|TOUR SUMMARY & ORE DEPOSIT DESCRIPTIONS
Image: Chromitites on Eastern Bushveld Traverse.
This tour was developed and organised by Porter GeoConsultancy Pty Ltd (PGC) to suit the specific requirements of Amazônia Mineração Ltda (AMZ). All mine visit approvals, and the planning and organisation of the tour, including the content and expert presenters for classroom and field workshops and traverses were undertaken by PGC. This was based on specifications provided by AMZ, who then approved the plan before organisation commenced.
The tour was planned to comprise the following workshops and mine visits in South Africa:
- Johannesburg Short Course, with Prof. Grant Cawthorn.
- Rustenburg Section PGE, Merensky Reef & UG2, Western Limb,
- Western Chrome Mines, Chromitite ores, Western Limb,
- Rhovan Vanadium Mine, Magnetite-Vanadium ores, Western Limb,
- Potgietersrust Platinum Mines, Platreef PGE ores, Northern Limb,
- Bushveld Eastern Limb Traverse, Sections across the Complex,
- Marula PGE Mine, UG2 & Merensky Reef, Eastern Limb,
- Nkomati Nickel Mine, Ni-Cu-PGE in the Uitkomst Complex,
- Elandsrand Witwatersrand Mine, for gold specialists of group.
A detailed itinery outlining the visit programs, arrangements, dates, times, points at which to report, instructions/maps showing how to get to the mines, accommodation locations and contact details (names, phone/fax/e-mail) for the mines, hotels, etc., was presented to AMZ prior to the commencement of the tour to manage themselves. Accommodation and transport recommendations were also provided (as instructed) for AMZ to arrange themselves.
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Johannesburg Short Course .......... Monday 1 May, 2006.
The first day of the tour was devoted to a classroom short course on the Formation, Geology & Metallogeny of the Bushveld Complex presented by the renowned expert, Prof. Grant Cawthorn, the Platinum Industry's Professor of Igneous Petrology, School of Geosciences, University of the Witwatersrand.
Bushveld Complex Overview - The composite Rooiberg-Bushveld Complex is located in northern South Africa to the north of Johannesburg and Pretoria,
The complex was emplaced into and over the Mesoarchaean to Palaeoproterozoic Kaapvaal craton, close to its northern margin, at 2.05 Ga.
The Kaapvaal craton comprises a Palaeo- to Mesoarchaean granite-greenstone basement, overlain by a thick stratified pile of late Mesoarchaean to early Palaeoproterozoic mafic volcanic and intracratonic sedimentary rocks. These include the ~8 km thick clastic successions of the Mesoarchaean Dominion Group and Witwatersrand Supergroup, the ~5 km of mafic volcanic rocks that constitute the Neoarchaean Ventersdorp Supergroup and the 15 km late Neoarchaean to early Palaeoproterozoic (2.64 to 2.05 Ga), Transvaal Supergroup, composed of intracratonic clastic, chemical (BIFs) and carbonatic sedimentary and lesser volcanic rocks. This pile remained relatively undeformed, because by the late Mesoarchaean, the craton was sufficiently rigid (stabilised) to retard the development of the unstable granite-greenstone tectonics seen at the same period on many other cratonic nuclei (Eglington and Armstrong, 2004).
The Transvaal Supergroup is unconformably overlain by the Rooiberg Group (sometimes included as the uppermost volcanic unit of the Transvaal Supergroup; e.g., Button, 1986). This group is almost entirely composed of volcanic rocks that total up to 3.5 km in thickness and are preserved over >50 000 km2, although they are estimated to have originally covered an oval-shaped 450 x 350 km (>110 000 km2) area (Kinnaird, 2005). The Rooiberg Group occurs directly over the centre of, and forms the intruded roof to the Bushveld Complex. It has been subdivided into four parts, as follows:
i). Dullstroom Formation, comprising at least three compositional groups, low- and high-Ti mafic to intermediate units, and high-Mg felsic units;
ii). Damwal Formation, marking the last high-Mg felsites and the first sedimentary intercalations and pyroclastics, accompanied by high-Fe, Ti, P volcanics, overlain by dacites and rhyolites;
iii). and iv). Kwaggasnek and Schrikkloof Formations, dominated by dacitic pyroclastics, rare rhyolites flows and intercalated sedimentary horizons. Precise U-Pb zircon dating of Kwaggasnek Formation felsites gives an age of 2059.9±1 Ma, consistently younger than the main layered mafic stage of the Bushveld Complex (zircons from the Merensky Reef and from late stage pegmatoids give ages of 2055.3±1.2 Ma and 2056.3±0.7 Ma respectively; Armstrong et al., 2010). The Rooiberg Group is conformably overlain by the Loskop formation clastic sediments with minor volcanic intercalations. Clastic sediments in this latter unit include eroded clasts of the main mafic stage of the Bushveld Complex (Schweitzer et al., 1995; Kinnaird, 2005).
Widespread, but not voluminous, 2061.8±5.5 Ma granophyre masses of the Rashoop Granophyre Suite are found below the Rooiberg Group, but above the layered mafic phases of the Bushveld complex which it predates (Kinnaird, 2005).
The main mafic to ultramafic phase of the Bushveld Complex, the 7 to 8 km thick Rustenburg Layered Suite (RLS), was emplaced as a series of sub-concordant sills along the unconformable contact between the top of the Transvaal Supergroup and the base of the Rooiberg Group, with the floor being the upper sections of the Pretoria Group. The RLS dips to the north, and covers an area of ~65 000 km2, elongated east-west parallel to the margin of the craton. It was formed in five chambers that were most likely connected with each other and with a sixth at depth. The RLS was fed by repeated injections of magma from depth, changing composition with time, and has a complex-wide "stratigraphic layering", including, from the base, the:
• Marginal Zone, medium grained, unlayered rocks which underlay much of the complex, mostly norite;
• Lower Zone, 1450 m thick of bronzitite, harzburgite and dunite;
• Critical Zone, 1400 m thick of a strongly layered lower pyroxenite series of orthopyroxene cumulates comprising norites, peridotites and chromitites (the main Cr ores), followed by an upper, strongly layered anorthosite series of harzburgite, pyroxenite, through norite to anorthosite with chromitite layers (containing PGE ores, e.g., the UG2, with lesser associated Ni-Cu);
• Main Zone, 2800 m thick containing the Merensky Reef (the main PGE host) then massive gabbronorite, with infrequent anorthosite and pyroxene bands; and
• Upper Zone, 2100 m thick, of strongly banded gabbro, Fe-olivine gabbro and magnetite layers (the main V-Fe ores).
The RLS occurs in four discrete lobes, namely the:
i). Western Lobe, which extends from near Pretoria westwards to Rustenburg and around the Pilanesberg alkaline complex, to and along the southern flank of the Makoppa Dome of the Archaean granitiods and gneisses;
ii). South-eastern or Bethal Lobe, which is largely covered by Mesozoic rocks;
iii). Eastern Lobe, that extends in a northwards arc west of Belfast up to Atok; and
iv). Northern Lobe that extends south of Mokopane to Villa Nora. In addition, a Far Western Lobe is indicated to the west of the Western Lobe.
In the eastern lobe of the complex, the Upper Zone of the RLS is characterised by banded gabbros with up to 25 layers of cumulate, massive and net-textured, titaniferous and vanadiferous magnetite, the thickest being 7 m, while the most consistent averages 2 m. Each layer has a sharp base and gradational upper margin (Kinnaird, 2005; Cawthorn and Molyneux, 1986). Sections of the Upper zone are also cut by magnetite breccia pipes of similar composition to the layers, reflecting upward expulsion of magnetite from the cumulate layers (Cawthorn and Molyneux, 1986).
The final phase of the Bushveld Complex is the 1.5 to 3.5 km thick, sheeted Lebowa Granite Suite (LGS) that has an areal extent of 30 000 km2, and with ages of 2054±2 Ma (Armstrong et al., 2010) postdates the RLS. The granite underlies the older, less dense Rooiberg Group and Rashoop Granophyre Suite, but overlies the denser RLS, through which feeder dykes are mapped. Wilson et al. (2000) suggest foliations and lineations are horizontal, reflecting vertical host-rock compression and horizontal magma flow during emplacement, with space being created for the granites by roof uplift and floor depression. They are predominantly alkali feldspar granites with iron-rich ferromagnesian minerals and have been categorised as A-type (Kleeman and Twist, 1989).
