CHEMICAL ALTERATION WITHIN THE VOLCANIC ROOF ROCKS OF THE BUSHVELD COMPLEX

Field relations, petrography, and geochemistry identify mobile and imm obile elements within the Rooiberg volcanic rocks of the floor and roo f of the mafic Rustenburg Layered Suite of the 2.06 Ga Bushveld Comple x. The floor rocks show ther mal metamorphism accompanied by dehydrati on. Two major alteration processes affected the roof rocks. The first followed extrusion of the felsic Damwal Formation and is marked by vei ns of pyrite and arsenopyrite. The second is related to the intrusion of the Rustenburg Layered Suite, which produced an aureole 1.4 km thic k in the overlying felsic rocks. Pb, Zn, and Mn attain maximum concent rations of 335 ppm, 929 ppm, and 0.45 wt percent, in tile metasomatize d roof rocks. The economic significance of elevated Pb and Zn concentr ations at the toy of the aureole deserves future attention. Primary co ncentrations of Si, Mn, Ca, Na, K, Fe, Mg, Sr, Th, Ba, U, Hf, Ni, Cu, Zn, Pb, Nb, Zr, and Y were modified to varying degrees, depending on l ocality and distance from the Bushveld intrusions. Ti, Al, P, Ga, Sc, ana heavy REE were immobile; concentrations of these elements in the R ooiberg volcanic rocks indicate that at least one geochemically distin ct magma type is present in both the floor and the roof sequences. Thi s confirms that the volcanic floor and roof successions originally for med as a continuous sequence. The color of the roof rocks is controlle d by the contact metamorphic processes and cannot be used for stratigr aphic subdivision, nor should the red color of these rocks be used for inferences about the evolution of tile Proterozoic atmosphere. Hydrot hermal alteration was also responsible for the aberrant radiometric da tes in the Rooiberg Group.