INTRODUCTIONThe Bushveld Complex, whichincludes the largest knownmafic intrusion in the world,reveals some spectacular geol-ogy, including the classic layer-ing of dark ultramafics andlight-colored anorthositic rocks(Fig. 1). Three distinct groups ofrocks make up the complex(Tankard et al., 1982), the old-est of which is the RooibergGroup (dominated by rhyolite)and the youngest, the LebowaGranite Suite (including a suiteof granophyres). The majorityof the ore deposits are, how-ever, restricted to the interven-ing group of ultramafic-maficrocks, or Rustenburg LayeredSuite. The Rustenburg LayeredSuite is intrusive into the sedi-mentary rocks and subordinatevolcanic rocks of the TransvaalSupergroup (ca. 2.5–2.1 Ga) and hasbeen dated at 2.055 Ga (Scoates andFriedman, 2008). The Bushveld containsthe greatest concentration of mineralwealth on the planet and includes, inaddition to the platinum group element(PGE) ores, base metals (e.g., chromium,iron, tin, titanium, and vanadium) andindustrial minerals (e.g., andalusite,dimension stone, and magnesite), asdescribed by Willemse (1969) andWilson and Anhaeusser (1988). Miningoperations have spawned extensivetowns and associated industrial com-plexes, including downstream process-ing plants with smelters and preciousmetal refineries, many of which usetechnology specifically designed for theBushveld ores.

THE RUSTENBURG LAYERED SUITEThe Rustenburg Layered Suite formsthree principal limbs (Fig. 2) together

with a number of smaller satellite bod-ies. The eastern limb is particularly wellknown owing to the excellent exposures

(Hall, 1932). Here, theTransvaal Supergrouphas been tilted anduplifted in response tovertical tectonicsinduced by intrusion ofthe mafic rocks (Daly,1926) to form a 2,000-m-high range, whereasthe Rustenburg LayeredSuite crops out in widevalleys associated withthe Olifants River sys-tem, and also forms arugged, 250-km-longescarpment. This areainfluenced developmentof the Great Escarpment,which in turn formed byuplift during theCenozoic era (Du Toit,1933) and has resultedin a discrete geographic-

botanic system known as theMiddleveld (locatedbetween the central

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Discovery and Geology of the Platinum Group ElementDeposits of the Bushveld Complex, South AfricaROGER N. SCOON (SEG 1985 F),† Postnet Suite 291, Private Bag X31, Knysna 6570, South Africa, and ANDREW A. MITCHELL(SEG 2001 F), Department of Geology, University of KwaZulu-Natal, Private Bag X54001, Durban 4000, South Africa

† Corresponding author: e-mail, rnscoon@iafrica.com

FIGURE 1. Classic layering at the contact between the Lower Criticaland Upper Critical zones in the Olifants River section, Eastern limb.Dark-colored layers of feldspathic orthopyroxenite and chromitite areintercalated with light-colored layers of anorthosite.

FIGURE 2. Schematic map of the Bushveld Complex depicting the three main limbs of theintrusion with mines and other localities referred to in the text: 1 = Winnaarshoek, 2 =Maandagshoek, Mooihoek and Driekop, 3 = Onverwacht, 4 = Brits, 5 = Kroondal, 6 =Union, 7 = Amandelbult, 8 = Sandsloot. Outliers and satellites not shown.

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plateau and coastal lowlands). In con-trast, the western and northern limbsare covered by thick soils on theHighveld. Knowledge of these areas islargely based on mining activity andextensive drilling programs, mostnotably in the western limb, where min-ing has revealed details of some of theeconomic reefs over many tens of kilo-meters. Despite excellent overviews ofthe Rustenburg Layered Suite (Wagerand Brown, 1968; Von Gruenewaldt etal., 1985) our understanding is inhib-ited by subjective interpretion of fieldrelationships between widely separatedchambers which are unlikely to beinterconnected.

