The Uis pegmatite hosted tin deposit is located 250 km NW of Windhoek and 150 km NNE of Swakopmund in the Erongo region of western Namibia.
The Uis tin mine was established in 1958, and closed in 1991 due to low metal prices.
Regional Setting
The mineralised pegmatites at Uis are found within the Khomas Series of the Swakop Group, part of the Damara Supergroup within the Damara-Katanga mobile belt. The Damaran section of the mobile belt is in general aligned in an ENE-WSW direction from Tsumeb to Cape Cross where it is rotated through ninety degrees to run NNW-SSE parallel to the Atlantic coast. A block of older basement is exposed in this elbow zone. To the north east, the older rocks are masked by the extensive Tertiary and Quaternary superficial cover of the Kalahari Desert. The Damara Supergroup can be subdivided as follows, from the base:
• Nosib Formation - This is composed largely of shoreline conglomerates, and shelf facies quartzites with lesser interbedded phyllites limestone and dolomite. It occurs around the margins of and along trend from the Archaean basement outcrops, below the less metamorphosed Outjo Group Facies. It is also present as an elongated belt to the north east or Walvis Bay.
• Swakop Group - The Swakop Group dominates the geology of the Damaran Mobile Belt. It can be subdivided into a lower Hakos and Upper Khomas Series. The Hakos Series is primarily composed at calcareous to pelitic sediments in which dolomites predominate. These are discordantly overlain by the 6000 m thick Khomas Series which comprises a basal tillite, overlain by marbles with amphibolite lenses, red porphyry tuff, pelitic calc-silicates with banded ironstones, amphibolites and graphitic schists which grade upwards into well bedded alternating psammitic and pelitic sediments forming the main part of the sequence. There are little or no carbonates and calc-silicates in the upper sections of the Khomas. These rocks are generally steeply dipping and strongly folded.
• Outjo Group - Along the inner, or northern margin of the Mobile Belt, a less metamorphosed facies comprising tillites, dolomites and limestones with iron formations shales and quartzites are found. These comprise the Sesfontein Phyllite and Otavi Series and are probably equivalents of the Swakop Group overlying the more stable margins of the Mobile Belt. To the north they are followed by the quartzite, slate, conglomerate and greywacke of the Mulden Series.
Salem Granite and Pegmatites - The Salem Granite, which yields age dates of 520 Ma, is best developed in the "elbow zone" of the Damaran Mobile Belt. In general it is a foliated, grey, porphyritic albite granite with abundant orthoclase phenocrysts and coarsely crystalline quartz, albite and biotite. Locally tourmaline is a common accessory. Sections of the granite are non-foliated and non-porphyritic. Structurally the Salem Granite is conformable with the country rocks being found mainly in anticlinal cores. The pegmatites of the Damaran Mobile Belt are also most prolific in the "elbow zone". They yield age dates of from 600 to 450 Ma with a peak at 520 Ma. In general they are developed perpendicular to the main ENE-WSW direction of the Mobile Belt.
Karoo Supergroup - The mainly cratonic Karoo Supergroup comprises tillite, boulder shale, shale, sandstone and coal measures, with extensive basic to intermediate volcanics and lesser rhyolites. These are of Permian to Lower Jurassic age.
Cape Cross Complex - The Cape Cross Complex is represented by a series of gabbro, granite, fayalite and dolerite intrusive bodies, elongated along the core of the Mobile Belt from Cape Cross almost to Tsumeb. These yield dates of 250 to 300 Ma, and are post to syn-Karoo.
Geological Setting
In the Uis area the Khomas Series comprises the following from the top:
• Quartzites and arenaceous sediments with lesser intercalated shales and phyllites. These are usually well bedded and are 500 to 1000 m thick.
• Calc-silicate and siliceous marl pods and beds distributed irregularly above the nodular schist, over an interval that is 5 to 10 m thick.
• Nodular schist - this varies from being a fine grained biotite schist to a sandy quartz biotite schist to a hard laminated biotitic quartzite. It is characterised by the presence of bands of from a few metres to several tens of metres in thickness containing small elongated spheroidal segregations of fine biotite with 20 to 30% fine quartz. These range from 1 to 10 or 20 mm across and weather out as individual entities. They usually comprise 5 to 10% of the nodular beds which are 100 to 300 m thick.
• Chlorite-biotite schist and argillaceous rocks, chiefly phyllites and micaceous shales, which are 2000 to 3000m thick.
• Inpersistant marble horizon, that is 100 to 500m thick.
These sediments are well bedded with the bedding being obvious as fine colour variations and macroscopically as more and less resistant bands reflecting frequent interdigitation of shale, schist and quartzite. Uis lies within one of the mineralised belts of the Damaran Mobile Belt as defined by a 10 km wide strip enclosing parallel zones of shearing elongated in a ENE-WSW direction. The largest of these zones passes through the Uis Mine area and is recognisable, to an experienced eye, by the increase in the degree of folding and a series of small faults seen on aerial photos. In addition, at the margin of the shear zones, sharp changes in the strike of the sediments are obvious.
