Mkushi - Munshiwemba, Coloquo, Munda, Mtuga
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The Mkushi Copper Project, including the Munshiwemba, Coloquo, Munda and Mtuga deposits and prospects, are located within the Central Province of Zambia, ~186 km NNE of Lusaka, 123 km SSE of Ndola, and ~45 km SW of the town of Mkushi (#Location: 13° 56' 45 S, 29° 08' 37" E).
The Mtuga prospect, the first in the district, was discovered in 1922, followed by Munshiwemba and Coloquo in 1923. Mtuga was developed as a small scale mine in 1924, including underground development to test the resource until 1926. After various sporadic exploration programs and initial development works, full scale underground production commenced in 1957 at Mtuga and sections of Munshiwemba to produce 0.0485 Mt @ 3.56% Cu and 0.02592 Mt @ 3.41% Cu respectively before closure. Open pit mining at Munshiwemba was commenced by and Italian company, Miniera di Fragne-Chialamberto in 1971 to close in 1975 after producing 2.2 Mt @ 0.98% Cu, when threatened nationalisation forced a cessation of operations, with resources remaining. No further production is reported to 2014.
The Mkushi deposits and prospects lie within the south-western part of the late Palaeoproterozoic to Mesoproterozoic Irumide Belt basement rocks that extend from central Zambia into northern Malawi (Dewaele et al., 2006). Regionally, these basement rocks are composed of a complexly deformed package of metamorphosed sedimentary, volcanic and intrusive lithologies. The Irumide sequence overlies a basement of 2.05 to 1.95 Ga Mkushi Gneiss Complex in the southwest and the Luwalizi Granite, Mwambwa River and Mulungwizi Gneisses in the northeast. The Mkushi Gneiss is characterised by banded biotite gneisses, augen gneisses and porphyritic granite gneisses (De Waele et al., 2006). These metamorphic rocks, which are probable correlates of the Lufubu Metamorphic Complex to the north, are structurally and in places unconformably overlain by 1.85 to 1.65 Ga metasedimentary rocks of the lower section of the Muva Supergroup (Stillman 1965). Dating reported by De Waele (2004) and De Waele and Fitzsimons (2007) suggest the Irumide rocks were deposited in two episodes, the first, an extensive quartzite-metapelite succession with minor carbonate (Mporokoso, Kanona [in the SW] and Manshya River [to the NE] groups), deposited in the Palaeoproterozoic at ~1.8 Ga (based on interlayered 1815 Ma volcanic rocks), and a second, more restricted sequence which consists largely of very mature, fluvial sandstones (Kasama Formation) at ~1.4 Ga that are recycled sediments from the first episode rocks (De Waele, 2004). The more restricted Muva Group of the Zambian Copperbelt are probably equivalents of this younger sequence, and assumed to be ~1.3 to 1.05 Ga (Selley et al., 2005), but may be older. These rocks have been deformed by the Mesoproterozoic tectonism and magmatism that forms part of the Kibaran Orogenic Cycle dated between 1.4 to 1.0 Ga. Peak metamorphism in the Irumide sequence was at 1.05 to 1.02 Ga (Rainaud et al., 2005). Voluminous syn- to post-kinematic Irumide granitoids were emplaced between 1.05 and 0.95 Ga.
These older rocks are bounded to the NW by the Neoproterozoic Lufilian Belt that includes the Katanga Supergroup, dated at 0.88 to 0.57 Ga, which hosts the Zambian Copperbelt to the north.
To the north and NE, partially separated by the Neoproterozoic sequence, largely undeformed Palaeoproterozoic basement lithologies of the Bangweulu Block are exposed, truncated to the NE by Mesoproterozoic and Neoproterozoic transcurrent shear zones, reactivating structures from the Palaeoproterozoic Ubendian Belt. To the SE and south, the Irumide rocks were reworked along the Neoproterozoic Mwembeshi Shear Zone separating the Lufilian and Zambezi Belts, and to the east by the East African Orogen or Mozambique Belt.
