Superimposed Triassic and Oligocene W mineralization in the Yangla deposit, Southwest China: Evidence from scheelite geochemistry and U-Pb geochronology

The Sanjiang region in southwestern China, located within the eastern Tethyan metallogenic domain, hosts the Yangla Cu deposit, a large-scale skarn system associated with Triassic granitoids. This study integrates mineral assemblages, U-Pb geochronology, and trace element compositions of scheelite to identify two discrete W mineralization events at Yangla. Three scheelite groups have been identified: (1) S-group scheelite, hosted in skarn ores, is coeval with chalcopyrite and pyrite, indicating its association with Triassic skarn Cu mineralization. Geochemically, it exhibits high Nb + Ta contents, low Sr/Mo ratios, and pronounced negative Eu anomalies, consistent with scheelite from granite-related W deposits. (2) B- and M-group scheelite, hosted in hydrothermal breccia and marble ores, respectively, formed during the Oligocene (M-group scheelite U-Pb ages: 28.1–30.0 Ma) and coeval with stibnite and calcite. They display distinct geochemical signatures, including low Nb + Ta contents, high Sr/Mo ratios, and slightly positive Eu anomalies, supporting a non-magmatic fluid origin. A comparison of the Yangla Triassic intrusions with other W polymetallic, Cu- and W-related granites in the Sanjiang region reveals significant similarities in both magmatic source and fractionation degree to those of granites associated with W polymetallic deposits. These granites exhibit mixed crust-mantle source and moderate to high degree of magmatic fractionation, which may be a critical control on the formation of W polymetallic deposits. The identification of Triassic skarn scheelite challenges the traditional view of Yangla as a solely Cenozoic W deposit, providing critical insights for regional exploration strategies targeting concealed W orebodies.