Geochronological and geochemical constraints on the formation of the Ruwai skarn Fe-Pb-Zn-Ag deposit in the Lamandau region, SW Borneo, Indonesia

The Ruwai skarn Fe-Zn-Pb-Ag deposit， situated in the Lamandau region of the South Schwaner zone within the Southwest Borneo block （SW Borneo）， is the largest base metal deposit in Indonesia. The exposed Sukadana granodiorite within the deposit is genetically linked to mineralization. Zircon U-Pb dating of the granodiorite yields an age of 95.0 ± 0.7 Ma， while garnet U-Pb dating of the skarn indicates an age of 94.7 ± 1.5 Ma， demonstrating that skarn mineralization is coeval with the intrusion of the Sukadana granodiorite. Biotite compositions from the granodiorite plot above the fayalite-magnetite-quartz （FMQ） buffer， and zircon-derived oxygen fugacity (logf_O2) ranges from 10^-16 to 10^-5 bars， indicating a highly oxidized magma. The initial Sr-Nd isotope ratios of the granodiorite are (⁸⁷Sr/⁸⁶Sr)_i = 0.704864-0.705641 and ε_Nd（t） = –1.03 to –0.75， while Pb isotopic compositions are radiogenic， with (²⁰⁶Pb/²⁰⁴Pb)_i = 18.448-18.691， (²⁰⁷Pb/²⁰⁴Pb)_i = 15.647-15.659， and (²⁰⁸Pb/²⁰⁴Pb)_i = 38.538-38.724. Zircons from the granodiorite exhibit slightly depleted Hf isotopic signatures， with ε_Hf（t） values from 0.44 to 4.50 and two-stage Hf model ages from 761 to 968 Ma. These data suggest that the Sukadana granodiorite originated from the partial melting of Neoproterozoic mafic crust with minor mantle input， and that the magma source was metasomatized by slab- or sediment-derived fluids from the subducted Tethyan plate. Sulfides are enriched in heavy sulfur isotopes (δ³⁴S_CDT = 3.13‰-8.00‰， average 6.04‰)， suggesting a magmatic source from the Sukadana granodiorite. The Pb， Zn， and Ag in the Ruwai deposit are primarily derived from the Sukadana granodiorite， while the Fe originates from various sources， including the Sukadana granodiorite， Kuayan Formation， or Ketapang Complex within the Ruwai deposit.