All of the Irumide Belt succession has undergone compressional tectonics, resulting in NW overthrusting onto the Bangweulu Block basement and extensive crustal shortening, but with minor SE-verging back-thrust structures locally developed within the overall NW-vergent tectonic regime. Some parts of the basement were affected by Irumide tectonism, although large-scale thrusting was mainly accommodated along a basal decollement at the basement-Irumide interface. Extensive shortening is reflected in tight- to isoclinal folding within the supracrustal sequence, varying from upright to recumbent, whilst thrusts were developed where shortening could not be sufficiently accommodated by tight folding, producing tectonic duplication, and rendering stratigraphic correlations tenuous.
Metamorphic parageneses show low- to medium pressure and medium- to high- temperature conditions, with metamorphic grades ranging from greenschist facies in the north-western foreland, to upper amphibolite facies in the SE, with local granulites. Peak Irumide metamorphism, as seen in metamorphic zircon rim overgrowths, has been dated at 1.02 Ga.
The Mkushi deposits are near the southwestern end of the Irumide Belt and correspond to an inlier of Mkushi Gneiss surrounded by porphyritic granitoids. The Mkushi Gneiss occurs as either porphyroblastic, finely banded or granitised gneiss. The former occurs as a 4 to 6 km wide, ENE-trending belt, bounded to the north and south by finely banded gneiss with a mass of granitised gneiss within these to the north. Within the centre of the filed, on the southern margin of the porphyroblastic gneisses, there is a ~2 km diameter granite body referred to as the Mtuga Granite.
A swarm of anastomosing post-metamorphic aplite and pegmatite dykes cut obliquely across the the porphyroblastic gneiss band along a SW to NE trending shear zone, aligned sub-parallel with the gneissic foliation. These dykes are also parallel to a biotite mylonite shear zone known as the 'phyllonite', which has been shown to be gradational to dolerite dykes, possibly reflecting progressive intrusion and deformation of mafic rocks along shear zones, which post-date the aplite-pegmatite dykes.
Geological mapping in the Munshiwemba open pit suggest the aplite-pegmatite dykes form a steeply dipping sinistral stepping en echelon pattern. The dykes have a broadly funnel shape in long section tapering downward, but have an inconsistent a plunge. Both the aplite and pegmatite dykes have gradational contacts with the enclosing gneisses suggesting replacement. Joints, clay-filled faults with minor displacement and phyllonite shears cross-cut the aplite-pegmatite dykes.
Four styles of alteration have been recognised, as follows:
&bull Intense silicic and hematite alteration is associated with the bulk of the mineralisation and is the most pervasive. It is commonly the earliest, overprinted other phases of alteration and mineralisation.
&bull Potassic alteration occurs as a result of K feldspar replacement of plagioclase feldspar, commonly associated with sericite and calcite, indicating loss of sodium to sericite and calcium to calcite from the plagioclase and enrichment of potassium. Biotite and chloritised biotite are also indicative of potassic alteration at Mkushi.
&bull Intense argillic alteration, reflected by complete alteration of all feldspars to kaolin, sericite, calcite and quartz (saussiterisation), whilst biotite is altered to chlorite with sericite and hematite. Chlorite, in turn, is partially altered to an assemblage of calcite, sericite, quartz and hematite. Where present, biotite may be replaced by an assemblage of chlorite, rutile, calcite, ankerite, zircon, pyrite, sphene and chalcopyrite. Argillic alteration is reflected by the presence of kaolinite veinlets and the ragged outlines of K feldspar, absence of plagioclase and widespread replacement of biotite to chlorite, sericite and hematite.
&bull Weak argillic to propylitic alteration is widespread, occurring as patchy kaolinite, formed at the expense of feldspars, hematite staining on rock fragments, propylitic epidote-chlorite-calcite and pyrite (with minor hematite), and a quartz assemblage. This alteration is also associated with pervasive hematite on fractures, probably related to magnetite or sulphide oxidation.
