Aggeneys - Broken Hill, Deeps
Northern Cape, South Africa
Pb Zn Cu
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The Broken Hill and Deeps mines at Aggeneys have exploited one of a cluster of three main ore deposits, the other two being Swartberg/Black Mountain and Gamsberg. It lies some 16 km to the west of Gamsberg and 6 km east of Black Mountain. Aggeneys is located 60 km East of Pofadder and 110 km West of Springbok within the Northern Cape Province of the Republic of South Africa.
All are hosted by the Bushmanland Group of the Namaqualand Metamorphic Complex, which in turn lies within the broader Namaqua Mobile Belt on the south-western and southern margin of the Kaapvaal/Kalahari Craton. This complex comprises an older metamorphic suite (locally a quartzo-feldspathic augen gneiss) dated at around 2000 to 1900 Ma, overlain by a 1700 to 1600 Ma (?) supra-crustal succession. The latter comprises a lower intrusive to extrusive leuco-gneiss, the Hoogoor Suite, and an upper schist/quartzite succession, the Bushmanland Group. The latter is extensive, and is largely preserved as infolded enclaves or thrust slivers, and is relatively thin, generally <1000 m thick.
Exploration in the Black Mountain-Aggeneys area was first recorded in 1929 with a prospecting shaft sunk on the Swartberg (Black Mountain) ~6 km west of Aggeneys. Sporadic investigations continued at intervals until 1970 when Phelps Dodge Corporation commenced a diamond-drilling program in the district. The main Swartberg deposit was intersected in 1971, followed by mineralisation at Noeniespoort (Broken Hill) in 1972, and in 1973 at the low grade Big Syncline mineralisation. The most promising was at Noeniespoort where an adit was driven in 1974 to collect bulk samples for metallurgical testing. In 1976, a feasibility study for the development of an underground mine was commissioned. In October 1977 Phelps Dodge concluded an agreement with Gold Fields of South Africa (GFSA) Limited which secured a 51% interest in the Black Mountain Mineral Development Company (Proprietary) Limited.
Subsequent to the feasability study, approval was granted for the mine development, followed by commencement of construction. The first production from the Broken Hill/Aggeneys mine was in 1979, reaching full production of ~1.56 Mt of ore per annum in 1980.
By the late 1990s reserves at Broken Hill were nearing depletion and the mine was scheduled to close by 2000. At this stage GFSA decided to divest its base metal assets, including Black Mountain and the undeveloped Gamsberg zinc deposit. At the end of 1998 Anglo-American Corporation purchased the Black Mountain Mineral Development Co. with the aim of developing Gamsberg and using the plant at Aggeneys to treat the ore from the new mine. Low-key exploration over the preceding decade principally aimed at finding extensions to the Broken Hill orebody, had not been fruitful, although potential still existed. With the change in ownership expected to result in the demise of this drilling program, the Chief Geologist requested funds for one final hole to test a target further down plunge. This hole, drilled in 1998, resulted in the intersection of high-grade mineralisation at a depth of just over 1000 m which became the Broken Hill Deeps deposit. Funds were immediately made available for an expanded drilling program. It showed the Deeps to be a redevelopment of the Broken Hill orebody which had pinched out at a depth of ~800 m below surface. The western extremity of the Deeps is ~390 m east of, and 240 m below the termination of the Broken Hill deposit. The results prompted a full scale feasibility study, culminating in the decision to exploit the Deeps ore body. A 1750 m shaft was sunk and equipped by 2007, and mining continued at Broken Hill to allow the treatment plant to continue production in the transition to the new deposit.
In May 2010, Vedanta Resources acquired Anglo American's zinc portfolio, including Black Mountain Mining and its resources at Broken Hill, Deeps, Swartzberg, Big Syn and Gamsberg, but also Skorpion in Namibia and Lisheen in Ireland.
In the Aggeneys area, the Namaqualand Metamorphic Complex sequence of the Namaqua Mobile Belt is as follows, from the base:
Augen Gneiss Formation, which occurs locally to the north of the Big Syncline prospect. This may be a member of the older 'heterogeneous lower sequence' of Palaeo- and Mesoproterozoic age, which has been dated elsewhere at around 2000 to 1900 Ma, although similar rocks 'intrude' the supracrustal sequence 20 km to the east. Locally it comprises leucocratic poikiloblastic microperthite augen set in a matrix of quartz and microcline (with minor perthite), plagioclase and biotite with occasional calc-silicate xenoliths.
