From bacterial to thermochemical sulfate reduction: Sulfur isotope constraints on the genesis of hyper-enriched black shales
Bibliographic record
Abstract
Hyper-enriched black shales (HEBS) are thin stratiform and stratabound sulfide-rich (~70 vol%) units that have percentage level concentrations of metals such as Ni, Mo and Zn. In the Peel River area of Yukon, Canada, HEBS are highly enriched in Ni (~3 wt%), Zn (~0.2 wt%), and Mo (0.2 wt%) hosted mainly by millerite (NiS), sphalerite (ZnS) and pyrite (FeS 2 ). The processes that control millerite-sphalerite precipitation in HEBS are poorly constrained. Here, we report the first in-situ sulfur isotope measurements of sulfide minerals in the Peel River HEBS. Early diagenetic pyrite (δ 34 S -31.9 ± 9.0 ‰) formed in response to open-system bacterial sulfate reduction (BSR), whereas later sphalerite (16.5 ± 5.1 ‰) and millerite (+10.0 ± 1.4 ‰) crystallized via thermochemical sulfate reduction. We have developed a three-stage model for HEBS formation. Stage 1 involves high primary productivity, a redox-stratified water column, a low sedimentation rate, and an active Fe-oxide shuttle, which promoted BSR-driven pyrite precipitation. Organic matter scavenged Ni and Zn from the water column and pore-water, producing metal-rich kerogen. Stages 2 and 3 occur during late diagenesis, with the infiltration of an acidic, oxidized, metal-poor, sulfate-bearing brine that interacted with the metal-bearing kerogen, triggering thermochemical sulfate reduction and millerite-sphalerite precipitation. Subsequent fracturing and increased fluid-flow during peak hydrocarbon generation caused acid-driven replacement of sphalerite by a second generation of millerite. This model accounts for HEBS formation in both Canada and China, demonstrating that, although late-stage fluids did not transport the metals, brines were essential to their genesis.
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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 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.008 | 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".