Platinoids in the Witwatersrand system C. A. Cousins. (visitor) SYNOPSIS Study of the composition, nature and mode of occurrence of platinoid minerals in the conglomerates of the Wit- watersrand System leads to some interesting conclusions. Genetic evidence strongly favours a placer origin, while comparison with modern alluvial platinoids supports this view but also indicates that they are very mature alluvials. The compositional patterns and distribution of the platinoids in the Witwatersrand basin suggest the water was saline. Study of the mineralogy was greatly facilitated by the electron-probe. SINOPSIS 'n Studie van die samestelling, natuur en manier van voorkoms van platinoiede minerale in die konglomerate van die Witwatersrand Sisteem lei tot bale interessante gevolgtrekkinge. Genetiese bewyse begunstig sterk 'n spoel- ertsafsettings oorsprong terwyl 'n vergelyking met huidige alluviale platinoiede hierdie meining onderskyf dit ook aandui dat dit baie verouderde alluviale was. Die samestellings model en verspreiding van die platinoiedes in die Witwatersrand kom doen aan die hand dat die water southoudend was. Die mineralogiese studie was grootliks moontlik gemaak deur die aanwending van die elektronebuis. INTRODUCTION Platinoids, predominantly osmium and iridium in approximately equal proportions, with lesser proportions of ruthenium and platinum and minor rhodium, occur in minute quantities in the conglomerate beds of the Witwatersrand System, associated with the gold and other heavy minerals. The presence of platinoids in the gold-bearing conglomerates was first noticed by Bettel in 18921. He succeeded in extracting osmiridium and platinum from the "black-sands", derived from the concentration of the gold-bearing ore at the New Rietfontein mine2 (later a portion of the Rietfontein Consolidated Mine, north of Germiston). Commercial extraction did not commence until 1919. The first ship- ment of 23 ozs of concentrate, pro- duced by the New Modderfontein mine, east of Benoni, was made in 1920. After 1923, when corduroy- concentration replaced plate-amalga- mation in the recovery plants of the gold mines, the recovery of platinoid concentrates commenced on a re- gional scale. Production averaged some 6 600 ozs of concentrate, con- taining some 90 per cent platinoids, from 1923 to 1956. At present, with major production from the four mines of the Evander area, the pro- duction is more than three times as *Geological Department, Johannesburg Consolidated Investment Co. Ltd. 'This paper was originally presented at a Symposium held by the National Institute for Metallurgy in February 1972.' 184 JANUARY 1973 great, but detailed statistics have not been published. The average recovery of the plati- noids from the gold-bearing con- glomerate ore in 1956 was only of the order of 3t parts per thousand million (3t mgJt) or I oz concentrate per 2 000 ozs of gold recovered. They are obviously only recoverable as a by-product and their recovery is a tribute to the efficiency of gravity concentration of high-density par- ticles. In addition to being present in the Witwatersrand conglomerates, al- most identical platinoid minerals are recovered from the gold-bearing basal conglomerates of two younger systems - from the Ventersdorp Contact Reef of the Ventersdorp System and from the Black Reef of the Transvaal System. In the Black Reef, concentrations several ti mes greater than in the usual Witwaters- rand ore have been reported from several widely separated localities. No comprehensive account of these platinoid minerals appears to have been published. The few papers written, many during the early years of production, are scattered through the literature relating to the Wit- watersrand formation. The present paper is an attempt to summarise this work, to study the regional patterns of composition and concentration and to attempt rational explanations of some of the problems which have been brought to light. Study of the mineralogy of the platinoid concentrates has been handicapped by the limitations of polished-section microscopy. The ad- vent of the electron microprobe has made possible a major step forward in this field. At present, my colleague, P. F. Barrass, is engaged in a study of the Witwatersrand platinoid mineralogy, using an electron-probe. He has generously consented to allow me to preview his mineralogical data. It should be noted that the data included in this paper are based on platinoid recovery values. Only on one mine, Geduld, has the per- centage recovery been estimated 3. In this case some 60 per cent was recovered. Rates of recovery may differ from mine to mine. Scatter in the patterns indicated may be partly the result of this cause. Acid treatment during cleaning processes may cause some loss of the less chemically resistant platinoid grains, but in general, the acids would only result in minor variations of composition. GENESIS The genesis of the platinoids in the Witwatersrand conglomerates is clearly related to that of the gold mineralisation of these beds. Two schools of thought exist re- garding the genesis of this minerali- sation. Most South African geologists, especially those intimately associated with the Witwatersrand mines, ap- pear unanimous in their belief that the heavy minerals are of placer origin4, 5, 6, 7. Many eminent overseas geologists, who have studied the ore, are JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY
Transcript
Platinoids in the Witwatersrand systemC. A. Cousins. (visitor)
SYNOPSISStudy of the composition, nature and mode of occurrence of platinoid minerals in the conglomerates of the Wit-
watersrand System leads to some interesting conclusions. Genetic evidence strongly favours a placer origin, whilecomparison with modern alluvial platinoids supports this view but also indicates that they are very mature alluvials.
The compositional patterns and distribution of the platinoids in the Witwatersrand basin suggest the water wassaline.
Study of the mineralogy was greatly facilitated by the electron-probe.
SINOPSIS
'n Studie van die samestelling, natuur en manier van voorkoms van platinoiede minerale in die konglomerate vandie Witwatersrand Sisteem lei tot bale interessante gevolgtrekkinge. Genetiese bewyse begunstig sterk 'n spoel-ertsafsettings oorsprong terwyl 'n vergelyking met huidige alluviale platinoiede hierdie meining onderskyf dit ookaandui dat dit baie verouderde alluviale was.
Die samestellings model en verspreiding van die platinoiedes in die Witwatersrand kom doen aan die hand dat diewater southoudend was.
