Depositional age and sediment source of the auriferous Moeda Formation, Quadrilátero Ferrífero of Minas Gerais, Brazil: New constraints from U–Pb–Hf isotopes in zircon and xenotime

Abstract

The Moeda Formation is a metasedimentary clastic sequence of Witwatersrand-like metaconglomerate and quartzite at the base of the Minas Supergroup, Quadrilátero Ferrífero of Minas Gerais, Brazil, which is also known for its world-class iron-ore deposits. The Moeda Formation can be subdivided into three units (Units I to III from bottom to top) with each unit consisting of metaconglomerates at their base, mostly auriferous, grading into quartzite and locally into phyllite. This study presents the first combined U–Pb and Lu–Hf isotope data from detrital zircons and xenotimes of auriferous metaconglomerates and quartzites from Units II and III. These data indicate that deposition of sediments occurred ≤2.68Ga for Unit II and ≤2.62Ga for Unit III, as supported by the youngest magmatic zircon grains. The data further provide evidence that the source area of sediments was affected by crust re-working during two major magmatic-metamorphic events at 2.89–2.87 and 2.72–2.69Ga, and minor events between 3.3 and 3.0 and at 2.80 and 2.65Ga. This interpretation is supported by mainly subchondritic ɛHft between −10 and +1.4, and large ɛHft variations including all magmatic events. A few superchondritic ɛHft obtained for the magmatic events at 3.3, 2.9, and 2.7Ga, point to limited juvenile crust addition. The new dataset indicates that the clastic sediments of the auriferous Moeda Formation were mainly derived from an evolved continental crust in contrast to primitive magmatic arc sources, which are suggested for the Mesoarchaean auriferous sediments of the Witwatersrand Basin of South Africa, and those of the eastern Goldfield province of Australia.