SELENIUM AND SULFUR CONCENTRATIONS IN COUNTRY ROCKS FROM THE DULUTH COMPLEX, MINNESOTA, USA: IMPLICATIONS FOR FORMATION OF THE Cu-Ni-PGE SULFIDES
Bibliographic record
Abstract
Various studies suggest that the formation of a magmatic Ni-Cu-platinum group element (PGE) sulfide deposit requires addition of S from the country rocks into the magma. Many sulfide deposits have S/Se ratios higher than primary magmas, which is attributed to the preferential addition of S from the country rocks. However, this has not been clearly demonstrated because most studies do not determine Se in the country rocks, partly because, although Se concentration in igneous rocks can be assessed by most routine analytical techniques, their detection limits are too low for measuring with precision the level of Se in sedimentary rocks. We have determined Se abundances, using thiol cotton fiber (TCF) combined with instrumental neutron activation analysis (INAA), S, and delta (super 34) S on the magmatic sulfides of the Duluth Complex and on the host-rock sediments of the Virginia Formation. Our data show that the pelitic and metapelitic rocks of the Virginia Formation are characterized by low S/Se ratios ( approximately 3,000), close to the mantle values, whereas the S-rich layer known as the bedded pyrrhotite unit has high S/Se ratios ( approximately 20,000). Consequently, most of the Virginia Formation sediments and xenoliths are unsuitable as a source of S because they have S/Se ratios and delta (super 34) S values lower than those of the mineralized rocks (<10,000). Only the bedded pyrrhotite unit, which is present at the contact with the intrusion and as partly digested xenoliths within the intrusion, has S/Se ratios and delta (super 34) S values higher than those of the magmatic sulfides and could thus be the source of the S. Furthermore, the presence of sulfides in the norites around the xenoliths suggests that the S was released from the xenolith during melting of the xenolith. Detailed sampling from the xenoliths into the norite shows that the S/Se ratio and delta (super 34) S decrease with distance from the xenolith. Our results favor a mechanism of crustal S contamination by in situ assimilation of bedded pyrrhotite unit xenoliths.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 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 teacher head, 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".