A fluid inclusion study of the Little Nahanni LCT-Type Pegmatite Group, NWT Canada: implications for the nature and origin of fluids in LCT-Type Pegmatites and Pegmatite evolution
Bibliographic record
Abstract
The Cretaceous (82 Ma) Little Nahanni Pegmatite Group , Canada, is a 15 km long swarm of LCT-type pegmatite dikes that intrude Precambrian to Lower Cambrian siliciclastic metapelites and carbonate rocks. The dykes are dominated by a primary assemblage of quartz, spodumene, K-feldspar ± muscovite lacking both graphic textures and core zones, but characterized by combtextured spodumene and K-feldspar intergrowths. Varying degrees of albitization and lesser phyllic alteration accompany ore-grade oxide enrichment (Ta-Nb and Sn, respectively).A comprehensive fluid inclusion study using a wide variety of methods (petrography , CL, microthermometry, SEM/EDS evaporate mound analysis, LA ICP-MS, laser Raman spectroscopy, δ13C) on minerals (quartz, K-feldspar, albite, spodumene, muscovite, garnet, and tourmaline) from barren and mineralized samples indicate the fluid inclusions are secondary and appear to post-date pegmatite crystallization. The presence of secondary fluid inclusions may instead represent entrapment during the late-stage (sub-solidus to post crystallization) metasomatic evolution stage of the pegmatites. Three fluid types occur, and listed in paragenetic order are: (A) an aqueous-carbonic fluid (≤1 wt. % eq. NaCl) with varying XCO2 (0.1 to 0.78), purity (TmCO2= -57o to -61oC), and density; (B) a low-salinity (≤2 wt. % eq. NaCl) aqueous fluid; and (C) an aqueous-methane-nitrogen fluid. SEM-EDS analysis indicates that the fluids are Na (-K) - dominant, but with significant S and enrichment in transition and chalcophile elements. Fluid inclusion extracts have δ13C values (-9.7 to -27.7‰; n=7) which indicates exchange of the fluids with the immediate, carbon-bearing wall rocks occurred. Collectively the data indicate: (1) fluids were trapped during the later, metasomatic stage of pegmatite evolution; (2) fluids were trapped at <2.5 kbars and <350°C with pressure fluctuating between lithostatic and hydrostatic; (3) variable degrees of fluid:rock interaction occurred with the exocontact zone, resulting in varying XCO2and CO2:CH4 values; and (4) metasomatism and rare metal mineralization occurred in the presence of meteoric water. These findings indicate that the LNPG pegmatites are not a closed system, some of the carbonic component of H2O-CO2 fluids is exotic, and that low-salinity fluids of meteoric origin infiltrated during the subsolidus stage of pegmatite evolution.
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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 | 0.002 | 0.001 |
| Science and technology studies | 0.002 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 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".