Hydrothermal fluid–rock interactions and subsurface Mn leaching controlling stratiform Mn mineralization in the Tizi-n’Isdid district (Western High Atlas, Morocco): insights from ore mineralogy, paragenesis, and geochemical evolution

The Tizi-n-Isdid manganese deposit, located along the western margin of the Ouzellarh Precambrian promontory in the western High Atlas, represents a key example of hydrothermally stratiform-type manganese ore deposit formed during the late Ediacaran to early Cambrian transition. This study integrates field observations, petrography, SEM-EDS analyses, automated mineralogical mapping and whole-rock geochemistry to reconstruct the mineralogical evolution and origin of the deposit. Macroscopic observations reveal massive, banded and brecciated Mn-rich bands hosted within lower Cambrian claystones (Adoudou Formation), with MnO contents ranging from 21 wt.% to 49 wt.%. The ore is dominated by braunite, piemontite, hollandite-group minerals, cryptomelane, pyrolusite, and subordinate amounts of barite and carbonate minerals. Textural relationships identify four successive stages: a detrital pre-ore stage; an early hydrothermal silicification stage; a carbonatation stage marked by brecciation and barite–carbonate veining; and a final near-surface oxidation stage.

Mn mineralization is closely related to hydrothermal fluid–rock interactions involving a mixed magmatic–meteoric fluid system. Meteoric waters infiltrated through permeable fault zones and sedimentary units, were progressively heated at depth by interaction with magmatic heat sources, and evolved into reactive hydrothermal fluids. During their ascent along fault-controlled pathways, these fluids efficiently leached Mn from the volcanic and crystalline basement rocks. Subsequent changes in temperature, redox conditions and fluid composition during discharge onto a shallow marine platform promoted Mn precipitation and the development of stratiform mineralization within clay-rich sediments.

Major and trace elements (Mn/Fe, Co/Ni, Co/Zn) consistently indicate a hydrothermal origin, while REE patterns, characterized by low ΣREE, strong negative Ce anomalies, positive Eu anomalies, HREE enrichment and high Y/Ho ratios, reflect the mixing of Mn-rich hydrothermal fluids with oxic seawater on a shallow platform. Structural and tectonic evidence links ore formation to late Ediacaran N–S extension, fault-controlled hydrothermal circulation and early Cambrian marine transgression. These combined mineralogical, geochemical and geological data support a genetic model in which Mn was leached from volcanic and basement rocks, transported upward along normal faults and precipitated syngenetically with clay-rich sediments in an oxygenated marine environment.