Multistage genesis of the late Cretaceous manganese karst-hosted Tasdremt deposit (High Atlas, Morocco)

The eastern part of the Souss Basin (Morocco) contains several Mn deposits in the Tasdremt district. ThreeMn orebodies occur within
the Cenomanian-Turonian dolostones and the Senonian (Coniacian to Maastrichtian) detrital series, the main orebody being located at
the boundary between them. The Mn ores consist of coronadite group minerals, mostly coronadite and hollandite, in a karstified
dolostone. New field observations, petrographic analyses, and geochemical data define the Tasdremt deposits as a karst-hosted
accumulation (11–60 wt.% Mn), particularly enriched in Ba (1.5–8.2 wt.%) and Pb (1.0–5.0 wt.%) with poor contaminations in Al,
Fe, and P. This study shows that the ore-forming process is similar to that occurring in the Imini C3 level, located ~ 100 kmto the northeast.
Such similarities with the high-grade pyrolusite-bearing ore suggest that the Tasdremt deposit is a lateral equivalent of the Imini
deposits. However, the scarcity of pyrolusite in Tasdremt results in lower Mn grades, the Tasdremt ores being considered an aborted/
incomplete system in comparison with the Imini deposits. 40Ar/39Ar geochronology of K-bearing Mn oxides yields late Cretaceous
ages, defining three phases at ~ 91.5 Ma, ~ 77.5–82 Ma, and ~ 65–67 Ma. Although the source of metals remains hypothetical,
mineralizing fluids were carried by O2-free groundwater that mixed with O2-rich shallow meteoric waters at the Tasdremt depositional
site. The dissolution of the host dolostones and the karst environment have provided suitable conditions for the precipitation of Mn
oxides, causing the coeval increase of pH and Eh, respectively. The Early Atlasic deformation during the Late Cretaceous is associated
with mineralization events and was responsible for creation of low stand reliefs from Tasdremt to Imini. This period enabled
karstification and mineralization. Connecting the Tasdremt deposits to other African Mn deposits is difficult since the latter consist
of laterite resting above PaleoproterozoicMn protores, and consequently formed under different conditions from karst-hosted deposits.
It is likely that other Mn occurrences formed along the Atlas belt in similar settings.