LOCAL GEOLOGICAL SETTING 1. Basal beds (K1) and coal measures (K2)

The oldest sediments of the Karoo sequence of the North Rukuru Basin have been termed ‘Basal Beds’ [2]. The Basal Beds include glacial and glacio–lacustrine sediments (K1) namely diamictite (tillite) with overlying flaggy sandstone and varved shale beds (Fig. 2(a)). This glacial unit has been dated as Lower Permian (Sakmarian) using palynological evidence [2].

Overlying coal measures and arkose were previously included in the North Rukuru Sandstone [2] but are here assigned to K2. The base of the coal measures is defined by a cross-bedded pebbly sandstone. Individual pebbly grit beds grade into fine-grained sandstone and are separated from the next grit bed by thin layers of fine, micaceous, flaggy sandstone. This overall

fining upward succession is overlain by a sequence of mudstone, carbonaceous shale and coal seams up to 1.5 m thick.

3.2. North Rukuru Sandstone (K3 to K5)

Overlying the Basal Beds with angular unconformity are arkosic sandstones and mudstones of the North Rukuru Sandstone, deposited in braided and meandering river systems [2]. Several informal units are recognized within the North Rukuru Sandstone [2]. These are the Upper Kalopa Arkose Member, Muswanga Red Bed Member and Kayelekera Member. The arkoses of the Muswanga Member are characterized by a hematitic matrix that is partially altered to goethite on weathering. A distinctive bed containing fossilized wood occurs at the top of the Muswanga Member. This bed defines the top of K3 in the Kayelekera area.

The Kayelekera Member (K4) is about 150 m thick and is the main uranium host. It is relatively well-known due to numerous drill hole intercepts and exposure in the open pit. At least 10 arkose units have been identified which range in thickness up to 14 m (Fig. 3). Each arkose is assigned a letter of the alphabet with production to date sourced mainly from the U, T and S units. Arkoses define the base of cyclothems and pass upwards into reddish to chocolate brown

‘oxide facies’ mudstone and then into ‘reduced facies’ grey-black carbonaceous and silty mudstone (Fig. 2(c)). Thin coal rich horizons are present at the top of some cyclothems. The redox interface defined by the change from oxide to reduced facies mudstone is bedding-parallel (Fig. 2(c)) and is probably indicative of fluctuations in redox potential during, or soon after, sedimentation. Several carbonaceous samples from the Kayelekera Member were dated as Middle Permian (Kazanian) using palynological evidence [2].

The arkoses contain poorly sorted clasts of subrounded to subangular microcline, perthite, plagioclase, quartz, chert, polycrystalline quartz, biotite, muscovite, mudstone pellets, cellular plant material and unidentified carbonaceous material associated with framboidal pyrite [2, 12].

Feldspars are typically pink to red, with the red coloration is interpreted to have been inherited from source [2, 12]. Carbonaceous debris occurs as fine layers, as disseminations and as individual woody fragments several centimetres in length. Discrete dark-colored layers of

<1 mm thickness are defined by higher concentrations of carbonaceous material or heavy minerals such as ilmenite, zircon and rutile.

The arkoses can be classified as arkose, litharenite and Fe–sand and the mudstones as shales and Fe–shale according to the scheme of Ref. [13]. Mudstone samples have been analysed by X ray diffraction and by short wave infrared spectroscopy [12]. Both techniques confirm smectite as the dominant clay constituent together with minor kaolinite and white mica.

Dominant kaolinite was found in surface and near surface samples and thus probably relates to relatively recent weathering. Little difference in mineralogy was noted between oxidized (red-brown) and reduced (grey) mudstones.

FIG 2. (a) Basal Beds (K1) near the basement unconformity along the Karonga–Chitipa road. Tillite at the base overlain by flaggy sandstone and varved shale; (b) Coal measures (K2) along the Karonga–Chitipa road. Flaggy micaceous sandstone, carbonaceous shale and coal seams; (c) S arkose and ST mudstone showing horizontal redox interface (dashed yellow line) between grey and chocolate brown mudstones. Open pit; (d) Carbonaceous mudstone; (e) Lacustrine sediments, grey–green mudstone and creamy white limestone (examples arrowed), possible lateral equivalent of K4.

A distinctive Karoo unit has in the past been assigned to K5. This unit is relatively rich in grey-green mudstone and discrete limestone beds (Fig. 2(d)). Recent mapping suggests that this is laterally equivalent to the K4 unit and represents transition from alluvial channels into a lacustrine environment.

3.3. Post–depositional history

The margins of the NRB are not well exposed, but there is little doubt that the eastern margin is defined by a major NW–SE trending fault, referred to as the Eastern Boundary Fault. The dip of this fault is poorly constrained, but is likely to be steep if not vertical, at least near the surface.

Sediments of the North Rukuru Basin generally dip at 35°E. Adjacent to the fault on the eastern margin of the basin, however, the dip is often 10 to 20°W. This dip reversal has been interpreted as the result of faulting [2].

The eastern boundary fault actually consists of several fault planes, one of which is exposed in the eastern part of the pit where normal displacement is evident. The deposit occupies a syncline axial zone which is a down-faulted block bounded by NNW trending normal faults. Transverse

faults with limited offset cut across this structure causing a dip reversal to the north and the creation of a basin structure bounded by faults on three sides.

The western boundary of the NRB is suggested by magnetic data to be shallower and could be a low angle fault or depositional unconformity, juxtaposing K1 glacial sediments or K2 Coal Measures against metamorphic basement rocks.

4. THE OREBODY