All of the data available suggests the whole complex, including the Rooiberg Group volcanic rocks, associated granophyres, A-type granites of the LGS, and the layered ultramafic complex of the RLS, was emplaced over a period of only 3 to 5 m.y. (Kinnaird, 2005; Armstrong et al., 2010). Kinnaird, 2005 discusses a range of opinion on the formation of the Bushveld Complex, but favours the Kaapvaal craton having undergone northeast-southwest extension and strike-slip reactivation, as suggested by the lack of pre-Bushveld deformation of the Transvaal Supergroup and its preservation over large parts of the craton, as well as the generation of A-type granites, usually associated with crustal extension. In addition, the preservation of the volcanic and shallow-level intrusive rocks of the Bushveld Complex indicates that the significant magmatic thickening related to the Bushveld event must have been compensated by coeval crustal thinning (Gibson and Stevens, 1998). However, as Late Proterozoic to Cretaceous diamondiferous kimberlites in the adjacent Kaapvaal contain a ~3.1 Ga diamond population (Richardson et al., 1984; Shirey et al., 2003), a lithospheric root in excess of 140 km must have existed beneath the craton in the Archaean and survived the Bushveld event (Gibson and Stevens, 1998). Gibson and Stevens (1998) suggest from a study of the deep section of the crust exposed by the 2.02 Ga (post-Bushveld) Vredefort Impact Structure, that the metamorphic profile revealed is consistent with heating by craton-wide intraplating of mantle-derived magmas when a mantle plume head reached the base of the Kaapvaal lithosphere at ~2.06 Ga and underwent partial decompression melting.
The heat released from these magmas resulted in regional metamorphism of the adjacent crust, and anatexis of the deeper, high-grade sections of this metamorphosed terrane to produced the magmas that rose to form the felsic volcanic rocks of the Rooiberg Group, and high-level granophyre intrusions. After the anatectic Rooiberg Group volcanic and Rashoop Granophyre intrusive rocks were emplaced, pulses of progressively fractionated and contaminated mafic to ultramafic magmas from the deep chamber were ejected and rose to be ponded beneath the less dense felsites. At a later stage, further anatectic felsic magma from the wall rocks of the deeper chamber produced the LGS granites, rising though the denser RLS, to be emplaced at their level of buoyancy immediately below the Rooiberg felsites.
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Anglo Platinum, Rustenburg Section PGE Mine .......... Tuesday 2 May, 2006.
The Anglo Platinum, Rustenburg Section operations, located to the east of the town of Rustenburg in North West Province, South Africa, exploit ores from both the Merensky and UG2 Reefs in the Critical Zone in the southwestern section of the Western Lobe of the Bushveld Complex. The Rustenburg Section comprises 5 adjacent mines Bathopele, Khomanani, Thembelani, Khuseleka and Siphumelele (#Location: 25° 41' 11"S, 27° 19' 16"E).
For background on the geology and mineralisation of the Bushveld Complex, see the separate Bushveld Complex record.
The Merensky Reef is a regular, persistent tabular body comprising a pegmatoidal, feldspathic pyroxenitic assemblage near the top of the Upper Group of the Upper Critical Zone of the Bushveld Complex. It is defined by thin top and bottom chromitite layers and is generally around 30 cm in thickness, seldom exceeding 1 m in the Rustenburg Section, with abundant associated, and generally highly disturbed, ovoid shaped (in plan) depressions, known as "potholes" making up about 15% of its area. "Potholes" represent areas where the base of the Merensky Reef disconformably transgress down, for distances of a metre to several tens of metres, into the underlying "stratigraphy" of the Critical Zone over widths of a few metres to hundreds of metres.
Some 75% of the PGEs and base metals are within the Merensky Reef, with the remainder occurring in the hangingwall and/or footwall. The highest grade encountered is around 8 ppm PGE+Au over a mining width of 90 cm (4.82 ppm Pt, 2.04 ppm Pd, 0.66 ppm Ru, 0.24 ppm Rh, 0.08 ppm Ir, 0.26 ppm Au), 0.12% Cu, 0.26% Ni. The pegmatoidal feldspathic pyroxenite of the Merensky Reef generally contains between 3 and 10% sulphides dominated by pyrrhotite, pentlandite, chalcopyrite, pyrite and cubanite with minor sulpharsenides, galena and sphalerite. Phlogopite is usually associated with the sulphides.
PGEs are present in a variety of forms, from Pt-Pd sulphides (30%), lesser tellurides and arsenides (11%) and sperrylite (6%). PGE-Au/Ag alloys are important, as are Ru sulphides (10%), etc. The PGEs occur in three associations in decreasing order of importance i). enclosed in or attached to base metal sulphides; ii). enclosed in silicates; or iii). enclosed in or attached to chromite or Fe-oxide. The composition of PGE is very similar in the Merensky Reef and the underlying UG2 at any point. To the east of the Rustenburg Section the Merensky Reef thickens appreciably to +10m, although the mineralised width is similar, but confined to the upper margin.
The UG2 chromitite occurs between 15 and 400 m below the Merensky Reef and like the Merensky Reef, can be traced over a strike length of more than 280 km. At Rustenburg it is around 70 cm thick, but averages 97 cm overall. "Potholes" are also common in the UG2. Chromite comprises 60 to 90% of the UG2 which averages 43.5% Cr2O3. The main base metal sulphides are pentlandite and chalcopyrite, with minor pyrrhotite, pyrite, arsenopyrite, bornite, chalcocite, covellite, galena & millerite. PGEs are found in the base, centre and top of the UG2 chromitite, averaging between 3.5 and 19.5 ppm PGEs, with approximately 3.6 ppm Pt, 3.8 ppm Pd, 0.33 ppm Ru and 2.26 others. Cu & Ni are typically <1000 ppm.
The ore reserves and resources at the Rustenburg Section Mines at the end of 2005 (Angloplats, 2006) were:
Proved + Probable Reserve - 50.6 Mt @ 5.63 g/t 4E (Pt+Pd+Rh+Au);
Measured + Indicated + Inferred Resource - 111.3 Mt @ 7.54 g/t 4E (Pt+Pd+Rh+Au);
Proved + Probable Reserve - 198 Mt @ 3.55 g/t 4E (Pt+Pd+Rh+Au);
Measured + Indicated + Inferred Resource - 291 Mt @ 4.73 g/t 4E (Pt+Pd+Rh+Au).
During 2005 some 11.85 Mt of ore (57% UG2) were milled at a head grade of 4.34 g/t PGE for a recovered production of 25.6 t (0.822 Moz) Pt; 12.8 t (0.401 Moz) Pd; 3.56 t (0.114 Moz) Rh; [47.46 t (1.53 Moz) Total PGE's]; 1.43 t (0.046 Moz) Au; 6300 t Ni, 3500 t Cu. The mine is owned and operated by Rustenburg Platinums Mines Ltd, a wholly owned subsidiary of Anglo Platinum Corporation Ltd, which is in turn 50.2% held by Anglo American plc.
The ore reserves and resources at the Rustenburg Section Mines at 31 December, 2011 (Angloplats, 2012) were:
Proved + Probable Reserve - 32.9 Mt @ 5.32 g/t 4E (Pt+Pd+Rh+Au),
Measured + Indicated + Inferred Resource - 78.1 Mt @ 6.55 g/t 4E (Pt+Pd+Rh+Au),
Proved + Probable Reserve - 186.9 Mt @ 3.63 g/t 4E (Pt+Pd+Rh+Au),
Measured + Indicated + Inferred Resource - 212.0 Mt @ 5.29 g/t PGE.
NOTE: All of the chromitites layers (7 in the Lower Group, 4 in the Middle Group and 2 in the Upper Group) of the Lower and Upper Critical Zone contain significant concentrations of PGE, although only the Merensky and UG2 are exploited primarily for PGEs. They are also the only reefs with significant associated sulphides.
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Samancor, Western Chrome Mines .......... Wednesday 3 May, 2006.
The Samancor Western Chrome Mines division, operates three mines centred around 40 km east of Rustenburg, distributed over much of a 50 km stretch of the southern rim of the Western Lobe of the Bushveld Complex in the North-west Province of South Africa. These are the Elandsdrift open pit and the two underground mines, Waterkloof-Millsell and Mooinooi.
For an outline of the setting, geology and distribution of mineralisation within the Bushveld Complex, see the separate Bushveld Complex record.
Western Chrome Mines division produces approximately 1.8 Mt of run-of-mine ore per year. The combined Western and Eastern Chrome Mines divisions of Samancor have proven reserves to a depth of 300 m, totalling 450 Mt of ore. The Eastern Chrome Mines division is developed over a 100 km section of the Eastern lobe and produces 2.2 Mt of ore per year. Samancor is an unlisted JV between BHP Billiton (60%), Anglo American Corporation (40%).
Within the three mines different chromitite layers are being mined, a reflection of the variation in thickness and grade along trend. Dips are generally 9 to 10° to the north. At Millsell and Elandsdrift the principal layer mined is the LG6 within the Lower Critical Zone of the Bushveld Complex. LG6 is regionally 0.5 to 1.05 m thick, containing 46 to 48% Cr2O3, and is the thickest of the 7 LG series chromitites.
