The Dullstroom basalt formation and the Rooiberg group : volcanic rocks associated with the Bushveld complex

Abstract

The aims of this study are twofold. First is to investigate the potential link between the predominantly volcanic Dullstroom Basalt Formation and the Rooiberg Group (both units formerly considered as part of the Transvaal Supergroup), now prised apart by the mafic rocks of the Rustenburg Layered Suite (RLS) and, second, to establish the possible relationship of these volcanic rocks with the Bushveld Complex. Field, petrographic, and geochemical investigations cover identified and potential Dullstroom volcanic rock occurrences (southeastern Bushveld Complex, Makeckaan and Rooiberg Fragments), as well as outcrops of the Rooiberg Group (Bothasberg Plateau, and Tauteshoogte, Rooiberg, and Loskop Dam areas). The Dullstroom volcanic rocks in the southeastern portion of the Complex are mapped documented and interpreted in detail, which also results in the first compilation of a comprehensive geological map. Extrusive volumes and mode of eruption are deduced for individual magma types. Three depressions in the vicinity of potential eruption centers are documented. A sand sheet overlies the generally flat unconfonnity that marks the top of the Transvaal Supergroup. Interaction between this sheet and the first extrusions is pronounced. The primary chemical composition of the rocks under consideration has been modified, in part as a consequence of their age but mostly by their proximity to major, acidic and mafic intrusive rocks. Field, petrographic and geochemical evidence are employed to identify mobile and immobile element concentrations. The floor rocks of the RLS (Dullstroom Basalt Formation) were subjected to thermal metamorphism and accompanying dehydration. Si, Mn, Ca, Na, K, Fe, Mg, Sr, Th, Ba, U, Hf, Ni, Cu, Zn, Pb, Nb, Zr, and Y were mobile in the roof rocks (Rooiberg Group). There, the mode and degree of alteration depends on the geological setting and the distance from major intrusions. Ti, Al, P, Ga, Sc, and heavy rare earth elements proved inert. Hydrothermal convection cells, active in the roof rocks, are expressed as a 1.4 kilometer thick zone of upward increasing hydration, topped by maximum concentrations of Pb (335 ppm), Zn (929 ppm), and Mn (0.45 wt.%). Alteration also affected the color of the rocks. Therefore color can not be utilized for stratigraphic subdivision nor to draw inferences about the evolution of the Proterozoic atmosphere. Employing immobile element concentrations it is deduced that one magma type, the High-Mg Felsite (HMF), is common to the volcanic floor and roof successions of the RLS, unequivocally establishing that the volcanic rocks were continuous before separation by the mafic rocks. The Dullstroom Formation therefore represents the basal portion of the Rooiberg Group and this fact creates the potential to establish a regionally applicable stratigraphy for the Rooiberg Group. This incorporates the regional review of previous field and geochemical work. It is recommended that the Rooiberg Group be redefined to include the Dullstroom Formation and that it is regionally subdivided into the Dullstroom, Damwal, Kwaggasnek, and Schrikkloof Formations (bottom to top). The Loskop Formation, overlying the Rooiberg Group, could potentially be included in the Rooiberg Group, but this awaits detailed study. Regionally persistent marker horizons, readily visible in the field, support the proposed subdivision. These are (i) a zone of quartzite xenoliths in the uppermost portion of the Kwaggasnek Formation, (ii) the Union Tin Member, which constitutes the top of the Kwaggasnek Formation, and (iii) the strongly flow-banded Schrikkloof Formation. commonly topped by a tuffaceous deposit. The lowermost Dullstroom succession is aiso present in the Makeckaan and Rooiberg Fragments. In the Rooiberg Fragment this succession is overlain by LMF flows of the Schrikkloof Formation, identifying intra-Rooiberg unconformities that onlap towards the north and northwest. The uppermost Dullstroom Formation is preserved in the Loskop Dam area. Synthesis of all available data facilitates the classification of previously undefined Rooiberg occurrences and a regional map of the Rooiberg Group detailing the distribution of the four formations is consequently compiled. It is suggested that the basal Rooiberg succession is also preserved beyond the present-day outcrop of the Bushveld Complex, e.g. in the Molopo Farms Complex. Six interstratified magma types constitute the Dullstroom Formation and these are named, in order of their extrusive sequence: Low-Ti (L Tl) Basaltic Andesite, Basal Rhyolite, High-Ti (HTI) Basalt, High-Mg Felsite (HMF), High-Fe-Ti-P Andesite, and Low-Mg Felsite (LMF). The latter two are continuous into the Damwal Formation, with the last HMF extrusion defining the top of the Dullstroom Formation. Two compositionally distinct LMF's constitute the Kwaggasnek and Schrikkloof Formations. In total then, eight magma types define the Rooiberg Group, with individual magma types exhibiting only minor signs of fractional crystallization. Sri is elevated and high concentrations of elements characteristically enriched in the crust are encountered. The HMF has a Proterozoic, upper crustal composition. The anomalously high Zn and Pb concentrations in the massive volcanic rocks in the roof zone of the RLS lead to consideration of the economic potential of the Rooiberg Group. Previously described deposits can be related to the newly established stratigraphic subdivision of the Rooiberg Group. Four mineralizing events, two linked to the intrusions of the RLS and one each to the intrusion of the Rashoop Granophyre and Lebowa Granite Suites, appear to have affected the Rooiberg Group. Initial intrusions of the RLS are proposed to have pneumatolytically-hydrothermally concentrated base metals, especially copper, into the Dullstroom floor succession (first mineralizing event). Pyrite and arsenopyrite in sedimentary rocks at the base of the Kwaggasnek Formation could be due to shallow granophyre intrusions (second mineralizing event). subsequently overprinted by base metal mineralization (mainly Pb and Zn), constituting the third mineralizing event. Fourthly, the Lebowa granites introduced tin and fluorspar, and this type of mineralization is restricted to the upper Kwaggasnek and Schrikkloof Formations. Sinter deposits, present at the top of the Schrikkloof Formation, are interpreted to be the surface expression of the hydrothermal convection cell and may be considered for their gold concentrations. Currently available age data suggest that the Rooiberg Group (2061+2Ma) is more closely associated with the Bushveld Complex (2054+2Ma for the Rusten burg Layered and Lebowa Granite Suites, 2053+12Ma for the Rashoop Granophyre Suite, and 2060+2Ma for the Rooikop Granite Porphyry) than the Transvaal Supergroup (about 2.4 - 2.6Ga). This is supported by a close geographic association of the volcanic and intrusive rocks, leading to consideration of the potential link between the acidic Bushveld suites and the Rooiberg Group. Comparison of granophyre and Rooiberg rhyolite chemistry confirms that the granophyre, by and large, is the shallow intrusive equivalent of the rhyolite, rather than remelted rhyolite or melted sedimentary rocks. The Rooikop Granite Porphyry is the shallow intrusive equivalent of the most evolved, and youngest Rooiberg magma. Further evidence supporting the notion that the Rooiberg Group forms part of the Bushveld Complex is found in the fact that the youngest RooihP.rg rhyolite compositions compare favorably with those of published Bushveld granites. The Rooiberg rhyolites, Rashoop granophyre and Lebowa granite are therefore all derived from a source similar to upper crustal composition. Granite intrusion terminated the Bushveld magmatic event but may have overlapped in time with the last extrusions of the Rooiberg rhyolites. Considering available evidence, a plume origin of the Complex is favoured over a meteorite impact. It is concluded that a mantle plume initiated the Bushveld magmatic event, with volcanic extrusions and silicic (Rashoop Granophyre and Lebowa Granite Suites) intrusions being related. The presence of Bushvel