|Abstract or Summary
- The Washougal Mining District is located on the western flanks of the southern Cascade Range of Washington, approximately 50 km (30 miles) northeast of Vancouver, Washington. The district has produced only $573 since 1903, derived from copper and silver. The presence of hydrothermal mineralization and breccia pipes similar to those associated with many porphyry copper type deposits elsewhere signifies an above average exploration potential for parts of the district. Bedrock in the southern part of the district consists of northwesterly dipping basalt and ande site flows, and volcaniclastic breccias of the East Fork (Ohanapecosh) Formation. Flat-lying basalt and basaltic andesite flows of the Skamania Formation unconformably overlie the East Fork units. These two formations range in age from late Eocene to early Miocene. The East Fork and basal members of the Skamania Formation are intruded by the Silver Star stock, which is of probable middle Oligocene to early Miocene age. This composite intrusion is comprised of small outlying plugs of intrusive andesite and diorite, and a larger mass containing quartz diorite, granodiorite, vesicular porphyritic quartz diorite, and granitic aplite, in probable order of emplacement. Chemical trends indicate differentiation from a single magma. An alkali-lime index of 60.8 percent indicates that the rock suite is representative of a highly calcic calc-alkaline magmatic sequence. Hydrothermal alteration associated with the Silver Star stock is analogous to that described for porphyry copper deposits, with the exception that the potassic and argillic assemblages are not present. Propylitic alteration affects most of the stock and forms a contact aureole in the adjacent volcanic country rocks. Minerals of the phyllic alteration assemblage are closely associated with shear zones, quartz veins, and breccia pipes. Albite-epidote hornfels metamorphism is recognized in one locality on the southeast margin of the Silver Star stock. Metallization in the southern Washougal District occurs as narrow quartz-sulfide veins within both the stock and the adjacent volcanic rocks. The veins generally trend northwest and have nearly vertical dips. Near the intrusive contact, sulfides of copper predominate over those of lead and zinc. The quartz veins are inferred to be the channelways for ascending hydrothermal fluids that formed the breccia pipes of the district.