REGIONAL METALLOGENY OF THE TERTIARY CASCADES VOLCANIC ARC IN WASHINGTON, OREGON, AND NORTHERN CALIFORNIA

We have systematically reviewed published data on mineral occurrences of the Pacific Northwest to identify those related to the Tertiary Cascades volcanic arc. The resulting database contains 1228 records for mineral occurrences that we believe may have a genetic association with Cascades magmatism. This data set, which includes metallic commodities and nonmetallic commodities related to hydrothermal alteration, but not placer deposits, is included in the USGS Mineral Resources Data System (MRDS).

The largest known deposits are porphyry Cu-Mo and porphyry Cu-Au systems. They are confined to Washington and northern Oregon, and are associated with phaneritic intrusive rocks that are abundant in this part of the arc. Polymetallic veins are the most wide-spread deposits; many are found near porphyry systems, but some are not. Breccia-pipe occurrences (Cu-Mo-Au-Ag) are distinguished from porphyry deposits on the basis of higher grades and smaller tonnages; several are found in porphyry copper districts. Many base-precious metal systems and their associated plutonic rocks formed during the period of high volcanic production rates in the Cascades arc (Oligocene to early Miocene), but some are younger.

Small Sado-type epithermal precious metal vein deposits are scattered throughout the Tertiary volcanic rocks of the Cascades and adjacent Basin and Range province. Comstock-type epithermal veins are much less common, and occur in both Cascades and pre-Cascades rocks. Acid-sulfate systems also are widely scattered through the Western Cascades terrane; several are known for non-metallic commodities including silica, clays, and alunite. Mercury occurrences that are hosted by Tertiary sandstone or shale in several districts do not readily fit the deposit model for hot-spring mercury. The few hot-spring type gold-silver deposits recognized are in Oregon and northern California. Epithermal deposits of all types are smaller and less numerous than they are in arc rocks of similar age in western Nevada.