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Adelaide Geosyncline, South Australia - Geochemistry & Petrology and Neoproterozoic Origin of Ironstones in the Eastern Part

The ironstone facies are a matrix to diamictites and as massive to laminated well-preserved glaciomarine sequences of the Sturtian Glaciation, about 750-700 Ma, are present in the eastern part of the Adelaide Geosyncline, that includes calcareous and dolomitic siltstone, manganiferous siltstone, dolostone and diamictite units, as well as associated Braemar Ironstone facies. The ironstones, comprising abundant iron oxides (haematite and magnetite) and quartz, minor silicates, that include muscovite, chlorite, plagioclase, tourmaline, carbonate and apatite, and detrital mineral grains and lithic clasts. Magnetite and haematite are indicated by micro-textures to be of metamorphic origin. They are intergrown with silicates and carbonates, the mineral assemblages being indicative of greenschist facies, biotite grade, metamorphism. Ironstones are formed with a large range of compositions with silica- and aluminium-poor ironstones being formed mostly by chemical precipitation processes, and silica- aluminium-rich ironstones with a significant component of detritus. Low concentrations of transition metals and large ion lithophile and high field strength elements and display REE signatures of modern coastal sea water are characteristic of ironstones that are silica-, aluminium-poor. The accumulation of the Braemar facies took place in a marine basin along the border of a continental glaciated highland and a low-lying landmass that was weathered. Glaciers that were wet-based, originating in metamorphic basement from the Palaeoproterozoic-Mesoproterozoic, debouched into the Baratta Trough, a fault-controlled depocentre. According to the authors1 a transgressive event during a post-glacial period could be an explanation for the intimate association of dolostones, manganiferous siltstones, ironstones and diamictites. Significant amounts of iron and other metals were added to seawater during the Neoproterozoic by hydrothermal exhalations. An influx of clastic detritus from the melting of floating ice and glaciomarine sedimentation from wet-based glaciers and the oxygenation of ferriferous (±manganiferous), carbonate and CO2 charged coastal waters. The oxygenated waters released CO2 to the atmosphere leading to the precipitation of carbonate as dolostones and the oxygenation of ferriferous (±manganiferous) waters resulting in the precipitation of Fe3+ oxides as laminated ironstones and as matrix for diamictic ironstones. Eh conditions resulted from further increases which led to the precipitation of Mn oxides or carbonates and the incorporation of them into clastic sediments. Thus chemical precipitation of dissolved Fe (and Mn) during a post-glacial transgressive period resulted in the Braemar ironstone facies, forming a near-coastal environment and a significant terrestrial influence . 

Sources & Further reading

  1. Lottermoser, B. G., and P. M. Ashley. "Geochemistry, Petrology and Origin of Neoproterozoic Ironstones in the Eastern Part of the Adelaide Geosyncline, South Australia." Precambrian Research 101, no. 1 (5// 2000): 49-67.
Author: M. H. Monroe
Last updated 05/08/2013
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