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La formation des minérais de platinoïdes et le rôle des magmas dans leur origine Le complexe du Bushveld Afrique du Sud J-F. Moyen University of Stellenbosch, South Africa.

De quoi on parle? Usages et production des platinoïdes Le complexe du Bushveld : géologie et formation Les minerais de platine dans le Bushveld Exploitation minière

Les platinoïdes « PGM » (ou PGE) =Platinum Group Metals (Elements) Pt-Platine Ir-Iridium Os-Osmium Re-Rhenium Pd-Palladium Rh-Rhodium Ru-Ruthenium

Propriétés Platine Palladium Densité 21.45 12.02 Point de fusion 1772 °C 1554 °C Rareté (lithosphère) 1.10-3 ppm 6.10-4 ppm Numéro atomique 78 46 Masse atomique 195.08 106.42

Usage des platinoïdes Industriels Bijouterie Catalyseurs (chimie organique, automobile) Electronique Bijouterie 1 once (oz) = 28.34 g, 7 mil. Oz ≈ 200 T. Pt densité 21, 1 Oz ≈ 1 cm3

Cours et données économiques 7 mil. Oz Pt ≈ 9 milliards US$ (cours 2007) 15 mil. Oz PGE ≈ 20 milliards US$ (cours 2007)

Importance relative Métal / ressource Production mondiale Cours (début 2007) « Poids » économique (2007) (US$) Diamant (industriel) 16 T (80 mil. Carat) Variable, 0.5$/ct 40 millions Argent 25 000 T (900 mil. Oz) $ 13 / Oz 12 milliards PGE 425 T (15 mil. Oz) $1250 / Oz 20 milliards Or 2500 T (90 mil. Oz) $ 700 / Oz 60 milliards Cuivre 15 mil. T $ 6 / kg 90 milliards 1 ct = 0.2 g; 1 Oz = 28.34 PIB France: 2500 milliards

Producteurs

Signification économique en Afrique du Sud Production mondiale 2007 Production Afrique du Sud 2007 Platine 212 T (7.5 mil. Oz) 165 T Palladium 219 T 86 T Autres PGM (Rh, Os, Ir, Ru) 78 T 58 T Total 509 T (15 mil. Oz) (20 milliards $) 310 T (11 mil. Oz) (14 milliards $) PIB 2007: 280 milliards = 5% (France 2500 milliards) En 2007, 1 Oz ≈ 1250 US$; 1 kg (35 Oz) = 44 000 $

2055 Ma

Le complexe du Bushveld Winter, 2001

Un complexe basique lité Cuestas de gabbros (Zone Critique) dominant des roches ultrabasiques (zone inférieure). Entre Polokwane et Burgersfort (Lobe Est)

Stratigraphie (!) Doc. J. Miller, Stellenbosch

Intermède: un peu de nomenclature Plagioclase Anorthosite Orthopyroxène + Plagioclase Norite Orthopyroxène (Ortho)Pyroxènite Olivine + Orthopyroxène Harzburgite Olivine Dunite Plagioclase Anorthosite 90 Cpx: Gabbro Cpx+Opx: Gabbronorite Opx: Norite Gabbro Olivine Clinopyroxene Orthopyroxene Lherzolite Harzburgite Wehrlite Websterite Orthopyroxenite Clinopyroxenite Olivine Websterite Peridotites Pyroxenites 90 40 10 Dunite Troctolite Olivine gabbro Plagioclase-bearing ultramafic rocks Pyroxene Olivine Winter, 2001

Intrusive sill of Bushveld dolerite in Transvaal sequence Near Burgersfort

Zone inférieure Doc. J. Miller, Stellenbosch

Lower zone Peridotites/pyroxenites (Jagdlust, Burgersfort-Polokwane Road)

Lower zone Peridotites/pyroxenites (Jagdlust, Burgersfort-Polokwane Road)

Zone critique Doc. J. Miller, Stellenbosch

Different pyroxenite types, Critical zone (Mandaagshoek, near Burgersfort)

Norite and anorthosite, Critical zone (Mandaagshoek, near Burgersfort)

Layered anorthosite, Critical zone (Tweefontein, Lydenburg-Burgersfort area)

Anorthosite and chromitite, Critical zone (Dwars River, Lydenburg-Burgersfort area)

Cyclicité de la zone critique Harzburgite (Ol + Opx) Anorthosite Chromitite (UG3) Pyroxènite Chromitite (Chr.) Anorthosite (Pl) Norite (Opx + Pl) Pyroxènite (Opx) Harzburgite (Ol + Opx) Toutes ces roches sont des cumulats!