Copper Mineralisation has been identified at seven deposits and prospects, which are connected by two separate mineralised shear zone trends. The first connects Mtuga, Munshiwemba, Botita and Coloquo, the second, to the south, controls Katunga, Fitalu and possibly Munda.
Hypogene copper sulphide mineralisation occurs as a series curvilinear, left stepping en echelon lenses of disseminated chalcopyrite. These lenses have strike lengths of from 80, up to 500 m, are 1 to 60 m thick, and extend between 80 to 200 m down a dip of ~60 to 70° in a direction of 300 to 350°. They are hosted by altered gneiss and granite of the Palaeoproterozoic Mkushi Gneiss Complex and have a close affinity with the intrusive aplite and pegmatite dykes. However, the mineralisation is not confined to the dykes, nor is it evenly spread through them, but transgresses all lithologies. Nevertheless, the dyke margins appear to be the preferred location of sulphide mineralisation. Three styles of hypogene mineralisation are recognised: i). disseminated sulphides; ii). sulphides in breccia zones, and iii). sulphides as tension gashes, late-stage veins along hair-line fractures, joints and minor faults, with increased concentrations of sulphides along structural intersections. The individual mineralised lenses are composed of varying proportions of these three styles of mineralisation.
Small fine- to medium-grained clusters of disseminated chalcopyrite varies in abundance from trace amounts to ~10 wt.%, and is often accompanied by idiomorphic pyrite in which the chalcopyrite partially replaces pyrite. The amount of disseminated sulphide is strongly dependent upon the concentration of dykes, with the chalcopyrite content decreasing away from the dykes. Chalcopyrite also preferentially replace biotite at aplite-gneiss contacts. Chalcopyrite grain size appears to be proportional to the immediate host rock, with coarser and finer grained varieties in pegmatite and aplite dykes respectively.
Pyrite is ubiquitous, typically averaging ~lt;5%, both as isolated grains within chalcopyrite and as globular aggregates completely surrounded by chalcopyrite. These aggregates may also contain bornite, tetrahedrite-tennantite, molybdenite, hexagonal pyrrhotite, cubanite and sphalerite as minute blebs. Chalcocite, sphalerite, native gold, galena, tetrahedrite-tennantite, acanthite and molybdenite may also occur within the matrix of the aplite dykes.
Gangue minerals include quartz, haematite, orthoclase, sericite, chlorite, biotite, tourmaline, calcite and magnetite.
Magnetite is common within the mineralisation, often replacing a pyrite, chalcopyrite and haematite assemblage, although in some instances, magnetite appears to be associated with chalcopyrite, suggesting two phases of chalcopyrite formation.
The paragenesis of copper mineralisation has been interpreted to be related to the development of the granite and gneiss host rocks and possibly coeval with the intrusion of the felsic dykes, but may, in part, be associated with the later stages of the post-kinematic (~1.05 Ga) Mtuga Granite, which is itself intruded by aplite and pegmatite dykes. However, there is also the possibility that the hydrothermal mineralisation event is significantly later than the magmatic event, with the felsic dykes providing zones of competency contrast, which focused structural development. Nimmo (2010) suggest it is possible the mineralising event may be related the events that also mineralised the Katangan Supergroup in the nearby Zambian Copperbelt.
Supergene copper mineralisation occurs over all of the deposits and prospects, mainly comprising malachite staining on joints and fractures of near surface and exposed rock. Chalcocite and covellite has also been recognised within weathered fracture zones, but to a lesser extent. Supergene mineralisation is confined to the top few metres of the subsurface profile within the regolith, and rarely to marginally greater depths where meteoric water has penetrated along deeper joints and fractures. The supergene mineralisation forms a very minor component of the known copper mineralisation.
Copper mineralisation occurs as disseminated chalcopyrite in aplite, pegmatite and porphyry that intrude biotite gneiss of the Mkushi Gneiss and the Mtuga Granite, and is found in close proximity to, and within shear zones, particularly in the Mtuga Granite. Mineralisation forms a series of sinistral en echelon lenses, individually been referred to as the X-Y-Z, E, F, G, H, and L zones. This pattern is considered to have ended in the central part of the H zone, which is located to the SW of the open pit, and split into a set of complex anastomosing and multiply branching bodies of mineralisation.
