Abstract
The Mineral Hill Mining District, located in Lemhi County, Idaho, contains several deposits of carbonatite-hosted rare earth element (REE) and Nb mineralization. Many of the carbonatite occurrences were discovered and first described in the 1950s during Ti and Nb exploration, and mapping in the 1980s shows that there are several prospects which follow a northwest trend that extends into Montana. Since the U.S. designation of REEs and Nb as ‘critical minerals,’ materials that are necessary to green technology, battery chemistries, permanent magnets, and other developing technologies, there has been a resurgence of interest in the district, and it is actively explored as a potential domestic REE source. The carbonatites are small, shallowly dipping bodies that outcrop on the sub-meter scale. Country rock predominantly consists of amphibolite and Augen gneiss previously dated at 1370 Ma, the protoliths of which are A-type granite and diabase, respectively. The carbonatites are exclusively emplaced in the mafic units (most often amphibolite) and often appear to be folded along with their host, concordant with the regional fabric. The mineralogy is dominated by calcite and dolomite, with varying amounts of amphibole, magnetite, ilmenite, rutile, monazite, and allanite. REE-phases are Ce-dominant with accompanying light rare earth elements and minimal Th.
Despite mention in several studies since their discovery, there has been no detailed characterization on the mineralogy of the Mineral Hill carbonatites and their REE-bearing phases since the resurgence of interest in strategic domestic deposits. Seven of the original prospects have been found and sampled, for the purpose characterizing the mineralogy, quantifying amounts of REEs and Nb, characterizing potential ore phases, and acquiring a date of mineralization. Continued mapping and thermobarometry will also help constrain the relationship between the carbonatites and regional metamorphism.