FLUID INCLUSION STUDIES OF RARE EARTH ELEMENT DEPOSITS

Most rare earth element (REE) deposits are associated with alkaline magmatism, including carbonatites, alkaline granites, and nepheline syenites. The deposits themselves are diverse in terms of the style of mineralization (vein vs disseminated), mineralogy, and the spatial relationship to the parent pluton. The role of fluids in these deposits varies from the in situ alteration of prexisiting magmatic concentrations of REE minerals to vein deposits and replacement bodies that are external to plutons. The environment of hydrothermal deposition and the scale of mobilization of the REE facilitated by fluids are thus quite variable.

Compared to many types of ore deposit, the number of fluid inclusion studies on REE deposits is limited. Salinities determined from microthermometric analysis are variable and mostly range from 6 to 28 wt % NaCl eq. A notable exception is the early fluids at the Bayan Obo deposit in China, which are between 1 and 5 wt % NaCl eq. Halite-saturated inclusions are not common, as are vapour-rich inclusions, suggesting that the formation depths and temperatures were sufficiently high that conditions were above the liquid-vapour solvus. Carbon dioxide contents cover a wide range, with some systems containing no detectable CO₂, and others, such as Bayan Obo, containing substantial CO₂. Methane has rarely been reported, with one notable exception: the Strange Lake Deposit, where methane and higher hydrocarbons are present in inclusions in granite pegmatite, and which have been explained through Fischer-Tropsch reactions catalyzed by magnetite. Leachate and laser-ablation ICP-MS analyses indicate total REE concentrations in the fluids of up to ~ 1000 ppm. Homogenization temperatures are variable, ranging from about 100 to over 600 °C, although most are less than 400 °C. In a number of systems there is evidence for a variety of fluids with different salinities and temperatures indicating that the fluid infiltration history was episodic and complex. The compositional and thermal characteristics of these deposits are diverse, which probably reflects the range of magmatic associations, from carbonatites to alkaline granites to nepheline syenites, and the likely involvement of non-magmatic fluids of varied provenance.