Summary:
The Lumwana copper deposits are large, tabular bodies of disseminated mineralisation, hosted within the Mwombezhi Dome of the Lufilian Arc, and are typically referred to as basement hosted copper deposits. The Domes Region is part of the Central African Copperbelt, which is a metallogenic province in the border region of Zambia and the Democratic Republic of Congo.
The main copper-bearing minerals at the Lumwana deposits are pyrite, chalcopyrite and occasional bornite, which is typically associated with higher grades. Copper mineralisation is hosted within either biotite or muscovite dominant schists.
The morphology of the deposits is controlled by pre- and syn-mineralisation structures. Later deformation of the structures may have reactivated the mineralising system and aided development of plunging mineralised shoots, such as those observed at Chimiwungo.
The copper deposits at Lumwana are in contrast to other deposits in the Central African Copperbelt, which are generally stratiform, sediment hosted deposits where mineralisation generally conforms to the orientation of the host rock strata.
There are four copper deposits (with subordinate uranium) at Lumwana:
1. Chimiwungo, located in the south of Mwombezhi Dome. This deposit strikes east-west and dips approximately 5° to 10° to the south. It measures approximately 8 km in length, 4 km in width, has variable thickness of between 75 m and more than 200 m, and is truncated by post-mineralisation east-west orientated normal faults. Copper mineralisation is present from surface, where it outcrops and continues to an approximate depth of 950 m. The dominant copper mineral is chalcopyrite hosted in mineralised schist (MS) and gneiss.
2. Malundwe, located on the west of the Mwombezhi Dome. This deposit strikes north-south and dips 10° to 15° to the west. It measures approximately 6 km in length, 1.5 km in width, and 14 m in thickness. Mineralisation is present from surface, where it outcrops and continues to a depth of approximately 380 m. The deposit is also impacted by east-west orientated post-mineralisation normal faulting. The dominant copper minerals are chalcopyrite and bornite hosted in MS and gneiss.
3. Kamisengo, located on the east side of Mwombezhi Dome. This deposit strikes NNW and dips 0° to 20° to the northeast. It measures approximately 4.5 km in length, 2 km in width, and from 70 m to 200 m in thickness. Kamisengo is composed of several MS separated by barren gneisses. Mineralisation is present from surface where it outcrops and continues to approximately 580 m in depth. The dominant copper mineral is chalcopyrite.
4. Kababisa, located on the internal west flank of the Mwombezhi Dome. This deposit strikes north-south and dips approximately 25° towards the west. It measures 2 km in length, 1 km in width, and from 15 m to 20 m in thickness. Mineralisation is outcropping at surface and has been drilled to a depth of approximately 250 m. While the dominant trend is north-south, the MS curves towards the northeast in the north of the deposit. The dominant copper mineral is chalcopyrite.
There are four weathering zones present: oxide, transitional, fresh, and a deep weathering zone around faults which extends into the fresh zone. The oxide zone is typically 10 m to 40 m deep and contains copper oxide minerals such as malachite, cuprite and chrysocolla. The transitional zone is approximately 15 m to 50 m thick and is a supergene enriched layer which is not fully developed. It contains a mixture of primary (bornite and chalcopyrite) and secondary (supergene chalcocite) sulphides with occasional oxide copper coatings. The fresh zone comprises primary sulphides (hypogene chalcocite, bornite and chalcopyrite). The deep weathering zone has mineralogy similar to the transitional zone.
Chimiwungo
The geology at Chimiwungo comprises four main units; a hanging wall gneiss, MS containing slithers of barren gneiss, and a footwall gneiss, which has a gradational contact with the MS.
Within the MS there are two high-grade mineralisation trends; the Roan Shoot to the west and the Equinox Shoot to the east. These are orientated north-south with an approximate dip of 5° to the south and an approximate plunge of 5° to the southeast. The Roan Shoot is approximately 5 km in length, 1 km wide with a thickness of approximately 150 m. The Equinox Shoot is approximately 5 km in length, 200 m wide with a thickness of approximately 200 m. The internal geometry and thickness of the MS package and the development of the high-grade shoots is controlled by boudin development with the high-grade shoots present as zones in the necks of gneissic boudins. Several east-west trending post-mineralisation normal faults offset the mineralisation and result in a series of fault blocks.
Malundwe
The geology at Malundwe is, in general, very similar to Chimiwungo and is subdivided into four lithologies; a hanging wall gneiss, barren gneiss within the MS, MS, and a footwall gneiss, which has a gradational contact with the MS.
Malundwe has a higher grade, thinner MS which strikes north-south, outcrops to the east, dips approximately 10° to 15 °to the west, and plunges approximately 5° to the south. It measures approximately 6 km in length, is up to 1.5 km wide, and extends to a maximum depth of approximately 380 m below surface.
Like Chimiwungo, the internal geometry and thickness of the MS package, and the development of the high-grade shoots, is controlled by boudin development with the high-grade shoots present as zones in the necks of gneissic boudins. In addition, the same east-west trending normal fault system that affects Chimiwungo extends further west into the southern reaches of Malundwe.
Kamisengo
The geology at Kamisengo comprises three major geological units: banded gneiss, foliated granitoid, and predominantly MS, with several variants and minor lithologies such as pegmatites and amphibolitic units.
Mineralisation is related to a large anastomosing shear zone with alternating schists and barren gneisses. There is a strong correlation between strain intensity and grade of mineralisation with high-strain zones concentrating the higher grades in boudin necks/pressure shadows. The shear zone strikes NNW and dips from surface approximately 10° to 20° to the east, sub-parallel to the average orientation of the gneissic banding and schist foliation. It has a strike length of over 4.5 km and is 2 km wide. Kamisengo is not impacted by post-mineralisation faults.
Kababisa
Kababisa is the simplest geologically of the deposits and is broadly separated into three units; a hanging wall gneiss, an MS, and a footwall unit consisting of mottled schist and footwall pink gneiss, which is particular to this deposit being both mappable at surface and a feature in magnetic and electromagnetic (EM) surveys.
Kababisa is boudinaged with variations in thickness influenced by flexures of the underlying footwall granite. The mineralisation strikes north-south and dips approximately 25° towards the west. A flexure in the north of the deposit has changed the strike in that location from north-south to north-northeast as well as generating a slightly flatter dip. The mineralisation has a length of approximately 2 km, a width of 1 km, and ranges from 15 m to 20 m thick.