The field trip covered a variety of gourmet pieces in economic geology and petrology:
The Mesoarchean Witwatersrand basin is the most important gold province on Earth. Since the discovery of gold near the future Johannesburg in the mid-eighties of the 19th century, about 50,000 tonnes of gold have been produced (equivalent to about 40 % of all historically mined gold on Earth) at an average grade of 8 g/t Au, and a similar resource is still in place. In addition, from the 1950s to 70s about 1.5 million tonnes of U3O8 have been produced at a grade of 270 g/t U3O8. The value of the historical gold production is equivalent to 740 billion USD at a present-day gold price of 460 USD/oz.
The ore consists of more than thirty quartz pebble meta-conglomerate layers within a low-grade metamorphic, mostly siliciclastic sequence of about 7000 m thickness. Clastic sedimentation (fluvial, deltaic, and near-shore) started at about 2.9 Ga and ended with flood basalt magmatism at 2.7 Ga. The large-scale spatial distribution of the gold mineralization is illustrated by the position of the main goldfields which are all located along the margin of the Witwatersrand paleobasin where the main entry points of braided river systems were located. On the local scale most gold is concentrated in paleochannels. The coarse-grained orebodies, dm up to a few meters thick, are controlled by local low-angle basal unconformities (erosional surfaces) related to regression/transgression of fluvial systems/shorelines, with highest grades (paystreak) near the erosional surface. The background value in finer grained sediments is <5 ppb Au. Gold occurs together with uraninite, pyrite, bitumen, and clastic heavy minerals such as zircon and chromite. The detrital nature of uraninite and, partly, of pyrite was demonstrated by ore microscopy fifty years ago (Ramdohr 1955), and since then has been a matter of great debate. The originally detrital nature of gold, with subsequent small-scale hydrothermal redistribution, is likely in view of old and new textural and geochemical observations (see summary in Frimmel et al. 2005).
The Witwatersrand mines are the deepest in the world, with active mining as deep as 3600 m below surface. In the more than one hundred years of underground mining, an average of 290,000 miners was employed (currently: about 170,000), and the extreme mining conditions have cost about 59,000 lifes. Mine safety has very much improved during the last twenty years, such as very efficient refrigeration systems which cool down rock temperatures from around 60°C to 30°C, modern backfill methods which reduce the risk from rock bursts, 5-hour face shifts, etc. Effective risk management over the last years has brought the Witwatersrand mines to safety levels similar to Canadian or Australian mines.
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The Mponeng mine works on the Ventersdorp Contact Reef (VCR) from depths between 2200 and 3300 m below surface. The VCR, which marks the base of the Ventersdorp Supergroup, is of fluvial origin and rests with a low-angle erosional unconformity of 3-4° on strata of the Witwatersrand Supergroup. The hangingwall consists of strong. competent, meta-basalt/andesite which has an U-Pb SHRIMP age on zircon of 2714 ±8 Ma. The VCR consists of pebble- and matrix-supported conglomerate with internal quartzite lenses and represents a braided fluvial environment which followed a prolonged episode of regional uplift. A series of fluvial regressions and transgressions, caused by continuing uplift, produced erosion and reworking of the sediments and superimposed terraces cut by channels. The fluvial activity was abruptly halted by extrusion of flood basalts which preserved the paleotopographic features. The conglomerate and quartzite horizons are affected by post-depositional argillic, chlorite and muscovite alteration. The gold grade is controlled by paleomorphology, with highest grades in the lower part of conglomerate units. Average ore grade is 9 g/t Au, cut-off is defined by gade times thickness, i.e. 1000 g/t x cm. Yearly production is around 13 t Au.
The TauTona mine works mainly on the Carbon Leader Reef which is stratigraphically 850 m below the VCR in the Central Rand Group of the Witwatersrand Supergroup. The reserves are 17.9 Mt @11.4 g/t Au, resources are 22.5 Mt @ 22.7 g/t Au. The total cash cost is about 200 USD/oz Au. The TauTona and Savuka Mines share a common processing plant (280,000 tpm) which uses a conventional milling and carbon-in-pulp (CIP) process with final electrowinning and smelting.
