An AMF-PGC International Study Tour
Developed & Managed by Porter GeoConsultancy
Africa-B 2001
Mineral Deposits of Southern & Central Africa
10 June - 5 July 2001 - In Two Separate Modules
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Merensky Reef PGE
DEPOSIT DESCRIPTIONS - MODULE 2

This tour, which was developed, organised, managed and led by TM (Mike) Porter of Porter GeoConsultancy Pty Ltd (PGC), as a joint venture with the Australian Mineral Foundation Inc. (AMF), included:

MODULE 2 - Mafic Intrusion Related PGE, Cr, V, Ni, Cu
Monday 25 June to Thursday 5 July 2001
The main focus of this Module 2 was Platinum Group Elements, Chromite, Vanadium-magnetite, Copper and Nickel of the Bushveld Complex and its satellite intrusions. It provided a unique opportunity to study the largest and most complete mineralised mafic system in the world and to learn more about the processes involved in the formation of these ores and their metallogenic and geologic inter-relationships.

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


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MODULE 2 - Mafic Intrusion Related PGE, Cr, V, Ni, Cu

Geological Overview

The Kalahari Craton of southern Africa embrace some of the largest and best developed mafic complexes in the world. These in turn host some of the most fabulous ore accumulations on the planet, particularly those of the PGE metals. In addition there are world class accumulations of chromite, vanadium and copper, and significant nickel.

The most important of these are in the vast Bushveld layered mafic (to acid) complex and associated outliers of the same age. Visits to the ore deposits of this region, the most outstanding examples in the world allow a study of how such deposits are formed and the best available opportunity to learn more about the distribution of, and inter-relationship between, the different metals that may occur in this environment.

The main Bushveld Complex covers an area of some 70 000 square kilometres in four generally circular overlapping lobes. The layered mafic rocks, the Rustenburg Layered Suite, comprise up to 9 km of gabbro, norite, pyroxenite, anorthosite, harzburgite and diorite that were emplaced at around 2050 Ma, followed by a thick upper segment of granophyres and granites dated at 2000 to 1900 Ma and granites as late as 1650 Ma.

The Bushveld Complex mafic suites comprise the following: (1). Lower Zone, 1450 m thick of bronzitite, harzburgite and dunite; (2). Critical Zone, 1400 m thick of a strongly layered lower pyroxenite series of norites, peridotites and chromitites (the main Cr ores), followed by an upper, strongly layered anorthosite series of norite, anorthosite and chromitites (the main PGE ores with lesser associated Ni-Cu); (3). Main Zone, 2800 m thick of massive gabbro, norite and anorthosite; and (4). Upper Zone, 2100 m thick, of layered gabbro, Fe-olivine gabbro and magnetite layers (the main V-Fe ores).

Some 75 km to the east of the south-eastern lobe of the main Bushveld Complex, an 8x1 km and 650 m thick, NW-SE trending, flat lying, poorly layered, sill like, 2025 Ma mafic body of harzburgite, gabbro, pyroxenite and gabbro-norite, the Uitkomst Complex, cuts Archaean basement and hosts the Nkomati massive sulphide Ni-Cu-Co-PGE deposit and the larger surrounding lower grade disseminated mineralisation of the same elements. The Uitkomst Complex is believed to be broadly coeval with, and related to, the Bushveld Complex.

To the north, some 150 km to the east of the eastern lobe of the Bushveld Complex, the 2060 Ma Phalaborwa Complex, a pipe-like 7x3 km mainly pyroxenite body with a much smaller core of carbonatite hosts large reserves of economic magnetite-copper mineralisation, the Palabora deposit, and would appear to be part of the larger Bushveld event.

Further north in Zimbabwe, the older 2450 Ma, 600 km long Great Dyke hosts significant PGE ores.

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Johannesburg Workshop

The Setting, Geology & Metallogeny of the Mafic Complexes of Southern Africa - Prof. Grant Cawthorn

The workshop will be led by internationally renowned expert, Professor Grant Cawthorn of the Witwatersrand University and include coverage of the following topics:
  • An overview of the geological and tectonic framework of Southern Africa, and the distribution, geological controls and characteristics of mafic complexes within the same region.
  • The metallogeny of these mafic complexes - which are mineralised, which are economically significant, with what elements, what styles of mineralisation are encountered, how do they compare/contrast with other mafic complexes around the world.
  • The Bushveld Complex, its formation history, conditions of emplacement, "stratigraphy", chemistry, characteristics, the development of the various mineralised layers and their characteristics, etc., and its relationship to other complexes of similar age in the region.
  • A comparison between the mines/deposits that we are to see and others found in the Bushveld Complex.


