Gold Deposits Related to Reduced Granitic Intrusions
Bibliographic record
Abstract
Abstract The role of intrusions in the formation of many types of gold deposits has been widely debated. Magmatic- hydrothermal processes are proposed by some authors for gold-rich porphyry systems, while many other authors claim that intrusions only serve as convective heat engines or structural hosts for mesothermal gold deposits. Based largely on well-documented examples from Alaska and the Yukon Territory, we recognize a class of deposits that display a spatial and temporal relation to reduced (low fO2) granitoids. Most intrusion-hosted and intrusion-proximal deposits and prospects of this class display a consistent and striking Au-Bi-Te-As (W,Mo,Sb) metal association; evidence for a series of alteration and mineralization events spanning a significant range of temperatures (>500°–<300°C); a consistent pattern of early feldspathic (albite and/or K feldspar) alteration and younger sericite-carbonate alteration; evidence for change in sulfidation state from well below pyrite-pyrrhotite (early) to pyrite-arsenopyrite (late); and the presence of high-CO2 and/or high-salinity fluid inclusions. Fluid inclusion and other geobarometric data indicate formation over a wide range of pressures. Deposits that formed at pressures >1 kbar generally lack evidence for rapid magmatic water exsolution (e.g., porphyritic textures, random stockworks, magmatic breccias). Deposits horizontally and vertically distal from intrusions within a given belt or district typically lack the strong Au-Bi association of proximal deposits and contain higher As, Sb, and base metals and only yield evidence for relatively low-temperature and low-salinity fluids. These latter types possess some characteristics of orogenic (mesothermal), epithermal, or Carlin-type deposits; however, their spatial and temporal association with the higher-temperature deposits suggests a common origin. Although the clearest examples of these reduced intrusion-related deposits (e.g., Fort Knox, Alaska; Dublin Gulch, Yukon Territory) are only of moderate size (<150 tonnes Au), worldwide deposits of apparently similar character and origin host major amounts of gold. Significant global examples include Mokrsko in the Czech Republic, Kidston in Australia, and possibly the world-class Murantau deposit in Kazakstan. Belts hosting deposits of this type occur in a variety of continental arc, back-arc, and collisional settings, most of which contain intrusion-related Sn-W deposits. A key characteristic of this class of deposit is the occurrence of a wide variety of mineralization styles, depending on formation depth, distance from the parent intrusion, and structural control. It is likely that new variants of this type of deposit are yet to be found in many areas of the world, an assertion supported by the relatively recent discovery of the unusually high-grade Pogo deposit in interior Alaska.
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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 | 0.000 | 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) | 0.001 | 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".