Rare-Earth Elements in the Nechalacho Deposit, NWT: Hydrothermal Controls on Mineralogy and Fractionation

The Nechalacho rare-metal (REE-Y-Nb-Ta-Zr) deposit, is hosted by a sequence of altered, layered sodic syenites. Three distinct whole-rock chemical signatures are evident. Two of these signatures have high P, of which, one has high light REE (LREE) but low heavy REE (HREE) (type 1), and one has high HREE (type 2). The third signature has low P and high HREE, LREE and Zr (type 3). Type 1 and 2 signatures represent abundant monazite-(Ce) (LREEPO4) and xenotime-(Y) ((Y,HREE)PO4), respectively. Type 3 represents zircon and a variety of non-phosphate REE minerals, such as allanite-(Ce), fergusonite-(Y) ((Y,REE)NbO4), or bastnäsite-(Ce) (REECO3F). The results indicate that the ore mineralogy can in part be predicted from whole-rock chemistry. Phosphates are an important reservoir for the LREE at the Nechalacho deposit. Two main textural types of monazite-(Ce) have been recognized: columnar and equant. These can also be distinguished chemically based on U and Th concentrations. The columnar monazite-(Ce) is associated with an early magnetite-biotite-quartz alteration, whereas the equant monazite-(Ce) is associated with later carbonate-chlorite-fluorite alteration. Xenotime-(Y) also occurs in various habits, such as rods and anhedral aggregates. 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 previously proposed that the LREE were transported from magmatic eudialyte in the Basal Zone to higher in the system by hydrothermal fluids and precipitated there as LREE minerals. A recent geochemical model predicted that a series of pulses of aqueous fluid transporting REE, passing through a P-bearing 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). The REE chemistry of monazite from Nechalacho, obtained using LA-ICP-MS, does not support this model for the origin of LREE mineralization because the concentrations of HREE in monazite show no consistent decrease upward in the intrusive body, which suggests, along with the textural variability of the phosphates and their association with different assemblages, that the genesis of the LREE mineralization was caused by multi-stage alteration and its genesis is more complex than the proposed models imply. Fluid inclusion microthermometry on primary fluid inclusions in quartz and xenotime-(Y) indicate that the alteration fluids had salinities of ~8 to 13 wt.% NaCl equivalent, and temperatures of ~240 to 590 °C, and were different from those that altered the nearby T Zone.