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Several rare earth element (REE) mine projects around the world are currently at the feasibility stage. Unfortunately, few studies have evaluated the contamination potential of REE and their effects on the environment. In this project, the waste rocks from the carbonatites within the Montviel proterozoic alkaline intrusion (near Lebel-sur-Quévillon, Quebec, Canada) are assessed in this research. The mineralization is mainly constituted by light REE (LREE) fluorocarbonates (qaqarssukite-Ce, kukharenkoite-Ce), LREE carbonates (burbankite, Sr-Ba-Ca-REE, barytocalcite, strontianite, Ba-REE-carbonates), and phosphates (apatite, monazite). The gangue minerals are biotites, chlorite, albite, ankerite, siderite, and calcite. The SEM-EDS analyses show that (i) the majority of REE are associated with the fine fraction (< 106 μm), (ii) REE are mainly associated with carbonates, (iii) all analyzed minerals preferably contain LREE (La, Ce, Pr, Nd, Sm, Eu), (iv) the sum of LREE in each analyzed mineral varies between ~ 3 and 10 wt%, (v) the heavy REE (HREE) identified are Gd and Yb at < 0.4 wt%, and (vi) three groups of carbonate minerals were observed containing variable concentrations of Ca, Na, and F. Furthermore, the mineralogical composition of REE-bearing minerals, REE mobility, and REE speciation was investigated. The leachability and geochemical behavior of these REE-bearing mine wastes were tested using normalized kinetic testing (humidity cells). Leachate results displayed higher LREE concentrations, with decreasing shale-normalized patterns. Thermodynamical equilibrium calculations suggest that the precipitation of secondary REE minerals may control the REE mobility.

Geochemical characterization including mineralogical measurements and kinetic testing was completed on samples from the Montviel carbonatite deposit, located in Quebec (Canada). Three main lithological units representing both waste and ore grades were sampled from drill core. A rare earth element (REE) concentrate was produced through a combination of gravity and magnetic separation. All samples were characterized using different mineralogical techniques (i.e., quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN), X-ray diffraction (XRD), and scanning electron microscopy with X-ray microanalysis (SEM-EDS)) in order to quantify modal mineralogy, liberation, REE deportment and composition of REE-bearing phases. The REE concentrate was then submitted for kinetic testing (weathering cell) in order to investigate the REE leaching potential. The mineralogical results indicate that: (i) the main REE-bearing minerals in all samples are burbankite, kukharenkoite-Ce, monazite, and apatite; (ii) the samples are dominated by REE-free carbonates (i.e., calcite, ankerite, and siderite); and (iii) LREE is more abundant than HREE. Grades of REE minerals, sulfides and oxides are richer in the concentrate than in the host lithologies. The geochemical test results show that low concentrations of light REE are leached under kinetic testing conditions (8.8-139.6 µg/L total light REE). These results are explained by a low reactivity of the REE-bearing carbonates in the kinetic testing conditions, low amounts of REE in solids, and by precipitation of secondary REE minerals.

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This work investigated a rare earth element (REE) natural biogeochemical cycle in an area with a light rare earth element (LREE)-rich ferrocarbonatite intrusion. An REE determination in this geological environment allowed us to trace REE natural transfers in order to better manage future REE mining exploitations. Our findings suggest that although REE concentrations in abiotic compartments (soil and freshwater systems) and biotic samples (terrestrial and aquatic plants) were low, the LREE fractionation observed in the parent material was maintained along compartments. Additionally, Nd anomalies observed in the sediment pore water suggest a potential different biogeochemical cycle of this element in aquatic systems. According to the potential bioaccumulation of REEs in the organisms of two studied plants belonging to terrestrial and aquatic compartments, Equisetum arvense L. and Typha latifolia L. (respectively), we observed that REEs were not accumulated and that they showed limited REE transfer inside plants, but with an increased uptake of Eu relative to the other REEs. Our results indicated a low mobility and transfer of REEs from REE-rich bedrocks in a natural area toward terrestrial and freshwater systems, but also pointed to a dilution of the REE content in the different compartments, maintaining the LREE fractionation. Our findings provide new knowledge about the REE biochemical cycle in a natural area (from rocks to plants) and represent a starting point for an environmentally friendly exploitation of future REE mining areas.

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