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The Vredefort Dome is a prominent 90-km-wide ring-shaped structure well exposed in the northern and northwestern sectors, and largely hidden under Karoo age rocks (300-160 Ma) in the south and southeast. The central area exposes granitic Archean basement, the circular collar consists of vertically and locally overturned metasedimentary and metavolcanic rocks of the Witwatersrand (2.9-2.7 Ga) and Ventersdorp (about 2.7 Ga) Supergroups. The Dome rocks display two particular deformation features:
(1) Pseudotachylitic breccia: veins and masses of dark grey or black extremely fine-grained silicate melt rock with numerous inclusions of non-melted host rocks. There is a spectrum from cataclasite to mylonite to pseudotachylite. The age of this unusual rock is dated at 2020 ±5 Ma (U-Pb on zircon).
(2) Shatter cones: Striated and curved fracture surfaces in shale, typical of meteorite-impact structures but not known from internal geological processes.
These features, together with mineral textures indicating shock metamorphism, point to an origin of the Vredefort structure by a meteorite impact at 2 Ga.
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The first diamond (21 ct) in South Africa was found in loose sediment of the Vaal river system in 1867, some 100 km south of Kimberley. A second alluvial diamond with 83 ct ("Star of South Africa") was found in 1869 and sparked a diamond rush to the Vaal river. The discovery of diamonds during sun-dried brick making from mud in a depression on the Bultfontein farm led to a rush of diggers from the river to this farm in what was then known as the dry diggings. Some more diamonds were found on neighboring farms. The diggers believed that they were still digging alluvials, but it turned out that these were weathered volcanic pipes which came to be known as the Bultfontein, Dutoitspan, De Beers and Kimberley pipes. The town that grew up around these pipes was called Kimberley, after Lord Kimberley the then British Governor General of the Cape Province, and the source rock of the diamonds became known as kimberlite.
The money from diamond mining in the Kimberley area was instrumental in the industrialization of South Africa, particularly the development of the gold mining industry after the discovery of the Witwatersrand gold in 1888.
The Kimberley pipe (now: Big Hole) started out as a garimpo, but then was brought under control of Barney Barnato. Cecil Rhodes owned the DeBeers pipe nearby, and bought the Kimberley pipe in 1888, as well as the Dutoitspan and Bultfontein mines which together formed the company known as De Beers Consolidated Mines. The Big Hole closed in 1914, when the diamond price collapsed. The De Beers mine closed in 1991, and Dutoitspan and Bultfontein closed in August 2005.
The Kimberley pipe was by far the richest and produced from its discovery in 1871 to its end of mining in 1914 about 14 Mct from 22.5 Mt of ore. This gives a recovered grade of 62 ct/100 t. The mine closed in 1914 at a depth of 1097 m below surface which was then the deepest mine on Earth.
The pipe cluster in the Kimberley area has an age of 90 Ma and belongs to Group I kimberlites which are derived from normal MORB-like asthenosphere. These calcite-serpentine-monticellite kimberlites have the full spectrum of mantle indicator minerals, i.e. pyrope, Cr diopside, Mg-Cr-rich ilmenite and Cr spinels.
The Boshoff Road dump is derived from tailings of the Bultfontein mine and is famous for its abundance of mantle xenoliths. These tailings still have a grade of 10-20 ct/100 t and are currently reprocessed at the De Beers treatment plant nearby. Some 100 years ago, each mine had its own treatment plant and kimberlite was mined and treated by a method known as "flooring". Flooring entailed mined rock to be laid out in the open on large tracks of flat ground and being exposed to the elements for a period of some four months. Steam tractos would run over the mined rock and the kimberlite would quickly weather and soften up without requiring crushing/milling. The loose and soft material would then be picked up and treated through a diamond processing plant. The tailings would be dumped, together with rock that had not broken down in the four months of flooring. These hard rock fragments were made up of all the xenoliths (mantle and crustal) typical of kimberlites. Pyroxenes and garnets in the mantle xenoliths allow reconstruction of the pT conditions during their formation which define a cool 40 mW geotherm with the deepest xenoliths coming from about 180 km depth (47 Kb, 1050°C).
