The Journal of Petrology, Volume 38, Issue 12: December 1997.

Constraints on magma degassing beneath the Far Southeast porphyry Cu-Au deposit, Philippines

H Shinohara^1,* and JW Hedenquist^1,2

¹Mineral and Fuel Resources Department, Geological Survey of Japan, 1-1-3 Higashi, Tsukuba 305, Japan, ²Institute of Geological and Nuclear Sciences, PO Box 31-312, Lower Hutt, New Zealand, ^*Corresponding author

K-silicate alteration in the Far Southeast (FSE) porphyry Cu-Au deposit formed at 1.4 Ma, concentric to dikes of quartz diorite porphyry. At 2 km paleodepth the hydrothermal system consisted of magmatic hypersaline liquid and vapor at 550°C and [le]50 MPa (lithostatic pressure). Advanced argillic alteration formed at the same time over the deposit at [le]1 km paleodepth from acidic condensates of the vapor. At [ge] 1.3 Ma, K-silicate alteration was overprinted by 350°C magmatic liquid ( 5 wt % NaCl equiv.) at hydrostatic pressure. Sericite alteration and much 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 [ge]800°C (saturated melt composition at 30 vol. % crystals, 5 wt % H₂O, 0.2 wt % Cl and [dgr]D of -40[permil]), 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.

Key words: magma; degassing; evolution; porphyry-copper; isotopes

Pages 1741-1752