz formed in the early and middle stages (Qz1 and Qz2) contains three compositional types of fluid inclusions, i.e. pure CO₂, CO₂-H₂O and NaCl-H₂O, but the late-stage minerals only contain the NaCl-H₂O inclusions. The inclusions trapped in the early, middle and late stages yield total homogenization temperatures of 237℃-386℃, 204℃ -310℃, and 125℃-235℃, respectively, with corresponding salinities of 1.4-9.2, 0.5-16.6, and 0.2%-10.6% NaCl equiv., respectively. The minimum trapping pressures estimated from CO₂-H₂O inclusions are 267 MPa and 208-253 MPa in the early and middle stages, corresponding to lithostatic depths of 10 km and 8~9 km, respectively. The isotope systematics shows the nature of metamorphic and meteoric fluids and the hostrocks (Palaeozoic clastic and carbonate sediments) to be a significant source of ore metals.

Conclusions The structural deformation features at Sawayardun indicate that the mineralization was associated with the tectonic transition from the early NW-SE-trending compression to the late strike-slip extension. The early compression caused the metamorphism and metamorphic dehydration of the Palaeozoic clastic and carbonate sedimentary rocks, resulting in the formation of the low salinity, CO₂-rich fluids and the early-stage barren quartz veins. Coupling with the tectonic transition from compression to strike-slip extension, the crust was rapidly uplifted and eroded, the fluid system depressurized and boiled, and mixed with and input by and mixed with the meteoric water, causing rapid escape of gases such as CO₂ and deposition of ore-metals such as Au.