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

Abstract

The Acupan epithermal gold deposit is one of the Philippines’ largest gold camps, having produced over 200 t Au in the last century with average grades of ∼ 6 g/t Au mostly from well-studied vein orebodies hosted by the Virac Granodiorite. However, vein and breccia mineralization (90/99 veins and GW orebodies, respectively) currently being mined in the northeastern portion of the Acupan deposit is hosted by the less-studied Balatoc Diatreme. This study demonstrates the role of the diatreme as a precursor to epithermal mineralization at Acupan, providing structural control in focusing the fluid flow along the diatreme margin. Mineralization in the Balatoc Diatreme formed across five mineralization stages. Stage I, the main gold mineralization stage, is characterized by gray quartz with pyrite + marcasite + arsenopyrite + electrum + sphalerite ± chalcopyrite. Stage II is typified by white quartz associated with pyrite + electrum + chalcopyrite. Stage III, a newly recognized stage for the Acupan gold deposit, is composed of clear quartz hosting pyrite + stibnite + chalcopyrite + galena + sphalerite + electrum (±pyrite + marcasite + arsenopyrite). Stage IV and Stage V are associated with calcite and gypsum, respectively, hosting trace amounts of pyrite and sphalerite. Fluid inclusions hosted by Stage I and Stage II vein quartz revealed homogenization temperatures (T h) ranging from 220 to 230 °C and 280 to 290 °C, respectively. The fluid inclusions in Stage III quartz breccia cement recorded bimodal homogenization temperatures, 230 to 240 °C and 280 to 290 °C. Fluid inclusion and textural evidence from Stage I quartz and Stage II quartz suggest boiling conditions during ore formation of veins transecting the Balatoc Diatreme. Fluid mixing, on the other hand, is proposed for the formation of the base-metal rich Stage III mineralization of the GW orebodies quartz cement. Pyrite and sphalerite δ 34S values (0.8 to 1.5 ‰) from Stage I to Stage IV veins and breccias indicate reduced ore-forming conditions in an H 2S-dominated system. Meanwhile, the negative δ 34S values (−1.6 to −1.5‰) measured from the pyrite of Stage V breccia, first reported in this study, imply partitioning of the heavier isotopes to the gypsum sulfate in an oxidizing environment and suggest possible spatial variations of sulfur isotope signatures across the Acupan epithermal vein system. This study emphasizes the significance of fluid mixing in diatreme-hosted epithermal deposits such as Acupan. The diatreme possibly provided pathways between the contrasting magmatic-hydrothermal and meteoric environments, resulting in ore precipitation.

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ORCID for Acer Jian Figueroa: orcid.org/0000-0001-9359-9160

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