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The iron and manganese districts of Sishen and Kuruman are located at the (present-day) western Paleoproterozoic passive margin of the Kaapvaal Craton.
The Sishen ore deposit is at the erosional contact between the Asbestos Hill Subgroup (ca 2.2 Ga) of the Transvaal Supergroup and the unconformably overlying Gamagara Subgroup (ca. 1.8 Ga) of the Olifantshoek Supergroup. The Asbestos Hill Subgroup consists of banded iron formation which near the paleoweathering surface is transformed into banded and massive iron ore. The base of the Gamagara Subgroup consists of conglomeratic iron ore and shale.
Current mining is at a rate of 200,000 t/day with a waste/ore ratio of about 2. The current annual production is 28 Mt of ore concentrate at 66 % Fe, and will increase to 39 Mt until 2009. The measured resource is 1.1 Gt, the measured + indicated resource is 1.6 Gt. Resource categories are based on drill grid density, i.e. inferred = 200-400 m drill grid, indicated = 100 m drill grid, measured = <50 m drill grid.
The mining cost is 35-50 R/t (half of this amount is for processing). Railway transport is about 40 R/t.
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These parameters define a value of ore per tonne of only 17.5 USD, i.e. marginally economic/subeconomic, particularly when compared to the recent mine developments in NW Canada with 200-800 USD/tonne ore.
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The Palaeoproterozoic Bushveld Complex, covers an area of 65,000 km2 and is the largest layered igneous complex on Earth. It consists of a mafic-ultramafic layered suite at the base (Rustenburg layered suite), a granite suite (Bushveld granites) and a sequence of heterogeneous predominantly felsic volcanic rocks of the Rooiberg Group. Both extrusive and intrusive Bushveld magmatism occurred with a time span of a few million years around 2057±3 Ma, and the layered sequence introuded below the volcanic blanket of the Rooiberg felsic suite.
The Rustenburg layered suite comprises a package of rocks, 7-8 km thick and more than 350 km wide, which range in composition from dunite to diorite. This layered suite is subdivided into Marginal, Lower (LZ), Critical (CZ), Main (MZ) and Upper (UZ) zones, although their exact boundaries have been the subject of much debate. Lateral facies variations within the sequence are common. The Marginal Zone is not always present, and the ultramafic Lower Zone varies from 800-1,700 m in thickness, or is absent. The distribution and thickness of the Lower Zone is controlled by floor topography and structure with the thickest sections developed in the eastern limb, where basal feldspathic pyroxenite with minor harzburgite, is succeeded by a lower pyroxenite, harzburgite and upper pyroxenite.
The Critical Zone, which is divided into a lower (CLZ) and an upper Critical Zone (CUZ), is of considerable economic importance as host to world-class chromite and PGE resources. The CLZ is characterized by a thick succession of orthopyroxenitic cumulates whilst the CUZ consists of partial or complete cyclic units from a base of ultramafic cumulates, chromitite, harzburgite, pyroxenite, through norite to anorthosite.
The Main Zone, which is the thickest of the zones, comprises a succession of gabbronorites in which olivine and chromite are absent and anorthosites are rare. Although not as spectacularly layered as the Critical Zone discrete packages of modally layered rocks can be identified.
The Upper Zone is some 2,000 m thick, and is characterised by up to 25 magnetitite layers in four groups.
