Schulz, et al., reported in 2019 that many rare earth element (REE) deposits have experienced multistage geological enrichment processes resulting in REE bearing mineral assemblages of considerable complexity and variability. Automated scanning electron microscopy (SEM) mineral liberation analysis of such REE ores is confronted by the difficult assignment of energy-dispersive X-ray (EDX) spectra to REE mineral names. To overcome and bypass this problem, a generic and reliable labelling of EDX reference spectra obtained from REE-bearing minerals based on their contents of Si, Ca, F and P in a bulk normalized analysis is proposed by the authors.
The labelled spectra are then combined into groups of REE-P (~monazite), REE-Ca-Si-P (~britholite), REE-Ca-F (~synchysite) and REE-F (~bastnaesite, parisite, fluocerite). Mixed spectra with low counts for REE from minute REE mineral grains are combined into a separate group. This classification scheme is applied to automated SEM mineral liberation analysis (MLA) data from beneficiation products by comminution and multistage flotation of REE carbonatite ores. Mineral modes, mineral grain-size distribution, mineral liberation, mineral locking and mineral grade versus recovery curves based on the analysis of >200,000 particles in a sample can be recognized and interpreted in virtual grain-size fractions. The approach as proposed by the authors will allow future process mineralogical studies of REE deposits to be robust and comparable.
Rare earth element (REE) ore deposits occur in a wide variety of geological contexts and are hosted by a considerable diversity of host rocks. Igneous host rocks appear mainly carbonatites and peralkaline plutonic rocks as nepheline syenites. Although pegmatites may also contain significant amounts of REE-bearing minerals, they are usually too small in volume to be of economic significance.
REE mineralisation also occurs in hydrothermal veins and stockworks. Recent and fossil mineral placers can be sedimentary REE mineral deposits. Significant enrichment of REE is also possible by chemical weathering with recoverable REE concentrations occurring in lateritic clays formed at the expense of magmatic REE deposits. Some of the largest known REE deposits owe their origin to a sequence of natural enrichment processes.
Primary igneous occurrences in alkali syenites and carbonatites as well as sedimentary heavy mineral accumulations and weathering crusts underwent metamorphic and hydrothermal overprint in depth, followed by weathering. Due to their possible complex geological evolution, the deportment of the elements in such REE mineral deposits are often ambiguous and the subject of scientific discussions.
REE ores are not only marked by geological complexity, but REE ore mineralogy is also very complex, with different minerals having complex chemical and crystallographic properties. More than 200 REE minerals are known. Actually, in the largest REE mineral deposit of Bayan Obo in Mongolia one can distinguish three ore types with Fe-REE-, dolomite-REE- and silicate-REE-ores. The economic REE minerals are bastnaesite and monazite. Other important REE mineral deposits are associated with carbonatite intrusions, as Mountain Pass (USA) and Mount Weld (Australia). The ore minerals in these deposits are bastnaesite, allanite (part of the epidote mineral group), monazite, apatite and pyrochlor. In lateritic clay deposits, REE released during chemical weathering of igneous host rocks may occur adsorbed in clay minerals.
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