The pegmatites are elongated normal to the shear zone and yield age dates ranging from 450 to 600 Ma with a peak near 520 Ma. They usually have an 'S' shape with the peripheries being outside of the main shear zone and roughly parallel to it. Most pegmatites are relatively steeply and variably dipping at greater than 45°, but not in any constant direction. In general they are laterally doubly plunging ie. the thicker section of the body increases in length laterally with depth. Within the Mine Leases, which cover an area of some 8 x 4 km, there are over 100 pegmatite bodies known. Eight of these pegmatites were large enough to be mined in 1978. These range in length from 400 m to 1 km and from 30 to 150 m, averaging 50 m in width. Their cumulative length is 6 km. There are a further 13 smaller bodies which are potential producers. These range from 300 to 1000 m in length with thicknesses averaging between 15 and 50 m. The remaining 79 pegmatites only have dimensions of a few tens to a few hundreds of metres by up to 15 m in width.
The Uis pegmatites have very irregular outlines, seldom having parallel sides. Their margins are often very irregular, undulose and ragged with many thin offshoots. Inclusions of the enclosing schist from 2 to 5 m across are often found within the pegmatite, especially along the margins. On their edges the pegmatite can be seen to have partially assimilated the schist, producing fine biotite bands in the outer 10 to 50 cm. The largest pegmatite currently being worked has a total length of 2.5 km. At the surface it comprised two parallel fingers, one 30 m and the other 5 m thick separated by 45 m of schist. Some 45 m below the surface these fingers abruptly coalesce to form a single 120 m thick pegmatite. A further 40 m down the main body is still 120 m thick. It dips overall at around 45° to the north and strikes at 90°, in contrast to the schist strike of 30°, and has very variably dip. The junction of the two fingers plunges outwards at a shallow angle. The 120 m thick section is currently exposed over a 400 m length but with depth will eventually be mined over a 700 m length. From the central pit section the two fingers can be traced at the surface in one direction for 1 km while in the other direction they only persist for about 100 m. There is then a gap of 50 m followed by the development of a pegmatite band some 30 to 50 m wide and 100 to 150 m long. This abruptly narrows into three 1 to 10 m wide fingers which persist for a further 50 m before terminating abruptly. The pegmatites are very coarsely crystalline, although crystal structures are not well developed. Quartz aggregates are up to 10 x 5 cms with white potash feldspars of similar size and muscovite books around l cm wide. In places some of the individual potash feldspar and quartz aggregates are up to 30 x 10 cm. These pegmatites are not zoned, ie. they are homogenous, although micas predominate on the outer 50 cms or so. Their mineralogical composition is as follows: Quartz - 30%; Muscovite - 8.5%; K feldspar - 30%; Clays, Fe+Mn+Li oxides - 1.4%; Plagioclase - 3%; SnO2 - 0.1%.
The overall chemistry of the pegmatites differs from the host schist in that it has more silica and lesser K2O and Na2O. Other wise the average chemistry of each is similar.
Mineralisation
Tin mineralisation occurs as cassiterite grains from 0.5 to 2 mm across with larger segregations up to 1 cm across being common. This mineralisation is confined to the sections of the pegmatites crossing the nodular schist. The mineralised pegmatites are invariably altered. Alteration takes the form of albitisation, fine mica development, chloritisation and tourmalinisation to varying degrees. The more highly altered pegmatites commonly have aggregates of relatively fine albite and muscovite enveloping the original quartz aggregates. However, mineralised pegmatites sometimes show very little alteration. The mineralised pegmatites have decomposed and bleached margins developed from 50 to 100 m outwards into the country rocks from their contacts. This zone is accompanied by an increase in iron and manganese oxides. These oxides may in part be due to the oxidation of the trace sulphides found in the nodular schists. The oxides occur as 0.5 to l mm thick veinlets parallel to schistosity and as 2 to 3 mm thick bands parallel to bedding. Tourmaline is very common in the mine area. It is found in three forms namely:
• Within bedding planes - here it occurs as dark 1 mm thick laminae, or as dark 'beds' up to 10 cm thick containing 20 to 30% tourmaline. This is found in biotitic quartzite and biotite schist. In other places beds of up to 10 m thick have diffuse tourmaline developments giving the whole rock a dark grey to black colouration. This bedded tourmaline development predominates in the nodular schists.
• Fracture fill - tourmaline is found as l mm thick coatings on cleavage planes developed at an angle to bedding.
• Irregular zones of tourmalinisation adjacent to pegmatites. These occur as rims to the pegmatite bodies, decreasing in intensity outwards. The tourmaline content of these rims can be mapped. In general the >5% tourmaline zone is 1 to 10 m wide, while the 1 to 5% zone occupies an area 20 to 30 m wide outside of this.
The pegmatites have only minor sulphides. The final concentrates occasionally have 1% sulphides, 90% of which is pyrite, the remainder being galena and chalcopyrite. The mineralised pegmatites in general carry between 0.11 and 0.15% Sn. In a particular pegmatite, the grade varies inversely to the thickness, such that if the pegmatite thins, the grade increases and vice versa. The host schist carries anomalous tin levels although insufficient work has been done to delineate a pattern. Values of up to 100 ppm are recorded with one sample yielding 2000 ppm Sn.
The deposit also contains tantalum, which was not recovered separately.
Reserves, as stated in 1978 (mine staff pers. comm.) were:
Proven reserves - ~20 Mt @ 0.13% Sn
Proven + probable + possible reserves - ~120 Mt @ 0.13% Sn.
(Source: Porter GeoConsultancy, www.portergeo.com.au, 2015)
