Stratigraphy, structure and mineralisation of the Akjoujt area, Mauritania

Abstract

The stratigraphy of the highly deformed and overthrust supracrustal rocks of the Akjoujt area consists of two sequences separated by an angular unconformity. A new stratigraphic framework has been proposed that is virtually the reverse of previously published schemes. The oldest recognisable supracrustal sequence consists of metabasalts overlain by banded iron formations and semipelitic and quartz-rich metasedimentary schists, for which the name Eizzene group has been proposed. This is overlain with angular unconformity by orthoquartzite followed by a suite of siliciclastic rocks, mafic to felsic volcaniclastic rocks, flows and banded iron formations. This well-layered sequence is overlain by poorly layered monotonous submarine metabasalts and coeval dolerites. All the rocks above the unconformity have been assigned to the newly created Oumachoueı̈ma group. The supracrustal rocks of the Akjoujt area are preserved as a complex system of overlapping thrust sheets, representing the disjointed limbs of a large-scale recumbent syncline. Igneous and metamorphic basement, with basal Oumachoueı̈ma group metasedimentary rocks attached, has been overthrust and is preserved in synformal remnants within the supracrustal domain. These are the erosional remnants of refolding by later, upright events. The main tectonic episodes consisted essentially of two periods of thrusting and recumbent folding followed by two episodes of thin-skinned upright folding above the sole thrust. Overthrusting of the suite onto the Archaean Amsaga basement to the northeast along the sole thrust is believed to be a late-stage event. The idiosyncratic Fe–Cu–Au–Mg carbonate mineralisation style of the Akjoujt area shows evidence of having been generated more than once during the evolution of the host rocks. Starting with pre to syn-early thrusting events, the carbonate-rich mineralisation recurred at least until the upright folding. There is strong field evidence for a genetic link to carbonate-rich iron formations by remobilisation, but no evidence of a synvolcanic or synsedimentary mineralising event.