Die mineralogiese studie was grootliks moontlik gemaak deur die aanwending van die elektronebuis.
INTRODUCTIONPlatinoids, predominantly osmium
and iridium in approximately equalproportions, with lesser proportionsof ruthenium and platinum and minorrhodium, occur in minute quantitiesin the conglomerate beds of theWitwatersrand System, associatedwith the gold and other heavyminerals.
The presence of platinoids in thegold-bearing conglomerates was firstnoticed by Bettel in 18921. Hesucceeded in extracting osmiridiumand platinum from the "black-sands",derived from the concentration ofthe gold-bearing ore at the NewRietfontein mine2 (later a portion ofthe Rietfontein Consolidated Mine,north of Germiston).
Commercial extraction did notcommence until 1919. The first ship-ment of 23 ozs of concentrate, pro-duced by the New Modderfonteinmine, east of Benoni, was made in1920. After 1923, when corduroy-concentration replaced plate-amalga-mation in the recovery plants of thegold mines, the recovery of platinoidconcentrates commenced on a re-gional scale. Production averagedsome 6 600 ozs of concentrate, con-taining some 90 per cent platinoids,from 1923 to 1956. At present, withmajor production from the fourmines of the Evander area, the pro-duction is more than three times as
'This paper was originally presented at aSymposium held by the National Institutefor Metallurgy in February 1972.'
184 JANUARY 1973
great, but detailed statistics havenot been published.
The average recovery of the plati-noids from the gold-bearing con-glomerate ore in 1956 was only ofthe order of 3t parts per thousandmillion (3t mgJt) or I oz concentrateper 2 000 ozs of gold recovered. Theyare obviously only recoverable as aby-product and their recovery is atribute to the efficiency of gravityconcentration of high-density par-ticles.
In addition to being present in theWitwatersrand conglomerates, al-most identical platinoid minerals arerecovered from the gold-bearingbasal conglomerates of two youngersystems - from the VentersdorpContact Reef of the VentersdorpSystem and from the Black Reef ofthe Transvaal System. In the BlackReef, concentrations several ti mesgreater than in the usual Witwaters-rand ore have been reported fromseveral widely separated localities.
No comprehensive account of theseplatinoid minerals appears to havebeen published. The few paperswritten, many during the early yearsof production, are scattered throughthe literature relating to the Wit-watersrand formation. The presentpaper is an attempt to summarise thiswork, to study the regional patternsof composition and concentration andto attempt rational explanations ofsome of the problems which havebeen brought to light.
Study of the mineralogy of theplatinoid concentrates has beenhandicapped by the limitations of
polished-section microscopy. The ad-vent of the electron microprobe hasmade possible a major step forwardin this field.
At present, my colleague, P. F.Barrass, is engaged in a study of theWitwatersrand platinoid mineralogy,using an electron-probe. He hasgenerously consented to allow me topreview his mineralogical data.
It should be noted that the dataincluded in this paper are based onplatinoid recovery values. Only onone mine, Geduld, has the per-centage recovery been estimated 3.In this case some 60 per cent wasrecovered. Rates of recovery maydiffer from mine to mine. Scatter inthe patterns indicated may be partlythe result of this cause.
Acid treatment during cleaningprocesses may cause some loss ofthe less chemically resistant platinoidgrains, but in general, the acidswould only result in minor variationsof composition.
GENESISThe genesis of the platinoids in the
Witwatersrand conglomerates isclearly related to that of the goldmineralisation of these beds.
Two schools of thought exist re-garding the genesis of this minerali-sation.
Most South African geologists,especially those intimately associatedwith the Witwatersrand mines, ap-pear unanimous in their belief thatthe heavy minerals are of placerorigin4, 5, 6, 7.
Many eminent overseas geologists,who have studied the ore, are
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Fig. I-Key to gold mines shown on plan equally certain that its origin ishydrothermal8, 9.
The evidence for placer origin liesin the clear stratigraphic correlationof the heavy minerals with thelayered conglomerates and the cor-relation of the content of each heavymineral with that of gold. Gold,pyrite, uraninite, platinoids and chro-mite all show close correlation, indi-cating that they were concentratedby similar agencies1 O. The solecommon factor, connecting theseminerals, is density - indicatinggravity control of concentration.
Evidence for hydrothermal originis largely the results of microscopicexamination. The suite of mineralsfound is markedly similar to thatfound in hydrothermal veins, ratherthan to that found in modern placers.With the exception of the platinoidminerals and some uraninite particles,the heavy minerals are devoid ofdetrital shape. The rounded particlesof the platinoid minerals, originallyconsidered to be water-worn, arealso suspect 11.
In the writer's view, the placerorigin fits the observations mostconvincingly and he therefore re-gards the platinoid minerals as detri-tal and believes that study of moderndetrital platinoid minerals will thrownew light on the composition andshape of the Witwatersrand plati-noids.
The apparent hydrothermalcharacteristics, visible under themicroscope, can be attributed tometamorphic action which has pro-duced recrystallisation and minorremobilisation.
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COMPOSITION
Table I lists typical platinoi analy-ses of concentrates from minesdistributed around the Witwaters-rand basin (Fig. I). These analysesare of the contained platinoid ele-ments only. The actual concentratescontain small amounts of gold andgangue minerals, together with tung-sten carbide, introduced from wearof the tungsten-carbide tipped drillsused during mining operations. Mostof the concentrates contain 5-15 percent of such non-platinoid material.
