The host rocks of the porphyry tin deposits in the Yangbin area are prince-pally topaz-bearing porphyry dikes about 2 km long and 2-20m wide.Three lithologic types are identified for the dikes:topaz-bearing potassium feldspar granitic porphyry,topaz-bearing monzonitic granitic porphyry and topaz-bearing quartz porphyry.The con-tent of topaz in the rocks ranges from 10 to 20 vol.%.Porphyritic texture is characteristic,with quartz,potassium feldspar and albite as main phenocryst minerals.The phenocryst occupies 10-20vol% of the rocks.The rock groundmass consists of subhedrak topaz,quartz and protolithionite.Topaz has a unit-cell parameter b=8.797(A)°,and F:OH=1.92:0.18, indicating a F-rich variety formed at high temperature .The topaz-bearing porphyries occurring in this area are strongly peraluminous (A/NKC=1.574-12.94)，with high ra-tios of F/C1(146-303) and Rb/Sr(5-122).They are rich in incompatible elements (Sn,313×10-6-1042×10-6;W,6×10-6-218×10-6;Nb,27×10-6-54×10-6),but poor in compatible elements (Sr,10×10-6-28×10-6;Ba,58×10-6-73×10-6;V,3×10-6-10×10-6,Cl,150×10-6-226×10-6).The rocks are also characterized by high total REE amount (281.69×10-6-319.76×10-6);with strong Eu depletion (&Eu=0.01-0.03)and low ratio of LREE/HREE(0.78-0.84).In summary ,the authors propose propose an idea of S-type genesis for the topaz-bearing porphyries with tin mineralization,instead of I-type.
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