THE CHARACTERIZATION OF HOST ROCKS AND THE ASSOCIATED EFFECTS OF ALTERATION AND MINERALIZATION AT COPPER MOUNTAIN PORPHYRY CU-(AU-AG) DEPOSIT, SOUTHERN BC, CANADA

The Copper Mountain porphyry Cu-(Au-Ag) deposit (reserves of 233Mt at 0.36% Cu) is temporally and spatially associated with several Late Triassic-Early Jurassic alkalic intrusions emplaced into the basaltic-andesitic, largely fragmental and brecciated, Nicola Group volcanic rocks. Intrusive rocks include the zoned syenite to diorite Copper Mountain Stock, and the largely diorite to monzonite Lost Horse Intrusive Complex (LHIC), which displays a predominant dike and plug geometry. Post-ore units comprise Cretaceous felsite dykes and Tertiary volcanic rocks and sediments of the Princeton Group.

Copper-(Au-Ag) mineralization is predominantly associated with small, but multiple intrusive units of the LHIC emplaced at different depths. Mineralization consists of sulfide vein-stockworks, and adjacent disseminated sulphides, with lesser matrix filling within hydrothermal breccias. Sulphide mineralogy includes chalcopyrite, bornite and hypogene chalcocite with pyrite and magnetite gangue. Mineralization is associated with sodic, largely albite, and potassic, K-feldspar – biotite, alteration. Both sodic and potassic alteration range from fractures to large zones of intense pervasive replacement. Potassic alteration typically overprints sodic alteration and has a closer temporal and spatial relationship to ore (Stanley et al., 1995; Holbek and Joyes, 2013). However, the relationship of alteration stages and mineralization to the various intrusive units is not well established.

This research project aims to characterize host rocks at Copper Mountain and assess their control on alteration, Cu, Au and Ag grade distribution and rock hardness. Fieldwork is focused along three NE-trending cross-sections across the super-pit, with detailed core logging and pit mapping. Preliminary field observations suggest the existence of several successive phases of Lost Horse intrusive unit, with variable alteration assemblages and intensities. Planned laboratory work includes petrography, whole rock geochemistry and U-Pb geochronology. Final results are expected to provide a basis for better understanding of the intrusive evolution and associated mineralization at Copper Mountain and provide a framework for magmatic complexity of similar known porphyry camps in BC.