The composition of the Witwaters-rand platinoids is surprisingly uni-
JANUARY 1973 187
Mine DistanceI
IrI
Os
1
Ru
I
Pt Rhmiles
iWinkelhaak 34,0 39,1 12,5 13,3 1,1
2SGovt. G.M. Areas 33,3 36,7 16,5 12,5 0,9
25Crown Mines 34,9 38,0 16,9 9,5 0,7
20Randfontein Estates 32,8 36,0 16,0 14,2 1,0
27Blyvooruitzicht 31,4 35,3 17,5 14,7 1,1
63Vaal Reefs. 38,S 44,3 12,3 7,1 0,8
53Freddies 33,8 39,6 13,6 11,8
I
1,220
Virginia 30,1 35,S 14,2 19,0 1,2
Mine Reef Zone Ir Os Ru Pt Rh
Crown Mines Main Reef Zone 34,9 38,0 16,9 9,5 0,72000'
Welkom Bird Reef Zone 34,0 40,2 14,3 10,4 1,11000'
Winkelhaak Kimberley Reef Zone 34,0 39,1 12,5 13,3 1,15000'
Western Areas
I
Elsburg Reef Zone 37,3 41,2 11,8 9,1 0,6S. Roodepoort M.R. Areas V.CR. 37,3 37,6 14,3 8,0 0,7Govt. G.M. Areas Black Reef 36,0 34,4 14,4 13,0 1,2
TABLE ntOM POSITION OF PLAljlNOIDS ALONG STRIKE OF WITWATERSRAND
TABLE IIICOMPOSITION OF PLATINOIDS IN VARIOUS REEF ZONES
%50
40 Os.
"
1\ , I'--,
,,\" I' I
"r"
I'll "1\:, ',1,
1'\ I, 1\ ~I' I,"V-- \ '/ \ '- I', ,\ 1\,' I --I ",~---,, 4 \ /\ \ I
form, both geographically (Table 11)over a strike distance of some 400 km(250 miles) and through a strati-graphic interval in the Upper Wit-watersrand beds, of some 2 500metres (8000 feet) (Table Ill) ex-tending from the Main Reef Zone ofconglomerates to those of the Els-burg series at the top of the suc-cession. Not only is the platinoidcomposition uniform in the Wit-watersrand succession, but the con-centrates recovered from theVentersdorp Contact Reef and theBlack Reef have a closely similarcomposition, in spite of these havingapparently passed through a secondcycle of erosion and deposition.Compositions of individual shipmentsof the platinoids, even from a singlemine, may however show somescatter.
Plots of the five platinoid elementspresent (Fig. 2) (palladium is absentor present in trace quantities only)show that the values for Os and Ircorrelate, as do Ru, Pt, and Rh:correlation coefficients are shown inTable Ill. However the two sets ofcurves are antipathetic. This fact,together with differences in thesolubility of the particles in aqua-regia!, shows that two groups ofminerals are present. Osmium andiridium, alloyed in various phases,form the major group, representing62 to 84 per cent of the total plati-noids.
SIZE AND SHAPE
Tyler screen analyses have beenmade for the writer, by courtesy ofthe various producing mines, for a
total of 30 different concentrates.These screen analyses indicate meanvalues of 38 to 80 microns (average58 micron) and the average 90%probability size ranges from 29 to105 microns.
It should be noted that this is thesizing pattern of the recovered con-centrates. Metallurgical recoverytechniques and some wear duringmilling may be involved in this sizingpattern, as this appears to be randomwith regard to geological features.
A considerable proportion of theplatinoid grains are rounded to well-rounded. Some consist of hexagonalcrystals, in various stages of rounding.A small proportion are knobby-shaped, showing embayments andprotuberances (Plate 2).
The earlier investigators12 con-sidered the rounding of the grains tobe the result of mechanical abrasionduring transportation in water.
Koen, after a microscopic studyof the particles, disagreed and sug-gested they were primary nodulargrains or perhaps had been reshapedby chemical action after depositionll.He showed that concentric phase-zoning was present in some of theosmiridium particles-the rim con-taining less osmium than the core.He drew attention to the mixtureof well-rounded and knobby particlesand remarked on the small size ofmany of the well-rounded grains,the size of which was below the lowerlimit at which mechanical roundingcould be expected in natural waters.
The writer feels that Koen's ob-servations show clearly that mech-anical abrasion cannot be the prin-
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cipal cause of the rounding of theparticles.
In many modern alluvial platinoidconcentrates, both well-rounded andknobby particles are found13(Plate I).The particle shapes of Witwaters-rand concentrates are clearly com-parable (Plate 2).
The writer believes that the prin-cipal cause of rounding was chemicalaction, with mechanical abrasion onlyas a secondary factor. He believesthat the rounding must have beencaused during weathering and trans-pcrtation.
Both attrition and accretion bychemical action might have beeninvolved14.
MODERN DETRITAL PLATINOIDMINERALS
Mineralogy and Composition
Mertie in describing detrital plati-noid minerals remarks that "it isworthy of note that no analogous(primary) deposits of platinum min-erals have ever been found"15. Inother words the mineralogy is mar-kedly different from that of all knownprimary deposits.
The majority of these mineralspresent in detrital deposits are alloys,often complex, of the platinoidmetals. Combined platinoid minerals,when present, are usually in minorproportion.
On the other hand the platinoidminerals of known primary depositsare largely in combination with otherelements-sulphur, arsenic, antim-ony, bismuth, tellurium. Platinummay also be present in the metallicstate, alloyed with iron. The majorityof primary deposits are closely as-sociated with concentrations of mag-matic base-metal sulphides. A pro-portion of the platinoids are con-tained in the sulphides in what hasbeen described as "solid solution"1.By "solid solution", it is implied thatindividual particles of the platinoidscannot be detected, even under thehighest possible magnification, inspite of the fact that chemical analy-ses show them to be present in thebasemetal sulphide particles.
