Diamictitic iron formation (DIF) deposits of the Neoproterozoic Nova Aurora Iron District (Macaúbas Group, Southeast Brazil)

The best-known Neoproterozoic glaciation-related iron deposits are the Rapitan-type banded iron formation (BIF) formed dominantly by chemical-sedimentary processes. We present a detailed study on a distinct type of glaciation-related iron deposit: the iron-rich metadiamictites of the Nova Aurora Iron District, comprising over 20 billion tons of iron oxide-rich rocks: the diamictitic iron formations (DIF >15 wt% Fe) and ferruginous metadiamictites (FD: 5–15 wt% Fe). Drill core logging, field mapping and analytical data revealed the protoliths, sediment sources, depositional environment, and ore enrichment processes on the iron district, and were used to discuss differences between the studied DIF and classic Rapitan-type BIF. U–Pb age spectra and Lu–Hf data for detrital zircon grains revealed a wide range of sediment sources and a maximum depositional age around 879 Ma. These data support correlations of the Nova Aurora iron deposits with the Cryogenian Macaúbas rift, a basin filled by glaciomarine successions related to the Sturtian glaciation event. The matrices of prevailing hematite-rich metadiamictites comprise (in vol %): hematite (7–55), quartz (17–57), muscovite (2–40), carbonate (<1–30), biotite (<1–18), chlorite (<1–23), epidote (<1–7), magnetite (1–5), and traces of tourmaline (<3), apatite (<3) and zircon (<1), with hematite, mica and stretched quartz outlining the metamorphic foliation. Clasts are free of iron-rich rocks. Hematite reconcentration (>50 wt% Fe) occurs in local mylonitic shear zones and tightly crenulated bands after gangue minerals removal during deformation. Late hydrothermal alteration formed magnetite-rich metadiamictite in high-strain zones. From base to top, the stratigraphic type-section of the Nova Aurora Iron District shows barren metadiamictite with quartz-rich matrix and graded-bedded quartzite lenses, followed by hematite-rich-matrix metadiamictite, then passing to muscovite-rich-matrix metadiamictite gradually richer in magnetite and sulfide, enclosing metapelite lenses. This section presents a fining-up graded-bedded succession with clasts size decreasing upwards and recurrent load structures, typical of debris flows and turbiditic sedimentation. The hematite/mica, Fe/Al, Fe/Ti and Fe/REE ratios, as well as the biotite/muscovite and silicate/sulfide ratios, which accompany variations in the oxidizing conditions, decrease to the top. Accordingly, the early protoliths were quartz-rich diamictites that passed to iron-rich diamictites and then to clay-rich, and under more reducing conditions, sulfide-bearing diamictites. Sedimentological and geochemical evidences suggest ferrous-iron accumulation followed by iron oxide precipitation from seawater when the conditions changed from anoxic to oxidizing by the input of oxygen-rich waters from deglaciation, followed by increasing reducing conditions accompanying marine transgression. These processes took place in a relatively restricted sector of the glaciomarine basin, probably a graben of the Cryogenian Macaúbas rift. Differences in relation to the classic Rapitan-type iron formations are the coarse-grained (ruditic) texture and the massive structure, the high-energy sedimentary environment, and the absence of true BIF in the Nova Aurora DIF deposits.