PETROGRAPHIC AND PARAGENETIC STUDIES OF THE AGBAJA IRONSTONE FORMATION, NUPE BASIN, NIGERIA

The Agbaja Ironstone Formation of the Lokoja district of Central Niger ia occurs within the Upper Cretaceous sedimentary sequence of the Nupe Basin which trends northwest-southeast. Three lithostratigraphical un its occur in the basin: the basal Lokoja Formation, overlain by the Pa tti Formation and the uppermost Agbaja Ironstone Formation. The Lokoja Formation rests unconformably on the Precambrian basement complex and grades upwards from conglomerate to arkosic sandstone. The Patti Form ation consists of interbedded sandstone, siltstone and carbonaceous mu dstone units. The Agbaja Ironstone Formation is made up of oolitic, pi solitic and argillaceous ironstones. Four petrographic facies are iden tified within the Agbaja Ironstone Formation: ooidal pack-ironstone, p isoidal pack-ironstone, detrital mud-ironstone and breccia mud-ironsto ne. Kaolinite ooids are spherical and fragmented, usually with pseudom orphs of goethite after pyrite at the core. The goethite pisoids are c emented in a kaolinitic to goethitic matrix, are elliptical to subsphe rical in shape, and are composite in nature. Constituent minerals of t he mud-ironstone are kaolinite, quartz and heavy minerals. Paragenetic studies indicate that pyrite, siderite, kaolinite, quartz, mica and h eavy minerals were the primary minerals of the ironstone deposit, wher eas secondary minerals were goethite, hematite, goyazite-crandallite, bolivarite and boehmite. Mineralogical studies provided evidence for a kaolinitic precursor for the ironstone deposit, contrary to the earli er proposed chamositic precursor. In addition, the presence of pseudom orphs of pyrite (in the nuclei and incorporated into the cortex of the ooids) are reliable indicators of a possible accretionary model for t he formation of ooids and pisoids prior to ferruginisation. Two ferrug inisation periods unrelated to lateritisation are indicated for the ir onstone deposit. (C) 1997 Elsevier Science Limited.