Strong positive fractionation of chromium isotopes in iron formation of the Jacadigo Group (Brazil) – A link to enhanced atmospheric oxygenation during the Late Neoproterozoic

Cr isotopes recorded in iron formations (IF) are considered to have the potential to reflect the isotope signatures in respective ambient surface seawater. The ∼600 Ma Fe and Mn deposits pertaining to the Banda Alta Formation (Urucum district, Mato Grosso do Sul, Brazil), comprise the world’s youngest and largest Neoproterozoic sedimentary Fe and Mn formations (MnF). Shale normalized Rare Earth Element and Yttrium (REY) patterns of drillcore samples show flat, positively sloped patterns with absent Europium anomalies and near- to supra-chondritic Yttrium-Holmium ratios with negative Cerium anomalies, which imply intermittent mixing of freshwater with seawater. Redox sensitive element enrichment factors (MoEF, UEF, CrEF) show positive correlation, indicating varying redox conditions across the Jacadigo Basin, with ephemeral euxinic conditions. The Cr isotope signatures across the Urucum IF (53Crauth at: Morraria Grande + 0.93 ± 0.34 ‰, 2, n = 28; Morro do Rabichão + 0.5 ± 0.4 ‰, 2, n = 3; and Morro do Urucum ca. + 0.64 ‰, n = 1) are statistically indistinguishable from previously published surface outcrop samples at Morro do Urucum. Our new data support: (1) a stable supply of oxidized Cr potentially from continental sources at the time of deposition, implying high atmospheric O2 levels in the Late Neoproterozoic; (2) insignificant alteration of the authigenic Cr isotope signals by tropical weathering despite surface iron up-concentration and leaching of carbonates, and (3) lack of isotope effects that would be associated with non-quantitative reduction processes and accompanying particulate transport to the chemical sediments in the Jacadigo Basin. The combined information from (isotope)geochemical data presented herein speak for a deposition of the Urucum IFs in restricted, periodically ice-covered and stratified sub-basins with partial connection to the open ocean, and for the presence of a sufficiently oxidative atmosphere which promoted a continuous supply of the surface waters with isotopically heavy Cr from the weathering landmasses at this time

Genesis of the Paleoproterozoic Ammassalik Intrusive Complex, south-east Greenland

The Paleoproterozoic Ammassalik Intrusive Complex of south-east Greenland is located at the margin between the Rae Craton to the north and the North Atlantic Craton to the south. The complex is part of the Nagssugtoqidian Mobile Belt emplaced at mid- to lower-crustal level and represents an eastward continuation of the Nagssugtoqidian Orogen of western Greenland. We present new whole-rock geochemical and isotopic data (whole-rock Rb-Sr, Sm-Nd, U-Pb, Lu-Hf and zircon U-Pb, Lu-Hf, O) acquired from the Ammassalik Intrusive Complex, with the aim of reassessing its petrogenesis and evolution. The intrusive rocks and surrounding host-rocks show typical calc-alkaline signatures as well as negative Ta, Nb and Ti anomalies. The new U-Pb age data indicate a protracted magmatic history over ca. 40 Ma, involving at least three successive magmatic pulses between ca. 1910 and 1870 Ma emplaced into a granulite facies biotite-garnet paragneiss with ca. ≤ 1990 Ma protolith ages. Whole-rock trace element and isotope geochemistry indicate that the magmas that formed the Ammassalik Intrusive Complex were derived from a mildly enriched (εNd0 = −0.8 to –3.8; εSr0 = +24 to +42) and deep, garnet-bearing (MREE/HREE fractionated patterns) mantle source, possibly the sub-continental lithospheric mantle. However, the observed decreasing zircon εHf values with time (εHf0 = +2.8 to −12.5) indicate increasing degrees of crustal contamination involving variably 18O-enriched crustal material (δ18O = 7–9‰) into the mafic magmas of the complex. This interpretation suggests that the mantle-derived magma do not have a deep, garnet-bearing source, but that the continental arc signature in the Ammassalik Intrusive Complex igneous rocks may have been inherited from the assimilation of partial melts of the garnet-bearing host-rocks. In addition, based on the new crystallisation ages broadly coeval with the Columbia supercontinent build up, marked by the formation of world-class Ni-Cu deposits globally, it is proposed that the Ammassalik Intrusive Complex and the south-east Greenland region were in direct continuity of the mineralised Kotalahti belt (Svecofennian Province, Finland). Together with the recent discovery of Ni-Cu mineralisation in the Ammassalik Intrusive Complex, our study further supports the high prospectivity of south-east Greenland for orthomagmatic Ni-Cu deposits