The Kansanshi sediment hosted copper-gold deposit was one of the earliest known significant copper occurrences in Zambia. It is located 10 km north of the town of Solwezi and 180 km to the NW of the Copper Belt town of Chingola.
The deposit had been mined by the 'Ancients' as evidenced by long abandoned iron age workings. Some 2900 t of Cu were extracted between 1908 and 1914, with further activity from 1952 to 1957, while a larger open pit was established in 1977. The current large scale operation commenced in 2005, with an annual production capacity of 110 000 tonnes of Cu, since expanded to 340 000 t Cu and 3.75 t Au per annum, with further planned expansion underway in 2013.
Regional Setting
For details of the regional structural and stratigraphic setting of Kansanshi, the Zambian Copper Belt and the Lufilian Arc, see the separate Zambian Copper Belt record.
The Kansanshi deposit is located in the "Domes Region" zone of the Lufilian Arc in the North-Western Province of Zambia, and is hosted within the Neoproterozoic Katanga Supergroup metasedimentary rock package, interpreted to belong to the Nguba (previously the Lower Kundelungu) Group Grand Conglomerat and the Roan Group Mwashya Subgroup. The Domes Region zone is characterised by relatively thick skinned deformation with upright folds, exposed basement domes (interpreted to represent culminations above thrust ramps), and upper-greenschist to upper-amphibolite facies metamorphism.
Kansanshi is located ~12 km to the north of the ~20 x 30 km, north-south elongated Solwezi Dome which is cored by a poly-deformed gneisses, migmatites and granites. Granitic intrusion within the dome have been dated at 1101.7±5.2 Ma (U/Pb; Barron 2003), consistent with Kibaran age granites within the Kasai shield. This dome is ~50 km east of the Mwombezhi Dome, the margin of which hosts the Lumwana deposits, and is ~160 km WNW of the larger Kafue Anticline dome which hosts the main Zambian Copper Belt deposits around its margins.
A sequence of metamorphosed quartz-mica schists, quartzites and conglomerates are exposed around the rim of the Solwezi Dome, and are assumed to be the chronological equivalents of the Lower Roan Subgroup, which hosts the main deposits of the Zambian Copper Belt on the margins of the Kafue Anticline, followed by dolomite-muscovite schists which may be correlates of the Upper Roan Subgroup. These are, in turn, overlain by marbles, calc-biotite schists, scapolite-calcite-biotite schists, marble, local ironstones, quartzites and phyllites, exposed along the northern edge of the Dome which are correlated with the Mwashya Subgroup (Arthurs, 1974). These rocks, which host the bulk of the mineralisation (described below), are overlain by metasediments of the Nguba Group, and the widespread middle to upper Nguba Group and possibly the Kundelungu Group. All of these sequences are cut by numerous ~750 Ma gabbro bodies.
Geology
The stratigraphic succession at Kansanshi has been interpreted to represent a symmetrically repeated sequence across a recumbent isoclinal fold, as follows, from the structural base:
Lower Dolomite - the Chafungoma Formation, an ~15 m thick sequence of intercalated calcareous biotite schists, marbles, dolostones and phyllites that is underlain by a greyish "dirty" dolostone that is >30 m thick. The dolostone often displays intense dolomitic alteration to a white saccharoidal dolostone. Arthurs (1974) assigned this unit to the Mwashya Subgroup. However, as the Mwashya Subgroup is typically clastic in nature, Gregory et al. (2010) suggest it is possible that at Kansanshi, it is absent and the Upper Dolomite should instead be correlated with the Upper Roan Subgroup, as also suggested by Naish (1979). However, on the basis of the interpretation presented in Hitzman et al., 2012, this unit stratigraphically overlies the Grand Conglomérat at the base of the Nguba Group and as such would equate to the Kakontwe Dolostone.
Lower Pebble Schist - a locally garnetiferous biotite schist containing up to 10% exotic clasts of dolostone, argillite, quartz and very rare granite, very similar to the Upper Pebble Schist, to which it is locally correlated (see below). It is locally considered to represent the Grand Conglomérat, which marks the base of the Nguba Group.
Lower Marble - a thick zone of grey, fine to medium-grained cryptocrystalline calcic marble, very similar to the Upper Marble.