The eastern and western limbs aresubdivided into sectors on the basis ofregional differences in the stratigraphy,although this has never been formal-ized. Sectors are separated by struc-turally complex boundary areas.Regular changes in the stratigraphy ofthe southern sector of the western limbled Eales et al. (1988) to recognize dif-ferent facies (broadly referred to asproximal and distal) on the basis of(theoretical) feeder localities. This inter-pretation is probably widely applicableand can assist with our understandingof similarities between, for example, thedistal components of both the westernand eastern limbs. Sectors are furthersubdivided into subchambers, typicallyfrom detailed knowledge of chromititelayers and PGE reefs. In the northernsector of the western limb, for example,the Union (Viljoen et al., 1986a) andAmandelbult (Viljoen et al., 1986b)mines comprise discrete subchambers.Facies changes may in part be ascribedto synBushveld tectonism (Scoon andTeigler, 1994), despite a conflict with awidely held belief that layered intru-sions occur in stable continental set-tings (e.g., Irvine, 1982). Structural dis-ruptions in the Bushveld includediapirism of the floor rocks (Uken andWatkeys, 1997), and development ofthe Rustenburg Layered Suite on theflanks of domes was influenced by asso-ciated episodic uplift (Scoon, 2002). Thisis ascribed to multiple phases ofmagma replenishment—an importanttenet of our understanding of theRustenburg Layered Suite, as proposedand discussed by Eales et al. (1988).

The Rustenburg Layered Suite attainsa maximum thickness of some 9 to 12km within the western and eastern

limbs, whereas the sequence in thenorthern limb is significantly truncated.On a smaller scale, each sector (andeach subchamber) reveals considerablevariability (South African Committeefor Stratigraphy, 1980). The uncon-formable relationship with the floorrocks, which is most apparent in theeastern limb (Sharpe, 1981), results inthe lowermost parts of the Suitesequence being spatially restricted intheir development, with only the upper-most layers being laterally extensive.Various parental magma types havebeen identified to explain the develop-ment of the Suite (e.g., Eales, 2002),over and above the effects of upwardfractional crystallization (Wager andBrown, 1968), although the mecha-nisms of magma intrusion are poorlyconstrained. Scoon and Teigler (1994)and Uken and Watkeys (1997) sug-gested that the thermal and loadingeffects of new influxes are important inenhancing floor rock irregulari-ties, which must be accountedfor in the interpretation of lat-eral variations between sectorsand subchambers.

The Rustenburg LayeredSuite is subdivided into zoneson the basis of a repetitivecyclicity and laterally extensivemarker layers (Fig. 3). The Mar -ginal zone consists of relativelyfine grained norite and felds-pathic pyroxenite with little dis-cernible layering. The Lowerzone is almost entirely ultra-mafic and is dominated by lay-ers of dunite, harzburgite, andorthopyroxenite. It is theCritical zone (Hall, 1932), how-ever, that reveals the most spec-tacular layering and containsthe PGE deposits and chromititelayers. The modern practice isto recognize a Lower Criticalzone, dominated by feldspathicorthopyroxenite, and an UpperCritical zone, with more com-plex layering of lithologicalunits, typically including felds-pathic orthopyroxenite andnorite-anorthosite. Chromititelayers, which are a definingcharacteristic of both the Upperand Lower Critical zones, aresubdivided into Lower, Middle,and Upper groups. Chromitemining is mostly concentrated

on the somewhat higher grade layers inthe Lower Critical zone and lower partsof the Upper Critical zone (Schurmannet al., 1998). The Main zone that over-lies the Critical zone is characterized byrelatively monotonous sequences ofnorite and gabbronorite, albeit withprominent layers of anorthosite. TheUpper zone includes numerous Ti mag-netite layers intercalated with magnetitegabbro, anorthosite, and ferrodiorite.

