Plumbotectonics of Moroccan ore deposits

Abstract

The synthesis of 240 lead isotopes analyses, measured on Moroccan ore deposits of Ediacarian to Neogene ages located in all geotectonic domains of Morocco allows a global reflection on the metallogeny of Morocco. The isotopic compositions vary widely, from 17.738 (Bou Skour) to 18.905 (Draa Sfar) for the 206Pb/204Pb ratio, and from 15.521 to 15.706 for the 207Pb/204Pb ratio. The source of lead in the studied deposits is located in the upper continental crust, except for those in the Anti-Atlas (Bou Skour, Imiter, etc.) and some in the High Atlas (Azegour) with a clear mantellic contribution. Isotopic variations noted at the scale of a district result either from the presence of several superimposed hydrothermal events calling upon different local sources as at Tighza, or from a single event disturbed by the segmentation of a volcanosedimentary basin, as for the Jebilet and Guemassa ore deposits. At the scale of the deposit (Draa Sfar, Bou Skour), isotopic variations result from the superposition of several hydrothermal events each with their own lead and associated metals. Overall, we can distinguish three generations of lead incorporated successively into the Moroccan geological basement by magmatism and/or hydrothermalism, characterized by their 206Pb/204Pb ratios: 17.74-17.90 (Panafrican), 18.10-18.40 (Hercynian) and 18.75-18.90 (Alpine). Panafrican lead is present in the Anti-Atlas, and very locally in the Meseta (Bouznika), and feeds in part on the mafic magmatism of Gondwana. Hercynian lead is the most represented and displays a definitive rupture in the source of metals, which is now exclusively crustal. It invades all Moroccan areas, including the Anti-Atlas, where it re-mobilizes and mixes with the panafrican lead. Alpine lead, more discreet, marks out the large scarf going from Agadir to Nador which traces on the surface the mantle plume of the Canaries and accompanies a Neogene magmatism which may also have acted as a simple engine remobilizing Hercynian lead, in particular to form MVT deposits from Touissit. The Hercynian and Alpine lead influxes are partly responsible for resetting the Neoproterozoic mineralization, as at Bou Azzer or Imiter. In the Sawkins's model, lead isotopic results support successive remobilisations of lead stored in primary and secondary tanks, as well as inheritance phenomena. Finally, the good transfer of the isotopic signature of lead from ore deposits to surface gossans shows that the isotopic geochemistry of lead is a useful tool for mineral exploration in Morocco, moreover for stratiform polymetallic sulphides ore deposits of Hajar type.