Use of QEMSCAN® to characterize oxidized REE ore from the Bear Lodge carbonatite, Wyoming, USA

Abstract

• Automated mineralogical analysis (AMA) of a complex REE deposit has characterized variations in ore types due to hydrothermal and supergene processes. • The complexity of the deposit necessitated a mineralogical survey to establish a realistic database of minerals to reference for AMA. • Resulting modes from AMA reconcile well with independent chemical assay data. • When correctly conducted, AMA provides useful data in developing mineral processing flowsheets for metallurgy, even on complex deposits with multiple ore phases. The Bear Lodge REE deposit is located in northeastern Wyoming. The Eocene carbonatite dyke and stockwork system intrudes trachytic-phonolitic rocks that contain multiple diatremes. The original magmatic characteristics of the REE-enriched carbonatite complex are strongly overprinted by carbo-hydrothermal and later supergene fluids. These fluids redistributed the REEs and created high variability in the ore mineral assemblage, as well as compositional variability within individual REE mineral species. The REE ore minerals at Bear Lodge can be classified into four types: 1. fluorocarbonates (bastnaesite, parisite, synchysite), 2. phosphates (monazite, xenotime, florencite, rhabdophane, churchite), 3. cerianite, and 4. ancylite. These minerals vary greatly in abundance, grain size, and morphology. REE distribution is heterogeneous throughout the deposit. Variations within a given REE mineral in terms of Ce depletion, Th content, degree of heavier REE enrichment, etc., create difficulties in the initial definition of discrete mineral species (i.e., by X-ray spectra) and their resultant species identification protocols for use in automated mineralogy (QEMSCAN Ⓡ in this study). Prevalent submicron-scale supergene mineralization result in hybrid spectra from multiple phases. Iterative work in reconciling QEMSCAN Ⓡ data interpretation with bulk assay, XRD, SEM, and optical petrography data allows for refinement of the protocols to quantify for both ore and gangue minerals. Use of automated mineralogy in the development of complex deposits requires rigorous review of these identification criteria in order to achieve results that can be applied with confidence to resolve mineral processing issues.