At Millsell LG6 is commonly around 0.80 to 0.85 m thick (but may be up to 1.15 m in potholes) with a second band, LG6A , separated by a low grade to barren parting of some 0.45 to 0.55 m thick, then the overlying 0.25 to 0.30 m LG6A. Potholes (see description and definition in Rustenburg PGE above) are generally <80 m in diameter. The LG6 chromitite, as mined at Millsell, averages 41.5% Cr2O3 with a Cr:Fe ratio of 1.5:1. LG6 is found within pyroxenite above the upper of the two Harzburgite units of the Lower Critical Zone. It is an accumulate-type, coarse granular chromite, comprising 97% of the band, with a 3% gangue of orthopyroxene, clinopyroxene, plagioclase and accessory biotite, quartz, sulphides, carbonate and chlorite. The chromite is generally friable, with grains varying from 50 µm to 2 mm. Contacts between the pyroxenite and chromitite may be either sharp or gradational.
In contrast, at Mooinooi the MG chromite layers, within the upper sections of the Lower Critical Zone, attain mineable widths of 1.35 to 1.5 m and 44 to 46% Cr2O3, and the LG6 bands are only present as a series of stringers.
The Cr:Fe ratio of the chromitites, decreases generally, but not regularly, upwards through the Critical zone, from around 1.50 in the LG6, to 1.43 in MG4 to 1.34 in the UG2. While the UG2 is a prime PGE source in the lobe, the lower grade Cr content means its chromite is not saleable. The number of layers and the Cr:Fe ratio both decrease towards the west within the Western Lobe.
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Xstrata, Rhovan Vanadium Mine .......... Thursday 4 May, 2006.
The Rhovan iron-vanadium mine is located approximately 35 km ENE of Rustenburg in the North West Province of South Africa, and lies within the Western Lobe of the Bushveld Complex.
It exploits vanadium bearing titaniferous magnetite bands in the Upper Zone of the Bushveld Complex. The mafic component of the Bushveld Complex (known as the Rustenburg Layered Suite) has been subdivided into five zones, from the base, the Marginal, Lower, Critical, Main and Upper Zones respectively. For background on the geology and mineralisation of the Bushveld Complex, see the separate record on the complex.
The Main Zone, which underlies the host Upper Zone, is composed of gabbros grading upwards into gabbronorites, with several anorthosites and a distinct Pyroxenite Marker two thirds of the way up the Zone. The base of the Upper Zone is defined by the first appearance of cumulus magnetite above the Pyroxenite Marker. This zone is composed of gabbro, anorthosite and a roof zone of olivine diorite, and has been divided into: i). Subzone A at the base, composed of 700 m of anorthosite and magnetite ferrogabbro, and more than 11 magnetite layers; ii). Subzone B, where cumulus Fe-rich olivine appears, composed of around 580 m of anorthosite, troctolite, and olivine and magnetite ferrogabbro containing 7 more magnetite layers; and iii). Subzone C where apatite is present as an additional cumulus phase in a 1000 m thick sequence of olivine diorite with anorthosite and magnetite rich diorite and a further 7 magnetite layers.
The Upper Zone layered mafic rocks carry approximately 8% magnetite disseminated in gabbroic rocks, plus a further cumulative 20 m of pure magnetite distributed in 25 discrete magnetite layers. The magnetite layers are concordant and of different thicknesses, V2O5 and TiO2 grade, but with persistence over considerable distances, both along strike and down dip. On the Eastern Lobe of the Complex there are also crosscutting plugs of almost pure magnetite in both the Main and Upper Zones (the largest had an ovoid plan dimension of 300x100 m).
Vanadium occurs in solid solution within the magnetite layers. The highest vanadium contents of around 1.6% V2O5 occur in the lowermost layers, decreasing to about 0.25% in the uppermost part of the Upper Zone. Titanium varies in an inverse manner from around 11% TiO2 in the lower layers to 18% in the uppermost.
The magnetite is characteristically black, highly magnetic, commonly coarsely granular (often up to 1 cm across) and closely packed. The approximately 5% ilmenite grains are interspersed and more variable in size, shape and distribution, being present as individual grains, elongated intergranular bodies or exsolution lamallae parallel to the octahedral planes of magnetite. The magnetite comprises intergrowths of both Ti-poor and Ti-rich magnetite phases.
The highest V2O5 grades over reasonable widths are confined to the Main Magnetite Layers which are approximately 130 m above the base of the Upper Zone, in Subzone A. In the Western Lobe of the Bushveld Complex, the lower magnetite layers, which include the Main Magnetite Layer, extend from north of Pretoria, westward to Rustenburg, then northwestward towards the Pilanesberg before trending north-easterly towards Thabazimbi. In the vicinity of Rhovan, the Main Magnetite Layer is actually three separate thick magnetite layers. The lower two are separated by less than 5 m, whereas the upper may be anywhere between 28 and 200 m higher. The lowest averages around 1 m in thickness with 1.8% V2O5, while the middle averages near 1.6 m at 1.3% V2O5, and the upper 1.6 m at 1.4% V2O5
Open pit reserves and resources at Rhovan in June 2005 included:
Proved Ore Reserves - 44 Mt @ 0.5% V2O5 + Probable Ore Reserves - 6 Mt @ 0.7% V2O5
Measured Mineral Resource - 77.5 Mt @ 0.5% V2O5 + Inferred Mineral Resource - 139.8 Mt @ 0.5% V2O5.
The Rhovan Mine is 100% owned by Xstrata Alloys.
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Anglo Platinum's Potgietersrust PGE Mine .......... Friday 5 May, 2006.
The Mogalakwena, formerly the Potgietersrust Platinums mine, exploits the Platreef on the Eastern limb of the eastern margin of the Bushveld Complex Northern Lobe in the Limpopo (Northern) Province of South Africa, some 240 km to the north-east of Rustenburg and 20 km NW of the town of Mokopane.
The Potgietersrus Limb extends for more than 100 km in a north-south direction. The base of the Bushveld mafic suite transgresses from the Pretoria Group quartzites and shales, through the Penge Iron Formation and Malmani Dolomite to basement granite gneiss in the north. The Lower Zone and the chromite bearing lower sections of the Critical Zone of the Complex are only fully represented in the far south. To the north they occur as disjointed masses in the basement rocks and disappears altogether near Potgietersrus.
The lowermost Bushveld unit over much of the limb is the upper sections of the Critical Zone. A sulphide bearing composite pyroxenite layer of variable thickness occurring near the base of the main layered mafic mass is known as the Platreef and hosts the important PGE deposits with associated Cu & Ni at Potgietersrus where it rests on basement. The 30 to 50 degree west dipping Platreef pyroxenite, which is not layered, has been correlated with the Merensky Reef found elsewhere in the Bushveld Complex. It has been variably and extensively contaminated by the basement, particularly iron formation, shales (graphitic in part) and anhydrite bearing dolomite, with abundant xenoliths which have significantly influenced its chemistry.
The Platreef varies from a few up to 150 m in thickness and is composed of three pyroxenite units, or 'reefs', as follows: i). the upper pyroxenite, or 'C' Reef which is usually barren; ii). the middle pyroxenite, or 'B' Reef - a coarse grained bronzitite with minor inter-cumulus plagioclase, very variable, but overall roughly equal orthopyroxene and clinopyroxene, virtually no olivine, some chromite and fair to good grades of base metal sulphides; iii). the lower pyroxenite or 'A' Reef - a highly feldspathic unit with a marked heterogenous texture and grain size, graphic plagioclase-quartz intergrowths, disseminated mineralisation and large blebs of composite base metal sulphides.
At the Potgietersrust Platinum Mine, the ore is hosted by the 'B' Reef. Four separate ore zones have been delineated based on a 3 g/t PGE cutoff. The uppermost of these zones is the thickest (2 to 39 m thick) and most consistent. Cu ranges from 0.1 to 0.25%, Ni from 0.15 to 0.35%, while PGEs vary from <0.25 to15 g/t, and locally up to 25 g/t, with a Pt:Pd ratio of around 1:1. Where Cu+Ni is from 0.2-2%, there is a strong correlation between base metals and PGEs. The PGE contents of the Platreef do not vary with the thickness of the mineralised pyroxenite.
Published ore reserves and mineral resources at the end of 2005, at a 1 g/t PGE cutoff were:
Proved + probable reserves - 335.9 Mt @ 3.23 g/t 4E (Pt+Pd+Rh+Au);
Measured + indicated + inferred resources - 2393.7 Mt @ 2.05 g/t 4E (Pt+Pd+Rh+Au).
The ore reserves and mineral resources at the Mogalakwena Mine at 31 December, 2011 (Angloplats, 2012) were:
Proved + Probable Reserve - 725.4 Mt @ 2.90 g/t 4E (Pt+Pd+Rh+Au);,
Measured + Indicated + Inferred Resource - 2785.4 Mt @ 2.18 g/t 4E (Pt+Pd+Rh+Au).