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Mining at Impala focuses on two reefs, the Merensky Reef and the UG2 Chromitite Layer which are part of the Rustenburg Layered Suite. Both reefs sub-outcrop on Impala's property and dip approximately 10° towards the center of the Bushveld Complex. The Merensky Reef displays a gradational mineralized rock sequence composed of a chromitite layer overlain by felspathic (fine to coarse-grained) pyroxenite, and underlain by anorthosite to anorthositic norite. Highest PGE grades are in the chromitite, but hanging- and footwall are also mineralized. The Merensky Reef defines the level of a major erosional (in the igneous sense) surface. The Reef transgresses the footwall layers and locally forms "potholes", roughly circular in shape, where the Reef slumps down by up to tens of meters.
PGE grades of the Merensky Reef correlate positively with nickel and copper sulfide contents (Ni+Cu <1 %). The most abundant PGE minerals are braggite [(Pt,Pd,Ni)S], cooperite [(Pt,Pd,Ni)S], moncheite [(Pt,Pd)(Te,Bi)2], and laurite [(Ru,Os,Ir,Fe)S2].
The metal ratios in the Merensky Reef (7-8 g/t PGE+Au) are: Pt:Pd:Ru:Rh:Ir:Au=57:25:8:4:2:4.
The UG2 Reef is about 70-120 m below the Merensky Reef. Thickness of the fine-grained chromitite layer is about 60-75 cm. The base of the UG2 Reef is marked by a coarse plagioclase-orthopyroxene layer, the top by porphyritic pyroxenite. PGE mineralization is confined to the chromitite layer; highest PGE grades occur in the upper and basal contacts of the OG2 Reef. The most abundant PGE minerals are sperrylite [(Pt,Rh)(As,Sb,S)2], laurite [(Ru,Os,Ir,Fe)S2], braggite [(Pt,Pd,Ni)S], and cooperite [(Pt,Pd,Ni)S]. The metal ratios in the UG2 Reef with 8-9 g/t PGE+Au are: Pt:Pd:Ru:Rh:Ir:Au=47:26:14:9:4:1.
The UG1 Reef is about 15-20 m below the UG2 Reef and contains sub-economic low-grade PGE mineralization within up to 1m thick chromitite which locally bifurcates with lens-like layers of either anorthosite or pyroxenite in between. Alternating layers of chromitite and plagioclase cumulates, with characteristic lateral pinch-out features, are typical of the immediate footwall of the UG1 Reef.
The mining width for both reef horizons, including dilution, is about one meter. Mining operations extend to a depth of around 1000 m, a very minor amount of ore from the Merensky Reef is mined in an open pit. There are three operational shaft systems of which each develops and mines about 8 km2. Some 27,000 people are employed by Impala.
Total reserves of Impala are 210 Mt @ 5.1 g/t PGE + Au, total resources are 420 Mt @ 8.4 g/t PGE + Au.
Current production is 16 Mt ore/year yielding about 2 million ounces of PGE + Au, including about 1.1 million oz Pt (price achieved in 2005: 841 USD/oz), 500,000 oz Pd (208 USD/oz), 130,000 oz Rh (1,168 USD/oz), 7,900 t Ni (14,600 USD/t). PGE recovery for all ore is 84 %; recovery of the metallurgically difficult UG2 ore is 78 %. The total mining cost per tonne milled is 48.5 USD. The cost per PGE ounce refined is 371 USD, recalculated as cash operating cost per refined platinum ounce at 687 USD.
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The rock succession in the northern limb of the Bushveld Complex differs from the eastern and western limbs of the Bushveld Complex and varies along strike. The Main Zone is poorly developed and together with the Lower Zone only occurs in the southern part of the northern limb. The Platreef is the predominant basal unit and comprises texturally heterogeneous and variably altered pyroxenites with peridotites, gabbros and norite cycles. These onlap northwards onto successively older Transvaal metasedimentary footwall units and eventually onto Archean granite.