Lower Calcareous Schist - a sequence of calcareous schists, calcareous biotite schists, marbles, knotted schists and phyllites, with considerable thickness variations, from 80 m in the NW pit to <40 m in the Main Pit. It displays abundant high ductile strain textures, e.g., S/C fabrics, 'C' shear bands and, most commonly, mineral differentiation banding resulting in a prominent mm-scale banding of biotite-rich and calcite-rich zones, and while it appears to represent a shear zone, does not crosscut stratigraphy.
Middle Mixed Clastics - a 30 to 100 m thick sequence of knotted schists, biotite schists and phyllites, very similar to the Upper Mixed Clastics. Considerable variations in thickness are evident.
Upper Marble - a thick (10 to 80 m) sequence of grey, fine to medium-grained cryptocrystalline calcic marble with bands of carbonaceous and calcareous phyllite towards the base. The marble is generally massive but becomes weakly foliated adjacent to external and internal contacts with clastic rocks. The grey colouration is due to homogeneously distributed fine carbon within the marble.
Upper Mixed Clastics - a thick (at least 250 m) sequence of phyllites and knotted schists.
Topmost Marble - massive grey calcic marble, which has considerable thickness variations, possibly as a result of boudinage.
Upper Pebble Schist - biotitic, commonly calcareous and occasionally garnet-bearing schists, containing 1 to 2% exotic clasts, which include dolostone, siltstone, sandstone, massive quartz, quartzite and very rare granite. These pebbles are strongly flattened within the dominant schistosity. This unit has been correlated with the Grand Conglomérat Formation marking the base of the Nguba Group.
Upper Dolomite - the Chafungoma Formation, a pale brown-grey to medium-grey saccharoidal iron-free dolostone and dolomitic marble. The footwall of the unit is marked by a 10 to 15 m thick dark grey to black, crystalline, massive and pyrrhotite-bearing, coarser-grained sedimentary rock.
Structure
The Kansanshi mineralisation occurs along the crest of a regional NW-SE (~310°) trending, broad anticlinal structure known as the Kansanshi Antiform, which can be traced along strike for ~12 km, but is separated from the Solwezi Dome by a WNW-ESE trending regional detachment structure. The antiform closes to the NW and probably also to the SE, where gentle southeasterly dips are seen. This broad antiform is superimposed on a tight isoclinal recumbent fold that had produced a gently SW-dipping structural sequence with a symmetrical repetition of stratigraphic units above and below a central shear that parallels stratification.
The Kansanshi Antiform has been re-folded, creating doubly-plunging, domed structures along its crest. The three major ore bodies (NW Pit, Main Pit and SE Dome) of the Kansanshi deposit are developed within two parasitic domal structures associated with the crest of the Kansanshi Antiform. These domes are asymmetric with gentle dips of no more than 25°.
Three deformation events have been recognised: D1, which produced a penetrative S1 foliation, east-west trending recumbent folding, and the L1 lineation, and was the result of NNE-directed shortening. The recumbent folding produced a generally flat to shallow dipping sequence that was (as a late D1 event ?) warped into the extensive NW trending Kansanshi Antiform. D2 resulted in crenulation folding, which plunges at low angles and trends north or NE, with associated cleavage. This phase is interpreted to reflect NW directed shortening. The doming of the Kansanshi antiform in the deposit area is interpreted to be the result of a local F2 anticline. A number of steep, north-south trending D2(?) shear zones cut the domes. D3 structures are restricted to kink bands with a similar orientation to F2, which may have developed during late D2 deformation (Broughton et al., 2002).
Mineralisation
The mineralised area originally outcropped as a conspicuous treeless hill with spectacular copper staining, due to impregnation of the host rocks with copper oxides that impart a distinctive blue-green colouration, known as the Green Wallrock.
