Note: This large file contains all sections of the U.S. Geological Survey Professional Paper 1049. The following abstract is for the first of 6 papers. The remaining 5 are cited after this abstract.
Staatz reported in the USGS Prof. Paper 1049-A in 1979 that the Lemhi Pass thorium district covers approximately 55 square miles in the central part of the Beaverhead Mountains. Approximately 250 veins were mapped in this rectangular district that lies astride the Continental Divide in Idaho and Montana. Thorium veins make up 87 percent of this total . Copper and barren quartz veins each make up 6 percent and iron veins less than 1 percent. Thorium veins range in length from a few feet to at least 4,350 feet and in thickness from thin seams to 40 feet. Fifteen veins are more than 1,000 feet in length . The longest vein, the Last Chance, is also the thickest and ranges from 5 to 40 feet. Much of it is 10-25 feet thick . The veins persist at depth, and the central vein on the lola property has been traced on the surface for a vertical distance of 560 feet. The veins may strike in almost any direction, but the strikes of more than half of them fall within either the N. 40°-60° W. or N. 70°-80° W. range. Most of them have a steep dip . The thorium veins are generally found along small fractures and shears that are related to faulting of Tertiary age .
Movement along these fractures commonly occurred before, during, and after the vein was emplaced . Although 45 minerals have been identified in various thorium veins, only a few are relatively common. Quartz and microcline are the principal gangue minerals. Hematite and limonite (chiefly goethite) commonly vein the earlier minerals and color the veins. Thorite is the most abundant thorium mineral, although monazite is present in many veins. Other thorium or rare-earth minerals found in a few veins are brockite, allanite, xenotime, synchisite, bastnaesite, and doverite. Not all the thorium occurs in discrete thorium minerals; some of it is in limonite. The ThO content of 420 samples from this district ranges from 0.0008 to 9.4 percent. Many of the larger veins have an average ThO2 content of more than 0.4 percent.
The overall total rare-earth oxide content of these veins is about equal to the thoria content, although in various veins the ratio between the two may vary widely. Neodymium is the most abundant rare earth in many of the veins. In addition, europium makes up a greater proportion of the total rare earths in about 70 percent of the Lemhi Pass veins than it does in other reported occurrences. The indicated thorium resources in this district are 176,500
short tons of ThO2; the inferred resources are 128,900 short tons of Th02.
The ten largest veins in the district having an average grade of 0.43 percent ThO, contain 96 percent of these resources. Rare -earth analyses are available for only 25 veins in the district . These veins contain indicated resources of 186,000 short tons rare-earth oxides and inferred resources of 122,000 short tons total rare-earth oxides. The ratio of total rare-earth oxides to thoria in these 25 veins is four to three. The thorium content of all the veins greatly exceeds the uranium content, and the latter is of little economic significance in this area. The only other veins of economic importance in the Lemhi Pass thorium district are copper veins.
Small amounts of copper ore have been produced from the Copper Queen, Bluebird , and Last Chance properties. The veins in this district are middle Tertiary or younger in age and were formed after both the Challis Volcanics and the diorite were emplaced. The copper veins are somewhat older than the thorium ones. The latter are believed to have formed from fluids derived from magma that formed buried alkalic rocks . These fluids followed major through-going fractures (and faults) but deposited their load along smaller subsidiary breaks. The temperature of formation is probably low, as suggested by the fine grain size of the veins and the distance that some of the fluids apparently traveled.
Precambrian Y sedimentary rocks, which are the host rocks for the thorium veins, are the oldest rocks in the Lemhi Pass district and directly underlie the greater part of the area. The older part of these Precambrian rocks lies below the Peterson Creek thrust fault. This part of the section has been divided by lithology into quartzite and siltite units. Above the thrust fault and in the Ramsey Mountain area the Precambrian Y rocks are divided from oldest to youngest into the Big Creek , Apple Creek , Gunsight , and Swauger Formations.
South of the Peterson Creek thrust fault , the Madison Limestone of Mississippian age is thrust over the Big Creek Formation. Challis Volcanics
of Tertiary age once covered the entire area but have been largely removed by erosion . The Challis is now preserved in downfaulted blocks and small
erosional remnants. The rocks of the Challis can be divided into two, and possibly three divisions. Rocks of the first division, which contains more than 90 percent of the Challis, consist of a thick basalt- rhyodacite sequence and associated tuffs and conglomerates. Rocks of this division have been divided into the following seven units in ascending order:
1 ) Basal conglomerate,
2 ) quartzite- bearing rhyolite tuff,
3 ) conglomerate of Flume Creek,
4) tuff of Lemhi Pass,
5 ) basalt- rhyodacite sequence,
6) tuff of Curtis Ranch, and
7 ) a flow - breccia unit.
Rocks a second division, which are separated from those of the first by an angular unconformity, consist of a younger basalt and a sanidine-quartz welded tuff. Rocks of a third division are light-colored quartz latite flows or intrusives in the northeast part of the district. These are poorly exposed, but the general relations of this rock type to the rest of the Challis Volcanics suggests that it is younger than the bulk of the Challis .
In Tertiary time, after the Challis Volcanics were deposited and uplifted, a sequence of lacustrine tuffaceous sedimentary rocks were formed in the large basin occupied by the Lemhi River and in a smaller basin in which Cow and Yearian Creeks now flow . They consist mainly of fine -grained tuttaceous sedimentary rocks with layers or lenses of conglomerate.
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USGS Prof. Paper 1049-B - Armbustmacker, T. J., 1980, "Abundance and Distribution of Thorium in Carbonatite Stock at iron Hill, Powderhorn District, Gunnison County, Colorado,"
USGS Prof. Paper 1049-C - Olson and Hedlund, 1981, " Alkalic Rocks and Resources of Thorium and Associated Elements in the Powderhorn District, Gunnison County, Colorado,"
USGS Prof. Paper 1049-D - Staatz, M. H., 1983, "Geology and Descriptions of Thorium and Rare Earth Deposits in the Southern Bear Lodge Mts., NE Wyoming,"
USGS Prof. Paper 1049-E - Staatz, M. H., 1985, "Geology and Descriptions of the Thorium and Rare Earth Veins in the Laughlin Peak Area, Colfax County, New Mexico,"
USGS Prof. Paper 1049-F - Armbustmacker, T. J., 1988, "Geology and Resources of Thorium and Associated Elements in the Wet Mts. Area, Fremont and Custer Counties, Colorado,"
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