Haramoep Gneiss, with a maximum thickness locally of 200 m - this is taken to be a member of the Hoogoor Suite of intrusive to extrusive leuco-gneiss which underlies, and is gradational upwards into, the Bushmanland Group. It comprises a pink heteroblastic quartz, microcline, perthite, plagioclase gneiss with minor biotite and opaques. It contains discontinuous lenses of schist, quartzite and amphibolite.
Generally a thrust contact in the immediate Aggeneys-Gamsberg area.
Bushmanland Group, comprising in the immediate Aggeneys-Gamsberg district,
• Aluminous Schist Formation (or Namies Schist), averaging 80 m, thick - composed of quartz, muscovite, K-feldspar and some sulphides, and in some areas, sillimanite and biotite, each constituting more than 25%. Virtually all of the sillimanite deposits within Bushmanland are within this formation. Common accessory minerals include tourmaline, spessartine-almandine garnet, rutile, anatase, hematite and ilmenite. This schist which weathers negatively, has a sharp 'concordant' contact with the overlying White Quartzite.
• White (or Pella, or Zuurwater) Quartzite, 5 to 900m thick - mainly a massive milky to grey quartzite with a glassy texture comprising 95% recrystallised quartz and minor muscovite, sillimanite, biotite, magnetite and rutile. Near the top there is often a discontinuous and irregular 5 to 10m thick zone of pelitic schist consisting of quartz, muscovite, biotite, sillimanite, magnetite and garnet. The negative weathering of this schist has the effect of dividing the White Quartzite into two approximately parallel ridges. At Tank Hill and Broken Hill, thin lenses of magnetite quartzite occur within the schist and contain small quantities of Cu mineralisation at Broken Hill. This may represent a shear/thrust contact on the interpretation of Colliston, et al., (1986), separating the main White/Pella Quartzite from the mineralised units. The upper contact of the upper quartzite and schist band with the overlying Aggeneys Ore Formation may be either sharp or gradational.
To the north of the Aggeneys-Gamsberg area there are a number of additional formations between the Zuurwater (Pella) Quartzite and the Aggeneys Ore Formation. While this contact is commonly a thrust plane in the Aggeneys-Gamsberg area, this is not always the case. Therefore if the mapping of Colliston, et al., (1986) and Strydom, et al., (1987) is correct, there was a hiatus or a thinning of the sequence southwards towards the Aggeneys-Gamsberg area and the position of the subsequent Swartberg-Zuurwater Thrust/shear Zone during the deposition of the Bushmanland Group.
• Aggeneys Ore Formation, 200 m thick - as described below. Almost all of the 'iron formation', sulphide and barite mineralisation in the Aggeneys-Gamsberg area occurs within this unit. The schist overlying the magnetite rich orebody at Broken Hill is characterised by the presence of 3 to 6% pyrite. Some authors interpret these pyritic schists which occur above the orebodies at Black Mountain and Broken Hill are the host to ore at Big Syncline.
Unconformity (probable), commonly a thrust contact.
Amphibolite and Leucocratic Grey Gneiss Formation - are found at the top of the sequence in the Aggeneys Mountains to the north of Broken Hill in the Big Syncline area. In this area they overlie the White Quartzite Formation which grades upwards into conglomeratic bands, which are in turn interbedded with a highly variable succession of gneisses and amphibolite. This is the Koeris Formation (or Nousees Mafic Gneiss) of Colliston, et al., (1986). The conglomerate has clasts which are predominantly quartzite in a quartz-mica matrix, while the leucocratic gneiss consists of quartz, feldspar and muscovite and the more mafic gneisses comprise biotite, amphibole and magnetite in a matrix of quartz and feldspar. The amphibolite in which hornblende predominates contains quartz filled amygdale like structures.
The Aggeneys Ore Formation is strongly deformed, and in the Broken Hill Mine area comprises the following generalised sequence, from the interpreted stratigraphic base, but structural top,
Hangingwall Schist, 5 to 15 m thick - a sillimanite-quartz schist found immediately above the White Quartzite unit, comprising 10 to 20% sillimanite, 30 to 40% quartz, 5 to 25% biotite, 5 to 15% garnet, with lesser muscovite and feldspar. Minor amounts of magnetite are present.
