Morrison - Hearne Hill copper-gold deposits Babine region, west-central British Columbia

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

Morrison and Hearne Hill are classic porphyry copper-gold deposits associated with a Tertiary continental magmatic arc in westcentral British Columbia. The deposits are located within and around dikes and plugs of an Eocene high-potassium calc-alkaline suite, the Babine Igneous Suite. Despite the calc-alkaline major element geochemistry, rocks of the Babine Igneous Suite possess immobile trace element ratios typical of alkaline rocks and may have been derived from an alkaline precursor or parental magma. Sulphide mineralization in both deposits occurs as disseminations, fracture fillings and quartz stockwork veinlets. Hearne Hill also hosts a deposit of chalcopyrite-cemented breccia which has characteristics that are unique among the Babine deposits. Stockwork mineralization on Hearne Hill formed at a minimum depth of 4 ± 1 km from highly saline hydrothermal brines. Temperatures of homogenization in complex fluid inclusions range from 164.5°C to >60fJOC, with salinities of 40% to 60%. The Hearne Hill breccia pipe formed at a minimum depth of < 100 m from dilute epithermal fluids. Temperatures of homogenization average 172.5°C, with salinities of 2% to 10%. The Morrison and Hearne Hill deposits are separated by a major regional structure, the Morrison Fault. Reconstruction of the Mesozoic-Eocene stratigraphic sequence suggests that Hearne Hill formed the root and that Morrison formed the upper slice of an originally contiguous deposit which was subsequently dismembered by extensional faulting and dextral shear. Faulting occurred before the cessation of magmatic and hydrothermal activity on the exposed root of the deposit. The circulation of highly corrosive hydrothermal brines resulted in solution, withdrawal of support and subsequent collapse to form the Hearne Hill breccia pipe. Indicated and inferred resources for the Morrison deposit, using a 0.30% Cu cutoff grade, are estimated to total190 million tonnes grading 0.40% Cu and 0.2 g!t Au to a depth of 300 m. Inferred resources for the Hearne Hill stockwork and breccia pipe mineralization, using a 0.10% Cu cutoff grade, are estimated at 60 million tonnes grading 0.16% Cu and 0.1 g!t Au to a depth of 100 m. Using a cutoff grade of 0. 75% Cu, the Hearne Hill breccia pipe contains an indicated resource of 143 000 tonnes grading 1. 73% Cu and 0.8 g/t Au to a depth of 100 m.

Mots Clés: Porphyry copper-gold deposits, Magmatic arc, Hydrothermal brines, Morrison fault