Abstract
Ironstone occurs in four landscape positions in the Dallol Bosso valley of Niger:
1.
on a high plateau,
2.
on terraces,
3.
in dunes and
4.
in partially or completely filled channels.
The Fe oxide mineralogy of the older plateau and terrace ironstones contrast with that of the newly formed pedogenic ironstone of the valley. Plateau ironstone is characterized by a hematiterich phase whereas terrace gravels are goethitic. In the valley, ironstone occurs at the base of dunal soils along the contact between medium to fine eolian sands and coarser fluvial sands; there it consists of quartz sand cemented by goethite. A maghemite-hematite association occurs at the top of the ground water table. Neoformation of the valley mineral phases occurs in an aquic soil environment and is driven by the large amount of surface evaporation in the semi-arid climate. The presence of Fe in the ground water of the Dallol Bosso suggests an active process of Fe movement. There is an abundant reservoir of Fe in the plateau ironstones that may be slowly released by physical and chemical weathering. Although the source of the Fe is speculative, secondary Fe phases are likely to continue to form as the Dallol Bosso landscape evolves.