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Tsumeb |
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Namibia |
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Cu Pb Zn Ag
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Super Porphyry Cu and Au
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The Tsumeb carbonate hosted, breccia pipe copper-lead-zinc-silver deposit is located in northern Namibia, some 430 km by road and rail from Windhoek and 600 km by rail from the Atlantic coast port of Walvis Bay (#Location: 19° 14' 22"S, 17° 42' 58"E).
The polymetallic orebody was present mainly on the outer peripheries of a near vertical, irregular, markedly transgressive, collapse-breccia pipe cutting dolomites of the Upper Proterozoic Damaran Supergroup. Sulphides were massive to semi-massive, and mineralisation was present over a vertical distance of nearly 1.8 km in the oval shaped pipe which is 20 to 180 m in diameter. The orebody has been exhausted and the mine closed.
The Tsumeb pipe transgresses carbonates of the upper half of the Otavi Group, a member of the Upper Proterozoic Damara Supergroup. It occurs in onlapping carbonates of the Northern Platform deposited over the Angola-Kasai Craton some 80 km to the north of the main northern margin of the Damaran/Katangan Rift Zone. The Upper Proterozoic sequence in this region and on the adjacent Northern Platform comprises, from the base:
* Nosib Group, 0 to 1200 m thick - comprising feldspathic quartzites, arkose and conglomerate; overlain by phyllitic agglomerate, tuff and epidotised andesite up to 300 m thick; capped by quartzite, conglomerate, arkosic mixtite, dolomite and ferruginous shale. The Nosib Group is not as extensive as the overlying Otavi Group which oversteps it onto the basement in the Northern Platform near Tsumeb.
* Otavi Group, 0 to 4800 m thick - is very widespread but restricted to the platformal fringes of the Damaran/Katangan Rift Zone. It is areally more extensive than the underlying Nosib Group and transgresses well onto the Angola-Kasai Craton. This group contains two units of predominantly carbonates, with minor argillites and no arenites, separated by a mixtite/tillite unit.
* Mulden Group. - On the Northern Platform, the Otavi Group is unconformably overlain by the Mulden Group with an angularity that varies from marked to virtually concordant. This group appears to represent an initial thicker coarse clastic phase, followed by finer grained clastics and carbonates.
The Tsumeb orebody transgresses at least 1000 m of stratigraphy of the upper part of the Tsumeb Subgroup carbonates of the Otavi Group. It is situated on the northern flank of the doubly plunging, slightly assymetric Tsumeb Syncline. Parasitic folds with varying plunges are mapped within this structure to the east and west of the orebody at surface and underground. An axial plane cleavage is present. Numerous bedding plane shears are obvious, and some prominent bedding thrusts caused by flexural slip folding are found in the host units. A major feature of the mine area is the North Break, a zone of alteration within the bedding of the lower portion of the T6 unit, a section rich in stromatolite beds. This zone, which is a major aquifer (and palaeo-aquifer), is recognisable at surface from 3 km to the west to 1 km to the east of the mine area as a discontinuous horizon of siliceous, ferruginous, calcitic and manganiferous dolomite which in many places contains secondary Cu, Pb, Zn and V minerals.
The main rock types within the pipe are as follows:
* Feldspathic Sandstone - which in part transgressively fills the pipe, is composed of a light to medium grey variety that petrographically and chemically resembles certain arenaceous facies of the Tschudi Formation of the overlying Mulden Group. Typically it consists of an equigranular clastic aggregate of angular to well rounded quartz and feldspar grains, usually with very little matrix. Shungitic carbon (an anthracite like pyro-bitumen) has been emplaced in part, resulting in darkening of the rock. The feldspathic sandstone has also penetrated the North Break having been found up to 300 m from the intersection with the pipe. Historically, because of the transgressive mode of its occurrence, the feldspathic sandstone has been regarded as an igneous rock and termed an aplite or pseudo-aplite. Petrographic work however indicates that it is a sediment and probably belongs to the Tschudi Formation that has flowed into a large solution collapse/dewatering structure in the unconformably underlying carbonates of the Tsumeb Subgroup.
* Dark Dolomite Breccia and Dolomite Breccia - the Dark Dolomite Breccia first appears at the 21 level at the change in plunge of the orebody pipe and persists to near the 28 level where it grades into the Dolomite Breccia. The Dark Dolomite Breccia consists of clasts which may be identified with lithologies cut by the pipe higher in the mine. For instance where it cuts the T6 unit, the T7 are evident, while where it cuts T5 the clasts are of T6 and lesser T7, while a clast of oolitic chert of the type found at the top of the T8 unit occurs 1000 m below that unit on the 40 level where the pipe cuts the T5 unit.
* Calcitisation - first appears on 20 level where it occurs in small areas, but increases to become more prominent by 24 level. Calcitisation is a replacement process which was initiated prior to the introduction of the ore and has affected dolomite, dolomite breccia and feldspathic sandstone.
