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cobalt-rich pentlandite ea sssling. Some of ttre pentland- ite is not stoichiometric; no systematic relation between departure from stoichiometry and metal ...

Canodian Mineralogist Yol.24, pp. 529-546(1986)

COMPOSITIONALVARIATION OF Co-RIGHPENTLANDITE:RELATIONTO THE EVOLUTION OF THE UPPERZONE OF THE WESTERNBUSHVELDCOMPLEX,SOUTH AFRICA ROLAND K.W. MERKLE aNn GERHARD VON GRTIENEWALDT Iwtitute for Geological Researchon the Bushveld Complac, University of Pretoria, Hillcrest, Pretoria 0002, Republic of South Africa

cation de l'existenced'un lien gdn6tiqueentre la formation des couchesde magndtite et le mClangede magm.as.

ABSTRAcT The uppermost [email protected] m of the upper zone of the western BushveldComplex at Bierkraal wereinvestigatedby wholerock geochemistryand mineral chernistryof olivine and sulfides. The rocks are depleted in Ni, owing to olivine crystallization and separation sf immissills sulfide melt tlroughout most ofthe upper-zonesequence.This resulted in a higb ColNi ratio in the sulfide melt and produced cobalt-rich pentlandite ea sssling. Some of ttre pentlandite is not stoichiometric; no systematic relation between departure from stoichiometry and metal contents or metal ratios is detectable. Variation of Co,/Ni in pentlandite is reflected by ColNi in coexisting pyrrhotite and in wholerock compositions. The upper zone doesnot representan unintemrpted differentiation of a homogeneous melt. Replenishmentby lessdifferentiated maena is reflected in cyclic changesin whole-rock chemistry, and in pentlandite and olivire compositions.The position of layers of magnetite at the base of geochemicalcycles strongly indicates a geneticlink betweenthe formation of maguetitelayers and magma mixing.

(Traduit par la Rddaction) Motstlds: penilandite, Complexe de Bushveld, zone sup6rieure, diffdrenciation, olivine, couchesde magndtite, mdlange de magmas, Afrique du Sud.

Ivrnopuc'rtoN

The importance of the state of evolution of the magma on the formation of economic and subeconomic sulfide mineralization in tle Bushveld Complex is the subjectof ongoing research.In the eastern BushveldComplex, the upper zone showsa systematic changein modal proportions of sulfide minerals as a consequenceof [email protected] (Von Gruenewaldt1970. Most membersof the standard Keywords: penilandite, Bushveld Complex, upper zone, as$emblage of magrnatic sulfides (shalcopyrite, differentiation, olivine, layersof magnetite,magmamixcubanite and pentlandite) exsolvedon cooling from ing, South Africa. the monosulfide solid-solution.As no extensivesolidsolution seriesexistsin the pure Cu-Fe-S systemat low temperature,differentiation will only be reflected SoMMAIRE in the modal proportions of the sulfides. Differentiation, however,not only influencesthe bulk comLes [email protected] m sup€rieurs de la zone sup6rieure du Com- po$ition ofthe sulfides, but also producesa continuplexe de Bushveld i Bierkraal ont 6td 6tudi6s par la 96o- ous compositionalchangein certain sulfide minerals. chimie (de la roche entidre) et la cristallochimie (de l,oliDuring microscope investigations of the sulfides vine et dessulfures).Les rochessont appauwic en Ni, 6tant in the upper zone of the westernBushveld, the optidonnd la cristallisation de I'olivine et la s6paration d'un cal properties (a9., color, reflectivit9 were found liquide sulfurd immissifls dans presque toute la s6rie de liazonesup€rieure.Commer6$ultat,on note le rapport 6lw6 to vary with the ColNi ratio in pentlandite. As Co ColNi dansle bain sulfurd ainsi que Ia formation de pent- and Ni behavedifferently during differentiation (Irvlandite riche en cobalt, au refroidissement. Une partie de ing 1978) and the separation of an immiscible sulla pentlandite n'est pas stoechiom6trique; nulle relation fide melt (Maclean & Shimazaki1976,Rajamani& systdmatiquen'est ddcelableentre l'dcart i la stoechioma Naldrett 1978),it was decidedto investigatethe extrie et la teneur en mdtaux ou les rapports entre m6taux. tent to which changesin pentlanditecomposition can La variation du rapport Co,tli drns la pentlanditesereflBre be related to petrological processesin the highly dansla valeur dudit rapport dansla pyrrhotine coexistante evolved melt of the upper zone. In addition, it was et dans la composition de la roche entiBre. La zone sup6hoped that the generation of a comprehensiveand rieure ne repr6ente pas la diff6renciation d'un bain fondu homogbne. La venue d'un magma moins diff6renci6 se internally consistent data-baseof pentlandite comrefldte dans des modifications cycliques d6as ls ghimigms positionswith different contentsof Co would prode la roche enti6re,ainsi que dansla composition de la pent- vide information on pentlandite stoichiometry and Iandite et de l'olivine. La position de couchesde magn6- the evolutionary trend of pentlandite compositions tite i la basedescyclesg6ochimiquesdonne une forte indiin highly differentiated magmatic rocks.

