Breccia and vein mineralization of the Balatoc Diatreme, Acupan gold deposit, Baguio Mineral District: An example of a diatreme-hosted epithermal deposit in the Philippines

The Acupan epithermal gold deposit is one of the Philippines’ largest camps, having produced over 200 t Au in last century with average grades ∼ 6 g/t mostly from well-studied vein orebodies hosted by Virac Granodiorite. However, and breccia mineralization (90/99 veins GW orebodies, respectively) currently being mined northeastern portion less-studied Balatoc Diatreme. This study demonstrates role diatreme as a precursor to at Acupan, providing structural control focusing fluid flow along margin. Mineralization Diatreme formed across five stages. Stage I, main stage, characterized gray quartz pyrite + marcasite arsenopyrite electrum sphalerite ± chalcopyrite. II typified white associated III, newly recognized stage for deposit, composed clear hosting stibnite chalcopyrite galena (±pyrite arsenopyrite). IV V are calcite gypsum, respectively, trace amounts sphalerite. Fluid inclusions I revealed homogenization temperatures (Th) ranging 220 230 °C 280 290 °C, respectively. III cement recorded bimodal temperatures, 240 °C. inclusion textural evidence suggest boiling conditions during ore formation transecting mixing, on other hand, proposed base-metal rich cement. Pyrite δ34S values (0.8 1.5 ‰) breccias indicate reduced ore-forming an H2S-dominated system. Meanwhile, negative (−1.6 −1.5‰) measured breccia, first reported this study, imply partitioning heavier isotopes gypsum sulfate oxidizing environment possible spatial variations sulfur isotope signatures emphasizes significance mixing diatreme-hosted deposits such Acupan. possibly provided pathways between contrasting magmatic-hydrothermal meteoric environments, resulting precipitation.