CONTROLS ON THE DISTRIBUTION OF RARE-EARTH ELEMENTS IN CARBONATITE-HOSTED FLUOROCARBONATE ORES

High tonnage carbonatite-hosted REE deposits are invariably light REE (LREE)-dominant and contain only nominal amounts of heavy REE (HREE) The full suite of REE is recovered during physical processing to produce a bulk REE oxide concentrate, prior to solvent extraction, thus insuring recovery of both LREE and HREE. Some carbonatite-hosted deposits are known to have slightly higher middle or heavy REE contents. This is sometimes attributed to a greater abundance of phosphate minerals like fluorapatite, monazite, or churchite. In other instances, there is a clear difference in HREE content, despite no obvious difference in phosphate abundance. We examined Ca-REE fluorocarbonate mineral chemistry acquired by EPMA for the Mountain Pass and Bear Lodge carbonatite-hosted deposits. A common trend emerged: the most Ca-rich varieties (parisite and synchysite) contain a greater proportion of middle and heavy REE.

In bastnäsite, the lanthanide atom (Ln) is coordinated in LnO₆F₃ polyhedra, whereas synchysite hosts a second cation site with alkaline earth metals (Ca and Sr) coordinated to 7 or 8 oxygen atoms in neighboring CO₃ layers. We hypothesize that HREE are more readily accommodated in the Ca-site because of its coordination geometry and atomic-scale vaterite (μ-CaCO₃) arrangement.

The uptake of HREE in more Ca-rich fluorocarbonate minerals might explain how some carbonatite-hosted deposits accommodate slightly higher concentrations of HREE. Carbonatites with a greater abundance of Ca-rich fluorocarbonate minerals (parisite, synchysite or intermediate varieties), are expected to have higher HREE to total REE ratios relative to bastnäsite-dominant ores, provided no other minerals are present that greatly affect REE concentrations. Carbonatite deposits with bastnäsite as the predominant ore mineral are expected to have the highest LREE to total REE ratios. This may be the cause of subtle, deposit-wide HREE:LREE zonations. For example, the Bear Lodge deposit hosts two major resources: 1) Bull Hill dominated by bastnäsite and the lowest HREE proportion, and 2) Whitetail containing abundant parisite and higher HREE contents. If the economic value of critical HREE (Tb, Dy) continues to be high, these atomic-scale substitutions may be important in how mining operations stage development and process ore.