PLATINUM DISCOVERIESDetails of the original discoveries ofplatinum are summarized here fromthe review by Scoon and Mitchell(2004a), which in turn was based onarticles and editorial comments in thepopular mining press of the time (e.g.,Merensky, 1925), as well as on a biog -raphy of Hans Merensky (Fig. 4) byLehmann (1955). Exploration started

FIGURE 3. Generalized vertical section of the lay-ered sequence from the eastern l imb of theBushveld Complex, depicting zonal subdivisionsand important marker layers.

Discovery and Geology of the PGE Deposits of the Bushveld Complex, South Africa (Continued). . . from 13

when a sample panned by AndriesLombaard from an ephemeral streamon his farm Maandagshoek, locatedsome 40 km from Lydenburg, was dis-patched in June 1924 to the Johannes -burg office of Merensky. The assayreported native Pt and Au, togetherwith iron oxide and traces of Rh and Ir.Merensky immediately undertook afield visit, during which he requestedLombaard, an experienced goldprospector, together with his cousinsSchalk and Willem Schoeman, to con-tinue the search. The “LydenburgPlatinum Syndicate” was formed byMerensky, privately funded by closefriends, with the objective of locatingalluvial and hard-rock PGE ores. Afteracquiring some mineral rights titles,Merensky returned to the field to findthat the Schoeman brothers hadpanned Pt in soils to the east of thestream. Three days later, on August 15,1924, the Syndicate located Pt in out-crops of “dark, lustrous crystallinepyroxenites and ultrabasic rocks” onMooihoek to the east of Maandagshoek.The mineralization occurred in a discor-dant body (pipe) over which the syndi-cate had to apply for claims. Discoveryof the Driekop pipe is credited to WillemSchoeman, who recollected seeing simi-lar rocks on a small hill to the north ofMaandagshoek. The Syndicate alsolocated the low-grade Twyfelaar pipe,but the Onverwacht pipe was discov-ered by Rand Mines (in October 1924),with geologist F.W. Blaine undertakingthe field program. No additional pla-tiniferous pipes have been discovered inthe Bushveld, despite the subsequentrecognition of many thousands of dis-cordant bodies!

In September 1924, the Syndicatemade the “far more important finding”of a layered reef at Maandagshoek. This

was initially credited to Lombaard, ashe undertook the rock chip sampling,but the prospecting team insisted it benamed the “Merensky reef.” Merenskywas aware that the pipes were of lim-ited size and was convinced that layeredrocks, specifically ultramafics with sec-ondary Cu, were a far more importanttarget. Wagner (1929) referred to theMerensky reef as the “Mother Lode”despite the pipes being successfullymined, and despite problems withexploitation of the Merensky reef asdescribed below. The Syndicate delin-eated the reef over much of the easternlimb, and several months later foundthe reef in the western limb; Merenskyalso assisted with discovery of thePlatreef in the northern limb. Addi -tional funding was acquired and thecompany was renamed “LydenburgPlatinum Ltd.” Evaluation of the threepipes, the Merensky reef, and somealluvial concentrations was undertaken,and many of the trenches and under-ground workings from this period canstill be examined, including several reefdeclines and drives at the Winnaarshoeklocality (Mitchell and Scoon, 2007). In1925, the company was purchased bythe Gold Fields group and floated onboth the JSE and LSE during a short-lived boom when the Pt price was fivetimes that of gold, driven by shortagesand stockpiling.

It is interesting that Merensky’s par-ents, who were German missionaries,passed through the discovery area whilefleeing an uprising in Sekhukhunelandprior to settling at Botshabelo, not farfrom the eastern limb of the intrusion,where Hans was born and lived as ayoung child. Merensky played a pivotalrole in a number of additional discover-ies, including the west coast diamondfields, the chromite deposits at Jagdlust,the apatite orebody at Phalaborwa, andthe southern extension of the Wit -watersrand gold fields, and several ofthe Lombaard and Schoeman familiesbecame successful geologists.

