WITWATERSRAND IRON-FORMATIONS AND THEIR SIGNIFICANCE FOR GOLD GENESISAND THE COMPOSITION LIMITS OF ORTHOAMPHIBOLE

In the West Rand Group of the 3.07-2.71Ga old Witwatersrand Supergroup , South Africa, a series of banded iron-formations occur. They are of chemical origin and were deposited in an offshore shelf environment. T he coarser-grained, in places pyrite-bearing, and partly auriferous me tasedimentary rocks forming the bulk of the Witwatersrand Supergroup a re regressive. The iron-formations, however, were deposited during tra nsgression. The presence of allogenic pyrite in the fluviatile metacon glomerates and that of magnetite and, in places, haematite in the mari ne iron-formations suggests a lower pH and higher sulfur activity for the Archaean meteoric environment than for recent hydrothermal fluids on the ocean floor. Post-depositional alteration of the Witwatersrand rocks includes burial metamorphism at temperatures between 300 and 350 degrees C and pressures around 2.5 kbar, and multiple hydrothermal in flitration events at slightly lower temperatures, coeval with the brit tle deformation of the basin fill during the deposition of the Transva al Supergroup and the Bushveld Vredefort events. Additional thermal me tamorphic overprint of the iron-formations around the Vredefort Dome c aused the growth of orthoamphiboles. They show a wide range of composi tions between ferro-anthophyllite and ferrous alumino-gedrite, suggest ing that the crest of the solvus curve for Fe-rich orthoamphiboles is below 500 degrees C. Chlorite and amphibole compositions, and the pres ence of Fe-oxide-bearing horizons between pyrite-bearing ones indicate that the fluid composition during post-depositional alteration was la rgely controlled by the bulk rock composition of the infiltrated strat igraphic horizons and not by some external source.