Mineral chemistry, fluid inclusion and stable isotope studies of the Suyoc epithermal veins: Insights to Au-Cu mineralization in southern Mankayan Mineral District, Philippines

The Suyoc prospect is an epithermal vein-type mineralization located at the southern part of the Mankayan Mineral District, Northern Luzon, Philippines. The prospect's epithermal veins are hosted in volcaniclastic rocks and conglomerate, which belong to the Late Oligocene to Early Miocene Balili Formation and the Middle to Late Miocene Suyoc Conglomerate, respectively. These veins are classified into three major types: quartz vein, quartz-sulfide vein, and quartz-carbonate vein. The quartz vein exhibits massive and cockade textures associated with pyrite + chalcopyrite. The quartz-sulfide vein consists of massive and comb quartz associated with pyrite + chalcopyrite + sphalerite. The quartz-carbonate vein has massive quartz associated with rhodochrosite and chalcopyrite + sphalerite + galena + Au/Ag ± bournonite. In addition, massive gypsum ± pyrite occurs as a minor vein-type. Fluid inclusion measurements in quartz from the three major vein-types revealed 230–250 °C formation temperatures and 1.0–3.5 wt. % NaCl equivalent salinity, which are values consistent with epithermal deposits. Quartz textures (e.g., massive, cockade, comb, crustiform) and dominance of liquid-rich fluid inclusions do not suggest that boiling is a primary mechanism of vein deposition. Possible mechanisms are inferred as meteoric water dilution based on homogenization versus salinity trend in quartz vein and presence of rhodochrosite in quartz-carbonate vein, and wall rock interaction based on FeS trend in sphalerite crystals of the quartz-carbonate vein. The presence of pyrite and chalcopyrite in the major vein-types and the FeS mole percent values (0.51 to 8.30) in quartz-carbonate vein indicate an intermediate sulfidation state. The alteration minerals illite, chlorite, pyrite and quartz indicate near-neutral pH condition. δ³⁴S values, which vary from −1.1 to +3.9‰, suggest reduced condition when compared to the representative bulk δ³⁴S value for the Mankayan Mineral District. The characteristics and conditions strongly indicate an intermediate sulfidation epithermal (ISE) style of mineralization. Moreover, the δD_water (−63 to −66‰) and δ¹⁸O_water (−3.9 to −1.1‰) values from the major vein-types suggest late stage formation in a magmatic-hydrothermal system similar to other ISE deposits in the Northern Luzon Segment (e.g., Victoria, Acupan and Antamok). Given these, the presence of the Suyoc ISE veins in the underexplored southern part of the Mankayan Mineral District implies potential presence of undiscovered cogenetic porphyry Cu and high sulfidation epithermal mineralization in the area.