s29

s30

THE CANADIAN MINERALOGIST

Georoctcel

Bacrcnouno

In 1974, the Geological Survey of South Africa drilled 3 boreholes on the farm Bierkraal in the LayeredSuite of the westernBushveldComplex @ig. l). The 3 holes(BK-l to BK-3) intersectedthe complete upper zone from the Main Magnetite Layer to the roof contact (againstmetasedimentsand granite). Rock types are mainly ferrogabbro, ferrodiorite, anorthosite and magnetite (Walraven & Wolmarans 1979). The data presentedin this paper all pertain to the BK-l core, which penetrated the upper 1200 m of the upper zone (Fig. 2). {n impsrtant characteristic of the upper zone of the western Bushveld, in contrast to the easternBushveld, is the presence of olivine tlroughout most of the stratigraphic succession. SauprrNc aNo ANaryucAL PRoCEDURE From the BK-l core, 139 sampleswith an average spacing of lessthan l0 m were taken for analysis.An additional 90 powderedsampla of the uppermost 400 m of the upper zone from the sameborehole were made availableby Dr. R.G. Cawthorn.

Whole-rock major-element contents of 139 samples were obtained at the University of Pretoria by XRF, using standard techniques of preparation (Norrish & Hutton 1969).Whole-rock trace-element concentrationsfor Co, Cu, Ni, Zn,Zr, Nb, Rb, Sr, Ba and Y of 217 samples were obtained by using pressed-powderbriquettes.Thce wereanalyzedwith multichannel wavelength-dispersionXRF equipment at the GeologicalSurveyof South Africa by courtesy of Dr. C. Frick. Sulfur contents were determined in all samples using a LECO CS244infrared-absorption sp€strometer with a [email protected] induction furnace (housedat the Geological Survey) in an oxygen stream at [email protected] l4O0oC. Results were confirmed with a gas chromatograph. A microscope investigation of the remaining slags showed that the rock powder was completelymelted, and that volatilization of tbe sulfur presumably was complete. The lower limit of detection for sulfur (3*igpa level) was calculatedto be 3 ppm, using the accelerator as a blank. Based on 7 to l0 replicateanalysc, reproducibility (1 siema) was determined to be 2 ppm at 18 ppm, 16 ppm at 330ppm, 61 ppm at32F ppm and 400 ppmat237 weight %. LECO reference steel was used as a standard.

Ftc. l. Position of the Bierkraal drillhole in relation to the general outline of the layered sequenceof the Bushveld Complex.

VARIATION IN Co.RICH PENTLANDITB.

The ore-microscope investigation of the core is based on 252 polished thin sections. Electronmicroprobe analyseswere carried out with a JEOL 733 Superprobe(with 4 spectrometers)at the University of Pretoria. Sulfides were analyzed for S, Co, Cu, Fe and Ni. As and Zn concentrations in pentlandite and pyrrhotite are below the detection limit. The following standards were used: natural troilite for S and Fe, natural arsenopyrite for As, synthetic cobalt-pentlandite for Co, natural chalcopyrite for Cu, natural millerite for Ni, and synthetic sphalerite for Zn. Accelera.tionpotential was 15kV with a beam current of 5 x l0-8 A, measuredand monitored on a Faraday cup. For full ZAF corrections, the program FZAFM was used. Counting time for eachelement was 50 seconds,resulting in lower limits of detection (3 siena) of 2fr) ppm for Co and 220ppm for Ni in pyrrhotite. Reproducibility was controlled on a routine basis by calculating the standard deviation from double analysesof identical spots with the formula given by Kaiser & Specker(1950. Calculated reproducibilities s (l sigma) are given in brackets after the approxinate r4nge on which the calculations are based (all values in atomic 9o). Reproducibility for pyrrhotite was: S, 50 to 53.3 (0.097);Fe,6.7 to 50 (0.095);Co, 0.M to 0.09(0.005 to 0.CI8); and Ni, 0.02to 0.03 (0.(X)4).For pentlandite, reproducibility was found to be: S, 47 (0.099); Fe, 5 to A (0.W1 to 0.102);Co, 8 to 46 (0.058to 0.144);and Ni, 2 to 2l (0.038to 0.137). Dependingon the frequency and sizeofpentlandite in a particular sample, either all grains present or at least l0 big grains wereanalyzed.Up to 30 spot analyseswere made on pyrrhotite coexisting with pentlandite. The results presentedin this paper are based on a total of 1306 pentlandite and 3133 pyrrhotite analyses.