Mineralisation occurs as irregular sulphide disseminations in aplite and gneiss (the most common); breccia veins in aplite; small, irregular sulphide veinlets in aplite and gneiss; banded sulphide disseminations in gneiss; and tension gashes. The banded sulphide disseminations occur near the margins of the aplite dykes within gneiss.
The en echelon lenses typically trend at about 55°, although they tend to swing to a maximum of about 75° with increased distance from the biotite phyllonites which typical trend at ~50°. The lenses dip moderately to steeply to the NW. On the southern end of the pit the lenses, collectively referred to as the G-zone, trend at ~20°.
Three sinistral en echelon lenses of copper mineralisation have been recognised, trending at ~54° over a strike length of ~500 m. The most south-westerly lense is ~85 m long and trends at ~70°, the central composite section consists of two sub-parallel lenses that trend ~60° over a strike length of 150 m, whilst the northern zone has a strike length of ~130 m and trends at ~58° NE. The individual lenses are typically 10 to 15 m wide, and dip steeply to the south. Mineralisation has only been tested to a maximum depth of some 100 m vertically below surface.
Mineralisation is associated with felsic intrusive rocks that include aplite, porphyry, leucogranite and pegmatite, which intrude biotite gneiss, and is primarily restricted to the intrusive rocks, although sulphide disseminations and veinlets also occur within the gneiss. The dominant sulphide is chalcopyrite, although pyrite is apparently more common than at Munshiwemba. The main styles of mineralisation are disseminated and veinlet sulphides. Minor banded sulphides zones occur where relict banding is evident when biotite in gneisses have been replaced. The sulphide breccias observed at the Munshiwemba prospect are not present at Coloquo.
Porphyroblastic gneiss, which are extensive at Munshiwemba, are only locally recognised at Coloquo, whilst achistose biotite phonolite zones appear to be relatively sparse and have poor continuity. Abundant tourmaline inclusions and segregations occur in aplites, along with common biotite spotting. The tourmaline is apparently antithetic to chalcopyrite. Epidote alteration is also common. Zones of oxididation are apparently more intensely and deeply developed than at Munshiwemba.
Copper mineralisation occurs in association with irregular lensoid bodies of actinolite-bearing granite and aplite intruding the Mtuga Granite, focused along narrow shear zones within the granite.
Mineralisation is hosted by granite within a NE to SW striking shear zone, dipping at ~70° NW, and the associated mineralised shear varies from 1.5 to 4.5 m in width. Pegmatite and aplite dykes are found within the shear zone, accompanied by sheared dolerite dyke and biotite-chlorite schist that extend the full length of the underground workings on the SE margin of the prospect. Oblique faults cross-cut the shear zone resulting in minor structural offsets. Alteration assemblages include sericite, chlorite, minor calcite, actinolite and tourmaline. Geological mapping suggests two lenses of copper mineralisation in a sinistral en echelon pattern. The northeastern and southwestern lenses dip at ~55° NW and are ~120 and ~80 m long, respectively, with the latter composed of two sub-lenses.
Estimated JORC 2004 compliant inferred resources (Nimmo, 2008) are:
- cut-off grade of 0.3 % Cu - 10.7 Mt @ 0.73 % Cu
- cut-off grade of 0.4 % Cu - 8.98 Mt @ 0.80 % Cu
- cut-off grade of 0.3 % Cu - 0.23 Mt @ 1.0 % Cu.
This summary has largely been drawn from: "Nimmo, M, 2008 - CGA Mining Limited: Mkushi Copper Project, Zambia, Project No. 7288; Independent Technical Report, August 2008, Reissued to Ratel Group Limited, November, 2010, prepared by Snowden Mining Industry Consultants, Perth, WA., Australia. 76p."
The most recent source geological information used to prepare this summary was dated: 2008.
Record last updated: 26/10/2015
This description is a summary from published sources, the chief of which are listed below.
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