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Zaaiplaats (Potgietersrus) Tin Field |
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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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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Zaaiplaats tin deposits are located ~27 km NW of Mokopane, in the Limpopo Province of South Africa (#Location: 24° 2' 47"S, 28° 46' 18"E).
In 1978, when visited, Zaaiplaats Tin Mining Co Ltd operated an underground and a small open cut mine with an attached smelter which treats the Zaaiplaats output, plus additional concentrates purchased from Rooiberg, Kamativi, Swaziland and local prospectors. Mining commenced at Zaaiplaats in 1908 and had continued un-interrupted until closure in 1989.
Geological Setting
The Zaaiplaats deposit is located within alkali feldspar granites of the Lebowa Granite Suite on the eastern margin of the Northern Lobe of the Bushveld Complex. In this region the granite is separated from the underlying layered Rustenburg Layered Suite by a sliver of Pretoria Group sedimentary rocks and is capped by Rooiberg Felsite. The layered basics, granites and felsites are all transgressively overlain by Waterberg Supergroup sedimentary rocks. A pronounced scarp has been formed along the eastern margin of the granite, with Quaternary coarse alluvial gravels and lacustrine deposits having been formed over the adjacent Pretoria Group sediments and basic complex.
For detail on the regional setting and geological map see the Bushveld Complex overview record.
Within the Zaaiplaats area the Bushveld Granites can be subdivided as follows:
Main Granite Suite - This comprises the earliest granitic body, comprising from the base upward of:
• Granulite - which is found in close contact with the Rustenburg Layered Suite and is a coarse grained granoblastic rock with quartz, perthite, anti-perthite and sodic plagioclase. Mafic minerals, mainly biotite, hornblende, magnetite, ilmenite, sphene and chlorite are segregated into nests.
• Granophyric granulite - a leucocratic, medium grained, non porphyritic rock, made up of quartz, perthite and anti-perthite with biotite and hornblende. This is found below the porphyritic granophyre.
• Porphyritic granophyre - this rock has phenocrysts of quartz and feldspar (as above) from 0.1 to 5 mm across, set in a quartzo-feldspathic matrix with segregations of dark minerals, mainly magnetite, ilmenite, chlorite, biotite, hornblende and sphene. This occurs below the ‘porphyry’.
• Porphyry - a fine porphyritic granite with 0.1 to 3 mm quartz and feldspar (perthite, minor plagioclase) phenocrysts in a fine quartzo-feldspathic matrix with variable amounts of calcite, chlorite and sericite. It is found at the top of the complex.
Foothills Granite - which occurs as irregular sheets, dykes and vein stockworks cross-cutting the Main Granite, the Pretoria Group metasediments and the Rustenburg Layered Suite. It is essentially a fine grained porphyritic granite with varying amounts of quartz and feldspar phenocrysts set in a fine grained quartzo-feldspathic matrix.
Bobbejaankop Granite Stage - which is the youngest phase in the area and is the host to the mineralisation at Zaaiplaats. It can be subdivided as follows:
• Main Bobbejaankop Granite, a medium-grained, pink to red granite with a grain size at around 3 mm. It is equigranular with quartz and red feldspar, and patches of chloritic aggregates up to 3 mm across,
• Lease Granite - which is almost identical to the adjacent Main Bobbejaankop granite, the only difference being in its 1 mm grain size. The contact between the two is not intrusive. It comprises a rapid, but gradational, grain size change over a 1 to 2 cm interval. It occurs as a number of thin sheets and dykes within the Bobbejaankop Granite, although it is mainly present as a sheet like margin, some 50 to 200 m thick, on the outer rim of the Bobbejaankop Granite, at the contact with the Main Granite Suite granophyres.
• Pegmatite, found predominantly within the Lease Granite and has a similar composition. They comprise 1 to 5 cm wide quartz and red feldspar crystals. These pegmatites range from a few cms up to 30 m in thickness. Within the Lease Granite, near the pegmatite bodies, segregations up to 30 cm across of similar coarse crystals are found erratically distributed. The density of these segregations increase as the pegmatites are approached.