531

BUSFIYELD COMPLEX

RESULTS ano ANAT,YTTCAL OBSERVATIoNS Immiscible sulfide liquid separatedfrom the silicatemelt during crystallization of a large part of the upper zone. A large portion of the sulfide is situated in intergranular spaces; however, drop-like inclusions of sulfidesin silicatesand oxidesare com-

Fenodiorirc

BIERKRML

IAGNETIIE

Anorthosite Mogr€tile gobbro lhgnelitiis ond [email protected] mognetilile Mognotite gobbro

kin Uogntlil, Lorcr

Mognefirs gobbro Mognalite gobbro Mognotite gobbo 5OOOm

Olivine was analyzed at 20 ky and 2 x l0-8 A lrJ using pure oxides as standardsfor AI, Si, Mg, Ti, 2 R Mn and Fe, Arenal hornblende (Jarosewich e/ a/. PTRAMID Gobbronorife z GABBRO-NORITE 1979)for Ca, natural millerite for Ni and synthetic = cobalt-pentlandite for Co. Counting time was 20 secondsexcept for Al, Co and Ni, where counting time was extendedto 50 seconds.The lower limits of detection (3 [email protected])in ttre iron-rich olivine were calculated to be 120 ppm for Co and 160 ppm for Ni. Reproducibility (1 sigma) for the 5Z olivine analysescarried out was also estimated from duplicate analysesand was found to be 0.CI27 for the Me,/(Me + Fe) atomic ratio and 57 ppm for cobalt concentration.If a standarddeviation of the analyses of more than 3 times the reproducibility is taken as the criterion to reject homogeneity of olivine in a Frc. 2. Simplified stratigraphy of the upper zone of the g'ain or sampl€,all samplesexceptfrom lll2.7 m westernBushveldComplex (modified after SACS 1980) depth in boreholeBK-l can safelybe treatedasbeing and the stratigraphic successioncovered by the BK-l drill core. homogeneousin their forsterite and cobalt contents.

532

THE CANADIAN MINERALOGIST

TABII'l.

(ATotltCt) ANDCoAt MTto (ATo!ilc)()F CoMLTCoNIENT PENILANDI1E COIqPOSINONS FROII IHE UPPER ZONE Colrr

4t&.5 538 563.5 589.5 607.5 620.3 638 726

d B a a

172.5 r 798 D 8!0.5 d 822.8 B 893 o 899 E 951 d 968.9 E 999.8 o 1009.6 u 1019.6 E 1030 B 1040.2 n 1050.2 E 1060.3 D 1068.9 d 10E2.2 u 1086.7 o 1089.8 B 109! D 1096.3 B l!00.8 B 1103.7 d lll2.7 o lllT

d

34.84 38.30 44.08 42.01 43.s0 tt4.95 28.45 26.22 n.63 43.84 29.45 18.62 25.66 24.80 u.43 29.28 30.16 34.57 29.08 28.55 29.82 31.30 42.A5 t&.a6 34.28 41.81 42.14 37.52 35.19 37.81 3t.49 37.42 34.t9 29.06 25.t4 24.n 25.56 12.08

: etthEettc @, Mai h!.gha6t @lc [email protected] la avdlabla

0.32 1.62 0.63 0.13 l.3r 2.19 2.14 2.46 [email protected] 1.60 2.4\ 3.30 0.49 0.50 4.05 L.84 0.18 0.26 l.l5 2.57 2.79 1.74 L.45 0.74 3.78 1.43 r.30 2.75 !.30 0.65 r.48 L.54 2.3L 5.63 5.54 6.72 4.46 3.02

niE

ltax

34.33 36.05 43.43 4t.8X 4r.85 3r.85 22.69 23.9 16.60 41.84 [email protected] t3.27 23.92 24.11 12.52 27.32 29.92 34.31 2a.r6 23.48 24.51 29.18 t&.92 39.41 26.72 39.54 39.42 30.84 t4.28 35.61 35.67 35.96 28.53 20.63 r5.9r 11.98 19.37 a.78

35.2! 40.16 44.66 42.L2 46.43 45.46 [email protected] 34.25 25.39 45.7t 34.r,2 23.59 25.21 25.64 z',m 32.72 g.3L 34.94 9.96 31.03 35.58 35.01 44.86 41.86 tt0.l7 44.45 44.24 39.70 37.85 38.80 40.10 40.50 3{t.58 37.00 33.14 46.03 33.45 L9.97

a

4.84 7.91 43.88 21.39 t6.42 12.05 2.64 2.28 1.55 20.81 2.86 1.25 1.89 1.78 1.08 2.9L 2.99 4.58 2.77 2.83 3.24 3.38 17.06 12.15 5.59 14.10 L7.64 t5.2L 10.50 13.32 9.09 u.07 7.24 4.83 3.22 3.49 2.62 0.70