During 1999 some 56.8 Mt of ore + waste was broken and 4.535 Mt of ore were milled at a head grade of 4.03 g/t PGE for a recovered production of 6.2 t (0.20 Moz) Pt; 6.5 t (0.21 Moz) Pd; 0.43 t (0.013 Moz) Rh; [13.78 t (0.443 Moz) Total PGE's]; 0.67 t (0.022 Moz) Au; 4600 t Ni; 2700 t Cu. The stripping ratio was 7.86:1. Mill feed is based on a 2.5 g/t cutoff, while stockpiled lower grade +1.7 g/t mineralisation is counted as ore in calculating the stripping ratio (1999). The mine is operated by Potgietersrust Platinums Ltd, a wholly owned subsidiary of Anglo Platinum Corporation Ltd, which is in turn 50.2% held by Anglo American plc.
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Eastern Bushveld Field Workshop & Traverses .......... Saturday 6 & Sunday 7 May, 2006.
A comprehensive series of traverses across the Eastern Limb of the Bushveld Complex to study its "magmatic stratigraphy", characteristics and the distribution and nature of the mineralised layers through the sequence.
Other locations of interest illustrating key points of the complex will also be inspected. The Eastern lobe was selected because of the better exposure available.
This series of traverses, which were undertaken in a very concentrated two day period, were designed and guided by the internationally known and published expert on the Bushveld, Dr Johan Kruger.
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Impala Platinum's Marula PGE Mine .......... Monday 8 May, 2006.
The Marula platinum mine, is located near the town of Burgeresfort in Limpopo Province, South Africa. The mine is operated by Marula Platinum Limited, a member of the Implats Group.
Marula has reserves of platinum mineralisation within both the Merensky Reef and UG2 Chromitite in the Critical Zone of the Eastern Limb of the Bushveld Complex. For details of the Complex and the position and characteristics of the Critical zone and the Merensky Reef and the UG2, refer to the Bushveld Complex entry.
At Marula the UG2 and Merensky orebodies have an average dip of 13° to the southwest,with a vertical separation of approximately 400 metres. While one prominent dyke and a dunite pipe are present, there is minimal faulting on the property.
The mining operation is initially concentrating on the UG2 orebody, with substantial resources having also been outlined in the Merensky Reef.
The Probable UG2 Mineral Reserve in June 2004 was 41.3 Mt @ 4.76 g/t PGE+Au.
The Indicated UG2 Mineral Resource in June 2004 was 22.4 Mt @ 9.27 g/t PGE+Au.
The Indicated Merensky Mineral Resource in June 2004 was 44.2 Mt @ 5.47 g/t PGE+Au.
The PGE+Au split is: Pt=36.8%; Ru=11.3%; Pd=40.4%; Ir=2.6%; Rh=7.7%; Au=1.2%. Overall recovery rates are in the region of 87%.
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LionOre - ARM's Nkomati Ni-Cu-PGE-Co Mine .......... Tuesday 9 May, 2006.
The Nkomati Ni-Cu-Co-PGE deposit is located within the Uitkomst Complex, an 8x1 km and up to 650 m thick, NW-SE trending, flat lying, poorly layered, sill like, 2025 Ma mafic body of harzburgite, gabbro, pyroxenite and gabbro-norite. Nkomati is approximately 325 km east of Rustenburg. The deposit is also some 75 km to the east of the south-eastern lobe of the main 2050 Ma Bushveld Complex, with which it is believed to be genetically associated. It plunges at around 4 degrees to the NW and cuts basement rocks of the Transvaal Supergroup, including quartzite, shales (in part graphitic), the Malmani Dolomite and Archaean granitoids.
The Uitkomst Complex comprises: i). a basal 6-30 m thick gabbro (possibly an earlier sill), overlain by ii). a sulphide bearing lower harzburgite (50-90 m) containing sedimentary xenoliths, iii). chromitiferous harzburgite (±60 m), iv). main harzburgite (±300 m), v). upper pyroxenite (60 m) and vi). upper gabbro-norite (±250 m). Drilling and field relations suggest the complex was initially emplaced discordantly at the base of the Transvaal Supergroup bounded by two NW-SE trending fracture systems.
The Uitkomst complex contains three disseminated, sulphide mineralised zones hosted by the basal gabbro and lower harzburgite, namely the Basal Mineralised Zone (BMZ), Main Mineralised Zone (MMZ) and the Chromititic Pyroxenite Mineralised Zone (PCMZ) for a resource in 2005 of 139 Mt @ 0.49% Ni, 0.19% Cu, 0.02% Co and 1.04 g/t PGE+Au.
Massive sulphide mineralisation is found in the immediate footwall of the Complex, within the Transvaal sediments and the Archaean granite gneiss. The orebody consists of three lenses separated by mafic/ultramafic intrusions and is accessed by means of two vertical shafts and a decline. The initially exploited orebody, the MSB, had an initial reserve of 2.86 Mt @ 2.04% Ni, 1.13% Cu, 0.10% Co, 1.65 g/t Pt, 4.18 g/t Pd, 0.16 g/t Rh, 0.18 g/t Au. The current (2005) resource of the MSB is 13.2 Mt @ 2.98% Ni, 0.97% Cu, 0.17% Co, 5.62 g/t PGE+Au.
The Combined Mineral Reserves for the current mine and expansion project (Dec 2004) including the MSB and MMZ zones was: 51.7 Mt @ 0.52% Ni, 0.2% Cu, 0.03% Co, 1.15 g/t PGE+Au.
In addition, the Uitkomst Complex hosts massive to semi-massive chromitite mineralisation within the chromitiferous harzburgite.
The Nkomati mine is a joint venture between African Rainbow Mines (50%) and LionOre (50%).
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Harmony's Elandsrand Gold Mine .......... Wednesday 10 May, 2006 - an additional visit for the gold specialists of the tour group.
The Elandsrand Witwatersrand gold mine is part of the Harmony Elandskraal operation on the Far West Rand Gold Field. The primary reefs exploited are the Ventersdorp Contact Reef (VCR) and the Carbon Leader which are separated by 900 to 1300 m, increasing from east to west. Secondary targets are the Middlevlei Reef (50 to 75m above the Carbon Leader) and the Mondeor Conglomerate Reef Zone, which subcrops on the western side of Elandsrand. Faulting and off-set of the reefs is generally less marked than in other Witwatersrand goldfields, with the structure being dominated by a series of east-west trending normal faults with throws of up to 40 metres, as well as a series of north-northeast striking normal faults with generally smaller displacements in the north-west.
The total reserve at Elandsrand in 2005 was 25.7 Mt @ 9.28 g/t for 238 t of contained gold. The total resource at Elandsrand in 2005 was 76.4 Mt @ 8.06 g/t Au, while the Elandskraal operation as a whole has a total resource of 120.8 Mt @ 7.00 g/t Au, for 845 tonnes of contained gold.
The Witwatersrand gold - uranium mines in north-central South Africa, between Johannesburg in Gauteng and Welkom in the Free State, have contributed over 50% of all of the gold produced in the world to date. These mines are distributed around the arcuate, active, fault bounded, north to north-western to western margins of the northeast-southwest elongated, 390 x 250 m Witwatersrand Basin which has a gently downwarping, more passive SE margin.
Production from the basin has come from more than 150 mines which have extracted over 4.5 Gt of ore to yield more than 42 000 t of Au at an average grade of 9.3 g/t Au. It has also produced nearly 145 000 t of U3O8 from 675 Mt of ore in 31 mines at an average grade of 0.215 kg/t U3O8. These figures relate to the period from 1887 to 1987 for Au and 1952 to 1987 for U (Pretorius 1991).
The mines are distributed within a series of gold fields that are, in anti-clockwise order from the north-east: the Evander, the over a gap of approximately 60 km across a basement granite to the contiguous East Rand, Central Rand (in the Johannesburg area) West Rand, South Deeps, Western Areas and Carltonville gold fields which represent around 150 km. After a gap of 80 km, the Klerksdorp gold field is located to the south-east of Klerksdorp. Approximately 110 km to the south the Welkom gold field covers and area of around 70 x 20 km centred on the city of Welkom on the southern tip of the basin.
The Witwatersrand Basin is one of five basins that occupy the central portion of the Archaean Kaapvaal craton. The axes of these generally northeast-southwest elongated basins progressively migrated from south-east to the north-west. They comprise the i). Pongola, represented by limited remnant sediments which are broadly contemporaneous with the lower parts of the Witwatersrand Basin to the north-west; ii). Witwatersrand (approximately 2985 to 2715 Ma); iii). Ventersdorp (approximately 2.7 to 2.65 Ma); iv). Transvaal (2.25 to 2.0 Ga); and v). Waterberg Basins (1.9 to 1.75 Ga).
The oldest basement rocks beneath these basins includes the ~3.64 Ga Ancient Gneiss Complex in Swaziland to the east, on the southern margin of the Barberton greenstone belt which represents 3.49 to 3.42 Ga basic to ultra-basic magmatism following 3.55 to 3.52 Ga tonalitic intrusion. Granite-greenstone and granitoid terranes, including the Murchison, Pietersberg and Giyani belts are extensively exposed or sub-cropping to the east and north of the Witwatersrand Basin as well as those immediately underlying the basin. The Murchison greenstone terrane to the north includes granitoids dated at 2901±12 Ma with some dates at 2820 to 2811 Ma. The Amalia-Kraaipan granitoid-greenstone to the west of the Witwatersrand Basin includes 3033±1 Ma mafic intrusives, a 2930 Ma volcanic arc and high level granitoids as young as 2767±6 Ma. The Kaapvaal greenstone belts include banded iron formations and pass upwards into a clastic sequence. A change from compressional to transtensional tectonics at around 3.1 Ga corresponded to the development of orogenic greenstone belt gold deposits of the Barberton gold field.