Upper Orebody Succession,
• Ferruginous Garnet-Quartzite, 5 to 25 m thick - thin bands of magnetite, comprising around 5 to 20% of the rock, and garnet (5 to 10%) set in a matrix of quartz. The magnetite content increases stratigraphically upwards, until near at the top of the unit it grades into the overlying quart-magnetite rock. The gradation is relatively sharp, over a few cm's.
• Magnetite-Quartzite and Amphibole-Magnetite rocks, up to 40 m thick - the quartz-magnetite rock predominates adjacent to the ferruginous garnet-quartzite with which it has a gradational contact. Towards the stratigraphic top it grades into an amphibole-magnetite rock.
Magnetite-quartzite, which is the predominant lithology within the Upper Orebody Succession, comprises thin, alternating bands (1 to 2mm thick) of fine to medium grained magnetite and quartz, with the magnetite content ranging between 40 and 60%. In places it grades into to a massive fine to medium grained rock without discernible separate quartz and magnetite bands. Other minerals, mainly garnet (up to 15%) and minor biotite are also present, while base metal sulphides, including galena, sphalerite and chalcopyrite are disseminated throughout the unit.
The banded amphibole-magnetite rock consists of thin alternating bands rich in quartz, spessartine garnet, magnetite, ortho-pyroxene, grunerite, cummingtonite and fayalite. The combined base metal sulphide content is slightly higher than in the magnetite-quartzite.
• Massive-sulphide and Sulphide-Quartzite, up to 20 m thick - sometimes commencing with a limited baritic massive sulphide which grades laterally into the main massive sulphides, but has a sharp vertical contact with the same rock type above. Massive sulphides are defined at Broken Hill as containing more than 25% sulphide by volume. In general they have less than 10% magnetite. In places the massive sulphides have a lensoid core of sulphide-quartzite with which they are vertically and laterally gradationally.
The baritic massive sulphides are localised in extent. They constitute 3 to 50% barite, but average 15%.
The massive sulphides are the host to the bulk, but not all of the chalcopyrite, galena, sphalerite and silver mineralisation within the orebody and form the core of the ore bearing zone. They generally contain between 15 and 50% combined pyrite and or pyrrhotite. The dominant gangue silicate is quartz. The unit is commonly coarsely recrystallised, and brecciation of the base metal sulphides occurs along the sharp upper contact with the stratigraphically overlying magnetite quartzite. The massive sulphides may be well banded and fine to medium grained, or massive and medium to coarse grained. The upper orebody massive sulphides are only present as isolated lenses, the largest of which is 240 m in strike length and 3 m thick.
The sulphide-quartzite which may be up to 5 m thick is predominantly composed of quartz with very finely disseminated pyrite, pyrrhotite, chalcopyrite, galena and sphalerite as accessory minerals, the contact with the massive sulphides is gradational.
• Magnetite-Quartzite, 5 to 30m thick - as described above, has a gradational contact with the adjacent massive sulphides. This may include a quartz-amphibole stage.
Intermediate Schist, 0 to 10 m thick - a sillimanite-quartz schist, similar to the Hangingwall Schist. It separates the Upper and Lower Orebody Successions, but lenses out to the west where the two ore sequences coalesce. As it thins to the west it becomes a biotite-garnet-magnetite schist before wedging out where its place appears to be taken by a thin laterally impersistent massive magnetite band, with minor garnet and tourmaline (Smith 1986).
Lower Orebody Succession, comprising,
• Ferruginous Garnet-Quartzite, 2 to 10 m thick - this is essentially the same as described for the Upper Orebody Succession.
• Amphibole-Magnetite and Magnetite-Quartzite, 5 to 10 m thick - again this is similar to that described for the stratigraphic footwall of the Upper Orebody section, although in general the ferruginous garnet-quartzite grades into amphibole-magnetite rock, instead of magnetite-quartzite, with the latter being less well represented. The boundary with the massive sulphides is often sharp, over an interval of 1cm or so, with a drop off in magnetite and corresponding increase in sulphides.
