Linking a fractionated magmatic system to skarn W-Mo mineralization in the Hahaigang deposit, Tibet: Implications for regional tungsten metallogeny and exploration

Abstract

In general, highly evolved and reduced magmatic systems are favorable for W-Sn mineralization and Mo-Cu mineralization can occur in low differentiated oxidized ones. However, it is not clear how coexisting W-Mo mineralization can form in a magmatic system. To characterize the genetic linkage between contemporaneous magmatic rocks in a magmatic system related to W–Mo mineralization, we investigated whole-rock geochemistry, geochronology, and Hf-Pb isotopic signatures from ore-related granodiorites and monzogranites in the Hahaigang W-Mo deposit, Tibet. The primary mineralization of the Hahaigang deposit shows a W → W-Mo → Cu-Pb-Zn mineralization zone along the Dalong fault zones from NE to SW. The stockworks and disseminated W-Mo orebodies are hosted in the skarn near the monzogranite and granodiorite. Zircon U-Pb ages of the monzogranite and granodiorite are 64.3 ± 0.3 Ma and 64.0 ± 0.3 Ma, respectively. These ages are consistent with the previous ages obtained from molybdenite Re-Os isochron dating, revealing that the Hahaigang W-Mo mineralization formed during the syn-collisional stage of the Indian and Asian plates. Metal sulfide S-Pb isotopes indicate that the ore-forming materials were mainly derived from granitic magma. The two granitoids that belong to the high-potassium and peraluminous calc-alkali I-type granites have similar Hf-Pb isotopic characteristics, indicating that they represent a felsic magma system and have been derived from a similar source. The monzogranite, with a spectacular tetrad effect in its REE distribution patterns, has higher negative Eu anomalies than the granodiorite, reflecting that it has more highly fractionated features. Meanwhile, it has lower whole-rock Fe2O3/FeO and zircon Ce4+/Ce3+ ratios than the granodiorite, suggesting that the granodiorite has higher oxygen fugacity than monzogranite. The formation of monzogranite is attributed to the fractionation crystallization of feldspar and Fe-Ti-oxide during magmatic evolution. Thus, this study clarifies that W-Mo mineralization can form in an evolved magmatic system undergoing different degrees of fractionation and having different redox state. Combined with the results of previous studies, at least four generations of W-dominated polymetallic metallogenic events are observed in the Lhasa terrane. Because the highly evolved magmatic system is common in the Lhasa terrane with crustal thickening, this system with existing mineralization may provide useful information about regional-scale tungsten potential and guide its exploration.