EXPLORATION METHODOLOGYOur research has led us to conclude thatSouth African geologists, includingMerensky, had speculated for manyyears about the possibility of finding Ptin ultramafic rocks of the BushveldComplex. Merensky first sampled andassayed rocks from the eastern limb in1904, including chromitite layers. Theassociation of PGE with the Bushveld

chromitites was discovered by Bettel(1925) in 1906 and it should be notedthat chromite was mined from theBushveld long before the platinum dis-coveries, including at the Winterveldmine, where the Onverwacht pipe islocated. Merensky also provided sam-ples for Hall and Humphrey (1908),who reported that one chromitite layeryielded a grade >6 g/t (the UG2?). Thisdid not constitute a “discovery,” as thefineness of the PGM meant extractionproblems were not resolved for manyyears (Vermaak, 1985). Discovery of thenickel-rich Vlakfontein pipes in 1923was also important, and the associationof Pt and chromitite as well as similari-ties between the pipes and the depositsof the Russian Urals were widely dis-cussed. During announcement of thediscoveries, Merensky was supported byboth A. L. Hall and P. A. Wagner, animportant point as financial scandalsresulting from earlier “discoveries,”together with problems with theWaterberg vein deposits, had hamperedprevious investment. In summary, werecognize three important componentsto the exploration methodology: knowl-edge of field relationships, an explo-ration model focusing on ultramaficrocks, and use of stream sediment sam-pling despite the polygenetic source(Oberthur et al., 2004).

SOME EARLY MINING HISTORYPrior to mining of the Bushveld ores,the main supply of Pt was from alluvialdeposits, mostly located in Russia. TheBushveld pipes are the oldest under-ground Pt mines (Onverwacht wasopened in 1925), with the exception ofsmall-scale workings at Solovyov Hill inthe Urals. The pipes were evaluated bycore drilling and trial mining. Theyyielded spectacular grades, notably atOnverwacht, where resources were ini-tially calculated as 55,000 t at 16 g/tPGE (>90% Pt) to a depth of 76 m (min-ing eventually attained a depth of 320m). They proved relatively easy to mineand process, as ore minerals, domi-nated by sperrylite and Pt-Fe alloy, werecoarse-grained and amenable to gravityconcentration (Wagner, 1929).Production costs amounted to half thePt price. In comparison, development of the Merensky reef proved far moredifficult, owing to higher mining costs,metallurgical problems caused by oxi-dation of near-surfaceore, and the presence of

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FIGURE 4. Photograph of Dr Hans Meren -sky “pointing out a diamond in matrixfrom Alexander Bay (from Lehmann, 1955).

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base-metal sulfides in deeper ore(Wagner, 1929). Some areas were trialmined, but low metal prices in 1929resulted in closure of most operations inthe Bushveld, including those on thepipes. The one exception was Kroondal,where mining of the thin yet extraordi-narily high grade Merensky reef (orewas hand sorted prior to milling)proved to be viable (Wagner, 1929).

THE PGE DEPOSITS

The UG2 reefThe UG2 reef is a textbook example of astratiform deposit: mineralization isconstrained to a layer of chromitite,albeit locally with one or more barrenpartings of feldspathic orthopyroxenite,which may vary from a few centimetersto several meters thick (Fig. 5A). TheUG2 is persistent in some sectors of theeastern and western limbs for tens ofkilometers along strike and to depths ofat least several kilometers. The footwalland hanging-wall rock units reveal con-siderable lateral variation, but, typically,the chromitite occurs in association withfeldspathic orthopyroxenite and norite-anorthosite. Mining of a narrow reef,particularly in high tonnage operations,

is a specialized business and in the caseof the UG2 is exacerbated by problemscaused by potholes (Lomberg et al.,1999), areas from a few meters to sev-eral hundreds of meters in diameterwhere part of the footwall sequence isabsent. Dilution resulting from hang-ing-wall collapse due to planes of weak-ness associated with thin chromititestringers (“leader seams”) is an addi-tional problem.

