Geology and supergene processes: Berg copper-molybdenum porphyry, west-central British Columbia

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

The Berg porphyry Cu-Mo deposit is in the Tahtsa Mountain Ranges approximately 84 km southwest of Houston, British Columbia. Mineralization is localized in and adjacent to one of several ca. 50 Ma quartz monzonite intrusions in the area. Two mineralized zones with a mineral resource of 250 million tonnes grading 0.40% Cu and 0.052% MoS2 (at a 0.25% Cu cutoff) occur in a highly fractured zone superimposed on homfelsed Hazelton Group volcanic rocks which occur adjacent to a quartz diorite intrusion. Hydrothermal alteration zones are spatially related to the central quartz monzonite stock and extend up to 1000 m from the intrusive contact. Hypogene alteration types are divided into facies (potassic, phyllic, argillic and propylitic), zones and subzones based on diagnostic mineral assemblages. Copper and molybdenum mineralization occur primarily in potassically altered rocks; the best grades are developed in the altered and homfelsed volcanic rocks close to the intrusive contact. Hypogene mineralization is characterized by several generations of veining. Disseminated mineralization is only important in the central part of the stock and in the adjacent quartz diorite where fracture intensities are low. Earliest veins, designated as types 1 and 2, contain much of the copper and molybdenum mineralization. Associated alteration envelopes are either potassic or non existent, implying equilibrium with the potassically altered wall rocks. Later veins (types 3 and 4), are typically poor in sulphides and are associated with phyllic and propylitic alteration assemblages. A well-developed supergene enrichment blanket is superimposed on hypogene mineralization. Three mineralogically distinct supergene zones are recognized: supergene sulphide (covellite, chalcocite and digenite), supergene oxide (malachite/azurite, cuprite, tenorite and native copper) and leached capping. The presence or absence of these zones is determined by several factors including, fracture intensity, abundance of hypogene sulphides and topography. Topography has the greatest effect on supergene profile development. Three different profiles corresponding to ridge top, slope and valley floor environments are recognized. In the ridge-top environment, the supergene profile is complex, consisting of a strong leached and oxidized zone underlain by a thick but poorly enriched supergene sulphide zone. In the valley floor environment, where the water table is at or close to the surface, leaching is minimal and fresh hypogene minerals occur at surface. The most complex profile is developed on steep slopes, where highly variable water table levels and a high rate of ground water migration have coupled to produce a strongly enriched supergene sulphide zone overlain by a zone of supergene oxide.
Keywords: Porphyry, Cu-Mo deposit, Quartz mazonite intrusion, facies, Hypogene mineralization.