The Itabirites of the Quádrilátero Ferrífero and Related High-Grade Iron Ore Deposits: An Overview
Bibliographic record
Abstract
Abstract The Quadrilátero Ferrífero district, located on the southern portion of the San Francisco craton in Minas Gerais, Brazil, comprises Archean greenstone terranes of the Nova Lima Supergroup and the Paleoproterozoic cratonic cover sequences of the Minas Supergroup that consist of quartzites, metaconglomerates, phyllites, dolomites, and banded iron formations. The Minas Supergroup was affected by two orogenic events—the Paleoproterozoic Transamazonian-Mineiro (2.1–2.0 Ga) orogeny and the Neoproterozoic to Early Paleozoic Brasiliano-Araçuaí (0.65–0.50 Ga) orogeny, resulting in complex deformation and metamorphic grades that increase from greenschist facies in the West to amphibolite facies in the East. Metamorphosed iron formations, referred to as itabirites, are found in three compositionally distinct lithofacies, namely quartz itabirite, dolomitic itabirite, and amphibolitic itabirite; these lithofacies are host to a large number of economically important high-grade iron ore deposits that give rise to the name Quadrilátero Ferrífero, or "Iron Quadrangle." High-grade iron ores replace itabirites in tectonically favorable, low-strain sites. faults acted as conduits while large fold hinges were sinks for mineralizing fluids. Hard and fine-grained hematite and/or magnetite orebodies are in the western low-strain domain of the Quadrilátero Ferrífero. Subsequent deformation led to recrystallization and development of distinctly schistose high-grade hematite ores characteristic of the eastern high-strain domain. A combination of hypogene and geologically recent supergene processes is thus invoked to explain the formation of the high-grade iron ores of the Quadrilátero Ferrífero. Three stages of hypogene ore formation are distinguished. The first two of these stages took place early during the Transamazonian orogeny (2.1–2.0 Ga) and are well preserved in the western low-strain domain. During the first stage metamorphic fluids leached SiO2 and carbonates and mobilized iron, which resulted in the formation of massive magnetite bodies, Fe oxide veins, and Fe-rich itabirite bodies; during the second stage, low-temperature, low-salinity fluids caused oxidation of magnetite and Fe-rich dolomite to hematite. The resulting ore is porous to massive and has a granoblastic fabric. The third and final hypogene stage of ore formation is related to thrusts of uncertain age (Transamazonian or Brasiliano orogeny), which dominate the tectonic structure of the eastern high-strain domain of the Quadrilátero Ferrífero. Crystallization of tabular hematite and large platy specularite crystals that overprint the preexisting granular fabric in the presence of high-salinity hydrothermal fluids are characteristic of this stage. During the Neogene, supergene residual enrichment processes gave rise to the formation of soft to friable hematite orebodies. The larger soft orebodies that surround some smaller hard high-grade orebodies are typically associated with dolomitic itabirite. Together, both ore types comprise the giant high-grade iron deposits typical for the Quadrilátero Ferrífero, resulting from the superposition of both hypogene and supergene processes. Pure supergene deposits are considerably smaller and do not extend to deeper levels below the erosion surface.
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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) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.009 | 0.005 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.002 | 0.001 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 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".