MT-ID Alkalic Belt

Region West, Northwest
Mineral systems
Deposit types
Critical minerals
Other minerals

Information leading to the delineation of this focus area

Basis for focus area From north to south, alkalic complexes of this focus area track the Idaho-Montana border and include: Rainy Creek, Haines Point, Skalkaho, and Crystal Mountain. Rainy Creek, Haines Point, Skalkaho; each of these includes bimodal felsic (syenite) + ultramafic (pyroxenite) intrusions, as well as localized alkali metasomatism (fenitization) of surrounding Proterozoic Belt Supergroup rocks. Crystal Mountain; fluorite-bearing granitic pegmatite hosted by metagabbro. The adjacent Sapphire batholith could contain REE and other critical elements.
Identified resources Rainy Creek: Past production of copper, feldspar, lead, vanadium, vermiculite, and zinc; Crystal Mountain: Past production of fluorite and scandium.
Production Rainy Creek: The Zonolite Company reported 1,500,000 lbs of mined vermiculite that was converted into heat-expanded zonolite; 100 tons were sold in crude form prior to 1929. Other commodities/minerals include vermiculite, fibrous amphiboles, feldspar, Cu, Pb, Zn, V, fluorite, aegirine, aegirine-diopside (both locally vanadiferous), local titanite and feldspar. Crystal Mountain (1952-1973): produced 240,185 short tons of fluorite between 1952 and 1958; Sc was recovered from mill tailing. Fluorite was used for flux by steel and ceramic companies, as well as for acid production.
Status Rainy Creek: Past mining; Crystal Mountain: Past mining with current owners attempting to reopen the mine; deposit was evaluated in 1984 as a potential Sc resource.
Estimated resources Unknown.
Geologic maps Rainy Creek: Harrison, Cressman and others (1986), scale 1:250,000; Johns (1970), scale 1:126,720; Pardee and Larsen (1929), scale 1:58,500; Boleneus and others (2001), scale 1:48,000; Crystal Mountain: Sahinen (1962), scale 0.75 inches = 100 feet.
Geophysical data A high-resolution Earth MRI airborne magnetic and radiometric survey was funded by Earth MRI in 2020. Flying is complete, but at this time the data have not been released.
Favorable rocks and structures Rainy Creek: Altered pods and lenses of igneous rocks associated with fault planes. Veins found mainly within pyroxenites have unusual mineralogy including vanadiferous aegirine, and quartz and fluorite veins that are stained with Cu-carbonate minerals. Crystal Mountain: Favorable rock for Sc resources is thortveitite-bearing gabbro hosting fluorite pods. Disseminated thortveitite covers a 1 km2 area.
Deposits Rainy Creek: Vermiculite Mountain mine (Libby Vermiculite mine) (MRDS dep_id: 10070453); Crystal Mountain: Crystal Mountain fluorspar mine (MRDS dep_id: 10069419).
Evidence from mineral occurrences MRDS.
Geochemical evidence Crystal Mountain: Scandium is reported to occur in thortveitite as disseminated grains in fluorite, diopside, edenite, and actinolite (Foord and others, 1993).
Geophysical evidence Unknown.
Evidence from other sources None.
Comments Crystal Mountain: The Late Cretaceous Sapphire batholith is composed of six comagmatic peraluminous intermediate plutons. The monzogranite and granodiorite located in the western part of the Sapphire Wilderness Study Area is reported to have high concentrations of fluorite, Sc and REE and should be investigated for possible critical element resources. Rainy Creek: REE, titanium, niobium, vanadium, fluorspar.
Cover thickness and description Crystal Mountain: The region is densely forested and soil cover may conceal other fluorite-bearing pegmatites.
Authors Kaleb C. Scarberry, Stanley L. Korzeb, Jay A. Gunderson, Virginia S. Gillerman, Reed S. Lewis, Christopher A. Tate, Joshua M. Rosera, Allen K. Andersen.
New data needs Crystal Mountain: High resolution aeromagnetics, lidar, geochemistry, geologic mapping and trace element analysis of intrusions may reveal other exploration targets for Sc and REE.
Geologic mapping and modeling needs Targeted 1:24,000 mapping and mine-site mapping would be beneficial.
Geophysical survey and modeling needs Inadequate Rank 5 airborne magnetic and radiometric coverage.
Digital elevation data needs Lidar variable over large focus area.