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Geoscience ›› 2006, Vol. 20 ›› Issue (4): 527-535.

• Engineering Geology and Environmental Geology • Previous Articles     Next Articles

Characteristics and Origin of Alkali-rich Porphyries from Beiya in Western Yunnan

XU Shou-min1,2, MO Xuan-xue1, ZENG Pu-sheng3, ZHANG Wen-hong4, ZHAO Hai-bin1, ZHAO Han-dong1   

  1. 1School of Earth Sciences and Resources, China University of Geosciences, Beijing100083, China;
    2Heilongjiang Geology Mineral Products Survey and Application Research Institute, Harbin, Heilongjiang150036, China;
    3Department of Resources and Environment, Yunnan University of Finance and Economics, Kunming,Yunnan650221, China;
    4Beiya Branch, Yunnan Geology and Mineral Resources Co.Ltd, Heqing, Yunnan671000, China
  • Received:2006-07-11 Revised:2006-10-17 Online:2006-04-20 Published:2006-04-20

Abstract:

This paper presents petrological and geochemical data of Au-mineralization-related alkali-rich porphyries in the Beiya gold deposit and its adjacent regions, western Yunnan. Geochemically, the alkali-rich porphyries from Beiya have relatively high contents of SiO2 (>68%) and Al2O3 (13.45%-15.22%), high Sr/Y and (La/Yb)N ratios, enrichment in Sr (365×10-6-930×10-6) but depletion in Yb and Y  (Yb:0.64×10-6-1.17×10-6; Y: 7.17×10-6-12.25×10-6), and weak Eu negative anomalies. All of those characteristics mentioned above indicate that alkali-rich porphyries from Beiya are somewhat similar to adakites in geochemistry. However, their lower contents of MgO, Cr and Ni are different from those of typical adakites. Petrological and geochemical characteristics of alkali-rich porphyries suggest that magmas might not be generated from subducted oceanic slab, but likely from the base of thickened crust, corresponding to eclogite-facies. High abundance of trace elements Rb, Ba, Th and K in alkali-rich porphyries may be attributed to fluid metasomatism. Dynamically, magma generation might be related to a face-to-face intra-continental subduction beneath Sanjiang blocks of the India continent versus the Yangtze continent, strike-slip movements of the Jinshajiang-Red River fault and tectonics-induced asthenospheric upwelling and metasomatism by fluids released from subducting slabs.

Key words: alkali-rich porphyries, adakite, lower crust, Yangtze block,  Beiya, western Yunnan

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