Chemical composition of detritalconcentrates shows a range varyingfrom a high platinum with a Iowosmium plus iridium content to amoderate to Iow platinum content
JANUARY1973 189
A
c
0I
SCALE2000
I3000
I MICRONS1000
I
A ColumbiaB & C. Good News Bay. AlaskaD & E . AdamsField, Tasmania.
B
D
E
PLATE I MODERN ALLUVIAL PLATINOIDS
190 JANUARY 1973 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY
A
c
SCALE100 zoo 300 400 SOD
I I I .I
1000I MICRONS
ABCDE
Main ReeF Zone - S A lands
Bird ReeF Zone - Wesbern Holdin~s
Kimberley ReeF - Kinross
ElsburQ ReeF Zone - Elsbur~Ventersdorp Contact ReeF - SoubhRoodepoort
B
D
E
PLATE 2 WITWATERSRAND PLATINOID CONCENTRATES
JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY JANUARY 1973 191
coupled with a high osmium plusiridium content.
All detrital platinoids show markedpalladium deficiency. Palladium is theleast resistant platinoid to chemicalattack by acids. It is readily solublein nitric acid and is even slowlyattacked by sulphuric acid.
In primary platinoid deposits, suchas the Merensky reef, a high pro-portion of the palladium content isleached away in the zone of weather-ing16.
The writer believes that the com-position of an alluvial deposit is re-lated to the "maturity" of thedeposit. By the term "maturity" isimplied the severity of the leachingof the primary deposit, the lengthof time of transportation and theextent of chemical attack duringtransportation.
Weathering of platinoid deposits
Very little study of the weatheringof deposits has been carried out.
Chemical reactions can occurduring the weathering of a depositand further reactions can occurduring transportation.
Although the platinoid elementsare very resistant to chemical action,it must be remembered that theymay be exposed to such attackthrough geological time-millions ofyears.
Another question is still unsolved-do the organisms, responsible forbacterial leaching, react with theseminerals and, if so, what reactionstake place?
During weathering the reactionis that of oxidation, possible bacterialleaching, the presence of sulphuricacid derived from base-metal sul-phides, and the attack by complexorganic acids.
During transportation the react-ions are connected with water, dis-solved salts and other containedchemicals.
During the process of weathering,oxidation of compounds leading toformation of the platinoid mineralsin their elemental state, appears tobe one of the primary reactions,accompanied by the leaching-out ofthe palladium content of the originalore.
The platinoids contained in thebase-metal sulphides will be liberatedas colloidal particles, possibly largely
192 JANUARY 1913
due to bio-chemical reactions. Duringthis process, accretion is a definitepossibility14. Chemical alteration andgrowth appear to be the prevalentreactions during weathering. Bothprocesses could lead to rounding ofthe particles.
During transportation, mechanicalwear and chemical leaching arelikely to be the major pattern.Studies of alluvial deposits suggestthat chlorides in the water may bea major maturing factor. Chemicalleaching could be a major cause ofrounding of the particles.
In order of resistance to chemicalattack by acids, the unalloyed plati-noid metals can be roughly listed asfollows:-
Iridium, osmium, ruthenium,rhodium, platinum, palladium. Pal-ladium is soluble in nitric acid, andin sulphuric acid at moderately hightemperature.
Platinum is soluble in aqua-regia.Rhodium is soluble in hot sul-
phuric acid.Ruthenium is not attacked by
acids, but dissolved halogens attackit at room temperature; it also formsa volatile oxide.
Osmium is attacked by strongoxidising acids, it forms a veryvolatile oxide.
Iridium is not attacked by acidsand is very resistant to oxidation.
"Maturity" of platinoid alluvials
A rough measure of the "maturity"of a deposit will be the ratio of(Os+lr) to the remaining platinoids.The higher this ratio becomes, themore mature the deposit can beexpected to be.
Primary platinoid deposits, ingeneral, contain platinum and palla-dium as their major elements. Ruth-enium, rhodium, osmium and iridiumare present in minor proportions.Thus loss of palladium will increasethe proportion of platinum, pro-ducing a platinum-rich alluvial. Grad-ual loss of platinum will increase theproportions of the minor elements.However, the loss of the major con-stituents will impoverish the placerin its total platinoid content and willtend to produce unpayable deposits.
A mature placer will containosmium and iridium as its majorplatinoids, while an immature onewill contain mainly platinum. The
composition of the Witwatersrandplatinoids will place them far in themature stage.
A sample from ColumbiCl (95% Pt,3% Os+lr, 2% Rh, Ru, Pd) consistsof flakes with minor rounding at thepoints. (Plate I A). A sample fromGood News Bay, Alaska (6°% Pt,12% Os+lr, 8% Ru, Rh, Pd) containsa proportion of well-rounded grains,others of knobby shape with embay-ments (Plate I B & C).
A sample, shown in a micrographby Young12, of osmiridium from GoldBeach, Oregon, U.S.A. shows a con-siderable proportion of rounded par-ticles, and includes a slightly wornhexagonal crystal of Os-Ir.
A sample from the osmiridium-rich placers of Adamsfield, Tasmania(Plate I, D & E) contains a mixtureof rounded and knobby particles.Microprobe examination shows atleast one particle is zoned in asimilar pattern to that of Witwaters-rand osmiridium grains.
These samples suggest clearly, thatthe more mature the platinoid alluvialhas become, (deduced from its chem-istry) the better rounded the grainshave become.
This correlation of rounding withchemical maturity suggests that therounding is at least a function of thematurity of the deposit. It couldtherefore be attributed to eithermechanical wear or to chemicalattrition, either of which processeswould tend to produce roundedparticles.
The evidence is strongly suggestivethat a process of chemical attritionis the major factor, with mechanicalwear as a supplementary cause.
This is underlined by Koen's ob-servation that particles, too smallfor mechanical rounding, are oftenparticularly well-roundedll. Chemicalrounding has no size-limit.