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Anglo Platinum Rustenburg Section - PGE

The Anglo Platinum, Rustenburg Section operations exploit ores from both the Merensky and UG2 Reefs. 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 (1). enclosed in or attached to base metal sulphides; (2). enclosed in silicates; or (3). 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 at Rustenburg Section of Rustenburg Mines in 1999 were: Merensky Reef, Proved - 27 mt @ 6.08 g/t, Probable 97.26 mt @ 6.21 g/t PGE; UG2, Proved - 13.05 mt @ 4.9 g/t, Probable - 67.86 mt @ 4.9 g/t PGE.

During 1999 some 7.7 mt of ore were milled at a head grade of 5.65 g/t PGE for a recovered production of 23.9 t (0.77 Moz) Pt, 10.2 t (0.33 Moz) Pd, 1.45 t (0.05 Moz) Rh, 1.58 t (0.05 Moz) Au, 8600 t Ni, 500 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.

NOTE: All of the chromitites (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 Elandsdrift - Cr

The Elandsdrift open pit is one of three mines currently comprising the Samancor Western Chrome Mines division, centred around 40 km east of Rustenburg. These are spread 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. The other two mines, Waterkloof-Millsell and Mooinooi are both underground operations. This 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 Ltd is owned by Billiton plc (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 degrees 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 0.80-0.85 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 0.45-0.55 m thick, then the overlying 0.25-0.30 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 Ám 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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Potgietersrust - PGE-(Au-Ni-Cu)

The Potgietersrust Platinums mine exploits the Platreef on the Potgietersrus limb of the eastern margin of the Bushveld Complex Northern Lobe in the Northern Province of South Africa, some 240 km to the north-east of Rustenburg. 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: (1). the upper pyroxenite, or 'C' Reef which is usually barren; (2). 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; (3). 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 (2-39 m thick) and most consistent. Cu ranges from 0.1-0.25%, Ni from 0.15-0.35%, while PGEs vary from <0.25-15 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.

Reserves at the end of 1999 at a 1.7 g/t PGE cutoff were: Proved - 47.43 mt @ 4.15 g/t, Probable - 296.5 mt @ 3.79 g/t, and Indicated - 167 mt @ 4.9 g/t PGE. During 1999 some 4.059 mt of ore were milled at a head grade of 4.59 g/t PGE for a recovered production of 6.2 t (0.20 Moz) Pt, 6.5 t (0.21 Moz) Pd, 0.47 t (0.015 Moz) Rh, 0.6 t (0.02 Moz) Au, 3900 t Ni, 1900 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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East Bushveld Field Traverses

A series of traverses and field inspections will be conducted over outcropping sections of the Bushveld Complex, from the layered lower mafic section to the upper acid rocks. In addition exposures of the immediate sequence enclosing the Merensky, UG2 and other chromitites and lithologies of interest will be inspected at surface.

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Palabora - Cu-Fe-P-etc

The Palabora Cu-Fe-P deposit is associated with a carbonatite core which occupies 6% of the area of the elongated 20 sq. km Phalaborwa Igneous Complex (2060 Ma), some 150 km to the east of the eastern lobe of the Bushveld Complex (2050 Ma). The Phalaborwa Complex is predominantly a pyroxenite composed of varying proportions of diopside, phlogopite and apatite, ranging in texture from fine and medium grained to pegmatoidal zones. A feldspathic pyroxenite zone occupies much of the outer rim with the intruded Archaean granite gneiss country rock. K-Na-Fe metasomatism of the granite gneisses has produced fenite and syenites along some sections of the contact and as a swarm of isolated bodies surrounding the complex.

Within the Phalaborwa Complex there are three cores of pegmatoidal pyroxenite with pyroxene and mica crystals up to 10-20 cm long. These are distributed along the long axis of the 7 km long Complex. The northern of these has associated zones of serpentine pegmatoid. The central of the three however encloses an inwardly zoned pipe like development of foskorite (olivine/serpentine-magnetite-apatite-calcite rock), banded carbonatite and transgressive carbonatite. This carbonatite bearing pipe is the main host to the Cu mineralisation.

