by Grain Wyson Phillip Malunga FIMMM Mining and Environmental Management Expert
Uranium in Malawi
Malawi has high potential for discovery of economic deposits of uranium in both intrusive granitic rocks and Karoo sediments. Recent country airborne geophysical survey (2013/14) will be the basis for analysis, interpretation and discovery of more potential exploration targets.
The paper tries to highlight the previous exploration work done so far and potential areas for further exploration.
Uranium is a chemical element, U, that is found in radioactive minerals. It occurs naturally in low concentrations in soils, rock and water. Uranium is used a high density penetrator for nuclear war heads and as fuel to nuclear power plants. Natural uranium is mainly U238 with U235 and U234 occurring in small amounts of 0.72% and 0.0052%. U235 is fissile and that is the one that is used in nuclear plants. One kilogram of Uranium-235 can release an equivalent energy amount of 1500 tonnes of coal.
Uranium’s average concentration in in soil, rock and water is between 0.7 to 11 ppm, 2 to 5 ppm and about 3 ppb respectively. Most large uranium reserves range from 100 ppm to 200 ppm. Some small reserves can be exploited with grades over 400 ppm such as the Kayerekera uranium deposit with an estimated tonnage of 1,500.
Geology of Uranium
Background concentration of uranium in rocks is between 2 ppm and 5 ppm. Any values above 100ppm are economic. In igneous rocks, U is associated with Zirconium (Zr), niobium (Nb), tantalum (Ta), titanium (Ti) and rare earths (REE or lanthanides). In Malawi uranium is associated with syenitic and granitic rocks. The main source of uranium is a mineral called pitchblende and uranite. Other uranium bearing minerals include zircon, betafite, allanite, zircon, davidite, uranothorianiteand davidite in aplites and pegmatites associated with nepheline syenite gneiss.
Malawi has sandstone hosted uranium deposits in Karoo rocks. These are secondary deposits dissolved in water after weathering of its minerals, transported in oxygen rich water and deposited in reducing (oxygen poor) environments. The Kayerekera uranium and Livingstonia uranium deposits were formed in such environments.
Intrusive Uranium deposits
Uranium pyrochlore deposit exist at Ilomba Hill (Table 1). The mineral occurs in pyroxenitelenticles in aegerinefoyaite. Uranium can be recovered as a by-product in the extraction of niobium.
Table 1: Igneous uranium deposits
Sedimentary Uranium Deposits
Uranium in Karoo sedimentary rocks is hosted in sandstone within the North Rukuru basin. The main mineralised unit at Kayelekera(Table 2)is a basal arkosic unit which lies above the coal measures sequence.
The site has also 1.6 million tonnes at 756 ppm U3O8in stock pile (PAL Project Brochure,2014)
Karoo sediments of Livingstonia which form basal beds of arkose and pebble bands contain uraniferousphosphatic material.
Figure 1 shows potential areas for uranium exploration in Malawi.
Area 1: Kirk Range Block (Map sheet 1434D, 1534B and 1534D)
This area is mostly occupied by basement gneisses of amphibolitic to granulitic metamorphic facies. Ultramafic to granitic intrusive rocks dominate the area. Target minerals include corundum and platinum while fault related mineralisation include sulphides and gold. Ring structures offer opportunities for discovery of rare earths, radioactive and sulphide minerals.
Area 3: Lengwe – Chiromo Block (Map sheets 1634B and 1634 D)
This block is dominated by Karroo rocks within a sedimentary basin with ultrabasic intrusivescharacterised by basalts and dolerites. This is an exploration target for agates, kimberlites, coal, uranium and hydrocarbon.
Area 6: Northern Karoo Basins (Map sheets 0933D, 1033B, 1034A and 1034C)
Area 6 is mainly underlain by amphibolites facies gneisses with cordierite hornblende and biotite. The western area hosts greenschist facies popularly known as Mafingi Group. Intrusive rocks include the Nyika granite. Karroo sediments overly these basement rocks in structural depressions known as North Rukuru, Ngana and Chilumba basins. Economic targets, worth pursuing, include coal, uranium, kimberlite (western edge of Chilumba Basin) and hydrocarbon within the lake.
Area 7: Nkhata Bay Block (Map sheets 1134A and 1134C)
This area is mainly underlain by basement gneisses containing biotite and grading into pyroxene granulite or charnockite. The Rumphi alkaline complex is in the northwest corner of the block. The Lunyangwa trough is believed to host Karroo sediments. Economic targets in this block are coal, uranium, pegmatite minerals and rare earths.
The piece “Technical File (Grain Malunga): Uranium in Malawi” featured above was initially published in Malawi’s Mining & Trade Review Issue Number 41 that is circulating this September 2016.
The full edition is available for download here. This monthly publication is edited by Marcel Chimwala.