NIOBIUM PYROCHLORE MINERALIZATION IN AN IRON-RICH DOLOMITE CARBONATITE, ELK CREEK, NEBRASKA (USA)
The mineral assemblage of interest is an aphanitic dolomite microbreccia dominated by small euhedral-to-subhedral magnetite cubes forming disseminations, aggregates, and veins. Along with minor apatite and biotite, this assemblage generally resembles a phoscorite. Secondary hematite dusting on all mineral surfaces is common to pervasive. This assemblage exists as two “lobes” near the center of the ECC with a maximum plan view area of 0.1km2 at approximately 700m below surface level. In individual core samples, intervals of magnetite dolomite are tens to hundreds of meters thick. These lobes are surrounded by a brecciated mix of magnetite dolomite and apatite-dominated dolomite. The magnetite dolomite is strongly associated with high niobium concentrations due to dense aggregates (occasionally disseminations) of very small (2-10 μm diameter) glassy cubic pyrochlore. Color is variable, mostly clear but occasionally pale green to pale tan. Zoning is not observed in petrographic thin-section. Aggregates occur in dolomite, but cathodoluminescent extinction suggests iron quenching where pyrochlore is present. Intergrowth with magnetite is occasionally seen at the margin of magnetite aggregates. Overall Nb concentration in bulk analysis is >10,000ppm with atypical depletion of light rare earths (La-Nd) compared to the other intrusive mineral assemblages in the Elk Creek deposit.