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

The eastern part of the Adelaide Geosyncline contains well preserved glacio marine sequences of the Sturtian glaciation (approximate to 750-700 Ma) inc luding calcareous or dolomitic siltstone, manganiferous siltstone, doloston e and diamictite units and the associated Braemar ironstone facies. The iro nstone facies occurs as matrix to diamictites and as massive to laminated i ronstones 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. T hey are intergrown with silicates and carbonates, with the mineral assembla ge indicative of greenschist facies (biotite grade) metamorphism Chemical c ompositions of ironstones vary greatly and reflect changes from silica-, al umina-poor ironstones formed by predominantly chemical precipitation proces ses 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 eleme nts and display REE signatures of modern coastal seawater. The Braemar faci es accumulated in a marine basin along the border of a continental glaciate d highland and a low-lying weathered landmass. Wet-based glaciers originate d from the Palaeoproterozoic to Mesoproterozoic metamorphic basement and de bouched into a fault-controlled depocentre, the Baratta Trough. The intimat e association of dolostones, manganiferous siltstones, ironstones and diami ctites can be explained by a transgressive event during a postglacial perio d. Hydrothermal exhalations added significant amounts of Fe and other metal s to Neoproterozoic seawater. Melting of floating ice led to an influx of e lastic detritus and deposition of glaciomarine sediments from wet-based gla ciers and to oxygenation of ferriferous (+/- manganiferous), carbonate and CO2 charged coastal waters. Release of CO2 to the atmosphere from the oxyge nated waters resulted in the precipitation of carbonate as dolostones and o xygenation of ferriferous (+/- manganiferous) waters led to the precipitati on of Fe3+ oxides as laminated ironstones and as matrix of diamictic ironst ones. Further increases in Eh conditions led to the precipitation of Mn oxi des or carbonates and their incorporation in elastic sediments. Thus the Br aemar ironstone facies is the result of chemical precipitation of dissolved Fe (and Mn) during a postglacial, transgressive period and formed in a nea r-coastal environment under significant terrestrial influences. (C) 2000 El sevier Science B.V. All rights reserved.