The Lumwana tenement is a Large Scale Mining License LML-49 that covers 1,355km2 of the Mwombezhi Dome and includes two major copper deposits, Malundwe and Chimiwungo, as well as numerous exploration prospects. The deposits are low grade Cu ± Co ± U with the largest known Zambian copper resource outside of the Zambian Copperbelt and will rank as Africa’s largest copper mine when mining commences in 2007. This chapter provides a detailed description of the geology of the Lumwana deposits, cross-sections constructed across the deposits and macro-scale structural observations made during field mapping and logging of core.
3.1.1 Field area characteristics
The topography of the project area is characterised by gently rolling hills incised by the Lumwana East River and its tributary streams. Elevations in the general vicinity of the Malundwe and Chimiwungo deposits range from approximately 1,270 to 1,410 metres above sea level. The predominant vegetation is miombo woodland with good vegetation cover although wetlands (dambo areas) are common along watercourses (Davis et al. 2006) (figure 3.1).
Figure 3.1 Photograph looking to the east of miombo woodland in the Lumwana East River valley (373056E 8646383N)
CHAPTER 3: GEOLOGY AND MINERALIZATION OF
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The region has distinct dry (May to October) and wet (November to April) seasons with the majority of the field work conducted during the dry season. Humidity ranges from 45% in the dry season to 90% in the wet season with humidity levels reaching their peak in October at the onset of the wet season. Rainfall mainly occurs in heavy thunderstorms producing typical precipitation events of 20 to 40mm (Davis et al. 2006).
The rock units of the Mwombezhi Dome are poorly exposed and outcrops are heavily weathered. The soils of the area are leached of nutrients and locally ferricrete laterite horizons form extensive terraces. The majority of the outcrop comprises a bedded quartzite unit that is resistive to weathering and forms topographic highs. Other rock units are exposed locally in river valleys and a structural traverse was conducted along the Lumwana East River in order to validate existing structural data.
3.1.2 Malundwe and Chimiwungo deposit characteristics
The Lumwana ore bodies are tabular and elongated in a north-south orientation with layer parallel disseminated sulphide zones. The ore bodies have high strike to thickness ratios with thicknesses measured in tens of metres and strike lengths measured in kilometres.
The mineralization is hosted within a thrusted tectono-stratigraphy. The dip and plunge of the ore bodies are parallel to the tectono-stratigraphy and to the foliation (S1) & stretching lineation (L1). Ore packages and individual ore schist
units pinch and swell forming north-south orientated lenses parallel to the foliation and stretching lineation.
The host rocks to the Lumwana deposits are migmatitic gneisses to amphibolite that grade into kyanite schists. The host rock to the ore bodies are dominated by two mineralogically similar but texturally distinct rock types, a granite to pegmatite gneiss and a banded to augen gneiss that both comprise quartz-feldspar ± biotite/phlogopite ± hematite. The ore bodies comprise muscovite-phlogopite-quartz-kyanite-sulphide schist, although
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mineralization can be observed locally within the weakly mineralized intervening gneiss units.
The Malundwe and Chimiwungo deposits are hosted by separate thrust sheets that are identified by their host rock lithologies and geophysical characteristics. In addition to the gneiss and schist units observed at Chimiwungo, the Malundwe footwall also includes units of unmineralized kyanite schist, talc ± epidote ± kyanite schist to quartzite. The kyanite schist to quartzite units are mineralogically similar to the “rimming quartzite” observed at the décollement between the basement rocks and overlying Katangan Supergroup meta-sedimentary rocks. Observation of sedimentary “way up” structures in the footwall quartzite at Malundwe indicates that it forms part of an overturned limb of a recumbent fold (Benham et al. 1976; Nisbett pers comm. 2004). However, no sedimentary structures were observed during this study. The footwall quartzite overlies emplaced calc-silicates and amphibolites that have been tentatively correlated with the Upper Roan (figure 2.17 & 3.2).
MALUNDWE SHEET CHIMIWUNGO SHEET HANGING WALL Basement Gneiss Basement Gneiss
Top D1 Shear Zone
Upper Ore Schist Middle Gneiss Main Ore Schist
Lower Gneiss
ORE PACKAGE Malundwe Ore Schist
Lower Ore Schist Mottled Schist
Epidote Schist Mottled Schist Speckled Schist
Basement Gneiss & Mottled Schist
Quartzite
FOOTWALL
Muscovite-Quartz Schist
D1 Thrust
UPPER ROAN Carbonates, amphibolites, pelites & gabbros
Not Present
Figure 3.2 Simplified stratigraphic comparisons of the Malundwe and Chimiwungo deposits as identified by Equinox geologists, and supported by fieldwork
The Chimiwungo deposit has three main ore horizons separated by intervening internal gneiss units that vary from quartz-feldspar-phlogopite gneiss to garnet-amphibolite. The Malundwe deposit consists of one orebody in the north and two in the south that are separated by an intervening internal gneiss unit. Observed contacts between ore and host rock units are
CHAPTER 3: GEOLOGY AND MINERALIZATION OF
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commonly transitional in nature with an increase in the mode of mica minerals correlating with a decrease in the proportion of feldspar minerals.
The primary sulphide mineralization at Lumwana commonly occurs in mineral pairs of copper sulphides, which exhibit a down section zonation in diamond drill core. The sulphide pairs consist of pyrite (FeS2) – chalcopyrite (CuFeS2),
chalcopyrite – carrollite (CuCo2S4) (Chimiwungo only) and chalcopyrite-
bornite (Cu5FeS4). Bornite and chalcopyrite are replaced by minor digenite
(Cu9S5) chalcocite (Cu2S) and covellite (CuS). Minor uranium mineralization
occurs locally cross-cutting the Cu-Co mineralization. Quartz veins which cross-cut both the mineralization and the barren internal gneiss units are commonly host to pyrite, chalcopyrite and/or bornite. Coarse, elongate, disseminated, aggregates and stringers of sulphide grains are observed parallel to the foliation & stretching lineation. Sulphides were deformed by the S1 fabric and overprinted by kyanite which formed at peak metamorphism.
This indicates that copper was introduced to the basement either syn or pre- peak metamorphism. Sulphides are commonly observed in microstructural dilatant sites such as pressure shadows around porphyroblast grains. This is interpreted as the result of sulphide remobilization during Dn+1 metamorphism
with associated quartz-muscovite alteration.