Characteristics of the Alkaline Picritic Volcanic Rocks in the Gejiu Superlarge Tin-Copper Deposit and their Continental Dynamic Implications

The Gejiu superlarge tin polymetallic deposit is located in the southeastern part of Yunnan Province,China,which comprises five separate orefields:the Malage,Songshujiao,Gaosong,Laochang and Kafang orefields. The pre-Triassic and Early Triassic sedimentations were marine source predominated,and shifted to terrestrial source dominated after Early Triassic. Multiple periods or stages of magmatic activities ranging from Proterozoic to Cenozoic along with main tectonic movements were recorded in the area. This paper reports the petrological and geochemical characteristics of the Triassic volcanic rocks which are closely related to the tin and copper mineralization in the Gejiu area. The volcanic rocks,which consist mainly of alkaline picrite,alkaline picritic basalt and basanite,are of alkaline ultrobasic with high content of titanium. The rocks are characterized by SiO2=34.62%~ 44.73%,K2O+Na2O=2.99%~7.15%,TiO2=1.83%~4.0%,high HFSE contents. The chondrite normalized REE patterns are smooth right-dipping lines without significant Eu and Ce anomalies,which are similar to those of the typical oceanic island basalt (OIB). Considering the rigional tectonic settings,the Triassic sedimentary basin in the Gejiu area was a backarc rift basin which was a favourable site for the formation of large-superlarge massive sulfid deposit.