BUBBLING MAGMA CHAMBERS AND PORPHYRY COPPER DEPOSITS

Porphyry copper deposits are the major source of copper and significant sources of molybdenum, gold, and other metals. They are associated with the near-surface intrusion of small stocks of intermediate composition. They can form when H2O-unsaturated magma is emplaced into wall rock that is cool enough that steep lateral thermal gradients create a narrow solidification front. At depths less than about 4 km, cooling and crystallization cause fluid saturation to occur within sidewall magma that is mobile because it contains less than about 25% suspended crystals. After a sufficient volume of bubbles forms, mobile sidewall magma buoyantly rises instead of sinking. The bubbles expand as they decompress and at depths of about 2 km, they become large enough to rise on their own, separate from the upwelled magma, and charge the cupola at the top of the stock with magmatic fluid. The partially degassed magma sinks into the interior of the stock.

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.