COMPARISON OF WITWATERS-RAND PLATINOIDS WITH
ALLUVIALS
The chemical composition of theWitwatersrand platinoids wouldplace them in the very mature cate-gory. Their composition, however,
JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY
I.1J::>-I 5
~ri 4wI-I.1J
~3<I:a:<I:a.
RELATIONSHIP aETWEENPLATINUM AND OSMIUM + IRIDIUM.
CONTENT OF ALLUVIAL PLATINOIDS
100:0.
90 "~,o
'<",80
70
'.::E 60::>Z
~ SO
-'D.40
30
20
10
0 ALLUVIAL PLATINOIDS.
+ WITWATERSRAND PLATINOIDS.
..
+ :I:++
e.10 20 1003D 40 SO 60 70
OSMIUM + IRIDIUM
80 90
Fig. 3-Relationship between platinum and osmium + iridium content of alluvialplatinoids
falls in the normal range of alluvialplatinoids. (Fig. 3). Owing to a some-what high content of ruthenium theyall scatter on the Iow Os+lr side ofthis range.
The shapes of the Witwatersrandgrains are also closely comparablewith those of any mature modernalluvial. (Plates I & 2).
The grains are generally appreci-ably smaller in size than the grainsof the three alluvial specimens whichthe writer has examined.
Arguments against the detritalorigin of the Witwatersrand plati-
noids based on their shape or com-position have obviously no factualbasis. The patterns are so markedlycomparable that a similar origin isstrongly indicated.
COMPOSITIONAL PATTERNS OFWITWATERSRAND PLATINOIDS
If the compositions of Witwaters-rand platinoids, from the analyses ofproduced concentrates, are studiedin terms of the parameters Osjlr,RujPt and (Os+lr)j(Ru+Pt+Rh) itis found that plots of these para-
(e) Osjlr shows vague antipathywith the other two para-meters.
(f) GoldjPlatinoid recovery ratiosshow some degree of correla-tion with RujPt and (Os+lr)j(Ru+Pt+ Rh) (Fig. 6 and TableIV). As other factors must in-fluence this pattern, appreci-able scatter could be expected.
Except for the last feature, thepatterns are in complete accordancewith the chemical resistance outlinedin an earlier section and the ratio(Os+lr)j(Ru+Pt+Rh) can be usedas a rough measure of the maturityof the deposit.
When the parameters RujPt and(Os+ Ir)j(Ru+ Pt+ Rh) are studiedin relation to the grade of platinoidsrecovered from the ore, there ap-pears to be a surprisingly gooddegree of correlation-antipatheticto the grade. This indicates that thericher ore is generally chemicallyless mature. When the grade re-covered and the two parameters areplotted in relation to their spatial
~ A"I, 1\
/ \ 1\ 1\ 1\/ \1\ 1\ /\ 1\ /1
I VI 1\" /, ,,1\1\ /\~ 1 \ 1\- 1\ 1\1,1\1\1\Ru+P\+Rh I \ 1 "\ / \ /, / \ I I / I / ',/,', , / 1__1 'I \/--'J \ I "11
~/ ,/"\ /"'\/
"\\
\ /v
I2
34
57
109
1413
1116
2220
2115
1218
2729
2831
3233
3537
3839 4041 42
4344
4547
5051
5253
5861
6264
6567
6869
7172
MAP REFERENCE No OF MINE
Fig. 4-Platinoids parameters Osjlr, Ru/Pt, Os + Ir/Ru + Pt + Rh, around Witwatersrand basin
JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY JANUARY 1973 193
~ . ~FREDDIES J-11>3Ea A. 3<!, B. I.ISi C. 2 ';[6F S GEDULQIA. 4.2 Ea-64B 1.46 ..C. 3,63 ~ELKOM -67
.WES,HOLD. ~,Ea A. 42A. 2iS Ea-6S, B. 1.07B. 'iSO
ft+C. 2.40
c. 3iOS4T. H~E NA PRES. STEYN. I
'. Ea-68 A. 3'9'\.66 j!Ea-69 B. 1.34 .1
~.:! PRES.BRAND C.. 2'S7 ....c' - \
Ql6S
"
.B. ,1,48
.HARMONY
'-, r7~, Ea-7! VIRGINIA\A. 1.7 . -nEa A. 7.7. 'e.,,31 B. 0.62
"c. 2'82 c. ,,70
\ MERRIESPRU'rT
\;: ~-73
Jc. - ,
,,-,/
position it is clear that the highergrade ore tends to occur on mineswhich outcrop or sub-outcrop. Whenfollowed down-dip, the grade de-creases and the maturity parametersincrease. Complications arise in manymines due to the extraction ofseveral reef horizons (Figs. 8, 9, 10),but where the ore is derivedfrom a single horizon, this patternis quite clear (Figs, 7 and 11).
The loss of platinum relative toruthenium and the increased pro-portion of Os-Ir relative to the otherplatinoids, with the decrease in theplatinoid content and distance fromthe shore-line appears to indicateclearly an increase in maturity, cor-relating with the distance of trans-
Fig. 11
portation within the Witwaters-rand basin.
This suggests that the water of theWitwatersrand sea was sufficientlychemically active to leach Pt and Ru.Sodium chloride is known to formsoluble double salts of the platinoidelements under oxidising conditions,suggesting that the water was saline.
Most of the chemical action on thesource platinoids must have takenplace during weathering and erosionand transportation from the hinter-land of the Witwatersrand basin, sothat the platinoids when they enteredthe Witwatersrand basin were ap-parently very mature, and were ap-parently of a generally uniform com-position (Table V). However, after
they entered the Witwatersrandbasin, further differential chemicalleaching must have taken place.
