PORPHYRY COPPER OREBODY FORMATION: STRIKE-SLIP FAULTING AND THROTTLING CUPOLAS

Porphyry copper deposits sometimes form during the solidification of stocks of relatively oxidized magma of intermediate composition. Why many plutons with similar chemistry are barren is a question of great interest. A simple answer is that while the necessary magma chemistry is common, the structural factors (plumbing) and cooling rate necessary for ore formation are not. Strike-slip movements can generate pull-apart pathways into which magma intrudes to form stocks. When bubble-bearing magma buoyantly rises along the sides of cooling stocks, a fluid-charged cupola can form when bubbles separate and pool between mobile magma and solidified igneous rock. In the hot (>500°C), weak rock above a fluid-charged cupola, the differential stresses must be small and the fluid pressure must be near lithostatic values. When seismogenic movement occurs on the strike-slip faults that bound a pull-apart zone, this part of the cooling pluton dilates by extension fracturing. Openings that propagate downwards and reach the fluid-charged cupola become filled with magmatic fluid that jets upwards, decompresses, and precipitates vein-forming minerals. The critical tectonic factor controlling the extent of mineralization is that strike-slip movements that dilate pull-apart zones above cooling stocks must occur with sufficiently short recurrence (decades to a few centuries?) that accumulating fluid escapes before becoming a pocket large enough (100-200 m wide?) to buoyantly hydrofracture its way upwards. This tectonic "throttling" acts as a safety valve that prevents explosive detonation. The characteristic alteration zones are created by the prolonged upward and outwards infiltration of the magmatic fluids. A porphyry copper ore deposit can form where strike-slip movements are concurrent with the early stages of deep-seated bubbling (> ~6 km) along the walls of a rapidly cooling stock of magma. Super-giant deposits (e.g., Grasberg) form where the bubbling front extends into the top of a batholithic chamber.