From sandstone- to carbonate-hosted stratabound deposits: an isotope study of galena in the upper-Moulouya district (Morocco)

The Upper Moulouya district displays three main types of lead ores in the Hercynian domain: (1) Aouli vein-type deposits hosted in the Hercynian basement, (2) Zeida Triassic sandstone-hosted, and (3) Mibladen carbonate-hosted stratabound and karstic mineralisations in the Mesozoic cover. Lead and sulfur isotope studies indicate that the Aouli vein-type and Zeida sandstone-hosted stratabound mineralisations are formed by a mixing of two fluids, one formed by the leaching of the Aouli granite massif, and the other by leaching of the Saharian basement. These deposits may correspond to the same Triassic metallogenic event, focused along fault systems or within permeable sandstone. Deep fluids were mobilised during the early extensional movements associated with the opening of the Atlas rifting basin. The Mississippi Valley type Mibladen mineralisation is related to a distinct metallogenic event superimposed on the earlier one, and represents a remobilization of earlier concentrations, or a more recent leaching of the same sources, but with a more pronounced contribution of the local organic matter. The Upper Moulouya area demonstrates the close relationship between vein type and sandstone-hosted lead-zinc mineralisations, and their major differences with Mississippi Valley type deposits.