The primary mineralisation occurs within the clastic stratigraphic units, chiefly interbedded graphitic shale, knotted schist, quartzite and lesser limestone of the Mwashya Subgroup, that have been subjected to supergene remobilisation and enrichment. The mineralised system being mined in the Main and Northwest Zones is classified into three dominant ore styles: i). vein-hosted, as steeply dipping, sheeted quartz-carbonate-sulphide veins; ii). sediment-hosted, occurring as mineralised haloes to the veins, but with an extent and character which has a strong lithological control; and iii). breccia mineralisation, as intervals of sulphide crackle and stockwork breccias, in zones of higher density of veining. The primary sulphide mineralisation in all three types is predominantly chalcopyrite, with local minor bornite and some gangue pyrite, the latter being more evident in NW Pit. Mineralisation is more prevalent in the clastic sedimentary rocks than in the carbonates, with the Middle Mixed Clastics unit host to the bulk of the mineralisation, both as vein- and sediment-hosted. Dating of accessory vein-hosted molybdenite and monazite has revealed two distinct mineralisation ages, ~512 Ma and ~502 Ma (Re-Os and U-Pb dating; Torrealday et al., 2000).
All of these primary styles are overprinted by supergene processes to produce i). oxide, ii). transitional (mixed oxide-sulphide), and iii). sulphide zones. Oxide mineralisation generally comprises malachite, wad and tenorite, with lesser chrysocolla, limonite and cupriferous goethite, while mixed mineralisation includes chalcocite, minor native copper and tenorite, and partially weathered chalcopyrite in addition to the oxide mineralogy listed above. Some copper appears to be carried in clay and mica minerals, where it is essentially refractory.
Vein-hosted ore
The veins are characterised by chalcopyrite-pyrrhotite-pyrite by with minor to trace amounts of brannerite, monazite, uraninite, pitchblende, molybdenite and rare bornite. Gangue minerals are dominated by quartz and/or carbonate, including quartz, ferroan carbonate minerals, rutile, and biotite, and range in thickness from a few mm's to 6 m, are dominantly vertical to sub-vertical with a number of orientations, although overall they form a NW-SE trending zone. Minor vein constituents, such as anhydrite and magnetite, as well as trace uraninite, appear to increase with depth. Molybdenite is a common accessory mineral present in both Main and NW Pit and appears to be associated with calcite dominated vein sets. Veins show a variety of textures, from laminated to brecciated to massive (Beeson et. al., 2008), while stringer veinlets of between <0.1 and 100 mm mostly occur within the sulphide zone and is the dominant style of mineralisation within the Lower Pebble Schist of the Main Pit. Veining is best developed in the southern and northern portions of the Main Zone, and throughout the Northwest Zone.
Veining consists of a series of generally north-south striking, undeformed, high-angle veins and vein swarms, with associated alteration halos, that are distributed in an en echelon pattern along the axis of the northwest trending Kansanshi antiform. The vein swarms have strike lengths of hundreds of metres to >1200 m, widths of up to 200 m, and vertical dimensions as much as 300 m. Individual veins have widths of <1 cm to >5 m.
Within the Main Zone, vein-hosted mineralisation is developed around the central dome, and is dominantly vertical to sub-vertical, occurring in three major orientations. The dominant vein minerals in all three sets tend to be very coarse-grained, such that the vein mineralogy varies dramatically in the relative proportions of quartz, carbonate and sulphide both vertically and along strike.
The dominant vein orientation in the Main Zone is a north-south set of metre-scale veins that have been traced over a strike length of up to 1.8 km, and were exploited in the old underground workings. The vein mineralogy is predominantly quartz-carbonate-sulphide with occasional molybdenite and magnetite. A secondary set, with a similar mineralogy, are oriented NNE-SSW, distinctly crosscutting the north-south veining and merging with them in places. A third, and final vein set occurs as spatially limited radial veins around the central dome of the Main Zone mineralisation, and are only present on the western side of the dome where they appear to be restricted to the Middle Mixed Clastics tectono-stratigraphic unit (Gregory et al., 2010; 2012). The first two vein sets are chalcopyrite rich and contain minor molybdenite, while the third has relatively abundant molybdenite with significant monazite, brannerite and minor chalcopyrite (Torrealday et al., 2000).
In the Northwest Zone, veining is considerably different, although three main, but more evenly distributed, vein orientations are present. The dominant vein set still appears to be north-south, with a second east-west set, represented by relatively few veins, and a third WNW-trending orientation. Veins in the Northwest zone are generally much wider than those in the Main Zone, reaching widths of up to 15 m, although the bulk mineralogy is similar in both zones, with quartz-carbonate-sulphide as the most common components. However, vein minerals are even coarser-grained in the Northwest Zone.
