Canadian Mineralogist Vol. 21, pp. 4l-58 (1983)
SPINELZONATION IN THEDE BEERSKIMBERTITE, SOUTH AFRICA: POSSIBLEROLE OF PHLOGOPITE
JILL DILL PASTERIS Departnrentof Earth and PlanetarySciences & McDonnell Centerfor the SpaceSciences, WashingtonUniversity, St. Louis, Missouri 63130, U.S,A.
ABSTRAcT tion d'enrichissement en Fe3+ ou Ti, sans grand changement dans le rapport Fe2+/(Ire!+ * Mg). Grains of zoned spinels from the well-mapped, Aucune kimberlite ne montre d elle seule la lign6e multi-intrusion De Beers kimberlite pipe, Kimber- complite. Les phases intrusives principales de la ley, South Africar were studied optically and zone profonde du complexe De Beers contiennent analyzed with an electron microprobe. Strict sam- en abondance des spinelles i texture en atoll, avec pling control made it possible to show that spinel noyau de chromite ou de titanomagn6tite. Les compositions and zoning trends are specific to lacunes de I'atoll 6taient originellement remplies de individual kimberlite intrusions. In general, the pl6onaste-Mg (spinelle Mg-Fe-Al), dont on trouve compositions of kimberlitic spinel define a parti les vestiges dans les cristaux de spinelle au contact cular trend in the spinel prism, from the Al-rich des roches encaissantes. La cristallisation tardive to the Cr-rich regions at the base of the prism, d'un spinelle alumineux r6fractaire pourrait r6sulter then up toward the Fe3+- or Ti-rich upper edge of de la fin de la cristallisation de la phlogopite lors the prism. commonly without great change in the de la mise-en-place et cristallisation fractionn6e de Fe2+71Fs:+ * Mg) ratio. No single kimberlite la kimberlite. La phlogopite est abondante dans expresses the entire trend. The main root-zone plusieurs de ces phas€s intrusives profondes, et intrusions of the De Beers kimberlite possess montre dans plusieurs cas des stades multiples de abundant grains of atoll spinel, with a core of cristallisation. C'est surtoui i une augmentation chromite or titanomagnetite. The gaps in atoll de la fugacit6 d'oxygdne dans le bain kimberlitique spinels were once filled with Mg-pleonaste (Mg- qu'on devrait attribuer Ia d6stabilisation et la 16- Fe-Al-spinel), remnants of which are found only sorption du pl€onaste partout sauf dans les zones in contact zones adjacent to wallrock, Late-stage de contact, oir la r6action est rest6e incompldte, precipitation of high-Al, refractory spinel may have been triggered by the cessation of phlogopite (Traduit par la R6daction) precipitation during rise and fractionation of the kimberlite. Phlogopite is abundant in several of the Mots-clis: kimberlite, Afrique du Sud, zoDe pro- root-zone intrusions, and commonly demonstrates fonde, spinelle phlogopite, texture en atoll. multiple episodes of precipitation. An increase in /(Og) and other changes in the kimberlite melt INtnopucrtoN subsequently caused breakdown and resorption of pleonaste everywhere in the pipe except in the contact zones, where the reaction did not go to The mineralogy and petrology of numerous completion. kimberlite intrusions are now documented in the literature (e.9., Haggerty L975, Mitchell Keywords: kimberlite, South Africa, root zone, 1978a, b, 1979, Mitchell & Clarke 1976, Mit- spinel, phlogopite. atoll texture. chell & Meyer 1980, Boctor & Meyer 1979). The present study concerns the root zone of SoMMAIRE the well-mapped De Beers kimberlite diatreme in South Africa. It currently is being mined, and On a 6tudi6 optiquement et analyse i la mi- therefore was accessible for extensive horizontal crosonde 6lectronique les spinelles zon6s du pipe and vertical sampling, in contrast to the more bien cartographi6 (Kimberley, de De Beers Afrique limited ac@ss available in many previous du Sud), i multiples venues kimberlitiques. Un studies of kimberlites. The De Beers diatreme contr6le strict de l'6chantillonnage perrnet de prou- consists of several intrusions, which ver que composition et type de zonation sont carac- are recog- nizable by t6ristiques de chaque venue. En g6n6ral, de tels their field relations and their p€trog- spinelles ddfinissent une lign6e caract6ristique dans raphy. le. prisme triangulaire des compositions qui part du The availability of fully documented samples p6le Al, va vers le p6le Cr i la base, et ensuite (drill core and personally collected specimens) vers l'ardte sup6rieure du prisme, dans une direc- from several intrusions (facies) within a single 41 42 TIIE CANADIAN MINERALOGIST
N vERTEALsfcTroil 0 SSTA€E I CRATERZONE- n rc!;TIJ EROTED 62On LEVEL
DNTR€TC ZONE
A,VERTICALSECTIOI{ F-At{vtEws 'fif;' C,D' ol@ . SCALE EASTPlPE,7zOm LR/EL
OATA@URTESY OF' DEBEERS CONSO-DATED MINES. PERPT€RAL KIMBERLM Enlorg€rnsnlof \ I Eo3l Pipo in \ Figurc D \------?\-_-/- BRECCIAZONE
AAB NOTTOSCALE
Frc. l. Diagrammatic vertical section (A) and cross sections (B,C,D) of the De Beers pipe, Kimberley, South Africa. Each of the three major regions of the diatreme (C,D) probably represents at least one separate intrusion. Fig. lB (enlargement of East Pipe in Fig. tD): according to the De Beers geologists. the East Pipe consists of an earlier Peripheral Kimberlite. a later Core Kimberlite, and the Intrusion Breccia (part of the "Breccia Zone"). The Intrusion Breccia probably is not a separate intrusion. but rather Peripheral Kimberlite that has undergone extensive physical (and to some degree. chemical) interaction with the walls of the diatremc. Figure from Pasteris (1981), copyrighted by American Geo- physical Union.
diatreme made it possible to compare petro- The opaque oxide phaseswere chosen for parti- logical relationships among the kimberlite facies. cular study becausethey are the best-preserved SPINEL ZONATION IN KIMBERLITE, SOUTIT AFRICA 43
minerals of t}le rocks, and because their com- samples analyzed in the present study come positional variability makes them good indi- from the dyke-like root zone, which has the cators of changing conditions in the parent best-preservedigneous features of the intrusion. melt. The De Beers root zone has three major parts A major goal of this study was to determine (Figs. I A,D,C) called the East Pipe, the West the extent to which the opaque phases in kim- Pipe, and a Neck, each comprised of one or berlite can be used as indicators of l) differen" more separateintrusions. The East Pipe consists tiation processeswithin a kimberlite facies, and of two major intrusions, the Peripheral (ear- 2) genetic relationships among the facies. lier) and Core (later) kimberlites, plus several kimberlite dykes, a kimberlite sill, and a peri- Gnorocy oF THE Dn Bmns KrMsrnrrre dotite dyke. Field relations in the root zone Cor"rpLEx suggestthere were tlree or four major episodes of intrusion: Peripheral, followed by Core, fol- The De Beers kimberlite pipe is one of five lowed by the Neck and the West Pipe (De major diatremes in the Kimberley district, South Beers Geology Department). The Intrusion Africa. The diatremes afe Cretaceous in age, Breccia ("Breccia Znne" of Fig. lB), which about 90 Ma old (e.g., U-Pb and pb--Fb dating occurs in both the Neck and the F.ast pipe, is by Allsopp & Kramers 1977, Davis 1977). The not considered a separate injection, liut rather basement rocks in the region are precambrian as kimberlite melt that has been affected by granite and amphibolite gneiss.They are locally interaction with large volumes of wallrock. One overlain unconformably by Ventersdorp vol- of the reasons for comparing the compositional canic rocks (-23M Ma) and by more than patterns of spinel among the different parts of 300 m of the sedimentary and volcanis rocks the root zone is to determine if they too define of the Karroo System (Carboniferous to Jurassic the samenumber and physical extent of separate in age). The Precambrian and Ventersdorp intrusions. rocks are the dominant wallrocks in the under- The East Pipe and Neck of the De Beers roor ground workings of the De Beers mine, where zone were sampled by horizontal and vertical mo-stof the samplesexamined in this study were traverses across the 105-m level to the 729-m collected. level (depths below the level of the present -The De Beers pipe has a geometry typical surfacein the mine). Contact zonesbetween the of many kimberlite pipes. It is dyke-[G at kimberlite and the wallrock were as well sam- depth, but expands abruptly into a funnel- pled as the middle of the intrusions. Drill-core shaped diatreme at about 20OO m below the samples (24) were studied from the 562-m level original Crelaceous land surface (Fig. l). Most through the 720-m level of the West Pipe.
