Fluid evolution and gold deposition at the Cuiabá mine, Se Brazil: fluid inclusions and stable isotope geochemistry of carbonates
Bibliographic record
Abstract
The gold-bearing quartz-carbonate vein deposits of the Cuiaba Mine, northern sector of the Quadrilatero Ferrifero, State of Minas Gerais, SE Brazil, are hosted by a sequence of banded carbonaceous metasedimentary rocks within tholeiitic basalt flows of the Archean Rio das Velhas Greenstone Belt. A regional greenschist metamorphic grade characterizes rocks in this section of the Belt. Episodes of hydraulic fracturing enhanced the focussing of fluids percolating along the host primary layering, which promoted pervasive carbonatization and sulfidation associated with crosscutting fractures and quartz-carbonate veins/breccias. The gold mineralization is closely related to the sulfiderich zones. Fluid inclusion investigations show that the mineralizing fluids were dominantly aqueous, of low salinity (3-6 wt.% NaCleq.), with variable concentrations of CO (0 - 11.4 mol%), CH 4 (1.5 - 10.2 mol%), subordinate N 2 (0 - 0.7 mol%), and traces of H 2 S/HS'. Carbonates in the carbonaceous host rock display heavier 8 13 C compositions (-4.3 to -0.2 0 / oo ) than carbonates in the gold-related, carbonate-sulfide altered zones and associated veins and breccias (-8.1 to -S.5 o / oo ). The range of carbonate a 18 O values in the carbonaceous host rock (12.8 to 16 o / oo ) is broadly similar to the carbonates in the mineralized zones (10.6 to 15.2 o / oo ), with a slightly larger variation in the latter. Collectively, fluid inclusion and carbon isotope data indicate that: (1) the mineralizing fluid was originally composed of a H 2 O-CO 2 , solution, with significant concentrations of CH 4 added via hydrolysis of the carbonaceous matter during vein-fluid/wallrock interaction; (2) sedimentary carbonates, probably of marine origin, are readily distinguished from those carbonates formed by reaction between CO 2 -bearing vein fluids and wallrocks; (3) gold-related vein-fluid CO 2 was relatively homogeneous in isotopic composition, suggesting equilibration with carbonate and reduced carbon reservoir (e.g. mantle-derived magma); (4) gold deposition may have been triggered by a sharp decrease in f O 2 caused by the CH 4 enrichment of the fluid.
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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 | 0.001 |
| 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) | 0.000 | 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 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".