Ore reserve calculations, excludinglosses due to potholes and dilution, are,however, relatively simplistic, despiteevidence of a pronounced nugget effect.The double peak of mineralization (bot-tom and either middle or top loaded,depending on layer thickness) is persis-tent throughout the intrusion, and isdefined by total PGE and interelementratios (Hiemstra, 1986). The antipa-thetic relationship between grade andthickness has been widely remarkedupon (e.g., Viljoen and Schurmann,1998). The footwall lithology of the UG2varies from anorthosite, leuconorite, ororthopyroxenite to pegmatoidal felds-pathic pyroxenite. Minor amounts ofPGE are located in the footwall, specifi-cally if it is pegmatoidal, but are typi-cally directly associated with dissemina-tions or stringers of chromitite. Theirregular basal contact of the chromitite

when the footwall is pegmatoidal con-trasts sharply with the planar uppercontact. The chromitite layer may bethinner in areas where the pegmatoidcontains abundant chromitite, anobservation the authors have made atthe Crocodile River mine, where largeareas of the UG2 are underlain by leu-conorite. The grade of the UG2 is typi-cally 4 to 8 g/t PGE+Au. The content ofbase-metal sulfides is typically very low(avg <200 ppm Ni and Cu). The PGEoccur as discrete, very fine PGM(Kinloch, 1982); the base-metal sulfidesare dominated by pentlandite and chal-copyrite (McLaren and De Villiers,1982). Fortuitously, the higher valuePGE (Pt and Rh) together with Pd occurin PGM located in interstitial sites,enabling recovery by fine grinding andsophisticated flotation techniques, asopposed to the bulk of the Ru and Ir,which occurs in laurite that remainslocked within the grains of chromite.

The Merensky reefThe Merensky reef is also laterally con-tiguous throughout large areas of theintrusion, but what is less well known isthat in detail it is extraordinarily com-plex, particularly in comparison to theUG2, and over large areas of the easternand western limbs the grade is too lowto justify mining. Resource estimationmodels are fraught with uncertainty.This is over and above problems causedby estimating loss of resources due topotholes (Viring and Cowell, 1999),some of which form catastrophic orregional structures, and discordant bod-ies of iron-rich ultramafic pegmatite(Viljoen and Scoon, 1985). Cryptic lay-ering of PGE in the reef is an enigmaticfeature and presents difficulties withoptimizing mining cuts. PGE typicallyoccur within a composite series of layersseveral meters wide, including twochromitite stringers (typically <10 mmwide), with the highest grades restrictedto a narrow part, or parts, of this reefzone, as typified at the Amandelbultmine (Viljoen et al., 1986b). Where aso-called thin-reef facies is developed(Fig. 5B), as described at the Brakspruitsection of the Rustenburg mine (Viljoenand Hieber, 1986), or where the entirereef is condensed into a single, rela-tively thick, chromitite stringer, as inparts of Impala mine (Leeb-Du Toit,1986), cryptic layering is difficult toidentify and some spectacular grades

FIGURE 5. Vertical profiles of some typical reefs: (A) UG2 reef from the Marula mine,Winnaarshoek; (B) Merensky “thin-reef” from the Brakspruit section, Rustenburg Platinummine (after Viljoen and Hieber, 1986); (C) Merensky “wide-reef” from the Marula mine,Winnaarshoek.

Discovery and Geology of the PGE Deposits of the Bushveld Complex, South Africa (Continued). . . from 15

may be preserved (>50 g/t). In someareas, PGE are associated with intersti-tial base-metal sulfides that extend intothe footwall (Cawthorn, 1999). In someparts of the eastern limb, cryptic layer-ing occurs on a scale of centimeters in awide-reef facies (Fig. 5C), as described inthe Winnaarshoek locality by Mitchelland Scoon (2007). In the wide-reeffacies, two chromitite stringers con-strain the width of the mineralized reefzone, as is typically the case, but theyare separated by a layer of feldspathicorthopyroxenite that is relatively thick(1.8 m on average) and includes a bar-ren middling. Moreover, the lithologybetween the chromite stringers through-out most of the western limb is a peg-matoidal orthopyroxenite, an unusualassemblage the origin of which hasbeen much debated (e.g., Cawthorn andBoerst, 2006), whereas in the easternlimb the principal layer of pegmatoidoccurs below the mineralized zone.

