Genesis of Sublayer, Footwall Breccia, and Associated Ni-Cu-Platinum Group Element Mineralization in the Sudbury Igneous Complex
Bibliographic record
Abstract
Abstract The Ni-Cu-platinum group element (PGE) sulfide mineralization in the 1.85 Ga Sudbury impact structure occurs in two discrete environments: (1) mineralization along or near the basal contact, including disseminated to semimassive mineralization in an inclusion-rich sublayer, semimassive to fragmental mineralization in the underlying anatectic footwall breccia, and veins and disseminations in the underlying pseudotachylitic Sudbury breccia, and (2) disseminated to semimassive mineralization in the inclusion-bearing quartz diorite phase of radial and concentric offset dikes that extend up to 20 km from the basal contact. The sublayer, inclusion quartz diorite, and footwall breccia all contain abundant mafic-ultramafic inclusions—local xenoliths derived from nearby country rocks, exotic xenoliths derived from unexposed upper-middle crustal target rocks and, locally, anteliths derived from an olivine melanoritic early border phase of the Sudbury Igneous Complex—but the quartz diorite margins of offset dikes contain very few local inclusions and no exotic ultramafic inclusions. The similar inclusion populations indicate a genetic relationship between the sublayer and inclusion quartz diorite and interaction between the sublayer and footwall breccia. But the sublayer is characterized by a cumulate noritic matrix, the inclusion quartz diorite by a noncumulate quartz dioritic matrix, and the footwall breccia by an anatectic felsic-intermediate matrix. The overlying main mass norite is very homogeneous in terms of Hf isotopes, indicating that the impact melt sheet was well mixed, but ores, sublayer, inclusion quartz diorite, and to a lesser degree overlying main mass norites vary widely in their Pb-S-(Os) isotope compositions. The majority of mafic-ultramafic inclusions, except for anteliths, contain no sulfide and exhibit no signature of Ni-Cu-PGE depletion caused by prior sulfide saturation, which indicates that the association between mafic-ultramafic inclusions and Ni-Cu-PGE mineralization is attributable primarily to the refractory nature of the inclusions and to a lesser degree their similar hydrodynamic behavior as the sulfide melt and, secondarily, to derivation of sulfides from the same sources as the inclusions. The very large Sudbury impact event volatilized much of the Pb-S-Zn-Cd-Se-Bi and significant amounts of Sb-Ag-Cu-Au-As from the target rocks. It generated large amounts of superheated impact melt and only minor fragmental debris during the compression and excavation stage, which is when the inclusion- and sulfide-poor marginal phase of the offset dikes was emplaced. Large amounts of debris, including sulfide-bearing Huronian basalts, Nipissing and East Bull Lake intrusions, and Archean mafic gneisses, were generated during isostatic rebound, formation of a central uplift, and collapse of the central uplift and crater walls, which is when the inclusion- and sulfide-bearing internal phase of the offset dikes was emplaced. Convective and gravity flow aided horizontal transport of residual exotic inclusions, local inclusions, and sulfide xenomelts into embayments and funnels to form the protosublayer. Olivine-saturated melts, generated by thermomechanical erosion of local olivine-bearing country rocks, locally crystallized an olivine melanoritic early border phase of the main mass, which was disrupted and preserved in the funnels of some North Range offset dikes. Continued thermomechanical erosion of country rocks enlarged dike funnels and exploited other fractures to generate footwall embayments and significant geochemical and Pb-S-(Os) isotope heterogeneities. As the rate of thermomechanical erosion decreased and the rate of heat conducted into the footwall rocks increased, contact ores and some offset ores fractionally crystallized to form residual melts that generated footwall vein systems.
Fetched live from OpenAlex and de-inverted. Abstracts are not stored in this database: the inverted indexes are 8.6 GB of the frame’s 9.3 GB of text, and the host has 13 GB free.
How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 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) | 0.016 | 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 teacher head, 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".