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Rooiberg |
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Limpopo (Northern) Province, South Africa |
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Sn
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Super Porphyry Cu and Au
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The Rooiberg tin mines were located ~50 km WNW of Bela Bela (previously Warmbad) in Waterberg District Municipality of the Limpopo province, South Africa.
The Rooiberg Minerals Development Company Limited (in 1978, a subsidiary of Gold Fields of South Africa Ltd.) operated two small and two larger mines in the Rooiberg-Leeuwpoort area when visited in 1877. The larger operations were the Rooiberg or 'A Mine' and the Leeuwpoort or 'C Mine'. Small productions were derived from the Vellafontein and Nieuwpoort (or 'B' Mine') deposits. These four mines are distributed over an area of 20 x 10 km. Rooiberg Minerals produced 2298 tonnes of tin metal in 1977. All but a small percentage of this production is as 'dirty concentrate' which was sent abroad for smelting by Berzelius in Germany and Capper Pass in the UK. Production had been almost continuous from one or more of these mines since 1905. Ancient workings dating back to 400 AD are also known at Rooiberg. The Rooiberg mine closed in 1993 due to declining tin prices rendering further operations uneconomic.
Geological Setting
The tin mineralisation of the Rooiberg district lies within a block of Pretoria Group sedimentary rocks, overlain by Rooiberg felsite, and surrounded by Bushveld Granites, and is interpreted to represent a roof pendant. The sedimentary and volcanic rocks of this block are folded into an arcuate NW to SW trending syncline with a superimposed NE aligned anticline near its centre. The stratigraphy at these sediments is as follows from the top of the sequence.
Transvaal Supergroup
Pretoria Group
• Leeuwpoort Formation
- Boshoffsberg Quartzite Member - This is the basal member of the Leeuwpoort Formation and comprises up to 1400 m of feldspathic quartzite termed an arkosite. The arkosite contains around 50% detrital pink feldspar and has red layers due to the presence of iron oxide, some of which is due to the oxidating of sparsely contained pyrite, while green to grey poorly chloritic layers are also interbedded within the sequence. Tourmaline is common as fine laminae, pods and specks throughout the sequence, particularly in the north. Conglomerates and inpersistant pebble beds and grits occur at irregular intervals as a minor constituent throughout the member. These have pebbles of chert and quartz. The arkosite displays widespread cross bedding while slumping scour and fill and ball and pillow structures are common particularly towards the top of the sequence. No ripple marks are obvious. In the mineralised area, thin discontinuous khaki coloured silt layers are observed. This colouration is due to the presence of chlorite and sericite. Deposition is believed to have been under shallow water conditions.
- Blaauwbank Shale Member - 300 m thick - Three facies are recognised. "The upper-most facies" is a thinly interbedded siltstone, mudstone and fine grained sandstone unit, with abundant cross bedding. Thin sandstone beds 5 to 10 cm thick are rhythmically interbedded with finely laminated grey to purple fissile micaceous siltstone and mudstone. Ripple-marks, minor slumping, mud pellet layers and rare mud cracks are observable.
The "middle facies" comprises a rhythmic alternation of sandstone with siltstone and shale. Bedding is lenticular and cross bedding, ball and pillow structures, ripple marks and layers of mud-cracks are common. Beds are usually from 2 to 50 cm thick.
The “lowest facies” are very similar to the underlying Boshoffsberg Quartzite and comprises a fine to medium grained feldspathic quartzite and sandstone with little or no interbedded siltstone. Cross bedding is common with scour and fill and slump structures. They are usually finely to thinly bedded.
• Smelterskop Formation, 280 m thick - four to five lenticular feldspathic quartzite and tuffaceous shale beds interbedded with thick andesitic lava flows. The quartzites are, in general, massive with crossbedding being evident, accompanied by local ripple marks and mud flake layers. Intercalated pebble beds and agglomerates are minor constituents. This unit disconformably overlies the Blaauwbank Shale Member and it taken to represent deposition on a coastal alluvial plain.
Rooiberg Felsite, 350 m thick - Massive dacitic to rhyolitic volcanics, capped by a pyroclastic unit.
The Pretoria Group sedimentary rocks are surrounded by Bushveld Granites, with granophyres developed to the west, parallel to the granite margins but cutting across the strike of the sediments. The area has been cut by extensive later faulting, the most significant being the South Parallel Fault which parallels strike and appears to localise some of the fracture mineralisation. This fault can be traced for some 21 km and passes through the Nieuwpoort Mine and heads towards Leeuwpoort.