Thrusting between the Upper and Lower Orebody successions (according to Ryan, et al., 1986) has removed sections of the sequence, including the Intermediate Schist and the lithologies between it and the Lower Orebody massive sulphides. Underground where the Intermediate Schist is absent, the contact between the Lower and Upper Ore Successions appears to be discordant in the stope walls.
• Massive Sulphide, 2 to 5 m thick - is very much thinner than the Upper Orebody, although it is more extensive laterally. Otherwise it is similar to the Upper Orebody, except as described in the 'Mineralisation' segment below.
• Amphibole-Magnetite and Magnetite-Quartzite, 5 to 10 m thick - this is similar to those described above, again passing from massive sulphides, through amphibole-magnetite to magnetite-quartzite to the succeeding ferruginous garnet-quartzite. In this position the amphibole-magnetite rock predominates again. In sections of the orebody the massive sulphides pass directly into the footwall schists with the intervening lithologies being absent. This has been interpreted as being due to thrusting.
• Ferruginous Garnet-Quartzite, 1 to 2 m thick - as described above.
Upper Footwall Schist, 5 to 20 m thick - generally comprising a finely banded, pyritic, sillimanite-quartz-biotite schist, which contains minor garnet.
The Weak Zone, 1 to 5 m in thickness - interpreted by Ryan, et al., (1986) to be thrust plane. It is occupied by a sheared, pyritic and graphitic biotite schist, which comprises 40 to 60% biotite, 20 to 40% quartz, variable amounts of clay and lesser muscovite, feldspar, graphite and pyrite. Numerous pegmatites occur in this zone and appear to have been emplaced along shears and other planes of weakness. This zone may be in contact with the Lower Orebody, or be separated from it by the intervening lithologies described above.
There are rapid thickness changes within the orebody sequence in the mine, with for instance over a strike length of 30 m within a stope, a magnetite-quartzite changing from 20 m to 5 m in thickness. Over the same interval the adjacent massive sulphide remains fairly constant at 3 to 4 m in thickness, although it may vary by up to 1 m.
Both the Upper and Lower Orebodies comprise i). a central core of 'massive sulphide' (>25% sulphide by volume), surrounded by ii). magnetite and amphibole rich rocks with disseminated sulphides, and iii). marginal magnetite and garnet rich quartzites. These lithologies are respectively well mineralised, moderately to poorly mineralised and essentially unmineralised (Smith 1986). Much of the 'massive sulphide' is better described as semi-massive to dense disseminations.
Pyrrhotite and galena are dominant in the massive sulphides, followed by sphalerite, chalcopyrite and pyrite. The sphalerite in the ore has a high Fe content and is black. The Lower Orebody has a higher pyrrhotite content, while the Upper Orebody has a lower amount of Fe within the massive sulphides, but more in the adjacent magnetite-quartzites. Quartz is the major gangue mineral, while garnet is frequently also present. Mica may be locally significant, and in places barite is seen in the upper massive sulphide. Magnetite is usually <10% within the massive sulphides, being found near the contact margin with the bordering amphibole-magnetite rocks.
In the Upper Orebody, the massive sulphides occur in separate, well defined lenses. They have a high total base metal content in excess of 20% and a high Pb:Zn ratio which frequently exceeds 5:1, and ranges up to 20:1. The Lower Orebody massive sulphide occurs as one continuous sheet with less than 20% combined base metal and a Pb:Zn ratio of 2:1. Texturally the massive sulphides vary widely, ranging from well banded varieties to less common fine grained sulphidic breccias containing wall rock inclusions. These brecciated wall rock inclusions range from wispy fragments to rounded, concentrically zoned nodules of from 1 to 2 cm in size. Ryan, et al., (1986) also note that within the Aggeneys Ore Formation there are thin bands of sulphide which sometimes contain rounded fragments of the enclosing rock (usually arranged parallel to foliation) and frequent hair like veins spread out from the breccia into the adjoining rock. These brecciated ores appear to represent "durchbewegung" textures. Elsewhere, particularly in the schistose baritic zones, flow type folding and small rootless fold noses are evident. Coarse grained recrystallised massive sulphide zones are particularly common in the Upper Orebody (Smith 1986).
The base metal grades within the magnetite and amphibole rich rocks generally increase towards the contact with the massive sulphide, or towards the laterally equivalent magnetite rich rocks along strike, which in turn have increasing base metals laterally towards the massive sulphides.
