Another PGC International Study Tour
Developed & Managed by Porter GeoConsultancy
Iron 2002-03
Key Iron Deposits of the World
September 2002 & March-April 2003
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Image:   Part of the Sishen Pit, Northen Cape, South Africa.    
Sishen Pit
DESCRIPTIONS of ORE DEPOSITS & PROGRAM

The program for this tour included:

MODULE 2 - AFRICA & SOUTH AMERICA
PART A, South Africa
- 26 to 31 March 2003

For information on the remainder of the tour, see the
Deposit Descriptions for   Module 1,   Module 2B


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MODULE 2 - AFRICA & SOUTH AMERICA,   PART A   - The Iron Deposits of South Africa

South Africa is Africa's leading iron ore producer, with an annual output of around 33 Mt, and 'reserves' of approximately 9 billion tonnes, 45% of which are located in the Northern Cape Province.   Production comes from two main areas, the Sishen mine in the Northern Cape Province, accounting for over 24 Mt of high grade 65% Fe ore per annum, and the Thabazimbi Mine in the Northern Province.   Thabazimbi generally provides iron to South African domestic smelters at Vanderbijlpark and Newcastle, at a rate of around 2 Mtpa, while almost all production from Sishen is exported through the port of Saldanha north of Cape Town.

Although 700 km apart, Thabazimbi and Sishen are both developed on iron formations in the Palaeoproterozoic Transvaal Sequence, which was deposited between sometime after 2400 Ma and 2100 Ma within an intracratonic basin on the Kaapvaal Craton.   The Kaapvaal Craton comprises a 3.4 to 2.8 Ga granite-greenstone basement, similar to the Zimbabwe Craton to the north which is separated by the 2.5 to 2.0 Ga Limpopo Mobile Belt.   The granite-greenstone basement terrane is over lain by a series of intracratonic basins, the axes of which migrated north with time from the early "upper" Archaean to the late Palaeoproterozoic Waterberg System to the north.

The oldest of these, the Pongola System is a 10 000 m thick, approx. 3.0 Ga succession comprising a lower basaltic volcanic and lesser quartzite sequence overlain by alternating argillaceous and arenaceous sediments with intercalations of BIF.   The overlapping ~9 000 m thick Witwatersrand Supergroup was deposited at between 3075 and 2715 Ma, commencing with early Dominion basaltic lavas, followed by a succession of mainly sandstones and shales with lesser locally gold bearing conglomerates.   These were in turn followed by the up to 5000 m thick Ventersdorp Supergroup composed dominantly of basaltic volcanics dated at 2714 Ma.

The up to 12 000 m thick Transvaal Sequence was deposited unconformably on the Ventersdorp Sequence, and occurs in two connected depo-centres, the Transvaal and Griqualand West sub-basins, which define a 1100x350 km, NE-SW elongated area of remaining exposure.   This sequence originally covered an area of approx. 500 000 sq. km.

In the Transvaal sub-basin, the Transvaal sequence commences with the up to 2000 m thick Wolkberg (or Buffalo Springs) Group of valley fill basalts and coarse clastics and lesser argillites.   These are followed by the few tens to 500 m thick Black Reef Quartzite, which grades up into the around 3000 m thick Chuniespoort Group which comprises the lower up to 2000 m thick Malmani Dolomite, variably composed of dolomite and chert, and the overlying up to 600 m thick Penge Iron Formation.

The Penge Iron Formation is the host to the Thabazimbi iron deposit and is composed of alternating carbonaceous shale and macro-, meso- and micro-banded BIF (quartz- magnetite- hematite- stilpnomelane- riebeckite- minnesotaite- grunerite and ferriferous carbonates).   The uppermost member of the Chuniespoort Group is represented by the dolomites, quartzites and shale of the locally preserved Duitschland Formation.   The Chuniespoort Group is unconformably overlain by the 7000 m thickness of quartzites, shales and minor basalts of the Pretoria Group and the 2-3000 m of rhyolitic lavas that make up the ~2100 Ma Rooiberg Group.

