ARSENOPYRITE MELTING DURING METAMORPHISM OF SULFIDE ORE DEPOSITS
Bibliographic record
Abstract
Arsenopyrite is present as a minor phase in many different types of ore deposits. Here we investigate a number of ore deposits metamorphosed to mid-amphibolite facies and above to show that in some environments, arsenopyrite is likely to melt during metamorphism, but in others it will persist until it is converted to lollingite + pyrrhotite. The fate of arsenopyrite is governed by the sulfur fugacity imposed by the surrounding mineralogy during metamorphism. At the Hemlo gold deposit, Canada, which contains a range of disseminated sulfi des, the breakdown of barite promoted conditions of high f(S2) and melting of arsenopyrite during prograde metamorphism. On the other hand, at several massive sulfi de deposits including Osborne Lake, Montauban and Geco in Canada, high f(S2) conditions were instead generated through pyrite breakdown on the pyrite–pyrrhotite buffer, also causing arsenopyrite to melt in favorable parts of the deposits. In contrast, metamorphic processes that inhibit high f(S2) through consumption of sulfur promote the solid-state conversion of arsenopyrite to lollingite and pyrrhotite rather than melting. In most mineral deposits, the strongest infl uence on sulfur fugacity is the pyrite-to-pyrrhotite reaction, which buffers f(S2) to increasingly elevated values as temperature increases. Once pyrite is consumed, however, f(S2) no longer is maintained at elevated values. If rocks hosting arsenopyrite are able to conserve pyrite to middle-amphibolite-facies conditions (beyond 491°C at 1 bar, or ~560°C at 5 kbar), arsenopyrite melting will occur. If not, arsenopyrite melting is unlikely, though still possible. Of the mechanisms that promote pyrite decomposition at metamorphic conditions below arsenopyrite melting, sequestration of sulfur by iron silicates or oxides (or both) to form pyrrhotite may be the most effective in many types of deposit. At the Calumet deposit and in some parts of the Geco deposit, this process was found to be effective in converting pyrite to pyrrhotite in magnetite-rich rocks. Pyrite consumption and low-f(S2) conditions are also promoted to a small extent by incorporation of sulfur in hydrothermal fl uids, such as an introduced fl uid or those generated by dehydration reactions, this effect becoming more signifi cant as temperature rises. Deposits where arsenopyrite is likely to melt during metamorphism include pyrite-rich massive sulfi de deposits as well as disseminated deposits lacking abundant iron silicates and oxides. The As-rich melts that result are highly effective in incorporating and mobilizing other metals, particularly gold and silver, as demonstrated at the Challenger deposit (Australia) and at Hemlo, Ontario.
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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 | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 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".