Stratabound Tungsten and Tin Related to Basic Rocks:the Precursor of the S-type Granite-related W-Sn deposits-Examples from the W/Sn deposits in Yunnan Province

Taking several W-Sn ore belts in Yunnan as examples and referring to the research results from China and abroad, the authors investigated the relationship between the W-Sn bearing stratabound W-Sn deposits and the W-Sn bearing S-type granites. The results show that all the W-Sn bearing granites occur in areas where the earlier strata are enriched in tungsten and/or tin, such as the Proterozoic tin (-zinc) belt from Ximeng of Yunnan to Wanleng of Myanmar, the Cambrian tungsten-tin belt in Dulong-Bainiuchang of southeast Yunnan, the Carboniferous tin-tungsten belt in Tengchong-Lianghe of west Yunnan, the Triassic tin belt in Gejiu of southeast Yunnan; and the Neogene tin belt in Haobadi of west Yunnan. In contrast, the S-type granites (such as the granites in Ludian and Jiaren of southwest Yunnan), hosted in the strata without dominant enrichment of W and/or Sn, have no tungsten and/or tin. An analysis of dating results, structures and textures of the rocks and/or ores and the property of the metamorphism of the host rocks shows that the essence of “the S-type granites’ affinities with mineralization of W, Sn, Mo, Bi” resulted from the sequence of the remelting of the preexistent strata with enrichment of W, Sn, Mo and/or Bi and redistribution of the ore-forming materials including W and/or Sn in precipitation order of the metals controlled by temperatures, under the condition of crust thickening or thermal disturbance of deep magma. Therefore,the S-type granites do not necessarily constitute the prerequisite of bearing W, Sn, Mo, and Bi. Correspondingly, the strata enriched in W and /or Sn are the key to mineralization of W and Sn. Consequently, the through analysis of the W and Sn-bearing strata and the setting of the formation is the key to enlarging the resources of W and Sn. A number of case studies of W-Sn ore deposits show that the stratabound W/Sn ore deposits are closely related to the (alkalic) basalts in the depth rather than to the S-type granites. Once there exists the early W/Sn–bearing sequences that are cut or engulfed, the mineral substances can be transported to specific spaces to reprecipitate and from ore deposits. The contact zone, especially the outer contact zone, and the related structure fractures, are favorable spaces for formation of the W and Sn deposits as a result of migration and redistribution of the ore-forming material. When the stratabound W/Sn ore deposits are involved in a metamorphic core complex, the ore-bearing sequence can be uplifted to a shallow level convenient for exploitation, and there might also exist the ore shoot at the contact of the concealed granite in the center of the core complex.