This change of tectonic regime marked the commencement of intracratonic basin formation. The lowest unit of this domain commences with a thin basal siliciclastic unit, overlain by a 13 000 sq. km, 2250 m thick, 3086±3 Ma to 3074±6 Ma bimodal volcanic sequence which comprises the Dominion Group. This sequence comprises 45% andesitic to rhyolitic rocks; 35% conglomerate, feldspathic quartzites and shaly quartzites; and 20% shales. The basal siliciclastics include a mineralised conglomerate with abundant uraninite and pyrite with relatively low gold. The Dominion Group was followed after a break of 100 m.y. by the lower unit of the Witwatersrand Supergroup.
The Witwatersrand Supergroup is divided into two groups:
West Rand Group - which extends over an area of 43 000 sq. km, is up to 5150 m thick and rests on the Dominion Group above an angular unconformity. It is thickest in the Klerksdorp area in the west, and thins to the north-east. Age dating restricts the deposition to between 2985±14 Ma and 2914±8 Ma, the age of volcanics in the upper part of the group. It has been sub-divided into three sub-group, based on varying sandstone/shale ratios and basinwide disconformities, namely the i). Hospital Hill Sub-group, is primarily composed of shallow marine deposits, mainly shales and orthoquartzites, comprising up to 2500 m of conglomerates, sub-greywacke, sub-arkose, ortho-quartzites, siltstones, shales, ferruginous shale and banded ironstone; with no volcanics, approximately 50% quartzites/conglomerates and 50% shales; the ratio of sandstone/shale increases upwards, with the sandstone being predominantly quartz-arenite; the unit contains no economic reefs; ii). Government Sub-group, characterized by more diverse lithologies which formed in a far greater variety of depositional settings, and include diamictites (possibly of glacial origin), interbedded beds of fluvial conglomerates and quartzite, shelf shales and quartzite, and iron formations. The base of the Subgroup is represented by a widely correlated diamictite which lies on a major, regional unconformity; the unit contains three subeconomic auriferous reefs; iii). Jeppestown Sub-group, which commences with a quartzite overlying a major regional unconformity and represents a return to a predominance of shelf deposits. It is up to 1200 m in thickness and comprises interbedded conglomerates, feldspathic quartzites, siltstones, shales, ferruginous shale, calcareous shale and andesitic volcanics; comprising approximately 30% volcanics, 30% quartzites/conglomerates and 40% shales. Two sub-economic reefs have been recognised within the unit.
Central Rand Group - which is up to 2880 m thick in the central part of the basin around the Vredofort structure and unconformably overlies the West Rand Group, covering an area of 9750 sq. km with a basinal extent of around 290 x 150 km. Similar to the West Rand Group, this group is composed of a series of cycles, each comprising fluvially dominated coarse grained siliciclastics overlying an erosional surface, although in contrast to the West Rand Group, the coarser lithologies predominate with subordinate shale or mudstone. Age constraints on the group places deposition between a lower limit of 2902±13 Ma and minimum age of 2840±3 to 2780±3 Ma. Two sub-groups have been defined, namely the:
Johannesburg Sub-group - The base of this unit is the first prominent conglomerate overlying quartzite of the Maraisburg Formation at the top of the Jeppestown Sub-group. This conglomerate is widely recognised and occurs as the North Reef in the Central Rand Gold Field (GF); Rock Tunnel Reef in the West Rand GF; North leader in the Carletonville GF; and the Ada May Reef in the Klerksdorp and Welkom GFs. This conglomerate and the overlying quartzite, which together comprise the Blyvooruitzicht Formation, are both locally truncated by an erosion surface upon which another widespread conglomerate was deposited.
The overlying Main Formation contains the economically important Main Reef, Carbon Leader and Main Reef Leader in the Central and West Rand and at Carletonville, and has been correlated by the prominent, shale-filled erosion channels (Green Bar of Carletonville, Black Bar of the Central Rand), which have been recognized in almost all of the goldfields. Although widespread, this unit was truncated by the overlying sequence in the eastern portions of the Witwatersrand basin.
The overlying Randfontein Formation consists of quartzite and conglomerates, which along the northern margin of the basin contains, at its base, the Nigel Reef of the East Rand, the South Reef of the Central Rand and the Middelvlei Reef in the Carletonville area. Several conglomerates are developed within the succeeding quartzite, which degenerated to pebbly quartzites regionally. This sequence of conglomerates and quartzite was accordingly grouped together in a single formation, termed the Randfontein Formation.
The Randfontein Formation is followed by the sequence of pebbly quartzite and locally developed conglomerates of the Luipaardsvlei Formation, which can be correlated with most of the Krugersdorp Formation of the Carletonville GF, the Stilfontein Formation of the Klerksdorp GF and the Welkom Formation of the Welkom GF, with some small changes in boundaries. This unit is absent on the East Rand, as it has been erosionally truncated prior to deposition of the overlying formation.
An extensive unconformity marks the top of the Luipaardsvlei Formation and the base of the Krugersdorp Formation which contains the Bird Reef and its correlatives Vaal and Basal/Steyn Reefs. This basal conglomerate is overlain by quartzite with locally developed pebbly quartzite or conglomerates, although locally, it fines upwards to siltstone or shale (e.g. Khaki Shale of Welkom). In the eastern sections of the basin, the formation contains the mafic Bird volcanics.
The succeeding Booysens Formation contains the only widespread shale sequence in the Central Rand Group and marks the top of the Johannesburg Subgroup. Both the upper and lower boundaries are generally transitional. The shale coarsens up into the locally prominent orthoquartzite of the Doornkop Member. The marks the top of the Johannesburg Subgroup.
The Turffontein Sub-group - The base of this unit is defined by the widespread unconformity-based conglomerate of the Kimberley Formation which comprises the 'B', Kleinfontein, 'C' and 'LK1' Reefs, and consist primarily of quartzites and conglomerates, deposited in a number of cycles of deposition and erosion. The the Beatrix Reef occurs in conglomerates of one these cycles. Shale-filled erosion channels occur at the base of several of these cycles. The upper boundary of the formation is defined by another widespread unconformity.
The basal conglomerate of the overlying Elsburg Formation contains the Intermediate Reef of the Evander GF, the UE1A of the West Rand GF, the Denny's Reef of the Klerksdorp GF and the VS5 of the Welkom GF. This basal conglomerate is overlain by upward coarsening quartzite throughout the basin. The lower, predominantly quartzitic portion, locally contains economic conglomerates which become increasingly more abundant upwards in the succession. Conglomeratic rocks dominate the upper portion of the sequence, which has been termed the partially equivalent Mondeor Formation. In the Welkom GF, the transition from the Elsburg to the Mondor Formation is characterised by a transition from predominantly oligomictic to polymictic conglomerates.
The top of the Witwatersrand Supergroup is defined by the unconformity which marks the base of the Ventersdorp Contact Reef (Venterspost Formation), where developed, or the base of the volcanic rocks of the Ventersdorp Supergroup.
Ventersdorp Supergroup, which is 5000 to 7800 m thick and forms an elliptical, north-east elongated basin covering an area of approximately 300 000 sq. km. It conformably overlies the clastics of the Witwatersrand Supergroup, is predominantly composed of andesitic composition rocks dated at 2720 to 2650 Ma and is divided into three groups, namely the: i). Klipriviers Group - around 1830 m of tholeiitic lavas which range from basaltic komatiite at the base, overlain by a succession of amygdaloidal basaltic lavas with an increasing Ni, Cr and Mg content with height within the sequence. It covers a very extensive area from south-west Botswana to the northern Cape Province. ii). Platberg Group - which is largely confined to narrow graben structures and is dominated by the 2.709 Ga quartz-porphyry rhyolites of the Makwassie Formation iii). Pniel Group - which includes arenaceous sedimentary rocks overlain by a further suite of mafic volcanics of the Allanridge Formation, which is predominantly fine grained amygdaloidal porphyritic tholeiites which filled small basins in the underlying sedimentary units.
Transvaal Supergroup, which is up to 15 000m thick and is composed of two main units, the lower 2642±2 to 2432±31 Ma Chuniespoort Group comprising predominantly carbonatic sediments, including dolomitic limestone, quartzite and shale, with lesser basaltic and rhyolitic volcanics, and tilloids, separated by a ~80 m.y. hiatus from the overlying Pretoria Group, a volcano-sedimentary succession with alluvial fan and fan delta deposits with minor marine components comprising up to 7000 m of andesitic lavas, banded iron formation and carbonates. The base of the Chuniespoort Group is marked by the Black Reef Quartzite Formation, a laterally persistent basal conglomerate and sandstone unit that rests on the post-Ventersdorp erosion surface and contains the Black Reef conglomerates which are indistinguishable from the auriferous, uraniferous and pyrite rich Witwatersrand reefs.
