MINERALOGY OF RARE EARTH ELEMENTS IN WYOMING ROLL FRONT URANIUM DEPOSITS

LREEs are significantly enriched over proximal barren sediments in the Lost Creek uranium roll-front deposit but mineralogy and modes of occurrence have not been previously characterized. Six pre-mining cores ranging from the oxidized zone above the deposit through the roll front and into the unaltered (reduced) zone below the deposit were studied. The host rocks are immature, coarse-grained arkoses of the Battle Springs Fm (55 – 47 Ma) in the Great Divide Basin of Wyoming. Observed LREE-bearing minerals include monazite and allanite. Monazite occurs as detrital grains and as inclusions in other detrital minerals in all samples. In addition, biotite altering to chlorite in the ore zone contains abundant 1 – 10 micron crystallites of monazite. Monazite-bearing biotite (MBB) is absent in reduced rocks below the ore deposit. Average ThO2, UO2, and PbO wt%s in detrital monazite are 7.36, 0.26, and 0.62 ; and 1.10, 1.27, and 0.02 in MBB grains. Based on these concentrations, the MBB must be less than 100 Ma and is inferred to be authigenic and coeval with the uranium mineralization. Allanite occurs as alteration products on detrital grain boundaries (including monazite) and as rare inclusions in detrital grains within the oxidized portions of the deposit and in the ore zone; it is virtually absent in the reduced zones of the deposits. Microprobe analysis of allanite reveals very low Th concentrations and Pb below detection limits. The low Th concentrations and the tendency of allanite to lose radiogenic Pb preclude constraint of age of crystallization, but the absence of allanite below the ore zone strongly suggests that much of the allanite observed in the ore zone is also authigenic. Uranium minerals may also contain appreciable REEs but crystals are too small to permit acquisition of microprobe data. We conclude that REEs are being transported and deposited along with uranium by some so-far unrecognized mechanism.