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Murchison Belt - Alpha Gravelotte, United Jack, Weigel Athens, Monarch Free State, Mulati

Limpopo (Northern) Province, South Africa

Main commodities: Sb Au
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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 contains a number of antimony deposits in what is known as the Antimony Line that have made the belt one of the world's largest antimony producing areas. Total production from 1940 to 1984 amounted to 0.446 Mt of contained Sb metal and 9.796 t Au.

A linear zone defined by 18 mines extends over a strike length of around 50 km, known as the Antimony Line. This zone is expressed as higher ground in the centre of the greenstone belt and immediately south of the resistant hills of the quartzitic 'Antimony Bar'. A second, but poorly exposed, weakly developed possible 'southern Antimony Line' is found 1 km to the south of the main Antimony Line. Both lines are hosted by the Weigel Formation, which is composed predominantly of quartz-chlorite and quartz-muscovite schists. Petrographic and geochemical studies suggest the chloritic schists are magnesium-rich pelites and that the quartz-muscovite schists represent felsic tuffaceous rocks.

The Weigel Formation is bounded to the south by the ultramafic to mafic volcanic-dominated Mulati Formation, and to the north by the mafic to felsic volcanics of the Rubbervale Formation. The latter hosts a group of small high grade volcanogenic copper-zinc mineralization in what is known as the 'Cu-Zn Line (see the separate record).

The Antimony Line is a linear shear or fault zone characterised by intense deformation, and represents a central, semi-brittle deformation zone within a broad ductile shear zone with oblique to dip-slip sense of movement. It is composed of a range of talcose and carbonate rocks in which stibnite mineralisation is developed in anomalous thickenings of quartz-carbonate rocks. The main rock types recognized often show a broad zonation and include (from the periphery to the centre of the Antimony Line) talc-chlorite schist, talc-carbonate schist, quartz-carbonate schist, green fuchsitic quartz-carbonate schist and chert-carbonate rocks. The talcose schists represent altered komatiites, while the quartz-carbonate rocks represent further, more intense alteration of the talc-carbonate schists. The conspicuous green fuchsitic carbonate schists results from potassium-metasomatism linked to the mineralisation.

There is a consistent pattern in the occurrence of mineralisation in nearly all workings, with stibnite-bearing quartz veins and stibnite impregnations in the competent, siliceous, cherty carbonate rocks and in associated green fuchsitic quartz-carbonate schist which generally occupies the centres of quartz-carbonate alteration zones within talcose schist. The antimonial ores are divided into three types, namely tetrahedrite ores, pure stibnite ores and complex antimonial ores. Only the pure stibnite ores are mined.

The maximum mineralisation age for sulphides from the antimony line has been dated at 3020±50 Ma by Pb-Pb model ages (Saager and Köppel 1976).

The principal mines include: Alpha-Gravelotte; United Jack; Weigel-Athens; Monarch-Free State; Mulati; BI Mine; and Cinnabar South.

The Alpha-Gravelotte complex is the largest antimony producer in the greenstone belt, having produced over 9 Mt of high grade stibnite ore (to 1986). It comprises two major, closely related, ore zones which are basically shear controlled, overlapping, but largely vertically and laterally separated zones. The two orebodies are associated with the tight to isoclinal, steeply plunging Alpha-Gravelotte fold, with the better mineralisation concentrated in what are thought to be cymoidal structures related to a major shear zone along the Antimony Line. The folding is reflected by the folded, competent, quartzitic Antimony Bar and has formed as a result of a sinistral shear couple with movement along the Antimony Line rocks immediately to the south of the former. These less competent rocks have been carbonatized, and the more competent carbonates still further deformed to provide the mineralised channels in tensionally created structural sites. The final location of the mineralisation within the controlling structures can be ascribed to multiple remobilisation and concentrating episodes applied to structurally located antimonial sulphides, possibly derived from a primary volcanic source.

Gravelotte, with a maximum strike length of 400 m, has a vertical extent of 650 m and is in the upper sections of the mine. Alpha occurs as a series of fracture controlled ore zones which are the down plunge extension of the Gravelotte mineralisation, continuing to well below 1200 m from surface. The lodes vary from a few cms to 1 m in thickness.

The main ore mineral at Alpha-Gravelotte is stibnite, with lesser berthierite, together with varying, but smaller, amounts of gersdorlfite, ullmanite, corynite (a mixture of gersdorffite and ullmanite), gudmundite, pyrite, arsenopyrite and scheelite. Native gold and aurostibite are important constituents.

The most recent source geological information used to prepare this summary was dated: 1986.    
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:
Abbot J E, van Vuuren C J J and Viljoen M J,  1986 - The Alpha - Gravelotte antimony ore body, Murchison Greenstone Belt: in Anhaeusser C R, Maske S, (Eds.), 1986 Mineral Deposits of South Africa Geol. Soc. South Africa, Johannesburg   v1 321-332
Boocock C N, Cheshire P E and Vearncombe J R,  1984 - Structural geology of the Gravelotte Shaft Quarry and Monarch antimony mine, Murchison greenstone belt, Transvaal: in    S. Afr. J. Geol.   v87 pp 315-325
Pearton T N and Voljoen M J,  1986 - Antimony mineralization in the Murchison Greenstone Belt - an overview: in Anhaeusser C R, Maske S, (Eds.), 1986 Mineral Deposits of South Africa Geol. Soc. South Africa, Johannesburg   v1 pp 293-320
Willson C and Viljoen M J,  1986 - The Athens antimony orebody, Murchison Greenstone Belt: in Anhaeusser C R, Maske S, (Eds.), 1986 Mineral Deposits of South Africa Geol. Soc. South Africa, Johannesburg   v1 pp 333-338


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