40 Ar/ 39 Ar geochronology and Pb isotopic evidence for the role of ridge subduction in generating orogenic gold mineralization in the Otago Schist Belt, New Zealand
Bibliographic record
Abstract
Orogenic gold mineralization in the Otago Schist Belt (OSB) of South Island, New Zealand, occurs as synto late tectonic veins and disseminations in brittle-ductile shear zones (e.g., Hyde-Macraes, Rise&Shine), as well as in late and posttectonic fissure veins. Gold is hosted within lower to upper greenschist facies metaclastic rocks of the Otago Schist that comprise part of a long-lived (Triassic to mid-Cretaceous) accretionary complex that developed along the active margin of Gondwana. Orogenic gold mineralization in the OSB is thought to have formed from fluids produced during prograde dehydration reactions within the Otago Schist at depth that also liberated and mobilized S and metals (including Au). Ar/Ar dating of muscovite within veins at the Macraes Mine (Hyde-Macraes shear zone) and the Glenorchy District yield ages in the range of 135-141 Ma, and are interpreted to represent the age of veining. Previous Ar/Ar dating of muscovite from the Rise&Shine shear zone and several late gold-bearing fissure veins elsewhere in the OSB suggest ages for that mineralization of ~101-106 Ma. Pb isotopic analyses of 48 sulphide separates from all of the various styles of gold mineralization within the OSB define two distinct clusters: a less radiogenic cluster of analyses that includes samples from Macraes and Glenorchy, and a more radiogenic cluster that includes samples from the Rise&Shine shear zone and other veins that have yielded younger Ar/Ar ages. Together the age and Pb isotopic data suggests that gold mineralization in the OSB formed during two discrete and relatively short-lived pulses, at ~135-141 Ma and ~101-106 Ma. The highly episodic nature of the gold mineralization argues that hydrothermal activity that produced orogenic gold mineralization in the OSB was not simply part of a steadystate process related to the normal development of an accretionary complex, but rather was related to specific tectonic events that affected the wedge. Age and geochemical studies of the associated arc complex suggest that a ridge subduction event occurred under the OSB at ~135-140 Ma, and reconstructions of probable plate motions in the region appear to require a younger ridge subduction event at ~100 Ma. We suggest that gold mineralization in the OSB was related to two discrete ridge subduction events, analogous to gold mineralization in the Chugach accretionary complex in southern Alaska. Zircon U-Pb age discordance and trace element alteration due to deep, post-impact flow: Implications for planetary chronology D.E. MOSER, W.J. DAVIS, S. REDDY, R.L. FLEMMING AND R.J. HART Dept. of Earth Sciences, Univ. of Western Ontario, London, Ontario, Canada N6A 5B7 (desmond.moser@uwo.ca) Geological Survey of Canada, 601 Booth St., Ottawa, Canada, K1A 0E8 (bidavis@NRCan.gc.ca) Dept. of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia (s.reddy@curtin.edu.au) iThemba labs, Johannesburg, South Africa (hart@tlabs.ac.za)
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 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 | 0.000 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.004 | 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".