Geology and mineralization of the north part of the Washougal Mining District, Skamania County, Washington Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/3197xr33t

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  • The Washougal Mining District is approximately 50 km (30 miles) northeast of Vancouver, Washington, along the western slopes of the Cascade Range. Although the district has produced the unostentatious amount of $572 in metals since 1903, the presence of porphyritic intrusions, hydrothermal alteration, breccia pipes, and base-metal mineralization typical of many porphyry copper-molybdenum deposits give this area increased exploration significance. Bedrock in the district consists primarily of mafic to intermediate flows and volcaniclastics of probable Eocene to Oligocene age, which have been intruded by the Silver Star plutonic complex. This complex is composed of a chilled border phase, diorite, quartz diorite, granodiorite, quartz diorite porphyry, gran.odiorite porphyry, and granite aplite, in probable order of emplacement. In addition, at least 21 tourmaline-bearing breccia pipes, interpreted to have formed by late-stage magmatic fluids, are associated primarily with the granodiorite phase. The phases of the complex display the systematic and continuous variations in mineralogy and chemical composition that are characteristic of a rock suite differentiated from a single magma which underwent late-stage silica and potassium enrichment. Northwest-trending folds and faults are found throughout the Washougal District. There is a close correlation between joint plane orientations, and lineaments determined from aerial photographs. This association suggests that pre-existing north-northeast- and no structures controlled the emplacement of the intrusion and localized the related hydrothermal mineralization. Prophylitic, phyllic, potas sic, and tourmaline alteration as well as the associated copper, molybdenum, and zinc mineralization accompanied and (Or) closely followed the emplacement of the granodiorite porphyry phase. The ore minerals, which are generally restricted to the hydrothermal breccias and the surrounding host rock, are sporadically zoned throughout the district. Anomalously high concentrations of copper and molybdenum are broadly antipathetic with those of silver, lead, and zinc. The association of calc-alkalic magma including porphyritic phases, tourmaline-bearing breccias, and erratic hydrothermal alteration and metallization suggests that the Silver Star plutonic complex as presently exposed is a high level porphyry copper-molybdenum deposit.
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