High grade ores of the Onverwacht platinum pipe, eastern Bushveld, South Africa
Bibliographic record
Abstract
ABSTRACT The platiniferous dunite pipes are discordant orebodies in the Bushveld Complex. The Onverwacht pipe is a large body (>300 m in diameter) of magnesian dunite (Fo80–83) that crosscuts a sequence of cumulates in the Lower Critical Zone of the Bushveld Complex. In a pipe-in-pipe configuration, the main dunite pipe at Onverwacht hosts a carrot-shaped inner pipe of Fe-rich dunite pegmatite (Fo46–62) which comprises the platinum-bearing orebody. The latter was ca. 18 m in diameter and a mining depth of about 320 m was reached. In the present work, a variety of ore samples were studied by whole-rock geochemistry, including analyses of platinum group elements, ore microscopy, and electron probe microanalysis. Olivine of the ore zone displays considerable chemical variation (range 46–62 mol.% Fo) and may represent either a continuum, or different batches of magma, or vertical or horizontal zonation within the ore zone. Chromite is principally regarded to be a consanguineous component of the pipe magma that crystallized in situ and simultaneously with olivine. The Onverwacht mineralization is Pt-dominated (>95% of the platinum group elements) and the ore is virtually devoid of sulfides. Platinum-dominated platinum group minerals predominate, followed by Rh-, Pd-, and Ru-species. Pt-Fe alloys are most frequent, followed by Pt-Rh-Ru-arsenides and -sulfarsenides, platinum group element antimonides, and platinum group element sulfides. Our hypothesis on the genesis of the Onverwacht pipe and its mineralization is as follows: After near-consolidation of the layered series of the Critical Zone, the magnesian dunite pipe of Onverwacht was formed by upward penetration of magmas that replaced the existing cumulates initially by infiltration, followed by the development of a central channel where large volumes of magma flowed through. Fractional crystallization of olivine within the deeper magma chamber and/or during ascent of the melt resulted in the formation of a consanguineous, residual, more iron-rich melt. This melt also contained highly mobile, supercritical, water-bearing fluids and was continuously enriched in platinum group elements and other incompatible elements. In several closing pulses, the platinum group element-enriched residual melts crystallized and sealed the inner ore pipe. Crystallization of the melt resulted in the coeval formation of Fe-rich olivine, chromite, and platinum group minerals. The non-sulfide platinum group element mineralization was introduced in the form of nanoparticles and small droplets of platinum group minerals, which coagulated to form larger grains during evolution of the mineralizing system. The suspended platinum group minerals acted as collectors of other platinum group elements and incompatible elements during generation and ascent of the melt. With decreasing temperature, the platinum group mineral grains annealed and recrystallized, leading to the formation of composite platinum group mineral grains, complex intergrowths, or lamellar exsolution bodies. On further cooling, platinum group minerals overgrowing Pt-Fe alloys formed by reaction of leached elements and ligands like Sb, As, and S mobilized by supercritical magmatic/hydrothermal fluids. Redistribution of platinum group elements/platinum group minerals apparently only occurred on the scale of millimeters to centimeters. Finally, surface weathering led to the local formation of platinum group element oxides/hydroxides by oxidation of reactive precursor platinum group minerals.
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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 | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| 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".