| Summary: | There is limited knowledge regarding the behaviour of naturally occurring bastnaesite [REE(CO3)F] and parisite [Ca(REE)2(CO3)F2] during mineral-fluid interaction. Bastnaesite and parisite comprise the rare earth element (REE) ore of the T-Zone at the Nechalacho Deposit, located in the Northwest Territories, Canada. This study characterizes the bastnaesite and parisite at this location and investigates the dissolution rates of bastnaesite and parisite at near Earth’s surface conditions. Analysis of the ore minerals of the T-Zone by electron microprobe analysis (EMPA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), indicated that the minerals are enriched in light rare earth elements (LREE). Bastnaesite and parisite are intergrown with each other and with fluorite and quartz as determined by scanning electron microscopy (SEM). Batch dissolution experiments were performed with a mixture of bastnaesite, parisite, fluorite and quartz in water to evaluate the effects of pH and grain size on the solubility of the REE minerals. Experiments were run at pH 2, 4 and 9 with three differing starting percentages of bastnaesite, parisite, quartz and fluorite. The dissolution of bastnaesite and parisite is largely pH dependent, with more REE released at acidic conditions than at alkaline conditions. In pH 2 conditions, precipitation of a lanthanide (Ln) fluoride phase is evident through X-ray diffraction (XRD) and SEM analysis. At an initial pH 4, the solution was neutralized within ~60 h to a pH of 7-8, exhibiting that bastnaesite and parisite can neutralize moderately acidic solutions but not highly acidic (pH 2) solutions. Mixed flow through dissolution experiments containing bastnaesite, parisite and quartz were performed at pH 2, 4 and 8.6 conditions. Results revealed that at pH 8.6 the release rate of total REE and Ca is likely non-stoichiometric whereas the release rate of total REEs and Ca at pH 4 is closer to stoichiometric. Formation of LnF3 was evident at pH 2, illustrating the ready ...
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