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UBC Theses and Dissertations

Metal- and alteration-zoning, and hydrothermal flow paths at the moderately-tilted, silica-saturated Mt. Milligan copper-gold alkalic porphyry deposit Jago, Christopher Paul

Abstract

The Mt. Milligan deposit is a tilted (~45°) Cu-Au alkalic porphyry located 155 km northwest of Prince George, B.C., Canada. It is the youngest of the BC alkalic porphyry deposits, all of which formed between 210 to 180 Ma in an extensive belt of K-enriched rocks related to the accretion of the Quesnellia-Stikinia superterrane to ancestral North America. Mt. Milligan has a measured and indicated resource of 205.9 million tonnes at 0.60 g/t Au and 0.25% Cu containing 3.7 million oz. gold, and 1.12 billion lb. copper. Shoshonitic volcanic and volcaniclastic andesites host mineralization. These have been intruded by a composite monzonitic stock (MBX stock), and associated sill (Rainbow Dike). Early disseminated chalcopyrite-magnetite and accessory quartz veins are associated with K-feldspar alteration in the MBX stock. A halo of biotite alteration with less extensive magnetite replaces host rocks within a ~150 m zone surrounding the stock, while K-feldpsar alteration extends along the Rainbow Dike and permeable epiclastic horizons. Peripheral albite-actinolite-epidote assemblages surround the K-silicate zone. Albite-actinolite occurs at depth, and epidote dominates laterally. Copper and Au grade are maximal where the albite-actinolite assemblage overprints biotite alteration. Gold grade is moderate in association with epidote, whereas Cu is depleted. The post-mineral Rainbow Fault separates the core Cu-rich zone from a downthrown Au-rich zone. A similar zonation of metals occurs in the hanging-wall (66 zone), where a Cu-bearing, potassically-altered trachytic horizon transitions to a funnel-shaped zone of pyrite-dolomite-sericite-chlorite alteration with elevated gold. Sulfide S-isotope compositions range from -4.79 δ34S in the central Cu-Au orebody to near-zero values at the system periphery, typical of alkalic porphyries. Sulfur isotope contours reflect the magmatic-hydrothermal fluid evolution, and indicate late-stage ingress of peripheral fluids into the Cu-Au zone. Carbonate C- and O-isotope compositions corroborate the magmatic fluid path from the Cu-Au rich zone to Au-rich zone with decreasing depth. Strontium isotopic compositions of peripheral alteration minerals indicate a laterally increasing meteoric fluid component. Changes in major- and trace element composition of epidote and pyrite across the deposit are also systematic. These provide additional vectors to ore, and confirm the kinematics of the Rainbow Fault.

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