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Sentinel, Kalumbila

Zambia

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The Sentinel or Kalumbila copper-nickel-cobalt deposit is located on the flanks of the Kabompo structural dome, 55 km west of the Lumwana mine, and 140 km west of the town of Solwezi, in the North West province of Zambia (#Location: 12° 15' 21"S, 25° 18' 28"E).

Regional Setting

For details of the regional setting of Sentinel, the Central African/Zambian Copper Belt and the Lufilian Arc, see the separate Zambian Copperbelt record.

The Sentinel 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 on the central-SE flank of the NE-SW elongated, 150 x 50 km Kabompo Dome, one of a string of Palaeo- to Mesoproterozoic basement inliers that characterise the Domes Region, surrounded by a thick succession of Neoproterozoic sedimentary rocks of the Katanga Supergroup. 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 Conglomérat), 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.

Geology

The key feature of the Sentinel deposit is a sigmoidal, boomerang-shaped, kilometre-scale, near-recumbent, north-closing syncline. Metre-scale recumbent isoclinal folds, consistent with the larger syncline, are developed within the phyllite that hosts the copper mineralisation. Copper mineralisation is largely confined to this structure, with the mineralised phyllite terminated to the NE by the NNW-SSE-trending Kalumbila Fault, and to the SW by a series of sub east-west cross-cutting faults. The strongest concentrations of copper mineralisation are located in lenses within the hinge of the recumbent syncline, with thin nickel-cobalt mineralised horizons associated with pyrrhotite.

The stratigraphic succession enveloping the Sentinel deposit is as follows, from the base:
Footwall Schists - a petrographically variable suite which tends to be foliated, micaceous schists, with a grey to brown matrix, dominated by biotite and quartz, with muscovite and prominent garnet porphyroblasts and aggregates. The biotite has a strong preferred orientation, which with abundant colourless muscovite, imparts a strong foliation to the rock. The protolith is interpreted to have been pelitic sediments recrystallised by regional amphibolite facies metamorphism in a strong directed stress regime. The constituent assemblage comprises quartz, biotite, muscovite, garnet, minor plagioclase, kyanite, ilmenite, rutile and pyrrhotite. The hanging wall contact with the overlying host phyllite is gradational, marked by a transition from biotite-muscovite rich schist to calcareous dolomitic bands, followed by the garnet porphyroblast-rich horizon typically found at the base of the phyllite;
Sentinel Phyllite - which is the host to "bedding-parallel", disseminated, semi-massive sulphide mineralisation, is a carbonaceous kyanite metapelite, locally referred to as a "phyllite". The unit varies from 300 to 500 m in thickness at its widest. Compositionally it is very fine-grained, with modal mineral abundances of ~50 to 70% quartz, 25 to 30% muscovite, ~10% biotite, and up to 5% sulphides and traces of rutile, tourmaline and cordierite.
   Internally the Sentinel phyllite can be subdivided into a lower micaceous and upper carbonaceous unit. Carbonaceous material, predominantly graphite, can constitute up to 20% of the rock (but averages 1.0 to 3.6% CTotal), concentrated in the first 200 m below the hanging wall schists. Ubiquitous porphyroblasts in the central 200 m of the unit are mostly phlogopite, kyanite, scapolite and garnet, found within particular horizons. Overall, the unit tends to be more carbonaceous towards the hanging wall, and more calcareous towards the footwall.
   The phyllite has a fine, centimetric, rhythmic layering. The sulphides form a vertical zonation from a pyrrhotite zone towards the footwall contact, followed upwards by a chalcopyrite-pyrrhotite and an overlying finely "laminated" pyrite zone. The sulphides tend to be stratabound, forming on the bedding-parallel S1 foliation. Minor sulphides are also evident on the S2 foliation.
   Alteration within the Sentinel Phyllite is represented by an abundance of quartz-carbonate veinlets with selvedges of light grey kyanite porphyroblasts. The host has also undergone alteration to produce phlogopite, silica and carbonate, and zones of intense sericitisation associated with quartz veining. In addition to the pyrite, pyrrhotite and chalcopyrite mineralisation, sphalerite is present in rare cases in the proximity of the Kalumbila fault. Zones of sericite alteration within the phyllite prevented or remobilised mineralisation and therefore are usually barren.
Hangingwall Schists - a 0 to 120 m thick sequence of quartz, plagioclase, muscovite, biotite ±kyanite ±pyrite schists, with local gneisses and minor quartzite. The unit is foliated, with a strong fabric, and petrographically comprises schistose metafelsic rocks of possible igneous origin, representing either a granitoid, porphyry, tuff or reworked tuffite protolith, rather than being of sedimentary origin.
   Quartz-feldspar veins within the hanging wall schists generally have strong feldspar, dominantly albite, selvedges. Silicification is one of the main wall rock alteration products, with lesser phlogopite, biotite and fine- to coarse-grained kyanite porphyroblasts. Some of the biotite rich metasediments in both the hanging wall and footwall are altered to dark green chlorite.