'I. TABLE SUI,{MARYOF PETROGRAPHICFEA'IURES OF ]ltE DE BEERSROOT ZONE 'I 'ee Region Features of Serpen- trix Phlogo. . Phlogo. inel Abund.(A) inel Zonlng ti nlzatl on Preservatlon(P) |cest rell consolldated. tepth-depend.,usu. v. abund.,usu. abund.i often -owA. PoorP Iype 2; atoll Pipe :oarse phlog. v. tt least partial. colorless;if altered rims; texture. rbund. Resernbles colored, R pleo zoned (R core) or ;ore K. unzoned (N or R) Core more{allrk. lncl. /ariable,usu. abund., R pleo iome, altered Low A. Poor P. Type 2; atoll KinberlI & serpentinlzation noderate texture. than Peripheral K. Periphera fell consolldated, rsu. partial. some: pale rbund.at wa]lrk. LowA. P better Type2; atoll Klmber'lit /. serpent. Feter color. lontactsi zoned than in Core. texture; sone {allrk. incl. than iR core) or un- Grainslargest at pl eonaste ;ore K. zoned(N or R). flallfk. contacts. remnants. Neck least altered & variable,usu. in some abund. at v{allrk \: l-39. P better Type l, often in Kimberlite friable mck of samples. contacts; lhan elsewhere. perovskite. Type 2; body. Homogeneous al tered .argest grains in pleonaste abund. at & flne-qralned. el seuhere. )ipe at contact. |{allrk; atolls. Intrusion flne-grai ned, variable, fron abund., often l: l-2%. P as in Type I, often ln Breccia basaltlc. Some none to complete. wel l preserved. {eck. Largest perovskite. Type 2i flov bandlng. yralns at wallrk pleonaste abund. at FeHwallrk lncl. :ontact. ml lrki atol ls. Dlkes& v. friablei extreneusu. coltDlete. matrix t phenosabund. zoned Moderate i A; usu. unzoned'1. titanomag & 5iil hydration & carbon- forn contlnuurn.(R core) or un- srnl l . Type Ilmenlte ation. Coarsephlog zoned (N or R). 'lncl. in titanomag. . nonna = inclusions €ochr0ism R = reverse pleochrolsm serpent. = serpentinized 44 THE CANADIAN MINERALOGIST
Pnrnocnerry oF TrrE Dn Bnnns Krtvrsenl-lre pite, the abundance of segregations of calcite and serpentine, and zonation in the grains of The different intrusions (facies) of the De spinel (Table 1). Beers kimberlite can be distinguished on the basis both of their silicate and opaque oxide ANALYTICAL PROCEDURES phasesClable l). Throughout, olivine and phlog- opite are the dominant phenocrysts, but they Analysis of spinel and phlogopite grains was typically also occur in the matrix, which may carried out using the automated wavelength- comprise more than SAVo of the rock volume. dispersion MAC 40O electron microprobe at the The matrix also contains calcite, serpentine, Geophysical Laboratory, Washington, D.C. apatite, monticellite and opaque oxide phases. (Tables 2,3,4). The operating conditions for The major opaque oxide-minerals are spinel, spinel analysis were 15 kV accelerating voltagen rutile, perovskite and ilmenite, which together 5b nA beam current, l-2'p,m diameter beam- represent at most a few percent of the rock. A size, and 30 seconds counting time or 50,000 minor portion of the kimberlite consists of counts per element. The standards used are mantle-derived xenoliths and xenocrysts, partic- natural minerals, synthetic oxide phases and ulary of garnet lhetzolite and ilmenite. synthetic glasses.Because of the small size of In hand specimen, different kimberlite facies the grains being analyzed, the elements Ca and can be distinguishedby color (hues of blue and Si were carefully monitored to ensurethat matrix green), friability, amount of crustal xenoliths, calcite and serpentine were not being excited and degree of serpentinization and carbonation. by the beam. The data-reduction program is All are porphyritic; some are obvious breccias. based on Bence-Albee correction procedures In thin section, fufther distinguishing features (Bence & Albee 1968, Albee & Ray 1970). are the abundance and preservation of phlogo- Fet* and Fe'* in the spinel (Tables 2, 3) were
TABLE2. REPRE5EI'IIATIVEZONED SPINELS FROM TltE DEBEERS PIPE
-_t 'l 1 -----2 3 ---) 4 5 ----i 6 7 ---) I 9 10 l------tl2-+I3 .10.46 21.42 13.26 1?.74 Mqo 4,23 13.83 ]s.96 1.1.6.1 13.14 11.56 ]?.q9 !Z'q8 19.89 1e.55 zt'.oi ii:te-0:64 zl.ag-0.99 e.64 e.67 15.23 3r.02 ;$ fi:d; zd.s5.l.or is.go ll.3e v.?1 0.67 0.57 0.77 !h0 0.94 0.70 0.46 0.e6 o.si q.gr 0.80 0.03 o.ii o:ti o.lC q'14 0't7 0'21 o'le 0'16 Nlo 0.zr 0.37 0,2't 0.00 'o.zi t 't+ o'tg c.o o.or o.ra o.:o o.so o.ao o.ii o.ii o'z+ o'gs o'zg 5.]8 7.3? 5^'22 A1203 11.32 '10.590.54 e.00 38.43 4.14 6.13 -9.q1 ^9.1! ?'ll ,9'9: Cr203 41.84 i:ii I:ii qi'.ii ;'.b, :i'.ii i'.41 y.e? ?7.!1 .1':: !?'12 tq.ts 30.q9 tt.tt' za'su q l'l .46 "t'ig Fe203 8.87 45.35 d:;: z\'.si io:G zri.oa '10.23 '17.56 8.09 8.93 5.01 20,70 Tl0z 4.91 17.21 J. v5 7.71 18.24 8.65 n to 0.24 0 0.97 0.I7 Sl0r 0.09 u.cq 0.26 0.39 0.01 0
n (oq 0.658 0.582 0.644 0.841 0.928 0.983 0,652 0.630 l'lq 0.514 0.233 0.682 0.800 0.4m 0.860 o.717 0,269 0.56I 0.759 0.610 0.777 0.322 0.252 0.252 F;2+ 0.576 0.921 0.017 0.016 0.022 0.026 0.032 0.020 0.011 0.028 0.0?6 0.018 0.020 0.024 0.021 l.h 0.004 0.004 0.005 0,005 0.004 Ni 0.006 0.011 0.006 0.000 0.001 0.003 0,006 0.004 0.007 0.013 0.015 5 0.015 0 Ca 0.002 'r.28r A1 0.440 0.024 0.351 0.168 0.243 0.?0q q.?9q 9'?19 9')?) g'lx? l'?ig R'?lg 0:t8t 0.03, 0.686 0.121 r.rr4 0'rr5 ti i:tfi ;::5; 6.0:o 0.052 1.166 o.iie 1.11e 0.284 0.626 r"r* o.zzo t.zoo o.z+s o.qzz o,zos o.idi d.i60 0.7i0 o.zoq o.sss 0.996 0.220 0.11? o.i:o 0'237 9'1?! 0'515 Ti o.1zz o.la7 o.4zs 0.084 o.lee 0.461 9.\92 . 0.006 si 0.003 0.016 o:d6d 0:0ii 0.000 0.d00 0:006 4.008 0.008 0.008 0.030. 0.00e T0TAL 3.00 3.00 3.00 3 'level, (l) 1-2t CoreKimber'lite; 720m neaf contact with Peripheral Kimber'llteand basenentgneissi dark core (2). andllght-colored rim 'i6vef, (4). 3-4: iteck firnber'litei OZOm near contact with Ventersdorpwal.lrock;.tan-core (3) andblack.rim 5-6: IntrusionBrecciai 720n level near Neckichromite core-(5) andtan rln (b)' pipe;720m blue-graycore (7) andtan rlm (8)-.. 7-8: l,leit ievel; (10)' 9-ll: per.ipheralfinoertrte;'Z2orn ievll near'c6nta"t tiitir'.doie'fiinler]ite; gray core (9), edgeof grav core (11). Mgnaintained throughout.zoning. '12-'13:and outef rim High KJrnberlitesiIi, blue ior'e (12) andedge of core (13). Note: colors refer to appearancein r€flected. light with oil-inmersion objectives;-Ferr calculated--assuming iiiiir'tiriii'v or ibinii'(FrnitC-progrimt; in all the core*im pairs and trJplets above,the Ms and Al contents remalnessentially constant or rise' SPINEL ZONATION IN KIMBERLITE, SOUTI{ AFRICA 45