In the Griqualand West sub-basin, the Transvaal Sequence is represented by the Ghaap Group, which is unconformably overlain by the Postmasburg Group.   The Ghaap Group is sub-divided into the lower interbedded silici-clastics and carbonates of the Schmidtsdrif Subgroup followed by the limestones and dolomites of the Campbellrand Subgroup.   These are overlain by the Asbesheuwels Subgroup which is sub-divided into the lower Kuruman Iron Formation, composed of interlayered carbonaceous shale and a chert- carbonate- stilpnomelane- magnetite- hematite- greenalite- siderite iron formation, and the upper Griquatown Iron Formation, comprising siderite-hematite and siderite-greenalite lutites.   The Asbesheuwels Subgroup is host to the giant Sishen iron deposit.   The Ghaap Group is unconformably overlain by the Postmasburg Group, commencing with the thin Makganyene Diamictite, the thick Ongeluk basaltic pillow lavas, followed in turn by the jasper, BIF and sedimentary manganese deposits of the Hotazel Formation, and finally the Mooidraai Dolomite.   These are in turn unconfoirmably overlain by the shales and red-bed sanstones of the Olifantshoek Group.

The Transvaal Sequence is unconformably overlain to the north by the extensive thick arkosic arenites of the 2000 to 1700 Ma Waterberg Group.

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Expert Workshop

The Geology & Iron Deposits of Southern Africa

The workshop was led by internationally renowned expert, Professor Nic Beukes, supported by Jens Gutzmer, both of Rand Afrikaans University in Johannesburg, and included coverage of the following topics:

  • An overview of the geological and tectonic framework of iron ore mineralisation in Southern Africa, and geological controls and distribution of significant iron deposits within the same region.
  • The factors influencing the development of ore grade iron mineralisation in southern Africa and the genesis of the key deposits.
  • Descriptions of other significant deposits not to be visited and a comparison with those that are on the itinerary.

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Field Workshop - Northern Cape Province

A one day field workshop concentrated on the surface expression, setting and geology of the Kumba Resources Sishen South deposits in the southern part of the Sishen-Kuruman-Postmasberg-Hotazel iron-manganese district of the Northern Cape Province of South Africa and was led by experts from Kumba Resources. It provided an overview of the regional setting and surface expression of iron ores and mineralisation in the Griqualand West sub-basin. The visit to Beeshoek also included a field workshop component with Assmang geologists.

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Sishen & Sishen South Deposits

The Sishen iron ore deposit is situated in the Northern Cape Province of South Africa, 280 km north of Kimberley and 800 km north-east of its export port at Saldanha Bay north of Cape Town, to which it is connected by a 1 m gauge rail line. It is just to the SW of the township of Kathu (#Location: 27° 45' 32"S, 23° 0' 0"E).

The mine is owned and operated by Kumba Resources Limited and produces around 27 Mt pa of 65% Fe hard hematite ore with a high lump proportion, and impurity levels of generally less than 0.05% P, 2.5% SiO
2 and 1.2% Al2O3.

Regional Setting

The major iron deposits of South Africa, which include Thabazimbi and Sishen that are 700 km apart, are developed on iron formations in the Palaeoproterozoic Transvaal Sequence, which was deposited between ~2.4 and 2.1 Ga within an intracratonic basin on the Kaapvaal Craton. The Kaapvaal Craton comprises a 3.4 to 2.8 Ga granite-greenstone basement, which is separated from the Zimbabwe Craton to the north by the 2.5 to 2.0 Ga Limpopo Mobile Belt. The granite-greenstone basement terrane is overlain by a series of intracratonic basins, the axes of which migrated northward with time from the early Neoarchaean to the late Palaeoproterozoic Waterberg System to the north.

The oldest of these, the Pongola System is a 10 000 m thick, ~3.0 Ga succession comprising a lower basaltic volcanic and lesser quartzite sequence overlain by alternating argillaceous and arenaceous sedimentary rocks with intercalations of BIF. The overlapping ~9000 m thick Witwatersrand Supergroup was deposited at between 3075 and 2715 Ma, commencing with early Dominion basaltic lavas, followed by a succession of mainly sandstones and shales with lesser locally gold bearing conglomerates.These were in turn followed by the up to 5000 m thick Ventersdorp Supergroup composed dominantly of basaltic volcanic rocks dated at 2714 Ma.

The up to 12 000 m thick Transvaal Sequence was deposited unconformably on the Ventersdorp Sequence, and occurs in two connected depo-centres, the Transvaal and Griqualand West sub-basins, which define a 1100 x 350 km, NE-SW elongated area of remaining exposure. This sequence originally covered an area of ~500 000 sq. km.

