APPLICATION OF MICROPROBE GEOCHRONOLOGY TO HYDROTHERMAL MONAZITE AND THORITE, LEMHI PASS DISTRICT, IDAHO

Electron microprobe Th-U-total Pb geochronology provides a means of deciphering age relationships in Th-REE ore deposits. The Lemhi Pass Thorium District (Beaverhead Mountains of Idaho and Montana) contains veins and shear zones with specular hematite, thorium and rare earth minerals, primarily monazite and thorite, hosted in Proterozoic (?) siltites and argillites of the Apple Creek Formation. Both Tertiary and Precambrian ages have been suggested for the mineralization, which may belong to the important Fe-Cu-U-REE class of deposits (Gillerman, 2000). Electron microprobe compositional mapping and dating was carried out on abundant monazite and thorite in relatively fresh samples from the Lucky Horseshoe Mine. The low Th (0.2-0.6 wt%) and U (<200ppm) of the bulk monazite makes precise age quantification difficult (Pb < 300ppm) by EMPA, but background regression techniques, gold coating (allowing high beam flux), and detailed mapping, improve results significantly. Results suggest the bulk of the monazite is 800-1100 Ma, exhibiting thin rims (< 6um) of higher Th monazite (0.8-1.6 wt.%) which appear considerably younger (200-400 Ma). The older monazite ages correlate well with a Re-Os age of 1055 ± 4 Ma on molybdenite from a nearby copper vein. Thorite occurs in fractures and partially replaces some monazite. Most thorite is altered, but irregular, embayed, minor domains (<20um) of nearly stoichiometric thorite remain and provide ages of 330-370 Ma, similar to monazite rims. Altered thorite exhibits increased Ca (and OH?), lower Th, Si and much lower Pb concentrations compared to unaltered domains. Age estimates of altered domains reflect this Pb loss (100-280 Ma) and emphasize the overall difficulty in using thorite in geochronology. Only careful compositional imaging and very high resolution in-situ analysis of the freshest possible samples can provide a means for extracting meaningful age information from such materials.