Le gisement de fer de Boukhadra (confins-Algérotunisiens)

Abstract

The Boukhadra massif belongs to the north-eastern part of the Saharan Atlas and to the mining district of the diapirs zone. It is a NE-SW orientated anticline characterised by formations of Lower Cretaceous age, cut in its SW part by a NW-SE orientated collapse trough in which Triassic evaporitic formations appear. These played an important role in the structuring of the region and in the emplacement of the Pb-Zn-Fe-Ba mineralisation found there. This work focuses on the carbonated (siderite) and oxidised iron mineralisation at Boukhadra, with the aim of describing and interpreting this type of mineralisation hosted in the Clansayesian limestones. Sedimentologically, the Boukhadra region resembles a barrier platform represented by a standard sequence of 9 microfacies indicating the passage from a frank marine environment (MF1) to a pre-barrier environment (MF2, MF3 and MF4), a barrier (MF5), a back barrier (MF6), then a lagoonal area (MF7) and finally a supratidal zone (MF8) capped by a pre-evaporitic sebkha-type environment (MF9), over an area limited in time and space. Most of the iron mineralisation occurs in the lagoonal facies with miliolae and rudists and in contact with Triassic evaporites, while small quantities of Pb-Zn mineralisation occur after the iron mineralisation and are found in the N-S fractures. The stable isotope geochemistry (C-O-S) carried out on the surrounding limestones, siderites and Triassic sulphates, as well as on a few sulphides, enabled us to define the conditions of deposition, namely that the carbon in the siderites is of mineral origin and that the sulphur originated in the evaporite formations of the Triassic by bacterial or thermochemical reduction. The results obtained at Jebel Boukhadras are comparable with those of the deposits in the diapir zone, both mineralogically and geochemically. These mineralizations are attributed to hydrothermal solutions that may have circulated through faults that acted as drains and favoured locations for Triassic extrusions. These diapiric formations and the Jurassic and Cretaceous cover could also be the source of the iron and lead-zinc elements that gave rise to the deposition of siderite by metasomatosis and the polymetallic veins. The post-Cretaceous periods saw the alteration of both the iron carbonates and the polymetallic veins, thus creating the oxidised mineralisation of the Pliocene period.