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Titanium-Bearing Oxide Minerals of the Critical Zone of the Eastern

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Titanium-Bearing Oxide Minerals of the Critical Zone of the Eastern American Mineralogist, Volume 64, pages 140_t50, 1979 Titanium-bearingoxide minerals of the critical zoneof theEastern BushveldComplex EuceNr N. CeNaenoN Department of Geology and Geophysics Uniu e rsity of Wis consin- M adison M adis on, IVis consin 5 3 706 Abstract The principaltitanium-bearing oxide minerals in theCritical Zone of theEastern Bushveld Complexare chromite,the most abundant,and rutile, widelydistributed as an accessory mineral.Ilmenite and a complexTi-rich oxide are rareminerals in the zone. The TiO, contentof one chromiteis 5.17percent, but the rangefor all otherchromites analyzedis 0.31percent ro 2.47percent. Rutile occurs as aciculaicrystalsin plagioclaseor bronzitein somelayers of the rock sequence,but mostly it is associatedwith chromite. Orientedacicular inclusions in chromiteand anhedramarginal to chromiteappear to have formed by subsolidusoxidation-exsolution from chromite.Prismatic crystals included in chromite,straddling chromite-silicate boundaries, or embeddedin plagioclaseor bronziteare interpretedas cumuluscrystals formed more or lesssimultaneously with chromite.Rutile interstitialto chromitecrystals in two chromititesapparently formed during the postcumulus stage. Rutile and chromitewere evidently stable as coexistingphases during accumulation of manyunits of theCritical Zone. The low TiO, contentsof chromitethus represent saturation levels at time of formation.A miscibilitygap may thereforehave existedin the series chromite-titanomagnetiteunder the conditionsof crystallizationof the Critical Zone.The occurrenceof cumulusrutile in a chromititefrom the Muskox intrusion and another from the Fiskenaessetintrusion suggests a similarconclusion for thosebodies. Introduction Previous work Two majorgroups of oxidemineral concentrations Available information on titanium in oxide miner- arepresent in thelayered rocksof theBushveld Com_ als in the Critical Zone is of two kinds: (l) analyses plex.One group consistsof vanadium-bearingtitano- reporting TiO, contents of chromites from various magnetitedeposits in the upperpart of theComplex. horizons, and (2) descriptionsof titanium-rich oxide Titanium-bearing oxidesin thosedeposits have been phasespresent in various rock units, mostly in chro- studiedin somedetail (Molyneux, 1972).The second mite-bearing rocks. All complete analysesof chro- group consistsof chromititesin the Critical Zone. mites from the Eastern Bushveld Complex show Titaniumis knownto be present in chromitein these TiOr, the amounts reportedranging from 0.20to 2.40 rocksand in associated silicate-richrocks, and other percent.Most analyses(cl DeWet and Van Niekerk, titanium-bearingoxide phases haveoccasionally been 1952; Cameron, 1964;Heiligman and Mikami, 1960; reported,but the occurrenceof titanium in oxide Cousins and Feringa, 1964;Waal, 1975)are of sam- mineralshas not been systematicallyexamined. ples from various chromitites, in which chromite is This paper describesthe occurrenceand composi- the principal mineral but silicatesare presentin vari- tionsof titanium-bearing oxideminerals in the Criti_ ous amounts. The exact significanceof the reported calZone of the Eastern BushveldComplex. The data values of TiOz is not indicated for any of the pub- help to explainthe compositional and stratigraphic lished analyses.Purification procedures used have gap betweenthe chromium-rich spinelsof the lower been designedprimarily to separatechromite from part of the Bushveldand the iron-titaniumspinels of associatedsilicates by crushing, elutriation, or mag- the upperpart. netic separation,and solution of remaining silicates 0003-004x/ 7 9 /0 | 02_0I 40$02.00 CAMERON: BUSHVELD COMPLEX t4l in hydrofluoricacid with or without sulfuricacid. diameter, but crystals 20 to 50 pm in length are The presenceor absenceof separateTiO, oxidesin found. the analyzedconcentrates has generally not beende- 2. As stout to elongate prisms included in chro- termined. mite, I to 4 inclusionsper chromite grain (in section). Thereare a few reportsof titaniumoxide minerals Control of orientation by the lattice of host chromite in various chromites of the Eastern Bushveld. may or may not be evident (Fig. 3B). Crystals range Younger(1958) described bodies of ilmenitein chro- up to 0.73 mm in length. mite in chromititesfrom Dwars River (Fig. l) and 3. As anhedra along margins of chromite crystals from the M unit (Fig.2) on Winterveld417 KS, and (Fig. 3C) or at intergrain boundariesof chromite. At tiny irregulargrains of rutilealong grain boundaries triple junctions of chromite grains, rutile may appear of chromite.Cameron and Emerson(1959) men- to occupy grain interstices(Fig. 3D). tionedthe occurrenceof rutilein