矿床地球化学国家重点实验室知识库

Although Zn-Pb deposits are one of the most important Cd reservoirs in the earth, few studies have focused on the Cd isotopic fractionation in Zn-Pb hydrothermal systems. This study investigates the causes and consequences of cadmium and sulfur isotope fractionation in a large hydrothermal system at the Tianbaoshan Zn-Pb-Cd deposit from the Sichuan-Yunnan-Guizhou (SYG) metallogenic province, SW China. Moderate variations in Cd and S isotope compositions have been measured in sphalerite cover a distance of about 78 m. Sphalerite has delta(114)/Cd-110 values ranging from 0.01 to 0.57 parts per thousand, and sulfides (sphalerite, galena and chalcopyrite) have delta S-34(CDT) values ranging from 0.2 to 5.0 parts per thousand. Although delta S-34(CDT) and delta(114)/Cd-110 values in sphalerites have no regular spatial variations, the delta S-34(CDT) values in galena and calculated ore-forming fluid temperatures decreased from 2.1 to 0.2 parts per thousand and from about 290 to 130 degrees C, respectively, from the bottom to the top of the deposit. Heavy Cd isotopes are enriched in early precipitated sphalerite in contrast to previous studies. We suggest that Cd isotopic compositions in ore-forming fluids are heterogeneous, which result in heavy Cd isotope enrichment in early precipitated sphalerite. In comparison with other Zn-Pb deposits in the SYG area, the Tianbaoshan deposit has moderate Cd contents and small isotope fractionation, suggesting differences in origin to other Zn-Pb deposits in the SYG province.

In the Tianbaoshan deposit, the calculated delta S-34 Sigma(s-fluids) value is 4.2 parts per thousand, which is not only higher than the mantle derived magmatic sulfur (0 +/- 3 parts per thousand), but also quite lower than those of Ediacaran marine sulfates (about 30 to 35 parts per thousand). Thus, we suggest that reduced sulfur of ore-forming fluids in the deposit was mainly derived from the leaching of the basement, which contains large amount of volcanic or intrusive rocks. Based upon a combination of Cd and S isotopic systems, the Tianbaoshan deposit has different geochemical characteristics from typical Zn-Pb deposits (e.g., the Huize deposit) in SYG area, indicating the unique origin of this deposit.