Mine Geology
The ore mineralisation within the Rooiberg-Leeuwpoort area is localised within arkosites of the upper sections of the Boshoffsberg member and within fractures cutting these sediments. In the 'A Mine' the arkosite is a massive pink rock above the ore zone where it carries about 50% pink feldspar, the remainder being clear to grey quartz. The grain size is around 0.25 to 0.5 mm. The stratabound pocket and bedding plane mineralisation is contained within a 15 to 20 m thick interval which is overall parallel to bedding and dips at 10 to 15°. However, in detail, it dips at a few degrees more steeply than the bedding, and pinches and swells. This is known as the 'tin zone' and has a slightly greyer colouration suggesting a lower feldspar content. Occasional thin siltstone beds a few cms thick are found in this zone. The most characteristic component of the tin zone and the sequence below it is tourmaline. The 'tin zone' rocks carry from 10 to 15% tourmaline as fine laminae from 0.5 to 2 mm in thickness and occasionally up to 2 or 3 cms. In other areas, the tourmaline content is up to 40% of the rock and is present as a diffuse dissemination giving the rock a dark grey to black colouration.
Sections at the arkosite in the mine area are carbonatic, while some graphitic layers have been recorded. The sequence in the mine is cut by a series of fractures and minor faults usually only having a displacement of a few cms to a metre of less. At 'B Mine' a few faults have throws of up to 50 m. Other than the variation in tourmaline content and possibly feldspar the sequence is reasonably monotonous.
Mineralisation
Mineralisation within each of the Rooiberg Mines may be summarised as follows:
Rooiberg or 'A Mine' - Ore mineralisation at A mine occurs in two main modes. These are stratabound and fissure controlled accumulations, as follows:
• Stratabound Mineralisation - As outlined above, the stratabound mineralisation at the ‘A Mine’ is principally confined to a 15 to 20 m thick portion of the Boshoffsberg Quartzite Member which is basically concordant with bedding. Within this horizon, known as the 'tin zone', mineralisation is distributed over an area of approximately some 2 km2.
Within this zone, apart from within the fissures which cut it, mineralisation is in two main forms, namely bedding plane and pocket accumulations. Individual bedding plane mineralisation pods are of the order of up to 2.5 m in thickness and 20 x 30 m in lateral dimensions. These zones carry up to 1 or 2% Sn as very fine disseminations within laminated tourmaline rich (up to 30%) arkosites. These zones usually have 1 to 2% pyrite as disseminated euhedra of 1 mm grain size. Within these zones conformable bands from 10 to 20 cm thick and 10 m long carry 10% pyrite and 30 to 40% tourmaline. These bedding plane pods apparently just fade out laterally into tourmaline rich arkosites of similar appearance which we were told were barren. However as Rooiberg's analytical technique has a lower tin detection limit of 0.1% Sn, significant values could persist between pods.
The pockets may occur within the bedding plane pods, but are also dispersed in zones through other parts of the ‘tin zone’. They occur as ovoid to rod shaped bodies ranging from a few cms in diameter to the largest encountered which was 2 to 3 m wide, 2 m thick and 80 m long developed parallel to bedding. These pockets usually have abrupt outer margins and often weather out at the surface after the surrounding arkosite has been decomposed. The main constituents of these pockets are tourmaline and pyrite with variable amounts of carbonates distributed concentrically within the pocket. In the examples sighted the outer margin of the pocket comprised dense black tourmaline over an interval of from 1 to 30 cm. Inside this a pyrite-tourmaline mixture was obvious with considerable cassiterite. The pyrite is present as disseminations to massive aggregates of from 5 to 50% pyrite euhedra from 1 to 3 mm in crystal size. The cassiterite occurs as 0.25 to l mm grains. Tin grades within the pockets are usually from 10 to 25% Sn, locally up to 70% Sn. In some faces inspected, these pockets comprise up to 5 or 10% of the exposures within otherwise barren tourmaline rich arkosite. Chlorite, silica and carbonates are usually found within the central core at these pockets, with occasional pyrrhotite, chalcopyrite, bismuthinite, sphalerite and galena, in association with the pyrite and cassiterite. The margins of the pockets are sharp and transgressive with respect to bedding. It is therefore likely that the pockets represent post sedimentation replacement in unconsolidated sediment by percolating mineralised media which is also responsible for the bedding plane mineralisation. Although the larger pockets are developed parallel to bedding, the smaller ones are irregular in shape and are randomly distributed with no preferred orientation.
