Experimental study of REE carbonate and fluorocarbonate synthesis as a basis for understanding hydrothermal REE mineralisation
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Many of the world's economic rare earth element (REE) deposits are formed from, or have been subsequently upgraded by, hydrothermal fluids. Some of the most important REE minerals are the light REE (LREE) enriched fluorocarbonates and carbonates, which are commonly found in carbonatites. Textural and mineralogical evidence from these and other sites point towards wall rock composition as a major control on the observed REE mineralisation, with the supply of carbonate, and possibly fluoride, thought to be the limiting factor. Despite theoretical and experimental studies focussed on REE speciation in hydrothermal fluids, and a few on REE mineral solubility, there remains a lack of understanding of the processes occurring at the uid-rock interface during REE carbonate and fluorocarbonate mineralisation. Many of the issues surrounding this topic stem from the difficulty of working at elevated temperatures, low REE concentrations, and with the corrosive fluoride ion. The synthesis of REE carbonates under simple, low temperature conditions is a useful starting point for understanding REE mineralisation, and as such has been the focus of research for decades. Despite this, cross-series trends are rarely assessed together under the same conditions, and multi-REE-bearing systems - useful for assessing REE fractionation - have scarcely been explored. Furthermore, wall rock experiments, whereby REE-rich fluids are reacted directly with carbonate rocks, are absent from the literature. The same is true for systems containing fluoride, necessary for studying the formation of fluorocarbonates. A fuller understanding of REE mineralisation cannot be achieved until empirical experimental results can be compared with theoretical data and field observations. This thesis documents the laboratory synthesis of single- and multiple-REE-bearing carbonates and fluorocarbonates, and compares the findings with a mineralogical and textural study of two REE-bearing carbonatite deposits. The REEs La, Nd, Gd, Er and Yb were investigated as representatives of the entire series. The experiments constituted titrations of REE chloride solutions with sodium carbonate, and `wall rock reactions' of REE chloride with dolomite, or dolomite plus fluorite. Batch and flow-through setups were used, and the experiments were performed, or the products aged, at temperatures ranging from ambient to 200 °C. Products were characterised by techniques such as PXRD and SEM to document their structure and morphology as a function of temperature, and assess the influence of single vs multiple REE on the final material (whether mixed or separate phases formed). Results showed that in titration experiments, the LREEs crystallised easily and at low temperatures (as low as room temperature), HREEs either do not crystallise (in some cases even at 200 °C) or are more diffcult to crystallise, and mixed LREE + HREE precipitates behaved more like HREE-only examples. The HREEs and LREEs + HREEs mostly produced X-ray amorphous materials, identified as carbonates using FTIR. These were analysed by XAS (XANES and EXAFS) to assess whether they possessed the same short-range structure as the crystalline phase into which are known to form, thus adding to the non-classical nucleation pathway argument as previously suggested for these materials. Results suggested the short-range order of most phases analysed were similar to known bulk phases, but that these were probably different to the earlier precipitates formed in solution. Additionally, in the mixed LREE + HREE systems (Nd+Er), REEs were well dispersed (as opposed to Nd- and Er-rich clusters). In contrast to the titration results were those of wall rock reactions, in which excellent crystallisation was observed for almost every REE configuration (single- or up to five- REE mix), or ageing duration. All but three of the phases produced were previously described natural or synthetic minerals. When fluorite was included in batch reactions the results were more varied: REE carbonates, fluorides and fluorocarbonates were all observed, but never together in the same sample (except in one example). A textural and mineralogical assessment of two carbonatite deposits, Bayan Obo, China and Tundulu, Malawi, which were analysed by EMPA, revealed multiple stages of hydrothermal activity, some of which related to REE fluorocarbonate mineralisation. REE fluorocarbonates, identified at both sites, were typically LREE enriched. No REE carbonates or fluorides were observed, despite the presence of fluorite (REE-barren) and carbonates at Bayan Obo, and carbonates (low REE content) at Tundulu. However, at both sites apatite contained considerable REE. The REE fluorocarbonates were not solely associated with carbonate wall rocks, although the Ca-REE fluorocarbonate synchysite was only observed in the significantly more carbonate-rock-rich Tundulu samples. At Bayan Obo, bastnasite and huanghoite (Ba-REE fluorocarbonate) were observed, the latter of which is reportedly replacing earlier Ca-REE fluorocarbonates. The results demonstrate the varying behaviour of REEs during precipitation under different conditions, and highlights the influence of dissolved carbonate supply rate to morphology, structure and crystallinity of the products. The occurrence of only one class of REE mineral (carbonate, fluoride or fluorocarbonate) in the synthetic experiments with fluoride may help explain the lack of natural REE carbonates and fluorides - and predominance of REE fluorocarbonates - in hydrothermal systems, as was observed in the natural samples studied. In addition, the lack (absence?) of naturally occurring HREE carbonates and fluorocarbonates in the studied carbonatites (and the literature) is suggested to result not from factors such as structural constraints, but instead from the relative crustal abundances of the individual REEs. It is shown that HREE carbonates and fluorocarbonates are valid species under certain conditions, but that these are not likely to occur naturally.