Genesis of the Hussainiyat ironstone deposit, Western Desert, Iraq |
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Authors: | K S Al-Bassam M Y Tamar-Agha |
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Institution: | (1) S.E. of Geological Survey & Mining (GEOSURV), P.O. Box 986 Alwiya, Baghdad, Iraq, IQ;(2) Department of Geology, College of Science, University of Baghdad, Jaddiriyah, Baghdad, Iraq, IQ |
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Abstract: | The Hussainiyat ironstone deposit (Jurassic) is mainly pisolitic, intraclastic and concretionary in texture, associated with
kaolinite mudstones and/or with quartzose sandstone. The ironstone consists mainly of goethite, hematite, kaolinite and quartz.
The deposits were derived from a variety of parent rocks that included low- and medium-rank metamorphics, intermediate igneous
rocks and pre-existing sediments of the Nubio-Arabian Shield. The source rocks suffered deep chemical weathering in the hinterland,
and the products (Fe-oxyhydroxides, kaolinite and quartz) were later transported by rivers to the depositional site. Iron
was mostly carried in association with the clay fraction and organic matter. Several genetic processes were involved in the
ironstone formation. Iron concretions were mostly formed by bacterial build-up in swamps and marshes, and were subsequently
embedded in kaolinitic mud. Large-scale development of groundwater laterite blanket (ferricrete) occurred later in the overbanks
and floors of wadis, under oxidizing pedogenic conditions. During this stage iron was mobilized from the kaolinitic deposits
and migrated upward in dry seasons and, to a lesser extent, downward in wet seasons. Pisolites and oolites grew in situ in
the kaolinitic soil at the upper limit of the fluctuating water table. This ferricrete blanket had a wide and continuous extension
within an elongated paleodepression. Seasonal heavy rain periods resulted in the flow of ephemeral streams and rivers, where
major parts of this ferricrete was reworked and deposited with quartz sand and mud clasts as channel deposits. The original
pisolitic-colloform ironstone was reworked continuously to form a semi-continuous sheet. In such wet seasons, additional Fe-enrichment
took place as cementing materials or overgrowths.
Received: 28 April 1995 / Accepted: 10 July 1997 |
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