The Platreef PGE mineralization was discovered by Hans Merensky in 1924, followed by small-scale mining which ceased in the thirties. Renewed exploration in the 1960s resulted in mine development at Sandsloot (open pit mining since 1929) and Zwartfontein South (open pit since 2002) by Anglo Platinum.
The PGE-base metal mineralization is at the contact of the Bushveld intrusion and its Paleoproterozoic country rock (BIF, dolostone), and the irregular ore zone is up to 100 m wide and dips along the contact with about 45° (traced by drilling to 2500 m depth).
The average PGE+Au grade is about 4 g/t + about 0.1 % Ni and 0.2 % Cu.
The footwall geology has a major effect on the mineralogy (silicate, base metal and PGE content) of the ore and its metallurgical behavior. Metal recovery ranges from <20 % in calcsilicate ore to 70-80 % in serpentinite (serpentinized peridotite) to 90 % in pyroxenite.
The grade has an erratic distribution, and there seems to be strong hydrothermal overprint.
Ongoing mineralogical studies demonstrate that PGEs are mostly associated with As, Te, Bi, Sb and not in (Fe,Ni)S, as conventional wisdom would suggest. The main precious metal minerals are Pt-Pd sulfides, tellurides, arsenides, Pt-Pd alloy, electrum. Coarse-grained base metal sulfides (mm size) are locally abundant and consist of pyrrhotite, pentlandite, chalcopyrite.
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The Phalaborwa Complex is a 2050 million years old, poly-phased igneous intrusion of three volcanic pipes into granitic rocks of the Kaapvaal Craton.
The outer portion of the Complex is composed of a diopside and phlogopite bearing pyroxenite. In places the rock bears apatite and has been mined since 1951 by the state-owned company Foskor from two open pits. Nowadays production is restricted to the North Pyroxenite pit with 26000 t/day at a grade of 7.2% P2O5. Reserves are estimated at about 1 Gt. Copper grade is 0.08% Cu. A second apatite operation is currently in development in the southern part of the Complex.
The central portion of the Complex consists of carbonatite and contains copper in the minerals bornite, chalcopyrite, and valleriite. Copper is mined since 1965 by the Palabora Mining Company (PMC) producing an open pit 760 m deep and 1,600 m in diameter. Current mining is from underground (block caving) which caused a large-scale slope failure in the open pit.
The carbonatite core is surrounded by a rock called foskorite. This rock type is characterized by carbonatite veins with magnetite and olivine reaction rims in pyroxenite. Some 200 million tons of magnetite have been stockpiled as a source of iron. By-products are uranothorianite (containing U, Th and REEs) and baddeleyite (used as abrasive).
Half of the world´s vermiculite (200000 tons p.a.) is mined at Phalaborwa. Vermiculite is formed by weathering of phlogopite and is used for thermal and acoustic insulation, in lightweight aggregates and as agricultural growing medium.
Small but steep conical hills (koppies) around Phalaborwa consist of syenite, which is cogenetc with the carbonatite magmatism and widely distributed in the neighboring Kruger Park.
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The Barberton greenstone belt is one of the oldest and best exposed Archaean greenstone belts on Earth. The greenstone belt comprises volcanic and sedimentary rocks formed between 3.5-3.1 billion years ago, and has long been recognized as one of the key regions for understanding the processes relating to early crustal evolution. Metamorphic grade increases from greenschist facies in the central parts of the belt to amphibolite facies at its margins, where the condensed isograds generally parallel the contact with the granitoid plutons. At least four episodes of magmatic activity have been documented from the granitoid terrain surrounding the belt, each of which being closely associated with periods of tectonism and metamorphism within the belt. Major episodes of tectono-magmatic activity have been dated at ca. 3.45, 3.23 and 3.1 Ga, the earliest of which are commonly interpreted as a result of subduction-related crustal shortening.