IABLE 3. I!|ICRoPR0BEAMIYSES S m BEERSSPINELS fmt PLEOMSTERIMS Trxrunel eNo CovposITIoNALReLertoNsnlps L23 501 Avroxc rnp Spruel MlNntels Mso 19.94 13.36 13.8:l L4.27 20.08 m.32 m.39 ?0.t2 20.40 Feo 9.51 24.75 25.90 22.& 10.36 9.74 10.54 10.76 9.98 lilno 0.84 o,tz 0.70 0.7x 0.30 0.29 0.32 0.29 0.28 Textural and zoning features rflo 0.09 0.16 0.2I 0.29 0.00 0.03 0.07 0.02 0.00 Cao 0.59 0.30 0.36 0.43 0.30 0.30 0.18 0.34 Spinelgrains in the De Beerskimberlite occur Al,0r 8.95 6.76 9.(m 15.93 47.L6 49.32 48.22 46.10 45.80 in the following forms: individual grains in the cr;0;21.15 9.09 l.16 2.30 2.03 2.56 1.17 1.62 2.86 Fe20i21.62 27.6 2s.93 29.9 15.80 14.38 15.51 L6.22 L6.20 groundmass, inclusions in perovskite, inter- Tl0, 9.83 14.86 t7.21 L2.66 3,18 2.36 3.09 3.46 2.72 growths with perovskiteand ilmenite (but with sroi 0.14 0.71 0.26 0.40 0.00 0.00 0.25 0.r7 0.23 no apparent reaction relationship), mantles on I0TAL 98.67 98.49 98.56 98.93 99.34 99.30 99,85 98,95 98.41 ilmenite macrocrysts, both as discrete grains lq21 0' 2 0.667 0.682 0.679 0.811 0.813 0.815 0,818 0.817 Fe-' 0.252 0.69X 9.7L7 0.603 0.235 0.219 0.2x 0.245 0.229 and as massive reaction rims, and symplectic ltr 0.023 0.023 0.020 0.019 0.C07 0.007 0.007 0.007 0.006 Nr 0.002 0.004 0,006 0.007 0.000 o.(X)l 0.002 0.000 0.000 (fingerprint-like) intergrowths with silicate Ca 0.020 0.0X1 0.013 0.@8 0.012 0.009 0.009 0.005 0.010 phases (Figs. 2-8). Spinel very rarely is seen At 0.335 0.267 0.351 0.599 1.505 1.560 r.524 1.481 1.479 Cr2- 0.530 0.24r 0.030 0.058 0.043 0,054 0.025 0.035 0.062 as inclusionsin olivine. Fe" 0.65a 0.697 0.745 0.710 0.322 0.2N 0,313 0.333 0.334 The spinel grains are relatively small ( ( 35 M.23! 0.374 0.428 0.304 0.065 0.048 0.062 0.071 0.056 and were studied sr 0.004 0.0?4 0.009 0.013 0.000 0.000 0.007 0.005 0.006 &m ) both in reflected and transmittedlight with high-poweroil-immersion IoTAL 3.00 3.00 3.00 3.00 3.00 3.00 3.oo 3.00 3.oo Analj6es f1-4: to tltilmagnetlte cores of sp'inels rlth black pleonaste objectives. They usually occur individualty r'lns. {1r cent.al region of Perlpheral Klrberlite, 720 n level; *2-3: [ec& Klnber]lte, 620 d lryel, near contact rlth rather than in clusters, and they commonly Ventersdorp rallmck; t4: tleck Klderlite, 620 o level, about 4.5 m fr@ aontact. exhibit resorptionfeatures. Analyses *5-9: bl6l, I'19-p'lenaste spJnel fron 720 m leyel of Intrusjon Both unzoned Brccld. Elack splnels rln te cores ll*e those ln AnalJ6es titanomagnetiteand zoned Cr- f,t-4. bearing spinel grains occur De Beers, Noter colors refer to appear@q ln reflsted llqht rlth oil- at but lmerslon obJectives; Fe$ catcul ated ass!6lng stolchl@etry zonedgrains are of spinel (FERRICproqrd). more common, The two typical patterns of zoning from core to rim are: (1) chromite+ (ilmenite)+ titanomagnetite(Fig. 4), and (2) chromite/titanomagnetite+ pleonaste (Mg-Fe-Al-spinel) -> magnetite (Figs. 5,6). TABLE4. MICROPROBEAMLYSES OF DE BEERSPHLOEOPITE GRAII{S Grains of type 1 are found as inclusions in L2345 perovskiteand individually in the matrix. How- Ia,o 0.03 0.00 0.06 0.26 t.17 Cation values nomallzed to ever. type-2 grains alwaysoccur separately.This Kzo 9.34 I0.24 10.46 9.89 9.45 11 oxjgens; ldeal fomula is 8 catlons rer 1l ornens. suggeststhat type-l zoning representsan earlier !490 24.43 26.55 24.83 24.17 19.64 Anatses ft-3 frm iiesent Feo 2.96 2.42 6.48 6.?t 4.67 study of De B€ers kimberl.ite. sequenceof precipitation, distinct from type-2 I'lno 0.09 O,02 0.01 0.01 0.04 Analyses f4 and 5 fr@ llio 0.06 0.05 0.08 n.d. n.o. Boettcher et al.,s (1979) zoning. Cao 0.71 0.29 0.02 0.92 0.86 data on the 0e Beers klnberllte, p, 175, smple In type-2 grains, the spinel core (euhedral Alr0115.79 12.54 10.73 9.49 14.27 Kb-5-1-A. ill: larqe, matrix cr;o;0.02 0.0r 0.11 0.2s 1.65 phlogoptte; sdple DB19; Neck to anhedral) commonly is separatedfrom its Kimberlltel 72tu. level. f2: Tio, 1.63 0.52 0.61 0.83 3.99 soal l, natrir phlogoplte; lighter-coloredouter rim(s) by a gap of several sr0;38.04 42.03 43.19 4r.98 40.38 smple DB3li Core Klmberlite: micrometres (Fig. 7). The 742 n. term "atoll spinel'l gg,oe level. f3i reerse rotlt g:.tt gs.5s gc.os go.12. pl@chrolc core of zoned was applied by Mitchell & Clarke (1976) to phlogoplte; smple D8428; lla 0.002 0.000 0.008 0.037 0.162 klmberlite dlke; 125 m. such features in a Canadian kimberlite. In the K 0.861 0.924 0.943 0.919 0.854 level. d4: reverse Dlschrolc core of zoned phloqoDlte. De Beers atoll spinels, the core phase commonly t49r.2,632 2,7& 2.613 2.623 2.074 f5: nomal olmchr6ii: rtm on Fe-'0.178 0.141 0.381 0.378 0.277 Iraln f,4. is euhedralchromite or anhedraltitanomagnetite. lln 0.004 0.C00 0.000 0.001 0.003 = llt 0.002 0.002 0.004 n.d. n.o. The outermost rim is so narrow ( 5 /,m) as Ca 0.053 0.021 0.000 0.077 0.069 tq make electron-microprobe analysis difficult, At 1.344 1.044 0,893 0.815 1.192 brit it clearly is rich in magnetite. The composi- Cr 0.(80 0.@ 0.006 0.01.70.093 tion and optical propertiesof the "gap" resemble Ti 0.087 0,027 0.031 0.046 0.213 sl 2.741t2.97t 3.050 3.056 2,861 those of the fine-grained, calcite-serpentinemix- Totll 7.9117,9L0 7.929 7.96L 7.792 ture that comprises most of the kimberlite groundmass(Fig. 7). The abundance,texture and composition of calculated by assuming stoichiometry (FERRIC the opaque oxide phases are as characteristic program, Geophysical Laboratory). Reconnais- of the different facies of kimberlite as are the sance microprobe analysis was performed on primary and secondarynonopaque phases (Table phlogopite grains, using a broader beam (> 5 l). In general,type-l and type-2 spinels occur pm) and a beam current of 40 nA. Many phlog- in the Neck and Intrusion Breccia.whereas the opite grains are at least partly chloritized or West Pipe and East Pipe (Core and Peripheral serpentinized, as reflected in the oxide totals kimberlites) contain only type-2 spinels. The in Table 4. West Pipe and Core kimberlites are recognized 46 THE CANADIAN MINERALOGIST
