Gold in the Farallones Block of the Shale-Hosted, Clastic-Dominated Castellanos Zinc-Lead Deposit (Northwest Cuba)
Bibliographic record
Abstract
The Zn-Pb ores of the Castellanos shale-hosted, clastic-dominated deposit in northwest Cuba average nearly 1 g/t Au, with local maximum concentrations up to 34 g/t Au. This deposit is stratiform with respect to the bedding in the host black shales and shows a bottom to top zoning of ore assemblages made up of a stockwork underlying the main orebody, a basal pyrite-rich zone and a disseminated to massive Zn-Pb ore zone capped by a discontinuous, thin barite-rich zone. Petrographic data and textural relations allow distinguishing five textural types of pyrite (framboidal Py I, colloform Py IIa, euhedral Py IIb, massive Py IIc and banded colloform Py III) successively formed during ore deposition. The main Zn-Pb ore formed after the crystallization of disseminated, sedimentary framboidal pyrite (Py I) in black shales by the superimposition of several crystallization events. The crystallization sequence of the main ore-forming stage evolved from the precipitation of colloform sphalerite and pyrite (Py IIa) with skeletal galena and interstitial dolomite-ankerite to similar ore assemblages but showing subhedral to euhedral crystal habits (Py IIb) and interstitial calcite-rich carbonates. This stage ended with the development of massive pyrite (Py IIc), mainly occurring at the base of the stratiform orebody. A late fracturing stage gave way to the development of a new generation of colloform banded pyrite (Py III) just preceding the crystallization of early barite. Au is mainly concentrated in pyrite showing variable contents in the different textural types of pyrite and a bottom to top enrichment trend. Minimum contents occur in massive pyrite (Py IIc) from the basal pyrite-rich zone (0.18 ppm Au average), increasing in pyrite IIa (from 0.29 to 2.86 ppm Au average) and in euhedral pyrite (Py IIb) (from 0.82 to 9.02 ppm Au average), reaching maxima in colloform banded pyrite (Py III) formed just before the crystallization of early barite at the top of the orebody. Au enrichment in pyrite correlates with that of Sb (0.08–4420 ppm), As (0.7–35,000 ppm), Ag (0.03–1560 ppm) and to a lesser extent Cu (3–25,000 ppm), Ni (0.02–1600 ppm) and Mn (0.6–5030 ppm). Au deposition should have taken place by oxidation and, probably cooling, of reduced (H2S-dominated) fluids buffered by organic matter-rich black shales of the host sedimentary sequence. The input of such reduced fluids in the ore-forming environment most probably occurred alternating with that of the main oxidized fluids which leached Zn and Pb from the large volume of sandstones and siltstones making up the enclosing sequence, thus being responsible for the precipitation of the majority Zn-Pb ore. Supply of Au-carrying reduced fluids might progressively increase over the course of ore formation, reaching a maximum at the beginning of the late fracturing stage. This evolution of Au supply is consistent with the early crystallization of barite since Ba can also only be transported at low temperature by highly reduced fluids. These results highlight the potential of medium-sized, shale-hosted, clastic-dominated deposits to contain economic (by product) Au amounts and show that ore-forming fluids can change from oxidized (SO42+ dominated) to reduced (H2S-dominated), and vice versa, throughout the evolutionary history of a single deposit.
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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 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.003 | 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".