Stratigraphy, geochemistry and platinum group element mineralisation of the central zone of the Selukwe Subchamber of the Great Dyke, Zimbabwe

The Selukwe Subchamber forms part of the South Chamber of the Great Dyke and is unique in its location and structure by its close association with the Selukwe greenstone belt. The contact with the greenstone belt (west side) is over 20 km in length and contrasts with the mainly granitic contact on the east side. The greenstone belt margin has caused a deflection of the magma chamber resulting in a narrowing of the subchamber in some parts. The narrowing of the chamber, combined with the contrasting marginal rocks, has resulted in several important petrological and geochemical features in the Selukwe Subchamber. These include abnormal compression of the stratigraphy from axis to margin by nearly 50%, geochemical differences in whole rock and mineral compositions, asymmetry in the forms and types of layers close to the east and west margins, and development of an extensive xenolith/autolith suite of rock fragments. The xenolith suite is derived from the greenstone belt, whereas the autolith suite resulted from fragmentation of the Border Group formed in contact with the wall-rocks early in the development of the magma chamber. The preservation of relics of the Border Group is the result of the more rapid cooling in the narrow chamber. In most areas of the Great Dyke, evidence of the Border Group has been largely eliminated. The style of the layering also contrasts with that in the larger North Chamber in that narrow layers of dominantly olivine pyroxenite characterise the sequence in the Selukwe Subchamber.The overall fractionation pattern of the Ultramafic Sequence in the central part of the chamber is represented by orthopyroxene compositions and, being similar to other parts of the Great Dyke, shows little change until the uppermost P1 Pyroxenite where marked fractionation is apparent. The platinum group element zone is associated with sharp compositional changes in orthopyroxene compared with the general trend of evolving pyroxene compositions in the P1 Pyroxenite layer.