In the Transvaal sub-basin, the Transvaal sequence commences with the up to 2000 m thick Wolkberg (or Buffalo Springs) Group of valley fill basalts and coarse clastics and lesser argillites. These are followed by the few tens to 500 m thick Black Reef Quartzite, which grades up into the around 3000 m thick Chuniespoort Group that comprises the lower up to 2000 m thick Malmani Dolomite, variably composed of dolomite and chert, and the overlying up to 600 m thick Penge Iron Formation. The Penge Iron Formation is the host to the Thabazimbi iron deposit and is composed of alternating carbonaceous shale and macro-, meso- and micro-banded BIF (quartz- magnetite- hematite- stilpnomelane- riebeckite- minnesotaite- grunerite and ferriferous carbonates). The uppermost member of the Chuniespoort Group is represented by the dolomites, quartzites and shale of the locally preserved Duitschland Formation. The Chuniespoort Group is unconformably overlain by the 7000 m thickness of quartzites, shales and minor basalts of the Pretoria Group and the 2 to 3000 m of rhyolitic lavas that make up the ~2100 Ma Rooiberg Group, marking the stage of emplacement of the Bushveld Complex.

In the Griqualand West sub-basin, the Transvaal Sequence is represented by the Ghaap Group, which is unconformably overlain by the Postmasburg Group. The Ghaap Group is sub-divided into the lower interbedded silici-clastics and carbonates of the Schmidtsdrif Subgroup followed by the limestones and dolomites of the Campbellrand Subgroup. These are overlain by the Asbesheuwels Subgroup which is sub-divided into the lower Kuruman Iron Formation, composed of interlayered carbonaceous shale and a chert-carbonate-stilpnomelane-magnetite-hematite-greenalite-siderite iron formation, and the upper Griquatown Iron Formation, comprising siderite-hematite and siderite-greenalite lutites. The Asbesheuwels Subgroup is host to the giant Sishen iron deposit. The Ghaap Group is unconformably overlain by the Postmasburg Group, commencing with the thin Makganyene Diamictite, the thick Ongeluk basaltic pillow lavas, followed in turn by the jasper, BIF and sedimentary manganese deposits of the Hotazel Formation, and finally the Mooidraai Dolomite. These are in turn unconformably overlain by the shales and red-bed sanstones of the Olifantshoek Group.

The Transvaal Sequence is unconformably overlain to the north by the extensive thick arkosic arenites of the 2000 to 1700 Ma Waterberg Group.

Deposit Geology

The Sishen deposits are hosted by the Palaeoproterozoic Transvaal Supergroup within the Griqualand West sub-basin. It is situated on the Maremane Dome which is defined by the carbonate sequence of the Transvaal Supergroup, the Campbellrand Subgroup and the overlying host iron formations of the Asbesheuwels Iron Formation. These units dip outwards at less than 10 degrees on the eastern margin of the dome. In the central and western sections of the dome, this part of the sequence is concealed by the unconformably overlying red-bed clastic Gamagara Formation of the Olifantshoek Group.

A unit of ferruginous chert breccia (the Wolhaarkop Breccia) which grades upwards into a distorted banded iron formation (the Manganore Iron Formation) is wedged between the underlying Campbellrand carbonates and the unconformity at the base of the Gamagara Formation. The Wolhaarkop Breccia is matrix supported and consists of unsorted angular chert fragments in a hematite to manganese bearing siliceous matrix.

Some 80 to 90% of the ore at the north-south elongated 12 x 1.5 km Sishen deposit is hosted by the Manganore Iron Formation, which is correlated with the Asbesheuwels Iron Formation, and is found immediately below the unconformity with the overlying clastic Gamagara Formation. Subsequent to deposition, Asbesheuwels Iron Formations are interpreted to have locally slumped onto a palaeo-sinkhole dominated surface developed in the underlying Cambellrand sub-group carbonates to produce the Wolhaarkop Breccia and Manganore Iron Formation during the period of erosion prior to the deposition of the Gamagara Formation.

Silica is believed to have been leached from the slumped and brecciated iron formation by alkaline supergene fluids at the time of slumping, while ferrous ions were oxidised to hematite, and additional transported supergene iron was added. Erosion of the hematite mineralisation and resultant accumulations of hematite pebble conglomerates in alluvial fan environments are preserved as the Doornfontein Conglomerate at the base of the Gamagara formation and account for around 10-20% of the orebody. The Doornfontein Conglomerate is best developed immediately adjacent to pockets of Manganore Iron Formation.

