Genesis and metallogenic significance of the Lugu Fe-Sn deposit, southwest China: Constraints from geochronology, geochemistry, and ore-forming physicochemical conditions

• Cassiterite and zircon U-Pb dating (827 Ma–802 Ma) directly links the Lugu Fe-Sn deposit to Neoproterozoic intra-Rodinia extension and the coeval Lugu composite granite. • Sulfur isotope and zircon Hf isotope compositions, combined with highly differentiated granite geochemistry indicate that Fe, Sn, and S were primarily derived from the magmatic-hydrothermal system of the Lugu composite granite. • Mineralization progressed from high-temperature, oxidizing to moderate-temperature, sulfur-rich hydrothermal environments, recorded by mineral chemistry. • Based on the geological characteristics of the ore deposit and combined with the element geochemical features of magnetite and sphalerite, the Lugu Fe-Sn deposit is classified as a magnetite-dominant, magnesian skarn (skarn-like) deposit. • This study clarifies the genesis of Fe-Sn deposits in the Panxi metallogenic belt and provides a scientific basis for regional Fe-Sn prospecting. The Panxi region, situated at the western margin of the Yangtze Platform, represents a significant Fe-Cu-Ni-(PGE)-Sn-Pb-Zn polymetallic metallogenic belt. This study investigates the representative Lugu Fe-Sn deposit in Mianning County, Sichuan Province, where orebodies occur as stratoid and lenticular lenses within the contact zone between the Neoproterozoic Lugu composite granite and Mesoproterozoic dolomitic marble (Darezha Formation). The major ore types are magnetite and cassiterite-dominant ores, with average grades of w (TFe) = 37.81 %–53.52 % and w (Sn) = 0.63 %. Mineralization is divided into three hydrothermal stages: magnetite (Ⅰ, hematite is scattered irregularly in magnetite in the form of fine droplets), magnetite-cassiterite-apatite (Ⅱ, the main Sn-enrichment stage, characterized by extensive development of magnetite and cassiterite, associated with serpentine and brucite), and quartz-sulfide (Ⅲ, with pyrite, chalcopyrite, and sphalerite). An integrated approach including field geology, petrography, LA-ICP-MS (zircon/cassiterite U-Pb dating; magnetite/sphalerite trace element), and in-situ sulfur isotope analysis was employed. The main findings are as follows: (1) New zircon U-Pb ages (813.9 ± 3.6 Ma, 801.8 ± 2.9 Ma) and a cassiterite U-Pb age (827 ± 13 Ma) constrain the magmatism and mineralization to the Neoproterozoic intra-Rodinia extension stage. The 15 Ma–25 Ma interval between earliest zircon crystallization and cassiterite precipitation reflects protracted magmatic-hydrothermal evolution of a composite pluton. (2) The highly differentiated signature of the Lugu granite ( w (Nb)/ w (Ta) < 5) and sulfur isotope composition of sulfides ( δ 34 S = 8.38 ‰–11.22 ‰) indicate that Fe, Sn, and S were primarily derived from the granitic magmatic-hydrothermal system. Zircon Hf isotopes (ε Hf (t) = −4.34–−0.57; T DM2 = 1.96 Ga–1.74 Ga) further demonstrate derivation from an enriched, Paleoproterozoic crustal source. (3) Magnetite trace elements indicate that Stages Ⅰ-Ⅱ formed under high temperatures (300 °C–679 °C) and high oxygen fugacity (FMQ + 1 to FMQ + 3). For Stage Ⅲ, sphalerite geochemistry constrains the temperature to 238 °C–241 °C, accompanied by high sulfur fugacity (log f S2 = −8.5–−8.0) and reduced conditions. (4) Magnetite (low Ti, V, Cr, high Mg, and Ni/Cr ≫ 1) and sphalerite (Ga/In < 1, Fe-Mn enrichment) trace-element discriminants, combined with the Mg-silicate-dominated alteration assemblage (serpentine, brucite, talc) confirm the Lugu Fe-Sn deposit as a magnetite-dominant, magnesian skarn (skarn-like) deposit. This study elucidates the genesis of Fe-Sn mineralization in the belt and provides a scientific basis for regional exploration.