ABSTRACT: Precious-Metal Deposits in Relation to Volcanic and Sedimentary Stratigraphy: Examples from the Oregon Plateau, U.S.A.
Michael L. Cummings
Miocene to Recent volcanic and sedimentary rocks of the Oregon Plateau host precious-metal deposits. The deposits, formed within an extensional tectonic setting, illustrate the relation among volcanic and sedimentary stratigraphy, regional structural development, and hydrothermal systems.
Gold deposits hosted in felsic volcaniclastic sediments in the Owyhee region formed during or shortly after sedimentation. Uniformly silicified tuffaceous mudstone and siltstone interbedded with siliceous and sulfide-rich sediments and bedded hydrothermal breccia, hydrothermal explosion craters infilled by breccia fragments, and stacked buried sinter deposits attest to mineralization near the time of sedimentation. The regional structures that controlled the distribution of volcanic activity and location of sedimentary basins changed through time. In the middle Miocene, basin-and range faults controlled the location of basalt and rhyolitic volcanism and fluvial and lacustrine basins. In the late Miocene and Pliocene, faults parallel to those of the Western Snake River Plain are more p ominent and interfingering volcanic and volcaniclastic sedimentary deposits characterize the stratigraphic section.
Fault-controlled mercury mineralization in quartz within opalized high-silica rhyolite flow-domes produces an obscure precious-metal prospect at Glass Buttes in the High Lava Plains. Glassy high-silica rhyolite flow-domes were erupted between 5.8 and 5.0 ma but alteration and mineralization occur along faults of the Brothers fault zone that cut the high-silica rhyolite sequence. Younger porphyritic rhyolite to dacite domes, spines, and intrusions controlled by faults of the Brothers fault zone are believed to be the heat source for the hydrothermal system. Precious-metal mineralization may occur in the subsurface where hydrothermal solutions boiled beneath the paleowater table.
AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990