Constraints on Magma Degassing beneath the Far Southeast Porphyry Cu-Au Deposit, Philippines

Abstract

K-silicate alteration in the Far Southeast (FSE) porphyry Cu-Au deposit formed at similar to 1.4 Ma, concentric to dikes of quartz diorite porphyry. At similar to 2 km paleodepth the hydrothermal system consisted of magmatic hypersaline liquid and vapor at 550 degrees C and less than or equal to 50 Mpa (lithostatic pressure). Advanced argillic alteration formed at the same time over the deposit at less than or equal to a km paleodepth from acidic condensates of the vapor. At greater than or equal to 1.3 Ma, K-silicate alteration was overprinted by 350 degrees C magmatic liquid (similar to 5 wt % of NaCl equiv.) at hydrostatic pressure. Sericite alteration and much of the Cu-Au mineralization formed at this later stage. Evolution of the magmatic fluid composition was simulated with a magma-chamber crystallization model. Homogeneous crystallization during early stage convection is assumed, whereas at 50 vol. % crystals the chamber becomes stagnant and crystallizes from rim to core over a narrow crystallization interval. The model calculation, based on a magma chamber with 2 km thickness at 6 km depth (150 MPa) and greater than or equal to 800 degrees C (saturated melt composition at 30 vol. % crystals, 5 wt % H2O, 0.2 wt % Cl and delta D of -40 parts per thousand), can reproduce the chemical and isotopic compositions of the early and late magmatic fluids. The most critical factor controlling the compositional evolution of the model hydrothermal system is the transition from convective to stagnant magma-chamber crystallization. There is also a sharp decrease in the rate of fluid exsolution associated with this transition, which can account for the thermal collapse of the FSE porphyry system, from K-silicate to sericite alteration.