The Formation and Preservation of the Witwatersrand Goldfields, the World’s Largest Gold Province
Bibliographic record
Abstract
Abstract The Witwatersrand goldfields in South Africa have been, and promise to remain, the world’s largest goldproducing province, at leat for the medium-term future. The majority of gold occurs, together with pyrite, uraninite, and local bitumen, on degradational surfaces of fluvial conglomerates that were laid down in the 2.90 to 2.84 Ga Central Rand basin. Integration of recent data on the tectonic evolution of the Kaapvaal craton with data on sediment provenance and paleocurrent directions indicate a strong tectonic control on sediment supply from vanished sources, whose equivalents are now exposed mainly in the Amalia-Kraaipan and Murchison granitoid-greenstone belts to the west and north, respectively. Sedimentation took place first in a foreland setting relative to collision between the Witwatersrand and Kimberley crustal blocks, with a subsequent change to a retroarc position in consequence of oceanic-basin closure to the north of the craton. Most of the gold is located within postdepositional microfractures and hydrothermal phases. Microtextural, mineralogical, geo-chemical, and isotopic data indicate that this hydrothermal gold was derived from the local mobilization of detrital particles, supporting a modified paleoplacer model for the genesis of these deposits. The distribution of heavy minerals with which gold is associated, the correlations between elements of detrital minerals, and the isotopic characteristics of uraninite, pyrite, and gold point to erosion of both granitic and mafic and/or ultramafic rocks in highly variable proportions, with the latter having contributed most of the detrital gold. The global and temporal distribution of orogenic gold deposits and Witwatersrand-type paleo-placer deposits suggests that extraction of gold from the mantle into the continental crust has decreased considerably since Mesoarchean times, probably because of decreased mantle heat-flux and partial melting rates. The apparent uniqueness of the Witwatersrand, in terms of total gold content, can be explained by a combination of intense reworking of sediments derived from a rapidly exhumed Paleo- to Mesoarchean greenstone-dominated hinterland by active braided streams in the absence of vegetation and intense chemical weathering under an acidic, reactive Archean atmosphere. Preservation resulted in the first instance from basin capping by a thick pile of extensive 2.71 Ga flood basalts. Discovery of Witwatersrand-style gold deposits in ter-ranes younger than Paleoproterozoic is highly unlikely. Only older foreland and/or retroarc basin fills on stable cratons with a significant greenstone component are considered good targets for future exploration. The expected gold content in Eoproterozoic source rocks is at least an order of magnitude less than in their Archean counterparts.
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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 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.004 | 0.005 |
| Scholarly communication | 0.002 | 0.001 |
| Open science | 0.000 | 0.002 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.007 | 0.001 |
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".