The outer margin of the foskorite is interleaved with pyroxenite in a series of concentric septa. The foskorite is similarly interleaved with the banded carbonatite on its inner margin, although in detail the interband contacts range from sharp to gradational. The elliptical-concentric fabric of the foskorite and banded carbonatite is cut at a sharp angle by the transgressive carbonatite masses.

Both phases of carbonatite, and the foskorite, contain abundant magnetite, whereas the pyroxenite is essentially free of magnetite. The foskorite has around 50% magnetite, the carbonatites together average 27%. Copper occurs in the foskorite and both phases of the carbonatite, but is best developed within the transgressive carbonatite. Bornite is the principal sulphide in both the foskorite and banded carbonatite, with some chalcocite in the foskorite. The main ore and highest Cu grades however are in the transgressive carbonatite where chalcopyrite predominates as stringers and veinlets, with lesser bornite and minor cubanite. Both chalcopyrite and bornite replace magnetite. Due to the large scale of mining and treatment, the low grade uranium (approx 30 ppm), PGEs, Au and Ag may be retrieved from the slime tailings.

The total original open pit +underground resource at Palabora has been quoted at 1200 mt @ 0.59% Cu. The open pit is nearing the end of its life to be replaced by the block cave underground operation currently nearing completion of development. The underground reserve is currently 245 mt @ 0.7% Cu. The mine is operated by Palabora Mining Co Ltd, owned by Rio Tinto (46.5%) and Anglo American (28.9%).

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Mapochs - V-Fe

The Mapochs iron-vanadium mine is located near the eastern margin of the Eastern Lobe of the Bushveld Complex, near its southern limit, some 140 km SSE of Potgietersrus and 280 km ENE of Rustenburg. It exploits one of the vanadium bearing titaniferous magnetite bands that are found within that section of the Complex, the Main Magnetite Layer within the Upper Zone of the Bushveld Complex.

The upper 1750 m of the layered mafic rocks of the Complex carry approximately 8% magnetite disseminated in gabbroic rocks, plus a further 20 m of pure magnetite distributed in 25 discrete magnetite layers. In addition 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). The magnetite layers are concordant and of different thicknesses with persistence over considerable distances, both along strike and down dip. On the Eastern Lobe for instance, the Main Magnetite Layer varies from 2.0 to 2.7 m only over a strike length of 150 km. Vanadium occurs in solid solution within the magnetite, and decreases upwards from around 1.6% V2O5 in the lowest, to 0.25% in the highest of the layers. TiO2, present as ilmenite and as titanium rich magnetite, increases inversely from 11% in the lowest to 18% in the highest layer. The magnetite is invariably black, highly magnetic and granular, occurring as closely packed, roughly equant grains, often up to a centimetre across. The approximately 5% ilmenite grains are interspersed and more variable in size, shape and distribution.

The Mapochs mine exploits the Main Magnetite Layer which in that area has an average thickness of 2.0 to 2.2 m, a grade of around 1.5% V2O5. and westward dip of 15 to 30 degrees. Proven ore reserves are stated at more than 100 mt. Mapochs is operated by Highveld Steel & Vanadium Corporation Ltd which is 73.9% owned by Anglo American plc.

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Nkomati - Ni-(Co-Cu-PGE)

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 (1). a basal 6-30 m thick gabbro (possibly an earlier sill), overlain by (2). a sulphide bearing lower harzburgite (50-90 m) containing sedimentary xenoliths, (3). chromitiferous harzburgite (▒60 m), (4). main harzburgite (±300 m), (5). upper pyroxenite (60 m) and (6). 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.

Three zones of disseminated sulphide mineralisation are hosted by the basal gabbro and lower harzburgite for a resource of 107 mt @ 0.56% Ni, 0.22% Cu, 0.03% Co and 1.08 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. These comprise the exploited orebody, with a 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.

In addition, the Uitkomst Complex hosts massive to semi-massive chromitite mineralisation within the chromitiferous harzburgite.

The Nkomati mine is a joint venture between Anglovaal Mining (75%) and Anglo American plc (25%).

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

This was another of the International Study Tours designed, developed, organised and escorted by T M (Mike) Porter of Porter GeoConsultancy Pty Ltd (PGC) in joint venture with the Australian Mineral Foundation (AMF).  While the reputation and support of the AMF contributed to the establishment of the tours, after it ceased trading at the end of 2001, PGC has continued to develop, organise and manage the tour series.

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