B 6t&dard oba6md, [ froB

th

lllo

0.13 l.t4 0.39 5.09 4.tfi 3.0E 0.47 0.57 0.40 9.09 0.45 0.35 0.06 0.08 0.40 0.66 0.01 0.12 0.3t 0.57 0.83 0.58 3.17 2.05 2.71 5.9r 4.53 4.4t 2.03 1.60 2.30 1.93 2.6L 2.93 t.52 7.95 L.to 0.30

devlatlotr, Ul! @Eb€r of eltse& euthof,s oa [email protected]

k

4.65 6.20 43.37 t6.97 12.05 7.70 t.75 t.10 1.03 11.76 2.13 0.74 1.78 1.65 0.67 2.14 2.99 4.43 2.5L t.95 1.90 [email protected] L3.O2 A.27 2.36 8.84 10.39 10.84 7.35 9.59 6.43 8.51 3.47 r.57 t.21 0.57 1.39 0.42

lores! I{016

4.91 a.97 41.X2 25.94 26.86 25.t9 3.70 4.32 2.29 35.r! 3.86 r.94 1.98 1.89 2.33 4.33 3,00 4.73 3.23 3.72 4.99 4.90 22.N 15.& L1.67 30.78 26.30 2L.29 t2.82 14.97 13.40 14.14 14.68 9.86 6.44 10.25 4.93 1.63

4 12 4 4 29 94 59 30 41 3 9l 133 L2 L2 A7 t2 4 4 t0 15 46 33 t2 18 30 34 50 L6 tl l4 22 t3 64 34 60 79 30 72

9e1u€ obsermd, ldfot aletall€al

mon. The sulfide minslslegy in the upper zone of the western Bushveld at Bierkraal is at first glance very uniform. It resernbleswhat is normally expected to havecrystallizedfrom a sulfide melt that unmixed from a highly evolved silicate melt constantly depletedin highly chalcophileelements,1.e.,it consists predominantly of pyrrhotite with only small amountsof chalcopyriteand minor pentlandite. Rare sulfides are sphalerite, molybdenite, loellingite, safflorite, cobaltite, mackinawite, valleriite, pyrite, marcasite, galena and cubanite. Sphalerite, molybdenite, arsenidesand galenaincreasetoward the roof of tlre complexand reflect tle enrichmentof Zn,Mo, As and Pb in the magmawith progressivefractional crystallization. Modal proportions of pyrrhotite, pentlandite and chalcopyrite vary with stratigraphic TABLE2.

REPRESE1TATIVE COIiIPOSITIONS* OF PENILAI{DIIE

D€pth

S

Fa

Nl

Co

Total

551.5E 493 o 1009.6a 1.040.2 o 1040.2! 1060.3! 1082.2E 10E2.2o 1100.8E 1103.7o lll2.7 n

33.86 33.69 33.16 33.83 33.03 32.7a 33.09 32.86 33.14 33.28 32.35

7.82 16.8r $.m L7.67 14.06 10.80 14.42 16.64 23.n 27.75 26.22

l.3r 15.17 13.00 t2.4t 9.02 2.42 3.78 4.88 12.06 14.06 25.18

57.94 35.67 tE.44 37.00 44.9! 54.19 48.61 4s.49 30.54 23.79 15.54

to0.r3 101.34 100.80 100.91 101.04 100.59 99.90 99.87 99.04 98.87 99.29

'ln

eeiSht l.

position and reflect the variations of whole-rock content of S, Co, Ni and Cu. Sulfidesin proximity of layers of magnetite tend to contain a much higher modal proportion of chalcopyrite. Mineral chemistry Pmtlandite: Althouelr pentlanditeoccursin all the sulfide-bearingrocks ofthe upper zone, amount and size of grains in the topmost 400 m are so small tlat proper documentation of compositional trends over the entire successionis not possible. Consequently, emphasiswas placed on a 225-m-thick successionof rocks between890 and 1ll7 m. In the BK-l samplesonly pentlandite u/ith a substantial content of Co was found. The pentlandite is granular in most casesand occasionally displays crystal faces. Fully idiomorphic pentlandite is rare and restricted to compositions with a high content of Co. However, eventhe most Co-rich grains are not necessarilyidiomorphic. In most sulfide ores, flame-like pentlandite exsolutions in pyrrhotite are, apart from granular pentlandite, the most common known form and indicate exsolution at low temperatures (Kelly & Vaughan 1983,Durazzo & Taylor 1982).In the upper zone at Bierkraal, however, only one sample (depth 620 m) contains pyrrhotite with flame-like exsolution lamellae of pentlandite. Formation and preservationof tlese pentlandite flames are thought to be related to a much stronger hydrothermal alteration at 620 m than in the other samples.Pentlandite grains are generallybelow 5 pn in diameter and rarely exceed15 pm. Becauseof the low bulk consentration of Ni and Co, the total amount of pentlandite in the rock is generallysmall, rarely exceeding 15 grains per section, even in the most sulfur-ris6 samples.Pentlandite may occasionally be present in sulfide inclusions in olivine or clinopyroxene, but most frequently occurs in intercumulus patches of sulfide. Pentlandite is readily altered fs minssalsof the linnaeite group or to disulfides, but despitethe hydrothermal alteration of some of the silicates, the pentlandite was not found to be affected. In the literature, Co-rich pentlandite has been describedfrom serpentinizedultramafic rocks (e.9., Harris & Nickel 1972, Misra & Fleet 1973), from hydrothermal veins (e.9., Petruk et ol. 1969), al.d from metamorphosed massive-sulfide ores (e.9., Kouvo et al. 1959,Huhma&Huhmal970, Lindahl 1973,Vaasjoki et al. 1974).The occurrenceof Corich pentlandite of magmatic origin in the Bushveld thereforeoffers a unique opportunity to investigate the changesof pentlandite composition in relation to changesin bulk-rock chemistry due to magmatic differentiation. The concentrationof Co determined and the ColNi ratio for the samplesinvestigatedare listed in Table 1; a more detailed listing of the

