Geochemistry of subsurface Late Quaternary ironstones in Rajshahi and Bogra Districts, Bangladesh: implications for genetic and depositional conditions
Rahman, M.S.; Hossain, I.; Biswas, P.K.; Rahim, M.A.; Hasan, A.S.M.M.; Adham, M.I.
Acta Geochimica 38(3): 404-413
2019
ISSN/ISBN: 2096-0956 Document Number: 213658
The present study deals with the geochemistry of Late Quaternary ironstones in the subsurface in Rajshahi and Bogra districts, Bangladesh with the lithological study of the boreholes sediments. Major lithofacies of the studied boreholes are clay, silty clay, sandy clay, fine to coarse grained sand, gravels and sands with (fragmentary) ironstones. The ironstones contain major oxides, Fe2O3* (* total Fe) (avg. 66.6 wt%), Si O2 (avg. 15.3 wt%), Al2O3 (avg. 4.0 wt%), Mn O (avg. 7.7 wt%), and Ca O (avg. 3.4 wt%). These geochemical data imply that the higher percentage of Fe2O3* along with Al2O3 and Mn O indicate the ironstone as goethite and siderite, which is also validated by Xrd data. A comparatively higher percentage of Si O2 indicates the presence of relative amounts of clastic quartz and manganese-rich silicate or clay in these rocks. These ironstones also have significant amounts of Mn O (avg. 7.7 wt%) suggesting their depositional environments under oxygenated condition. Chemical data of these ironstones suggest that the source rock suffered deep chemical weathering and iron was mostly carried in association with the clay fraction and organic matter. Iron concretion was mostly formed by bacterial build up in swamps and marshes, and was subsequently embedded in clayey mud. Within the coastal environments, the water table fluctuates and goethite and siderite with mud and quartz became dry and compacted to form ironstone.