Black Mountain, Swartberg
Northern Cape, South Africa
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Super Porphyry Cu and Au|
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The Black Mountain or Swartberg deposit is some 6 km west of the Broken Hill deposit within the Aggeneys District of the Northern Cape Province of the Republic of South Africa.
The deposit occurs on Swartberg (or Black Mountain), a hill characterised by a black colouration due to the magnetite (and to a lesser extent the manganese) content of the exposed mineralisation.
Black Mountain comprises two conformable 'orebodies', separated by 0 to >30 m of waste. Both 'orebodies' comprise large high-grade sulphide lenses, enveloped by lower-grade magnetite-rich rocks, which, in turn, are over- and underlain by weak schist and quartzite. The main sulphide minerals are galena, sphalerite, chalcopyrite and pyrrhotite.
The host sequence has been subjected to four phases of deformation, resulting in variable dips and thicknesses. A tight, isoclinal F2 fold nose has been developed in the vicinity of the orebody. Higher grade, coarser, mineralisation is present as an elongate body concentrated down the plunge of the F2 fold axis and occurs in garnet-quartzite, magnetite-quartzite and amphibole-magnetite rocks. In this zone ore occurs as heavily disseminated pyrite and ore sulphides (galena, sphalerite and chalcopyrite) in magnetite and amphibole rich quartzose metamorphics, with finer grained, banded sulphides on the limbs aligned with the foliation of the magnetite-quartzite and amphibole-magnetite hosts.
There is a zoning from a Cu bearing garnet-quartzite, to chalcopyrite-galena rich magnetite-quartzite to an upper galena-sphalerite dominant amphibole-magnetite in the core of the fold hinge,
The regional geology is as described in the Aggeneys (Broken Hill) record.
At Black Mountain, the host Aggeneys Ore Formation overlies a sequence of variable interbanded schists, quartz schists and quartzites. The schists are predominantly muscovite, biotite and sillimanite bearing, with quartz in varying amounts, becoming dominant in the quartz schist layers. Garnet is present in some of the quartzites. It is possible that, as at Broken Hill the underlying schists and quartzites are separated from the Aggeneys Ore Formation by a folded thrust/shear which would also account for the marked transgressive nature of the contact.
These underlying schists and quartzites in turn overlie coarse pink leucocratic gneisses with a granitic texture, and amphibolites.
Within the Black Mountain deposit the Aggeneys Ore Formation comprises the following generalised sequence, from the interpreted stratigraphic base (after Black Mountain Mineral Development Co., 1988, and Ryan, et al., 1986):
► Lower Orebody, approximately 5m thick - this zone of mineralisation is only present on the lower limb of the main F2 fold which hosts the main zone of mineralisation at Black Mountain, occurring immediately above the postulated thrust/shear separating it from the underlying barren schists and quartzites.
This unit is predominantly a baritic schist with disseminated magnetite and sulphide minerals, of which pyrite is dominant with subordinate galena and sphalerite, and traces of chalcopyrite. In the vicinity of the F2 fold closure the Lower Orebody grades laterally into an amphibole-magnetite quartzite and garnet quartzite also. The garnet quartzite is identical to that described below, although the amphibole magnetite rock differs from that in the Upper Orebody in that it consists of dominant grunerite and subordinate spessartine, with magnetite being rarer.
The Lower Orebody may either be a separate unit, or part of the Upper Orebody which has been thrust into its present position. It is similar in composition to the barite-magnetite schist which is found near the Upper Orebody position (see below).
► Mixed zone of Schists and Garnet Quartzite, 30 to 65m thick - this interval includes a range of lithologies forming a progression from garnet-quartzite to garnet-quartz schist to banded schist with increasing distance from the nose of the F2 fold where the main high grade mineralisation is concentrated. The main rock types are as follows, not necessarily described in the order in which they appear.
• Garnet-Quartzite - composed predominantly of fine to medium grained quartz, generally around 80%, with subordinate almandine garnet (Fe rich) and biotite, and accessory cordierite (Mg, Al) and sillimanite. Magnetite increases towards the margins with the magnetite quartzite. The rock is generally massive with indistinct banding in the core of the unit away from the gradational contacts with the schists. The garnet quartzite is thickest, and is concentrated in the vicinity of the high grade mineralisation in the nose zone of the F2 fold. Towards the NW, up dip along the axial plane of the F2 fold, there is a gradual decrease in the ferro-magnesian minerals and it grades into a glassy quartzite (see plan no. AFRa azk). The adjacent magnetite quartzite, a lithotype of comparable competence, displays far less thickening in the same structural position.
