Genesis of the Longfengchang polymetallic sulfide deposit in the southwest Fujian depression, southeast China, with a comparative study of the “Makeng-Type” iron deposit

• Extensive mineralogical and in situ sulfur isotope analysis was conducted to determine the origin of the LFC deposit. • The LFC deposit belongs to the skarn-type polymetallic sulfide deposit. • The LFC deposit’s weaker structural position, stratigraphic preservation, “Si-Ca” interface development, and mantle-derived magma influence lead to differences in mineralization scale and type compared to the “Makeng-type” deposit. • Emphasizing the significance of disconformity between the Jinshe and Lindi Formations within the window areas of the thrust-nappe structure system in large iron deposit exploration. The Southwest Fujian Depression Belt is a prominent metallogenic zone for skarn-type iron polymetallic deposits in China, with the Longfengchang (LFC) sulfur polymetallic deposit representing a medium-scale, sulfide-dominated deposit in the region. This study conducted a detailed analysis of the LFC deposit, focusing on its mineralogy, mineral composition, and in-situ sulfur isotopes, alongside a comparative study with the “Makeng-type” deposit. The study aims to elucidate the genesis of the LFC deposit, its relationship with the ’Makeng-type’ deposit, and the factors underlying differences in dominant economic minerals and resource scale. The LFC deposit is hosted within the skarn above the fault contact zone between the Lindi Formation sandstone and the Chuanshan–Qixia Formation carbonate, with mineralization stages classified as skarn-magnetite, quartz-sulfide, and carbonate. LFC garnets are primarily composed of CaO, TFeO, and SiO 2 , with minor Al 2 O 3 and trace amounts of MgO and MnO, classifying them as distal exoskarn andradite. The presence of Mn 3+ substituting for Fe 3+ in garnet suggests that the ore-forming fluid during the garnet skarn stage was likely oxidizing and weakly acidic. LFC pyrites exhibit Co/Ni ratios primarily ranging from 1 to 10, decreasing from Py 1 to Py 3 . In-situ sulfur isotope δ 34 S values range from −1.48 to 3.51 ‰, centering around 0 ‰, and increase from Py 1 to Py 3 , suggesting a magmatic-hydrothermal origin and a cooling metallogenic process. Thus, the LFC deposit is classified as a magmatic-hydrothermal skarn-type deposit, consistent with the genesis of “Makeng-type” deposits. The absence of the Jinshe Formation, and mantle-derived magma contribution, and less developed “Si-Ca” interface may explain the smaller scale and different mineralization type in the LFC deposit compared to the “Makeng-type” deposit. The key prospecting area for large iron-sulfur polymetallic deposits in the Southwest Fujian Depression Belt should feature a nappe structural window, well-preserved Jinshe Formation, developed “Si-Ca” interface, Yanshanian high-K calc-alkaline to shoshonitic intrusions, and coeval mantle-derived magma.