Isotope geochemistry of the Huize Zn-Pb ore field, Yunnan Province, Southwestern China: Implication for the sources of ore fluid and metals

抄録

The Huize ore field, which is the most famous high-grade Zn-Pb ore field in China, comprises the Kuangshanchang and Qilinchang deposits. The Zn and Pb reserves of these two deposits are more than 5 Mt with ore grades ranging from 25% to 35% in weight. Lead, sulfur, carbon, oxygen, hydrogen and strontium isotope geochemistry is reported to help understand the sources of the ore fluid and metals. The ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb values of the ores range from 18.251–18.530, 15.663–15.855 and 38.487–39.433, respectively. These values are similar to those of the wall rocks. The pyrites disseminated in the wall rocks have indistinguishable Pb isotope composition with the ores. These data indicate that the wall rocks provided metals to the ore fluid. Most δ³⁴S values of the ores range from 13 to 17 per mil. The sulfur of the ores originated by in situ reduction of sulfate. Three kinds of gangue calcite from the ores have similar isotope compositions, which have δ¹³C values in the range of −2.1 to −3.5 per mil with respect to PDB and δ¹⁸O values in the range of 16.8 to 18.6 per mil with respect to SMOW. The δD_FI values of fluid inclusions in the three kinds of gangue calcites have a narrow range of −50 to −60 per mil and the δ¹⁸O_H2O values calculated from δ¹⁸O values of calcite range from 7.0 to 8.8 per mil at 200°C. These data suggest that the ore fluid was a basinal brine that passed through shale, clastic rocks and mudstone underlying the host rock. Initial ⁸⁷Sr/⁸⁶Sr values of the pyrite, sphalerite and calcite from the ores range from 0.714 to 0.717. The initial ⁸⁷Sr/⁸⁶Sr values of unaltered host rock (0.7083–0.7093) are lower than that of the altered host rock (0.7106). It suggests that the ore fluids have higher initial ⁸⁷Sr/⁸⁶Sr values than the wall rocks. These high initial ⁸⁷Sr/⁸⁶Sr values may be due to the reaction between the ore fluid and the shale, clastic rocks and mudstone underlying the host rock or the fluid might have originated from these rocks.