The Manganore Iron Formation is more restricted in areal extent than the Wolhaarkop Breccia. It is composed of 7 zones/bands from the base, namely: Zone 1 - a spotted carbonaceous and dark brown shale with chert pillows and hematite nodules; Zone 2 - hematite micro-banded white chert with interbeds of intercalated cherts and black-brown shale; Zone 3 - chert banded hematite ferhythmites with cycles of hematite-lutite to hematite-microbanded chert, to hematite ribbon, wave and pillow-rhythmite; Zone 4 - hematite rhythmites that represent the bulk of the banded ore; Zones 5 and 6 of hematite-greenalite banded lutites. The first 6 zones were derived from the Kuruman Iron Formation, the lower of the 2 Asbesheuwels units, while a zone 7 composed of hematite lutite with meso-bands of peloidlutite is correlated with the overlying Griquatown Iron Formation. Chert bands within the Manganore Iron Formation are porous, with partially infilling platy hematite. Boundaries of high grade ore cut across primary sedimentary boundaries.

Three types of ore are present in the Manganore Iron Formation, namely:
i). Laminated ore - composed of both thickly laminated alternating massive, porous hematite meso-bands with dull and bright micro-banded 2 to 15 mm thick meso-bands, and of thinly laminated micro-banded hematite comprising very thin microbanded high lustre hematite with equally thin earthy hematite meso-bands;
ii). Massive ore - massive to very poorly bedded, fine grained, porous aggregates of platy hematite;
iii). Breccia ore - of two types which may be developed anywhere in the iron formation, comprising oligomictic hematite rhythmite breccia derived from laminated ores, and oligomictic hematite-lutite breccia.

The Makganyene diamictite and Ongeluk Lava of the Postmasburg Group which unconformably overlies the Ghaap Group and unconformably underlies the Gamagara Formation of the Olifantshoek Group, were subsequently thrust over the deposit and host sequence from the west. Much of the sequence in the region is concealed by around 50 m of Tertiary Kalahari Formation cover.

The 2001 mineral reserve estimate for the Sishen deposit was 877 Mt, and the mineral resource estimate was 1724 Mt. Some 70 km to the south at the Sishen South (or Welgevonden) deposit there is a high quality resource of 259 Mt of high lump ore to be developed by 2006.

In 2005, the proved + probable reserve at Sishen totalled 1021 Mt @ 59.0% Fe, plus a quoted resource of 1957 Mt @ 57.2% Fe.
The proved + probable reserve at Sishen South in the same year was 167 Mt @ 64.2% Fe, plus a resource of 248 Mt @ 65% Fe.

In 2008, the proved + probable reserve at Sishen totalled 956.9 Mt @ 59.6% Fe, plus a quoted resource of 1628 Mt @ 56.4% Fe.
The proved + probable reserve at Sishen South in the same year was 214.1 Mt @ 64.1% Fe, plus a resource of 153.2 Mt @ 64.3% Fe.
Reserves and resources from Kumba Iron Ore website and annual report.

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Beeshoek Deposits

The Beeshoek operation is located some 70 km to the south of Sishen in the Northern Cape Province of South Africa and 7 km west of Postmasburg (#Location: 28° 17' 6"S, 23° 0' 27"E).

It comprises four pits which feed ore to a central crushing, screening and washing plant that produced 5 Mt of lump, DRI and fines ore in 2001 for rail transport to, and export from Saldanha Bay near Cape Town.

Like Sishen, Beeshoek is associated with the Manganore Iron Formation, localised at the unconformity between the Campbellrand Formation dolomites of the Ghaap Group and the unconformably overlying Gamagara Formation, but on the opposite (or southern) limb of the Maremane Dome.

For more detail on the setting and stratigraphy see the separate Sishen record.

Four types of ore are found at Beeshoek, as follows:   i). Thaba ore - hematised cherty banded iron formation near the base of the Manganore Iron Formation;   ii). Laminated ore - hematised finely banded iron formation from near the top of the Manganore Iron Formation;   iii) Conglomeratic ore - of the Doornfontein Conglomerate, the basal unit of the Gamagara Formation; and   iv) Detrital ore.

The bulk of the ore is the conglomeratic type, deposited in a NNE trending palaeo-channel developed in the erosional surface upon which the Gamagara Formation was deposited, and forms the basal unit of that formation.   It comprises a poorly sorted mixture of rounded and angular pebbles in a highly ferruginous matrix.   The angular clasts imply relatively short transport distance from source.   The Doornfontein Conglomerate most likely represents an alluvial fan filling solution hollows on a palaeo-karst surface, close to the source hematised Manganore Iron Formation.   Maximum ore thickness reaches 55m, but rapidly thins on the uneven palaeo-karst basement requiring substantial drilling programs to outline pre-mining reserves.

