The Great Dyke of Zimbabwe

Abstract

The Great Dyke of Zimbabwe is unique within the family of large layered intrusions by virtue of its highly elongate form. Apart from the tectonic controls that gave rise to the series of linked magma chambers which together comprise this intrusion, the width to length ratio profoundly affected the layering style, rock-types, mineral compositions and the form of mineralized ore bodies. The intrusion developed as a series of initially isolated chambers which became linked at progressively higher levels during the filling process. The dynamic interplay of crystallization and magma emplacement gave rise to the succession of cyclic units within the ultramafic sequence. The entire length of the Great Dyke (some 550 km) was linked at a level corresponding to the top of the Ultramafic Sequence and at this stage influxes of new magma effectively ceased. The initial magma of the Great Dyke was high magnesian (15.6% MgO), relatively enriched in silica, but with low initial 87 Sr/ 86 Sr indicating low crustal contamination. The various primary processes of magma mixing resulting from emplacement of new magma into an expanding chamber gave rise to economically important chromitite layers, while fractionation combined with influx of magma caused the formation of base metal sulphides enriched in platinum group elements. This paper considers the following aspects of the Great Dyke: its tectonic setting, structure, form and development of the magma chambers, initial magma composition, emplacement of magma, crystallization and fractionation, and mineralization.