Region 4: The Columbia Plateau

The Columbia Plateau, dominated by the Miocene-aged Columbia Flood Basalts, is present in only a small area of the Northwest Central US, in far west-central Idaho. This area does not contain any mineral occurrences of note. The Snake River Plain of southern and central Idaho, which marks the movement of the North American plate over the Yellowstone hot spot, has only a few small associated gold placers. However, the volcanic and igneous activity associated with the formation of this feature may have contributed to the formation of hydrothermal gold deposits in nearby mining districts. Gold and other precious metals, as hydrothermal deposits, are also found in the hot springs of the Yellowstone Plateau, which is the terminus of the Snake River Plain (Figure 5.20).

Figure 5.19: The Carissa Gold Mine, which operated from 1867 to 1954. In 2003, the state of rock high in abundance of Wyoming restored the mine and mill as a historic attraction

The most notable mineral deposit near the Snake River Plain is the Silver City-De Lamar District, a remote area in southwestern Idaho. This district has produced over 28 million grams (1 million ounces) of gold and more than 910 million grams (32 million ounces) of silver from selenium-rich ores emplaced about 16 million years ago in the middle Miocene. Common minerals and metals found here include gold, silver, naumannite, aguilarite, and argentite, and the ruby silver minerals cerargyrite and acanthite. Today, De Lamar and Silver City are both ghost towns, largely abandoned after their nearby mines were depleted.

Bruneau Canyon, in Owyhee County, southwestern Idaho, produces large quantities of jasper. This silicate mineral precipitated within the cavities and fractures of rhyolite flows, and it ranges in color from brown to reddish cream.

Figure 5.20: Principal mineral resources of the Columbia Plateau.

Zeolites—porous alumino-silicate minerals with cation-exchange properties that can transform hard water into soft water—are mined along the Idaho-Oregon border. These deposits were created from alkaline volcanic ejecta that was deposited into a fresh or salt water source.