Super Porphyry Copper & Gold Deposits - A Global Perspective

Copper-molybdenum and copper-gold porphyry deposits in the Quesnel terrane occur in association with either calc-alkalic or alkalic intrusive suites respectively emplaced within a succession of island arc volcanic rocks that are of Late Triassic to Middle Jurassic or Late Cretaceous to Eocene age. The Highland Valley porphyry district, in southern British Columbia, consists of five major copper-molybdenum deposits, Valley, Lornex, Bethlehem, Highmont and JA, located within a fifteen square kilometre area in the centre of the Guichon Creek batholith. The batholith is a Late Triassic calc-alkalic intrusion that REE data suggest was derived from either subducted oceanic crust or depleted mantle. The crystallisation of the batholith, based on the U-Pb zircon analyses, is 210 Ma. Mineralisation occurred in late magmatic and early post magmatic time. Oxide analyses suggest a single source magma but younger phases were locally injected into older due to tectonic forces. The earliest deposits occurred after separation of a fluid phase that is marked by a sharp discontinuity in the evolution path of the alkali oxides.
The batholith is elongated northward and segmented by major northerly and northwest-striking faults that are intimately related to mineralisation. Most of the sulphide mineralisation is in fractures, veins, faults or breccia bodies, and all the deposits are hosted entirely within the rocks of the granodiorite batholith. In general, early potassic and propylitic alteration are overprinted by later phyllic and argillic alteration. In more detail, complications in cross-cutting alteration and vein relationships suggest local influxes of hotter aqueous solutions. Phyllic alteration is characterised by formation of both microscopic sericite and medium grained 'flaky sericite' (muscovite). Principal hypogene metallic minerals in the deposits are bornite, chalcopyrite, molybdenite and pyrite. Today only the Lornex and Valley deposits remain in production. For these two deposits, the total ore milled to the end of 2002 has exceeded a billion tonnes. As of December 31, 2002, remaining reserves in the Lornex and Valley mine deposits are 296 million tonnes grading 0.42% copper and 0.008% molybdenum.
The area is veneered by glacial deposits. Induced polarisation surveys have proven to be the most useful exploration tool for Highland Valley deposits, but silt, soil and lithogeochemical surveys, and alteration mapping can be effective. A regional lithogeochemical study showed generally high copper values (120 to 100 ppm) in the older rocks of the batholith and less than 50 ppm in the younger rocks. A zone in which copper abundance is less that 10 ppm lies several kilometres south of the large, younger deposits offers a potential source for much of the copper in them.