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.