which is related to nucleation of the magma in the roof zone of the intrusion, the ... Geologic map of the eastern Bushveld Complex showing the locality of the ...
EconomicGeology Vol. 80, 1985, pp. 1075-1088
The Geochemistryof Titanomagnetite in Magnetite Layers and Their Host Rocks of the Eastern BushveldComplex D. D. KLEMM, J. HENCKEL, R. DEHM, Institutfiir AllgemeineundAngewandte Geologic,Universit&'t MfinchenLuisenstrasse 37, 8000 Munich2, WestGermany AND G. VON
GRUENEWALDT
Institutefor GeologicalResearchon the RushveldComplex,Universityof Pretoria,Pretoria,Republicof SouthAfrica Abstract
More than 430 analysesof magnetite separatesrevealed pronounced differences in compositionbetween magnetite from magnetite layers and disseminatedmagnetite from the host rocks.Magnetite from the massivelayers is amongothersenrichedin elements suchasTi, Mg, A1, and Si, which suggests that this magnetitecrystallizedunder conditions of disequilibriumwith the magma,i.e., by spontaneous nucleationand rapid crystallization. Lower vanadiumvaluesin the magnetitesfrom the massivelayerssuggestcrystallizationat higherfo2 than from the rockscontainingthe disseminatedmagnetite. Variation diagramsreveal a cyclicity in compositionaltrends upward in the sequence which is related to nucleationof the magmain the roof zone of the intrusion,the periodic collapseof this crystal-ladenmagma,and its incorporationin the zone of crystallizationat the baseof the magmachamber.Deviationsof this normaltrend are consideredto reflect a stratificationof the roof zone and a gradualincorporationof the stratifiedroof zone liquid into the convectingmagma.A pronouncedupward increasein the Cr contentof magnetite is tentatively ascribedto a combinationof olivine crystallizationat the expenseof pyroxene in the upper subzonesand a possibleincreasein the partition coefficientof chromium between spineland liquid with a decreasein the crystallizationtemperature. Introduction
tained from samplingtraversesacrossthe complete upper zone north of Roossenekalin the eastern Bushveld (Figs. 1 and 2). Closely spaced samples
ON the basis of experimentalinvestigationof the intrinsicoxygenfugacityof chromitefrom chromitite were also taken across several sections of all the layers and their host rocks of the critical zone as magnetitelayers. In addition, severalsampleswere well as detailed mappingof the magnetitelayers in alsotaken of the main magnetitepipes which were the upper zone, Klemm et al. (1982) concludedthat emplaceddiscordantlyto the layeringof the complex, the formation of the oxide layers in the Bushveld to yield a total number of more than 430 samples Complex was causedby spontaneousincreasesof for this investigation. the oxygenfugacity at the solid-liquidboundary at The aim of our investigationwas,therefore, twothe baseof the magmachamber.Exactmeasurement fold. First, we were interestedin studyingthe comof the intrinsic oxygen fugacity behavior of the plete chemicaldifferentiationbehaviorof the titanodifferent manifestationsof magnetitein the upper magnetitesin the three typesof environments(layers, zone, as was done with the chromite (Snethlageand disseminated,and pipes), and second,we wanted to Von Gruenewaldt, 1977; Snethlage and Klemm, determinewhether any significantdifferencesin the 1978), is not possible,however,becauseof extensive chemicalevolution of these three titanomagnetites postcumulusequilibration related to oxidation of exist. the titanomagnetites.For this reason a different approachhad to be adoptedin order to find evidence The Upper Zone in support of or againstthe hypothesisthat an The area to the northeast of Roossenekal is one increase of the oxygen fugacity was an important factor in the formation of massivetitanomagnetite of the relativelyfew areaswhere rocksof the upper layers. This was done, first, by an extensive ore zone are extremely well exposed. The area was petrographicalinvestigationof the titanomagnetites mappedon a scaleof 1:50,000 by Von Gruenewaldt within massivelayers and as disseminationsin the (1973a) and subsequentlyremapped on a scale of gabbroicrocks(Von Gruenewaldtet al., 1985), and 1:15,000 by Dehm (1980), Henckel (1981), and second,by a geochemicalinvestigationof the titano- Schmidt-Thom•(1981). Thesedetailedmapsconstimagnetites.Samplesfor this investigationwere ob- tute the basis for location of the traversesalong 0361-0128/85/417/1075-1452.50
1075
1076
KLEMM,HENCKEL, DEHM,ANDVONGRUENEWALDT
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o•o•'?.:.• '-,, '- . ',...'.....•'.•
•Karoo ß Sequence '"'•x•Waterberg Group
;•Nebo Granite -:•:•Res•op Granophyre
•Bushveld Complex
..... MainMagnetite Layer
•TransvaalSequence......Merensky Reef
•Groblersdal Formation ;- - - Chromitite Layers
• •lArchaean
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Fault
Rustenburg
-Layered Suite
.•:•Gabbro,Norire,Pyroxenite.
