Epithermal Au-Ag-Te mineralization, Acupan, Baguio district, Philippines: Numerical simulations of mineral deposition

In this study we discuss the effects of cooling, boiling, fluid mixing, and water-rock interaction on a low-sulfidation chloride water. Our water comp osition is derived from fluid inclusion and mineralogical studies of the Ac upan gold mine, a large gold-silver-tellurium-bearing low-sulfidation epith ermal deposit in the Philippines. Our numerical modeling results show that a single mineralizing water (300 degreesC, 0.5 wt % NaCl + KCl, 0.41 m Co-2 ) will evolve along different reaction pathways in response to different ph ysicochemical processes, and that these pathways are difficult to predict i ntuitively in many cases. Acidity and redox can evolve dramatically and in different directions, with boiling resulting in oxidation and pH increase, cooling resulting in pH decrease at a relatively constant sulfate/sulfide r atio, and mixing with sulfate-bearing ground waters causing oxidation and a cidification. Based on the correlation of predicted and observed ore and gangue minerals, boiling is concluded to have resulted in the deposition of most of the pre cious and base metals at Acupan. Continuous boiling, boiling with intermitt ent gas loss, and throttling probably all occurred at various times during the evolution of the hydrothermal system. The loss of gases during boiling (e.g., H2S, H2Te, Te-2) enhanced electrum and base metal sulfide deposition and inhibited the precipitation of hessite and calaverite. Mixing of low-t emperature ground waters with the high-temperature chloride water resulted in mineral assemblages that are similar to those observed in shallow levels of the mine and in deep-level, late-stage barren vein fill. Mixing with gr ound water could account for the observed transition from adularia-carbonat e vein assemblages in deep mine levels to sericite-bearing assemblages in s hallow levels. Late-stage anhydrite could have formed via mixing with or he ating of near-surface ground waters. We predict tellurium to be transported preferentially in a gas phase. Becau se tellurium solubilities are predieted to be low in auriferous chloride wa ters, telluride and native tellurium deposition in low sulfidation environm ents may result from condensation of magmatically derived H2Te(g) and Te-2( g) into deep-level chloride waters. The minor amount of tellurium that diss olves into chloride waters will be deposited effectively by cooling or flui d mixing. Aqueous tellurium will partition strongly into the gas phase in b oiling low-sulfidation systems and could precipitate via condensation into lower temperature ground waters. This could lead to vertical zonation of el ectrum and tellurium-bearing minerals, which may be of significance to mine ral exploration.