Other pockets, similar in appearance but devoid of tin, are found over an interval of up to 300 m below the 'tin zone'
• Fissure Mineralisation - Fissures are developed at right angles to bedding and usually only represent filled cracks across which very little movement has taken place. These mineralised fissures range from 1 to 50 cm in thickness and carry pods up to 10 m long with up to 50% pyrite and 50% tourmaline with high grades (up to 10%) of tin as fine cassiterite. Elsewhere they contain up to 50% tourmaline with lesser chalcopyrite. These fissures are mineralised for up to 100 m vertically, centred on the stratabound mineralisation. Mineralisation within the fissure just fades out vertically. In some parts of the 'A Mine' fissures from 1 to 20 cm thick are found separated by from a few metres to a few tens of metres and can be bulked together when mining rather than by individual extraction. These densely spaced fissures individually have lateral dimensions of 10 to 20 m and are 20 to 30 m in vertical extent.
There are four zone within the 'A Mine' with lateral dimensions of up to 100 x 50 m where there is a sufficient density of fissure veins and pockets to allow bulk extraction over thicknesses in excess of 10 m. As a general ‘rule of thumb’ it is considered that the better size and grade fissures are found adjacent to better pocket and bedding plans developments, although good stratabound occurrences do not always have adjacent mineralised fissures. Close spaced drilling of portion of the surface outcrop of the tin zone has indicated to date three blocks which, when the veins, pockets, disseminations and bedding plane mineralisation is bulked give grades of 0.3% Sn in 2 to 300 000 tonne blocks amenable to open cut mining. Copper levels are found to increase in the lower parts of the mine.
Leeuwpoort or 'C Mine' - At the 'C Mine', concordant mineralisation predominates. This mineralisation is found as bedding plane deposits and concordant vein-like accumulations of banded coarse cassiterite in a gangue of pyrite, ankerite and tourmaline, with lesser magnetite, hematite, pyrite and chalcopyrite. Conformable mineralisation is developed in two areas that are adjacent but vertically off set, each with dimensions of about 2000 x 800 m. Within these areas there are four or five stratigraphic levels over an interval of around 60 m that are mineralised and within each level a number of higher grade shoots. These shoots are up to 300 x 100 m in lateral dimension and are separated by up to 50 m of lower grade ore. These shoots are mineable over 1 m vertical intervals and comprise 10 to 15 cm thick developments of up to 3.5% Sn. These concordant mineralised bands are cut by mineralised fissures as in the ‘A Mine’. One set of larger fissure veins connects the vertically off-set concordant layers at right angles and may fill a fault which has displaced the two halves of the tin zone.
Nieuwpoort or 'B Mine' - Mineralisation is found within the South Parallel Fault and the steeply dipping Stewart Lodes. The best mineralisation is at the intersection of the two lodes.
Vellefontein Mine - Bedded mineralisation accumulations are cut by three separate non-parallel mineralised faults.
Blaauwbank Mine - Cassiterite is found disseminated through portions of the arkosic quartzite.
Mining at 'A Mine' takes the form of following the vertical mineralised fissures with 1 to 2 m wide overhand open stopes and mining outwards erratically into indicated zones of pocket or bedding plane mineralisation via random cross cuts. The concordant lodes at Leeuwpoort are mined by low, scraper-open stopes. A small open cut operation was working portion of the outcrop of the tin zone at the 'A Mine' in 1978. Ore was treated by crushing, heavy media separation, gravity tables and flotation of sulphides.
Pocket and fissure fill mineralisation is very difficult to outline by drilling. An area containing 10 to 15% pockets each of 5 to 30 cm in diameter and each having about 25% Sn (ie. total grade 2.5% Sn +) over a 5 m thicknesses was not detected by diamond holes which missed virtually all of the pockets. Similarly the fissure mineralisation because of its irregular nature is hard to evaluate. Fissures, mineralised or not, are located by drilling and followed up by underground development. If mineralised, they are stoped and pockets searched for adjacent to the fissure by blast hole drilling. The ‘open cuttable’ low grade bulk reserves were out lined with a 10 x 10 m drilling grid centred on the outcrop of the 'tin zone'.
Mining statistics for 1977 were:
Total mined - 0.235 Mt from 'A mIne'; 0.277 Mt from 'B Mine' for a total of 0.512 Mt of ore.
Total treated - 0.2036 Mt from 'A mIne'; 0.2297 Mt from 'B Mine' for a total of 04333 Mt of ore.
Head grade - 0.67% Sn from 'A Mine'; 0.70% Sn from 'B Mine' for an average of 0.68% Sn.
HMS plant extraction - 92.1%.
Gravity plant extraction - 76.5%.
Tin from gravity - 2088.8 tonnes.
Tin from flotation - 205.5 tonnes.
Total contained tin produced - 2294.3 tonnes.
The most recent source geological information used to prepare this decription was dated: 1978.
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.
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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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