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The Enterprise nickel-copper deposit is located on the flanks of the Kabompo structural dome, 15 km NW of the Sentinel copper deposits, 65 km west of the Lumwana copper mine, and 140 km west of the town of Solwezi, in the North West province of Zambia (#Location: 12° 10' 52"S, 25° 11' 42"E).
For details of the regional setting of Enterprise, the Central African/Zambian Copper Belt and the Lufilian Arc, see the separate Zambian Copperbelt record.
The Enterprise deposit is situated within the Domes Region, towards the western end of the Pan-African Lufilian Arc structural belt that extends from the Zambian Copper Belt in the east to the Democratic Republic of Congo (DRC) in the north and northeastern Angola in the west. It lies within an enclave of Neoproterozoic sedimentary rocks of the Katanga Supergroup on the central-SE margin of the NE-SW elongated, 150 x 50 km Kabompo Dome. This dome is one of a string of Palaeo- to Mesoproterozoic basement inliers that characterise the Domes Region, surrounded by a thick succession of Katanga Supergroup rocks. The enclave in which Enterprise is hosted is connected to the main Katanga Group to the south, representing a SSW-plunging, NNE-SSW elongated synformal keel overlying the basement. Enterprise is located on the northern closure and the western and eastern limbs of the structure. Both basement and cover rocks were deformed and metamorphosed during the ~550 to 450 Ma Lufilian/Pan-African Orogeny. Metamorphic grades reached amphibolite facies but have been locally retrogressed to green schist facies (Appleton 1973).
The basement rocks within the Kabompo Dome include fine- to coarse-grained biotite gneiss and local amphibolites, ultramafic and granite gneiss bodies, and more widespread schist
units containing variable amounts of phlogopite-muscovite and kyanite. These rocks have been subjected to abundant pink potassic feldspar "flooding" representing potassic alteration, elsewhere in the Copper Belt taken to have accompanied the main Katangan mineralisation event.
Katangan metasedimentary rocks flanking the Kabompo Dome have historically been correlated with the Lower Roan stratigraphy of the Zambian Copper Belt. An alternative interpretation suggests it may actually be the time equivalent of the Upper Roan-Mwashia stratigraphy, with the Lower Roan being thin or absent in this interval. The sequence commences with a basal sandstone, siltstone and conglomerate sequence that grades up into a mixed suite of siliciclastic-carbonate-evaporite rocks, which have been extensively altered and modified to quartz-feldspar-phlogopite schists, dolomites, talc-anhydrite rock and kyanite-quartz white schists. This sequence appears to grade upwards into marbles and then the carbonaceous phyllite, host to the Sentinel mineralisation. These rocks are capped by a shaly diamictite (Grand Conglomerat), a basin wide marker unit. A series of ~750 Ma gabbroic bodies intrude the Katangan sedimentary sequence along the eastern margin of the Kabompo Dome, part of a broader cluster throughout the western Domes Region, some of which are as large as 20 km in diameter.