::.:: ..::l:
:: : ::tl
perovskite grain. Peripheral Kimber- Fro. 2. Dark grey, subhedral titanomagnetite inclusions in light grey lite. Reflected light, oil-immersion objective. Scale bar 20 y.m' SPINEL ZONATION IN KIMBERLITE. SoUTH AFRICA
Frc. 3. Major phase (light grey), ilmenite, has been fractured and altered. Dark grey, highly titaniferous magnetite forms part of symplectic replacement zone. Small, highly reflecting inclusions in the ilme- nite are sulfides. Neck Kimberlite. Reflected light, oil-immersion objective. Scale bar 20 pm. Ftc. 44. Zoned atoll spinels attached to or partly enclosed in perovskite. 4B. Sketch. p perovskite, T tilanomagnetite, M magnetite-rich spinel, C chromite. A anatase or perovskite. Neck Kimberlite. Re- flected light, oil-immersion objective. Scale bar 10 3r,m. Ftc. 5. Zoned atoll spinel. Medium grey titanomagnetite. black (see arrow) Mg-pleonaste, white rims of magnetite-rich spinel. Core Kimberlite. Reflected light, oil-immersion objective. S"ate bar 6 p,m. Fto. 6. Multirimmed grains of atoll spinel (left) and zoned spinel (right). In zoned grain, tan titano- magnetite core' Mg-pleonaste (black) inner rim, magnetite-rich (white) outer rim. Note partial re- sorption of both titanomagnetite and pleonaste. Note also juxtaposition of highly resorbed (left) and. slightly resorbed (right) grains of atoll spinels. Neck Kintberlite. Reflected Jight,-oil-immersion ob- jective. Scale bar 20 p,m. otj- Ftc. 7. Atoll spinel with wide "gap" filled with kimberlite groundmass of serpentine and calcite. Note the small, euhedral chromite core and magnetite-rich spinel rim. Peripheral Kimberlite. Reflected light, oil-immersion objective. Scale bar 6 p,m.
Frc. 8. Sympletic intergrowth of zoned spinel (chromite cores and magnetite-rich rims on the digits) and altered silicate. Such symplectite usually forms part of what appeir to be mantle xenoliths. Core Kimberlite. Reflected light. oil-immersion objective..scale bar 50 um. petrographically by the low abundanceand poor Compositional patterns preservation of their spinel grains; atoll grains (type-2 spinel with no pleonaste zone) are the A principal feature of spinel grains in the most common. The kimberlite dykes and sill De Beers and other kimberlites is a composi- contain small single-phase euhedra of titano- tional discontinuity betiveen the core phases, magnetite and type-l zoned spinel. Type-2 znn- which are usually Cr-rich, and the rim phases, ing and atoll spinel grains aie absent in the which are Ti-rich. This gap occurs both within dvkes. grains and within kimberlite suites (e.g., Hag- 48 THE CANADIAN MINERALOGIST gerty 1975); it also is apparent in magmatic are too Mg-rich for the Buddington & Lindsley - ore deposits, in igneous complexes and, to a (1964) geothermometer oxygen barometer to certain extent, in lunar rocks (e.9., Haggerty be applicable(Pasteris 1980a). One must there- 1976). The discontinuity may be due to a fore regard the zoned grains of kimberlitic peritectic relationship between spinel and spinel in themselves as indicators of magmatic pyroxene (e.g., Irvine & Smith 1969), to parti- trends. In the following paragraphs,spinel com- cular l(Or) conditions during crystallization of positional patterns in the De Beers root zone the melt (e.g.,Ulmer 1969,Hill & Roeder 1974), are compared among the various facies and or to gaps in solid solution in the spinel system with patterns observed in other kimberlites. (e.s., Muan et al. 1972). Plots of microprobe-derived compositions Unfortunately, equilibrium coexistence of show three distinct compositional groups on the specific silicate and spinel phases cannot be spinel compositional prism (Figs. 9,lO), namely inferred for the De Beers kimberlite nor, prob- (l) chromite (base of prism), (2) titano- ably, for most kimberlitic pipes. Standard geo- magnetite (toward top of prism), and (3) Mg- thermometry and geobarometry techniques pleonaste (almost exactly on the spinel-hercy- therefore, are not applicable. It is also difficult nite join). There is a wide spread in composition to prove equilibrium between specific grains of in groups I and 2, but not in grouP 3. ilmenite and magnetite. Where the association Two major compositional trends are evident does occur in kimberlites, tle phases usually in Figures 9 and 10. The first is a normal mag-
o. lnlrusionBreccio oFRE3708 OFRB3?OC AFRBs?O8. OFRB37 7 ocnc t coorrr Ocnc t Rnr
Fctr%
MgAl"o.
')/-, MgCr.O.
Frc. 9. Compositional plot for the spinels of the Intrusion Breccia; Fe2+ only plotted (Pasteris 1982). The lines ind arrows represent core-to-rim zonation. Each type of symbol (e.g., circle, square, trian- gle) represents a particular sample. All samples came from the 720-m level of the mine, except for FRB377, which came from the 620-m level (Fig. 1). Letter notations on the diagram refer to colors in reflected light and to unusual (and perhaps xenocrystic) grains (l,h,Q). Note the distinctive zonation pattern (type 2): chromite/titanomagnetite + Mg-pleonaste + magnetite; in some casesnnot all three separatezones are present. SPINEL Z)NATION IN KIMBERLITE, SOUTH AFRICA 49
WestPipe o DBW42A o oBw47 AD8W55A oDBW/C58
hc1o.
MeCr.O.
