Petrologic And Geochemical Characteristics And The Function In Gold Mineralization Of Alkali-rich Porphyries Of The Beiya Area In Western Yunnan Province

Beiya gold-polymetallic deposit is one of typical deposits related to alkali-rich intrusion of the Cenozoic in western Yunnan, so many researchers focus on the research on the alkali-rich porphyry for decades. On the basis of the full reference and absorption of the previous research results, we combine the widespread field and the indoor synthetic research work. In order to further study on the origin and petrogenesis of alkali-rich porphyries, this paper systematically makes a study of mineralogy, petrology, geochronology, petrology and geochemistry characteristics of alkali-rich porphyries. To discuss the relationship between mineralization and alkali-rich porphyries, and investigate genetic relation among various ore types, this article studies geologic features, fluid inclusions, extensive field investigation of different ore blocks, and the source of the mineralization fluid and ore-forming materials. Based on the above-mentioned research, the following conclusion is reached:1)In this paper SHRIMP U-Pb age, TY-1(31.5±1.1Ma),TY-3(31.34±0.73 Ma) of Tanyao quartz syenite porphyry in Beiya is reported for the first time, combined with the ages reported by former researchers, which suggests that the intrusion of Beiya is formed in Cenozoic.2) The results show that the alkali-rich porphyries in the Beiya area are shoshonitic rocks, and this suite of rocks have relatively low contents of∑REE( 40.04×10-6~298.30×10-6), enrich in LREE and lack Eu negative anomalies. Besides, they have relatively high contents of Sr, and enrich in LILE (such as Rb, Ba ,K etc.), but lack in Yb, Y and HFSE(such as Nb,Ta etc.). All characteristics mentioned above indicate that alkali-rich porphyries from Beiya are somewhat similar to adakites in geochemistry.3)Systemtical studies on Sr, Nd, Pb isotopic characteristics in this thesis, indicate that the source region has geochemical characteristics of EMII enriched mantle. On the basis of discussion on the origin and petrogenesis, we propose that the alkali-rich porphyries in Beiya, are very likely to be originated from partial melting of the base of thickened crust, which is induced by strike-slip movement of the Jinshajiang-Red River fault caused by the late-collision of India to Asia, or related to upwelling, thermal erosion of asthenospheric melts initiated by the oblique collision and face to face intra-continental subduction between the India-Myanmar continent and the Yangtze continent. In this process, the source region has been extensively metasomatized by the fluid released from old subducted slab.4) During the metallogenic process, the alkali-rich porphyries not only provide power and thermal energy, but also are main source for the mineralization resources and fluid resources. And we further propose that all the Beiya gold-polymetallic mineralization is controlled by a porphyry-hydrothermal metallogenetic system.