On the basis of the lithologies, specifically a thick (>100 m) host carbonaceous pelite, the absence of significant carbonate rocks, and the transitional footwall contact with further pelitic schist, rather than a fluviatile-terrestrial clastic sequence, Gregory et al., 2012, and Hitzman, 2010, suggest the host sequence correlates with the Upper Roan-Mwashia stratigraphy rather than the lower Roan Group.

Structure & Metamorphism

Folding is characterised by two temporally separate, but similarly oriented structural styles:
i). upright folds, in both the basement (where S
2B 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 S
1K 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.

Kyanite porphyroblasts within the carbonaceous phyllite, and locally in the upper micaceous phyllite, are the earliest metamorphic mineral phase in the Katangan rocks at Sentinel. These porphyroblasts are overprinted by S
1K, and have also been mostly retrogressed to sericite. The retrogression took place in 2 steps during S1K, with an early acicular kyanite-phlogopite/muscovite-quartz, and a late muscovite/phlogopite-quartz assemblage, indicating a progressive decrease in metamorphic grade during the development of S1K. Biotite-phlogopite veinlets parallel to S1K in the mineralised zones, are restricted to the carbonaceous phyllite. Fabrics in the micaceous and carbonaceous phyllite are characterised by a drop in metamorphic grade with time, with early biotite-phlogopite-muscovite being overprinted by sericite along transposition planes. The ~600 to 450°C and 4 to 7 kb P-T conditions of the pelitic mineral assemblage indicate lower amphibolite to greenschist facies metamorphism.

Mineralisation

Mineralisation at Sentinel is hosted by the carbonaceous facies of the Sentinel Phyllite, over a strike length of ~11 km, within the NNE to east-west to NE striking (from west to east) hinge zone of a recumbent fold. This hinge zone closes, down-dip to the north, and both the lithologic defined limbs and S
1K axial plane cleavage dip moderately to shallowly to the north. Within the recumbent, isoclinal synform, the (stratigraphically) upper carbonaceous phyllite, which hosts the ore, forms the core of the structure, and the lower micaceous phyllite forms an 'outer rind', locally transitional into the biotite schist country rock of the Footwall Schists in the upper and lower limb. Metre-scale recumbent folds are evident, and are a prominent feature of the carbonaceous phyllite. Original bedding, S0, is only locally preserved, mainly in the hinge zone of the large-scale fold, where it is folded at a high angle to the limbs, but is transposed parallel to S3 (and S1K) along the limbs of the fold.

The copper mineralisation, mainly chalcopyrite, at Sentinel is hosted in the metasedimentary carbonaceous phyllite, while the surrounding quartz biotite feldspar schists are generally unmineralised, although rare low grade mineralisation does occur close to lithological contacts with the carbonaceous phyllite and in the proximity to fold noses.

