CHEMISTRY OF FLUID INCLUSIONS IN THE RARE-ELEMENT-RICH T-ZONE DEPOSIT, THOR LAKE, NWT, CANADA

The world-class Thor Lake rare-element (REE-Zr-Nb-Ga) deposit is hosted by alkaline syenite and granite that belong to the Blatchford Lake Complex, Northwest Territories, Canada. The T-Zone deposit, one of major mineralized bodies at Thor Lake, is a zoned pegmatite that has seen a strong hydrothermal overprint.

Primary (P) and pseudosecondary (PS) fluid inclusion assemblages (FIA) associated with bastnäsite-group minerals (bastnäsite, synchysite, parisite), zircon, columbite, xenotime, thorite, phenakite, anatase and rutile are dominated by liquid-rich, liquid-vapor ± solid inclusions. All solids are trapped solids as they have inconsistent phase ratios, do not dissolve during heating, and have been identified using Raman spectroscopy to be the same as solids in the host quartz (zircon, rutile, and anatase). Raman spectroscopy also reveals that the LV±S inclusions contain minor CO₂, and, in some inclusions, CH₄.

P and PS fluid inclusion assemblages can broadly be classified into two populations: one characterized by moderate salinity (~20 to 25 wt. % NaCl eq.) and low homogenization temperatures (~100 to 150°C) and the other by lower salinity (~ 5 to 10 wt. % NaCl eq.) and higher homogenization temperatures (~250 to 300 °C).

SEM-EDS analysis of decrepitates of the primary inclusions shows that, in both fluid populations, the principal dissolved components are Na and Cl, but that S and REE are present in minor to trace amounts. Fluorine and Ca, which are commonly considered to be important for the transport and precipitation of REE, respectively, were generally not detected, and only were present in trace amounts in a small number of decrepitates.

LA-ICP-MS analysis reveals major-element fluid chemistry that is consistent with the decrepitates. Fluids related to bastnäsite and xenotime show relatively high total REE concentrations (up to 1000 ppm). Those related to xenotime and zircon contain the highest Y and Zr concentrations (up to 30 ppm), respectively. These values are comparable to the few data reported from other intrusion-related mineralizing fluids, including porphyry deposits.

The data indicate that chloride was the main complexing agent for REE and Zr and a model for precipitation that involved mixing with Ca-rich fluids is unlikely to have been important for the assemblages studied.