Iron-rich ooids, their mineralogy and microfabric: clues to their origin and evolution

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Abstract
The controversy surrounding oolitic ironstone origins has been prolonged by a lack of published discriminatory characters which could be applied to determine the sedimentary processes responsible for the genesis of individual iron-rich ooids. This is in part due to the limitations of optical petrography and has led to confusion between authors as to the nature of the ooids each is describing. A new and simple descriptive system which documents ooid mineralogy and fine structure has been used to distinguish ooids of different origins and a classification of iron-rich ooids into three broad mineralogical categories is proposed in this paper. These Fe-oxide/hydroxide, berthierine/chlorite and siderite dominated groups are further divided by characteristic textural criteria into 15 minor categories. The abundance of ooids which easily fit the three bulk mineralogical categories probably reflects common diagenetic modifications, rather than preservation of discrete primary phases. The wide variety of ooid microfabrics observed by transmitted light microscopy, secondary electron, backscattered electron and X-ray emission imagery reflect differing modes of ooid accretion and early diagenesis, in addition to later diagenetic and weathering processes.

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