VARIATION IN Co-RICH PENTLANDITE,

BUSHVELD COMPLEX

533

analytical results is available from the authors on request.Someexamplesof compositionsrvith different contents of Co and cation ratios in pentlandite are given in Table 2.

is intemrpted by reversals, so that a ryclic pattern in the Co content of pentlandite can be recognized; the most obvious one is developedbetween 890 m and ll17 m in borehole BK-I. Higher up in the sequence,trends of Co enrichment can be recogFigures 3 and 4 summarizethe Co content deter- nized; however, the cyclic pattern is not as evident, mined and the ColNi ratio in pentlanditegraphically, owing to rock sequencesin which pentlandite grains and allow comparisonwith the compositional trends were not found or are too small to analyr-e. of pyrrhptite, olivine, and the whole-rock ColNi Riley (1977)summarizedanalytical data on pentratio. The pentlandite displays significant changes Iandite from the literature and found two different in composition with stratigraphic height. The trend trends of cobalt substitutions: one is restrictedto ser-

qaih lnm

+ Fer ff

6

14 2.30 38 46 0 tO ?O 30 Q 50 1234 50 46 4 5678 ofo.qtgpacgniCoinp€nthndileCo/Ni stomicrotioinpentlordileolomicpercent ColNiotomicrotio voriotionondorilhmeticrneon \Eriolionord qithmdticme([t Fein pjrrhotite ih inlermediotepyrrhotite orilhnrelicmem orilhmelicmeon ondstondorddwiolion

FIc. 3. The total variation and arithmetic meansof Co content (in atomic 9o) and ColNi ratios (atomic) in pentlandite in the topmost [email protected] m of the upper zone at Bierkraal. Also presentedare the arithmetic means of Fe contents (in atomic 9o) and arithmetic meansand I statrdard deviation of the ColNi ratio (atomic) of pyrrhotite coexisting with pentlandite.

534

THE CANADIAN MINERALOGIST

|B

.t;

FEI-

t

l

0.3 200 utoo 600 800 [email protected] 6 I lO O.l 02 4 ptn Coin olivine Co/Nl rofio Mg/(Uq"ft) otomC rnriotftn md iolio in divinc in rhole-rodronotyses wiotbn ond srilhmelic mecr ailhrEfic [email protected] variFrc. 4. Comparison of tie Co content in penllandite (seeFig. 3) with the ColM -ratio in the rvhole rock, the total mean ation and arithmetic mean of tle Mg/-Ofe + Fe) aiomic iatio in olivine, and the total variation atrd arithmetic of Co concentration in olivine. o 6 t4 22 303846 otomicpa canlCoh p€nllondite voriolionondorithmeticmecn

?

pentinites and hydrothermal veins, where Co substitutes for Fe; in the other, a feature of met?morphosed massive-sulfide ores in Scandinavia, Co replacesNi. Cobalt-rich pentlandite from the Bushveld also follows a trend of preferential replacement of Ni @ig. 5). However, becauseof the very high Fe/Ni ratio in rocks of the upper zone, this trend is set off toward higher Fe,/Ni valuesthan the trend from published data (Fig. O. The analysesof the pentlandite show that in the upper zone, the pentlandite is consistently Co-rich. The lowest observedvalue is 8.2 atomic 90, the highest,46.4, and the overall mean,28.7 atomic go (Fig. 7). This high content of cobalt is seenas the

reason for the virtual abs€nceof exsolution flames of pentlandite in pyrrhotite. Co increasesthe upper thermal stability-limit of pentlandite (rraasjokr et al. 1974), so that exsolution of cobalt-rich pentlandite from the monosulfide solid-solution starts at higher temperature than for Co-poor pentlandite. The higher rates of diffusion at elevated temperature would favor granular exsolution. It also seemsthat cobalt-rich pentlandite has a much strongertendency toward idiomorphism than ordinary Fe-Ni-pentlandite, so that flame-like exsolution lamellae (which might have formed in analogy to normal pentlandite) could have recrystallized to minute cranular grains during the slow cooling of the Bushveld.

