Stratigraphy and sedimentary petrology of an Archean volcanic-sedimentary succession at Mt. Goldsworthy in the Pilbara Block, Western Australia: implications of evaporite (nahcolite) and barite deposition

A approximately 3.3 Ga sedimentary succession at Mt. Goldsworthy in the northeastern Pilbara Block, Australia contains beds composed of silicified pseudomorphs of nahcolite (NaHCO3) or barite (BaSO4). This sedimentary succession correlates with the Corboy Formation and consists of lower, middle and upper sedimentary units, which conformably overlie mafic to ultramafic volcanic and volcaniclastic rocks. The lower and middle units are predominantly siliciclastic, whereas the upper unit is characterized by ferruginous cherts and banded iron-formations with minor proportions of intercalated mature sandstone. The succession defines an overall upward fining and deepening trend. The detrital materials were derived from older greenstone successions, volcaniclastics and reworked precipitative beds. Silicified pseudomorphs of nahcolite crystals up to 40 cm in length occur in a 20 m thick bed in the upper portion of the lower unit. The formation of this unit was initiated by deposition of immature terrigenous elastic sediments and followed by the precipitation of nahcolite from Na+-HCO3- brines in a closed or semi-closed evaporitic basin. Bladed barite deposits occur mainly in the middle unit, which was deposited in a sub-aerial to shallow marine environment. Barite may have precipitated as a result of mixing SO42--rich seawater and Ba2+-rich hydrothermal fluids. The Mt. Goldsworthy sedimentary succession records an Archean shallow to sub-aerial sedimentary environment that probably developed in a continental margin setting. (C) 2002 Elsevier Science B.V. All rights reserved.