The entrant points of the plati-noids into the Witwatersrand basincould be indicated as positions wherethe platinoid concentration is a maxi-mum and the maturity factor is Iow.
The Witwatersrand gold field canbe divided into various units:-
(a) Evander Area.(b) East Rand Basin.(c) Central Rand.(d) West Rand(e) West Wits Line.(f) Klerksdorp.(g) O.F.S. goldfield.The platinoid patterns indicate the
following entrant points:(a) Entrant from south at Bracken
Mine.(b) Entrants from north at Modder
"B" and from south east atSub Nigel, and from south eastat Wit. Nigel.
(c) No evidence of entrant.(d) Entrant at north of Randfon-
tein Estates.(e) Entrant from north west at
Blyvooruitzicht.(f) Insufficient data. (On only one
mine are platinoids recovered).(g) Entrants from west at Lorraine
and from east at Virginia.Regional variations in average com-
position are small. Table V gives theaverage composition (not weightedfor yield) for mines in the unitslisted earlier. Scatter for these aver-age compositions is small. As themines are spread along 400 km (250miles) of strike it is obvious that thesource material must have beensurprisingly uniform in composition.
PLATINOID CONTENT RELATEDTO STRATIGRAPHY
Very little reliable evidence isavailable regarding the relative plati-noid content of individualreefs. The
Ru Pt Rh
1,20,80,80,80,81,2
196 JANUARY 1973
14,3 11,6 0,9
JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY
content is generally far too Iow forindividual assay results and data areonly available for all the ore milled.
An impure sample of concentratereputed to come from DominionReefs mine, working the pre-Wit-waters rand Dominion reef, collectedby the late Mr C F. M. Bawden atGoldfields laboratories, containedsperrylite and minor osmiridiumgrains.
No information is available re-garding reefs froni the lower Wit-watersrand beds. There is no recordof recoveries from the Afrikandermine, in the Klerksdorp area, whichworked reefs in this formation.
In the Main reef zone, consistingof the Main Reef, Main Reef leader,Carbon leader, South Reef andvarious minor reefs, evidence for theplatinoid content of individual reefsis lacking.
In the East Rand the so-calledMain Reef leader, in all probabilitythe South Reef, showed markedplatinoid enrichment near entrantpoints.
The Bird reefs are only worked inthe west of the Central Rand, in theWest Rand, in the Klerksdorp andO.F.S. gold fields. In the West Rand,at the East Champ D'Or mine, thegrade of platinoids fell markedlywhen only Bird reefs were mined,compared with the period duringwhich both the Main and Bird reefswere milled.
The Bird reefs of the Klerksdorparea appear to be Iow in platinoidsbut those in the O.F.S. gold field arevariable. Near entrant points theirplatinoid content is well above aver-age.
The Kimberley reefs are workedin the Evander area, the East Rand,at the Durban Deep mine and in theWest Rand.
Those in the Evander area havethe richest grade in platinoids, sofar found within the whole Wit-watersrand gold field.
Papenfus records that at Govern-ment Gold Mining Areas, the Kim-berley "A" reef was much lower ingrade than the Kimberley "C" reefin which very high platinoid valueswere found byassay17.
At the Durban Deep, inclusion of:t 4°% ore from Kimberley reef didnot improve the platinoid content-it appears to have impoverished it.
The Elsburg reefs of WesternAreas contain only a Iow platinoidcontent.
The Ventersdorp Contact Reef ofSouth Roodepoort Main Reef Areasis interesting. Its platinoid contentis above average and the maturityfactor is Iow. This suggests that itmay not be re-worked Witwaters-rand ore, as all the surroundingmines, except Randfontein, are moremature.
The Venterspost, West Driefonteinand Western Deep levels mines allwork V.CR. in addition to Mainreef zone ore. All are relativelyimmature. The Black reef is appar-ently generally rich in platinoids.Concentrates figured by Young arefrom the Eastleigh mine at Klerks-dorp12. At Randfontein Estates, Swie-gers found a platinoid particle in apolished section of Black Reef ore18.Wagnerl mentions high values fromthe Vesta Mine (S.W. of Johannes-burg) and records that platinoidswere recovered from working thedumps of small mines near Natal-spruit (SE ~f Johannesburg). A mar-ked increase in the platinoid contentwith increa~ing proportion of Blackreef ore mined was noted at Govern-ment Gold Mining Areas17. Analyti-cal evidence from this mine indicatedno major change in platinoid com-position or maturity with increasein Black reef ore content relativeto Main Reef ore. Unfortunatelyno analyses of platinoids, from minesworking Black Reef only, are avail-able. The micrograph published byYoung12 shows that the particles ofplatinoids recovered from the Blackreef were closely similar to thosefrom the Witwatersrand.
MINERALOGY
Although it appears that early in-vestigators realised that more thanone platinoid mineral was presentin the concentrates, they apparentlybelieved that only alloys were pre-sent.
Wagnerl found that when a con-centrate was treated with aqua-regia nearly all the platinum andruthenium content reported in thesolution, while the osmium andiridium proved insoluble.
Koenll, after cleaning the concen-trate with aqua-regia, recognised
JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY
three different Os-Ir alloy phases.He found that some of the grainswere concentrically zoned. Hiem-stra19 recognised sperrylite in theconcentrate. de Kock2O describingthe concentrates from mines in theWest Wits line, mentions the pres-ence of sperrylite, cooperite, brag-gite, platinum, platiniridium andosmiridium. Braggite is however,suspect, as it contains palladiumwhich is virtually absent in Wit-watersrand platinoids. The listedoptical and mineralogical propertiesof this mineral and those of cooper-ite were reported by leonard et al21to be confused and over-lapping.This may explain the reported re-cognition-the mineral was probablycooperite.
