Geochronological and Petrochemical Study of Non-Glaciogenic Neoproterozoic Banded Iron Formations (Anti-Atlas, Morocco): Insights into Their Formation in a Suboxic Arc-Related Basin

The resurgence of Banded Iron Formations (BIFs) during the Neoproterozoic, following a billion-year hiatus, reflects significant geodynamic and climatic transition. Newly discovered Neoproterozoic BIFs in the central Anti-Atlas region of Morocco provide key insights into these processes. The studied BIFs units are exposed within the Bou Azzer-El Graara inlier (Central Anti-Atlas), an oceanic paleo-suture zone between the Paleoproterozoic West African Craton and remnants of a Neoproterozoic magmatic arc. This inlier comprises 750 to 680 Ma magmatic arcs and ophiolitic remnants, both intruded by ~650 Ma dioritic plutons and overlain by Ediacaran metasedimentary sequences. The studied BIFs are hosted in meta-volcano-sedimentary units, intercalated between magmatic arc and ophiolitic complexes, and locally intruded by igneous bodies. Neither the BIFs nor their host volcano-sedimentary schists are associated with glacio-derived sediments.

Petrological, geochemical, and geochronological analyses were conducted to reconstruct the paleo-depositional environment and identify the mechanisms of BIF formation. In situ U-Pb dating on hematite yielded a crystallization age of 641 ± 41 Ma. Hematite dating could be interpreted as an early diagenetic age probably close to BIF deposition.

The whole-rock major and trace element composition of the Bou Azzer BIFs exhibits a high correlation among terrigenous proxies (e.g., Al, Zr, Hf) and silica content, with trends strongly aligning with the felsic host rocks. This suggests that the BIFs’ whole-rock geochemical signature, specifically the siliceous layers, is predominantly controlled by detrital inputs. Multi-element geochemistry, (e.g. mean La/Yb_SN ratio of 0.36, low TiO₂ content of 0.24 wt%, Y/Ho ratio of 26, Nb-Ta depletion) combined with Nd-Sr isotopic data from the host rocks (εNdᵗ +4.0 to +4.5), indicates a juvenile arc source, consistent with presence of igneous minerals, such as feldspar, epidote, and amphibole, in both the host rocks and BIF samples.

Hematite in BIFs show two habitus: large euhedral grains surrounded by platy hematite. Petrographic evidence suggests that euhedral hematite precipitated at a more precocious stage, while platy hematite is distinctly aligned with the foliation of the host sediments. In situ LA-ICP-MS analyses of hematite from the two habitus reveal distinct geochemical signatures from each other and from the whole-rock compositions. Overall, hematite exhibits significantly lower ΣREE and superchondritic Y/Ho ratios up to 42, with a median value of ~28. Large euhedral hematite displays a pronounced negative Ce anomaly, indicative of precipitation from oxygenated seawater and distant from hydrothermal sources, as shown by low positive Eu anomaly (~1.06). The chemical composition of platy hematite shows no Eu or Ce anomalies, suggesting anoxic conditions during diagenetic crystallization. 

The Bou Azzer BIFs are Cryogenian and were deposited in an arc-bounded basin, with no evidence of glaciogenic influence. This paleo-depositional context emphasizes the role of limited arc-related basins during the Neoproterozoic, which facilitated the development of unique suboxic conditions.