The Beeshoek mine is owned by Assmang Ltd (50% ARM, 45.5% Assore) and commenced operation in 1961.   Beeshoek South, a new southern extension, was commissioned in 1999 and currently provides 70% of the ore treated. Production form the mine in 2014 was 2.1 Mt of ore.

Remaining measured + indicated rsources in 2014 (ARM Resources) were:
    117.5 Mt @ 63.75% Fe

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Thabazimbi

The Thabazimbi iron ore deposits are situated in the Northern Province of South Africa, 200 km north of Johannesburg.   #Location: 24°36'S, 27° 23'E.

These mines have been in production since 1932 and are currently owned and operated by Kumba Resources Limited. The operation produces around 2.4 Mt per annum of lump and fines iron ore in roughly equal proportions exclusively for Kumba's South African steel operations at the Vanderbijlpark Steel Plant, outside of Johannesburg and the Newcastle Steel Plant in northern KwaZulu-Natal.

The Thabazimbi deposits are hosted by the Palaeoproterozoic Transvaal Supergroup within the Transvaal sub-basin.   They occur in the basal Penge Formation, immediately above the lowest shale unit, a 10 m thick chert rich band which immediately overlies the thick dolomite and chert succession of the Malmani Dolomite.

The Penge Formation, is 350 m thick and is composed of thick iron formations alternating with thin units of orthochemical iron formation.   The iron ore lenses (+60% Fe) are restricted to the basal 80 m section of the Penge Formation rhythmites, occuring as irregular, tabular bodies distributed over a strike length of 12 km.   Individual bodies lens out along strike, separated by sterile gaps of un-economic iron formation and vary in thickness from 2 to 100 m, averaging around 20 m.   Both the host and ore dip at around 50°.

At depth the ore grades laterally down dip into talc-hematite and then to carbonate-hematite rocks, while lenses of primary iron formation are also found within the orebodies closer to surface.   The ore zones have a gradational upper boundary with the un-enriched overlying iron formation, while the degree of iron enrichment in the ore appears to be directly proportional to the amount of brecciation of the iron formation resulting from solution collapse in the underlying chert poor dolomites of the uppermost Malmani Dolomite.

Most of the ore is brecciated, occuring as primary hematite fragments (ie. the hematite, magnetite and martite of the ferhythmites in the original iron formation) set in a fine secondary hematite matrix.   The primary hematite clasts have a steel-grey to blue-grey colour, metallic lustre and a dense compact texture.   In contrast the fine seconday hematite matrix has a variable colour from steel-grey to black-grey, and is locally red-brown.

The brecciation and content of clasts versus matrix is variable as is the hardness and friability of the ore and the degree of removal of chert and replacement by goethite and hematite.   The iron enrichment is believed to be post (Palaeoproterozoic) Waterburg tectonism with a second post (late Mesozic) Karoo phase.

The mineral reserve and resource estimates for the deposit in 2001 were -   Proven: 16.3 Mt and Probable: 4.1 Mt.   The mineral resource estimates were -   Measured: 32.43 Mt, Indicated: 42.00 Mt and Inferred: 19.80 Mt.

Total production from 1932 to 2005 was 118 Mt of saleable ore.

The mineral reserve and resource estimates for the deposit in 2006 were -   Proven: 10.0 Mt and Probable: 4.0 Mt, for a total reserve of 14 Mt @ 64% Fe, 0.07% P.   The mineral resource estimates were -   Measured: 23 Mt, Indicated: 18 Mt and Inferred: 20 Mt, for a total resource of 61 Mt @ 61.3% Fe, 0.06% P (Kumba Resources website, 2009).

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The summaries above were prepared by T M (Mike) Porter from a wide range of sources, both published and un-published.   These are listed in the Literature Collections pages for this tour.

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For more information contact:   T M (Mike) Porter, of Porter GeoConsultancy   (mike.porter@portergeo.com.au)

This tour was designed, developed, organised, managed and escorted by
T M (Mike) Porter of Porter GeoConsultancy Pty Ltd.

Porter GeoConsultancy Pty Ltd
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LINDEN PARK, 5065
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Telephone: +61 8 8379 7397
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