Summary:
Mount Polley is an alkalic porphyry copper-gold deposit. It lies in the tectono-stratigraphic Quesnel terrane or Quesnellia, which is characterized by a Middle Triassic to Early Jurassic assemblage of volcanic, sedimentary and plutonic rocks which formed in an island arc tectonic setting outboard of the ancestral North American continental margin. Quesnellia hosts several major porphyry copper deposits such as Highland Valley, Copper Mountain, AftonAjax, Gibraltar and Mount Milligan, all generated by early Mesozoic, calc-alkalic or alkalic arc magmatism.
In the Mount Polley region, the Triassic arc rocks are assigned to the Nicola Group and comprise alkalic basaltic to andesitic volcanics and sedimentary rocks, which are intruded by sub-volcanic stocks; all are overlain by post-Nicola, Early Jurassic clastic rocks and rare volcanics. Mount Polley itself is a complex of alkalic intermediate porphyritic intrusions and related magmatic-hydrothermal breccias. It was emplaced into the Nicola Group in the Late Triassic around 205 million years ago. The intrusive complex is about 6 km long (north-northwest) and 3 km wide, lying between Polley Lake in the east and Bootjack Lake in the west. The intrusions range from diorite (oldest) to monzonite (youngest) and are marginally undersaturated in silica. The Mount Polley Intrusive Complex is in the centre of the Mount Polley property; the remainder of the property is underlain mainly by Nicola Group volcanics and post-Nicola conglomerate, and small intrusions in which no economic mineralization has been found to date.
Mineralization in the Mount Polley Intrusive Complex (“MPIC”) is primarily hosted by irregular zones of hydrothermal breccia, which are closely related to the porphyry intrusions and were formed by magmatic devolatization processes. Mineralization and brecciation were accompanied by potassic or calc-potassic, albite, and magnetite alteration; the MPIC is bounded on most sides by propylitic country rocks. As in many alkalic porphyry systems, there is no single or simple zoned mineralization pattern, but instead a number of copper-gold zones of various size, shape and grade characteristics, distributed around the MPIC from the far north to the south. There is no clear structural control on the location of these mineralized breccia zones, although the greatest continuity and the bulk of the past and present resources occur in the centre of the MPIC (e.g. Springer, Cariboo, Bell zones) between two pre-mineral diorite intrusions. Dimensions of mineralized breccias in the MPIC range up to many hundred metres in length and width, such as in the Springer zone. Elsewhere, smaller zones (generally less than 100 m across) may form mineable bodies if grades and other factors are favourable. Post-mineral faulting probably did not disrupt the continuity of mineralized zones very significantly, except in the Northeast zone where deeper mineralization was offset along a fault a few hundred metres laterally and dropped vertically slightly.
In the deposits, the degree of brecciation and associated hydrothermal alteration is usually a reliable guide as to grade. There is relatively little post-mineralization dike dilution. Chalcopyrite is the dominant copper mineral, typically accompanied by pyrite; bornite is relatively uncommon in the centre of the MPIC. Here, copper sulfides occur as disseminations or veins and fracture coatings in brecciated intrusion, or they are disseminated in the matrix of breccias, in both cases precipitated along with alteration minerals. Mineralization has been traced by deep drilling in the Springer zone to a depth of around 900 m (from pre-mining surface).
In the north of the MPIC are much higher grade orebodies, namely the Northeast (mined in the Wight pit, 2005- 2009) and Boundary zones, where copper grades can reach several percent per tonne. Chalcopyrite and significant bornite form coarse-grained infill in breccias, and intense vein and microvein stockworks. As in the zones in the centre of the MPIC, gold and silver occur mainly as microscopic inclusions in the copper sulfides and in pyrite.
Exploration has always proceeded alongside mining at Mount Polley, leading to the expansion and deepening of known deposits, or to the discovery of new zones, or raising the status or resource category of marginal prospects, potentially towards feasibility for profitable mining. Geological and geotechnical logging of drill core is integrated with down-hole assay data and used with software for computation of the resource block model and mine design. In addition, exploration and academic research since the restart of operations in 2004-2005 have considerably advanced understanding of geology, structure and deposit genesis at Mount Polley, improving interpretation of mineralization geometry and the design of drill programs. New underground development is followed where appropriate by wall mapping and rib sampling to further characterize the mineralization, fill gaps in the resource model, and help guide stope design. Airborne and ground magnetic signature are regarded as the most important geophysical tools for identifying new mineralization, preferably integrated with induced polarization data.