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Investigating the high Zr values in the fine fraction of stream sediments in Nigeria

Key, Roger M.; Johnson, Chris C.; Horstwood, Matthew S.A.; Lapworth, Daniel J.; Knights, Kate V.; Kemp, Simon J.; Adekanmi, Michael; Arisekola, Tunde; Everett, Paul. 2011 Investigating the high Zr values in the fine fraction of stream sediments in Nigeria. [Poster] In: 10th International Congress for Applied Mineralogy (ICAM), Trondheim, Norway, 1-5 Aug 2011. (Unpublished)

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Abstract/Summary

Regional geochemical surveys in Nigeria have consistently found very high concentrations of Zr in the fine (<150μm) fraction of stream sediments irrespective of underlying bedrock in upstream catchment areas. For example, mean/maximum Zr values of about 0.2%/2.3% and 0.3%/2.6% were determined in about 1600 analysed stream sediments collected from two 160X160 km2 Global Reference Network (GRN) cells[1] in central (the ‘Minna Cell‘) and SW (the ’South-western Cell‘) Nigeria respectively [2, 3]. X-ray diffraction (XRD) studies on a sub-set of the analysed stream sediments showed that Zr is predominantly found in detrital zircons grains. The main proximal source rocks for the analysed stream sediments are the widespread Pan-African, late Neoproterozoic ‘Older Granites’ of Nigeria, their Palaeoproterozoic-Neoproterozoic migmatitic gneiss country rocks and a local cover of Cretaceous siliciclastic sedimentary strata. However, these rocks are not enriched in zircon so the zircon enrichment in the stream sediment must be due to surficial processes. A combination of tropical/chemical weathering and physical weathering involving a combination of wind winnowing during the dry season and flash flooding during the rainy season has effectively broken down bedrock silicate minerals and removed much of the resultant clay phases. The weathering has formed quartz-rich stream sediments enriched in resistant ‘heavy’ minerals, including zircon. U-Pb LA-MC-ICP-MS dating with cathodoluminescence (CL) imaging on detrital zircons from four stream sediment samples (and on zircons extracted from underlying Pan-African ‘Older Granites’) confirms a local bedrock source for the stream sediment zircons. None of the examined zircon grains show significant signs of rounding which you might expect from transported distal material. Detrital zircon grains collected from two stream sediment samples with Older Granite-dominated catchment geology give 630-640 Ma age populations identical to the determined ages of the underlying granites. Detrital zircons from two stream sediment samples collected from migmatitic gneiss terrains yielded a range of Proterozoic ages (580- 640 Ma, 1200 Ma, >2150 Ma, 2200 Ma) to match published absolute ages from the migmatitic gneisses of Nigeria. The ‘anomalous’ high values of Zr, as well as other elements concentrated in resistant ‘heavy’ minerals in Nigeria’s streams is predominantly due to in-stream winnowing processes and may not reflect anomalous concentrations of these elements in the bedrock of catchment areas of the collected stream sediment samples. This conclusion has important implications for using stream sediment chemistry as an exploration tool for primary metal deposits associated with heavy minerals. However, Zr appears to be a good pathfinder element for placer deposits of heavy minerals.

Item Type: Publication - Conference Item (Poster)
Programmes: BGS Programmes 2010 > BGS Corporate
Additional Information. Not used in RCUK Gateway to Research.: Poster presented at the official poster session of the conference following peer review by the conference organisers
NORA Subject Terms: Meteorology and Climatology
Earth Sciences
Atmospheric Sciences
Chemistry
Date made live: 09 Aug 2011 15:16
URI: http://nora.nerc.ac.uk/id/eprint/14863

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