Kemess South and Kemess North porphyry gold-copper deposits, northern British Columbia

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

Exploration on the Kemess property has delineated two porphyry gold-copper deposits: Kemess South and Kemess North. The property is underlain mainly by Upper Triassic to Lower Jurassic Takla Group mafic flows, breccias and minor sedimentary rocks and by Lower Jurassic Hazelton Group pyroclastics and epiclastics. Lower Jurassic intermediate to felsic plutons intrude the volcanoclastic and sedimentary strata. Several large hydrothermal alteration zones that host porphyry-type mineralization and a number of skarn and vein-type mineral occu"ences are spatially and genetically related to some of these intrusions. Outliers of Cretaceous continental clastic sedimentary rocks and minor basalt flows that unconformably overlie older rocks are preserved in grabens. The Kemess South deposit is hosted by a flat-lying porphyritic quartz monzodiorite intrusion. Pyrite, the dominant sulphide, occurs as veins and fracture coatings accompanying quartz stringers. Chalcopyrite occurs as disseminated grains and in quartz stockwork veins. Native gold is included within or is peripheral to grains of chalcopyrite, and gold grades correlate closely with those of copper in the hypogene zone. The highest grades of gold and copper mineralization correlate with zones of intense quartz stockwork development accompanied by intense potassium feldspar selvages and local magnetite stringers and disseminations. This potassic alteration is strongly developed in the western two thirds of the deposit where it overprints earlier sericite and calcite alteration. Secondary biotite forms an aureole under the potassium feldspar-quartz stockwork zone that correlates with a decrease in gold-copper grade, gold:copper ratios, and in the amount of groundmass calcite. Pervasive groundmass sericitization is the dominant form of alteration within the intrusion. Sericitization does not show a consistent association with gold or copper mineralization. A supergene zone, comprising 20% of the deposit, formed during a period of arid weathering synchronous with the formation of the Late Cretaceous Sustut Basin. Copper grades within this zone are locally leached or enriched while gold concentrations remain unchanged. Native copper is the dominant secondary copper mineral except near the base of the supergene zone where chalcocite becomes more abundant. Exotic native copper mineralization was locally deposited in the Sustut Group sedimentary rocks beyond the exposed and weathering deposit during continued subsidence of the basin and on-going clastic sedimentation and volcanism. The Kemess North deposit is hosted by potassic-altered Takla Group volcanic rocks and is centred around porphyritic monzodiorite dikes. Higher grade gold-copper mineralization is characterized by secondary biotite in volcanic host rocks. Potassium feldspar is present in and adjacent to monzodiorite dikes. Outward from the potassic zone the onset of the propylitic alteration is marked by a pronounced decrease in gold-copper concentrations. Many porphyry copper deposits feature one dominant economic metal with other secondary metals (gold, silver and molybdenum) occurring in less important concentrations. The Kemess property contains gold and copper as two major commodities. To assess the dual nature of the Kemess property and to rank its importance against other producing and potentially-producing copper and gold deposits, Prorata and normal costing methods need to be applied. At the Kemess South deposit favourable economics are indicated in a perceived low-grade porphyry copper deposit.

Mots Clés: Kemess, Porphyry, Copper-gold deposits, Lower Jurassic, Native copper