Deposits of monazite and Nb-bearing rutile in a belt more than 10 mi. long in northern Lemhi County, Idaho, are distinguished by their notable concentrations of such elements as Th, Ti, Nb, Ba, Ca, Fe, P, S, and the rare earths, and by their abundance of carbonates, chiefly calcite. These deposits occur as structurally controlled replacements of crystalline limestones, amphibolites, and porphyroblastic gneisses which form part of the complex metamorphic border zone of the Idaho batholith. Most of the deposits, which are small and tabular, show a sporadic and irregular distribution of minerals, the most abundant of which are actinolite, allanite, apatite, monazite, calcite, barite, ilmenorutile (Nb-bearing rutile), ilmenite, and magnetite. Other minerals, some of them sparse, include biotite, phlogopite, garnet, epidote, glaucophane, ancylite, bastnaesite, sphene, rutile, chlorite, pyrite, pyrrhotite, chalcopyrite, and quartz. Silicates, except sphene, were deposited first and were followed successively by phosphates, carbonates, sulfates, oxides, and sulfides. Most of the Th entered the allanite instead of the somewhat later monazite. The deposits possess all the compositional characteristics of the carbonatites, but differ from the type carbonatites in their seeming lack of association with alkalic igneous rocks. Although the deposits were regarded by others as replacements of marble beds and as products of metamorphic differentiation, the unique association and concentration of elements so characteristic of the carbonatites, coupled with the paragenetic development, suggests that they, like the carbonatites, are of magmatic rather than of metamorphic origin.

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