Deposition of the Pretoria Group was followed after a hiatus by the generally contemporaneous extrusion of the Rooiberg Group felsic volcanics pile and the intrusion of the 2059±1 Ma Rustenberg Suite of up to 9000 m thick gabbro, norite, pyroxenite, anorthosite, harzburgite and diorite mafic to ultramafic layered complex and the 2054±2 Ma Lebowa Suite granitic to granophyre phase. Both intrusive phases form the Bushveld Igneous Complex which covers an area of some 70 000 sq. km in four generally circular overlapping lobes to the north of the Witwatersrand Basin.
The structural pattern of the central eastern section of the Witwatersrand Basin has been modified by the ~75 km diameter Vredefort Dome that has a core of older Archaean basement and is interpreted to represent a 2023±2 Ma meteorite impact.
Waterberg Supergroup, up to 8 000m thick, distributed over an area of around 40 000 sq. km - was deposited along the northern margin of the Kaapvaal Craton between 1.9 and 1.75 Ga and comprises marine and continental volcano-sedimentary successions, including conglomerate-quartzite units, red-beds, shales and thin units of trachyandesitic and quartz-porphyritic lava.
Seismic profiling shows that the northern and western margins of the Witwatersrand Basin are characterised by thrusting and faulting, and block faulting throughout the basin. There are several stages evident in the evolution of the Witwatersrand Basin, which was initiated as a rift at ~3.07 Ga, during Dominion Group times, followed by post-rift thermal subsidence during deposition of the early parts of the West Rand Group. Thermal subsidence is interpreted to have been completed by late West Rand Group times (~2.9 Ga), although minor volcanic interludes within the West Rand Group sequence may indicate phases of extension. The onset of compression and thrusting outside of the thermal basin generated a flexural load and clastic input into the basin as it evolved from thermal sag to foreland basin during the late West Rand Group (~2.95 Ga). Foreland basin development culminated during deposition of the Central Rand Group, with an increasingly coarse-grained clastic input, and thrust systems that progressively encroached on the basin margins, profoundly influencing structural styles. Thrusting was interrupted during the early Ventersdorp Supergroup by the accumulation of Klipriviersberg Group basic volcanic rocks (2.71 Ga). Thrusting recommenced at the end of Klipriviersberg Group. The basin then returned to an extensional tectonic setting during the Platberg Group rifting (2.709 Ma), with major preserved grabens filled with immature Platberg Group diamictites and other mass flow sediments. This extension represents a major rift event across much of the region, with post-rift thermal subsidence during the late Platberg Group. The overprint of Platberg Group extensional faults breaks up the structural continuity of the Central and West Rand Group sediments within, and adjacent to, the basin, making it is difficult to differentiate the earlier compressional thrust-fold structures.
The overlying Transvaal Supergroup is relatively un-deformed, with structural deformation characterised by broad, open to overturned folds, low angle southerly dipping faults and small periclinal folds. Faulting of the underlying Ventersdorp and Witwatersrand Supergroups is developed both parallel and normal to the basin margins. These faults have been the locus of normal displacement of the Klipriviersberg Group volcanics by as much as nbsp;m, as well as forming the margins of the Platberg Group grabens. Many of these same faults were also active during the deposition of the Witwatersrand Supergroup and controlled the thickness and distribution of the units of the sequence. Monoclines are associated with some of these faults, with drastic thickness changes evident across the faults perpendicular to strike, but little to no change parallel to strike. In addition, significant erosion is indicated with the Klipriviersberg Group volcanics overstepping the monoclines and associated faults to lie directly on West Rand and Basement rocks within 3 to 4 km outboard of the monoclines. The resultant apparent vertical displacement across some of these faults in the Johannesburg area during, and just prior to deposition of the Central Rand Group, was as much as 7000 m, with an implied reverse component of movement, downside into the basin. Some of these same faults had a moevement in the opposite sense, ie. downthrow outboard of the basin, by as as much as 2.5 km during deposition of the Platberg Group. The development of these monoclinal structures resulted in steep to overturned dips and local thrust displacement of the reefs. In general, along these structures, dips decrease stratigraphically upwards. In addition, the faults normal to the basin margins have displaced the margins of the basin and form many of the boundaries of the individual gold fields.
Gold has been mined from at least 30 reefs within the Witwatersrand Basin sequence, 95% of which are within the Central Rand Group, with a general upward decrease in both total Au yield and average grade (eg. the Carbon Leader Reef at the base of the Central Rand Group averages 25 g/t Au, while the Venterdorp Reef at the top of the same group carries 5 to 12 g/t Au). The most important reefs within the Central Rand Group are at 6 stratigraphic positions, 3 within the Johannesburg and 3 within the Turffontein Sub-group. In additon there are three sub-economic reefs within the Government Sub-group and a further two in the Jeppestown Sub-group, both within the West Rand Group. The reefs are typically composed of a variety of fluvial lithofacies ranging from clast-supported oligomictic conglomerate to loosely packed conglomerate, pebbly arenite or pebble-lag surface associated with trough cross-bedded quartz-arenite. Rarely, ore occurs within debris-flow lithofacies.
The individual orebodies range in thickness from a few tens of centimetres to several metres and are confined to the interval between a basal degradation surface, which is normally an angular unconformity, and an upper planar bedding surface marking the contact with overlying quartzwackes or siltstones. Both the upper and lower margins of the orebodies are marked by a sharp change in gold content from several grams per tonne to <20 ppb. The orebodies occurs as lens-like fluvial fluvial bar and channel beds with unimodal palaeo-current directions. Thicker bodies occur as multi-channel sequences of conglomerate and quartz-arenite representing flood and waning stage flows. The depositional environment of the orebody hosts range from proximal alluvial fans, to terraced fluvial, braid plain and braid delta, merging into shoreline environments. On a regional scale the orebodies are distributed along the margin of the Central Rand Group basin, at the main entry points of complex river systems into the original basin. On a mine scale, the highest grades are usually, but not exclusively in the channel facies, with higher grades where the channels are thickest. Individual orebodies may be of the order of 3 km or more wide and 5 to 10 km long.
Gold, together with uraninite and pyrite are spatially associated with detrital minerals such as zircon and chromite which are concentrated on degradation surfaces marked by pebble lags at the base of clast supported conglomerates, but also on crossbedded foresets, bottomsets and coset boundaries. Uranium occurs as uraninite, brannerite and leucoxene, with a distinctive decrease in the uraninite:brannerite ratio up-section, from 8.7 in the Steyn Reef towards the base of the section, to only brannerite in the younger Beatrix Reef, both examples from the Welkom District. There is also a systematic trend in U:Au ratio up-section, from the gold-rich Central Rand Group reefs to theDominion Reef which does not contain significant gold. In addition, there is a systematic trend in the U:Au ratio doen the palaeoslope in the Welkom Goldfield, with uraninite being enriched in the more distal facies and the ratio ranging from 0.001 on the basin margin to 10 in the distal facies. Pyrite is the most common iron minerals, with iron oxides (mainly magnetite) only being found in the shale facies.
There is an intimate association between gold and carbonaceous matter, which occurs as stratabound seams and a spherical glassy globules. The carbon seams are absent from proximal high energy deposits, and preferentially in more distal environments, and occur in conglomerates and sandstones that contain rounded pyrite. These seams are found along micro-fractures and appear to represent hydrothermal pyrobitumen.
Gold is found in association with carbon and/or pyrite within siliceous quartz pebble and cobble (rounded white and smoky quartz) conglomeratic rocks, banded pyritic quartzites and orthoquartzites, as well as in low energy environments of distal fans where thin bituminous bands carry fine gold and uranium. Within these rocks gold is present in three separate forms; as fine free gold in the clastic matrix with quartz, chlorite, pyrophyllite, fuchsite or sericite; with thucholite (bituminous material); or as a coating on pyrite.
Major mines operating in 2007 in each of the main gold fields, icluded:
Evander Gold Field
Harmony Evander Operations, are located in Mpumalanga Province, some 120 km ESE of Johannesburg and include the Evander 2, 5, 7 and 8 Mines, and the Rolspruit and Poplar Projects. These operations are situated within the Evander basin, a discrete easterly extension of the main Witwatersrand Basin, where the only economic reef mined is the Kimberley Reef. In addition to the faulting of the reef horizon, there are numerous dykes and sills that complicate continuity, the most significant of which is an extensively developed dolerite footwall sill that occasionally intersects the Kimberley Reef, causing displacements (SRK, 2007).
In 2006 reserve and resource figures (SRK, 2007) totalled:
Proved + probable reserves - 56.169 Mt @ 7.6 g/t Au for 450 t Au,
Measured + indicated resources - 55.455 Mt @ 12.4 g/t Au for 685 t Au,
Inferred resources - 29.32 Mt @ 10.9 g/t Au for 335 t Au.
