RARE EARTH MINERALOGY OF THE MINERAL HILL CARBONATITES, EAST-CENTRAL IDAHO

Northern Lemhi County, Idaho contains several rare earth mineral-bearing carbonatites that trend from the Mineral Hill district northwest into the Sheep Creek district in Montana. The area was prospected beginning in the 1950s, but the carbonatites’ potential to contain energy-critical rare earth elements (REEs) has prompted current study by the Idaho Geological Survey (IGS) under the United States Geological Survey-funded Earth Mapping Resources Initiative (Earth-MRI). Mineral Hill geology consists of interlayered augen gneiss and amphibolite that have screens of schist and gneiss. The carbonatites are generally associated with the amphibolite and appear concordant with its foliation, suggesting they were either emplaced with the amphibolite’s protolith, or are preferentially precipitated along the regional fabric. Garnet-biotite geothermometry performed on an associated schist indicates that regional metamorphic temperatures reached 598°C. Scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) and X-ray diffraction (XRD) have confirmed that the carbonatites generally consist of intermixed calcite, dolomite, and actinolite, with lesser monazite, allanite, magnetite, barite, and Nb-rutile. Monazite and allanite are abundant in some of the prospects, and form homogeneous, millimeter-scale crystals. Electron probe microanalysis (EPMA) of monazite provides a formula of (Ce_0.49La_0.32Nd_0.12Pr_0.04)_S0.97(P_1.01Si_0.01)_S1.02O₄ averaged from four prospects. Laser ablation inductively-couple plasma mass spectrometry (LA-ICP-MS) on monazite show that there are also appreciable amounts of economically significant LREEs. Further geochemical analysis including whole rock geochemistry and dating of zircon and monazite will pair with the IGS’s continued mapping efforts to better describe the petrogenesis of the carbonatites as well as their potential as a source for REEs.