In outcrop the garnet quartzite appears as a foliated pink and black medium grained quartzite with a black surface staining/varnish.
• Garnet-Quartz Schist - which has a composition intermediate between the garnet quartzite and the banded quartz schist. The K-feldspar, sillimanite and muscovite contents are higher than in the garnet quartzite, although it contains 40 to 70% quartz and garnet. Traces of pyrite are disseminated throughout the rock, particularly near the gradation to garnet quartzite, where sporadic minor chalcopyrite concentrations are also present.
• Banded Quartz Schist - is characterised by the alternation of quartzose and schistose material, the former being similar to the white quartzite, while the latter is an aluminous schist.
► Barite-Magnetite Quartzite and Barite-Quartz Schist, 0 to 25m thick - these two lithologies are separate and grade laterally into one another.
• Barite-Magnetite Quartzite - has from 25 to 70% barite accompanied by quartz and around 10% magnetite with accessory fine grained orange garnet and micas. Where it approaches the lateral extremities of the ore zone it is coarse grained and exhibits a moderate compositional banding. The sulphide content is negligible.
• Barite-Quartz Schist - this lithotype is similar to the Lower Orebody host. It appears to represent a gradational change from the barite-magnetite rock, with a decrease in barite and magnetite relative to quartz, while muscovite, biotite and chlorite increase until the rock has a schistose texture.
► Magnetite-Quartzite, 0 to 20m thick - composed dominantly of quartz and magnetite which together make up 80 to 100% of the rock, with accessory garnet (almandine-spessartine), biotite, chlorite and rare apatite. Towards the extremities of the magnetite-quartzite on the limbs of the F2 fold magnetite decreases relative to quartz. On the main sections of the two limbs it displays a millimetric compositional banding, which is obliterated in the nose zone by rotation.
► Amphibole-Magnetite Rock, 0 to 25m thick - this unit, which is also concentrated in the nose of the F2 fold, tapering and lensing out on the limbs, comprises magnetite with cummingtonite/grunerite, accompanied by, in decreasing order of abundance, pyroxmangite, quartz, hedenbergite, garnet (spessartine rich), fayalite and apatite.
Laterally in the lower fold limb this has been interpreted as grading into the barite-magnetite quartzite described above.
► Leptite and Amphibolites, 1 to 30m thick - are found discordantly cutting the Upper Orebody zone. The leptite is a medium grained quartzo-feldspathic rock composed predominantly of quartz and microcline with subordinate plagioclase and muscovite. The amphibolite, which almost invariably accompanies or borders the leptite, is predominantly hornblende.
The regional structural features and deformation stages are as described at Aggeneys/Broken Hill deposit.
Within the mineralised zone there is a strong foliation that has been folded by both F2 and F3 structures. This foliation is represented by both the schistosity and by compositional banding within the garnet quartzites, magnetite quartzites and magnetite amphibole rocks. It is uncertain whether this foliation represents the original bedding or is a regional F1 metamorphic/structural feature.
The isoclinal F2 folding is best displayed by the main tight fold whose nose is occupied and intimately followed by the mineralisation. This fold is mappable at the surface, with abundant clear parasitic folds in the folded magnetite-quartzite whose vergence supports the interpreted fold closure. Strong L2 mullions are also present within the amphibole-magnetite in the nose of the fold. This fold is corroborated by the drill intersections.
Mineralisation ocurs in two positions, as preserved at present. These are the,
• Lower Orebody - which is found at the interpreted base of the Aggeneys Ore Formation, possibly following a thrust contact with the underlying schists and quartzites. It is predominantly a baritic schist with disseminated magnetite and sulphide minerals, of which pyrite is dominant, with lesser galena and sphalerite, and traces of chalcopyrite. Towards the nose of the F2 fold it grades into amphibole-magnetite rock and garnet quartzite. All three lithologies are mineralised.
The garnet quartzite envelopes the magnetite rich rocks in the hinge zone. The dominant sulphide is pyrite, followed by chalcopyrite with accessory galena, pyrrhotite and sphalerite.
The amphibole magnetite rock of the Lower Orebody differs from that of the Upper Orebody in that it has dominant grunerite, subordinate spessartine and rare magnetite. The main sulphide minerals are sphalerite and chalcopyrite, in contrast to the dominant galena in the Upper Orebody.
