Coexistence of Algoma- and Superior-type BIFs in back-arc systems: Evidence from Xincai, North China Craton
Bibliographic record
Abstract
Superior-type and Algoma-type banded iron formations (BIFs) generally developed in distinct tectonic environments and have unique geological characteristics. Superior-type BIFs were deposited on passive continental shelves or intracratonic basins and are laterally extensive, whereas Algoma-type BIFs were formed near volcanic arcs and spreading centers. Here, we present a case where late Archean (∼2.5 Ga) Superior- and Algoma-type BIFs co-developed in a back-arc setting at Xincai, along the southern margin of the North China Craton. The Algoma-type BIFs at Xincai exhibit high Ge/Si ratios (4.84 to 8.84) and a broad range of εNd(t) values (−7.2 to +1.2). These geochemical signatures, along with similar εNd(t) values (+1.2) in contemporaneous amphibolites, indicate a significant influence of hydrothermal fluids associated with submarine volcanism. In contrast, the Superior-type BIFs at Xincai are characterized by positive La anomalies, low Ge/Si ratios (0.21 to 4.60) and narrow range of εNd(t) values (−6.1 to −1.0) suggesting a major contribution from terrigenous sources. The coexistence of Algoma- and Superior-type BIF types at Xincai is related to the interplay of multiple fluid systems in a back-arc environment. The Algoma-type BIFs formed from the confluence of hydrothermal fluids originating from enriched subcontinental lithospheric mantle (high Ge/Si and low εNd(t)) with those emanating from the adjacent back-arc spreading centers (depleted mantle like—high Ge/Si and εNd(t)). On the other hand, the Superior-type BIFs formed near the continental margins of the back-arc basins, where surface seawater carrying terrigenous substances (low Ge/Si and εNd(t)) mixed with hydrothermal fluids from the back-arc spreading centers. The distinct co-existence of Algoma- and Superior-type BIFs at Xincai underscores the dynamic nature of back-arc environments as crucibles for diverse BIF mineralization processes, offering new perspectives on the contributions of submarine volcanic activity and terrigenous inputs to the BIFs.
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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 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.001 |
| 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".