The Tashisayi nephrite deposit from South Altyn Tagh, Xinjiang, northwest China

Abstract

The Tashisayi nephrite deposit is located in South Altyn Tagh, in Qiemo County, Xinjiang Province, northwest China. It is a recent discovery in the vast, well-known Kunlun-Altyn nephrite belt distributed along the south of the Tarim Basin, producing more than half of the nephrite from the whole belt in 2017. Field investigations revealed that it is a dolomitic marble-related (D-type) nephrite deposit, but little is known about its age of formation and relationships between the granites and marble. Here we report field investigations, petrography of the nephrite, as well as petrography, geochemistry, geochronology of the zoisite-quartz altered intrusive rock and adjacent granites. An A-type granite is identified with a SHRIMP U–Pb zircon age of 926 ± 7 Ma, suggesting it was emplaced in an extensional tectonic environment at that time. The altered intrusive rock has a cluster of U–Pb zircon age of 433 ± 10 Ma, with similar trace element features to the A-type granite, suggesting it was formed in an extensional regime at this later time. Nephrite formed because of the metasomatism of dolomite marble by hydrothermal fluids. It is inferred that Ca2+ was released from the dolomitic marble by metasomatism forming Ca-rich fluids, which caused alteration of both the intrusive rocks (6.00–8.22 wt.% CaO) and granite (1.76–3.68 wt.% CaO) near the nephrite ore bodies. It is also inferred that Fe2+ from the granite migrated towards the dolomite marble. The fluids gave rise to the formation of Ca-minerals, such as zoisite, in the nephrite and altered intrusive rock, and epidote in the granite. Based on the contact relationships, similarity in hydrothermal processes, and consumption of Ca2+, the Tashisayi nephrite is considered to have formed at the same time as the alteration of the intrusive rocks, i.e. ∼433 Ma. The geochronological similarity (∼926 Ma, 433 Ma) of South Altyn and North Qaidam may suggest that tectonically they belong to one single complex in the past, which was offset by the Altyn Tagh fault (ATF). The similar formation ages of the nephrites from Altyn Tagh (433 Ma) and the previously studied areas of West Kunlun (378–441 Ma) and East Kunlun (416 Ma) indicate that these nephrites formed during the closure of Proto-Tethys and in the accompanying post-collisional, extensional environment.