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

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 SiO_(2)(68%) and Al_(2)O_(3)(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.