The nature and origin of the Nechalacho rare metal deposit (REE, Nb, Zr), Northwest Territories, Canada
Bibliographic record
Abstract
The Nechalacho REE-Nb-Zr deposit at Thor Lake, Northwest Territories, Canada, consists of layers of hydrothermally altered eudialyte syenite. With combined measured, indicated and inferred resources of 269 Mt grading 1.47 wt. % REE-oxide, 0.35 wt. % Nb2O5 and 2.59 wt. % ZrO2, the deposit is one of the largest REE deposits in the World. The principal primary magmatic REE-Nb-mineral is kentbrooksite (eudialyte). Strong hydrothermal alteration led to its replacement by fergusonite-(Y), samarskite-(Y), columbite-(Fe) and -(Mn), xenotime-(Y), monazite-(Ce), bastnäsite-(Ce), parisite-(Ce), synchysite-(Ce), allanite-(Ce) and zircon. The host aegirine-nepheline syenite was metasomatically replaced by a phlogopite-quartz-magnetite-ankerite-fluorite assemblage. Peripheral pegmatites, the T-, R- and S-Zones, contain high-grade Li, Be, REE and Nb mineralization. The host to the Nechalacho REE-Nb-Zr deposit is an agpaitic nepheline syenite intrusion, the Nechalacho Layered Suite, which crosscuts the anorogenic 2176 Ma Blatchford Lake Igneous Complex. The laccolithic intrusion has a diameter of ~1.9 km, is >1.1 km thick and displays modal layering of nepheline, sodalite, end-member aegirine and annite, K-feldspar, albite and eudialyte. A U-Pb zircon concordia age of 2176.0 ± 2.7 Ma shows that the nepheline syenites are contemporaneous with the other units of the complex. The initial εNd(2176 Ma) was between 1.1 and 3.1, averaging 2.0, and δ18Omelt and δDmelt values calculated from magmatic minerals are 5.1 to 7.8‰ and 60‰, respectively. In the earlier alkaline granite and quartz syenites of the complex (Grace Lake Granite, Thor Lake Syenite, Rim Syenite), δ18Omelt and δDmelt were 6.7 to 8.1‰ and -99 to 93‰, respectively. Combined with whole-rock trace element profiles, this indicates that the units of the complex formed from melt batches extracted from a deep crustal feeder magma chamber in which a mantle-derived, heterogeneous mafic parental melt, that was similar to modern ocean island basalt, underwent fractional crystallization. The Nechalacho Layered Suite formed from the youngest and most evolved batch, which was magnesium-free and enriched in Zr, REE, Nb, carbon species, fluorine and chlorine. Mineral equilibria indicate that at ~800 to <500 °C and a pressure of ~4 kbar, oxygen fugacity was initially below FMQ+0.2 to FMQ+1.3, and later increased in response to auto-oxidation of the melt by annite crystallization. In situ fractional crystallization proceeded upwards and downwards and REE, Nb and Zr were enriched in an iron-rich residual melt until the magma became saturated with respect to eudialyte. Mineral fractionation led to variations in light/heavy REE, Zr/Hf and Nb/Ta, with the Basal Zone recording the strongest heavy REE enrichment. Magmatic-hydrothermal auto-metasomatism associated with ankerite-bastnäsite-(Ce) mineralization is indicated by trends of δ13C from -6.3 to 2.7‰ and of δ18O from 11.3 to 15.0‰ for hydrothermal carbonates. Ankerite from the T-Zone has similar values, demonstrating a genetic link to the peripheral pegmatites at the magmatic-hydrothermal interface. U-Pb and Th-Pb ages of hydrothermal monazite-(Ce) from the Basal Zone (1871 ± 14 to 1835 ± 57 Ma) indicate a late regional hydrothermal overprint associated with faulting along the southern margin of the Slave Craton. Hydrothermal temperatures determined using oxygen isotopes are ~300 °C and the fluids had δ18O of 3.1-9.3‰ and δD of 24‰, suggesting a crustal origin. Mineral equilibria indicate that the alteration fluid was reducing (FMQ+2.0 to +3.7) and that pH was moderately acidic to mildly alkaline (between 3.7 and 5.7). Likely, the alteration fluid was initially acidic and neutralized upon reaction with the nepheline syenites. This external crustal fluid was Mg-bearing and caused the in situ replacement of eudialyte by multiple REE-Nb minerals and phlogopite-magnetite-quartz alteration.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.002 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".