The Research Of Syenite And Its Relationship With Metallogenesis In Bengge Gold Deposit, Yunnan Zhongdian Area

Bengge Syenite has complex structural environment and it is located in the south of Yidun island-arc, which is the intersection of Lanping-Simao plate、Sanjiang fold belt and Yangtze plate. During Kainozoic era, the inner plate orogeny of Jinshajing-Ailaoshan happened in this place. The geochemical characteristics reveals that the Bengge syenite is rich in alkaline, with the higher content of K2O (6.42%～11.63%) compared with Na2O. Na2O+K2O=8.29%～12.85%and the content of SiO2=50.65%～63.65%. Chondrate-normalized REE are right-inclined and display the enrichment of LREE. Trace elements characteristics display that the LILE such as Rb、Ba、Th、U、K are positive anormaly and Nb、Ta、Ti are negative anormaly, indicating the characteristics of island-arc magmatite. There are complicated zircon crystal populations in Bengge syenite, and LA-ICP-MS U-Pb dating shows that there are four different age groups in Bengge syenite, they are2261±5.8Ma、213±0.81Ma、135±1.5Ma�'�35±1Ma respectively. Combining with the tectonic environment, we speculate that Bengge syenite generated in35±1Ma, mixing the materials of Indo-Chinese Epoch、Stage of Yanshan and the basement of Yangtze, so there are different zircon crystal populations in Bengge syenite.The mineralogy research of clinopyxene suggests the composition of syenite changed when the clinopyxene was crystallizing, and it can be shown from several respects:the core of big pyroxene are rich in MgO and CaO, while the edges are rich in Na2O and FeO, where the aegirine-augite crystallized at; The crystallizing direction of clinopyxene is toward tholeiitic series, but the characteristic of syenite is alkaline-parlkaline; F1-F2diagrams of clinopyxene display different tectonic background between big pyroxene crystal and small crystal; the Al-Ti and Al-Si diagrams reveal the disaccord of big pyroxene and small pyroxene; all of the phenomena can be explained by the injection of alkline fluid into the syenite magma. The biotite in syenite are magnesian, and their characteristics of composition suggest they have a mixed source of crust and mantle, and they crystallized at a high temperature about 700℃～750℃and high oxygen fugacity.By confirming the position of gold deposit、the relationship of syenite and gold deposit, as well as the discovery of cryptoexplosive breccia, we can conclude that the metallogenesis of gold deposit happened after the formation of syenite, and the bengge gold deposit controlled by structure is hydrothermal deposit, not the porphyry deposit claimed by the former researchers. Syenite has no contribution to metallogenesis, and it is like a tabular intrusion with the function of shielding layer, the intrusion triggering the metallogenesis is at deeper location and indicates the potential of gold prospecting.