The Berg porphyry copper-molybdenum deposit is in the Tahtsa mountain ranges, approximately 84 km southwest of Houston, British Columbia. It is localized in and adjacent to one of several mineralized ca. 50-m.y.-old quartz monzonite intrusions in the area. Two potential orebodies with estimated reserves of 308 million metric tons of 0.348 percent Cu and 0.052 percent MoS 2 (with a 0.25% copper equivalent cutoff) are centered in a highly fractured zone. Hydrothermal alteration zones are related spatially to a central quartz monzonite stock and extend up to 1,000 m from the contact. Hypogene alteration types are divided into facies, zones, and subzones based on mineralogic associations. Potential orebodies occur in potassically altered, hornfelsic volcanic rocks close to the stock. Potassium enrichment of the hornfels is demonstrated using major element ternary and binary plots that compare rocks from the mineralized zone with fresh, albeit distant, regional equivalents. Hypogene mineralization is characterized primarily by several generations of veins; disseminated sulfides are important only in the central part of the stock and in the quartz diorite to the northeast, where fracture densities are low. The earliest veins, designated as types 1 and 2, contain much of the copper and molybdenum mineralization. Associated alteration envelopes are either potassic or nonexistent, implying equilibrium with the potassically altered wall rocks. Later veins defined as types 3 and 4 are characterized by low sulfide contents and phyllic or propylitic mineral assemblages.--Modified journal abstract.