Lemhi Pass District

Location: Idaho, United States (44.919, -113.495 approx.)

Commodities: rare earth elements; thorium

Geology

“The Tertiary rocks consist of the Challis Volcanics and the tuffaceous sedimentary rocks. The former are made up principally of dark flow rocks but also contain some tuff and conglomerate. These rocks were emplaced in the early Tertiary and at one time probably covered the entire area. Directly following this period of volcanism was a major period of diastrophism, which uplifted and down dropped various parts of the Challis Volcanics. Subsequent erosion has stripped away most of these volcanic rocks, and only about 15 percent of the area now is underlain by this rock type, which is found principally in downfaulted blocks (fig. 2). The tuffaceous sedimentary rocks are mainly of lacustrine origin and were deposited in mountain basins during middle Tertiary time. They consist mainly of fine-grained tuffaceous sedimentary rocks containing layers or lenses of conglomerate. These rocks formed after most of the faulting and they are found still in broad topographic lows. Diorite dikes and small intrusives, also of Tertiary age, intrude the Precambrian Y sedimentary rocks. The Quaternary rocks are widespread and entirely mask the underlying rocks in about 20 percent of this district.” (Staatz (1979)) “Rocks of the Lemhi Pass thorium district are of four principal types: (1) Fine-grained clastic sedimentary rocks of Precambrian Y age, (2) limestone of Mississippian age, (3) volcanic rocks and sedimentary rocks having a volcanic component, of Tertiary age, and (4) unconsolidated deposits of Quaternary age. The Precambrian Y rocks consist principally of micaceous or feldspathic quartzite interbedded in places with siltite. These rocks underlie the entire area but are poorly exposed over about 60 percent of it. Above the Peterson Creek thrust fault and west of the Yearian Creek graben the Precambrian Y rocks resemble those mapped by Ruppel (1968) in the adjoining Leadore quadrangle to the south, and were subdivided similarly. Below the thrust the Precambrian Y rocks are divided on the basis of their lithology. The Mississippian rocks are the Madison Limestone and are principally confined to small areas southeast of the Peterson Creek thrust.” (Staatz (1979)) “Four kinds of veins are found in the Lemhi Pass thorium district: (1) Thorium veins, (2) copper veins, (3) iron veins, and (4) barren quartz veins. Thorium veins greatly outnumber the other types of veins, making up 87 percent of the 251 veins found in this area. Copper and barren quartz veins each make up 6 percent of the total and iron veins less than 1 percent. With but one exception, all the veins in the district intrude rocks of Precambrian age. The veins lie north of the Peterson Creek thrust fault, and the country rock is chiefly a fine-grained micaceous quartzite. The 14 veins at localities 1, 14, 135, 149, 182,183, 188, 190, 191, and 192, however, occur in siltite. As the quartzite is much more widespread than the siltite, either rock type appears equally favorable as a host rock for veins.” (Staatz (1979))

Published resource estimates

As compiled by USGS USMIN from the cited studies. Resource classes follow the source; "approximate" flags values USMIN reports as approximations. These are not current company reserve statements.

Record imported from USGS USMIN ver. 4.0 (June 2019). Developments after that date appear on facility records, not here; see the methodology.