The average grade of the Merenskyreef is similar to the UG2, al though it isfar more variable. Typically, a muchlarger number of PGM species occur(Kinloch and Peyerl, 1990) and lateralvariation of the PGM contrasts with theregularity of the primary layering. PGMare spatially associated with base-metalsulfides (1–2 %), constituting approxi-mately equal abundances of pyrrhotite,pentlandite, and chalcopyrite; the rela-tive paucity of pyrrhotite is unusual(Liebenberg, 1970). Base-metal sulfidesare an important by-product from theMerensky and assist with downstreamsmelting operations, whereas smeltingof sulfide-poor UG2 concentrates isproblematic.

The PlatreefThe Platreef is restricted to a relativelysmall area of the northern limb, and istypically located a few tens of metersabove the base of the intrusion, wherethe floor ranges from Archean graniteto sedimentary rocks of the TransvaalSupergroup. The lowermost zones of theRustenburg Layered Suite are not devel-oped in this area. The Platreef is farmore irregular than the UG2 andMerensky, is typically much thicker (upto 100 m or more), and contains PGEthat are invariably associated withbase-metal sulfides throughout (Viljoenand Schurmann, 1998). The Platreef isalso characterized by a much lowerPt/Pd ratio than the Merensky and UG2reefs, although the lower average gradesare offset by the width of the reef. The

Platreef may constitute a localizedequivalent to the Merensky reef, and atthe Tweefontein locality, a few kilome-ters to the south of the Sandsloot mine,the mineralized sequence is relativelythin and includes both chromititestringers and a pegmatoidal pyroxenite(Viljoen and Schurmann, 1998).

We find the importance of footwallcontamination to the Platreef has beenoverstated, in part as earlier studieswere restricted to an area where thefloor rocks are dolomite (White, 1994).A new terminology may assist withexplaining this: the sequence below the(barren) Main zone should be catego-rized as a Platreef unit rather than the“Platreef.” This consists of gabbronorite,feldspathic websterite, feldspathicharzburgite, and reconstituted felds-pathic pyroxenite, the latter revealing apegmatoidal or glassy texture. Theserock units are in discordant intrusiverelationship with one another and withthe overlying Main zone. Thus thesequential stratigraphy (A, B, and Creefs of earlier workers) is inappropri-ate, yet despite this, a Main mineralizedlayer (analogous to the “B” reef?) cangenerally be identified. At the Akananilocality, where deep drilling hasrevealed a relatively thick Platreef unitdowndip from the open pit Sandslootdeposit, we found PGE to be most abun-dant in sinuous layers of harzburgiteand, to a lesser extent, reconstitutedpyroxenite; the gabbronorite and felds-pathic websterite are relatively weaklymineralized.

The pipe depositsThe pipe deposits are no longer of com-mercial interest and, paradoxically,have a negative affect on reef-typemines, as they disrupt the layered wallrocks. PGE were concentrated in smallcore zones (max diam 24 m) of coarse-grained, iron-rich dunite and wehrlite(Wagner, 1929), now mostly mined out.The bulk of each pipe (diam >300 m),however, is dominated by barren mag-nesian dunite. Barren outer envelopesalso occur, and our unpublished map-ping at Mooihoek demonstrates thiscomponent is even more extensive thanthe magnesian dunite. Bushveld pipesthat do not reveal this zonation are typ-ically barren (Viljoen and Scoon, 1985).

