Metallogenic mechanism of cobalt in the Zhuchong cobalt -rich skarn iron deposit in the Middle -Lower Yangtze River Valley Metallogenic Belt: Constrained by in -situ sulfur isotopes and zircon U-Pb dating

Cobalt, being a vital strategic rare and precious metal, is primarily produced within diverse deposit types through co association. The cobalt -rich skarn iron deposit is a key deposit type with considerable development potential. However, the source and enrichment mechanism of cobalt in skarn iron deposit remain unclear. The Zhuchong cobalt -rich skarn iron deposit is located in the Anqing-Guichi district of the Middle -Lower Yangtze River Valley Metallogenic Belt ( MLYB). It is the first large-scale skarn iron -rich deposit discovered in the metallogenic belt in recent years. Its associated cobalt resources have reached a medium scale. Meanwhile, the cobalt grade in the Zhuchong cobalt -rich skarn iron deposit is the highest among similar deposits in the MLYB, reaching 190g/t. In this study, the zircon U-Pb dating of the ore -forming intrusions and the spatial variation of pyrite in -situ sulfur isotopes in the vertical direction ( drilling section) of the Zhuchong cobalt -rich skarn iron deposit were carried out. The results indicate that the U-Pb ages of the two deep concealed magmatic rock samples are 139. 6 +/- 1. 0Ma and 138. 9 +/- 0. 6Ma, respectively, which occurred during the Early Cretaceous. Vertically, the overall range of pyrite sulfur isotope is +5. 3%similar to + 13. 9%o, and delta S-34 varies greatly from top to bottom, and shows strong heterogeneity, which is obviously different from the sulfur isotope compositions of Anqing skarn Cu -Fe deposit. The pyrite delta S-34 in the cobalt -rich magnetite ore body in the bottom -up mineralization -alteration zone of the Zhuchong deposit ranges from 5. 3%o to 10. 0%o. In the diopside skarn belt near the ore, the delta S-34 range of the vein pyrite spans from 12. 1%o to 13. 9%o. Within the altered diorite, the delta S-34 values of vein pyrite vary from 7. 1%o to 10. 8%o. In the outer skarn, the delta S-34 range of vein pyrite ranges fall between 6. 8%o and 7. 6%o. Within the siltstone of the Yueshan Formation, located in the gypsum -bearing salt layer, the delta S-34 values of pyrite range from 12. 0%o to 12. 7%o. Similarly, the uppermost siltstone vein pyrite of the Tongtoujian Formation exhibits a delta S-34 range of 6. 1%o to 7. 8%o. The sulfur isotope composition of pyrite within the Zhuchong cobalt -rich magnetite ore exhibits an intermediate range between that of the Yueshan pluton and the gypsum -salt layer of the Yueshan Formation. This suggests that the sulfur isotope signature within the ore reflects a mixture of two distinct end members. Statistical analysis of scale and grade within the MLYB cobalt -rich skarn deposits ( Fe -Cu -Co) indicates no apparent correlation between cobalt mineralization and the type of Fe -Cu skarn deposits. Furthermore, the involvement of sulfur through the gypsum -salt layer in the ore -forming hydrothermal fluid does not exhibit a discernible