Geochemistry, petrology and origin of Neoproterozoic ironstones in the eastern part of the Adelaide Geosyncline, South Australia

Abstract

The eastern part of the Adelaide Geosyncline contains well preserved glaciomarine sequences of the Sturtian glaciation (≈750–700 Ma) including calcareous or dolomitic siltstone, manganiferous siltstone, dolostone and diamictite units and the associated Braemar ironstone facies. The ironstone facies occurs as matrix to diamictites and as massive to laminated ironstones and comprises abundant Fe oxides (hematite, magnetite) and quartz, minor silicates (muscovite, chlorite, biotite, plagioclase, tourmaline), carbonate and apatite, and detrital mineral grains and lithic clasts. Micro-textures indicate that magnetite and hematite are of metamorphic origin. They are intergrown with silicates and carbonates, with the mineral assemblage indicative of greenschist facies (biotite grade) metamorphism. Chemical compositions of ironstones vary greatly and reflect changes from silica-, alumina-poor ironstones formed by predominantly chemical precipitation processes to silica-, alumina-rich examples with a significant detrital component. Silica-, alumina-poor ironstones are characterised by low concentrations of transition metals and large ion lithophile and high field strength elements and display REE signatures of modern coastal seawater. The Braemar facies accumulated in a marine basin along the border of a continental glaciated highland and a low-lying weathered landmass. Wet-based glaciers originated from the Palaeoproterozoic to Mesoproterozoic metamorphic basement and debouched into a fault-controlled depocentre, the Baratta Trough. The intimate association of dolostones, manganiferous siltstones, ironstones and diamictites can be explained by a transgressive event during a postglacial period. Hydrothermal exhalations added significant amounts of Fe and other metals to Neoproterozoic seawater. Melting of floating ice led to an influx of clastic detritus and deposition of glaciomarine sediments from wet-based glaciers and to oxygenation of ferriferous (±manganiferous), carbonate and CO 2 charged coastal waters. Release of CO 2 to the atmosphere from the oxygenated waters resulted in the precipitation of carbonate as dolostones and oxygenation of ferriferous (±manganiferous) waters led to the precipitation of Fe 3+ oxides as laminated ironstones and as matrix of diamictic ironstones. Further increases in Eh conditions led to the precipitation of Mn oxides or carbonates and their incorporation in clastic sediments. Thus the Braemar ironstone facies is the result of chemical precipitation of dissolved Fe (and Mn) during a postglacial, transgressive period and formed in a near-coastal environment under significant terrestrial influences.