Bell and Granisle porphyry copper-gold mines, Babine region, west-central British Columbia

Special Volume, Vol. SV 46, No. 1995, 1995

The Bell and Granisle open pit mines exploited two porphyry copper-gold deposits in the Babine Lake region of central British Columbia. The Bell mine produced 303 277 tonnes copper, 12 794 kg gold and 27 813 kg silver from 77.2 million tonnes of ore, while the Granisle mine produced 214 300 tonnes copper, 6833 kg gold and 69 753 kg silver from 52. 7 million tonnes of ore. The deposits are associated with intrusive rocks of a Tertiary continental magmatic arc known as the Babine Igneous Suite. This suite consists of the remnants of volcanic edifices constructed on rocks of the Stikine terrane during Eocene time. Tertiary extension and transtensional faulting resulted in the formation of a series of northwesterly trending grabens in the Babine Lake region. The most prominent of these structures is the Morrison Graben which is bounded on the east by the Morrison Fault and its offset, the Newman Fault. Dikes and plugs of intermediate to felsic calc-alkaline porphyritic intrusive rocks were emplaced along these faults with extrusive equivalents preserved in downdrop basins as flows, debris flows, hornblende crystal tuff and piles of poorly consolidated volcanic rubble. Volcanism was locally explosive, with coarse breccias plugging volcanic vents. The Bell deposit is a classic high-level porphyry copper-gold deposit with symmetrical zones of biotite-magnetite and propylitic alteration overprinted by pervasive quartz-sericite alteration. The principal sulphides are chalcopyrite and pyrite occurring as disseminations, fracture fillings and in an intensively developed quartz stockwork. Symmetry was subsequently disrupted by explosion and collapse, resulting in the partial destruction of the upper part of the southeastern quadrant of the deposit. The Granisle deposit also has well developed biotite magnetite propylitic alteration zoning, but less extensive development of a pervasive quartz sericite overprint. The principal sulphides are chalcopyrite, bornite and pyrite. The Granisle deposit appears to be exposed at a lower level than the Bell deposit and may represent the root zone of a porphyry system. Operating practices at Bell and Granisle are described with emphasis on their An extensive examination of the Bell deposit, beginning in 1988, identifred a potential open pit resource of 70.4 million tonnes grading 0.44% Cu and 0.20 gl t Au at a strip ratio of 1.9:1. Economic considerations prevented development of this resource at the time of the mine closure in 1992.

Mots Clés: Porphyry, Copper-gold deposits, Intrusive rocks, Babine Igneous Suite, Grabens, Biotite-magnetite, Propylitic alteration