The Tianger (Bingdaban) shear zone hosted gold deposit, west Tianshan, NW China: Petrographic and geochemical characteristics

Abstract

The Wangfeng–Tianger–Saridala ore district consists of over 20 orebodies with total proven gold reserves exceeding 15 tonnes. The district is located on the EW-striking Tianger (Bingdaban) shear zone in west Tianshan Mountains, northwest China. The lensoid orebodies are distributed along the shear zone in a region about 25 km in length and 1 km in width. Enhanced fluid flow and fracturing controlled location and orientation of mineralized zones during deformation along narrow shear bands oriented oblique to the overall shear zone margins. Fabrics in fractures and strain shadow overgrowths generally plunge steeply, parallel to the lineation in the surrounding cataclasite matrix consisting of new grains of quartz and minor amounts of micas. Native gold occurs in mylonitized granites or mylonitized sulfide–mica–quartz veins. Gold grade is closely related with the occurrence of sulfide–quartz veins. Pyrite and pyrrhotite in gold-bearing sulfide–quartz veins contain inclusions of native gold. Chondrite-normalized rare earth element (REE) distribution patterns for pyrite from the orebodies show light REE enrichments and negative Eu anomalies. Relationships between gold contents and Th/U, Zr/Hf and (La/Yb) N values for pyrites imply that gold-rich pyrites are different from gold-poor pyrites, thus indicate that the pyrites in the orebodies have a different origin compared with those in wall rocks. A Rb–Sr isochron for the mylonitized Au-bearing quartz veins indicates an age of 224 ± 14 Ma with an initial 87Sr/ 86Sr ratio of 0.7294 ± 0.0089 (MSWD = 1.1). This age is conformed by 40Ar/ 39Ar dating on muscovites (220.9 to 222.5 Ma), which is interpreted as the age of ore-formation in the Tianger deposit. High initial 87Sr/ 86Sr ratios (0.7294), high δ 34S values (+ 11.2 to + 16.5‰), low δ 18O H2O values (1.67 to 3.07‰) and low δD values (− 84 to − 104‰) indicate that the ore-forming fluid was unrelated to any magmatic process.