Geology, alteration and origin of hydrothermal breccias at the Mount Polley alkalic porphyry copper-gold deposit, south-central British Columbia
Creators: name: Fraser, Theresa M.
Publisher: name: University of British Columbia
Year: 2009

description: Mount Polley is an alkalic porphyry copper-gold deposit of Lower Jurassic age within the Quesnel terrane in south-central British Columbia. The Mount Polley intrusive complex is assumed to be coeval with the regional Nicola Group volcanic rocks in which it is emplaced. Some volcanic rocks are silica undersaturated, contain feldspathoids and are chemically similar to the plutonic rocks. The Nicola Group rocks have trace and rare earth signatures of a volcanic island arc environment. The Mount Polley deposit is characterized by multiple intrusions that compositionally vary from diorite to crowded plagioclase porphyry to monzonite. Minor intrusion and abundant hydrothermal breccias are emplaced in a northerly trending diorite host. Hydrothermal breccias are the main host to mineralization and are associated with the highest concentrations of copper and gold. Hydrothermal breccias are subdivided into four distinct types based on the dominant hydrothermal mineral in the matrix. Actinolite breccia is developed in an elongate zone within the core of the system; it is parallel and lies east of a northnorthwest trending structure, the Polley fault. Actinolite breccia grades laterally and vertically into biotite breccia in the southeastern part of the deposit. Magnetite breccia is irregularly distributed and is relatively sparse. West of the Polley fault, albite breccia is dominant. Pervasive and vein-related alteration correlates with the breccia types. A zonal distribution of alteration minerals has been mapped. The core of the hydrothermal system at Mount Polley is subdivided into three zones: actinolite, biotite and potassium feldspar-albite. The actinolite zone is typified by development of actinolite-pyroxene-magnetite-sulfide veins with potassium feldspar envelopes. A biotite alteration zone is characterized by the formation of secondary, coarse grained biotite within open spaces of the hydrothermal breccias. Arcuate around these two zones is a region of pervasive potassium feldspar and locally intense albitic alteration, generally spatially related to hydrothermal breccias. The margins of the potassic zone are overprinted by a discontinuous zone of calc-silicate minerals. A complicated assemblage of garnet, epidote, albite, potassium feldspar, chlorite, magnetite and sulfides is present. This intermediate zone passes outwards into propylitic alteration. Mineralization is most prominent within hydrothermal breccias and is generally present as disseminations, blebs within the matrix and in abundant veins. Metals are outwardly zoned from a core of chalcopyritemagnetite- bornite to magnetite-pyrite-chalcopyrite. Constant copper-gold ratios indicate that chalcopyrite and gold are probably co-precipitating from the same fluids; the pyrite-dominated assemblage forming at lower temperature. Hydrothermal breccias are genetically related to the emplacement of the crowded plagioclase porphyry melt. Crystallization of the melt was probably accompanied by volatile/aqueous exsolution, foiming a water saturated carapace. Decompression of the chamber in response to magma withdrawal, fracture propagation and possible fault movement may have allowed hydrothermal brecciation to occur at the apex and margins of the intrusion. Alteration and mineralization appears to be controlled by fluids migrating away from the plagioclase porphyry.