Hydrothermal Geochemistry and Mineralizing Processes in the T Zone, Thor Lake Rare-element Deposit, Northwest Territories

The Thor Lake rare-metal (Y-REE-Nb-Ta-Zr-Be) deposit, located about 100 kilometers southeast of Yellowknife, Northwest Territories, Canada, is regarded as one of the largest high field strength elements (HFSE, including Nb, Ta, Zr, Hf, Ti, Y, and the lanthanides) deposits and hosted by alkaline granite and syenite. The T Zone deposit, one of the main mineralized zones at Thor Lake, is characterized by HFSE mineralization that has intimate connection to hydrothermal activities. The T Zone at Thor Lake provides an excellent opportunity to assess the mobility and precipitation of HFSE in magmatic-hydrothermal systems.;The T Zone has been identified as a pegmatite that experienced multiple alteration stages. Most HFSE minerals were hydrothermally formed, as indicated by their occurrence in pseudomorphs. The most important precursor minerals for those HFSE-rich and Be-rich pseudomorphs are aegirine and mica-group minerals. The HFSE concentrations in aegirine and mica are not sufficiently high. Therefore, a dissolution-reprecipitation model cannot explain the presence of zircon and other HFSE minerals that pseudomorph aegirine. Rather, the addition of HFSE by hydrothermal fluids is required.;Mass transfer calculations show that HFSE were added or removed during different alteration events. Related to the identified alteration events, the remobilization patterns of HFSE reveal that transport and precipitation of HFSE in the T Zone was likely caused by fluids with different characteristics.;Magmatic and various types of hydrothermal zircon are characterized by different trace element chemistry. Titanium-in-zircon geothermometer was employed to constrain the mineralizing temperatures of the T Zone. The crystallization temperatures for the magmatic zircon from the host granite range from 792 to 1195 °C. Extremely high Ti concentrations in hydrothermal zircon from the T Zone prevent the application of Ti-in-zircon geothermometry.;Primary fluid inclusion assemblages (FIAs) related to precipitation of various HFSE minerals in the T Zone were identified. Microthermometric analysis has revealed that there were likely two populations of fluids in the T Zone. According to fluid inclusion LA-ICP-MS analysis and EDS decrepitate analysis, the fluids responsible for HFSE transport in the T Zone contain significant HFSE, are aqueous and dominated by Na and Cl with trace amounts of CO 2 and CH4.