East and Central Rand Gold Field
East Rand Proprietary Mines, (ERPM) is located 25 km to the east of Johannesburg, near the town of Boksburg, and is the eastern extension of the Central Rand District of Gauteng Province. The upper levels of ERPM have a relatively complex structure determined by a combination of strike faults and NE-SW trending faults. Reef dips are generally regular, decreasing from an average of 50° near surface to an average of 35° at around 30 level. The 'Composite Reef', as defined and exploited at ERPM, comprises the Main Reef and the Main Reef Leader, with the South Reef occurring sporadically in the hanging wall. All three reefs are found near the base of the Central Rand Group. These three principal auriferous conglomerates are well developed in the west, where they are characterised by partings of about 1 m between the Main Reef and Main Reef Leader, and 30 m between the Main Reef Leader and the South Reef. Towards the east, the reefs merge progressively and at ERPM, the lower two reefs are completely cut out by the South Reef. The lowermost Main Reef is generally a poorly sorted conglomerate with pebbles of up to 5 cm in diameter. The Main Reef is the most prolific gold producer in the Central Rand and is better sorted and graded but thinner than the Main Reef Leader. Pebbles are, on average, coarser being up to 8 mm in diameter. The reef is often dark in colour, mainly due to chlorite in the footwall. The South Reef is the most persistent of the conglomerate layers but is of slightly lower grade than the Main Reef Leader. It is less well sorted than the other two reefs and generally occurs as a number of individual pebble bands with arenaceous partings. Many dykes and faults have been identified in the lease area. The regional distribution of high-grade areas at ERPM is restricted to two broad zones, the Central Payshoot and the Northern Payshoot, with the latter having been extensively mined.
In June 2006 reserve and resource figures (Durban Roodepoort Deeps, 2007), both underground and open pit, totalled:
Proved + probable reserves - 19.466 Mt @ 1.88 g/t Au for 36.37 t Au,
Measured + indicated + inferred resources - 262.74 Mt @ 1.96 g/t Au for 515 t Au.
West Rand Gold Field
West Wits Operation, in the Carltonville District, is situated near the town of Carletonville in North West Province, south-west of Johannesburg, straddling the boundary with the province of Gauteng of South Africa, which includes the Mponeng, Savuka and TauTona mines.
Two reef horizons are exploited at the West Wits operations: namely the Ventersdorp Contact Reef (VCR), which is located at the top of the Central Rand Group, and the Carbon Leader Reef (CLR) near the base. Due to non-conformity in the VCR, the separation between the two reefs dencreases from west to east, from 900 to 400 m. TauTona and Savuka exploit both reefs while Mponeng only mines the VCR. The structure is relatively simple with fault displacements of greater than 70 m being rare. The CLR consists of one or more conglomerate units and varies from several cm to more than 3 m in thickness. Regionally, the VCR dips at approximately 21°, but may vary between 5 and 50°, accompanied by changes in thickness of the conglomerate units. Where the conglomerate has the attitude of the regional dip, it tends to be thick, well-developed and accompanied by higher gold accumulations. Where the attitude departs significantly from the regional dip, the reef is thin, varying from several centimetres to more than 3 m in thickness (AngloGold Ashanti, 2007).
Measured + indicated + inferred resources at 31 December, 2006 were (AngloGold Ashanti, 2007):
Mponeng - 24.08 Mt @ 8.75 g/t Au for 210.8 t Au,
Savuka - 0.95 Mt @ 5.69 g/t Au for 5.4 t Au,
TauTona - 13.76 Mt @ 11.27 g/t Au for 155.1 t Au.
Harmony West Wits Operations are some 85 km SW of Johannesburg Gauteng and North West Provinces and comprises the Elandsrand, Cooke 1, Cooke 2, Cooke 3 and Doornkop Mines. Three main reefs have been identified in the district, the Ventersdorp Contact Reef (“VCR”), the Carbon Leader Reef (“CLR”) and the Mondeor Reef. Only the VCR is economic to mine and has been exploited at depths of from 1600 to 2800 m below the surface, extending to 3300 m at Elandsrand Mine. The VCR and CLR consist of
narrow (20 to 200 cm) tabular orebodies of quartz pebble conglomerates hosting gold, with extremelateral continuity.
At the Elandsrand Mine, the vertical separation between the VCR and CLR increases east to west from 900 to 1300 m due to the angle between the VCR unconformity surface and the regional strike and dip. The CLR strikes west-southwest and dips to the south at 25°. The VCR strikes ENE, with a regional dip of 21° SSE. Local variations in dip are largely due to the terrace-and-slope paleotopography surface developed during VCR deposition. The dip of the VCR at Deelkraal Mine is relatively consistent at 24°
The “Randfontein Section” (collectively the Cooke 1, 2, 3 and 4 Mines and Doornkop Mines) is situated in the West Rand Goldfield of the Witwatersrand Basin, where the Witpoortjie and Panvlakte Horst blocks are superimposed over
broad folding associated with the southeast plunging West Rand Syncline. The structure in the north part of the Randfontein Section is dominated by a series of NE trending dextral wrench faults. The Randfontein Section contains six identified main reef groupings: the Black Reef; the VCR; the Elsburg Formations; the Kimberleys; the Livingstone Reefs; and the South Reef. Within these, several economic reef horizons have been mined at depths of between 600 and 1260 m below the surface. The reefs comprise fine to coarse grained pyritic mineralisation within well developed thick quartz pebble conglomerates or narrow single pebble lags, which in certain instances are replaced by narrow carbon seams (SRK, 2007).
In 2006 reserve and resource figures (SRK, 2007) totalled:
Proved + probable reserves - 40.252 Mt @ 7.3 g/t Au for 293 t Au,
Measured + indicated resources - 63.289 Mt @ 10.3 g/t Au for 652 t Au,
Inferred resources - 50.74 Mt @ 8.8 g/t Au for 446 t Au.
Blyvoor is situated to the south of the town of Carletonville on the West Wits line and commenced operation in 1942. It exploits two main gold-bearing horizons: the Carbon Leader Reef (CLR), and the Middelvlei Reef, some 75 m above the CLR, both within the lower Central Rand Group.
In June 2006 reserve and resource figures (Durban Roodepoort Deeps, 2007) totalled:
Proved + probable reserves - 39.779 Mt @ 4.0 g/t Au for 159 t Au,
Measured + indicated + inferred resources - 185.81 Mt @ 3.37 g/t Au for 626 t Au.
Carltonville Gold Field
Driefontein - in the Carltonville District, Jauteng Province, in the far West Rand Gold Field. Gold mineralisation occurs at depths of between 1000 and 4000 m below surface from two main reefs; the Carbon Leader Reef (CLR) and the Ventersdorp Contact Reef (VCR). A third reef, the Middelvlei Reef (MR), at the base of the Central Rand Group, is a minor contributor to the resource base. The VCR is at the contact between the Central Rand and overlying Venterspost Formation. The CLR comprises various facies from single carbon seam to single and multiple band conglomerates and is a high-grade reef near the base of the Central Rand Group. At Driefontein the CLR dips at 25°SE.
Measured + indicated resource figures at 31 December 2006 were (Gold Fields, 2007): 111.6 Mt @ 12.5 g/t Au for 1390 t Au.
Kloof - lies approximately 60 km SW of Johannesburg and 20 km from Carletonville, in Mpumalanga province, in the West Rand Gold field. The mine is limited between the north-trending Witpoortjie Fault to the east and the Bank Fault to the west and is divided into four sections, the Kloof, Libanon, Venterspost and Leeudoorn mines. These mines exploit the Ventersdorp Contact Reef (VCR) at the top of the Central Rand Group at the base of the Venterspost Formation, at depths of between 2500 and 3700 m with minor production from the Kloof (KR), Libanon (LR) and Main (MR) reefs lower within the Central Rand Group.
Measured + indicated resource figures at 31 December 2006 were (Gold Fields, 2007): 177.6 Mt @ 12.0 g/t Au for 2135 t Au.
South Deep - is situated adjacent to Kloof, in the Carltonville District, Jauteng Province. It comprises two shaft systems, the South Shaft Complex and the South Deep Twin Shaft Complex. Production at South Deep is currently from the Ventersdorp Contact Reef (VCR) at the base of the Venterspost Formation, as well as the Upper Elsburg Reefs in the Turffontein Sub-group in the upper Central Rand Group. The VCR occurs in the western extremity of the mining authorization. The Upper Elsburgs occur to the east of a north-northeast striking subcrop with the overlaying VCR and form part of an easterly divergent clastic wedge. In general terms, the Upper Elsburg succession represents an easterly prograding sedimentary sequence.
Measured + indicated resource figures at 31 December 2006 were (Gold Fields, 2007): 288.7 Mt @ 7.2 g/t Au for 2077 t Au.
