Depositional setting and hydrothermal alteration of Paleoproterozoic manganiferous metasedimentary rocks in the Ampanihy district (Southern Madagascar)

The Paleoproterozoic is known to host significant sedimentary manganese accumulations deposited in peri-cratonic areas. The depositional environments, the source of Mn and the geometry of the basins are often poorly understood, although they are key aspects in understanding the origin of these strategic resources. We investigate the Mn-rich graphitic metasediments of the Ampanihy district (South Madagascar) to characterize depositional conditions and to identify potential hydrothermal alteration. The results show that the ore is predominantly composed of Mn silicates (spessartine, rhodonite) with accessory jacobsite and pyrophanite varieties rich in Zn close to the Ampanihy shear zone. Geochemical data suggest that the source material in the eastern domain (Ankara) of the district is more crustal in origin than sediments in the western part of the district (Vohidrakitsy, Bekily). The Ankara deposits have chemical characteristics that are close to many other Paleoproterozoic Mn deposits and can be considered as relatively unaltered and fits with the euxinic model. The western domain of the Ampanihy district shows strong hydrothermal overprinting, with significant enrichment in Zn, Ba and Sb, resulting from post-depositional hydrothermal alteration along the proto-Ampanihy shear zone. This event took place at some stage between the deposition of the sediments (c. 1.9‐1.7 Ga) and the suture of the Vohibory and Androyen domains (c. 0.63–0.60 Ma). Based on our data, we propose a new discrimination plot, (La/Lu)_PAAS vs Zn/Ni, which helps in the identification of sediment sources and the recognition of hydrothermal overprinting.