Summary:
Mining and exploration occur in an area where tectonically induced salt (evaporite) mobilisation took place, followed by uplift and erosion which resulted in today’s geological framework. The bedrock map shows the intersections of northwest striking and plunging synclines and anticlines and north-striking faults. The Kipoi anticline is a major structure and its core is faulted by a reverse fault or thrust which displaced the southwest flank over the northeast flank. The faulted anticline is followed north and south by synclines occupied by Nguba Group and Kundelungu Group sediments.
Kipoi Central Copper (Cobalt-Silver) Project
Kipoi Central mineralisation is hosted in a fragment consisting of Mwashya Series (R-4), the youngest of the Roan Group, overlain by diamictites of the Nguba Group. Both units form one allochthonous fragment removed from its original geological context and surrounded by breccias of the RAT.
The diamictites of the Nguba Group rest disconformably over the Mwashya Subgroup sediments and are referred to as the “grand conglomérat” stratigraphic formation. The contact is tectonically overprinted. The “grand conglomérat” consists of a massive unstratified, poly-lithic matrix supported breccia/conglomerate including well rounded, randomly distributed pebbles and boulders of quartzite, granites and schist. Its contact with Mwashya Subgroup is a concave surface (as mapped within the pit area and interpreted from drilling) that truncates layering in the Mwashya Subgroup at high angle.
The epiclastic rocks are overlain by undifferentiated, finely laminated dolomitic siltstones with interbedded dolostone. Two such dolomitic horizons can be mapped in the pit and are both about 15–25 m thick. The siltstones are bleached near surface and moderately graphitic and sulphidic at depth.
Dolomitic siltstones grade upward into fine-laminated, parallel, thin- to medium-bedded, highly graphitic siltstones. Siltstones are generally undeformed however show strong folding and faulting near the contact with RAT breccia. Graphitic siltstone hosts about 75% of mineralisation.
The brèche hétérogène is developed in RAT and has a talcose, calcareous matrix with sub-angular to rounded clasts of grey-green and light-red purple calcareous siltstone. The brèche hétérogène “body” has very irregular contacts. It may form dykes cutting across layered strata and may incorporate large (>1 m) fragments of Mwashya Series rocks.
At Kipoi Central, hypogene chalcopyrite, bornite and chalcocite are oxidised to malachite, azurite, pseudomalachite and chrysocolla. As is the case with most Congolese copper deposits, the wealth of many deposits is in the supergene enrichment zone of the regolith profile. Most mineralisation is hosted in brittle and brittle-ductile trap sites in graphitic, sulphidic, or dolomitic siltstones and dolomites of the upper R-4 sediments with subordinate stratabound mineralisation. Mineralisation commonly occupies interconnecting, conjugate and bedding parallel veins, the matrix of crackle and mosaic breccias, as well as in the matrix of tectonic rubble breccias.
Kipoi North Copper Deposit
The Kipoi North deposit is located about 0.8 km north of the Kipoi Central mine. Despite this proximity, the geology at Kipoi North differs in terms of host rocks from all other deposits in the project area. Kipoi North is hosted in R-2 rocks. The R-2 (Mines Series) is stratigraphically older than the R-4. The R-2 series rocks are known to extend for about 800–900 m in an eastwest direction. The eastern limit is confirmed by drilling and the western extent is open. The R-2 rocks are not closed off at depth. The R-2 rocks of the Kipoi North deposit are preserved akin to the Kipoi Central deposit as a fragment of the Roan Group tectonically isolated from its original context.
The Lower Mines Series (R-2) is a succession that includes, from the stratigraphically oldest to youngest (north to south at Kipoi North), a massive grey-green and red unit of graded and cross bedded, laminated calcareous siltstones and sandstones of unknown thickness referred to as the Grey RAT. The sandstones develop upwards into fine laminated siltstones with evaporitic textures and develop into a stratified cherty, nodular, centimetre-scale banded dolomitic unit, about 4–7 m thick (“DSTRT” or Roche Stratifee). The cherty dolomite transitions into a very finely laminated (millimetre-scale layers) of stromatolitic or cryptalgal laminated dolomite (Roche Siliceuse Feuilletee or “RSF”), which measures 3–5 m thick on average. There is a rapid but uninterrupted transition into the overlying vughy, cellular, massive, stromatolitic, silty dolomite (Roche Siliceuse Cellulaire or “RSC”). This unit can be up to 40 m thick. Its characteristic circular (tube or column-like) texture, with cement-filled inner tubes reflects formation as a bioherm under shallow, but rising, sea level conditions. Near the hangingwall, the columnar stromatolites are intermittently and temporarily, probably also locally capped by a thin (˜1 m thick) layer of calcareous laminated siltstones interpreted to indicate a sudden change in depositional conditions (reef drawing) before the bioherm is re-established. The presence of the SD unit (dolomitic siltstone) overlying the RSC is established soon after and indicated the longer-term transition to deeper water siltstones and carbonates in the Kipoi area.
The style of mineralisation at Kipoi North differs in line with the lithology from that at Kipoi Central. The mineralisation at Kipoi North is, despite its predominantly secondary nature, primarily stratabound and concentrated in the DSTRT, RSF, and RSC units. In the DSTRT, malachite occurs parallel to the thin laminated, algal matted silty dolomite layers. In the RSF, mineralisation occurs pervasively through the thin bedded layers of the rock, while the mineralisation in the RSC is associated with the dolomitisation/dissolution vughs, veins and fractures of the rock occurring immediately above the contact with the RSF. The mineralisation extends, in places, over several tens of metres of this unit, however it tends to be variable.
Kileba Copper Deposit
The host rocks of the Kileba deposit form a northwest to southeast trending ridge that extends from Judeira north of Kipoi past Kipoi Central to Kileba in the southeast. The deposit area includes two in-line ridges separated by a gap that gives access to the prospect area. The modelled and estimated Kileba mineralisation is confined to the south-eastern ridge segment but extends for at least 600 m to the northwest. The ridge is a reverse faulted anticline with southwest side over northeast side.
The Kileba deposit is about 5 km southeast of Kipoi Central. At Kileba, the southwest flank of the ridge exposes diamictities of the Nguba Group overlying a thick interbedded dolomitic and shaley siltstones unit in contact with talcose, evaporitic calcarenite unit and interbedded cherty carbonates has been intersected in drilling. It is inferred that this succession is structurally composited. This succession has been interpreted as upper Mwashia (R-4.2) where it gradually transgresses into diamictites to the southwest and Lower Mwashia (R-4.1) in the footwall.
Copper mineralisation is localised in a variety of brittle settings at Kileba. This includes primarily vein-hosted, disseminated and strata bound, and fault breccia-controlled mineralisation. A significant proportion of the mineralisation occurs within the halo of strong brecciation and silicification surrounding the faulted R-4/Nguba-Kundelungu footwall contact. In the hangingwall of the fault, the stratabound disseminated mineralisation is hosted within a graphitic, sulphidic calcarenite unit with abundant evaporite textures (gypsum needles and chicken wire texture). The siltstone unit above the calcarenite features abundant vein-controlled mineralisation.