Down dip to the southeast the amphibole magnetite rock grades rapidly into the baritic, sulphidic schist over a lateral distance of a metre or two. This latter rock type often becomes a massive sulphide. Pyrite is dominant, with subordinate galena and sphalerite, and traces of chalcopyrite. Barite is irregularly distributed, reaching 40% in places, while gahnite is a common gangue silicate, together with quartz, white mica and hematite.
• Upper Orebody - Mineralisation is hosted by the garnet quartzite, magnetite quartzite and amphibole magnetite rock. Galena and sphalerite are contained throughout the amphibole magnetite rock and the magnetite quartzite, mainly as low grade disseminations, but also within the high grade core.
Within the amphibole magnetite rock the principal sulphide is pyrrhotite, followed by pyrite, while galena and sphalerite are the most abundant base metal sulphides. Chalcopyrite is an accessory, with minor amounts of arsenopyrite.
Within the magnetite quartzite the dominant sulphide present is pyrite, accompanied by subordinate pyrrhotite. Galena is the main base metal sulphide, followed by chalcopyrite and accessory sphalerite. The greatest concentrations lie adjacent to the high grade mineralisation within the garnet quartzite.
The dominant sulphide within the garnet quartzite is generally chalcopyrite which is disseminated, and not ubiquitously or regularly distributed through the host rock. Although the sulphide content is variable, there is a general increase in sulphide towards the core of the F2 fold closure where concentrations approaching massive sulphides are found locally, with pyrite being the dominant sulphide and subordinate chalcopyrite. Away from the high grade core the sulphide minerals decrease in all directions, with pyrite persisting beyond chalcopyrite. Minor sphalerite, galena and pyrrhotite are encountered near the contact with the magnetite quartzite.
Massive sulphides are virtually unknown at Black Mountain, with predominantly disseminated pyrite and ore sulphides, within magnetite and amphibole rich quartzose metamorphics. Much of the ore is finely banded on the limbs of the F2 ore fold, with sulphides often present as small grains aligned along the micro banding of the magnetite quartzite and amphibole magnetite rock. In the vicinity of the hinge zone however the ore is recrystallised to form coarse aggregates.
The high grade rod like core of mineralisation follows the plunge of the F2 antiform in the fold hinge. This core has been traced for some 1300 m down plunge, within all three lithologies. In detail this mineralisation displays asymmetry with respect to the F2 hinge trace, corresponding to the maximum development of the garnet quartzite.
There is a vertical 'stratigraphic' zonation of metals, or alternatively a zonation from the outer to the core sections of the fold hinge. The stratigraphically lower garnet quartzite on the outer sections of the hinge is Cu rich, followed by the galena-chalcopyrite in the magnetite quartzite and galena-sphalerite in the upper amphibole magnetite rock in the core of the fold.
There appears to be a strong relationship between the distribution of sulphides, ore, magnetite and garnet and the presence of a tight isoclinal F2 fold within a major shear/thrust zone. In detail there is a high grade core of mineralisation forming a rod like body that is 100 to 200 m long, closely following the F2 fold hinge down plunge for more than 1300 m. Within this core there appears to be a lithological control on the sulphide zonation. Away from the high grade core, lower grade finely banded to disseminated sulphides follow the magnetite and amphibole bearing lithotypes, but like those rock types, pinch out on the limbs. On a broader scale, the magnetite rich rocks also lens out on the limbs of the F2 structure over a distance of a km or so. The stratigraphic equivalents of the ore bearing lithotypes do contain weak mineralisation and occasional more intense developments sporadically between Black Mountain and Broken Hill.
Reserves, Resources and Production
The deposit has been developed and ore is extracted via a shaft to truck to the Black Mountain Mining treatment facility at Broken Hill, 6 km to the east. Mining capacity has been ~350 000 tpa of ore to produce 13500 tpa of metal in concentrate. Plans are underway (2018) to deepen the shaft and increase capacity to 1.6 mtpa of copper and lead ore, and 60 ktpa - 70 ktpa of metal in concentrate (Vedanta Resources website viewed Jan, 2019).
The deposit had an original geological resource of 82 Mt @ 0.75% Cu, 2.7% Pb, 0.6% Zn, 30 g/t Ag (a cut-off of 0.5% Cu equiv.; Ryan, et al., 1986).
Remaining Ore Reserve and Mineral Resources at the end of 2018 (Vedanta Resources Annual Report, 2018) were:
Proved + Probable Reserve - 2.33 Mt @ 0.62% Zn, 3.26% Pb;
Measured + Indicated Resource - 35.68 Mt @ 0.84% Zn, 3.70% Pb;
Inferred Resource - 26.49 Mt @ 2.19% Zn, 3.04% Pb.
