Murchison Belt - Maranda J, LCZ, Romotshidi, Mon Desir, Solomons, Mashawa

Limpopo (Northern) Province, South Africa

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The Murchison greenstone belt is located in the northern part of the Limpopo Province of South Africa, approximately 400 km north-east of Johannesburg. It hosts 12 massive sulphide copper-zinc deposits in what is known as the Cu-Zn Line that occurs to the north of the "Antimony Line" (see separate record) that has made the belt is one of the world's largest antimony producing areas. The more significant of the "Cu-Zn Line" deposits include Maranda J, LCZ, Romotshidi, Mon Desir, Solomons and Mashawa.

The Murchison greenstone belt is located in the eastern part of the Limpopo Province of South Africa, on the northen margin of the Archaean Kaapvaal craton. It comprises a narrow, ~140 x 4 to 15 km, ENE-trending zone that forms a distinct mountain range of variably altered bimodal basaltic to rhyolitic volcanic rocks. The belt is flanked to the north and south by intrusive Archaean granites and granite gneisses. To the west, it is overlain by early Proterozoic sedimentary and volcanic units of the Transvaal sequence Wolkberg Group, while to the east, the extension is more diffuse, marked by a belt of xenolithic amphibolite facies volcanic lenses surrounded by later granitoids. These Archaean suites are overlain by Upper Palaeozoic and Mesozoic rocks of the Karoo sequence, and is interpreted to be part of the Thabazimbi-Murchison Tectonic Lineament, which extends across most of the Limpopo Province below cover to join the Thabazimbi belt to the west (McCourt 1995). It represents a regional zone of ENE-trending fractures approximately 600 km in length, subparallel to the Limpopo mobile belt about 100 km to the north. To the north, the Murchison greenstone belt is bounded by the Rooiwater layered mafic igneous complex (Vearncombe et al., 1987, 1992).

The deposition of the rift-related volcanic sequence has been dated at between 3.08 and 2.97 Ga. The belt is divided into two major lithological domains: i). the felsic Rubbervale Formations and ii). diverse ultramafic to mafic volcanic and siliciclastic Murchison schists comprising the Weigel, MacKop, Leydsdorp, Mulati and La France formations (Van Eeden et al., 1939; Taylor 1981; Minnitt 1981; Vearncombe et al., 1992). The Silwana Amphibolite and the Rooiwater Complex on the northern rim of the belt have a minimum age of emplacement of 2740±4 Ma (Poujol et al., 1996). The central Murchison greenstone belt is a greenschist-facies bimodal volcanic suite, with minor intercalated volcaniclastic and siliciclastic sedimentary rocks. Ultramafic komatiite-Mg-rich basalts sequences of the Weigel Formation (3.087±21 Ga) and minor siliciclastic sediments predate andesitic to rhyolitic volcaniclastic rocks, rhyolitic lavas and quartz-feldspar porphyritic rhyolites of the Rubbervale Formation (2.971±10, 2.969±20, and 2.967±1.3 Ga; Poujol et al., 1996; Brandl et al., 1996; Poujol 2001). Field and age relationships indicate quartz porphyritic lavas and domes represent the initial products of felsic volcanism in the Murchison belt and locally cut the former komatiitic to basaltic crust (Poujol et al., 1996). The Murchison belt is cut by late-stage granite plutons, adjacent to the southern margins and in the central part (2.901±20 Ga minimum age; Poujol et al., 1996); Vearncombe et al., 1992; Poujol et al., 1996).

Structural deformation in the central Murchison belt represents NNE-SSW compression and a strong subvertical east-west extension with major WSW-ENE D1 and east-west D2 tectonic events that resulted in an isoclinally folded and strongly foliated stratigraphic succession. Isoclinal folding is vertical with east- and west-dipping axes and strongly associated with steep stretching lineation, vertical elongated micro- to macro-scale boudinage, and outcrop scale flexural-slip folds (Vearncombe et al., 1992). DD3 and D4 follow the NNE-SSW compression forming kinkbands, small-scale faulting with limited, centimetre-wide shear component.

The antimony line (Pearton and Viljoen 1986) defines a linear zone up to 250 m wide and 35 km long of discontinuous Sb-Au deposits, which are hosted by talc-chlorite- and talc-carbonate-altered schists within the komatiitic sequence of the Weigel Formation (see separate record).

Stratabound volcanic-hosted massive sulphide deposits occur along a distinct stratigraphic horizon, the "Cu-Zn line" of the Rubbervale Formation, close to the contact with the mafic sequences of the Weigel Formation, a few kilometres north of, and (sub)parallel to the "antimony line". The "Cu-Zn line" forms a poorly defined, discontinuous, about 500-m wide unit that is characterised by variably altered volcaniclastic sedimentary rocks and felsic flows.

The deposits mostly occur as 100 to 1000 m long, highly deformed, stratabound, sulphide lenses, hosted within felsic lapilli-sized and finer-grained pelitic hyaloclastic volcanic rocks. They are located ~200 to 500 m to the SE of felsic volcanic centres, which are mainly composed of massive flows and associated cryptodomes. The deposits are spatially and temporally confined to the emplacement of quartz porphyritic and massive rhyolite volcanic domes, rhyolite to andesite flows, and volcaniclastic rocks. The volcanic domes probably outline former active volcanic centres, and form flat circular morphological hills with heights varying between 50 m (e.g., Romotshidi) and 140 m (e.g., Rita Kop, Townlands Kop). They are mainly situated on the lower southern slopes of the central and western Murchison mountain range and are often morphologically and tectonically separated from their quartz-muscovite schist carapaces. Individual deposits are spaced at 7 to 14 km along the "Cu-Zn line".

