Contribution à l'étude du métamorphisme des gisements cupro-cobaltifères stratiformes du Sud-Shaba, Zaire. Le district minier de Lwishia

Abstract

The late precambrian copper-bearing strata of southern Shaba (‘Série des mines’) have undergone metamorphism during the latest proterozoic to early paleozoic times. Structural, textural and crystallochemical analysis of this metamorphic event shows a very peculair evolution. During a first phase, prior to folding, a static recrystallization is accompanied by several chemical chanes such as Na, then KMg metasomatism. This hydrothermal character is due to ‘hydrostatic’ stress conditions and fluid circulation linked with surrounding evaporites. During this prograde stage of metamorphism, kyanite + carbonate + sulphide paragenesis suggest low water partial pressure and high pH-low Eh conditions. Geothermometers and fluid inclusions indicate moderate to medium temperatures (300–400°C). Sparse synergetic CuCo sulphide mineralization is overconcentrated to its present disposition. A second hydrothermal event following the main folding phase is probably linked with halokinetic or halotectonic structures. It is essentially retrograde with oxidizing and low-temperature (200°C) conditions, resulting in the total leaching of evaporites and the forming of clay minerals, such as kandites or smectites.