FIc. 10. Plor of compositions of spinel from the West Pipe. Tie lines and arrows connect core and rim of a zoned grain of spinel; each symbol tlpe represents a particular drill-core sample. The speci- mens represent the following depth zones: DBW42B (620 m), DBW47 (660 m), DBW5sA (7lO m), DBW/C58 (72o m).
matic pattern, extending from chromite cores Thus, compositional differences are evident to titanomagnetite rims (Fig. lO). Between in the spinel grains from different parts of the these end-members,data points are few, a fea- De Beers pipe (Table 2). The major distinctions ture that is common to many kimberliles and are as follows: the presenceor absenceof trend mafic complexes. The second trend appeam 2, the extent (length in composition space) of unique to the De Beerspipe. In this case,titano- the spinel compositional trends, the values of magnetite-rich cores are directly mantled by the Fe2"/(Fe" + Mg) ratios (FFM ratios), pleonasterims (Fig. 9), and many of the titano- and the slope of the chromite-Jitanomagnetite magnetite compositions appear to lie on a tielines in compositional space. The typical mixing line with pleonaste. Ifowever, the latter range in FFM ratios for each part of the root feature is not a function of integration of ad- zone is as follows: Core 0.20-0.45, Peripheral jacent spinel zones by the electron-microprobe 0.20-0.55, Neck O.20-0.55, Intrusion Breccia beam; many of the titanomagnetites plotted are 0.20-0.80, West Pipe 0.4(H).55, and kimberlite atoll cores with no pleonaste rims present. Dykes and Sill O.35-0.9O.It is also clear from Although trend I (chromite -r titanomag- the data that spinel grains in close proximity netite) is representedthroughout the pipe, trend to the wallrock commonly have compositions 2 (titanomagnetite -> pleonaste) is recorded and textures distinctive from those in their host only in the Intrusion Breccia and Neck. (Rem- intrusions. nant pleonaste was optically identified in the The apparently'aberrant data points on Fig- Core and Peripheral kimberlites, but in grains ures 9 and l0 (see captions) represent sym- too small for electron-microprobe analysis.) plectites and other spinel grains derived from 50 THE CANADTAN MINERALOGIST mantle xenoliths. They are usually richer in Al Spinel data were plotted on Fe'+/(Fe!+ * and Mg than groundmass spinel in the kimber- Fe'*) versasFe"+/(Fe'* * Mg) or FFF ver,ras lite. Additional compositionally distinctive spinel FFM diagrams (not shown; see Pasteris l980a) grains in the kimberlite are those that mantle to monitor how the increase in Fes+ correlates large crystals of ilmenite; they usually are richer with the change in total Fe. Becauseof the con- in Ti and Fe than all other grains. There also straints that logically can be placed on the is an optically distinct, salmon-colored (in re- system under investigation, the FFF ratio prob- flected light), Ti-enriched spinel that is com- ably can be used as an index of relative oxida- monly present in veins between fragments of an tion within the pipe. The iron ratios show ilmenite grain, or between an ilmenite and its distinctions among pipe facies and changes massive perovskite rim (Fig. 3). throughout differentiation. As FFM increases,
REPRESENTAT I VE SP/ruELS
.4 .6 .8 t.o t.2 F2+ re E cotions= 3 Frc. I l. Cr-Fe2+ plot of representativespinel compositions from each major region or intrusion in the De Beers pipe. For the sake of clarity' Mg- pfeonaste compositions (0.15
FFF decreasesin almost all cases. Thus, al- thin magnetite-rich zone (Figs. 5,6). Away though Fe'* increases with reference to Mg from contact regions in the same kimberlite during fractionation, it increases more slowly facies, the intermediate zone of Mg-pleonaste than Fe3+. Note here that Fes+ and Fee+ were usually does not exist. Instead there is a gap calculated by assuming stoichiometry. Assuming that gives rise to the atoll spinel grains de- that groundmass ilmenite was precipitated as scribed previously as a variation of type-2 zon- spinel crystallized (Pasteris 1980b) and that ing (see Fig. 7; cl. Mitchell & Clarke 1976). the melt was oxygen-buffered,the negative slope Zoned spinel grains with Mg-pleonaste rims of the spinel differentiation trend on an FFF- surrounding titanomagnetite cores occur almost FFM plot may represent a decreasein tempera- exclusively in the Neck and the Intrusion Brec- ture along a single l(CI) buffer. cia. where the kimberlite is in direct contact In an effort to compare spinel compositions with wallrock. Although Mg-Al-spinel rims are among the several kimberlite facies at approxi- common here, one can see, in any given thin mately the same degree of fractionation, the section from the two regions, not only fully FFF values were determined at selected FFM preservedrims of pleonaste,but also atoll spinel ratios. Chromite grains from the Core and grains that have only a partial pleonaste rim Peripheral kimberlites have a higher content of (Fie. 6). Fet* than chromite from other parts of the The following textural evidence strongly sug- pipe, an effect probably due to bul\-composition gests that Mg-Al-spinel rims originally were differencesor perhaps to high l(O,). present in all the major root-zone facies. In As a first approximation, one can assumethat the Intrusion Breccia and Neck, some grains all the De Beers intrusions have the same or a with chromite or titanomagnotite cores have similar parent kimberlite melt. If this is true, thick rims of pleonaste. In these cases, the the degrees of fractionation of the facies are pleonaste is subhedral and has no outer mag- indicated by the relative Fe-enrichment in their netite rim. More common in this region of the spinel compositions (Fig. I I ). A plot of Cr pipe are fragmentary patches of pleonaste versus Fez+ (Fig. 1l) in spinel indicates subtle around a chromite or titanomagnetite core. In differences between some facies, and shows a such cases, a discontinuous, poorly developed clear distinction between the Core and Peri- outer rim of magnetite partly enclosesthe grain pheral kimberlites and all the other facies. The (Fig. 6). Only very rarely is (fragmentary) diagram indicates that the apparent sequence pleonaste identified in the Core and Peripheral from least to most differentiated is Peripheral kimberlites (Fig. 5). Such fragments usually Kimberlite, Core Kimberlite, Neck, Intrusion are too small for quantitative analysis. On the Breccia, West Pipe, Kimberlite Dykes and Sill. other hand, in those samples (from the East This interpretation is in accord with the field Pipe, Neck, and West Pipe) in which pleonaste evidence suggestingthat the major episodes of is not found, a black, translucentmaterial oc- intrusion at De Beers were the Peripheral, fol- curs in some cases between the chromite core lowed by the Core, followed by the Neck and the and magnetite rim. Microprobe analysesof the West Pipe. Spinel compositions also support the translucent material indicate variable quantities field interpretation that the contact kimberlite of Si, Ca and Ti (not titanite), but almost no called the Intrusion Breccia was not a separate Al. Much more commonly, this zone is filled intrusion. It probably is Neck Kimberlite with with the same calcite-serpentine mixture that a distinct cooling history. For instance, rapid, comprisesthe kimberlite matrix (supported by nonequilibrium cooling may have produced the microprobe analysis). It is the latter grains of strong Fe-enrichment recorded in Intrusion spinel that possessan atoll texture (Fig. 7). Breccia spinel grains. Textural and compositional relationships thus suggestthat in the main intrusions of the root Sppcler RErarroxsHrps rN THE Coxtecr ZoNns zone, Mg-Al-spinel rims originally were present. Resorption of pleonaste is believed to have oc- In the case of both phlogopite and spinel, curred in all but the contact kimberlite of the not only do compositional and textural rela- Neck and Intrusion Breccia. tionships differ among the kimberlite facies, but The tgxtural features of phlogopite pheno- they also are strongly dependent upon the crysts also are related to their proximity to the proximity to the wallrock. For instance,kimber- wallrock. At increasing distances from the lite in the contact region contains type-2 spinel diatreme wall, phlogopite phenocrysts show en- grains with an intermediate zone of Mg-pleo- hanced developmentof thick, black rims (opaque naste, which, in turn, usually is rimmed by a to transmitted ligho of a fine-grained, inhomo- 52 THE CANADIAN MINERALOGIST