The host Rubbervale Formation comprises a thick pile of felsic volcaniclastic schists, massive flows, and volcanic domes (Taylor 1981; Vearncombe et al., 1992) representing an atypical sequence of voluminous felsic rocks with kilometre-scale flows and volcaniclastic sedimentary rocks covering more than 50% of the Murchison volcanic sequences (SACS 1980). It comprises a northern unit of quartz-rich volcaniclastic schists (Van Eeden et al., 1939) and a southern unit of strongly foliated quartz and rarely feldspar porphyritic to massive rhyolite domes and lavas and intercalated thin horizons of felsic volcaniclastic rocks hosting the deposits of the "Cu-Zn line" (Pearton and Viljoen 1986; Vearncombe et al., 1992; Terblanche and Lewis 1995; Schwarz-Schampera et al., 2001). The southern unit is characterised by in situ coherent quartz porphyritic and massive rhyolite volcanic cryptodomes, andesitic to rhyolitic lava flows and locally bedded volcaniclastic rocks. The coherent domes consist of massive, grey, very fine-grained rhyolite showing minor hydrothermal alteration.

The main orebodies along the "Cu-Zn line" are tabular, stratabound lenses, up to several hundreds of metres in length, 250 to 500 m wide, and between 10 and 30 m thick. The present shape of the orebodies is controlled by close F
2 folds and to a lesser extent by smaller F2 folds, but they have largely retained their original lens-shaped character. They are generally affected by boudinage, with ocal thickening of the ore lenses apparent at the fold hinges and in single boudin bodies. The orebodies are locally bound by subvertical faults, and contacts between the massive sulphides and adjacent altered wall rocks are commonly slightly sheared. The occurrence of a thin sulphide “mylonite” containing fine-grained rounded chert and chlorite schist fragments along the ore contacts is described by Terblanche and Lewis (1995).

The ores are composed of of finely banded, massive, and commonly coarse-grained recrystallised sphalerite, pyrite, pyrrhotite, and chalcopyrite, with traces of magnetite, cassiterite, stannite, tennantite, and secondary digenite, bornite, chalcocite, and native copper. The ore passes into more pyrite-rich banded assemblages towards the chloritic and siliceous deposit margins, with hydrothermal replacement of layered volcanic hyaloclastites evident along the contacts. The primary mineral zonation and textures were largely destroyed during deformation and intense remobilisation and recrystallisation, although distinct zones of chalcopyrite-rich mineralisation and associated chlorite-dominated alteration, as well as sphalerite-pyrite-galena assemblages with quartz-sericite alteration can be distinguished. Six ore types have been defined: i). massive pyrite, ii). banded pyrite-sphalerite, iii). layered sphalerite with pyrite porphyroblasts, iv). massive coarse-grained sphalerite, v). massive sphalerite-chalcopyrite, and vi). massive chalcopyrite.

The "Cu-Zn line" comprises 12 massive sulphide occurrences and deposits, with a total tonnage of ~3 Mt, with ~1.8 Mt in the Maranda J and LCZ (Letaba, Platveld) deposits; and a further 1.2 Mt in the Romotshidi mine, and several smaller deposits including, from west to east, Ngwabu River, Rita Kop, Ka Khuweni, Ka Phokwe (satellite orebody of Maranda J), Carrol's Peak, Mon Desir, Solomons, Vlaklaagte and Mashawa (Taylor 1981; Pearton and Viljoen 1986; Vearncombe et al., 1992).

Apart from early mining of oxidised ores for pigments (Pearton and Viljoen 1986), small-scale mining of the massive sulphide orebodies commenced in the 1950s at the Letaba and Mashawa mines, followe by the solomons mine in the 1970s. Small-scale underground and open pit mining took place in the 1980s at the Mon Desir deposit. In total, about 27 000 t @ 27% Zn and 0.4% Cu were recovered. The Maranda J deposit was discovered in 1978, and mining started in 1991, with operations largely suspended in 2005. Subsequent exploration had increased the reserves to a tonnage of about 1.1 Mt of massive sulphides @ 23% Zn and 3.0% Cu. The Romotshidi deposit, about 7 km to the west of Maranda J, produced about 0.3 Mt @ 23% Zn and 1.5% Cu between 1996 and 2004. The old Letaba mine (LCZ) was first developed on two parallel mineralised horizons between 1948 and 1952, comprising five ore lenses with a combined tonnage of 0.73 Mt @ 1.5% Cu and 9% Zn. Reevaluation in 1995 delineated 0.24 Mt @ 19% Zn and 1.9% Cu in one of the lenses, before mining restarted in 1998 but was suspended in 2000 due to metallurgical problems.

The most recent source geological information used to prepare this summary was dated: 2010.    
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:
Jaguin J, Poujol M, Boulvais P, Robb L J and Paquette J L,  2012 - Metallogeny of precious and base metal mineralization in the Murchison Greenstone Belt, South Africa: indications from UPb and PbPb geochronology: in    Mineralium Deposita   v.47 pp. 739-747
Schwarz-Schampera U, Terblanche H and Oberthur T  2010 - Volcanic-hosted massive sulfide deposits in the Murchison greenstone belt, South Africa: in    Mineralium Deposita   v.45 pp. 113-145

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