Formation de cycles de cumulats Winter, 2001

Formations des chromitites Kinnaird et al 2002 (EGRI 369) Naslund & McBirney 1996

Réalimentations en magma Sri = marqueur de magmas différents Sri très variable, change à chaque cycle Multiples injections de petites quantités de magmas de caractéristiques distinctes

UG3 Chromitite, Critical zone (Mandaagshoek, near Burgersfort)

Zone principale Doc. J. Miller, Stellenbosch

Massive black gabbros (« Belfast black »), Main zone (Near Stoffberg, Belfast area)

Modal layering in gabbros, Main zone (Lydenburg-Burgersfort area)

Zone supérieure Doc. J. Miller, Stellenbosch

Gabbros and magnetite (Main Magnetite Layer), Upper zone (Magnet Heights)

Upper zone gabbronorite (Magnet Heights)

Main magnetite layer, Upper zone (Magnet Heights)

Refroidissement in-situ Sri varie peu Deux (?) injections de grands volumes de magmas qui évoluent in-situ Ces roches sont des gabbronorites à texture “ordinaire” (non-cumulative) Eales & Cawthorn 1996

Modèle de construction du complexe du Bushveld Krüger 2004 (EGRI 377)

RUSTENBURG LAYERED SUITE: Emplacement Model 1. Development of Hertzian Fracture 4. Injection of Critical Zone 2. Marginal Zone and Sill Phase 5. Injection of Main Zone 3. Injection of Lower Zone 6. Injection of Upper Zone

Et les platinoïdes? MERENSKY REEF UPPER GROUP CHROMITE SEAM 2 (UG2) PLATREEF

Localisation des principaux gisements Platreef Merensky + UG2 Merensky + UG2

“reefs” Merensky UG2 Doc. J. Miller, Stellenbosch

CRITICAL ZONE: Stratigraphy

UG2 (Northam, lobe Ouest) UG2 CHROMITITE Lumière réfléchie chr Sulfures de PGE sil sulf Doc. J. Miller, Stellenbosch

Merensky Reef (Northam, lobe Ouest) chr chr chr Chromitite sup. chr Pyroxenite pegmatoïde Chromitite inf. Doc. J. Miller, Stellenbosch

Teneurs en PGE – Merensky Reef Wing & Cowell 1999

Minéraux à PGE – Merensky Reef chr pxite pl px Doc. J. Miller, Stellenbosch

Minéraux à PGE – Merensky Reef SULFURES Pyrrhotite Fe8S9 Chalcopyrite CuFeS2 Pentlandite NiFeS2 P G M Braggite (Pt, Pd, Ni) S Doc. J. Miller, Stellenbosch

Merensky Reef : variations latérales Smith et al. 2003

“potholes” Smith et al. 2003

Faciès du Merensky Reef (Northam) Smith et al. 2003

Cartes du Merensky Reef (Northam) Epaisseur PGE total Smith et al. 2003

Doc. J. Miller, Stellenbosch

Exploitation minière

PPL (Potgietersrus Platinum, ltd.) Zwartfontein Sandsloot

Sandsloot

Zwartfontein Sandsloot

Zwartfontein (PPL) Février 2007 Novembre 2008

Blasting

“composite stacking” Contact loading G3 G1 W

Délimitation de différents grades Different coloured types are used to demarcate the different grade categories on the muckpile

Equipement minier

Northam Doc. J. Miller, Stellenbosch

Comparaison entre mines souterraines et ciel ouvert Potgietersrus Platinum Ltd. (PPL) Northam Localisation Lobe Nord Lobe Ouest Niveau exploité Platreef Merensky, UG2 Propriétaire Anglo Platinum Northam Platinum ltd. Mode d’exploitation Ciel ouvert Souterrain Tonnes de roches extraites (2007) 87 mil. 2.7 mil. Tonnes de minerai extraites 4.2 mil. 2.3 mil. (400 000 m2 de “reef”) Grade 3.5 g/T 5.1 g/T Production de platinoïdes 520 000 Oz 350 000 Oz Coût par tonne de minerai R 282 R 601 Coût par Oz. R 2287 R 3834 Investissements R 4100 mil. R 340 mil.

