Gold and Trace Element Release From Pyrite During Prograde Metamorphism of Carbonaceous Metapelites: Implications for Orogenic Gold Formation
Bibliographic record
Abstract
Abstract The Neoproterozoic carbonaceous metapelites of the Mandhali Formation in the inner Lesser Himalaya contain diagenetic pyrite with above-background levels of Au (>200 ppb) when analyzed by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). These metapelites exhibit progressively metamorphosed zones (from south to north), ranging from the chlorite-muscovite zone (~372°C), to the muscovite ± biotite (~520°C) zone, to the greenschist-amphibolite transition (~570°C, >5 kbar). Within this regional framework, we document a hitherto unknown paragenesis of pyrite growth from diagenesis to peak metamorphism along with a localized phase of contact metamorphism, which predates regional metamorphism. A comparative assessment of pyrite trace element concentrations from the progressive metamorphic zones reveals that >80% of Au and ~65% of As is released from diagenetic pyrite during early chlorite dehydration at ~372°C, that is, in the chlorite-muscovite zone. Further into the muscovite ± biotite zone, at the terminal chlorite breakdown stage (480°–520°C), pyrite still occurs as the major sulfide with subordinate pyrrhotite. However, by this stage, ~93% of Au; ~75% of As; and ≥90% of Mo, Cd, Pb, Sb, W, Cu, Bi, Te, and Tl are released from pyrite. With the transition of pyrite to pyrrhotite (500°–550°C), almost all trace elements are released from pyrite into metamorphic fluids, except Co that is incorporated into the metamorphic pyrrhotite. This documented grain-scale elemental mobility is also reflected in bulk-rock Au assays; that is, mean Au of 9.8 ppb in the chlorite-muscovite zone and 1.1 ppb in the muscovite ± biotite zone. Our results suggest that pyrite recrystallization during chlorite dehydration is relatively more significant than pyrite-pyrrhotite transition in terms of Au and As release, and by extension, orogenic gold formation.
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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 | 0.000 | 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".