Discrepant chemical differentiation and magmatic-hydrothermal evolution of high-silica magmatism associated with Pb-Zn and W mineralization in the Lhasa terrane

High-silica (SiO₂ > 70 wt.%) granites (HSGs) are the main source of W, Sn, and rare metals. However, abundant HSGs, temporally, spatially, and genetically associated with Pb-Zn mineralization, in the Lhasa terrane (LT), provided an ideal opportunity to study the key factors responsible for Pb-Zn enrichment, instead of W-Sn enrichment. Here we contribute to this topic through U-Pb dating of zircon and garnet, and whole-rock and Sr-Nd-Hf isotopic geochemistry of ore-related quartz porphyries in the Bangbule deposit and compared these results with published data from large and giant Pb-Zn and W deposits in the LT. The magmatism-alteration-mineralization event in the Bangbule deposit was recorded by robust zircon U-Pb ages of 77.3 ±0.9 Ma and hydrothermal garnet U-Pb ages of 75.7 ±4.8 Ma, which is 10-15 Ma earlier than the main Paleocene metallogenic event and the first record of late Cretaceous Pb-Zn polymetallic mineralization in the LT. The late Cretaceous-Paleocene magmatism and mineralization events are a response to the subduction of Neotethyan oceanic lithosphere, which occurred as a result of the collision of the Indian and Asian plates. These HSGs related to Pb-Zn mineralization, with high total-alkalis and low magnesian contents, are enriched in Ba, Th, and Rb, but depleted in Ti, Eu, Sr, and P. They belong to either the S-type, or I-type granites. The Sr-Nd-Hf isotopic compositions of the Pb-Zn mineralized granites demonstrate that they were generated by the partial melting of Proterozoic basement with or without mantle-derived melt input. This was consistent with the postulated source of W enrichment in the LT. The Pb-Zn and W related granites have similar zircon-Ti-saturation temperatures, comparable low whole-rock Fe₂O₃/FeO ratios, and zircon oxygen fugacity. This indicated that the Pb-Zn-W enrichment in the high-silica magma system could be attributed to a relatively reduced magma. The Pb-Zn related HSGs, abundant quartz and feldspar phenocrysts, and weak fractionation of twin-elements in whole-rock analysis, can be used to reconstruct a model of the magma reservoir. We postulate that these features could be reproduced by silica-rich crystal accumulation in a magma reservoir, with a loss of magmatic fluids. The magma associated with W mineralization exhibited a higher level of differentiation compared to the Pb-Zn related magma; however, different groups of zircon texture with varying rare earth elements and concomitance of rare earth elements tetrad effect and high fractionation of twin-elements in whole-rock are formed by a magmatic-hydrothermal transition in highly evolved system. As the source and oxygen fugacities of the Pb-Zn and W related magmas are similar, the absence of a giant W-Sn deposit in the LT may indicate that parent magmas with a low degree of evolution and magmatic-hydrothermal transition are not conducive to their formation. This implies that the rocks that originated as highly evolved silicate-rich parent magmas, with a high degree of magmatic-hydrothermal alteration, would need to be targeted for W-Sn mineral exploration in the LT. In summary, our results emphasize that variations in chemical differentiation and the evolution of high-silica magmatic-hydrothermal systems can lead to differences in Pb-Zn and W enrichment. This has implications for the evaluation of the mineral potential of high-silica granites and hence their attractiveness as targets for mineral exploration.