•RooibergFelsite 0
40km
FIG. 1. Geologicmap of the easternBushveldComplexshowingthe locality of the area from whichthe sampleswere derivedfor thisinvestigation.
which the more than 400 sampleswere collected for this investigation. The base of the upper zone is defined by the SouthAfrican Committeefor Stratigraphy(SACS) (1980) as being the level where magnetitemakes
The upper zone consistsof a well-layered se-
quenceof magnetitegabbro,anorthosite,magnetite layers,andolivine-bearing gabbroicrocks.Magnetite
is a common constituent in virtually all the rock types and constitutes,on averagebetween 8 to 10 its appearance in the succession.This committee percent by volume of the rocks.The basalsubzone divided the sequenceinto three subzoneson the (subzone A) is close to 200 m thick and consists basisof the appearanceof new cumulusmineralsin essentiallyof magnetitegabbrointerlayeredby some the sequence.We have retained the division into thin anorthosite;there are usuallythree magnetite four subzonesas was originallyproposedby Von layers.These have been referred to as lower layers Gruenewaldt(1973a) for reasonsthat will become I to 3 by Molyneux (1974), who alsoidentified on evident later. The only differencebetween the two olivine-bearinglayer some 50 m below the Main subdivisions is that subzones A and B as used in this Magnetite layer in the Magnet Heights area, 40 km
papertogetherconstitutesubzoneA of SACS(1980).
to the north of Roossenekal.
TITANOMAGNETITE GEOCHEMISTRY
1077
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!
•
Granite 09' -
'::":-• Subzone D t • Alluvium etc.
!•
Subzone C
•
Magnetite Lair no.
::• Subzone B UPPER ZONE i•'• Main Magnetite L. ay•r
'_'• Subzone A --$2 Sampling Profile no. • Main Road ::• Subzone C•-MAIN ZONE•0 ,I 12 51km FIG. 2. Simplifiedgeologicmap of the area north of Roossenekal showingthe positionof the samplingtraverses.
The baseof the Main Magnetite layer is taken as the compositionof the plagioclase(Yon Gruenewaldt, 1973a).Apartfromthe fluctuations in the plagioclase subzonealsoconsistsof magnetitegabbroand inter- compositionreferred to above,the major silicates layered anorthositesand magnetite layers. Apart plagioclase,olivine, orthopyroxene,and clinopyroxfrom the Main Magnetite layer, magnetitelayers 1 ene display a systematicchange in composition to 7 occur within this subzone.The appearanceof toward the lessrefractory end membersupward in iron-rich olivine in the sequencemarks the base of the succession(Atkins, 1969; Von Gruenewaldt, subzone C. Olivine gabbrosare abundant in the 1973a; Molyneux,1974). Similarly,availablewholelower 200 m of this subzonebut make way for a rock compositionsalso displaya systematicupward 450-m-thick succession of magnetitegabbroswhich increase in the less compatible components(Von contain magnetite layers 8 to 14. Some olivine is Gruenewaldt, 1973b; Cawthorn and McCarthy, presentin this part of the subzone,but it is only an 1985) asis to be expectedof a fractionatingmagma, irregular and very subordinateconstituent.One of the courseof crystallizationof whichwasnot unduly the most prominent markers in the whole upper interrupted by, e.g., influxes of undifferentiated zone sequenceis magnetite layer 13 together with magma. the immediatelyoverlyingmottled anorthosite. Analytical Procedure The reappearanceof cumulus olivine together with cumulusapatite marks the base of subzoneD. In order to obtain the purest possiblemagnetite Plagioclase within theseolivine-apatite-bearing rocks concentrates,all the samples were crushed and has a composition below Ans0 so that these rocks subsequently groundin a Vidia powdermill to ___200 are referred to as diorites. Olivine and apatite are /•m. Magnetite was separatedstepwisefrom the absentin those parts of the succession containing silicateswith a hand magnet and an additionalfine magnetitelayer 15 and magnetitelayers 16 to 21. separationwas achieved with the aid of ultrasound It is of interest that the plagioclasecomposition equipment. The final sieve fraction of
