Chert in the Palaeoproterozoic Bartle Member, Killara Formation, Yerrida Basin, Western Australia: a rift-related playa lake and thermal spring environment?

Abstract

Palaeoproterozoic chert in the Bartle Member of the Killara Formation, Yerrida Group, Yerrida Basin of Western Australia, contains an anomalous association of crystal structures and rock fabrics that are difficult to interpret. The association appears to indicate an evaporitic–pyroclastic–thermal-spring environment associated with rifting at about 2.2 Ga. The chert member contains silica pseudomorphs after evaporite minerals that in places enclose relict isolated crystals and aggregates of crystals of gypsum and anhydrite. The evaporite minerals are associated with minerals such as barite and analcime. The association of these minerals, together with palaeoenvironmental evidence, invites comparison with rocks of the Afar region and Lake Magadi in the East African Rift System. The Bartle Member locally contains anomalous gold (up to twelve times the average crustal values of 4 ppb) and barium. It also contains finely disseminated kerogen together with numerous spheroids (classified here as microdubiofossils) and some curious structures showing organisation into complex petal-like structures (classified here as putative microfossils). The Bartle Member chert has many characteristics of playa lake and thermal-spring deposits, and may host epithermal precious-metal mineralisation.