VARIATION IN Co-RICH PBNTLANDITE,

During the analysesof pentlandite from the Bushveld, it becameobvious that individual gains deviate considerably from the assumedstoichiometric formula of MerS", although the mean value of 47.12 atomic 9o sulfur for all compositions (Fig. 8) was found to be very close to the theoretical value (47.06).It has previouslybeenassumedthat a relation betweenmetal ratios and sulfur contentsin Copoor pentlandite might exist (Rajamani & Prewitt 1973). In contrast, Page (1972), Harris & Nickel $n2), and Riley (197) did not observea [email protected] variation in the extent of nonstoichiometry in pentlandite. This, however, could be due to relatively large analytical errors or inconsistenciesin the analytical procedurefor the data compiledfrom the literature. The analytical data in this paper are consistent within tle analyticalreproducibility given above. To rule out analytical problems as the reason for deviations toward anomalously low contents of sulfur, somenonstoichiometricgrains werere-analyzed. The resultswere found to be reproducible within the expectedlinits of analytical uncertainty. Analyses with high valuesof sulfur might haveaccidentlybeen influenced by the pynhotite surrounding tle small Crains of pentlandite. If this were the case, then a correlation betweenFe contentin the pentlanditeand the S content should result. Table 3 givesthe Spearman correlation matrix for the metals, the metal ratios and sulfur in atomic proportions. No relation betweensulfur and iron can be detected.Figures 9A to C also demonstrate tlat there is no specific reason for noustoichiometry in cobalt-rich pentlandite.

a

BUSFMLD

Pyrrhotite: In the samples investigated, the pyrrhotite consists essentially of mixtures of varying amounts of intermediate pyrrhotite (Morimoto et al. 1975) and troilite, resulting in a bimodal frequency-distribution of the iron content, as determinsd wift the microprobe. Some pyrrhotite compositions with different contents of iron are given in Table 4. Becauseof this bimodal distribution of iron contents, the arithmetic mea:rs for pyrrhotite coexistingwith pentlandite (Fig. 3) in different samples cannot be compared quantitatively. They do, however, indicate differencesin the averagecontent of iron in the pynhotite betweensamples.The concentrations of Co and Ni in this pyrrhotite depend linearly on the iron content and are lowest in troilite, with Ni contents generally below the limit of detection (Merkle, in prep.). Consequently,in order to allow comparison of the ColNi ratio of pyrrhotite with that in pentlandite, only intermediate pyrrhotite compositions, with an iron content of 47.8 I 0.3 atomic 90, were selected.Thesedata pertain to the composition of the most iron-rich intermediate pyrrhotite and the interval whereD.70/o Q sigma) of all analysesof pyrrhotite with this composition can be expected. Only 667out of 3133pynhotite compositions fall into the required interval of composition, and of these only 354 contain Ni in detectable amounts. Becauseonly pyrrhotite coexisting with pentlandite wasincluded in this study, and becauseat somelevels (mainly abovemagnetitelayers) none of tle pyrrhotite could be usedbecauseof its high content of Fe,

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COMPLEX

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Ni/(Ni +Fe)otomicroflo Fto. 5. Variation of the Ni/(Ni + Fe) atomic ratio with increasing Co concentration in the pentlandite from the upper zone.

536

THE CANADIAN MINERALOGIST

Co r A o * + 0 e x a * o r

Fialdof Shishkinetol. (1974)bosadon 3!t4 doto points

ffiffi Fieldof uppar-zone ts$l pentlondi?c

Lindohl(1973) Strishkinet d. (19741 Honis I Nickel(1972) Voosjokiel ol. (l9il4l Noldrettetol.(19721 Rilcy 09rA SpringerA Croig(1975) Poge(192 Misro I Flaat (1973) Stumpfl 8 Clork(1964) Kowo 6t oL (1959) RoiomoniA Prswilt(1973)

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o

i'*' Fe Ftc. 6. Field of upper-zonepentlandite (shadedarea) and projection points of selectedpentlandite data from tie literature in the Fe-Co-Ni cation plot. The stippled lines gives the limits of known compositions of pentlandite, after Knop & 151ahim(1961).

gapsin the documentation of values of Co,/Ni ratio in pyrrhotite could not be avoided. The small number of compositions do, however, show strong similarities in the trends of ColNi in pentlandite and in pyrrhotite. Except for one sampleat a depth of lll2.7 m, which for no obvious reason does not follow the generaltrend, the pyrrhotite in samplesoverlying the layers of magnetiteis enrichedin iron. With increasing distance above layers ofmagnetite, the average content of iron decreasesbut increasesagain significantly abovethe next layer of magnetite.This pattern is bestdevelopedabovethe two magnetitelayers at depth 783.8and 823.0m. Olivine: Resultsof microprobe analysesshow that

the composition of olivine is, with one exception, very constant on the scale of a thin section. Analyses of large g:ains of olivine (about I mm) reveal no compositional inhomogeneityor zonation that exceedsthe analytical uncertainty. Large grains of cumulus olivine are chemically indistinguishable from interstitial olivine in the same section. The cobalt content of olivine is generally in the range of [email protected] to 700ppm (FiC.4), whereasthe nickel content is always below the detection limit (160 ppm: Table

s).

