STRUCTURAL ANALYSIS OF THE MOUNT POLLEY PORPHYRY CU-AU DEPOSIT: RESTORATION OF POST-MINERAL DEFORMATION USING ORE BODY GEOMETRY

The Mount Polley porphyry copper deposit comprises three breccia-hosted ore zones associated with a series of Late Triassic monzodiorite to monzonite intrusions emplaced into the Nicola Group of the Quesnellia Terrane, located in Central British Columbia. There are few planar features in the intrusive complex that can be used as strain markers to evaluate the magnitude of structural deformation. The relative sequence of folding, faulting, and tilting events can be resolved, however, by integration of new structural mapping and use of the geometry of sulfide zones and alteration assemblages as strain markers. During a period of 8 million years, from 205 Ma to 197 Ma, the Mount Polley intrusions were emplaced and mineralized, exhumed to the surface, and subsequently buried beneath a sequence of conglomerates and sandstones, including a thin quartz latite ignimbrite horizon. Structural analysis of ore body geometry to characterize fault separation indicates that the oldest faults in the district are NNW-striking reverse faults, which displace intrusions, mineralized zones, and porphyry dikes. These faults parallel and are likely coeval with a northwest-plunging open synform formed as a result of regional shortening. The youngest faults in the district are the southwest-striking sinistral strike-slip and west-striking normal faults. A tilting event that postdates deposition of the clastic rocks and latite ignimbrite produced a 30-35° NW tilt of the sedimentary beds, ore bodies, and porphyry dikes. Two new apatite fission track ages indicate that the district was exhumed and cooled during the Paleocene to Eocene. Whereas extensional exhumation is known elsewhere in the Cordilleran hinterland in the Early Cenozoic, it is not clear what mechanism initiated the exhumation in the Mount Polley region.