Direct measurement of metal concentrations in fluid inclusions, a tale of hydrothermal alteration and REE ore formation from Strange Lake, Canada

Granites and pegmatites in the Strange Lake pluton underwent extreme enrichment in high field strength elements (HFSE), including the rare earth elements (REE). Much of this enrichment took place in the most altered rocks, and is expressed as secondary minerals, showing that hydrothermal fluids played an important role in HFSE concentration. Vasyukova et al. (2016) reconstructed a P-T-X path for the evolution of these fluids and provided evidence that hydrothermal activity was initiated by exsolution of fluid during crystallisation of border zone pegmatites (at similar to 450-500 degrees C and 1.1 kbar). This early fluid comprised a high salinity (25 wt% NaCl) aqueous phase and a CH4+H-2 gas. During cooling, the gas was gradually oxidised, first to higher hydrocarbons (e.g., C2H6, C3H8), and then to CO2, and the salinity decreased to 4 wt% (similar to 250-300 degrees C), before increasing to 19 wt%, due to fluid-rock interaction (similar to 150 degrees C). Here, we present crush-leach fluid inclusion data on the concentrations of the REE and major ligands at different stages of the evolution of the fluid. The chondrite-normalised REE profile of the fluid evolved from light REE (La-Nd)-enriched at high temperature ( similar to 400 degrees C, Stages 1-2a) to middle REE (Sm-Er)-enriched at 360 to 250 degrees C (Stages 2b-3) and strongly heavy REE (Tm-Lu)-enriched at low temperature (150 degrees C, Stage 5). These changes in the REE distribution were accompanied by changes in the concentrations of major ligands, i.e., Cl- was the dominant ligand in Stages 1, 2, 4 and 5, whereas HCO3- was dominant in Stage 3. Alteration of arfvedsonite to aegirine and/or hematite contributed strongly to the mobilisation of the REE. This alteration released middle REE (MREE) and heavy REE (HREE), which either partitioned into the fluid or precipitated directly as bastnasite-(Ce), ferri-allanite-(Ce) or gadolinite-(Y). Replacement of primary fluorbritholite-( Ce), which crystallised from an immiscible fluoride melt and altered to bastnasite-(Ce), was also important in mobilising the REE (MREE). This paper presents the first report of the distribution of the REE in an evolving hydrothermal fluid. Using this distribution, in conjunction with information on the changing physicochemical conditions, the study identifies the sources of REE enrichment, reconstructs the path of REE concentration, and evaluates the REE mineralising capacity of the fluid. Finally, this information is integrated into a predictive model for REE mobilisation applicable not only to Strange Lake but any REE ore-forming system, in which hydrothermal processes were important.