Selenium enrichment in the Dingtoushan carbonate-hosted epigenetic Pb-Zn deposit in the western Yangtze Block, SW China

Abstract

Selenium (Se) is a typical dispersed element (crust Clarke value only 0.05 × 10-6) and is mainly enriched in black shales (>20 × 10-6). Selenium can also be associated with Pb-Zn and U deposits. In carbonate-hosted epigenetic hydrothermal Pb-Zn deposits, the selenium contents in sulfides are commonly < 50 × 10-6, and the selenium is mainly enriched in galena, whereas the selenium contents in sphalerite are often < 10 × 10-6. There are few studies on Se-rich carbonate-hosted epigenetic hydrothermal Pb-Zn deposits, and the origin of such deposits needs more in-depth exploration. In this study, we selected the Dingtoushan Se-rich Pb-Zn deposit in the western Yangtze Block, SW China, and utilized in-situ trace elements and S-Pb isotopes of Se-rich sphalerite to investigate this issue. There are two textures of sphalerite formed in the Dingtoushan deposit, of which subhedral sphalerite (Sp-I) precipitated during the early stage, whereas euhedral sphalerite (Sp-II) generated during the late stage. The results of trace elements show that the Dingtoushan deposit is significantly enriched in selenium (>22 × 10-6) compared with other carbonate-hosted Pb-Zn deposits (<22 × 10-6, generally < 10 × 10-6) in the Sichuan-Yunnan-Guizhou (SYG) Pb-Zn metallogenic province. In detail, a positive correlation between Se, Cd, In and Cu contents and sulfur isotopes of sphalerite is identified at the deposit scale, but no relationship is observed at the grain scale. In addition, the late stage Sp-II with δ34S values of −7‰ to −3‰ is rich in selenium and other trace elements, whereas the early stage Sp-I with isotopically light sulfur (δ34S = -29 ‰ to −8‰) is depleted of them. This indicates that in the early stage, insufficient addition of hydrothermal fluid resulted in relatively low temperature, and sulfur formed by bacterial sulfate reduction (BSR) was mainly involved in mineralization. With the addition of a large amount of hydrothermal fluid, thermochemical sulfate reduction (TSR) was induced, which leads to the formation of sphalerite in the late stage with relatively high δ34S values. Furthermore, the Pb isotopes reveal that the ore-forming metals were likely derived from the Cambrian black shales. Hence, we infer that fluid mixing is the main sulfide precipitation mechanism and may cause the positive correlation of selenium concentrations and δ34S values. On the other hand, at the grain scale of sphalerite, the variation of selenium contents shows a negative correlation with temperature, suggesting that temperature may play a critical role in selenium enrichment in sphalerite. This study highlights that selenium enrichment in sphalerite is jointly controlled by fluid mixing and temperature. The degree of fluid mixing dominated the selenium concentration in hydrothermal fluid, but high temperature may not be conducive to selenium incorporation into sphalerite. Our new results indicate that sphalerite in the carbonate-hosted epigenetic hydrothermal deposits is also a major host for selenium and such type deposits may have a great economic potential for selenium resource.