Away from the fold nose, the lower section of the Sentinel Phyllite, is mostly devoid of copper, while the centre and top of the carbonaceous core to the major fold hosts most of the mineralisation. The copper is concentrated in sheet like bodies that are sub parallel to bedding and foliation. These bodies can vary from a few, to 200 m in thickness. The orebody outcrops in the south, where it strikes NNE-SSW and dips at ~30°NW. In the NE, the strike is NE-SW and the dip is as steep as 45°, but flattens to only a few degrees in the east-west striking central sections of the deposit.

The main ore zone is situated in the central 5 to 6 km of the 11 km mineralised strike length. To the SW, the grade decreases very rapidly, and to the NE is also much lower, but is coincident with higher nickel and cobalt values that increase towards the Kalumbila fault that terminates the host phyllite and deposit in the NE. The nickel is thought to be related to a hydrothermal system centred on fluids moving up the Kalumbila fault, and occurs as the nickel sulphides of pentlandite, siegenite and violarite associated with magnetic pyrrhotite blebs and pyrrhotite-rich layers. Peak Ni and Co levels in metallurgical samples were ~0.02% and 0.04% respectively. Gold averages 0.03 to 0.2 g/t within the orebody, although locally 0.2 to 0.7 g/t Au have been identified accompanying higher Cu grades. Partitioning of the ore grade along the trend of the deposit is related to several steep, WNW trending, crosscutting faults concentrated in intervals where the strike of the deposit changes.

Copper sulphide mineralisation is exclusively chalcopyrite, with extensive zones of pyrite being found, particularly in the structural hanging wall, while pyrrhotite occurs throughout, but is generally within the structural footwall. Chalcopyrite has two main habits; i). clots, blebs and disseminations, predominantly as coarse chalcopyrite, intergrown with pyrite and/or pyrrhotite in blebs and overgrowths, which are in close spatial association with, and overprinting kyanite porphyroblasts, and ii). accompanying biotite-phlogopite growth along veinlets following S
1K foliations and selective S0 bedding layers, that can locally be seen to cut foliation and porphyroblasts (Hitzman 2010; Outhwaite et al., 2010). The sulphide mineralisation appears to be relatively late (Hitzman 2010).

Oxide mineralisation is only minor, generally limited to within 10 to 20 m of the surface within the weathering profile, where it occurs as minor malachite and native copper.

Published NI 43-101 compliant mineral resources and ore reserves at 22 March 2012 (First Quantum Minerals Ltd, 2012) at a 0.2% Cu cut-off, were:
  Measured resource
      oxide - 4.1 Mt @ 0.67% Cu;
      mixed - 20.7 Mt @ 0.51% Cu;
      sulphide - 489.2 Mt @ 0.55% Cu;
  Indicated resource
      oxide - 25.0 Mt @ 0.53% Cu;
      mixed - 22.8 Mt @ 0.43% Cu;
      sulphide - 465.5 Mt @ 0.47% Cu;
  TOTAL measured + indicated resource - 1027.3 Mt @ 0.51% Cu
  Inferred resource
      mixed - 2.9 Mt @ 0.38% Cu;
      sulphide - 162.3 Mt @ 0.42% Cu;
  TOTAL inferred resource - 165.6 Mt @ 0.42% Cu
The mineral resources include
  TOTAL proved + probable reserve - 774.2 Mt @ 0.50% Cu (excluding oxide, but including 40.6 Mt of mixed oxide-sulphide ore)
These reserves are extractable at a strip waste:ore ratio of 2.23:1 with the removal of 1727.3 Mt of waste.

This summary is largely drawn from the Technical Report "Ni 43-101 Technical Report on Mineral Resources of the Sentinel Copper Prospect" by Gregory et al. (6 May, 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.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Hitzman M W, Broughton D, Selley D, Woodhead J, Wood D and Bull S,  2012 - The Central African Copperbelt: Diverse Stratigraphic, Structural, and Temporal Settings in the Worlds Largest Sedimentary Copper District: in Hedenquist J W, Harris M and Camus F, 2012 Geology and Genesis of Major Copper Deposits and Districts of the World - A tribute to Richard H Sillitoe Society of Economic Geologists   Special Publication 16, pp. 487514


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