Geochemical characteristics and genesis of topaz-bearing porphyries in Yangbin area of Taishun County, Zhejiang Province

Abstract

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 lithologie 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, quartz and protolithionite. Topaz has a unit-cell parameter b=8.797 (Å), 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/Cl (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.