Mineralised veins in both zones usually have associated alteration haloes dominated by albite replacement and bleaching of host stratigraphy. The veins and vein swarms are surrounded by alteration halos of albite, ferroan dolomite, ferroan calcite, quartz, green muscovite (V-rich roscoelite) and rutile. These haloes vary from millimetres to several metres in width, depending on the lithology hosting the vein, typically large and diffuse within carbonaceous units and more restricted within the garnetiferous and knotted schists. Disseminated mineralisation is often associated with these haloes and is commonly present in the phyllites, carbonaceous phyllites and marble units.
Sediment-hosted mineralisation
Strata-bound, sediment-hosted mineralisation usually occurs within the sulphidic veined zone, occurring within altered (primarily albitised) haloes developed around veins. The widths of the alteration haloes vary considerably, controlled by the host lithology. Haloes are broad in carbonaceous phyllites, but much more restricted in schists and marbles. These haloes are asymmetrically developed around veins. Relatively dense vein swarms (with ~1m spacing) will generally have alteration and mineralisation over the entire intervein vein interval, although the outer margin of the swarm with only be altered and mineralised over a few cms. This apparent asymmetry to mineralised alteration haloes may reflect lateral fluid flow between veins as a result of fluid pressure differentials.
The sediment hosted mineralisation occurs as bedding-parallel disseminated and stringer chalcopyrite which is dominantly present within phyllites and carbonaceous phyllites. Organic carbon within the phyllites acted as a redox boundary, promoting the precipitation of sulphides. Disseminated sulphides are also found within dolomites, though less frequently. Sulphide species are typically chalcopyrite, bornite, pyrite and pyrrhotite.
Sediment-hosted copper sulphide mineralisation is present in the Main Zone, but is almost non-existent in the Northwest Pit, where veins are considerably thicker, but tend to have a wider spacing. Hence the mineralisation is not continuous between veins and only occurs as thin selvedges.
Breccia mineralisation
Sulphide stockwork-breccia style mineralisation is structurally controlled and corresponds to zones of greater intensity of veining (Hanssen et al., 2010), occurring as brecciated veins and breccia zones of altered wall rock and vein fragments in a matrix of ferroan dolomite, calcite, albite, quartz, rutile and rare chalcopyrite. It is predominantly found in the Main Zone dome region, where all of the stratigraphic units are mineralised, and extends deep into the Lower Pebble Schist directly below the centre of the original dome, forming wide zones of altered and mineralised wall rock in addition to the sulphide veins themselves. Some of the main developments of sulphide breccias include a north-south elongate 5400 zone slightly to the east of, and overlying the Main Zone doming, and at depth, as the 4800 zone, another linear north-south trending structure interpreted to represent a strike-slip fault.
Supergene mineralisation
Weathering profiles beyond of the mineralised zones at Kansanshi, where vertical faults and veins are largely absent, are simple, with a depth of weathering generally of <100 m. However, as the primary sulphide mineralisation of the deposit is intimately associated with vertical faults and veining which allow local oxidation to depths of as much as 500 m, the weathering profiles within the ore zone are complex and deep.
The weathering profile comprises an:
Upper leached zone - which contains little copper, other than refractory mineralisation, where elemental copper is contained in the matrix of smectite clays, and in iron oxides and hydroxides (e.g., cupriferous goethite). Although grades can range from 0.5% to 1% Cu, typically <30% is acid soluble, and as a result it is not regarded as being economically viable. Refractory mineralisation is found predominantly in the northern area of Main pit, in the saprolite zone of the Upper Mixed Clastics.
Oxide zone - the proportion of soluble copper "oxides" (malachite, tenorite, cuprite, chrysocolla and rare azurite) gradually increases with depth, mixed with progressively lesser amounts of refractory mineralisation. The top of the oxide zone is regarded as where the proportion of total copper that is acid soluble is > 80%. Oxide mineralisation is developed in veins, alteration haloes and dispersed into the surrounding lithologies, with more diffuse margins than the primary sulphide zoning due to the mobilisation and both lateral and vertical dispersion of the copper minerals during weathering. Gangue minerals are predominantly smectite clays, calcite, marble, iron oxides and hydroxides. In general the oxide zone tends to follow the morphology of the primary sulphide vein sets.
