Geothermal Systems in the Cascade Range in Oregon: Insights from a Fossil System, North Santiam Mining Area, Western Cascades

Abstract

Two volcanic sequences bounded by erosional unconformities compose the stratigraphy of the North Santiam mining area, Western Cascades, Oregon. Diorite, granodiorite, and leucocratic quartz porphyry dikes, stocks, and sills contact metamorphic aureoles, breccias, and complicated stratigraphic relations chracterize a volcanic center for the older sequence in the center of the mining district. The younger volcanic sequence is associated with a volcanic center located southeast of the center of the district. Alteration destroyed porosity within the rocks of the older sequence. Circulation of geothermal fluids has been restricted to northwest-trending structures and the margins of dikes and intrusions that occupy these structures. Zones of rising hot water are chracterized by silicification associated with potassic and phyllic alteration and tourmalinization. Fossil geothermal systems can serve as a model for exploration in the Cascade Range.