Genesis of W–Mo mineralization in the Xiaoliugou and Ta’ergou ore fields, North Qilian Orogen (NW China): Constraints from fluid inclusions and S–Pb[sbnd]H[sbnd]O[sbnd]Nd isotopes

Abstract

Tungsten–molybdenite (W–Mo) polymetallic mineralization are intensely developed in the western section of the North Qilian Orogen (NW China), represented by the Xiaoliugou and Ta’ergou ore fields. In this study, fluid inclusion microthermometric and S–PbHONd isotopic analyses were conducted on the W–Mo ores to shed insights into the genesis of the mineralization belt. The ore fields are characterized by abundant liquid-vapor two-phase inclusions which have medium–high temperature-salinity (200–320 °C and 4–10 wt% NaCl equiv.). The consistent δ18OV-SMOW (7.7‰ to 13.1‰) and δ2HV-SMOW (−88.1‰ to −49.4‰) values of quartz and muscovite indicate that the ore-forming fluid originated from granitic intrusion but mixed with meteoric water. Typical δ34S values of their sulfide ores range from 6 to 18‰, and the ore-related granites (206Pb/204Pb = 18.18–26.61, 207Pb/204Pb = 15.51–16.02, 208Pb/204Pb = 37.60–39.77) and sulfides (206Pb/204Pb = 17.97–20.06, 207Pb/204Pb = 15.42–16.64, 208Pb/204Pb = 37.39–41.06) have variable Pb/Pb ratios, indicating both magmatic rocks and meta-sedimentary rocks contributed to the mineralization. The negative Nd values (εNd(t) = −9.0 to −3.2) and old two-stage Nd model ages (TDM2 = 1517 to 1901 Ma) of the scheelites suggest that the ore-forming materials may have been derived from old crust together with the granitic magma. An isochron age of 422 ± 42 Ma (initial 143Nd/144Nd = 0.511701 ± 0.000046) has been yielded by the scheelite Sm-Nd dating from the Ta’ergou ore field, implying the 450–420 Ma magmatic–hydrothermal event resulted in the intense W–Mo mineralization in the North Qilian, especially in the western section of this orogen. Consequently, we propose a multi-metal coupled mineralization model to illustrate the mineralization process of the ore fields, which involving the formation of ore source beds (wall-rocks) in Proterozoic and the intrusion of granites in Early Paleozoic. This research highlights the use of multiple isotopic methods in revealing complex fluid evolutionary process of large-scale W–Mo deposits.