CONSTRAINTS ON MAGMA DEGASSING BENEATH THE FAR SOUTHEAST PORPHYRY CU-AU DEPOSIT, PHILIPPINES

K-silicate alteration in the Far Southeast (FSE) porphyry Cu-Au deposi t formed at similar to 1.4 Ma, concentric to dikes of quartz diorite p orphyry. At similar to 2 km paleodepth the hydrothermal system consist ed of magmatic hypersaline liquid and vapor at 550 degrees C and less than or equal to 50 Mpa (lithostatic pressure). Advanced argillic alte ration 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 th an or equal to 1.3 Ma, K-silicate alteration was overprinted by 350 de grees C magmatic liquid (similar to 5 wt % of NaCl equiv.) at hydrosta tic pressure. Sericite alteration and much of the Cu-Au mineralization formed at this later stage. Evolution of the magmatic fluid compositi on was simulated with a magma-chamber crystallization model. Homogeneo us crystallization during early stage convection is assumed, whereas a t 50 vol. % crystals the chamber becomes stagnant and crystallizes fro m rim to core over a narrow crystallization interval. The model calcul ation, 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 compo sition 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 compos itions of the early and late magmatic fluids. The most critical factor controlling the compositional evolution of the model hydrothermal sys tem is the transition from convective to stagnant magma-chamber crysta llization. There is also a sharp decrease in the rate of fluid exsolut ion associated with this transition, which can account for the thermal collapse of the FSE porphyry system, from K-silicate to sericite alte ration.