A Method for Detrital Corundum Characterization in Sediments: Case Study of the Gem Mountain Mine Placer Sapphire Deposit (Rock Creek, Montana, USA)
Bibliographic record
Abstract
Abstract A new, comprehensive approach to characterizing detrital corundum in sediments and placer deposits is outlined and applied to a sample of poorly sorted, mud-supported colluvium rich in rhyolite tuff clasts from the sapphire-producing layer at Gem Mountain Mine, Rock Creek, Montana. High angularity and the predominance of inherited surface features indicate that corundum was not abraded during transport, consistent with deposition by mass wasting. Corundum grain abundances per 10 kg of sediment (rounded to nearest whole number) are as follows: 4–8 mm (1 grain), 2–4 mm (12 grains), 1.0–2.0 mm (10 grains), 0.5–1.0 mm (43 grains), 0.25–0.50 mm (90 grains), 0.125–0.250 mm (480 grains), and <0.125 mm (55 grains; underrepresentation due to poor recovery). The ∼ 50× abundance of corundum in <2 mm size fractions relative to the >2 mm size fraction of potential economic interest suggests that fine-grained corundum may have potential as a pathfinder for gem corundum deposits in heavy mineral surveys. Corundum grains are transparent and divided into four types by surface texture and extent of crystal face development: (1) rounded with frosted surface, anhedral/subhedral; (2) angular with hillocky/irregular surface, anhedral/subhedral; (3) angular subhedral grains with glassy curved faces; and (4) angular, idiomorphic blue grains (all <1 mm). Surface texture Types 1–3 occur in pastel-colored corundum (green, yellowish-green, yellow, colorless) and are the result of dissolution subsequent to euhedral crystal growth. Mineral inclusions in corundum include: pastel corundum – biotite (variable Fe, Mg, Ti, F contents), plagioclase feldspar (andesine-labradorite), rutile, hercynite; blue corundum – alkali feldspar, rutile, zircon, ilmenite, apatite, spinel. Corundum uncommonly occurs in composite grains with biotite, plagioclase, and K-feldspar. Rhyolite occurs as a partial encrustation on one corundum grain. Rock Creek corundum may have originated from partially melted metapelite in a metamorphic basement, and incorporated and transported to the surface by rhyolitic magmas. Due to scant geologic context and an absence of radiometric age data, the proposed genetic model remains highly speculative.
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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 | 0.001 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 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".