Genesis and paleoenvironment of the neoproterozoic iron formations of the Mwashya Kansuki in southern Katanga, democratic Republic of Congo

Abstract The Neoproterozoic iron formations of the Mwashya Subgroup and Kansuki Formation in southern Katanga can be categorized into three main types according to petrographic analysis: hematitic quartzites with a granoblastic texture, clastic or brecciated cherts, and ferruginous quartzites with a banded-like structure. The granoblastic texture in ferruginous quartzites often displays diagenetic zoning, characterized by iron oxides or subrounded quartz grains embedded in a fine ferruginous cement, indicating a partly allochemical origin. These formations are primarily hosted in siliceous shales and exhibit features such as intraformational cracks and folds (including convolutions and slumps), which suggest deposition on a submarine slope probably in a subtidal environment. The results of ICP-MS analysis showed that the iron concentration is associated with low trace element contents of detrital origin (e.g., Rb, Zr, Pb), pointing to an absence of significant continental material. Moreover, elevated Zr/Hf ratios, higher than those typically observed in zircon grains, further suggest a non-continental source. The rare earth element composition of the sediments exhibits a pronounced positive Eu anomaly, indicating hydrothermal input in a relatively deep marine environment. A slight negative anomaly of Ce observed in the clastic and brecciated cherts might indicate transitioning from anoxic to oxic conditions due to changes in oxidation state and depth. The interaction of hydrothermal fluids with pyroclastic materials derived from submarine basal volcanism in a continental rift setting explains the similarity to the Algoma type, even though it has more light rare earth elements.