An important part of the oxide resource at Kansanshi is the so-called "residual ore-body", which formed as the result of the interaction between carbonate (marble) units and acid solutions derived from the sulphide mineralisation by supergene processes. While normal weathering processes result in volume loss in carbonate units, dissolution is considerably enhanced by the interaction with sulphide derived acids, resulting in almost total removal of carbonates, leaving a deflated residual layer of the non-acid soluble components of the carbonate bodies, expressed as a 4 to 15 m thick, soft, micaceous (biotite) siltstone containing Cu oxide mineralisation. Where oxidised meteoric water reacts with copper-bearing sulphides (generally chalcopyrite at Kansanshi), acidic copper-bearing fluids are produced, which on encountering marbles, lead to dissolution and removal of soluble carbonate minerals. In the process, the fluid is buffered, and reacts with the carbonate, to form malachite which remains in the deflated residual layer. This has produced a band of high-grade supergene oxide enrichment at Kansanshi. This mineralised residual layer may be either contiguous with the main ore zone or separated by up to 16 m.
Transition zone - mixed mineralisation occurs as a wide transitional zone between the base of complete oxidation and the top of fresh sulphide zone, comprising a mixed mineralogy containing primary copper sulphides, secondary copper sulphides and copper oxides. The copper ore mineralogy within this zone is complex, consisting of chalcocite, native copper, digenite, covellite, cupriferous Fe-oxyhydroxides and partially weathered sulphides in addition to malachite and tenorite, and partially weathered chalcopyrite and rare bornite (Köttgen & Bastin, 2009). The boundaries above and below are diffuse, often over an interval of up to 10 m.
Summary
The orebodies are located in F2 domal culminations along the axis of a broad, NW-SE trending (late ?) F1 anticlinal fold, due to warping of an earlier, tight, isoclinal, recumbent, F1 folding that had produced a gently SW-dipping structural sequence with a symmetrical repetition of stratigraphic units above and below a central shear that parallels stratification. A number of D2(?) north-south steep shear zones have controlled the introduction of mineralised magmatic derived fluids that are interpreted to have leached and scavenged elemental copper and gold from the basement and/or the intervening sedimentary rocks. Oxidised copper enriched fluids migrated upwards through the stratigraphy along these structural discontinuities, which are filled with breccia-stringer stockwork sulphide mineralisation. Deposition of copper mineralisation is dependent on both the dome shaped trap structures and lithological redox boundaries, specifically the carbonaceous clastic rocks and graphitic phyllites of the Mwashya Subgroup (Hitzman et al., 2012) of Nguba Group (Gregory et al. 2010; 2012). Ore occurs within these rocks as steep, mainly north-south trending quartz-carbonate veins, with a number of other overprinting trends in different areas, and as stratabound stringer mineralisation within the host sedimentary rocks as selvedges to the veins. Ore was deposited during the Cambrian (~512 Ma and ~502 Ma) within the Neoproterozoic host rocks. Subsequent supergene processes have produced oxide ores (Gregory et al., 2012).
Published ore reserves and mineral resources at December 31, 2012 (First Quantum website, 2013) were:
Proved + probable Leach ore reserve - 106.5 Mt @ 1.34% CuT, 0.84% CuAs, 0.17 g/t Au (0.25% Cu cut-off);
Proved + probable Mixed Flotation ore reserve - 144.1 Mt @ 0.75% CuT, 0.16% CuAs, 0.14 g/t Au (0.25% Cu cut-off);
Proved + probable Sulphide ore reserve - 503.7 Mt @ 0.65% CuT, 0.13 g/t Au (0.25% Cu cut-off);
Measured + indicated resource (Main, NW and SE Dome) - 727.03 Mt @ 0.86% CuT, 0.21% CuAs, 0.14 g/t Au (0.3% Cu cut-off);
Inferred resource (Main, NW and SE Dome) - 365.24 Mt @ 0.71% CuT, 0.04% CuAs, 0.11 g/t Au (0.3% Cu cut-off).
Note: CuT = Total copper, CuAs = Acid soluble copper.
(Source: Porter Geoconsultancy, http://www.portergeo.com.au/, 2013)
