Petrographic and paragenetic studies of the Agbaja Ironstone Formation, Nupe Basin, Nigeria

Abstract

The Agbaja Ironstone Formation of the Lokoja district of Central Nigeria occurs within the Upper Cretaceous sedimentary sequence of the Nupe Basin which trends northwest-southeast. Three lithostratigraphical units occur in the basin: the basal Lokoja Formation, overlain by the Patti 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 Formation consists of interbedded sandstone, siltstone and carbonaceous mudstone units. The Agbaja Ironstone Formation is made up of oolitic, pisolitic and argillaceous ironstones. Four petrographic facies are identified within the Agbaja Ironstone Formation: ooidal pack-ironstone, pisoidal pack-ironstone, detrital mud-ironstone and breccia mudironstone. Kaolinite ooids are spherical and fragmented, usually with pseudomorphs of goethite after pyrite at the core. The goethite pisoids are cemented in a kaolinitic to goethitec matrix, are elliptical to subspherical in shape, and are composite in nature. Constituent minerals of the mud-ironstone are kaolinite, quartz and heavy minerals. Paragenetic studies indicate that pyrite, siderite, kaolinite, quartz, mica and heavy minerals; were the primary minerals of the ironstone deposit, whereas 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 earlier proposed chamositic precursor. In addition, the presence of pseudomorphs of pyrite (in the nuclei and incorporated into the cortex of the ooids) are reliable indicators of a possible accretionary model for the formation of ooids and pisoids prior to ferruginisation. Two ferruginisation periods unrelated to lateritisation are indicated for the ironstone deposit.