The Green Bar: a 2.8 Ga Ultramafic Exhalative or Impactite?

Drill core samples of an anomalous shale layer in the 2.8 Ga Witwatersrand Supergroup, known as the Green Bar, were analyzed for PGE abundances to determine whether the layer may represent an Archean impactite. The Green Bar occurs in the lower part of the fluvial quartzites and conglomerates of the Johannesburg Subgroup of the Central Rand, approximately 1 to 2 meters above the Au-bearing Carbon Leader. It is comprised of 1 m of chloritoid-rich, laminated to massive siltstone that disconformably overlies a cross-bedded quartz arenite and is disconformably overlain by a cross-bedded quartz wacke. The Green Bar can be correlated over a distance of ~10 km in the northern margin of the Witwatersrand basin. DeKock (1949) proposed that the Green Bar might be a volcanic tuff, perhaps not unlike the stratigraphically younger, chloritoid-rich, Bird Reef Marker, interpreted as a volcanic tuff associated with the Bird Amygdaloidal lava. PGE analyses of laminated and massive Green Bar samples yielded Ir concentrations ranging from 0.17 to 0.22 ppb. The concentrations of Ir, Ru, Rh, Pt, Pd and Au normalized to C1 chondrites are consistent with those of ultramafic rocks. The laminated facies exhibited some relative enrichment of Au that may be related to its mobilization during metamorphism. These data are more consistent with an ultramafic exhalative origin for the Green Bar. The presence of ultramafic eruptives during Witwatersrand deposition may also explain the origin of the detrital diamonds associated with some of the conglomerates. An impactite origin is not necessarily precluded by the PGE data if the impact occurred in ultramafic oceanic crust, but it does require some other evidence to support that case. DeKock, W.P. The Carbon Leader on the Far West Rand. Trans. Geol. Soc. S. Africa, 51, 1949.