Metal trap development and formation mechanisms of clastic-dominated Zn-Pb±Ag mineralization in Paleozoic strata of the Macmillan Pass (Selwyn Basin, Canada)
Bibliographic record
Abstract
The growing global demand for metals needed for the green energy transition has led to renewed exploration efforts. Ancient sedimentary basins contain a number of important resources such as zinc (Zn) and lead (Pb), and sometimes germanium (Ge), gallium (Ga), and indium (In). Some of the largest Zn deposits are called clastic-dominant (CD-type) deposits because they are hosted in clastic rocks like mudstones and siltstones. These deposits are thought to have formed when metal-rich fluids from deep in the Earth’s crust were expelled towards the surface along faults. However, many aspects of this overall model are not well understood. For example, the composition of the ancient metal-rich fluids is largely unknown, along with the mechanisms by which metals are deposited to form economic enrichments in certain locations. The Selwyn Basin (Canada) contains a number of important CD-type deposits that formed hundreds of millions of years ago. The rocks hosting these deposits are now incorporated into the Canadian Cordillera, meaning they have been uplifted and deformed. This can make it challenging to interpret the rocks and understand how the deposit formation (mineralization) occurred. As a result, it is crucial to carefully examine samples that preserve some of the key primary features of the host rocks and the deposits. This project evaluates aspects of the CD-type deposit model using various approaches that utilize two sample sets. 1) Barite- and pyrite-rich samples from the Late Devonian Canol Formation in Canada, which contain no Zn, Pb sulfides and formed at the same time as those rocks containing the deposits at other locations, were used to determine how the ancient environment was before the deposits formed. 2) Mineralized rock samples from a newly discovered CD-type deposit (Boundary Zone, Canada) were utilized to evaluate i) how these deposits formed, ii) the physicochemical properties of the metal-rich fluids, and iii) what essential metals are present. To answer some of the abovementioned questions, these two sample groups were used to make petrographic, mineralogical, and geochemical observations across various scales, from hand specimen to microscopic levels. Data obtained through detailed petrographic and isotopic analyses indicate that the barite and pyrite in the Canol Formation formed during early diagenesis and that biological activity was critical for converting sulfate to sulfide. Similar mineral phases are observed in the samples from the Boundary Zone, where sulfide formed during early diagenesis likely reacted with metal-bearing hydrothermal fluids during an initial stage of ore formation. The first stage is dominated by fine-grained sphalerite formed as layers due to the replacement of quartz (and barite) components of the rocks. A second ore stage consists of several sphalerite types forming in cracks within the same rocks after fracturing. Critical metals, including Ge and deleterious components like Hg, occur in high amounts in the sphalerite from both the mineralized stages. Furthermore, experiments conducted on tiny droplets of fluids trapped within sphalerite and quartz indicate that the mineralizing fluids consist of variable salinity and homogenization temperature ranging from low (around 120 °C) during the early ore-forming stages to high (around 260 °C) at a later period. This suggests that the Boundary Zone deposit formed due to mixing of these fluids at some point. Altogether, this thesis provides significant insights into components of the CD-type deposit model. It shows how vital microbial activities were during the formation of rocks that later host these deposits and also highlights prolonged hydrothermal fluid flow that could form multiple mineralization types. These findings are valuable for exploration strategies in the Macmillan Pass district and similar geological settings.
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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 | 0.002 |
| Science and technology studies | 0.002 | 0.002 |
| Scholarly communication | 0.002 | 0.000 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 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".