The end

POTGIETERSRUST PLATINUMS LIMITED(PPRUST) Grade: 3.49 g/T Tonnes mined: 87 mil. Tonnes milled: 4.2 mil. Production: 163 500 Oz Pt, 520 000 Oz PGE Mine operation cost: 1200 Mil. ZAR Cost per Oz: 7233 ZAR (1000$) 2287/PGE Oz Cost per ton milled: 282 ZAR Capital expenditure: 4100 mil. ZAR

The critical zone overlooking the lower zone (left) (Burgersfort-Polokwane Road)

NORTHAM Grade: 5.6 g/T (Merensky), 4.4 g/T (UG2) Tonnes mined:. Tonnes milled: 2.3 mil. (400 000 sq. m) Production: 350 000 Oz PGE Mine operation cost: 1.7 Mil ZAR Sales: 3.7 mil. ZAR Cost per Oz: 3834 ZAR Cost per ton milled: 601 ZAR Capital expenditure: 340 Mil. Profts (after tax) 1.3 mil. ZAR

Bushveld Complex The World’s Biggest Layered Igneous Intrusion Floor LZ LCZ UCZ MZ UZ Panorama of the Olifants River Valley, NE Bushveld Jodie’s lecture - 2006

PPL: Secteurs actifs et projets

Deux niveaux minéralisés MERENSKY REEF UG2 CHROMITITE

PLATREEF Outcrop and Target Areas PPRust North Zwartfontein South Sandsloot Mine Technical Services PLATREEF Outcrop and Target Areas Tweefontein North The 5 target areas on the Platreef; Anglo holds total strike length of just over 17kms on the Platreef – it is estimated that over 60 years worth of open pitable reserves available . The area of investigation includes the basal transgressive contact of the Potgietersrus limb of the Bushveld Igneous Complex, with the underlying sedimentary sequences of the Transvaal Supergroup. This contact exhibits a marked “overlapping” relationship and northwards successively lower units of the Transvaal sediments from the floor to the complex. Quartzites of the Daspoort Stage from the floor to the complex in the Potgietersrus Townlands. Successive lower units of quartzite, shale and dolomite of the Timeball Hill formation form the footwall to the B.I.C. on the farms Macalacaskop and Turfspruit. The penge banded ironstone formation forms the footwall upon Tweefontein, whilst the Malmani dolomite forms the floor, on the farms Sandsloot and Zwartfontein. Upon Overysel the basic rocks abut against the Archaean Granites. This transgressive relationship is of primary importance as the degree of metamorphism, metasomatism, and assimilation of the floor rock is directly related to the nature of these original sedimentary units. The contamination of the Bushveld rocks and the style and distribution of the mineralisation within the magmatic units, are thus directly related to the footwall rock types. The succession within the Potgietersrus limb of the B.I.C. differs significantly from the sequences of the eastern and western lobes of the complex. No units of the Lower Zone, or lower parts of the Critical Zone are recorded within the Potgietersrus limb. And, it is the mineralised pyroxenitic unit of the Critical Zone (the local equivalent of the Merensky Reef) that forms the base of the complex. The interaction of this pyroxenitic unit with differing sedimentary sequences, has resulted in a highly complex and unique suite of rock types. It is this package of partially contaminated, mineralised, pyroxenitic rocks that has been called the “Platreef”. A basic stratigraphic succession has been recognised for the “Platreef”, particularly within the “Prospect Area”. The mineralisation encountered within the “Prospect Area” is variable in width and tenor and the “style” is closely related to the underlying footwall sedimentary rock type. A change in nature of the floor rock producing a change in the style of the mineralisation within the Platreef. The mineralised horizons within the Platreef attain considerable thickness and frequently exceed tens of meters in width and contain appreciable quantities of copper, nickel and P.G.E. The “Platreef” pyroxenitic suite is overlain by gabbro-norite units of the Main Zone. The contact is comparatively uniform and may be marked by a mottled textured anorthositic norite phase. In other cases the hanging wall to the Platreef is disturbed and is represented by an interlayered sequence of norites and pyroxenites. This frequently occurs in areas close to known faults. It may thus reflect an early local disturbance within the magma chamber, which subsequently culminated in rupture and faulting of solidified B.I.C. units. The first occurrence of an igneous layering is recorded within the gabbro units above the “Platreef”. The “Platreef” pyroxenitic suite does not exhibit any igneous layering of any note. Irregularly shaped bodies of pyroxenite have been intruded into the Archaean granite footwall to the east of the main part of the complex. There are five major intrusives and these are known as “Satellite bodies”. These pyroxenitic bodies are considered to be remnants of the Basal Zone of the B.I.C. The rocks of Potgietersrus limb of the B.I.C. strike NNW and dip approximately 40 to the SW. The underlying sediments of the Transvaal Supergroup strike NW and dip to the SW and conditions for an overlap at the base of the complex are thus satisfied. The regional geological pattern is disturbed by NE trending faults. These structures are fairly common and are predominantly normal faults with a downthrow of the southern limb. Within the southern area a fairly major fault striking NNW has been recognised. This strike fault marks the contact between the Bushveld and the sedimentary floor rocks over a distance of in excess of a 1 kilometer section on Macalacaskop. Numerous microgranite (aplite) veins and the pyroxenite satellite bodies, have a similar orientation and may reflect the existence of some underlying structural control on these intrusives. The contact of the basic rocks and the sedimentary units exhibits a slightly sinuous pattern. The contact is disturbed and interrupted by the “dolomite tongue” on Sandsloot and a promentary of quartzite on Macalacaskop. Both features represent truncated “diapirs” of footwall material which has been injected into the magma chamber. The Platreef “pinches and swells” in thickness and exhibits an irregular footwall contact and a slightly “rolling” hanging wall contact. Local dip values are thus quite variable although an average dip of approximately 45 may be assumed. Tweefontein Hill