Several mechanisms were considered that might have produced the zoning sequence of titanomagnetite core, Mg-pleonaste inner rim, magnetite outer rim. Atomic-absorption spec- troscopy, performed on bulk samples of kim- berlite throughout the pipe, indicate no signif- icant Al-enrichment due to wallrock contami- nation. Thus, the formation or preservationof pleonastecannot be linked to the observable bulk-chemistryof the present rock. The possibility was also considered that the zoning could reflect magma mixing [cl. Mit- chell & Meyer (1980) for the Jos kimberlite dykel, i.e., the introduction of a separate,some- what Al-richer kimberlite melt caused precipita- tion of the pleonaste.One problem with this hypothesisis that the contaminant melt would have to be injected into only certain intrusions in the pipe. lt also seemsunlikely that the pleo- naste-rimmed grains of spinel represent an entirely foreign suite of opaque phases;their size and the composition of their central chromite and titanomagnetitefractions match those of the other populationsof spinel at De Beers. A complexprocess of exsolutionand recrystal- lization of an original aluminous titanomag- Frc. 12. Pleochroically zoned phlogopite that is netite was consideredas a possible origin for possibly (owing size). a mantlex€nocryst to large the zonation sequencetitanomagnetite -+ pleo- It is rounded (possiblydue to physicalattrition) -> magnetite. ternary solvus exists in and chemicallyaltered to a black rind that extends nastc A inward along somecleavage planes. Matrix is cal- the two titanium-bearing systemsFeCrrO+-Fet cite, serpentineand serpentinizedolivine. Speci- TiOr-FeAlsOa and MgCr:On-MgeTiOn-MgAl:Or, men D844, kimberlite sill: transmitted light; at leastup to l300oC (Muan et al. 1972); the scalebar 200 g,m. solvus configuration is similar to compositional patterns in the De Beers spinel grains, as in- dicated in the triangular (Al, Cr, Fe'* + 2Ti) geneousproduct (Fig. l2). However, phlogo- plot in Figure 13. Although the compositionsof pite phenocrysts are well preserved in the kim- the grains of zoned spinel at De Beers (Table berlite nearest to the wallrock contacts. 3) are compatible with a solvus relationship, the rimming textures are not. Furthermore, if GuNErrc CoNsrDERATroNs:PossrBLE Onrctus titanomagnetiteand pleonasterepresent two limbs phases oF SPINELZoNTNc of a solvus,one would expect the two to be equally restricted in composition. However, Spinels grains with a chromite core and a Figure 13 showsthis not to be true. titanomagnetite rim can be explained by crystal- Although the widespread serpentinization in lization during normal magmatic differentiation. the De Beers kimberlite probably did not However, several questions arise concerning the affect either the precipitation or the resorption pleonastezones around titanomagnetite cores of of pleonaste,it was consideredas a means of spinel grains from certain regions of the kimber- producing the thin outer rims of magnetite on lite pipe: l) Is the observedzonation in Al in the spinel, However, electron-microprobe anal- spinel produced by magmatic precipitation?2) ysesshow that.the pseudomorphicserpentine is How did late-stage enrichment occur in both approximately as iron-rich as unaltered olivine, Mg and Al" which is a reverse of the normal obviating the need for expulsion of a magnetite magmatic trend? 3) Why are pleonaste-rimmed componentduling serpentinization. spinel grains confined to certains regions of the Owing to the relative geochemical immobility pipe? 4) If resorption of pleonaste occurred of Al, it appears to be a magmatic processthat elsewhere in the pipe, what was the reaction produced the zonation in Al. Because Al con- mechanism? tents of the De Beers population bf spinel not SPINEL ZONATION IN KIMBERLITE. SOUTH AFRICA 53
ZTi r FcSi
O FRB 37OB O FRF 37OAI AFRB 37OB' OFRB 377 O cRc I corae OcRc 1 Fiue o o o o 10ll|
AI Cr Ftc. 13. Plot of spinel analyses from the De Beers pipe Intrusion Breccia. Tie lines connect core (Cr-, Ti-, Fe3+-rich) and rim (Al-rich) in a grain. Symbols are the same as in Fig. 9. Note that Al contents are low and fairly constant except in the late-stage pleonaste, in which Al is high and rclatively constant.
only are very low, but also almost invariant phlogopite, the relative coefficients'reverse of absorp- (Fig. 13) throughout their precipitation (pleo- tion are o
Frc. 14A,B: Pleochroically zoned phlogopite phenocryst, which is a com- mon feature in the De Beers kimberlite. Polarizers rotated 90o between photographs A and B. Reverse-pleochroic core, normal-pleochroic rim. Transmitted light; specimen DB35B, kimberlite dyke; scale bar 100 pm. distinguishable types of phlogopite. However, pite grains may be zoned and have reverse- at least in some cases, groundmass phlogopite pleochroic cores, or they may be unzoned, in tends to be lower in Fe and Ti, and higher in which case they have either reverse or normal Al and (in some cases) Mg than either reverse- pleochroism(Table 1). or normal-pleochroic phlogopite phenocrysts. This represents a reversal of the normal mag- PnoposEpRslerroNsHrp BrrwErN SplNn matic trends toward late-stage enrichment in eNn PHLocoPttr Fe and Ti. Groundmass blades in the major root-zone kimberlite facies characteristically are Relationships observecl in the De Beers pale to colorless and, where colored, they ex- kimberlite hibit reverse pleochroism and appear unzoned. The typical chronological sequence for phlog- Both spinel and phlogopite occur as zoned opite precipitation in the main root-zone (as grains in the kimberlite groundmass. They are distinct from the kimberlite dykes) appears to also the only major aluminous phases in the be (1) reverse-pleochroicphenocryst, (2) nor- rock. Thus, possiblecorrelations were investigated mal-pleochroic phenocryst, and (3) reverse- between the compositionand abundanceof all pleochroic or colorless groundmass phlogopite. available generationsof phlogopite and spinel The reversal in the pleochroic scheme and the (cl. Rimsaite l97l). Compositional variations compositional differences Clable 4) between the in phenocrysr phlogopite (for instance, its Cr three defined types of phlogopite suggest a content) do not appear to correlate with those hiatus in crystallization between core and rim in spinel.Furthermore, no correlationwas found phenocrysts (cores commonly rounded) and between the presenceof groundmass phlogopite between phenocryst and groundmass precipi- and the presenceof rims of Mg-Al-spinel. Thus, tation (cl. Farmer & Boettcher 1981). the incorporation of Mg and Al into one of In the kimberlite dykes and the sill that post- these two matrix phases did not preclude the date emplacementof the main diatreme, phlogo- formation of the other phase.As described in a pite is very abundant. The grain:size distri- previous section, however, there is a strong bution is not bimodal, however. Matrix and positive correlation between the preservation of phenocryst grains form a sontinuum. The phlogo- Mg-pleonaste rims on zoned grains and the SPINEL ZONATION IN KIMBERLITE, SOUTH AFRICA 55 preservation of phlogopite phenocrysts, both tion of normal-pleochroic rims on reverse-pleo- being enhanced at wallrock contacts. Phlogo- chroic phlogopite phenocrysts. The following pite phenocrysts show increasing alteration model is a possible history of subsequentpro- away from the contact, but their serpentinized cesses.In the root-zone