Whole Rock Geochemical Signatures and REE Mineralogy of the Nechalacho Rare-Earth Element Deposit, NWT

English Abstract:

The Nechalacho rare-metal (REE, Y, Nb, Ta, Zr) deposit, located in the Northwest Territories, is hosted by an altered layered nepheline syenite. Three distinct whole-rock geochemical signatures are evident. Two of these signatures have high P, of which, one has high heavy REE (HREE) (type 1) and one has low HREE, but high light REE (LREE) (type 2). The third signature has low P and high HREE, LREE and Zr (type 3). The type 1 and 2 signatures were found to represent abundant xenotime ((Y, HREE)PO₄) and monazite (LREEPO₄), respectively. The type 3 signature represents zircon and a variety of non-phosphate REE minerals, such as allanite, fergusonite ((Y, REE)NbO₄), or bastnäsite (REECO₃F). The results support the hypothesis that the ore mineralogy can largely be predicted from whole rock geochemistry. Phosphates are an important reservoir for the LREE at Nechalacho. Two main textural types of monazite have been recognized; one that is rod shaped and another that is blocky. Xenotime also occurs in various habits, such as rods and anhedral patches. The deposit is divided into Upper and Basal zones, the Basal Zone having a higher HREE/LREE ratio than the Upper Zone. It has been proposed that the LREE were transported from the Basal Zone to the Upper Zone by hydrothermal fluids and precipitated there as LREE minerals. Williams-Jones et al. (2012, Elements, v. 8, p. 355-360) predicted that a series of pulses of aqueous fluid transporting REE and P, passing through a nepheline syenite would precipitate monazite crystals that were progressively more depleted in HREE (relative to the starting fluid) with increasing distance from the source (height in the system). Preliminary LA-ICP-MS data for monazite does not support this model because the concentrations of HREE in monazite increase upwards in the intrusive body, which suggests, along with the textural variability of the phosphates, that the genesis of the LREE mineralization is more complex than the proposed models imply.