Most of the deformation recorded within the belt, however, is believed to have occurred during the short-lived compressional episode at ca. 3.23 Ga, which coincides with the syndeformational deposition of the upper Fig Tree and Moodies Group sediments and the intrusion of the ca. 3227 Ma old Kaap Valley tonalite. This event was responsible for the upright, tight to isoclinal folding, the formation of thrust faults, and eventually resulted in the amalgamation of the northern and southern part of the greenstone belt along the Saddleback-Inyoka fault system. The emplacement of large volumes of sheet-like potassic granites, namely the ca. 3.1 Ga Nelspruit, Piggs Peak and Mpuluzi batholiths, marked the final stabilisation of the greenstone belt and its surroundings. It has been suggested that this late-tectonic history of the belt was associated with a change from convergent to transtensional (extensional) tectonics.
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The 3.1 Ga-old shear-zone related (orogenic) gold mineralization in the Barberton greenstone belt represents the oldest gold mineralization known on Earth. It is thought that similar ore deposits, now eroded, were the source of the Witwatersrand placer gold. There are currently six gold mines active in the Barberton greenstone belt: Fairview, Consort, Sheba (all with Barberton Gold Mines), and Agnes, Lily and Barbrook. The historical production of the Barberton area is 100 Moz = 300 t Au.
The mineralization consists of the mineral assemblage of pyrite-arsenopyrite-quartz-carbonate-fuchsite. The green fuchsite alteration and quartz flooding are very pronounced. Gold is mostly invisible (submicroscopic) and bound to the sulfide minerals.
The Sheba mine works on small-scale structures in low-grade metamorphic clastic and volcanic rocks with a production of about 10,000 t ore/month at a grade of 12-17 g/t Au. The output (CIP processing) is 100-200 kg Au/month.
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We visited some key outcrops along the Spinifex Creek and the Komati river in the Mpumalanga Park.
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We thank for financial contributions by the excursion fund of Technical University of Clausthal, and by the Rudolf-Vogel-Stiftung.
We thank for help in planning and organization of this field trip (in alphabetical order):
Carl Anhaeusser (Wits University, Johannesburg)
Grant Cawthorn (Wits University, Johannesburg)
Gaynore Cele (De Beers, Kimberley)
Hartwig Frimmel (Würzburg University, Würzburg)
Christian Gauert (University of the Free State, Bloemfontein)
George Gilchrist (AngloGold Ashanti, Carletonville)
Jens Gutzmer (Johannesburg University, Johannesburg)
Lorraine Hutton (Kumba Resources, Sishen Mine)
Dave Hutchinson (Johannesburg)
Cornelia Kleyn (Anglo Platinum, Mokopane)
Jochen Kolb (Technical University of Aachen, Aachen)
Johan Kruger (Wits University, Johannesburg)
Roelf Le Roux (Barberton Mines, Barberton)
Pierre Marais (Kumba Resources, Sishen Mine)
Glen McGavigan (Kumba Resources, Sishen Mine)
Jan van der Merwe (Foskor, Phalaborwa)
Nathi Mntungwa (Kumba Resources, Sishen Mine)
Ted Nohajer (Anglo Platinum, Mokopane)
Thomas Oberthür (BGR, Hannover)
Dominique Plouffe (De Beers, Kimberley)
Johan Pretorius (Kumba Resources, Sishen Mine)
Cobus van Rensburg (Vredefort Dome Tours and Trails, Parys)
Chris Rippon (Sheba Mine, Barberton)
Jock Robey (De Beers, Kimberley)
Trevor Rowlands (De Beers, Cullinan)
Alfred Sarila (Anglo Platinum, Mokopane)
Wynand Smit (Kumba Resources, Sishen Mine)
Sollie Terblanche (Implats, Rustenburg)
Bill Trewick (AngloGold Ashanti, Carletonville)
Karel Venter (Anglo Platinum, Mokopane)
Seef Vermaak (Implats, Rustenburg)
Pieter van Zyl (AngloGold Ashanti, Carletonville)