Tracing gold mobility using in situ Pb isotope geochemistry of early and remobilized sulphide minerals, Meliadine gold district
Bibliographic record
Abstract
The late timing of gold is a characteristic feature of orogenic gold deposits. For many of these deposits, gold and other metals were liberated from the host greenstone belt and/or early sulfides concomitant with the syndeformational and fluid-assisted metamorphic transition of pyrite to pyrrhotite and remobilized into late, high-grade ore shoots. However, this metamorphic-driven gold-upgrading model is usually inferred, at least in part, from mineral textures, which can be equivocal in the absence of geochemical and/or isotopic constraints. Herein, we report new bulk mineral separate and in situ Pb isotope results for minerals in key textures in the Paleoproterozoic Meliadine gold district. Arsenopyrite and pyrite porphyroblasts occur in Paleoproterozoic quartz ± ankerite veins and are enveloped by remobilized pyrrhotite, chalcopyrite, and galena crystals that accompany gold in late, microtextural sites, a texture that forms the basis for the gold-upgrading model. Early, sieve-textured arsenopyrite and pyrite are gold rich and tend to yield the least radiogenic Pb isotope compositions (high 207Pb/206Pb and 208Pb/206Pb ratios), suggesting these relict domains escaped complete recrystallization during later reworking. Late, inclusion-free arsenopyrite and pyrite overgrowths scatter to more radiogenic Pb isotope compositions (low 207Pb/206Pb and 208Pb/206Pb ratios) and are typically gold poor, which we attribute to reworking and precipitation from a younger and more radiogenic fluid prior to and/or during the precipitation of free gold in variably plunging high-grade ore shoots at 1.90 to 1.85 Ga. Remobilized sulfides are the youngest sulfide phases, but locally yield low 206Pb/204Pb and 207Pb/204Pb ratios and, thus spurious old model mp;lt;"ages,mp;gt;" which overlap with the least radiogenic, sieve-textured arsenopyrite/pyrite domains. The isotopic signature of these remobilized sulfides was likely inherited from Pb liberated during reworking of early arsenopyrite and pyrite and a second, crustal Pb source that was introduced during late remobilization. Our results highlight the importance of element recycling from early sulfides and/or the Neoarchean deposit host rocks during significantly younger Paleoproterozoic reworking and gold remobilization. The in situ Pb isotope results of early and remobilized sulfides document the transition of gold from source to sponge to sink, which is likely a feature that occurred during the formation of other orogenic-style gold deposits.
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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 | 0.001 | 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".