The Goedgenoeg Formation, partition from the Makwassie Formation, Platberg Group, Ventersdorp Supergroup

The Goedgenoeg Formation is here described as a separate formation from the Makwassie Formation of the Platberg Group, Ventersdorp Supergroup. The Goedgenoeg Formation is visually distinguished from the overlying Makwassie Formation by the occurrence of green dacitic large-feldspar porphyritic ash flows, compared to the much paler coloured rhyolitic quartz- porphyritic ash flows of the overlying Makwassie Formation. Both formations also contain dark green andesitic lavas, which cannot easily be distinguished. The two formations usually occur together, but either may be absent. Goedgenoeg Formation dacite samples contain highly variable amounts of magmatic and metamorphic zircon xenocryst grains ranging from 3.1 to 2.7 Ga in age, but the magmatic autocrysts formed during extrusion are now dated at 2 723 ± 4 Ma on four samples of the Goedgenoeg Formation. This is within error of the published Makwassie Formation 2 720 Ma ± 2 Ma extrusive age on four samples. The Goedgenoeg Formation is widespread in the Platberg Group depository in the Witwatersrand and Kimberley terranes of the Kaapvaal Craton as documented by boreholes, but is poorly exposed. Exploration boreholes intersected it in the Bothaville Gap, between the Free State and Klerksdorp goldfields and westwards to Vryburg and Kimberley. The type section is defined in borehole VE1 and supplementary sections in boreholes LL1, BES1 and DHK1, all in the Bothaville Gap and stored at the Orkney core yard of Harmony Gold Mining Company Ltd. Together they show the main characteristics and lithological diversity of the Goedgenoeg Formation and how it is distinguished from the Makwassie Formation. Three poorly exposed and weathered outcrop localities are also documented. The volcanic succession varies greatly in thickness, with maxima of more than 3 000 m in major grabens. Coeval dykes and possible volcanic correlates occur in the Swaziland Terrane of the Kaapvaal Craton. The Goedgenoeg Formation overlies the sedimentary Kameeldoorns Formation, which forms the base of the Platberg Group. Minor sedimentary units may occur close to the lower contact and near the upper contact with the overlying Makwassie Formation quartz porphyries. The volcanic rocks have undergone greenschist grade burial metamorphism and generally minor alteration. Their geochemical composition includes basaltic andesite, andesite lavas and high-and low-Zr groups of dacitic feldspar porphyry ash flows. The volcanic rocks are, geochemically ascribed to a calc-alkaline volcanic arc in a continental margin setting. Magma source signatures suggest an enriched mantle source with added continental material, probably related to subduction. The Goedgenoeg Formation volcanic rocks are easily distinguished from the underlying Klipriviersberg Group and the overlying Allanridge Formation on geochemical diagrams such as Ti-Zr-P and P/Ti vs Zr/P. They overlap geochemically to some extent with rocks of the Makwassie and Rietgat formations, but the Ti/Zr ratio allows partial discrimination. The origin of the Goedgenoeg Formation and Ventersdorp Supergroup may be ascribed to either a long-lived plume beneath the Kaapvaal Craton, or to subduction of an Archaean ocean beneath the craton before the 2.7 Ga continent-continent collision with the Zimbabwe Craton, which formed the Limpopo Belt. The Goedgenoeg Formation should now generally be distinguished from the Makwassie Formation on maps and borehole logs.