The Guelb Moghrein Fe-Oxide Copper-Gold-Cobalt Deposit and Associated Mineral Occurrences, Mauritania: A Geological Introduction

The Guelb Moghrein deposit is located within the Mauritanide Mobile Zone, north east of Nouakchott in the Islamic Republic of Mauritania, West Africa. The copper, gold and iron deposits of the Maurilanides fold/thrust belt of the Akjoujt area have many of the characteristics of the hydrothermal iron oxide copper-gold class of deposits as well as showing distinctive features of their own. Exploration by General Gold International SA since 1994 has concluded that they represent, perhaps, a carbonate-rich sub class and show considerable diversity of form and setting.

The Mauritanides in Mauritania incorporate a great diversity of rocks in multiple, thrust-bounded domains including metamorphosed siliciclastic sediments, meta-felsic volcanics, meta-basic volcanics, serpentinite, and bodies of granitoid rocks.

The Akjoujt area features a supracrustal suite of metamorphosed mafic to felsic volcanic, volcaniclastic and epiclastic rocks, with a number of banded iron formation (BIF) horizons, that have been overthrust northwards and eastwards onto the Archaean Amsaga Basement and the Taoudeni Basin edge. The supracrustal stratigraphy has recently been revised by the authors because previously published schemes were found to be inconsistent with field observations. Carbonate alteration is widespread in the supracrustal suite and the BIFs are commonly carbonate rich.

All of the Fe-oxide copper-gold mineralisation of the Akjoujt area is associated with coarse-grained Fe-Mg-Ca carbonate occurrences. These are widespread throughout the Akjoujt area and they range in size from clusters of carbonate porphyroblasts in BIF, through small pods and lenses, to large lenses, irregular sheets, and from single narrow veins to extensive stockworks and vein systems. Some form of structural control is usually apparent and emplacement has occurred at various stages in the tectonic history from early to syn-thrusting through to upright folding and doming.

The Guelb Moghrein Cu-Au-Co mineralisation within both the Occidental and Oriental bodies is hosted by a massive magnetite-sulphide bearing iron-magnesium carbonate representing a C02 metasomatised shear zone or 'tectonite and is characterised by prominent magnetic anomalism. A total measured and indicated resource of 23.6 Mt @ 1.88 % Cu, 1.41 g/t Au and 143 ppm Co has been calculated and currently the mineralised zone is open at depth, presenting a significant geophysical target for the discovery of additional down dip and satellite deposits.

At El Khader the carbonate vein system is interpreted to belong to a different stage in tectonic evolution than at Guelb Moghrein. There the brittle fracture event is considered to have occurred later than the ductile shear setting of Guelb Moghrein.

Given the abundance of carbonate in the country rocks, and the structurally controlled settings of the coarse grained occurrences it is likely that the carbonate bodies in the Akjoujt area evolved by remobilisation from within the system during tectonism. This would probably have involved recycling of basin fluids, however there is no evidence that magmatism played any role in this. The introduction of the metallic components is assumed to be along the same structural channels but the source of metal is unclear. The most likely origin is, like the carbonate, from within the pile, in this case from the abundant, leachable mafic volcanic rocks.