INVESTIGATING CARBONATITES AND RARE EARTH MINERALIZATION IN LEMHI COUNTY, IDAHO

The Mineral Hill district in northern Lemhi Country, Idaho, is underlain by a complex of interlayered gneiss, paragneiss, and amphibolite rocks that extends northwest into Montana, where it is known as the Sheep Creek-Mineral Point mining district. Both districts contain multiple horizons and bodies of rare-earth and Nb-bearing carbonate rocks, characterized generally by several historic studies: Sharp and Cavender (1953), Abbot (1954), Sturm (1954), Anderson (1958, 1960), Heinrich and Levison (1961), and Spence (1984). However, the area’s potential as a source of energy-critical minerals has led to many of the deposits becoming subjects of updated research. This study focuses on the Upper and Lower Roberts, Lee Buck, and Cardinal prospects, with the goals of chemically characterizing mineral phases of interest, determining rare-earth abundance to classify the carbonatites, and better understanding the episodes and controls on mineralization. Samples of carbonatites and their related rocks from these properties will be studied with optical petrography, scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), and powder X-ray diffraction (PXRD) in order to characterize the general mineralogy, phase relations, and textures present. SEM-EDS study has already revealed large monazite-(Ce) grains in multiple samples, many of which are rimmed with fluorapatite and allanite in corona-like structures. These structures will potentially provide insights into the pressure and temperature of crystallization, while the monazite itself is a candidate for geochronometry. Electron probe microanalysis (EPMA), wavelength dispersive X-ray fluorescence (WDXRF), and inductively coupled plasma mass spectrometry (ICP-MS) will provide geochemical information, including trace element abundances that will help determine the amount of REEs hosted in the carbonatites. This data will be used to determine the type and abundance of REE phases, study the movement of Ce in the system, and understand the extent of mineralization in the trend.