Mine Technical Services Generalised section from Macalacaskop in South to Nonneworth in the North showing the footwall lithology changes.(Kinnaird et al.,2005 Mineraluim Deposita in press)

Airborne magnetic survey Mine Technical Services Zwartfontein South PPLNorth Project Airborne magnetic survey

MERENSKY REEF: A DEFINITION (LEE, 1996) A plagioclase-bearing (feldspathic) orthopyroxenite, olivine orthopyroxenite, or harzburgite layer, located at the base of the Merensky unit, and enriched in economic amounts of base metal sulphide and platinum-group elements. The texture is coarse-grained pegmatoidal, partly pegmatoidal or medium grained. Thin chromitite layers (two to four) are associated with the upper and lower limits of economic mineralization. The Merensky Reef is conformably overlain by medium- to coarse-grained poikilitic feldspathic pyroxenite, constant in thickness. The Merensky Reef is paraconformable to the uppermost units of the Critical Zone. In the case where these units are plagioclase cumulates, the Merensky Reef may be directly underlain by an anorthosite, conformable with the Merensky Reef, and variable in thickness.

WESTERN BUSHVELD: Merensky Reef Facies

Grade drops with thickness Merensky Reef Types: Metal Profiles Swartklip Facies Cu, Ni and PGE correlate Grams Constant Grade drops with thickness Rustenburg Facies

Normal Reef Sub-facies Merensky Reef Types: Northam Normal Reef Sub-facies HW chr sul chr sil Base Metal Sulphide: Pyrrhotite Pentlandite Chalcopyrite sulf chr

Regional Pothole Sub-facies Merensky Reef Types: Northam Regional Pothole Sub-facies NP2 Reef P2 Reef Significant PGE in the footwall due to Merensky magma infiltration reactions

MERENSKY REEF The PGEs are concentrated in the base metal sulphides within the basal chromitite Mostly chr, white grain = sulf (reflected)

Platinum Group Metals

UG2 CHROMITITE: Metal Profiles

Minéralisations

Mine Technical Services Distribution of rock types and ore zone Norite Pyroxenite Calc-silicate PGE mineralisation cuts across the lithologies – rafts/xenoliths of f/w within the pyroxenitic package not uncommon Parapyroxenite Dolomite

PGM Types - Platreef vs Merensky PGE Alloy PGE Sulphides PGE Tellurides PGE Arsenides Laurite Gold Merensky Platreef

PGM + Au Recoveries on the Platreef Serpentinite/Parapyroxenite 68 - 77% Pyroxenite 86 - 93% Serpentinised Pyroxenite 72 - 80% Calc - Silicate 17 - 20%