* Marble Breccia - where calcitisation is most intense on the 34 level, the entire core of the pipe has been thoroughly altered to a coarsely crystalline, grey, calcitic rock which is in places ferruginous and has locally also been bleached. The resultant rock has been termed Marble Breccia. The breccia clasts are composed of foliated, crystalline and calcitised grey dolomite, together with subordinate grey chert and unaltered dolomite. The matrix is as described for the calcitised dolomite above.
* Silicification - which is observed from 26 level downward, is restricted to the southern margin of the ore zone to 34 level, but intensifies and becomes more widespread deeper down until it dominates.
* Silica Dolomite - comprises dolomite or dolomite breccia of which more than 40% has been reconstituted by siliceous alteration. All silica-dolomite development is below the North Break. Within the silica-dolomite breccias there is a variety of results of the alteration, ranging from large white crystalline interclast siliceous veins, to pink silicification of clasts. In places there is a strong sharply defined breccia texture, while elsewhere there are only diffuse boundaries between the matrix and clasts. In general the clasts predominate, while in places massive sulphides rim the silicified clasts.
The principal primary sulphide minerals at Tsumeb are galena, tennantite, sphalerite, chalcocite, enargite and bornite, with lesser chalcopyrite, germanite and renierite. Pyrite is widespread, normally in small amounts. Supergene chalcocite, djurleite, digenite and covellite are important in the upper section of the mine, while the same minerals are present in a lower oxidation zone between 24 and 38 levels. A large number of sulphides and sulpho-salts occur as erratic traces throughout the deposit. Limited amounts of non-metallic gangue minerals are present, principally calcite, quartz and dolomite and rarely barite and fluorite. Some 213 minerals are recorded in the mine, for 40 of which it is the type locality.
There are four types of mineralisation present, as follows:
* Massive Peripheral Ore - these complex Pb, Zn and Cu ores contain up to 40% total metal. Massive ore from the surface to 20 level is typically concentrated peripheral to feldspathic sandstone, reaching its maximum lateral extent in adjoining dolomite breccia. This ore is generally present as medium to coarse grained sulphides, comprising variable amounts of galena and sphalerite, together with tennantite, enargite (in the upper sections of the mine), bornite and chalcocite.
* Manto Ore - this comprises wing like appendages to the massive peripheral ore that are present between the 26 and 30 levels on the southern and northern side of the main pipe. Most of these very rich concordant to sub-concordant lenses occur near the base of the T6 unit in which many stromatolitic beds are located. Most of the manto ores are present over the whole strike of the orebody and are often very rich. The S95 Manto for instance had 75 000 t @ 23% Cu. On the southern side of the pipe the manto ores are generally bornite, chalcocite, djurleite, tennantite, galena and sphalerite, except for the S95 which is mainly chalcocite. On the northern side they comprise galena and sphalerite with subordinate tennantite. As with the massive ores, textures strongly indicate a replacive nature.
* Disseminated and Stringer Ores - these ores are hosted by feldspathic sandstone, unaltered to altered bedded dolomite and by various dolomite breccias. The disseminated ore is most commonly within feldspathic sandstone with the equigranular sulphides evenly scattered, preferentially replacing feldspar. Bornite, chalcocite and tennantite are dominant, galena is scarce and sphalerite is rare. Discontinuous Cu rich veins are also present in the feldspathic sandstone, and where replacement is more advanced, the disseminated and stringer mineralisation grades into massive ore.
* Oxide Ores - these are present in the upper zone of supergene enrichment in the upper parts of the mine and in a zone of 'oxidation' from the 24 to 38 levels.
The Tsumeb ore deposit contained: 27 Mt @ 4.3% Cu, 10% Pb, 3.5% Zn, 95 g/t Ag (historic production).
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 1995.
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.
Tsumeb
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Selected References:
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Haest, M. and Muchez, P., 2011 - Stratiform and vein-type deposits in the Pan-African orogen in central and southern Africa: evidence for multiphase mineralisation: in Geologica Belgica v.14, pp. 23-44,
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Kamona A F, Leveque J, Friedrich G, Haack U 1999 - Lead isotopes of the carbonate-hosted Kabwe, Tsumeb, and Kipushi Pb-Zn-Cu sulphide deposits in relation to Pan African orogenesis in the Damaran-Lufilian Fold Belt of Central Africa: in Mineralium Deposita v34 pp 273-283
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Lombaard A F, Gunzel A, Innes J, Kruger T L 1986 - The Tsumeb lead-copper-zinc-silver deposit, south west Africa/Namibia: in Anhaeusser C R, Maske S, (Eds.), 1986 Mineral Deposits of South Africa Geol. Soc. South Africa, Johannesburg v2 pp 1761-1787
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Schneider J, Melcher F and Brauns M, 2007 - Concordant ages for the giant Kipushi base metal deposit (DR Congo) from direct Rb-Sr and Re-Os dating of sulfides : in Mineralium Deposita v42 pp 791-797
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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