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Geologic setting, mineralization, and aspects of zoning at the Berg porphyry copper-molybdenum deposit, Central British Columbia Panteleyev, Andrejs

Abstract

Berg copper-molybdenum deposit is in mountainous terrain of the Tahtsa Range in west-central British Columbia. It is a type example of Lowell and Guilbert's (1970) model of porphyry copper deposits. The deposit is in thermally metamorphosed and hydrothermally altered Middle Jurassic volcanic rocks and Tertiary quartz diorite adjacent to a weakly mineralized Eocene stock. The stock is about 2,100 feet in diameter and consists of four major quartz monzonitic phases. Unmineralized volcanic and sedimentary strata of the Cretaceous Skeena Group crop out east of the deposit. Hydrothermal alteration and sulphide minerals are arranged in concentric, annular zones around the composite stock. From the interior outward pervasive alteration zones are potassic, phyllic, and propylitic. Argillic alteration is rare. A biotitic alteration zone that surrounds the stock is largely a thermal aureole but is also partially of hydro-thermal origin. A multistage vein stockwork is superimposed on the pervasively altered rocks. Early veins, many with alteration envelopes, were deposited from saline fluids at temperatures in excess of 400 degrees centigrade. Later veins were deposited from cooler, less saline fluids and commonly have retrograde alteration margins. A gypsum-filled sub-horizontal fracture cleavage cuts all alteration minerals and veins. The molybdenite zone follows closely the intrusive contact where a quartz stockwork is well developed. Chalcopyrite is most abundant in the biotitic alteration zone. Pyrite forms a broad halo that extends outward for at least 2,000 feet from the stock. Minor elements In pyrite are concentrated in disseminated pyrite from well-mineralized copper zones. Leaching and supergene mineralization cause pronounced vertical zoning in the deposit. In the leached capping copper has been removed to a depth of 125 feet but molybdenum remains and is locally concentrated in limonite. In the supergene zone, which overlies the entire deposit and is up to 300 feet thick, copper is enriched by a factor of 1.25 times primary grade. Supergene mineralization is a contemporary ongoing process that was initiated after Pleistocene glaciation.

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