Mineralisation
Ore mineralisation at Zaaiplaats occurs in three main forms namely:
• Disseminated sheet like orebodies - The main disseminated orebody at Zaaiplaats occurs as a series of 5 parallel, semi connected shoots occupying around 70% of a zone of some 750 x 650 m. Cassiterite, commonly associated with scheelite and sulphides, is interstitial to the major silicate minerals in the disseminated ores. The ores are confined to a favourable zone that can be defined on geochemical-statistical grounds as a 55 to 66 m thick tabular body of varying thickness. These tabular bodies range from 50 to 100 m in width, with the largest being 150 m. In general they have a down dip extent of 400 m with the largest being 750 m. In the central zone the individual shoots attain within the favourable zone have thicknesses of 15 m with a single maximum of 30 m. Down dip they decrease in thickness and width. This main zone of mineralisation is within the Bobbejaankop Granite, although at Roodepoort and Groenfontein 1.5 and 3 km along trend respectively, similar mineralisation is found within the Lease Granite. The sheet like disseminated bodies are developed parallel to the contact of the Bobbejaankop and Lease Granites but some 66 to 83 m below, the pluton roof. Cassiterite occurs as a primary constituent of the granite with a grain size of less than l mm. It is not accompanied by any form of alteration. Grades within the extracted disseminated zone range from 0.15 to 0.4% Sn, averaging nearer 0.25% Sn.
• Cylindrical bodies - This style of mineralisation is very high grade and takes the form of long, very irregular, erratic, cylindrical bodies which change direction at random. They range in diameter from 25 cm to 15 m and have lengths from 10 m up to the largest encountered which continued for 750 m. These cylindrical bodies are found within and in close proximity to the sheet like orebodies. They exhibit intense alteration comprising an outer diffuse surrounding zone of several metres of kaolinisation, enclosing a narrower zone, a few cms thick of silicification. Inside of this zone there is a marked ring of dark tourmalinisation from 1 cm to 1.5 m thick. The core is occupied by a zone of intense sericite and/or chlorite alteration accompanied by disseminated cassiterite crystals and pyrite (1%) up to l mm in diameter. The central core often carries from 10 to 30% Sn with, in some cases, up to 70% cassiterite. The outer diffuse kaolinised zone is not common.
• Lens shaped bodies - This type of body predominates in the Lease Granite and is usually found closely below pegmatite developments. It is similar to the cylindrical bodies except that tourmaline is absent, as is any circular banded zonal appearance. The host granite within the lenses is sericitised with disseminated cassiterite, while minor pyrite, chalcopyrite and scheelite are found on the lens margins. These lenses are usually only a metre or so thick and several metres to a few tens of metres in width. Down plunge they persist for up to 200 to 250 m, but are usually less extensive. Grades similar to, but usually lower than, those of the cylindrical bodies are obtained.
Geochemical halos around the ore-bearing zone are absent or poorly developed and, except for the anomalous tin contents in this zone, there are few obvious differences, other than the degree of alteration, between mineralised and barren Bobbejaankop Granite. The granitic rocks at Zaaiplaats are more heavily altered than equivalent granites elsewhere in the Bushveld Complex. Granite in the disseminated tin zone is more altered than barren Bobbejaankop Granite. A sequential alteration paragenesis can be recognized, beginning with high-temperature feldspathic (albite and microcline) alteration, followed by chlorite, sericite and lastly, silicification. Compared with Bobbejaankop variants in the southeastern Bushveld Complex, the entire Zaaiplaats suite is enriched in K, Ca, Rb, Cu and Sn, and depleted in Na. Within the mine area, granite from the mineralized zone contains higher Na, Sn, W, As, S and F, but lower K and Rb contents than the barren granite. These differences reflect increasing degrees of fluid-controlled precipitation and alteration (Coetzee and Twist, 1989).
