Title: Hydrothermal fluids phase separation triggers base-metal sulfide mineralizations in an epithermal system: insights from the Central Seruyan Pb-Zn-Cu deposit, Kalimantan, Indonesia

[Abstract]
The Central Seruyan Pb-Zn-Cu deposit, located within the Central Borneo metallogenic belt, represents an epithermal system where hydrothermal phase separation played a critical role in mineralization. To establish a genetic model and guide future exploration, we integrated petrography, fluid inclusion microthermometry, trace element geochemistry of sulfide minerals, and Pb isotope analyses. Mineralization is divided into four hydrothermal stages, with the primary base metal sulfide deposition occurring during the main ore stage. This stage, characterized by the precipitation of galena, sphalerite, and chalcopyrite, along with minor covellite and tetrahedrite-tennantite, occurred under temperatures of 170–320 ℃ at an estimated depth of 0.1–0.4 km from the paleosurface.
Petrographic and fluid inclusion analyses indicate that hydrothermal phase separation— driven by fluid boiling and decompression-induced cooling—was the dominant mechanism controlling base metal sulfide precipitation. The heterogeneous trace element distribution in pyrite and distinct color zonation in sphalerite suggest rapid thermal fluctuations, reflecting abrupt physicochemical changes during mineralization. Zoned arsenic and gold in pyrite, along with indium and gallium variations in sphalerite, provide additional evidence for episodic boiling. Microanalytical data from fluid inclusions in sphalerite indicate that ore-forming fluids originated from magmas undergoing batholith-scale fractionation. Future exploration should focus on SW-NE trending faults, which likely acted as conduits for fluid ascent, facilitating decompression, phase separation, and subsequent ore deposition.