Archaean turbidites and banded ironstones of the Mt. Belches area (Western Australia)

Abstract

Contrasting with well-known eugeosynclinal sequences of the Archaean Kalgoorlie System, typified by the rocks of the “Golden Mile” at Kalgoorlie and bordering Lake Lefroy, are areas such as the Mt. Belches area, described here, in which monotonous sequences of typical miogeosynclinal metasediments are exposed. Metagreywackes of fysch character, showing associations of primary structures and lithologies widely accepted as indicative of turbidite activity predominate. The primary structures are remarkably well preserved despite regional metamorphism which has produced mineral assemblages of the upper sub-facies of the greenschist facies. These structures are described in some detail. The microscopic primary texture is largely obliterated by recrystallisation and cataclasis, the coarser metasediments having now a semischist texture. Rudites are absent. Banded ironstones are evident in the form of quartz-magnetite rocks and gruneritic/ hornblendic metasediments. A prominent sequence of these ironstones picks out an interesting chevron-fold pattern, on the ground, on air-photographs and on aeromagnetic maps. There are some pre-metamorphic basic dykes, entirely converted to amphibolitic rocks, and also some post-metamorphic doleritic and pyroxenitic dykes of the east-west trending Widgiemooltha suite. The latter are dated as 2,420 (±30) m.y. and the metamorphism as about 2,700 m.y.