Alluvial Deposits - The main stream cutting the Zaaiplaats-Bobbejaankop zone carries 'high grade' alluvial tin accumulation in its lower sections. These have been mined in the past, but are currently below the tailing disposal. It is not known how far downstream these alluvials have been tested.
The background levels of tin in the various granites have been quoted by a number of authors. There is a great variation, depending largely on the individual authors sampling bias. In general the Main Granite appears to carry from 3 to 10 ppm Sn, while the Bobbejaankop Granite in general has from 20 to 30 ppm, with 40 to 90 ppm and higher in mineralised areas. The Lease Granite carries 5 to 15 ppm with 30 to 50 ppm in the mine area.
Mining and Production
Mining within the disseminated sheet like orebodies was by open, flat lying scraper stopes. These had very irregular outlines following payable ore. During mining, the sludge from each blast hole is collected and panned. If insufficient tin is indicated it is not charged and that section is left. After each blast the ore brought down is sampled by the shift boss to determine which sections should be worked for the shift to get the correct blend and grade. The sub-grade faces are grid drilled with horizontal 2 to 4 m holes on a 3 x 2 m pattern to delineate zones that may become payable with price fluctuations.
The cylindrical bodies and lenses, once located, were followed down plunge, resulting in irregular snake like tunnels with the same diameter as the pipe. These pipes, because of their dimensions, are hard to locate with drilling and apart from those found outcropping, which alone accounted for the first 40 years of production, they are only found during the extraction of the disseminated sheet like bodies. The cylindrical bodies and lenses acted as sweeteners raising the overall grade of the mine to a profitable level. No reserves are calculated. Drilling has indicated the dimensions of the disseminated zone and it is assumed to have a similar grade throughout. A small quarry operation is being carried out on the outcrop of the disseminated zone.
The ore was crushed, screened and gravity separated, with flotation to remove remove sulphides, before passing over final cleaning tables for dressing. This product was then smelted onsite to produce 99.9% Sn with 0.03% Pb and 0.01% Cu.
Statistics for the year ended July 31 1977 were:
Ore treated - 73 458 tonnes,
Head grade - 0.4% Sn,
Recovered grade - 0.285% Sn,
Production of 66.3% Sn concentrates - 319.13 tonnes,
Tin metal produced - 211.58 tonnes.
Over 100 000 tonnes of tin were produced from hard-rock mining at the Zaaiplaats deposit between 1906 and 1989.
The most recent source geological information used to prepare this decription was dated: 1990.
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.
Zaaiplaats
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Coetzee J, Twist D 1989 - Disseminated Tin mineralization in the roof of the Bushveld granite pluton at the Zaaiplaats mine, with implications for the genesis of magmatic hydrothermal Tin systems: in Econ. Geol. v84 pp 1817-1834
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Crocker I T 1986 - The Zaaiplaats Tinfield, Potgietersrus district: in Anhaeusser C R, Maske S, (Eds.), 1986 Mineral Deposits of South Africa Geol. Soc. South Africa, Johannesburg v2 pp 1287-1299
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McNaughton N J, Pollard P J, Groves D I, Taylor R G 1993 - A long-lived hydrothermal system in Bushveld Granites at the Zaaiplaats Tin mine: Lead isotope evidence: in Econ. Geol. v88 pp 27-43
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Pollard P J, Andrew A S, Taylor R G 1991 - Fluid inclusion and stable isotope evidence for interaction between granites and magmatic hydrothermal fluids during the formation of disseminated and pipe-style mineralization at the Zaaiplaats Tin Mine: in Econ. Geol. v86 pp 121-141
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Pollard P J, Taylor R G, Taylor R P, Groves D I 1991 - Petrographic and geochemical evolution of pervasively altered Bushveld granites at the Zaaiplaats Tin mine: in Econ. Geol. v86 pp 1401-1433
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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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