certainchromitites, 4. As euhedraalong margins of chromite crystals; and Guilbert (1962)reported rutile as an accessory the chromite is molded against rutile. Some rutile mineral(trace amounts to 0.2percent) in rocksof the crystals straddle chromite-silicate boundaries (Fig. C and D units (Fig. 2) on Winterveld417 KS and 44). Jagdlust418 KS. Rutilewas also found in acicular 5. As independentcrystals up to 0.32mm in length crystalsin labradoriteand bronzitein feldspathic (Figs. 48, 4C), mostly associatedwith chromite but pyroxenite.Cameron and Glover (1973) briefly com- largelyor completelydetached from it and embedded paredthe TiOz contents of spinelsin maficpegmatites in postcumulusplagioclase, less commonly biotite. with TiOzcontents of spinelsin the normalrocks of 6. As aggregatesinterstitial to chromite (Fig. 4D). the EasternBushveld. 7. As acicular crystals in plagioclase,less com- monly in bronzite. Materialsstudied Acicular inclusions in chromite and marginal an- Materialsstudied are outcrop samplesand drill hedra (l and 3 above) are commonly associatedand coresrepresenting the full 1600-meterthickness ofthe are found in various chromitic rocks from the B unit CriticalZone on FarmsJagdlust 418 KS, Umkoanes to the top of the Critical Zone. Prismatic euhedral Stad412 KS, andWinterveld 417 KS (Figs.I and2), inclusionsin chromite, marginal anhedra, and inde- plus collectionsfrom numerousother farms in the pendentcrystals (2,3, and 5 above) occur togetherat EasternBushveld. Polished sections representing all a number of horizons from the D unit upward. They major units of the Critical Zone, as well as many are especially abundant from the F unit upward. individuallayers within units,have been examined in Acicular and prismatic inclusions may occur in a the courseof the presentstudy. single grain of chromite; in such grains the portions adjacent to large inclusions are devoid of acicular Occurrenceof titaniumin oxide minerals inclusions.Types 1, 2,4,5, and 6 may alI occurin the same rock, but type 6 is the least common and has Generalslqtement been noted thus far only in two chromitites. In rocks of the Critical Zone, tiranium occursin The acicular, oriented inclusions in chromite are four oxideminerals-rutile, chromite, ilmenite, and presumablydue to separationof TiOz from the chro- an oxideof complexcomposition (Cameron, 1978). mite structure.The processis not simple exsolution, Chromiteand rutile are by far the mostwidely dis- however,since charge imbalance in the spinel(cation tributed,and chromiteis by far the mostabundant. excess)would result. More likely is an oxidation- exsolution similar to that proposed as the origin of Rutile ilmenite-titanomagnetite intergrowths (Verhoogen, Rutile is readilyrecognized by meansof its white 1962;Buddington and Lindsley, 1964).ln terms of a color, reflectivity,anisotropism, bireflectance, twin- hypothetical ulv6spinel component of the chromite ning in largercrystals, crystal habit, and internal solid solution, the reaction reflections(yellow to amber,less commonly reddish). 3 FerTion * 02 - 2 Feso^+ 3 Tio, Rutile occursin the CriticalZone in sevendifferent ways: may be written. l. As acicular inclusions in chromite, parallel to The separationof TiOz from chromite appearsto (l1l) of chromite (Fig. 3A). Most suchinclusions are have beenvery uneven.Chromite-rutile intergrowths 5 pm or less in length and are less than I pm in have beenobserved in polishedsections of rocks rep- t42 CAMERON: BUSHVELD COMPLEX LISI OF FARMS 1 Scheiding 4O7 KS 2 Zeekoegot 421 KS 3 Jogdlusi 418 KS 4 Wrntenveld 417 KS 5. Moeijelijk 412 KS KS 6, Umkoones Stod 412 KS 7. Zwortkoppies 413 KS I Woterkop 113 KT 9 Poschos Krool 466 KS 1O Mecklenburg ll3 KT 11, Hockney 116 KT i2 Surbiton t15 KT 13, Fofest Hill 117KT 14. Clophom 118 KT 15- Winnoorshoek 25O KT 16 Groothoek 256 KT 17 Driekop 253 KT 18. Mdondoogshoek 254 KT 19 Mooihoek 255 KT 20. Hendriksploqts 28t KT 2l Onverwocht 292 KT 22. Doordbsch 294 KT 23. Wlnte.veld 293 KT 24, Eee.ste Gcluk 322 25. Goudmyn 337 KT 26. De Goede Verwochtinq J32 KT 27 Annex Gnootboom 335 KT 28, Grootboom 336 KT 29. Spitzkop 333KT 30. Kennedys Vole 361 KT 3l Tweefontein 360 KT 32. Frischgewoogd 362 KT 33. Kolkfontein 367 KT 34, Dwors River 372 KT 35. Dc Grooteboom 373 KT 36 Rietfontein 375 KT 37 Richmond 37O KT 38 Thorncliffe 374 KT 39. Heleno aJT M)Z\Z NePhclire syenrte BushwH groniie ond oss@ioted nocks E6tem Bushveld Complex F_l Moan_ond Upper zones Criticol Zone fL_-.l Lorcr Zone Merensky Reef vr7 Pfetorio series or----:-to Kilomete.s Fig. I Generalized geologic map of the Eastern Bushveld Complex. CAMERON.' BUSHVELD COMPLEX 143 u t ii-- uJ o o- z6 Norite intenloyered with ononthosite(o) ZN 363 Noritc, onorthosite(o), ond feldspothic bronzitlte (b) 47 Gobbro (obove) ond norite r.il I l5 Feldspothlc bnonzltite z Anorthosite, norite(n). gobbro (g), ond feldsPothic o
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