A tectonic origin for ironstone horizons in the Zimbabwe craton and their significance for greenstone belt geology

Metre-thick horizons of ironstone lithologically similar to ‘sulphide-facies’ banded iron formation, but interpreted as silicified and sulphide-impregnated shear zones, are a common component of the greenstone stratigraphy of the Archaean Zimbabwe craton. Such tectonic ironstones separate different lithostratigraphic units commonly regarded as autochthonous rock sequences. On outcrop scale, shearing along ironstone horizons is indicated by anastomosing foliation domains, folding, boudinage and mylonitic fabrics, and, on a regional scale, by truncation of bedding and/or foliation, an anastomosing geometry of the horizons and duplication or juxtaposition of lithostratigraphic units. Tectonic ironstone formation is attributed to in situ silicification and iron (sulphide)-impregnation of rocks, commonly sediments, by mineralized fluids that penetrated the shear zones and their immediate wall rocks. The shear zones formed as a result of thin-skinned thrust tectonics that gave rise to horizontal accretion, imbrication and juxtaposition of volcanic and sedimentary rock units before deformation associated with granitoid diapirism. As a result, ‘layer-cake’ stratigraphic models of greenstone sequences containing tectonic ironstone ‘layers’ have to be treated with care, and the use of ironstone horizons as stratigraphic markers should be discouraged.