BUBBLING MAGMA CHAMBERS AND PORPHYRY COPPER DEPOSITS
Upwelling of saturated sidewall magma entrains deeper-seated, near-saturated magma which decompresses and saturates as it rises. As the system cools, the depth of H2O saturation and sidewall upwelling increases. Bubbles of copper-rich fluid are generated where the saturation front extends to depths of about 6 km or more. Overall, the system is cooling, but the upwards advection of heat maintains the cupola region at roughly constant position for the life of convective upwelling along the sidewalls.
Porphyry copper ore deposits can form where draining of the fluid pocket beneath a cupola is steady and a large volume of magma is cycled through the system. Magma in the stock that escapes to intrude commonly has a porphyritic texture because crystal growth is enhanced, and nucleation is suppressed when the magma is H2O-saturated. Porphyry copper deposits of common size can form during the solidification of large stocks. Super-giant porphyry copper deposits can form where the saturation front propagates from a stock into an underlying batholithic chamber with a magma volume on the order of 1000 km3 and a top at depths of 10 to 15 km.