Region 5: The Basin and Range

The Basin and Range region, with its distinctive horst and graben features formed by extensional tectonics, is present only in southeastern and east-central Idaho. Aside from a few gold placer deposits associated with the southern margin of the Snake River Plain, the Basin and Range region in Idaho contains only one metallic ore deposit of even marginal significance: the Mount Pigsah District in the Caribou Mountains, which produced some 454,000 grams (16,000 ounces) of gold from ore bodies intruded into Mesozoic sediments. In addition, this area produces industrial minerals such as pumice and phosphate for use in fertilizer and the making of phosphoric acid and dimension stone (Figure 5.21). It also produces perlite, an amorphous hydrated volcanic glass often found as small hollow spheres embedded within obsidian (Figure 5.22). Perlite is used in horticulture, water filters, lime, and cement.

Figure 5.21: Principal mineral resources of the Basin and Range.

Figure 5.22: An outcrop of flow-banded perlite (amorphous hydrated volcanic glass) in obsidian. Perlite occurs as small hollow spheres called “spheruloids.”