intrusions. volatile re- and chloritized pseudomorphsare readily recog- lease or simple depressurization (both decreas- nized (and recorded in Table l). ing activity of HsO) interrupted the precipita- The positive correlation of the presence of tion of titanornagnetiteand phlogopite.Mg and these two phases appears to rule out late-stage Al increasedin concentration in the melt during Mg-Al-enrichment in the melt as primarily the phlogopite hiatus, permitting pleonaste to being due to thebreakdown of phlogopite pheno- precipitate. In the dykes, however, this disrup- crysts. However, another type of reaction rela- tion mechanism was not active. Continuous tionship is possible between phlogopite and precipitation of titanomagnetitekept Fee+ and spinel. Precipitation of phlogopite ddpletes the Ti concentrations low in the melt. and normal- melt in K, Mg, Al, Si and HrO. The activity pleochroic (presumably low-Fe3*) phlogopite of one of these components may have de- continued to precipitate. As a consequenceof creased sufficiently to eliminate phlogopite pre- continuousphlogopite precipitation,Mg and Al cipitation temporarily, as demonstraied by the concentrations in the dyke melt never reached distinct generations of phlogopite in most the saturation point of pleonaste, regions of the root zone. If the activity of K Subsequent.resorptionof Mg-pleonastein the or H:O were the limiting factor, phases such root-zone intrusions is difficult to explain owing as Fe{i-oxides could continue to crystallize. to its refractory nature. The ambient l(Or) may The concentration of Mg and Al eventually have risen during late-stage precipitation of would rise in the melt, causing precipitation of spinel, causingoxidation of the initially ferrous other silicates and oxides by reactions such as plbonaste.The presenceof at least a moderate the following: l(Or) is supportedby the magnetitecomponent 4KMgsAlSiso,o(OH), -> 5Mg:Sior + cleterminedin the pleonaste (about ll wt.Vo phlogopite forsterite FeO and 15 wt.Vo Fe:Or in preservedgrains). 2MgALOr + (2K,O + 4H,O 1 7 SiO,) An increasein the Fe'* componentcould have spinel melt shifted the pleonasteto a composition within The simplified reaction indicates that decreases the ternary extension of the known magnetite- in KzO and SiO, in the kimberlite melt could hercynite solvus (Turnock & Eugster 1962), stabilize the assemblageforsterite * spinel * causing segregation of a magnetite* and a melt speciesover phlogopite. spineL"component. The Fe'* component,which As explained above,both the phlogopite and was oxidized to magnetiteealw&S precipitated spinel relationships differ between the dyke "epitactically", whereas Mg and Al were re- and the main root-zone kimberlites (Table l). sorbed by the remaining melt. This gave rise to These distinctions could be a function of dif- the common atoll texture, and explains why the ferences in bulk composition, in pressure of outer rim of magnetite occurs only where pleo- emplacement, and in reactions with volatiles. nasteis partly resorbed.The mechanismmay be Some mechanism that caused discontinuous pre- analogonsto that documented by Price & Putnis cipitation of spinel and phlogopite apparently (1979) in low-temperature, apparently non- was active (perhaps episodically) in the root- equilibrium oxidation-exsolution of pleonaste zone intrusions, but not in the dykes. Textural granules associatedwith titanomagnetite. evidence (Pasteris 1980a) indicates that, unlike the main intrusions in the root zone, the dyke Possible broader implications for kimberites did not undergo fluidization or any kimberlite melts similar rapid evolution of a gas phase (separable from the melt). For instance, the spinel grains Examination of these and other available in the dyke are not rounded (cl. Mitchell data on spinel in kimberlites, especially those | 978b) as are the titanomagnetite cores of of Mitchell (1978a,b, 1979), leads to the con- the root-zone spinel (Fig. 6); large segregations clusion that, broadly speaking, there is one of calcite and serpentine are more common in general trend with several variations for kim- the dyke rocks, and much less rounding of berlitic spinel compositions (cf. Fig. 9). The large grains is evident in the dykes. initial trend is along the base of the spinel The histories of phlogopite and spinel devel- prism from the MgAlzOe-rich corner toward opment in the dyke and root-zone kimberlites more Fe-rich chromite, with the addition of a apparently began to diverge after the precipita- small amount of Ti [seen in the Peuyuk and 56 THE CANADIAN MINERALOGIST
Elwin Bay kimberlites, Northwest Territories Survruanv oF CeNctusloNs (Mitchell & Clarke 1976, Mitchell 1978a) and From this study of ,ftimberlitic spinel, it is in the Bellsbank kimberlite, South Africa @octor clear that spinel compositions and textures differ & Boyd 1979)1. Subsequently, Ti (and Fe"*) systematically within h single pipe. With respect increase, commonly without significant increase to the De Beers.pipe, field relations and com- in the Fe2+/(Fe'* Mg) ratio. Compositions + positional patterns of the spinel are compatible greatly enriched in the ulviispinel-magnetite with the hypothesis that the main kimberlite component can be attained in late-stage spinel facies of the root zone represent successive or in rims on earlier Cr-rich spinel. intrusions from a single, fractionating parent The following are several of the most obvious meh at depth. The obviously later-injected kim- modifications to this general statement: l) En- berlite dykes and sill are more fractionated tire compositional clusters of spinel data from than the main root-zone facies. The dykes and one pipe commonly are displacedalong the FFM sill may have arisen from a different parent axis of the prism diagram with respect to data than the main root-zone rocks, or may rep- points from other pipes. 2) In most cases,spinel resent the late-stageresidual liquid of the same compositions from a single pipe will not de- parent kimberlite. Textural and compositional lineate the entire trend, e.g., a) the initial features of the kimberlite dykes and sill suggest phase may be an Al-poor chromite, as a Tun- that they, unlike the main root-zone intrllsions, rae, N.W.T. and Kirkland Lake, Ontario did not undergo fluidization. lMitchell 1978b, 1979) and at De Beers (this study), or b) the final Ti-enrichment may not It also appears, from the data in this study be attained, as at Elwin Bay. N.W.T. (Mitchell and others, that there may be a single trend 1978a). or c) only the final part of the trend defined by spinelcompositions precipitating dur- may be represented, as at the Benfontein sill, ing normal differentiation of kimberlite. If so, South Africa (Dawson & Hawthorne 1973). it may be possible to place in sequential order Green Mountain, Colorado (Boctor & Meyer the separate intrusions of multi-injection kim- 1979), and in the Jos dyke, N.W.T. (Mitchell & berlite pipes. It also may be possible to better Meyer l98O). 