The Lower Orebody magnetite and amphibole rich rocks have a very low Cu content and Pb may be the only base metal present in significant amounts. In the Upper Orebody magnetite and amphibole rich rocks mineral banding is frequently poorly defined and is often disrupted while coarse grained remobilised patches of chalcopyrite and galena are common in the hangingwall of the orebody.
Smith (1986) notes that strongly developed, lateral base metal zoning causes wide variation in Cu:Pb:Zn ratios, while Mourant and Smith (1986) note that Cu values decrease systematically from >1% in the west to <0.2% in the east, while the Pb:Zn ratio increases from <1 to> 2 over the same interval.
Massive magnetite developed at the position of the Intermediate Schist where that unit has pinched out (or been over-thrust) to the west is poorly mineralised.
The quartzites on the outer margins of each of the orebodies vary from relatively pure quartzites to garnet-biotite varieties in the Lower Orebody, while those adjacent to the Upper Orebody characteristically have minor amounts of magnetite. Texturally these lithologies include layers, lenses or angular fragments of white quartzite in a darker, impure quartzitic matrix (Smith, 1986). Only traces of mineralisation are present, with generally <1% base metals, although systematic lateral zoning is again present. Minor non-stratabound garnet quartzites occur as coarse grained pods near the margins of both the upper and lower orebody massive sulphides.
Within the massive sulphides and the magnetite and amphibole rich rocks of the Lower Orebody there are well defined regular mineral bands mm's to cm's thick, rich in garnet, amphibole or galena which may be traced laterally for tens of metres. Galena rich bands are sometimes visibly remobilised into discordant veinlets.
The Deeps deposit is a down plunge redevelopment or extension of the Broken Hill deposit to the NE. The western extremity of the Deeps is ~390 m east of, and 240 m below the current deepest level of the of the Broken Hill mine which is at 800 m below surface. The latter plunges to the at ~55° from near surface. The Deeps deposit has a down plunge extent of >1100 m, with the deepest ore at ~1680 m below surface (Vedanta, 2013).
The Deep deposit is sub-divided into five geologically distinct zones each composed of iron formation and massive sulphide. Lead-zinc-copper-silver mineralisation occurs as fine to coarse disseminations within, or as interbanded layers in the iron formations. Mineralisation in the massive sulphide is fine-grained and often brecciated. Economic ore in all of the five ore zones and is predominantly concentrated at or close to the footwall of each. The deposit is located in a synformal structure, mostly on the extensive, 63 to 70°S dipping southern limb. The shape and disposition of the deposit has been determined by both folding and thrusting.
Reserves and Resources
Reserve and resource figures when visited in 2000 included:
Initial pre-mining resource - 85 Mt @ 0.34% Cu, 3.6% Pb, 1.8% Zn, 48 g/t Ag,
including the reserve of 38 Mt @ 0.45% Cu, 6.4% Pb, 2.9% Zn, 82 g/t Ag.
Reserves in 2000 were ~8.7 Mt @ 0.5% Cu, 5.5% Pb, 2.9% Zn, 78 g/t Ag.
Remaining Ore Reserve and Mineral Resources at the end of 2018 in the Deeps area (Vedanta Resources Annual Report, 2018) were:
Proved + Probable Reserve - 5.59 Mt @ 2.98% Zn, 2.24% Pb;
Measured + Indicated Resource - 10.60 Mt @ 2.88% Zn, 2.97% Pb;
NOTE: Reserves are additional to resources.
The most recent source geological information used to prepare this summary was dated: 2013.
Record last updated: 28/1/2019
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.
Bailie R H and Reid D L, 2005 - Ore textures and possible sulphide partial melting at Broken Hill, Aggeneys, South Africa I: Petrography : in S. Afr. J. Geol. v108 pp 51-70|
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|
McClung C R, Gutzmer J, Beukes N J, Mezger K, Strauss H and Gertloff E, 2007 - Geochemistry of bedded barite of the Mesoproterozoic Aggeneys-Gamsberg Broken Hill-type district, South Africa : in Mineralium Deposita v42 pp 537-549|
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, Smith C B, Watkeys M K, Welke H J and Betton P J, 1997 - Whole-rock radiometric age patterns in the Aggeneys-Gamsberg ore district, central Bushmanland, South Africa : in S. Afr. J. Geol. v100 pp 11-22|
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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