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Prieska, Copperton

Northern Cape, South Africa

Main commodities: Cu Zn
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The Prieska Cu-Zn deposit at Copperton in the Northern Cape Province of South Africa, is located in the Northern Cape province of South Africa, ~190 km SSE of Upington, and 600 km NE of Cape Town (#Location: 29° 57' 3"S, 22° 17' 50"E).

It lies within the Kakamas Terrane of the Namaqua Mobile Belt, some 350 km to the east of Gamsberg-Aggeneys within probably equivalent, but more strongly metamorphosed rocks belonging to the Middle Proterozoic Areachap Group of the Korannaland Supergroup.

Geology

According to Theart, etal., (1989) the host sequence in the Copperton district is as follows, from the base:

 Smouspan Gneiss Member, 800m thick - which is considered to be the lowest member of the Smouspan Gneiss Member. It is a homogeneous, granoblastic, equigranular, fine grained (0.3mm) gneiss of intermediate composition, consisting of the following minerals in order of decreasing abundance; plagioclase (44%), quartz (28%), hornblende (9%) and biotite (8%), with or without orthoclase, sphene, magnetite-ilmenite, apatite and zircon. The gneiss shows very little variation in mineral mode and chemical composition and is homogeneous over a considerable thickness and strike distance. No primary features have survived metamorphism. On the basis of chemical composition and trace element ratios, Theart, etal., (1989) suggest this gneiss may have originally have been dacitic in composition.

 Prieska Copper Mines Member, 0 to approximately 120m thick - comprising a composite unit of per-aluminous silicate rocks which contains the main sulphide orebody at Prieska. Essentially it is a quartz-cordierite-sillimanite-andalusite-muscovite-biotite rock which is strongly foliated and very fine grained (Herriman, etal., 1977). It displays great compositional and mineralogical variation and includes some unusual rock types. Contacts between the individual component lithologies and the underlying Smouspan Gneiss are gradational, although in some drill holes a massive amphibolite separates these two members (Theart, etal., 1989).

Theart, etal. (1989) distinguish two major rock types within the member, which are characterised by their mineralogy and silica content. The first of these is what is known as the 'gedrite fels', a gedrite, anthophyllite, cummingtonite-grunerite, phlogopite rock with variable dravite, which contains, 50% silica. This lithotype is found within the upper Smouspan Gneiss Member as unevenly distributed patches of coarse grained phlogopite and gedrite, becoming more abundant as the main gedrite fels is approached within the Prieska Copper Mines Member. The gedrite fels is depleted in Si, Ca, Na, Sr and Zr, and relatively enriched in Al, Mg, Fe, Mn, K, Ti, V, Sc, Ni, Ba Cu and Zn, while heavy rare earth concentrations are also found in some samples.

The second main lithotype is a quartz-perthite-sillimanite gneiss that contains between 70 and 80% silica. The abnormally high Mg content of both the gedrite fels and the quartz-perthite-sillimanite gneiss distinguishes them.

The massive sulphide ore at Prieska consists mainly of the following, in decreasing order of abundance; pyrite, sphalerite, chalcopyrite, pyrrhotite and minor galena. A carbonate-sulphate zone occurs on the upper side of the sulphide body, comprising mainly calcite, dolomite, anhydrite and barite with associated Mn rich rhodonite bearing assemblages dominated by magnetite. Dravite bearing layers are found below the sulphide zone at the base of the unit just above the contact with the Smouspan Gneiss Member.

Theart, etal., (1989) consider this unit to represent an altered volcano-sedimentary unit.

 Vogelstruisbult Member, >2000m thick - comprises a variety of rock types, including banded hornblende gneiss of intermediate composition, laminated amphibolite and meta-pelites.

Layering in the banded hornblende bearing gneiss ranges from 1 cm to 25 m in thickness, comprising fine grained leucocratic layers of quartz and oligoclase with minor biotite, hornblende and magnetite. Near the top of the Prieska Copper Mines Member these felsic bands often carry layers of fibrous sillimanite, plates of garnet and unaltered cordierite. The dark layers are strongly laminated (except the dominantly hornblende layers) and comprise hornblende, andesine, quartz, biotite and magnetite. Whereas the Smouspan Gneiss has limited chemical variation, this gneiss is considerably variable, but also of intermediate composition. The main band of this lithotype follows the Prieska Copper Mines Member and is up to 500 m thick.

The laminated amphibolite layer forms a conspicuous unit, distinguished by its 5 to 10mm thick relatively continuous colour banding of darker plagioclase-hornblende layers, alternating with paler laminae commonly containing diopside, plagioclase and sphene, and less frequently grossular, calcite and scapolite. It has a high Ca content and is tholeiitic in composition. The grain size is generally <1mm and in places the banding grades to zones where boudinage is prevalent. The main development of this lithotype is higher in the sequence and is up to 1250m thick.

Pelitic gneisses of the Vogelstruisbult Member have a flaser fabric, with composite augen consisting of cordierite, garnet and biotite within a matrix of K-feldspar and quartz. The grain size varies from 3mm to several cm's. The best development of this lithotype is 50m thick between the banded hornblende bearing gneiss and the laminated amphibolite.

