Regional Geochemistry of Tertiary Igneous Rocks in Central Chile: Implications for the Geodynamic Environment of Giant Porphyry Copper and Epithermal Gold Mineralization
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Bibliographic record
Abstract
The giant porphyry copper-molybdenum deposits of central Chile formed within a thick sequence of Cretaceous \nto Pliocene volcanic rocks. The predominantly calc-alkaline basaltic andesites of the Cretaceous Las \nChilcas Formation are characterized by La/Smn ratios of 1.8 to 2.5 and Sm/Ybn of 1.8 to 2.8. The upper \nOligocene to lower Miocene Abanico Formation (variously defined as the Los Pelambres, Abanico, or Coya \nMachali Formations) ranges from basalt to rhyolite in composition and exhibits a broad southward transition \nfrom calc-alkaline to tholeiitic. All samples from this formation are characterized by LREE enrichment and \nmoderately, or locally strongly, fractionated HREE (La/Smn = 1.3-4.1; Sm/Ybn = 1.5-5.8). The basaltic andesites \nand andesites of the middle Miocene Salamanca Formation have REE chemistry similar to that of the \nUpper Cretaceous strata (La/Smn = 1.5-2.7; Sm/Ybn = 1.6-2.7). The overlying middle Miocene Farellones Formation \nranges from tholeiitic to calc-alkaline and from basalt to andesitic and has a similar LREE enrichment \nand fractionated HREE (La/Smn = 1.7-2.5; Sm/Ybn = 1.7-3.3). The Pliocene La Copa Rhyolite Complex, however, \nis strongly LREE enriched and HREE depleted (La/Smn = 3.8-3.9; Sm/Ybn = 4.2-4.7). Overall, the trace \nelement geochemistry of the Cretaceous to middle Miocene volcanic rocks is characterized by enriched LREE \nand negative Nb anomalies, consistent with an arc setting, with only minor differences in the abundance of \nmost elements. Crustal thickening during the Miocene in central Chile has been suggested to have been responsible \nfor a transition from an amphibole- to garnet-dominated residual mineralogy resulting in the release \nof fluids that enabled the formation of giant copper porphyry deposits. However, the gradual increase in La/Yb \nthrough the early, middle, and late Miocene reported in earlier studies and interpreted to be a response to \ncrustal thickening is not observed in the regional data. Instead, a rapid change in the geochemical signature between \nthe end of the eruption of the Farellones Formation and the eruption of the high La/Yb La Copa Rhyolite \nComplex implies a more abrupt change in the tectonic environment. Isotopic data broadly support this, \nalthough lower epsilon Nd 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 Fernandez Ridge may have been the key geodynamic process responsible for the genesis of the three middle Miocene to \nlower Pliocene giant porphyry copper deposits in central Chile, possibly by promoting crustal-scale faulting and \neven acting as a source of metals.
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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) | 0.002 | 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 it