NOTE: Reserves are additional to resources.
The most recent source geological information used to prepare this summary was dated: 2001.
This description is a summary from published sources, the chief of which are listed below.
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Cawood, T.K. and Rozendaal, A., 2020 - A Multistage Genetic Model for the Metamorphosed Mesoproterozoic Swartberg Base Metal Deposit, Aggeneys-Gamsberg Ore District, South Africa: in Econ. Geol. v.115, pp. 1021-1054|
Colliston W P, Praekelt H E, Schoch A E 1991 - A Progressive Ductile Shear Model for the Proterozoic Aggeneys Terrane, Namaqua Mobile Belt, South Africa: in Precambrian Research v49 pp 205-215|
Cornell D H, Pettersson A, Whitehouse M J and Schersten A, 2009 - A New Chronostratigraphic Paradigm for the Age and Tectonic History of the Mesoproterozoic Bushmanland Ore District, South Africa: in Econ. Geol. v104 pp 385-404|
de Bruiyn H, van de Westhuizen W A, Beukes G J 1994 - Secondary Zn-Pb-Cu Minerals, an Aid to Mineral Exploration in the Aggeneys Area, Bushmanland, South Africa: in J. of African Earth Sciences v18, no.1 pp 61-71|
Moore J M, le Fur F 2000 - Relationships Between Conglomerates, Iron Formations and Gahnite-rich Rocks at Gamsberg and Aggeneys East: Implications for the Exhalative Origin of Broken Hill-type Base Metal Deposits (Abstract): in Geocongress 2000: 27th Earth Science Congress of the GSSA (abstracts) J. of African Earth Sciences pp 53-54|
Moore J M, Watkeys M K, Reid D L 1990 - The Regional Setting of the Aggeneys/Gamsberg Base Metal Deposits, Namaqualand, South Africa: in Spry P G, Bryndzia L T (eds), Regional Metamorphism of Ore Deposits and Genetic Implications: Proceedings of the 28th International Geological Congress, Washington, US. VSP, Utrecht, Netherlands pp 77-95|
Mourant D, Smith P 1986 - Addendum to the Paper by Ryan et. al. (1985) on the Aggeneys Base Metal Sulphide Deposits, Namaqualand District: in Anhaeusser C R, Maske S, (eds), Mineral Deposits of Southern Africa Geol. Soc. of South Africa, Johannesburg v2 p 1475|
Praekelt H E, Schoch A E, Visser J N J 1997 - The Metasediments of the Aggeneys Terrane in the Namaqua Mobile Belt: Sedimentary Response to Extensional-compressional Variations in a Continental Environment: in S. Afr. J. Geol. v100, no.1 pp 101-110|
Reid D L, Welke H J, Smith C B, Moore J M 1997 - Lead isotope patterns in Proterozoic stratiform mineralization in the Bushmanland Group, Namaqua Province, South Africa: in Econ. Geol. v92 pp 248-258|
Reid D L, Welke H J, Smith C B, Moore J M 1997 - Lead Isotope Patterns in Proterozoic Stratiform Mineralisation in the Bushmanland Group, Namaqua Province, South Africa: in Econ. Geol. v92 pp 248-258|
Ryan P J, Lawrence A L, Lipson R D, Moore J M, Patterson A, Stedman D P, Van Zyl D 1986 - The Aggeneys Base Metal Sulphide Deposits, Namaqualand, South Africa: in Anhaeusser C R, Maske S, (eds), Mineral Deposits of Southern Africa Geol. Soc. of South Africa, Johannesburg v2 pp 1447-1473|
Stalder M, Rozendaal A 2000 - Rare Earth Element Geochemistry of Garnet Associated with the Aggeneys-Gamsberg Cu-Pb-Zn Deposits, South Africa, Using Laser-ablation Microprobe ICP-MS: in Geocongress 2000: 27th Earth Science Congress of the GSSA (abstracts) J. of African Earth Sciences pp 74-75|
Stalder M, Rozendaal A 2000 - Metamorphism of the Polymetallic Sulphide Ores from the Mesoproterozoic Aggeneys-Gamsberg Sedex Deposits, Namaqua Province, South Africa (Abstract): in Geocongress 2000: 27th Earth Science Congress of the GSSA (abstracts) J. of African Earth Sciences pp 73-74|
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