## 6 Discussion
### Exploration for the Sanjiang Tethyan Orogenic Belt
West Yunnan has a large distribution of Himalayan copper-bearing porphyry bodies, and porphyry alteration and strong Cu mineralization are commonly seen. It is a favorable area for exploring porphyry Cu(-Mo-Au) deposits. The spatial distribution of porphyry Mo-W(-Cu) deposits and magmatic hydrothermal Sn-W deposits are strictly restricted to the low \(\varepsilon\)Hf(t) region (Figure 3). Proven porphyry Mo-W(-Cu) deposits (Xiuwacu (No. 17), Relin (No. 18), Tongchanggou (No. 19), and Donglufang (No. 20)) are concentrated at the edge of reworked old crust or near the ancient crust. Similarly, the identified magmatic hydrothermal Sn-W deposits (Xiaolonghe (No. 24), Dasongpo (No. 25), Lailishan (No. 29)) are concentrated in the ancient T\({}_{\text{DM}}\)\({}^{\text{c}}\) crustal zone or developed along the \(\varepsilon\)Hf(t) value isotopic boundary of ancient crustal blocks. These features suggest that the spatial distribution of Sn-W and Mo-W deposits is closely related  
Figure 6: Cenozoic alkali-rich porphyry showing distribution in western Yunnan. Adapted with permission from Ref. [152]. Copyright 2020 Wiley Online Library. to the compositional heterogeneity and remelting/modification of ancient crustal blocks (Figure 5). Late Cretaceous magmatic rocks are developed in Xiuwacu, Relin, Donglufang, and other areas on the southern margin of the Yidun Island Arc (Figure 2). The magmatic activity in this period is that the Yidun Island Arc is in the extensional background, due to the thinning of the lithosphere. The asthenosphere material upwelled, the temperature of the lower part of the rock group increased, and the thickened lower crust was partially melted [55]. During the partial melting of the lower crust, chalcophile elements (such as Cu) were removed, but siderophile metals (such as Mo) were left as a residue in the cumulate zone of the deep arc crust and/or in the metasomatized mantle lithosphere. This mechanism facilitated porphyry Mo-Cu mineralization in the southern Yidun Terrane (Figure 5) [156]. Elemental and chemical anomalies of Mo, W, Sn, and Bi in the Yidun Island arc region are mainly distributed in the Xiuwacu, Yaza-Lantang-Pulang, and Gelu areas of the Garze porphyry belt [56]. Among them, the Mo, W, Sn, and Bi anomalies located in the Xiuwacu promotion site are large in scale, high in intensity, and obvious in concentration centers, with tertiary concentration zoning, representing the strong Mo-W mineralization in the area [56]. These areas of chemical anomalies are generally consistent with the transformation sites of the low \(\epsilon\)Hf(t) isotope anomalies in zircon. The western Yunnan Sn belt is mainly developed in the Teng-Liang area, Baoshan block, Changing-Menglian, and Lincang combined belt, all in the region of low zircon \(\epsilon\)Hf(t) values. The Sn bearing granites in western Yunnan can be divided into three granite belts: Yunlong-Mengnan (East Asian belt), Changing-Ximeng (Central Asian belt), and Tengchong-Lianghe (West Asian belt) [181]. The comparative research revealed that the Sn-bearing granite belt in western Yunnan belongs to the northern extension of the Sn-bearing granite belt in Southeast Asia [182]. The Sn-bearing granites are produced by regional tectonics driven by micro-plate subduction and collision [115]. At present, most of the discovered deposits in the Sn belt in western Yunnan are related to the remodeling-remelting granite magmatic activity [115]. The remodeling-remelting of the crust enriches the metallogenic metal elements initially dispersed in the crust (mantle). Regional geophysical studies show that Sn polymetallic deposits in western Yunnan are mainly located in the low-gravity anomaly zone (\(-2\) mGal--10 mGal), and the range of weak positive aeromagnetic anomalies (0-100 nT) reflects that the Sn polymetallic deposits in western Yunnan are mainly associated with granites and sedimentary rocks [115].