The atomic ratio Mg/(Mg+Fe) is low and varies between0.ll and 0.29 n the stratigfaphic interval presentedhere. Theseextremevalues were found at depthsof lll2.7 and,1103.7m, respectively(Fig.4),

VARIATION IN Co-RICH PENTLANDITE,

BUSHVELD COMPLEX

537

g

o (, t-

o'

CL

.e 30 E o

o o

u

fo 20 30 40 50 60 70 80 90 too ilo t20 FRECU.JENCY Flc. 7. Frequency distribution of Co contents in upper-zone pentlandite.

where an abrupt reversal in olivine composition appears sliehtly above the 6-cm-thick layer of magnetite at lllT m. Below this reversal in the Mg/Ug * Fe) ratio, olivine follows a strong upward iron-enrichment trend. Above the reversalthe magnesium content declines rapidly from about l2go MgO to slightly more than 690 at about 50 m above the reversal.Olivine with a slightly hieher magnesium content occurs at higher stratigraphic levels, but the density of olivine sampling doesnot allow localization of any possible sharp or gradational reversals. Despite the large scatter in cobalt content of the olivine grains analyzed, a vague relation to the [email protected] * Fe) ratio is indicated (Fie. a). However, the most iron-enrichedolivine is from a sampleabout 2 m above the layer of magnetite at lllT m. This is explained as being due to a processof infiltration metasomatism(rvine 1980).

Whole-rock chemistry If the slight variation in the sulfur content of pyrrhotite and the other minor sulfides is ignored, the bulk content of sulfur of the upper zonecan be taken as being proportional to the modal content of sulfides. The sulfur determinationscoverthe BK-l core in great detail, with sample spacing in some parts

c o o o Ct

TABI.! 3.

6 fr tu re

ro/M e/rt

SPEAR'IAN IIATRIT MR i3O6 PEI'IILAI{DITECOI.IPOSITIONS CORRELATION

{.9619 [email protected]

-0.0607 0.0283 0.0032 0.9079

{.9620 0.m0r 0.8653 0.0001 0.0997 0.m03

0.0A3t [email protected] -0.2021 [email protected]

E o

r€/&.

6/m

o.7t54 [email protected]

0.9$r 0.0001

4.9908 0.0001

-0.9933 0.m0r

0.9242 [email protected]

-0.85r5 0.m0r

.9

0.1095 0.0001

0.0335 o.2268

{.0233 0.3993

0.0837 0.0025

-{.0424 0.1255

-{.5298 0.000r

{.9066 0.0001

0.9889 o,ml

0.2555 0.0001

0.1565 0,0001

{.06e 0.0163

0.8012 o.otx)r

-0.6343 [email protected] -4.9547 0,@01

C'

o

FREOUENCY Flc. 8. Frequencydistribution of sulfur Oontentsof upper zone pentlandite. The theoretical value is 47.06 atomic a/0.

E

o (, o, CL

.9

E o o

rn

2

3

5

4

8

7

6

lO ll

9

12 13 14 15 16 17 18 19 20 21 2223

24

Fe olomlc per oenf

c o

(,

47.7

47.4 o ct 47.1 .9 E 46.4

o

o zl€. U'

4e 45.6

13 f5

lT

i9

2t

23 a

27

2e 31

33 3{t trl

39 41 43

Co otomlc per cenl

48.6 It8.3 4Ao c 471 o (, 473-

t

ql.l

ll

E

u16.8

sC' 216.5 C"

46.2 45.9

45.3

o

|

2

3

4

5

6

7

8

9

lO ll

12 13 14 l5 16 17 l8

19 204

22za24

Ni. olomlc Percent Fro. 9. Unsystematicvariation of sulfur in pentlandite as a function of Fe (A), Co @), and Ni content (C) (in atomic go).

