) from Minas Gerais, Brazil. Note the thin bands of hematite, jasper, and specular hematite|alt= The iron ore deposits of the Iron Quadrangle are hosted within the
Cauê Formation, part of the Minas Supergroup. The Minas Supergroup was initially deposited approximately 2.5 Ga ago on the edge of the
São Francisco craton, the geologic core of southern Brazil. The larger Iron Quadrangle contains the following five
lithostratigraphic units: • Archean Basement rock (3.2–2.61 Ga):
Gneiss and
migmatites of the São Francisco craton. At the time, this craton would have been connected to the
Congo craton and would not have been associated with the rest of South America. Also included in this unit are
calc-alkaline and
granitic plutons emplaced during the Late
Archean. •
Rio das Velhas Supergroup (2.86–2.6 Ga):
Greenstone and intercalated sedimentary rocks, including
Algoman-type
banded iron formations (BIF) • Minas Supergroup (2.6–2.12):
Metamorphosed sedimentary units deposited on the shelf of the São Francisco craton. The lowermost units (
Tamanduã and
Caraça Groups) show a typical
marine transgression sequence, with
conglomerates and
sandstones fining upward into marine
pelites. These are overlain by the Cauê Formation, Lake Superior-type banded iron formations which are in turn overlain by
carbonates of the
Gandarela Formation and shallow water to
deltaic sediments of the
Piracicaba Group. Lastly, the whole sequence is
unconformably overlain by the
Sabará Group, a mix of
turbidites,
volcaniclastics, conglomerates and
diamictites. • Post-Minas intrusives (various ages):
Pegmatitic dikes and
granitoid plutons which intruded into the Minas Supergroup. It is these pegmatites which host the gemstones being mined in the region. •
Itacolomi Group (2.1 Ga): coarse sandstone and conglomerates which overlie the Minas Supergroup. This unit contains banded iron formation clasts which might be derived from the Cauê Formation. The area shows evidence for a collisional event (the
Transamazonian Orogeny) approximately from 2.2 to 1.8 Ga, based on mapped
folds and
thrust faults. It has been suggested that this event relates to other contemporaneous
orogenies around the world, leading to the assemblage of the
Supercontinent Columbia. The iron ore in the region is of the type known as
itabirite. Itabirite is a metamorphosed variant of
banded iron formation in which the original bands of
quartz and
jasper have been recrystallized to macroscopically distinguishable quartz grains. The iron minerals (
hematite and
magnetite) are typically present in thin bands. This material typically produces a high-grade iron ore, as impurities such as
sulfur or
phosphate were removed during the metamorphic processes. ==IUGS geological heritage site==