LOW-GRADE PGE DEPOSITSAn understanding of the economicdeposits is incomplete without mention

of low-grade ores in the Bushveld. Theseinclude all the chromitite layers locatedbelow the UG2 reef, as well as thePseudoreefs that are situated betweenthe UG2 and Merensky in the north-western Bushveld. The Bushveldchromitites reveal regular upwardtrends in both their composition (e.g.,decrease in the Cr/Fe ratio) and the PGEgrade and tenor (Scoon and Teigler,1994). These chromitite layers, however,report very low contents of sulfide. Thesedata are important as they demonstratethat the UG2 and Merensky reefs do notoccur in isolation but represent the cul-mination of a general upward increasein the PGE content of mineralized layerswithin the Rustenburg Layered Suite.This trend is disrupted by the low-grade,harzburgitic Pseudoreefs. The bifurca-tion and subsequent elimination of dis-crete layers of harzburgite (bounded topand bottom by stringers of chromitite) isindicative of rejuvenation of more prim-itive magmas that were intruded later-ally and which may be interpreted asprecursors to the Merensky reef (Scoonand De Klerk, 1987).

GENESIS AND CONCLUDING REMARKSThe holistic approach advocated byWagner (1929) is important to ourunderstanding of the PGE deposits inthe Bushveld Complex. There is amarked spatial association of PGE withchromitite and other ultramafic rocksthat have sharply defined and demon-strably discordant basal contacts, andnot uncommonly sharp upper contacts.Norite-anorthosite is almost invariablybarren, except in isolated cases. Theserelationships, in addition to the restric-tion of economic ores to the UpperCritical zone, are unlikely to be coinci-dental. Rather than the model of Camp -bell et al. (1983), in which the residentmagma column provides the PGE, webelieve field relationships and mass bal-ance considerations are consistent withthe “lateral mixing hypothesis” ofScoon and Eales (1989) and Scoon andTeigler (1994). Thin, hot layers of fresh,ultramafic magma, enriched in PGE(and chromite: Eales, 2000) streamedlaterally into the different chambers ofthe intrusion. In the case of theMerensky reef, as well as some otherunits, the new, U-type magma wasintruded into an earlier-formed crys-talline substrate ofnorite-anorthosite

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(Mitchell and Scoon, 2007), an impor-tant consideration if, as suggested byEales et al. (1986), the lowermost part ofthe Main zone was emplaced after thereef. A similar hypothesis is applicableto the Platreef: U-type magma intrudedearlier-formed gabbronorite and web-sterite, forming mineralized harzburgiteand pegmatoid, which in turn wasemplaced prior to the Main zone.Chromitite layers formed by mixing ofU-type and more evolved magma (A-type), a hypothesis presented by Sharpeand Irvine (1983), albeit we have sug-gested the latter, at least in the UpperCritical zone was derived by partialmelting of plagioclase cumulates underthe influence of the new influxes of U-type magma. The spectacular grade ofsome chromitite stringers, as well asPGE-bearing chromitites with low sul-fide contents, suggests two processes—nucleation of PGM triggered by crystal-lization of copious amounts of chromiteand S-saturation—were necessary togenerate economic deposits. U-typemagmas also intruded the complex invertical conduits (Scoon and Mitchell,2004b). Partial melting of earlier-formed reefs, triggered by heat associ-ated with the magnesian dunite, gener-ated a PGE- and iron-rich melt whichdrained downward into the cores of spe-cific pipes, a hypothesis supported bysimilarities with so-called replacedMerensky reef (reef invaded by discor-dant iron-rich ultramafic pegmatite:Scoon and Mitchell, 2004a). In sum-mary, we find that hydrothermal fluidsare of little importance in formation ofPGE deposits in the Bushveld, as dis-cussed by Barnes and Campbell (1983),and recognition of orthomagmatic pro-cesses is fundamental. The uniquelylayered Upper Critical zone, containingthe world’s premier PGE ores, resultedfrom episodic replenishment by U-typemagmas that persisted long after theonset of the crystallization of norite-anorthosite.

ACKNOWLEDGMENTSWe acknowledge reviews by WolfgangMaier and an anonymous reviewer,and are particularly grateful to the for-mer for his constructive comments. Wepay tribute to Hugh Eales for his men-torship early on in our careers, and forhis ongoing interest in our research.

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Discovery and Geology of the PGE Deposits of the Bushveld Complex, South Africa (Continued). . . from 17