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Sedimentary Petrology
Published in Supriya Sengupta, Introduction to Sedimentology, 2017
Iron-bearing sediments include banded (bedded) iron formations (BIF), ironstones, ferruginous ooids and fossil fragments. The bedded iron formations consist of alternating thin layers of silica and ferruginous minerals of chemical origin. The iron-bearing minerals are mostly oxides (magnetite and hematite). Silica occurs as quartz, chert or jasper. The bedded iron rocks, found in all the Precambrian cratons of the world, are often modified by diagenesis and metamorphism. The ironstone formations, which are of younger age, are composed mostly of iron carbonate (siderite) with some iron silicates (chamosite). Ooids and fossil fragments, often replaced by iron oxide minerals, are also known to occur within ironstones. The siderites are often altered to limonite near the surface.
Geological setting of exceptional geological features of the Flinders Ranges
Published in Australian Journal of Earth Sciences, 2020
The Braemar ironstone facies and its equivalent in the central Flinders Ranges, the Holowilena Siltstone, occur as matrix to diamictite and as massive to laminated ironstone and comprise abundant Fe-oxides (hematite, magnetite of metamorphic origin) and quartz, minor silicates, carbonate and apatite, and detrital mineral grains and lithic clasts. Lottermoser and Ashley (2000) concluded that the ironstone is the result of chemical precipitation of dissolved Fe (and Mn) during a post-glacial, transgressive period and formed in a near-coastal environment under significant terrestrial influences. Hydrothermal exhalations added significant amounts of Fe and other metals to the seawater, while oxygenation of ferriferous (±manganiferous) waters led to the precipitation of Fe3+ oxides. Lechte and Wallace (2016) propose deposition under an ice shelf to explain interbedding with massive ice-proximal diamictite.