539

VARIATION IN Co-RICH PENTLAND|ITE, BUSFTVELD COMPLEX

parablebehavior throughout the investigatedpart of theupperzone. Thesegroupsare: l) Si, Al, Ca, Na, K, Ba, Rb and Sr, 2) Fe, Ti, Mn, Nb, Zn andZr, SfeNtCo tot6l 3) P and Y, and 4) S, Co, Cu and Ni. 38.89 60.60 0.064 0.t22 99.64 38.m 60.98 0.058 0.109 100.05 Elements of group I obviously reflect the silicate 38.53 6r.12 0.056 0.106 99.52 componentin the rock; they correlatenegalivelywith 38.37 61..60 0.064 0.108 t00.14 a7.84 62.21 0.059 0.08r 100.19 most of the elementsof group 2, which include tle t7.84 62.36 0.0470.101 r00.35 36.73 62.Ar O,OO4: O.lr4 99.66 componentsof titaniferous magnetite. Phosphorus 36.3S 63.09 0.005' 0.080 99.56 reflects the apatite content of the rock. The correla36.78 63.39 0.03r. 0.076 100.28 36.74 61.61 0.000' 0.09r 100.46 tion between P and Y suggeststhat Y has a high affinity for apatite, a relationship also found by . [email protected] dotGctlon llEl.tTowell et al. (1965). Group 4 representsthe sulfide portion of the rock. The elements of this group closelycorrelatewith tlose elementsincorporated in as small as2 metres.The sulfur contentin theserocl6 magnetiteand thereforeare also negativelycorrelated varies considerably over short distanc€s (Fig. l0). with the first group. Zirconium not only correlates Despitethe small-scalevariation, however,a roughly with the elementsof the magnetite and, therefore, cyclic distribution of the sulfur contents in the core also the sulfide group, but also with Mo. Molybdeis indicated. This cyclic pattern is more clearly visi- num is presentin the rocks as molybdenite, but does ble in parts of the core with higher frequencyof sam- not follow the general sulfide trend. It is enriched ples. It is difficult to clearly define tle sizesof these in the course of differentiation; its positive conelacyclesfrom sulfur data alone, but rapid increaseof tion with Ba, Nb, Rb, Zn and Zr rcflegtsthe similaxsulfur content from lessthan [email protected] ppm to about 2ffi0 ities in their enrichment trends toward the top of the ppm or more could be taken as the baseof a sulfur Bushveld, irrespective of cyclic patterns at lower cycle, as is the caseat drilling depths 445, 638, 846, stratigraphic levels.Yttrium showsan enrichment in lll2, 1494and 1613m. Within every one of these the uppermostpart as well, but the affinity with apacycles, fluctuations in the sulfur content may inditite is too pronounced to allow a strong correlation cate superimposedsmaller cycles. It should also be coefficient with Mo. pointed out tlat layers of magnetite or rock units The elementsof the sulfide group also correlate with increasedmagnetite content, tend to coincide with Mg. As the Mg content in the silicatesis a measwith the inferred baseof the sulfur cycles.However, ure of the degreeof fractionation, similar"ity to the wheremany layersof magnetiteoccur closetogether, trends of sulfur in the cyclesimplies that sulfur and e.g., between7M and 893.m,no clearcyclesare deve- magnesiu'n were replenished in the magma by the loped. Many of the other elementsthat could be ana- sameprocess. lyzed with sufficient accuracy show cyclic patterns The variation of someelementswith height in the that coincide with the sulfur cycles. For some ele- BK-l core is presentedin Figure 10. The variation ments, however, a general increasetoward the top of the Sr contents is given as an example of the of the complex is superimposedon these cycles. behavior of a group-l component, whereas Ti According to the Spearman correlation matrix representsthe elementsincorporated in magnetite. (Table Q, four major Ctroupsof elementsshow com- Phosphorus and yttrium are presentedto show the (wt.%) OF PYRRHOTITE TABLE4. SELECTED COMPOSITIOI{S CoEXISTING IIITH PENTIANDIIE AT DEPIH1009.6 m

TASLE5. hptb

b

Sto" -FoO

ft.

'IIIAT REPRESEIITATIVE COI.IPOSITIONS OFOLIVINEFROIiI SELECIED SAI,IPLES COVER IIIE OBSERVED RAXGE 0F l4g/(tils + Fe) VATUES

u8o llno CaO EOz Total

899

r0r9.E

1050.2

t068.9

1086.7

lr.o3.7

'r2a.7

T.44 s9.7s 8.35 0.92 0.08 0.00

30. t9 59.42 9.!5 l.0l 0.03 0.02

30.4t 61.70 6.44 1.08 0.oo 0.01

29.26 63.19 5.85 t.tt 0.01 0.01

29.25 63.30 6.15 r.05 0.00 0.00

30.78 60.95 7.58 0.95 0.06 0.07

30.19 [email protected] 9.33 0.99 0.07 0.00

30.93 56.10 11.91 0.E6 0.10 0.00

30.09 61.67 7.t2 1.03 0.01 0.00

99.54

99.82

99.64

99.13

100.03

100.39

99.rE

99.90

99.92

560

[email protected]

772.2 Z

940.1

ppE

llt Co

610

belor 730

690 carloIs

0.98 1.60 0.41 0.02 0.205

u8 Utr lasl(!tg+P€) a

total

Lio!

e

treO

o.97 1.60 0.44 0.03 0.215

d€tect!,ot 470

ltdit 5m

otr mE BASIS 0r 4 oxlctN

0.99 1.68 0.3r 0.03 0.157

0.97 1.75 0.29 0.03 0.142

o.97 1.73 0.30 0.03 0.148

3X

660

atoa

0.99 r.63 0.36 0.03 0.rE2

o.97 l.5E 0.45 0.03 0.22t

0.97 1.47 0.56 0.02 0.274

0.98 1.67 0.34 0.03 0.17t

s40

TTIE CANADIAN MINERALOCIST g(D oJ5

3a

-l

r+(D (D

€ = (Doo

|D50 :' ct= AE C c-t t . a+ o,o(D o=oo

€6

oo

o uro JJ

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