Petrographic-microchemical studies and origin of the Agbaja Phanerozoic Ironstone Formation, Nupe Basin, Nigeria: a product of a ferruginized ooidal kaolin precursor not identical to the Minette-type |
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Authors: | A Mücke T A Badejoko S O Akande |
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Institution: | Mineralogisch-Petrologisches Institut, Georg-August-Universit?t, Goldschmidtstrasse 1, D-37077 G?ttingen, Germany, DE Department of Geology, University of Ibadan, Ibadan, Nigeria, NG Department of Geology, University of Ilorin, Ilorin, Nigeria, NG
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Abstract: | The Campanian-Maastrichtian Agbaja Ironstone Formation of the Nupe basin, Nigeria, forms a major part of the about 2 billion
tons of iron ore reserves of the Middle Niger Embayment. The ironstone deposits were previously reported to be similar to
the Minette-type ironstones because of their depositional patterns, composition and inferred origin. Four rock-types are recognized
within the Agbaja Ironstone Formation: ooidal pack-ironstone, pisoidal pack-ironstone, mud-ironstone and bog iron ore. In
the ironstones, kaolinite of both the groundmass and the ooids/pisoids is of lateritic origin, whereas the associated quartz,
mica and heavy minerals are of detrital origin. Ooids and pisoids were formed by mechanical accretion of platy kaolinite crystals
by rolling on the sea floor in a near-shore environment, and were subsequently transported and deposited together with a fine-grained
kaolinitic groundmass. Pyrite (mainly framboidal) and siderite (both exclusively occurring as pseudomorphs of goethite and/or
hematite) are diagenetic whereas goethite is post-diagenetic in origin, resulting from the ferruginization of the kaolinitic
precursor. Crandallite-gorxeicite-goyazite, bolivarite and boehmite are also post-diagenetic in origin. Hematite was formed
from the dehydration of goethite, whereas gibbsite (restricted to the upper part of the deposit) is of recent and in situ lateritic origin. The presence of newly formed authigenic pyrite and siderite (now replaced by hematite and goethite) are
indicators of a reducing environment during diagenesis. The absence of diagenetic chamositic clay minerals, evidently caused
by a low Mg concentration, suggests that fully marine conditions were not established during sedimentation. This is supported
by the lack of fossils, brecciated shell materials and bioturbation features in the deposit. Reworking and redeposition of
the primary constituents are inferred from broken pisoids, nuclei of pisoidal/ooidal fragments in pisoids and high iron concentrations
present in the pisoids and ooids compared to that of the groundmass. These observations indicate that the Agbaja ironstone
deposits of the Lokoja study area exhibit some environmental and mineralogical characteristics that are markedly different
from other known deposits of Minette-type, where primary chamositic clay minerals generally form the protore for the ironstones.
The recognition of kaolinite as the precursor constituent and the occurrence of similar deposits of the same age (Late Cretaceous)
in Nigeria, Sudan and Egypt have implications for the paleoenvironmental interpretations of Phanerozoic ironstone deposits.
Received: 16 February 1998 / Accepted: 8 July 1998 |
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