The Kayelekera uranium deposit, northern Malawi : past exploration activities, economic geology and decay series disequilibrium

The present paper describes the exploration and evaluation work carried out by the Central Electricity Generating Board on the Kayelekera uranium deposit in Northern Malawi between 1983 and 1991. This is one of the largest Karoo age sandstone hosted uranium deposits yet discovered. Approximately 200 boreholes, y60% of which were fully cored, were drilled into the deposit during this evaluation. An important part of the ore reserve estimation undertaken by the Central Electricity Generating Board at Kayelekera was gaining an understanding of the uranium decay series distribution within the deposit. Being located in a near surface environment the deposit is subject to weathering effects caused by oxidising groundwater. Three ore types are recognised: reduced facies ore, oxidised facies ore and transitional facies ore containing both oxidised and reduced material in varying proportions. Being more mobile under oxidising conditions uranium tends to be leached from the oxidised parts of the deposit and re-deposited in more reducing parts. However its gamma emitting daughters tend to be less mobile in an oxidising environment leading to a marked disequilibrium between uranium and its daughters with the oxidised facies ore being depleted in uranium relative to its daughters and the reduced facies ore often showing relative enrichment.