The Role of Gypsum Salt Layer in the Mineralization Process of Skarn Type Iron Deposits: A Case Study of Hanxing Area

The Skarn-type iron deposit is an important type of iron-rich deposits in China with reserves accounting for about 60% of all iron-rich ores. In recent, the role of gypsum salt layer in the mineralization process of Skarn iron deposits has been the focus of attention of scholars. The results show that the δ³⁴S values of sulfate and sulfide in the Skarn-type iron deposits in the Hanxing area range from 24‰ to 29‰ and from 11‰ to 20‰, respectively, and have the characteristics of sulfur isotope composition of marine sedimentary rocks. These data imply that sedimentary gypsum-salt layer material is involved in the mineralization of skarn-type iron deposits. Taking the skarn iron deposit in the Hanxing area in this study, the compositions of δ³⁴S of the sulfides, gypsums, and other sulfur-containing minerals in this area were systematically analyzed, and the role of the gypsum salt layer in the mineralization process was clarified. ① Na⁺, Cl⁻ and other substances, as mineralizer, in the gypsum salt layer are added to the gas-water hydrothermal fluid. The alkali-rich gas-water hydrothermal fluid causes the albitization of the diorite near the contact area, which precipitates Fe and migrates iorn in the form of sodium chlorate complex and iron halide. ② the sulfate in the gypsum salt layer has high oxidation activity under high temperature conditions, and it would cause a redox reaction with the reducing iron-rich hydrothermal fluid to increase the oxygen fugacity. The Fe²⁺ in the hydrothermal fluid, therefore, is oxidized to Fe³⁺, resulting in the formation of magmatite (i.e., Fe₃O₄). Hence, the magnetite is enriched and precipitated among the surrounding rock layers to form an iron-rich deposit. ③ various dissolution erosion voids are formed in the carbonate rock layers after the dissolution of the gypsum salt layer material, which provides a favorable mineralization space and is one of the important geological conditions for the formation of skarn magnetite deposits. In conlusion, gypsum salt layer has important significance in providing mineralizer, oxidant and ore-contained space in the process of skarn iron mineralization.