AN EARLY PROTEROZOIC PLAYA IN THE PRETORIA GROUP, TRANSVAAL, SOUTH-AFRICA

A 2.2-Ga-old occurrence of jasper-banded iron-formation forms a very thin layer of restricted extent, closely associated with a palaeosol developed on basaltic lava. Early diagenetic polygonal cracking, plastic microdeformation and quartz spherulites suggest a sodium silicate precursor for the cherty phase of the banded iron-formation. The same bed contains cyclic layers of well-preserved pseudomorphs after evaporites, forming radiating blades which were probably mirabilite. It seems that the sediments were deposited in an alkaline (presence of sodium silicate precursor) playa in a semi-acid but probably cool (presence of mirabilite) climate. The iron and silica of the banded iron-formation, as well as the salts of the evaporites, originated from ground water that leached lava. This occurrence provides further evidence that banded iron-formations had a sodium precursor. © 1990.