Mesoproterozoic base metal sulphide deposits in the Namaqua Sector of the Namaqua-Natal Metamorphic Province, South Africa: a review

Abstract

The Mesoproterozoic Namaqua Sector of the east-west trending Namaqua-Natal Metamorphic Province (NNMP) located in the Northern Cape Province is underlain by supracrustal rocks and abundant felsic intrusive rocks that have been subjected to medium to high grade metamorphism and polyphase deformation. It is the host to world class base metal sulphide resources that have been mined for more than a century. Most of these mines are no longer in production such that the Cu-Pb-Zn-Ag mines in the Aggeneys-Gamsberg Ore District (A-GOD), with a total output of 1.8 million ton (Mt) per annum, are the only current significant producers. Development of the supergiant Gamsberg zinc deposit commenced in 2015 and will add a further 4 Mt per annum (250 000 t of zinc) to the output with the mine eventually becoming one of the larger zinc producers in the world. The Namaqua Sector of the NNMP has been subdivided into several lithologically distinct, structural discontinuity bounded Subprovinces and Terranes, each with its characteristic base metal sulphide deposits. The most extensive and best mineralized of these is the Bushmanland Subprovince where the ~1300 Ma stratabound polymetallic Broken Hill-type meta-SEDEX deposits contain total resources exceeding 500 Mt of Zn-Pb-Cu-Ag ore. Further east much smaller and subeconomic stratabound deposits occur at Putsberg (Cu-Zn-Pb-Ag) and Adjoining Geelvloer (Zn-Cu-Pb-Ag-Au). Magmatic ore deposits include minor magnetite-copper skarn/iron-oxide-copper-gold (IOCG) deposits associated with syntectonic granitoids (~1200 Ma) in the east and small syntectonic gabbronorite hosted Ni-Cu-(Co) occurrences at Hondekloof (~1170 Ma) to the southwest. The mined-out Okiep Copper District in the west hosts an abundance of small, syn- to late-tectonic mafic cupriferous intrusions (~1030 Ma), which collectively contributed some 100 Mt of ore yielding ~2 Mt of blister copper. The Areachap Terrane in the far eastern part of the Namaqua sector of the NNMP has a number of stratabound volcanogenic massive sulphide (VMS) Zn-Cu (Ag-Au) deposits of variable tonnage and hosted by intermediate to mafic meta-volcanic and meta-sedimentary rocks (~1300 Ma). The largest of these, the world class Prieska deposit (47 Mt) has been mined out. The remainder of the deposits are small (1.5 to 9 Mt), of low grade and uneconomic. The Terrane has been intruded by a suite of poorly mineralized mafic to ultramafic rocks (~1180 Ma) of which the high tonnage (114 Mt) but low grade Jacomynspan Ni-Cu deposit, is the most significant. The Richtersveld and Gordonia Subprovinces are poorly mineralized whereas the Kaaien Terrane has no Mesoproterozoic mineralization of any significance. The Gordonia Subprovince hosts the small magmatic Oup Ni-Cu-(Co) deposit (~1150 Ma) and is the only massive sulphide occurrence in the Subprovince. Both domains however, have minerals deposits associated with the regionally extensive post-tectonic Namaqualand pegmatite belt (~1000 Ma) which is a resource of beryl, tantalite, wolframite, cassiterite, micas, and feldspar. The Mesoproterozoic stratabound deposits of the Namaqua Sector of the NNMP are all of similar age and terrane bounded. Their different characteristics are the result of relative position within a subduction related plate tectonic setting. The siliciclastic hosted deposits of the Aggeneys-Gamsberg Ore District are interpreted to have formed in a continental sub-basin whereas the more chemogenic-volcanogenic hosts of the Putsberg and Adjoining Geelvloer deposits have the lithological characteristics associated with a back arc basin. The VMS deposits of the Areachap Terrane are linked to a mature island arc and were located more proximal to the subduction zone. The younger magmatic, mainly Ni-Cu and Cu deposits are syn- to post-tectonic, formed over a timespan of ~150 Ma, do not appear to be terrane specific and are considered to have been generated from basaltic magma during partial melting of subcontinental lithospheric mantle. Localization of intrusive suites can be the result of crustal thinning or proximity to major zones of dislocation such as the Pofadder Shear Zone and Boven Rugzeer Shear zone. The exploration success within the Namaqua Sector of the NNMP has been significant and effectively all known deposits, apart from those of the Okiep Copper District, were discovered using basic airborne and ground geophysical methods, soil geochemistry, lithological mapping and limited remote sensing during the 1970’s and early 1980’s. Future exploration should focus on the use of advanced airborne, ground and down-the-hole geophysics supported by remote sensing, soil and lithogeochemistry, the use of heavy mineral tracers and a workable geological ore deposit model. Over the last 35 years, the area has shown a decrease in the number of operating mines and commodity production, in particular copper. Although several areas show exploration potential, the Aggeneys-Gamsberg Ore District offers the best brown fields target whereas initiation of mining of the known smaller deposits is dependent on the revival of the currently depressed commodity demand and price.