Regional Geochemistry of Tertiary Igneous Rocks in Central Chile: Implications for the Geodynamic Environment of Giant Porphyry Copper and Epithermal Gold Mineralization
Bibliographic record
Abstract
The giant porphyry copper-molybdenum deposits of central Chile formed within a thick sequence of Cretaceous to Pliocene volcanic rocks. The predominantly calc-alkaline basaltic andesites of the Cretaceous Las Chilcas Formation are characterized by La/Smn. ratios of 1.8 to 2.5 and Sm/Ybn of 1.8 to 2.8. The upper Oligocene to lower Miocene Abanico Formation (variously defined as the Los Pelambres, Abanico, or Coya Machalí Formations) ranges from basalt to rhyolite in composition and exhibits a broad southward transition from calc-alkaline to tholeiitic. All samples from this formation are characterized by LREE enrichment and moderately, or locally strongly, fractionated HREE (La/Smn = 1.3-4.1; Sm/Ybn = 1.5-5.8). The basaltic andesites and andesites of the middle Miocene Salamanca Formation have REE chemistry similar to that of the Upper Cretaceous strata (La/Smn = 1.5-2.7; Sm/Ybn = 1.6-2.7). The overlying middle Miocene Farellones Formation ranges from tholeiitic to calc-alkaline and from basalt to andesitic and has a similar LREE enrichment and fractionated HREE (La/Smn = 1.7-2.5; Sm/Ybn = 1.7-3.3). The Pliocene La Copa Rhyolite Complex, however, is strongly LREE enriched and HREE depleted (La/Smn = 3.8-3.9; Sm/Ybn = 4.2-4.7). Overall, the trace element geochemistry of the Cretaceous to middle Miocene volcanic rocks is characterized by enriched LREE and negative Nb anomalies, consistent with an are setting, with only minor differences in the abundance of most elements. Crustal thickening during the Miocene in central Chile has been suggested to have been responsible for a transition from an amphibole- to garnet-dominated residual mineralogy resulting in the release of fluids that enabled the formation of giant copper porphyry deposits. However, the gradual increase in La/Yb through the early, middle, and late Miocene reported in earlier studies and interpreted to be a response to crustal thickening is not observed in the regional data. Instead, a rapid change in the geochemical signature between the end of the eruption of the Farellones Formation and the eruption of the high La/Yb La Copa Rhyolite Complex implies a more abrupt change in the tectonic environment. Isotopic data broadly support this, although lower εNd values in the Farellones Formation imply a greater role for crustal contamination in younger suites. In the absence of gradual crustal thickening, it is suggested that the subduction of the Juan Fernández Ridge may have been the key geodynamic process responsible for the genesis of the three middle Miocene to lower Pliocene giant porphyry copper deposits in central Chile, possibly by promoting crustal-scale faulting and even acting as a source of metals. © 2005 Society of Economic Geologists, Inc.
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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 | 0.002 | 0.002 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| 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".