Preliminary report on the geology and deposits of monazite, thorite, and niobium-bearing rutile of the Mineral Hill district, Lemhi County, Idaho

Deposits of minerals containing niobium (columbiull), thorium, and rare earths occur in the Mineral Hill district, 30 miles northwest of Salmon, Le,nhi County, Idaho.Monazite, thorite, all-nite, and niobiumbearing rutile form deposits in mettmorphic 1L:lestone layers less than 8 feet thick.The known deposits are small, irregular, and typically located in or near small folds.Minor faults are :omr.,on.Monazite generally is co.rsely rystAlline and .ont_dnsless than one percent thoriuu.Rutile forms massive luAps up to 3 inches across; it contains between 5 and 10 percent niobium.Rutile occurs in the north western half of the district, thorite in the central and southeastern parts.Monazite occurs in all deposits.Allinite is locally abundant and contains several percent thorium.Magnetite and ilmenite are alsc locally abundant.A major thrust fault trending northwest across the map-area separates :oderately folded quartzite and phyllitic rocks of Belt age, on the northeast, from more intensely metLmorphosed and folded rocks on the southwest.The more metamorphosed rocks include amphibolite, porphyro bl_stic feldspar gneiss, quartzite, and limestone, all probably of sedimentary origin, and probably also of Belt (late Precainbrin) age.The only rocks of definite igneous origin are rhyolite dikes of probable Tertiary age.The more metamorphosed rocks were formed by metasomatic metamorphism acting on elastic sediments, probably of Belt eic;e, althouLh they may be older than Belt.Netamorphiam doubtless was part of the episode of emplacement of the Idaho batholith, but the history of that episode is not well understood.The rare-element deposits show no evidence of fracture-controlled hy,!rothermal introduction, euch as special fracture systems, veining, and gangue material.They may, however, be of hydrothermal type.More likely they are metamorphic segregations or secretions, deposited in favorable stratigraphic and structural positions during regional metamorphism.