The first stages of the basalt calcitization were studied along a weathering profile located near Elhajeb city, within the Moroccan Middle Atlas. This study allowed us to distinguish between smectitic weathering of basalt and its calcitization. The weathering front is up to 35 m deep. It is characterized by the transformation of primary minerals to smectites with a strong depletion of Ca. Calcitization is developed until 1.5 m deep and the lower limit of the calcitization front is marked by an increase of the Ca contents. 10 to 15% of this calcium is inherited from basaltic material and is in situ incorporated in secondary products. The 85 to 90% remained are of external origin to the profile and attributed to either Quaternary carbonate (travertine) surrounding the profile or to Liassic carbonates outcropping above Elhajeb basin.

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