The stratigraphic succession enveloping the Enterprise deposit is as follows, from the base:
Basement - the dominant lithology is a Palaeo- or Mesoproterozoic medium to coarse grained quartz-biotite-muscovite±hornblende schists, with local
development of pink-red garnet and/or blue-green kyanite porphyroblasts. These rocks have a ubiquitous flat-lying, biotite-defined schistosity, with common boudinaged layer-parallel quartz veins. The complex also includes local amphibolites, ultramafic and granite gneiss bodies;
Décollement Zone - represented by a strongly foliated quartz-phlogopite/biotite rock, that is essentially a structural transition from the basement to the Katangan sequence, which may be derived from either at any one point (Stoltze and Wood, 2011);
Quartzites - a hard, massive bedded quartzite with porphyroclastic texture and occasional clasts of quartz and feldspar (Ng’ona, 2009). The gradation from the décollement zone is marked by a lower suite of mafic schist, biotite-quartz rock and talc schist;
Mixed clastics - an upwardly fining succession of sandstones, siltstones, shales and carbonates, all of which have been intensely metasomatised and as a consequence significantly altered and recrystallised. The upper part of the sequence includes black shales. The carbonates at the top of the pile are interpreted to be part of an evaporitic sequence, and are dominated by talc-anhydrite rock (Wood and Outhwaite, 2010);
Upper Carbonates - Composed of calc-silicate units, mainly silicified dolomites and marbles, frequently containing kyanite porphyroblasts. Fluidised breccia zones that are common at Enterprise, are thought to result from the dissolution of carbonate or evaporite material within the matrix. A conglomerate band is evident high in the sequence (Wood and Outhwaite, 2010);
Carbonaceous shale - which has a turbiditic texture, comprising finely interbedded units of shale and pebbly sandstone. The finer grained shale units are frequently rich in carbon, mainly graphite (Wood and Outhwaite, 2010);
Shaley diamictite - correlated with the 'Grand Conglomerat', composed of a dark, fine grained massive micaceous rock with rare coarse cobbles of quartz rich sedimentary rock. Although the matrix may be locally carbonaceous, the majority of the dark material is fine grained micritic biotite (Wood and Outhwaite, 2010).
Folding is characterised by two temporally separate, but similarly oriented structural styles:
i). upright folds, in both the basement (where S2B related to this folding, re-orients the S1B migmatite foliation) and in the overlying Katangan rocks. In higher strain zones, these folds are transposed, and have a rootless and intrafolial fold geometry.
ii). recumbent folds, restricted to the mineralised phlogopite-quartz veined ore zone in the basal Katangan siliciclastics, located ~50 m above the basement contact. These folds are also intrafolial, and have open to closed hinge geometries. The axial planar fabric S1K of these folds, is co-planar with their limbs. In the hinges of the recumbent folds, later formed S1K fabrics overprints early formed S1K, a common feature of folds developed in shear zones. The axes of the recumbent folds plunge shallowly, parallel to the multiple mica-quartz mineral lineation. Locally developed asymmetric recumbent folds indicate a north directed tectonic transport direction.
The Enterprise deposit lies on the southern limb of a ~5 km wide syncline that has gentle to moderately dipping limbs and a shallow plunge to the SSW. It is hosted by Katangan, possibly of the Mwashia Group siliciclastic rocks, and is along strike from a large gabbro that intrudes along a NNE-trending structure (Wood and Stoltze, 2011).
The host sequence has been subjected to a semi-regional alteration, the mineralogy of which is not significantly different to the alteration proximal to sulphide mineralisation. The siliciclastic rocks of the host sequence are modified to a quartz-talc-kyanite-magnesite assemblage, with the black shale members converted to quartz-kyanite±talc, containing sulphide mineralised veins. Alteration of the black shale to a quartz-kyanite rock in mineralised zones has been so intense as to render the original lithotype difficult to recognise.
A layered specular hematite-talc rock, which is characteristic of the host sequence, is thought to represent an early stage of the alteration paragenesis, with hematite being progressively removed/destroyed where sulphide mineralisation occurs. Phlogopite occurs as a halo of clots and/or veins for tens of metres around sulphide mineralisation.
The host sequence to the nickel mineralisation is strongly altered, and in many places has obliterated any indication of the original lithotype. What is interpreted to have been an upward fining sequence of quartzo-feldspathic sediments, black shale and evaporite has been intensely altered to quartz-talc-kyanite-magnesite-hematite, with lesser albite, tourmaline, chlorite, garnet and actinolite. Black shale is locally recognisable, but has been recrystallised to a granoblastic quartz rock, grading into a quartz-kyanite-talc rock (±sulphide) where alteration is more intense.
Intensely altered chlorite-actinolite±magnetite±hematite rocks, interpreted to be of mafic origin are abundant, and are spatially associated with sulphide mineralisation. These rocks occur as sills within the siliciclastic sequence, forming irregular discontinuous bodies. Basaltic lava, with a local flow banded base, vesicles and a chilled upper margin, is found within the Enterprise stratigraphy, located in carbonate rocks below the Grand Conglomerat, occurring as a long narrow flow band, with a variable orientation.
