METASOMATIC PHENOMENA IN THE GRANITE CONTACT ZONES OF THE GEJIU TIN DEPOSIT, YUNNAN PROVINCE, CHINA
云南个旧锡矿床花岗岩接触带的交代现象

The Gejiu tin deposit is one of the largest tin deposits in the world. The mineralizations mainly occur in the metasomatites in the contact zone between Yanshanian granites and Triassic carbonate country rocks or in the carbonate country rocks adjacent to the contact zone.There exist six ore-bearing metasomatic formations in the Gejiu mine district, namely 1) calciskarn formation, on which are superimposed Sn, Cu and W (S, As, Bi and Fe) mineralizations; 2) magnesioskarn formation, the mineralizations superimposed on it being the same as those on the calciskarn formation; 3) alkalimetasomatite formation, associated with W, Sn and Be mineralizations; 4) greisen formation, associated with Sn and W (B, Li, Nb, Ta and TR) mineralizations; 5) fluor-boron-metasomatite formation, associated with Sn and W (Be and Li) mineralizations; and 6) carbonate-fluorite-quartz-metasomatite formation, associated with Sn, W and polymetallic mineralizations.Each metasomatic formation contains special metasomatic facies (metasomatic mineral association). All the abovementioned metasomatic formations occur in different geological settings, but they are closely related with each other in origin, time and space, showing various distinct zones, and may constitute a special metasomatic series.Tin mineralization is cheifly relasted to the evolution of post-magmatic metasomatism. During magmatic crystallization, tin exists mainly in biotite probably in the isomorphous form. Occasionally, fine-grained cassiterite as a accessory mineral may be found within biotite. In the skarn stage, most of tin is concentrated in the garnet, forming Sn-bearing andradite. According to the abundant occurrence of metasomatic minerals containing higher amounts of volatile components (F, OH, Cl, B and S) in the metasomatites, such as scapolite, boro-vesuvianite, chondrodite, phlogopite, fluorite, tourmaline, amphi-bole, fluoborite, axinite, danburite, datorite, tremorite, nordenskiSldine, cuspid-ine, sulfides etc., the authors hold that volatile components play a very important role in the ore formation.