Geochronology and origin of high-grade Cu mineralization at the Nulasai deposit, western Awulale Cu district, Tianshan, NW China

Abstract

The western Awulale copper district, located within the heart of the Chinese Western Tianshan, formed in close association with regional Permian volcano-plutonic activity. The Nulasai deposit, the earliest mined area in the district, exhibits a relatively high copper grade (averaging 1.34% Cu). Ore minerals of the Nulasai deposit mainly comprise bornite, chalcocite, and chalcopyrite, with a Cu grade that can reach 3.27% in the main mineralization stage. In this study, we carried out a combined geochronological, lead isotope, and rare earth elements (REEs) investigation to explore the timing and origin of high-grade Cu mineralization at the Nulasai.

The Re-Os isochron age obtained from bornite and chalcocite in the main mineralization stage is 282.8 ± 3.3 Ma, which is broadly coeval with the hydrothermal apatite U-Pb age of 284.3 ± 7.6 Ma from the ore-bearing quartz monzonite porphyry, confirming their contemporaneity. Lead isotope ratios and REEs distribution patterns of bornite and chalcocite are comparable with the coeval quartz monzonite porphyry and the host Wulang volcanic formation. Contrasting δEu values of bornite and chalcocite indicate that their precipitation may have been controlled by variable oxygen fugacity of the evolving ore-forming fluids. Based on these findings, we propose that the mixing of magmatic hydrothermal fluids and meteoric water led to a decrease in temperature and pressure of the mineralizing fluids, triggering fluid immiscibility and the development of a relatively oxidizing environment. The combination of metal-sulfur ion complexation and the subsequent precipitation of metal-rich sulfides may have formed high-grade copper mineralization in Nulasai.