Barrass22, after examining a rep-resentative collection of concentratesfrom the present producing mines,has confirmed the presence of threephases of Os-Ir alloys. Some of thegrains show the concentric zoningnoted by Koenll.
Sperrylite frequently forms thes.kin of a platinum rich particle, inaddition to more complex zonepatterns. laurite also appears tofavour the skin of Os-Ir particles.
Cooperite forms complex inter-growths with platinum.
Platinum grains show iron-rich andiron-poor phases, frequently inter-grown.
The mineralogy is currently beingstudied by Barrass22 and doubtlessfurther interesting data will bebrought to light.
PHASE-ZONING OF Os-IrPARTICLES
A small proportion of the Os-Irgrains show phase-zoning. The mostcommon type is concentric zoning,having a characteristic pattern. Aroughly circular (sometimes hexa-onal) core of an Os-high phase issurrounded by a phase in which Osis roughly equal to Ir and this issurrounded by a shell of an Os-Iowphase.
The cause of this zoning has notbeen determined.
Chemical accretion could be a pos-sible cause. Zoned grains show fea-tures resembling those of agates.Desborough23 says that he has notfound concentric phase-zoning in
JANUARY1973 197
Os-Ir particles from Goodnews ~ay,Alaska which he is currently studying.He suggests the zoning may be ofmagmatic origin or may be the resultof leaching of osmium from the peri-pheral parts of the grains.
Leaching could not account formany of the other zone patternsfigured by Koenll. There is also diffi-culty in accounting for the enrich-ment of osmium in the core-phase.The general tenor ofthe osmium andiridium content indicate that thegrains contain approximately equalproportions of Os and Ir.
Koenll described three phases ofOs-Ir alloys and these have beenconfirmed and studied by Barrass22using the electronprobe.
Iridosmine i high in Os and Iow inIr is found in the core-zone of con-centrically zoned grains. Only in-significant proportions of other plati-noids are present in this phase.
Iridosmine ii roughly equal pro-portions of Os and Ir. Ru is a minorconstituent. Found in the inter-mediate zone of concentrically zonedgrains.
Osmiridium Iow in Os, high in Ir.Pt and Ru are significant but minorconstituents. Found as the outerphase in concentric zoning.
In the Witwatersrand particles thezone contacts appear sharp, but azoned particle from the Tasmanianalluvials had numerous fine radialvein lets of the outer phase passinginwards into the inner phase.
Occasional particles of hexagonalshape occur both in Witwatersrandconcentrates and in at least somealluvialsl2. If reference is made toPlate 2 E, it will be noted that two ofthe grains, shown in the micrograph,have roughly hexagonal holes in themiddle of the grains. It seems pro-bable therefore that the hexagonalgrains may be core-portions of theseOs-Ir particles, broken out, eitherduring transportation or duringtreatment of the ore. Many of thehexagonal particles show sharp out-lines, suggesting that they are muchless mature than the more commonrounded particles. Examination ofthese hexagonal particles with X-raydiffraction equipment indicate thatthey are composed of randomlyoriented crystals.
The zoning therefore may be dueof some process of accretion - pos-
198 JANUARY 1973
sibly resembling that involved in thegrowth of agates and similar concre-tionary bodies.
The source mineral of the Os-Iralloys is not certain. Os-Ir particleshave been observed in primarydeposits, but generally the majorquantity of these elements is con-tained in the ruthenium mineral,laurite (RuS2)'
Laurite has been observed in theTasmanian and Witwatersrand plati-noids22 and has been found in theGood News Bay alluvials21.
Laurite is a difficult mineral todecompose under ordinary analyticaltreatment, so there are difficulties inthe way of suggesting that the Os-Irparticles are derived from this source.However, if it was decomposed,growth zoning might occur. Thatplatinoid accretion might take placeduring the weathering of platinoidswas suggested by Ottemann andAugustithisl4.
SOURCE OF THE WITWATERS-RAND PLATINOIDS
The platinoids, together with chro-mite, have most probably been de-rived from basic to ultrabasic rockswhich occur in the lower portion ofthe Archean formations which pre-date the Witwatersrand.
From the evidence of modernalluvials it is believed that the Wit-watersrand platinoids represent avery mature stage of the chemicalattrition of the source platinoids.
Thus the source ore might havehad a completely different compo-sition. It was most probably as-sociated with magmatic sulphide con-centrations and the writer believeswas most likely platinum and palla-dium rich, with only minor pro-portions of the rarer metals. It couldvery easily have contained combinedplatinoid minerals and have beenpoor in metallic alloys. The Laurite-Roseite14 mineral series could havebeen the source of the osmium,iridium and ruthenium.
The very Iow content of platinoidscompared with that of gold in the orecould have been due to the loss of themore plentiful, but less chemicallyresistant, platinoid elements.
ACKNOWLEDGEMENTS
The writer wishes to express his
siricei'e thanks for the invaluableassistance given to him by all themining Groups. Their ready co-operation is another instance of theaid they invariably give to scientificinvestigation. He wishes therefore tothank the Consulting Engineers andConsulting Metallurgists of the fol-lowing groups:
African Exploration Co. Ltd.Anglo-American Corporation of
SA Ltd.Anglo-Transvaal Consolidated In-
vestments Co. Ltd.General Mining & Finance Co. Ltd.Goldfields of SA Ltd.Rand Mines, Ltd.Union Corporation Ltd.Specimens of alluvial platinoids
were generously supplied to thewriter, who wishes to thank:
Matthey Bishop, Inc.Tasmanian Dept. of Mines.Or Murray of Goldfields Labora-
tories and Mr Evans, ConsultingGeologist of Union Corporation, Ltd.are thanked for their assistance inobtaining specimens.
