Chemical alteration within the volcanic roof rocks of the Bushveld Complex

Pretoria University, Geology Department, Pretoria, South Africa

Field relations, petrography, and geochemistry identify mobile and immobile elements within the Rooiberg volcanic rocks of the floor and roof of the mafic Rustenburg Layered Suite of the 2.06 Ga Bushveld Complex. The floor rocks show thermal metamorphism accompanied by dehydration. Two major alteration processes affected the roof rocks. The first followed extrusion of the felsic Damwal Formation and is marked by veins of pyrite and arsenopyrite. The second is related to the intrusion of the Rustenburg Layered Suite, which produced an aureole 1.4 km thick in the overlying, felsic rocks. Pb, Zn, and Mn attain maximum concentrations of 335 ppm, 929 ppm, and 0.45 wt percent, in the metasomatized roof rocks. The economic significance of elevated Pb and Zn concentrations at the top of the aureole deserves future attention. Primary concentrations of Si, Mn, Ca, Na, K, Fe, Mg, St, Th, Ba, U, Hf, Ni, Cu, Zn, Pb, Nb, Zr, and Y were modified to varying degrees, depending on locality and distance from the Bushveld intrusions. Ti, Al, P, Ga, So, and heavy REE were immobile; concentrations of these elements in the Rooiberg volcanic rocks indicate that at least one geochemically distinct magma type is present in both the floor and the roof sequences. This confirms that the volcanic floor and root successions originally formed as a continuous sequence. The color of the roof rocks is controlled by the contact metamorphic processes and cannot be used for stratigraphic subdivision, nor should the red color of these rocks be used for inferences about the evolution of the Proterozoic atmosphere. Hydrothermal alteration was also responsible for the aberrant radiometric dates in the Rooiberg Group.

This record provided courtesy of AGI/GeoRef.