Theart, etal., (1989) consider the Vogelstruisbult Member to have originally represented immature sediments derived from a basaltic to intermediate volcanic source.

 Massive Amphibolites consisting primarily of hornblende and plagioclase with less than 55% silica and relatively high concentrations of Ca Mg and Fe. The major chemical composition of these rocks is similar to that of tholeiitic basalts. They are present in a form that suggests a sequence of dykes and sills that cut the sequence, after the alteration that affected the Smouspan Gneiss and Prieska Copper Mines Members.

Note that this succession is inverted when compared to that of Wagener and van Schalkwyk (1986), although it also relies on a different structural interpretation.

Locally within the immediate Prieska mine the Prieska Copper Mines Member comprises the following, from the base, after Wagener and van Schalkwyk (1986), inverted to be consistent with the Theart, etal., (1989) sequence above,

 Tourmaline zone, 0 to 5m thick - abundant black tourmaline, generally as randomly oriented laths up to 2cm long set in a matrix of ferro gedrite, phlogopite and quartz which are characteristic of this zone. It grades locally into the black hornblende gneiss normally found in the same stratigraphic position.

 Quartz zone, 10 to 100m thick - this zone is the most extensive and generally flanks the ore on both sides and for a considerable distance laterally and down dip. It is fine grained, strongly laminated with bands and stringers of pyrite, but only traces of Cu or Zn. Barely visible layers of galena contribute several hundred ppm Pb levels overall. The rock has a fine ribboned and flaser texture implying shear movement. The constituent minerals are mainly quartz with phlogopite, sillimanite, pyrite, cordierite and tourmaline with minor barite and galena. With increasing phlogopite the rock becomes gneissic with augen of sillimanite and cordierite. There are no mafic minerals or magnetite.

 Ore zone, 0.5 to 35m thick - the ore zone has sharp contacts with both the quartz zone and the banded gneiss that form its walls. The orebody was present as a single zoned tabular massive sulphide body with an elongate Cu rich core flanked on either lateral margin by Zn rich strips, but not on the up or down dip sides, or above or below. The zonation of the ore plunges at around 45° to the horizontal, parallel to the large number of shoots of different mineralogy which together make up the coherent tabular ore grade body. Each of these shoots has sharp or gradational boundaries with its neighbour. These lenses together however, define large consistent zones of higher and lower grade plunging parallel to the overall trend of the orebody. There is a tendency for calc-silicates, pyrrhotite (including massive pods), tremolite and diopside to predominate in the stratigraphic footwall. The ore is typically granular and not foliated. Pyrite (averaging 45%) is the predominant and earliest sulphide. Carbonates are the main non sulphide, while there is virtually no free quartz and a 'silica' content of <5%. Anhydrite and barite are apparent in thin section.

 Carbonate zone, 0 to 10m thick - patchy with persistent but irregular layering. The most common minerals are dolomite, calcite and serpentine with rare remnant forsterite, iron rich olivine and grossularite. A metre wide lens of scapolite with an NaCl component and diopside, with millimetric layers of anhydrite and calcite, has been recorded at one locality.

 Manganese rich magnetite zone, 0 to 10m thick - this has a limited distribution forming a central strip parallel to the general plunge of the orebody, on the contact of the banded gneiss and the orebody. It is composed of strongly variable proportions of magnetite, carbonates, spessartine and andradite, rhodonite (white and pink), tephroite, bustamite, diopside and green amphibole. Spessartine is usually associated with barite and magnetite, while andradite and rhodonite occur together. Any sulphides are present as discrete lenses or layers with sharp margins.

The deposit originally contained 47 Mt @ 1.7% Cu, 3.8% Zn

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1994.    
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.


Prieska

  References & Additional Information
   Selected References:
Cornell D H, Hawkesworth C J, Van Calsteren P and Scott W D,  1986 - Sm-Nd study of Precambrian crustal development in the Prieska-Copperton region, Cape Province : in    S. Afr. J. Geol.   v89 pp 17-28
Humphreys H C, van Bever Donker J M, Scott W D, and van Schalkwyk L,  1988 - The early deformational history of the eastern Namaqua Province; new evidence from Prieska copper mines : in    S. Afr. J. Geol.   v91 pp 174-183
Schade J, Cornell D H, Theart H F J  1989 - Rare earth element and isotopic evidence for the genesis of the Prieska massive Sulfide deposit, South Africa: in    Econ. Geol.   v84 pp 49-63
Theart H F J, Cornell D H, Schade J  1989 - Geochemistry and metamorphism of the Prieska Zn-Cu deposit, South Africa: in    Econ. Geol.   v84 pp 34-48
Wagener J H F, Van Schalkwyk L  1986 - The Prieska zinc-copper deposit, North-Western Cape Province: in Anhaeusser C R, Maske S, (Eds.), 1986 Mineral Deposits of South Africa Geol. Soc. South Africa, Johannesburg   v2 pp 1503-1527


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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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