The Consulting Engineers and Con-sulting Metallurgist of the Johan-nesburg Consolidated Investment Co.have given the writer every assistanceand encouragement in the prepara-tion of this paper.
The writer wishes to thank hiscolleagues in the Metallurgical, Geo-logical and Mineralogical departmentsfor assistance and helpful discussions.
He especially wishes to thankMr P. Barrass for a preview of hismineralogical research into the Wit-watersrand platinoids, for valuablediscussions and for supplying photo-graphs of the concentrates andalluvials.
He wishes to thank Mrs Gower fortyping the manuscript and Mr Swane-poel for preparing the plans.
REFERENCESI. WAGNER, P. A. (1929). "Platinum Deposits and
Mines of South Africa". Oliver and Boyd, Edin-burgh 1929.
2. BARING HORWOOD C. (1912). "Iridosminefrom the New Riedontein Mine". Trans. geol.soc.S.Afr.IS, p.SI, 1912.
3. WILLIAMSON, J. E. and SAVAGE, J. A. (196S)."The determination of Osmiridium in Wit-watersrand Ores". Jour. S. Afr. Insl. Min. andMelal/. Jan. 1965.
4. YOUNG, R. B. (1917). "The Banket of the S.African Goldfields". Gurney and Jackson, London.p.26.
S. LIEBENBERG, W. R. (l9S5). "The Occurrence andOrigin of Gold and Radioactive Minerals in theWitwatersrand System the Ventersdorp Contactreef and the Black Reef". Trans. geo/. soc. S. Afr.S8. pp. 101-227 (195S).
6. LIE8ENBERG, W. R. (1960). "On the Origin of
JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY
'Uranium, Gold and Osmiridium in the Conglo-merates of the Witwatersrand Goldfields". N.J.jb. Mineral. Abh. 94, pp. S31-867 (1960).
7. RAMDOHR, P. (195S). "New Observations on theOres of the Witwatersrand in 5. Africa and theirgenetic significance". Trons. geol. soc. S. Afr. An-nexure. vol. 59 (l95S).
S. GRATON, L. C. (1930). "Hydrothermal Origin ofthe Rand Gold Deposits". Part I. Econon. Geol. 25,supplement.
9. DAVIDSON, C. F. (1953). "The Gold-UraniumOres ofthe Witwatersrand". Min. Mag. Feb. 1953.
10. COUSINS, C. A. (1956). "The Value Distributionof Economic Minerals with special reference to theWitwatersrand Gold Reefs". Trons. geol. soc. S.Afr.59, 1956.
11. KOEN, G. M. (1964). "Rounded Platinoid grains inthe Witwatersrand Sanket". Trons. geol. soc. S. Afr.67, p. 139. 1964.
12. YOUNG, R. B. (1912). "Note on the origin ofIridosmine in the Banket". Trons. geol. soc. S. Afr.IS, p. 113, 1912.
13. LINDGREN, W. (1933). "Mineral Deposits",McGraw Hill Book Co. Inc. New York, 1933,p. 246, 77S.
14. OTTEMAN, J. and AUGUSTITHIS, S. S. (1967).
;'Geochemistry and origin of 'platinum nuggets' inlateritic covers from ultra basic rocks and bir-birites of W. Ethipoia". Min. Deposita I, pp. 269-277, 1967.
IS. MERTIE, Jun. J. B. (1969). "Economic Geology ofthe Platinum Metals". U.S. Geol. Surv. Profession.Paper 630, Washington, 1969.
17. PAPENFUS, J. A. (1964). "The Black Reef in theWitwatersrand Basin". Geology of some ore de-posits of Southern Africa, vol. I, p. 212. Geol. Soc.S. Afr. 1964.
IS. SWIEGERS, J. V. (1940). "Gold, Carbon, Pyrite andother sulphides in the Black Reef". Trons. geol. soc.S. Afr. 42, p. 36. 1940.
19. HIEMSTRA, S. A. (1964). Discussion on Koen'spaper. Trons. Geol. Soc. S. A(r. 67. p. 288.
20. DE KOCK, W. P. (1964). "Geology and EconomicSignificance of the West Wits Line". Geology ofsome ore deposits of Southern Africa. Vol. I,p. 358. Geol. Soc. 5, Afr. 1964.
21. LEONARD, B. F., DESBOROUGH. G. A. andPAGE, N. J. (1969), "Ore Microscopy and ChemicalComposition of some Laurites". Am. Mineral 54.
1969.22. BARRA5S,P. F. (1971). Personal communication.23. DESSOROUGH, G. A. (1970). Personal communi-
cation.
BI BLlOG RAPHY
ELLlSTON, 1. and HUGHES, T. D. (1965). "PlatinoidDeposits of Tasmania". Geol. of Australian OreDeposits, Vol. I, Sth Commonwealth Congr. Min.and Metall. Australia and New Zealand, 1965.
LAWN, J. G. (1925). Anniversary Address. Proc.geol. soc. S. Afr. 27, 1925.
MINERAL RESOURCES OF THE UNION OF SOUTHAFRICA. Govt, Printer, Pretoria, 1959.
PRENTICE, T. K. (1940). "Precious Metal constituentsof Witwatersrand Ores". Proc. Assoc. Scientific andTech, Sacs. S. Afr. 1939-40.
PRETORIUS, D. A. (1964). "The Geology of theCentral Rand Goldfields". Geology of some oredeposits of Southern Africa. Vol. p. 101. Geol.Soc. S. Afr. 1964.
QUIRING, H. (1962). "Platinmetalle" Ford. Enke,Stuttgart, 1962.
The Federation of Societies ofProfessional Engineers
TO THE SECRETARIES OF ALL MEMBER AND
AFFILIATED SOCIETIES
Dear Sir/Madam,
SALARY SURVEY 1973
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