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Evolution of the Northeast zone breccia body, Mount Polley mine, British Columbia Jackson, Meghan Lea

Abstract

The Mount Polley deposit in north-central British Columbia is a Cu-Au porphyry system related to alkaline magmatism. The deposit is composed of numerous discrete zones of Cu-Au mineralization related to bodies of hydrothermal breccia, including the Northeast zone which hosted a pre-mining proven and probable reserve of approximately 11 Mt of ore averaging 0.88% Cu and 0.28 g/t Au. The polylithic breccia body at the Northeast zone is irregular in shape and intruded by multiple generations of post-mineral dykes. Post-mineral faults cut the breccia, making the original breccia geometry unclear. 11,000 meters of core logging along two vertical sections perpendicular to the long axis of the orebody show that a significant percentage (locally >50%) of the breccia is composed of K-feldspar-phyric monzonite porphyry clasts. Many of these have globular shapes, implying that this material was ductile at the time of brecciation. Cu-sulfides are most abundant in and immediately above zones containing the greatest concentration of K feldspar-phyric monzonite porphyry clasts. This relationship suggests that the fluids responsible for both mineralization and brecciation originated during this phase of intrusion. Most of the Cu-Au-bearing sulfide minerals form hydrothermal chemical infill in the breccia. Where fine-grained clastic material is abundant or where movement of clasts was minimal, sulfides are generally less abundant or absent. The entire breccia body of the Northeast Zone at Mount Polley appears to be the result of a single brecciation event. Variation in breccia character within the body is attributable to variations in fluid flux, pre-existing rock character, and fluidization processes. Furthermore, the permeability structure established during the brecciation event exerted the fundamental control over ore distribution within the breccia body.

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