Klerksdorp Gold Field
Vaal Rivers Operation, in the Klerksdorp District, is located near the towns of Klerksdorp and Orkney in the North West and Free State provinces of South Africa, which includes the Great Noligwa, Kopanang, Tau Lekoa and Moab Khotsong mines.
In order of importance, the Witwatersrand reefs mined at the Vaal River operations are the Vaal Reef, the Ventersdorp Contact Reef (VCR) and the C Reef:
• the Vaal Reef contains approximately 85% of the reserve tonnage with mining grades between 10 and 20 g/t and comprises a series of oligomictic conglomerates and quartzite packages developed on successive non-conformities. Several distinct facies have been identified, each with its own unique gold distribution and grade characteristic;
• the VCR has a lower grade than the Vaal Reef, and contains approximately 15% of the estimated reserves. The economic portion is concentrated in the western part of the lease area and can take the form of a massive conglomerate, a pyretic sand unit with intermittent pebble layers, or a thin conglomerate horizon. The reef is situated at the contact between the overlying Kliprivierberg Lavas of the Ventersdorp Super Group and the underlying sediments of the Witwatersrand Super Group, which creates a distinctive seismic reflector. The VCR islocated up to 1km above the Vaal Reef; and
• the C Reef is a thin, small-pebble conglomerate with a carbon-rich basal contact, situated approximately 270m above the Vaal Reef. It has less than 1% of the estimated reserves with grades similar to those of the Vaal Reef, but more erratic. The most significant structural features are the north-east striking normal faults which dip to the north-west and south-east, resulting in zones of fault loss (AngloGold Ashanti, 2007).
Measured + indicated + inferred resources at 31 December, 2006 were (AngloGold Ashanti, 2007):
Great Noligwa - 23.72 Mt @ 13.94 g/t Au for 330.6 t Au,
Kopanang - 20.60 Mt @ 16.57 g/t Au for 341.4 t Au,
Moab Khotsong - 19.66 Mt @ 18.24 g/t Au for 358.6 t Au,
Tau Lekoa - 41.84 Mt @ 5.31 g/t Au for 222.4 t Au.
Harmony Orkney Operations, are located in the North West Province, near Klerksdorp, some 175 km SW of Johannesburg and comprise the Orkney 2, 4, 6 and 7 Mines, where the Vaal Reef is the most significant reef mined. This reef strikes NE, dips SE and is heavily faulted to form aseries of graben structures. Dips are generally <30° but vary locally in direction and magnitude to >45°. The VCR Elsburg Reefs are also exploited. There are several major faults in the lease area, typically with throws of tens of metres which further divide the reef into blocks of up to 100 m in width (SRK, 2007).
In 2006 reserve and resource figures (SRK, 2007) totalled:
Proved + probable reserves - 6.751 Mt @ 6.0 g/t Au for 40 t Au,
Measured + indicated resources - 27.271 Mt @ 9.6 g/t Au for 262 t Au,
Inferred resources - 15.93 Mt @ 6.1 g/t Au for 96 t Au.
Welkom Gold Field
Beatrix - in the Free State Province, some 240 km southwest of Johannesburg, near Welkom and Virginia. The Beatrix operation has four shaft systems and is a shallow to intermediate depth mining operation, at depths of between 700 and 2200 m below surface. The mine exploits the Beatrix Reef (BXR), at Shaft Nos. 1, 2 and 3, and the Kalkoenkrans Reef (KKR), at Shaft No. 4 (the former Oryx mine). Both reefs are within the Central Rand Group and are developed on the Aandenk erosional surface, dipping to the north and north-east at between 4 and 9°. In general, the BXR occurs at depths of between 570 and 1380 m, while the KKR occurs at between 1800 and 2200 m. Both the BXR and KKR reefs are markedly channelised and comprise of multi-cycle, upward fining conglomerate beds with sharp erosive basal contacts. A general east-west trending pay-zone, some 800 to 1000 meters wide, has been identified east of Shaft No. 4 and is known as the main channel Zone 2. In addition, surface exploratory drilling and underground development has exposed additional reserves to the south of Beatrix's main channel Zone 5.
Measured + indicated resource figures at 31 December 2006 were (Gold Fields, 2007): 84.2 Mt @ 6.9 g/t Au for 580 t Au.
Harmony Freegold Operations - comprises a series of mines located in the Free State Province, on the south-western edge of the Witwatersrand basin. These include the Tshepong, Bambanani, Eland and St. Helena Mines which are located in and around Welkom, while the Joel Mine is approximately 50km south of Welkom. Mining at Tshepong Mine and Bambanani Mine, is primarily conducted on the Basal Reef, with limited exploitation of secondary reefs. Mining at Joel Mine is primarily conducted on the Beatrix-VS5 Composite Reef. The reefs generally dip towards the east or northeast while most of the major faults strike north-south (SRK, 2007).
In 2006 reserve and resource figures (SRK, 2007) totalled:
Proved + probable reserves - 56.710 Mt @ 7.5 g/t Au for 423 t Au,
Measured + indicated resources - 71.903 Mt @ 11.8 g/t Au for 633 t Au (?),
Inferred resources - 72.42 Mt @ 9.1 g/t Au for 657 t Au.
Harmony Target Operations, are located in the Free State Province, some 270 km SW of Johannesburg. The gold mineralisation exploited by the Target Mine is hosted within a succession of Elsburg and Dreyerskuil quartz pebble conglomerate reefs hosted by the Van Heeversrust and Dreyerskuil Members of the Eldorado Formation, respectively, while additional resources occur in the Big Pebble Reefs (“BPR”) of the underlying Kimberley Formation. All of these units are within the Turffontein Sub-group of the Central Rand Group. The bulk of the resources in the Target Mine are within the Eldorado fan, a structure with dimensions of some 135 m vertically, 450 m down-dip and 500 m along strike. This fan is connected to the subsidiary Zuurbron fan, located between the Target Mine and Loraine, by a thinner and lower grade sequence of Elsburg reefs termed the Interfan area. To the north of the Eldorado fan, a number of fans are known, including the Siberia and Mariasdal fans. A number of faults displace the reefs of the Target Mine, the most prominent of which are the north-south trending Eldorado fault and the east-west trending Dam and Blast faults which offset and divide the mine into a number of mining areas, the Paradise, Siberia and Mariasdal areas. An additional north-south trending fault, the Twin fault has uplifted the distal portions of the reefs. North of the Mariasdal fault, the reef horizons are at depths of >2500 m below surface. Mineral Resources have been delineated over a strike lenght of up to 15 km north of the Target Mine. Approximately 40 km north of Target Mine, drilling has intersected gold bearing reefs in the Oribi area close to the town of Bothaville, where mineral resources have been delineated on the VCR and Elsburg Reefs at depths of ~2750m below surface (SRK, 2007).
In 2006 reserve and resource figures (SRK, 2007) totalled:
Proved + probable reserves - 19.281 Mt @ 7.0 g/t Au for 135 t Au,
Measured + indicated resources - 133.624 Mt @ 7.4 g/t Au for 985 t Au,
Inferred resources - 127.203 Mt @ 7.0 g/t Au for 895 t Au.
Harmony Free State Operations, are located in the Free State province, some 270 km SW of Johannesburg, within the Free State Goldfield on the SW edge of the Witwatersrand Basin. They are located on the SW and SE limb of a synclinal closure, with the Brand, Saaiplaas and Masimong shafts occupying northerly extensions of the same structure. The reefs dip inwardly from their sub-outcrop positions in the east and south of the mining area to a position close to the western boundary of the original Harmony mine, where the reefs abut against the De Bron fault. To the west of the De Bron faulted zone, faulting is generally more intense (SRK, 2007).
In 2006 reserve and resource figures (SRK, 2007) totalled:
Proved + probable reserves - 25.548 Mt @ 5.0 g/t Au for 125 t Au,
Measured + indicated resources - 37.835 Mt @ 7.2 g/t Au for 272 t Au,
Inferred resources - 132.2 Mt @ 6.0 g/t Au for 795 t Au.
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Amazônia Mineração Ltda - background
Amazônia Mineração, (AMZ) is principally a gold exploration and mining company based in Goiania, Goias, Brazil.
The company's principal asset is a high-grade, low-cost, gold resource in the state of Goiás, the Sertão deposit, which the company acquired from WMC in 2000 before completing the delineation of, and expanding the economic resource. This deposit has subsequently been developed and successfully mined by Sertão Mineração Ltda. AMZ, has retained 30% of the operating company, which is 70% controlled by the partner with gold mining experience AMZ introduced to the project, Troy Resources of Australia. AMZ continues to be responsible for exploration on behalf of the joint venture on and around the mine leases at Sertão.
AMZ also operates a basalt aggregate mine in the state of Roraima through its subsidiary Amazônia Mucajaí Mineração Ltda, and undertakes exploration in its own right and as a service to associated companies through Amazônia Technical Services, principally for gold, but also nickel and PGE's.
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T M (Mike) Porter, of Porter GeoConsultancy
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T M (Mike) Porter of Porter GeoConsultancy Pty Ltd on behalf, and to the specification, of the client.
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