摘要
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.
The host rocks of the porphyry tin deposits in the Yangbin area are principally 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 content 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 10-20 vol% of the rocks. The rock groundmass consists of subhedral topaz, quartzand 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 ratios 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, 3×10-6-12×10-6, 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 (δEu=0.01-0.03) and low ratio of LREE/HREE (0.78-0.84). In summary, the authors propose an idea of S-type genesis for the topaz-bearing porphyries with tin mineralization, instead of I-type.
基金
National Natural Science Foundation of China
