PAN Ping1,2, CHANG He1*
(1. School of Land Resources Engineering, Kunming University of Science and Technology, Kunming 650093, China; 2. Geomatics Engineering Faculty, Kunming Metallurgy College, Kunming 650033, China)
Abstract: The Daxiao Pb-Zn deposit in Dongchuan, Yunnan is located in the southwestern Yangtze Block. Its orebodies, occurred as veined, lenticular, or stratiform forms, are hosted in argillaceous dolostone of the Heishan Formation of the Meso- to Neo- Proterozoic Kunyang Group. Ore minerals of the deposit are mainly galena, and secondly sphalerite and pyrite. Gangue minerals are mainly calcite, dolomite, and quartz. Ore minerals have granular, metasomatic and other textures. Ores have massive, veined, and disseminated structures, with characteristics of the epigenetic mineralization. The δ34S values of galena samples range from +6.1‰ to +8.5‰, which are significantly higher than those of mantle-derived sulfur (-3‰ to +3‰). Thus, the contribution of sulfur mainly derived from the mantle magma can be ruled out. However, the δ34S values of sulfides vary within range of Meso- and Neo-proterozoic metamorphic rocks/fluids (δ34S=-20‰ to +20‰), so the contribution of sulfur derived from the ore-bearing strata cannot be ruled out. In addition, compared with the sulfur isotope compositions of marine-derived sulfate minerals in the overlying Ediacaran-Permian sedimentary rocks (δ34S=+11.8‰ to +28.3‰), the δ34S values of sulfides are significantly low. However, the thermochemical sulfate reduction (TSR can result in up to +15‰ isotope fractionation) could lead to the sulfur isotopic compositions of sulfides in the Daxiao Pb-Zn deposit, so the contribution of sulfur from the evaporate rocks in the overlying strata should not be ruled out. Therefore, the most reasonable explanation is that the sulfur in the ore-forming fluid of the deposit is most likely to be provided by the ore-bearing strata and the overlying sedimentary rocks, and the TSR played an important role for the formation of reduced sulfur. The Pb isotope ratios of individual particles of galena vary in small ranges with 206Pb/204Pb ratios varying from 16.5687 to 16.6058, 207Pb/204Pb ratios varying from 15.4978 to 15.0026, and 208Pb/204Pb ratios varying from 36.3700 to 36.4067. Such Pb isotopic ratios are plotted in the field between the upper crust and orogenic belt average lead evolutionary lines, and are similar to those of ore-hosted rocks of the Neo-Proterozoic Kunyang Group, indicating that the Pb source of the deposit has a crust characteristics and Pb was mainly sourced from the ore-hosted strata. In summary, the ore-forming elements of the Daxiao Pb-Zn deposit have characteristics of mixed sources. It is believed that the main ore-forming elements (i.e. metals and parts of sulfur) could be sourced from ore-hosted strata of the Kunyang Group, a part of sulfur could be sourced from the overlying sedimentary rocks. The deposit belongs to the type of epigenetic hydrothermal deposit.
Keywords: Sulfides; S and Pb isotopes; sources of ore-forming elements; fluids mixing; the Daxiao Pb-Zn deposit
ACTA MINERALOGICA SINICA Vol. 40, No.4, 2020, page 466-474