The host sequence is intensely altered, but has a reasonably consistent stratigraphy, and preserved primary lithological contacts. The siliciclastic sequence is underlain by a strongly foliated quartz-phlogopite/biotite rock, interpreted to represent a major décollement between basement and Katangan rocks, which is essentially a transition
zone of interleaved slivers of both (Wood and Stoltze, 2011).
The nickel mineralisation is composed of sulphide minerals include vaesite (NiS2), millerite (NiS), nickeliferous pyrite
([Fe,Ni]S2), bravoite ([Ni,Fe]S2) and carrollite (Cu[Co,Ni]2S2). Millerite, vaesite and nickeliferous pyrite are the most abundant minerals that are be treated. These sulphides occur within, or as an alteration halo to, quartz-kyanite±talc veins and vein breccias, and are concentrated within and proximal to altered black shales and carbonates, but can occur in lesser amounts in siliciclastic rocks.
The mineralisation is hydrothermal, and is interpreted to be related to a series of deep seated extensional structures within a local graben basin that broadly corresponds to the main synclinal structure. The graben was infilled with Katangan sedimentary rocks then overlain by a carbonate-evaporite package. The intensity and offset of faulting is greatest low in the sequence, reducing higher in the sedimentary package, indicating syn-sedimentary growth faults. The greatest offset always appears to be on the north-dipping fault which bounds the graben to the southeast.
The location of mineralisation appears to be dominantly controlled by a combination of structural, rheological and chemical factors. The black shales hosting the bulk of the mineralisation have a brittle-deformation rheology, and act as a reductant to probable oxidised hydrothermal fluids. Carbonate units proximal to black shales sometimes host bonanza grades of Ni. Other siliciclastic units may be locally mineralised, but typically have much lower concentrations of Ni than the black shale (Wood and Stoltze,2011).
Copper mineralisation has been encountered at depth, beneath the known nickel mineralisation. Copper mineralisation of up to 2.80% has also been
intersected occurring as chalcopyrite, bornite and chalcocite (Wood and Stoltze, 2011).
The 'Enterprise Main' resource occurs as a 1000 x 500 m block in the northern nose of the main fold, and as a contiguous 'Enterprise SW' Zone of 800 x 200 to 400 m in plan on the SE limb of the same fold. The main resource occurs as up to 6 flat lying (0 to 10°), 10 to 30 m thick blocks distributed over a thickness of ~180 m in the centre of the 'Enterprise Main' deposit, thinning rapidly to only one or two bands on the peripheries. Mineralisation appears to occur in veins within lenses that generally parallel bedding.
Published NI 43-101 compliant mineral resources and ore reserves at 28 November 2012 (First Quantum Minerals Ltd, 2012) at a 0.15% Ni cut-off, were:
Measured resource - Enterprise Main - 2.7 Mt @ 1.51% Ni;
Indicated resource - Enterprise Main - 34.3 Mt @ 1.08% Ni;
Indicated resource - Enterprise SW - 3.1 Mt @ 0.60% Ni;
TOTAL measured + indicated resource - 40.1 Mt @ 1.07% Ni;
Inferred resource - Enterprise Main - 2.5 Mt @ 0.92% Ni;
Inferred resource - Enterprise SW - 4.6 Mt @ 0.58% Ni;
TOTAL inferred resource - 7.1 Mt @ 0.70% Ni.
The mineral resources include
TOTAL proved + probable reserve - 32.7 Mt @ 1.10% Ni.
This summary is largely drawn from the Technical Report "NI 43 -101 Technical Report for the Mineral Resources of the Enterprise Nickel Prospect" by Gregory et al. (31 December, 2012) for First Quantum Mineral Limited.
The most recent source geological information used to prepare this summary was dated: 2012.
This description is a summary from published sources, the chief of which are listed below.
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