PORPHYRY COPPER DEPOSITS: STRIKE-SLIP FAULTING AND THROTTLING CUPOLAS

Porphyry copper deposits sometimes form during the solidification of stocks of intermediate composition magma. Strike-slip movements can generate pull-apart pathways into which intrusions rise from batholithic magma chambers to form stocks. Upwelling of buoyant, bubble-bearing magma along the sides of the chamber brings it to shallow depths where large bubbles separate and pool under the cupola separating solidified igneous rock from magma. Where rapid seismogenic movement on a bounding strike-slip fault extend the pull-apart containing the solidifying intrusion, there is a different mechanical response near the surface and at depth near the cupola. The differences are due to the local state of stress and ambient fluid pressure conditions. Faults form near the surface where the rock is relatively cool and strong. Near the surface, pore fluid pressures are at, or near, hydrostatic values. Extension fractures form at depth where the rock is solidified, but so hot and weak that differential stress is small. Pore fluid pressures near a fluid-charged cupola approach lithostatic values. Downward propagating extension fractures can tap and drain accumulations of magmatic fluid. Decompression and cooling of fluid that jets upward into the fractures causes mineral precipitation. Where strike-slip fault offsets cause pull-apart movements to occur with sufficient recurrences (decades to perhaps a century or so), the episodic draining of a fluid accumulation acts as a safety valve. This "throttling of the cupola" prevents the buildup of fluid to the point that the system detonates as an explosive volcanic eruption. Over time, the steady upward and outward infiltration of magmatic fluid away from the cupola generates the characteristic alteration zones. Porphyry copper ore deposits 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. Giant deposits form where the bubbling front extends into the top of a parent batholith.