Controls on the Ore Fertility of Continental Arc Magmas: New Constraints from Silicate Melt Inclusions
Bibliographic record
Abstract
Upper-crustal magmatic-hydrothermal systems associated with volcanic arcs form the globally important porphyry-type Cu-Mo-Au deposits. However, the overall scarcity of these ore deposits has generated considerable debate regarding the tectono-magmatic conditions that may favour ore genesis. The composition of magmas from which ore fluids are released has received special attention, leading to the controversial concept of unusually ‘fertile’ magmas. Fueling this controversy is the traditionally heavy reliance on whole rock analyses, which are incapable of fully constraining the complex evolution of magmatic reservoirs, and also the inherent challenge of constraining magmatic processes operating at increasing depths. This thesis addresses these limitations by presenting novel data including the concentrations of key ore-related elements (S, Cl, Cu, Mo, Ag, Au, Pt) from silicate melt inclusions (SMI) trapped in phenocrysts from fertile and barren arc magmatic systems, which offer unique records of the silicate melt composition during magma differentiation. At the giant Bingham Canyon deposit, SMI data support a magmatic evolution dominated by the accumulation of co-genetic hydrous and oxidized magmas. This contrasts with previous models involving important contributions to the magmatic ore-fluid element budget by mafic alkaline magmas, and to the ore fluids by magmatic sulfide decomposition. In the Andean Southern Volcanic Zone (SVZ), mafic SMI establish a baseline compositional variation through the arc, and for continental arcs in general. The thicker-crust fertile northern SVZ has lower chalcophile element concentrations but higher S and Cl concentrations than the barren south. These differences are attributed to contrasting extents of deep crustal degassing and magmatic sulfide saturation. As differentiation proceeds to felsic compositions, SMI from different volcanoes exhibit different co-variations in volatile and ore metal concentration. Numerical modelling supports the interpretation that in at least one location, the magma was saturated in fluids with similar compositions to those measured in natural barren and mineralized hydrothermal systems. The findings of this thesis offer new support for a paradigm of magmatic fertility wherein magmas with ‘typical’ ore metal endowments are capable of forming porphyry deposits, with greater emphasis on the availability of magmatic volatile elements and local conditions permitting focused fluid flow in the subsurface ore-forming regime.
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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) | 0.001 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.000 | 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".