Abstract

The emerald deposits of Muzo and Coscuez hosted within the western flank of the Colombian Eastern Cordillera formed during thrust tectonics affecting series of lower Cretaceous black shales. An unusual polygenetic breccia fills opened fractures and dilatant sites within thrusts and tear faults. Detailed field and petrographic studies show that the breccia formed during a fluidization process in which a hydrothermal pulp was injected into dilatant sites. Fluidization is synchronous with fault propagation. Multistage brecciation is observed and appears to correspond to successive faulting-fluid flow pulses. The opening of dilatant sites during compressive tectonics and the combination of fluidization and hydraulic fracturing suggests that these pulses may be related to successive build-up and drop of the fluid pressure.