Typology of Fe-Coated Grains and Diagenetic History of Agbaja Ironstone Formation Exposed at Mount Patti and Koto – Karfi, Southern Bida Basin, Nigeria
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Date
2022
Journal Title
Journal ISSN
Volume Title
Publisher
Journal of Mining and Geology
Abstract
Ooidal ironstones from theMaastrichtian Agbaja Formation, Bida Basin in the north central Nigeria were investigated
and sampled around Mount Patti and Koton – Karfi where they formed consistent cap over the older sedimentary
sequence. The ironstone samples collected from the outcropping sections were subjected to microscopic and
mineralogical studies in order to examine their texture diagenetic history and origin. The ironstone iswell indurated,
dark to reddish brown and occurs as oolitic – pisolitic, laminated – massive and concretional lithotypes. The
ironstones as well as the associated lithologies are characterized by cross and parallel laminations, ripple marks with
burrows of Thalassinoides and Skolithos ichnogenera. Thin section and Scanning Electron Microscopy images
reveal scars on the surface of some Fe – coated grains which is interpreted to have been due to attrition, abrasion and
impart during transportation. The main textural components include: true and superficial ooids, oncoids, matrix,
pores, lithorelicts and bioturbations with varying level of iron replacement. X – Ray Diffraction result reveal that
kaolinite and quartz are the main detrital precursors in the ironstone while goethite, hematite and magnetite form the
diagenetic Fe-bearing minerals. Finning upward depositional characteristics, bioturbations, well as broken and
scoured surface coated grains suggest a high energy shelf environment with agitated hydrodynamic conditions.
Diagenetic processes responsible for the formation of the ironstones include precipitation, replacement,
recrystallization and dehydration of Fe-oxide. The study shows that the ironstones are mainly post-depositional in
origin with evidences of progressive replacement of the precursor kaolinitic coated grains and groundmass by iron
oxide.