ORIGIN,EVOLUTION AND PROSPECTING SIGNIFICANCE OF GRANITES IN THE GEJIU TIN FIELD,YUNNAN PROVINCE
WANG Xinguang ZHU Jinchu (Department of Earth Sciences, Nanjing University, 210008)
Based on the distribution, mineralogy, petrochemistry, trace and rare earth elements, Rb-Sr, Sm-Nd and oxygen isotopic characteristics of the granites in Gejiu, it can be concluded that the granites are the products of two stages of magmas: the Longchahe granite is product of the melted earth crust, in the, early Yanshanian; the Mashong, Shenxianshui, Baishachong and Laoka granites are differentiated products of the late Yanshanian magma. They display certain inheritance and evolutional relations from early to late stages. This may indicate that they have the similar source rocks and the products of the selected melting of these rocks. The late Yanshanian granites are all tin-bearing granites compared with the known tin-bering granite. This may give a good explanation to existence of large quantity of stream tin distribution in the eastern Gejiu fault. Tt may also have some significance to find tin deposits at the southern (or southcastern) area of the Shenxianshui and Baishachong granite bodies.