3) The late-stage spinel may compare kimberlites from different districts with becomevery rich in Fet* without much Ti en- respect to their degreesof fractionation. A better richment, as in the massive micaceous kimber- understanding of spinel compositional patterns lite facies at Tunraq (Mitchell 1979) and in is especially important in highly carbonated some atoll spinel rims at De Beerr. (secondarily replaced) kimberlite-like intrusions, Of particular interest are the exceptions to .suchas those from Kansasand Missouri (Pasteris, the above trend, such as late-stage enrichment unpubl. data), in which zoned spinel is the only in Mg and Al in the pleonaste rim on spinel primary phase remaining. grains in some facies of De Beers (this sfudy) Evidence in the De Beers kimberlite suggests and late-stageenrichment in Al over Cr in the that the aluminum content of the melt was con- spinel grains of the Kao kimberlite (Haggerry trolled over much of the interval of crystalliza- 1975), By a reaction process, aluminous spinel tion by the coexistence of phlogopite and low- compositions developed in garnet kelyphites aluminum spinel. During the rise of the kim- in the Monastery pipe (Haggerty 1975). These berlite melt, phlogopite is believed to have be- variations attest to the sensitivity of spinel to come unstable, and much of the available alum- changesin its environment during both primary inum partitioned into an aluminous spinel phase precipitation and subsequent re-equilibration, lpleonaste). Subsequentincrease in l(Or) in the and to the need to assess_the extent of spinel- late-stage kimberlite melt caused breakdown of silicate interaction. the pleonaste and precipitation of a magnetite- The relationships describedabove suggestthat rich rim on the grains of residual spinel. The compositions of groundmass kimberlitic spinel latter process produced the atoll texture char- can be used to distinguish different kimberlite acteristic of the spinel. In the De Beers pipe, intrusions of a pipe and perhaps to order them Mg-pleonaste zones in the spinel are inferred to with respect to their age of emplacement. In have formed initially throughout the body, very complex multi-intrusion diatremes like wherever the atoll texture now is observed.How- the Premier pipe, South Africa, this would ever, pleonastewas preseryedonly in kimberlite be no small feat. The spinel compositions also directly adjacent to the wallrock, which is also offer a means of comparing and contrasting the region of best preservation of phlogopite kimberlite pipes with respect to degree of frac- phenocrysts. Presumably the pleonaste and tionation. phlogopite breakdown reactions did not go to €
SPINEL ZONATION IN KIMBERLITE, SOUTTI AFRICA 57
completion here owing to rapid cooling. Bocron, N.Z. & Bovo, F.R. (1979): Petrology of The mechanism described above for the devel- kimberlite from the De Bruyn and Martin Mine, opment of grains of atoll spinel is proposed spe- Bellsbank, South Africa. Carnegie Inst. Wash, cifically for the De Beers kimberlite. Cursory Year Book 78, 496-498. microscope observationsof about 25 other kim- & MpyEn, H.O.A. (1979): Oxide and sul- berlites from North America and southern Afri- fide minerals in kimberlite from Green Mountain, ca suggestthat the atoll texture in spinel is very Colorado. /n Kimberlites, Diatremes, and Dia- common. However, it could be due to a variety monds (F.R. Boyd & H.O.A. Meyer, eds.). Amer. of mechanisms. The only other kimberlite-like Geophys. Union. Washington, D.C. suite reported to have spinel with a titanomag- netite core and a Mg-pleonaste rim are the un- Boerrcunn, A.L.. O'Nen, J.R., Wnoolr, K.E., usual extrusive rocks of the Igwisi Hills, Tan- Srpwnnr, D.C. & Wnsnrnr, H.G. (1979): Meta- zania (Reid et al.1975, Pasterisunpubl. data). somatism of the upper mantle and the genesis At Igwisi, the spinel grains are much larger than of kimberlites and alkali basalts. In The Mantle Sample: Inclusions in Kimberlites and Other those of the De Beers kimberlites. pleonaste Vol- The canics (F.R. Boyd & H.O.A. Meyers, eds.). rims usually are preserved,but apparent resorp- Amer. Geophys. Union, Washington, D.C. tion effects, including atoll textures, are ob- served. The above resorption mechanism is pro- BuoomcroN, A.F. & LtNosLEv, D, (1964): Iron; posed for atoll spinel at De Beers becauseof the titanium oxide minerals and synthetic equivalents. optically traceable, progressivedisapparance of J. Petrology 5, 310-357. Mg-pleonaste throughout the kimberlite root- zone. DAus, G.L. (1977): The ages and uranium con- tents of zircons from kimberlites and associated rocks. Carnegie Inst. Wash. Year Book 76, 631- AcKNowT.EDGEMENTS 635.
Much of this paper representswork done for DAwsoN, J.B. & HewnronNp, J.B. (1973): Mag- a Ph.D. thesis at Yale University under the ad- matic sedimentation and carbonatitic differentia- visorship of Brian J. SkinnernPhilip M. Orville, tion in kimberlite sills at Benfontein. South John V. Walther, and Robert J. Tracy. Electron- Africa. Geol. Soc. Lotrd. .1. 129, 6l-85. microprobe analyseswere carried out at the Geo- physical Laboratory, and a travel grant for field FARMER,G.L. & BoETTcHER,A.L. (1981): Petrol- work was provided by the Carnegie Institution ogic and crystal-chemical significance of some deep-seatedphlogopites. Amer. Mineral. 66, ll54- of D.C. sampleq Washington, Kimberlite were I 163. obtained personally by the author through the courtesy of the De Beers Geology f,bpartment, Haccrnlv, S.E. (1975): The chemistry and genesis Kimberley, South Africa. To the individuals in- of opaque minerals in kimberlites. Phys. Chem. volved in the above organizations and to Roger Earth 9, 295-307. Mitchell and an anonymous reviewer, who read earlier versions of the manuscript, the author (1976): Opaque mineral oxides in terrestrial expressesher sincere appreciation. However, she igneous rocks. /r Oxide Minerals (D. Rumble, retains full responsibility for all opinions and III, ed.). Mineral, Soc. Amer., Rev. Mineral. 3, Hg-l0l-Hg-300. interpretations expressed. Htrt, R. & RoEorn, P. (1974): The crystallization of REFERENcES spinel from basaltic liquid as a function of oxygen fugacity. J. Geol. 82, 709-729. ArBEr, A.L. & Rev, L. ( 1970): Correction factors for electron-probe microanalysis of silicates, lRvrNE,T.N. & SutrH, C.H. (1969): Primary oxide oxides, carbonates, phosphates and sulfates. Anal, minerals in the layered series of the Muskox in- Chem, 42, 1408-1414. trusion. Econ. Geol,, Mon, 4, 76-94,
ALLsopp, H.L. & Kne.urns, J.D. (1977): Rb-Sr MITcHELL,R.H. (1978a): Mineralogy of the Elwin and U-Pb age determinations on southern Afri- Bay kimberlite Somerset Island, N.W.T., Canada, can kimberlite pipes. Second lit. Kimberlite Anter, Mineral. 63, 47-57. Conf. (Santa Fe), Extended Abstr. (1978b): Composition of spinels in mi- BBNcE, A.E. & Ar.snr, A.L. (1968): Empirical cor- caceous kimberlite from the Upper Canada minen rection factors for the electron microanalysis of Kirkland Lake, Ontario. Can. Mineral. 16, 591- silicates and oxides. l. Geol. 76, 382-403, 595. 58 THE CANADIAN MINERALOGIST
(1979): Mineralogy of the Tunraq kim- (1982): Representationof compositions in berlite, Somerset Island, N.W.T., Canada. lz Kim- complex titanian spinels and application to the berlites, Daremes, and Diamonds (F.R. Boyd De Beers kimberlite. Anrer. Mineral. 67, 244-250. and H.O.A. Meyer, eds.). Amer. Geophys. Union, PurNts, A. (1979): Oxidation pheno- Washington, D.C. Pnrcr, G.D. & mena in pleonaste-bearingtitanomagnetites. Confi . & Cr-ems, D.B. (1976): Oxide and sulfide Mineral. Petrology 69, 355-360. mineralogy of the Peuyuk kimberlite, Somerset A.M., DoNerosoN, C.H., Dewsott, J.B., Island. N.W.T.. Canada. Contr. Mineral, Petrol- REro, BnowN, R.W. & RIDLEv,W.I. (1975): The Igwisi ogy 56, 157-172. Hills extrusive "kimberlites". Phys. Chem. Earth & Mrvrn, H.O.A. (1980): Mineralogy of 9. 199-218. micaceous kimberlite from the Jos Dyke, Smer- (1971): Distribution of major and set Island. N.W.T. Can. Mineral. 18, 241-250. RruselrE, J. minor constituents between mica and host ultra- MueN, A., Heucr, J. & Lorer.r, T. (1972): Equil- basic rocks. and between zoned mica and zoned ibrium studies with a bearing on lunar rocks. spinel. Contr. Mineral. Petrology lg' 259-272. Proc. 3rd Lunar Sci. Conf. l, 185-196. TuRNocK, A.C. & EucstEn, H.P. (1962): Fe-Al PAsrERrs,J.D. (1980a): Opaque Oxide Phases oI oxides: irhase relationships below 1000'C. ,. the De Beets Kimberlite (Kimberley, South Afri- Petrology 3, 533-565. ' ca) and Their Petrologic Significance. Ph,D. (1969) Experimental investigations thesis, Yale Univ., New Haven, Conn. ULMER, G.C. : of chromite spinels. Ecotr.,Geol,, Mon, 4, ll4-131, (1980b): The significance of groundmass ilmenite and megacryst ilmenite in kimberlites. Contr. Mineral. Petrologlt 75, 315-325. (1981): Kimberlites: strange bodies'! Amer, Received April 6, 1982, revised manuscript accepted' Geophys. Union Trans. 62, 713-716, luly 5,1982.