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Carwdian Mineralogist Vol. 17, pp.147-t63 (1979)

THE FIELDCHARAGTERISTICS AND PETROLOGVOF ARGHEANAND PROTEROZOICKOMATIITES

NICHOLAS T. ARNDT Deportnxentof Geological Sciences,University o! Saskatchewan, Saskatoon,Saskatchewan S7N 0W0

DONALD FRANCIS AND ANDRBW J. I{YNES Depafintent of Geological sciences,McGilt (Jniversity, 3450 university street, MontrCal, QudbecH3A 2A7

AssrRAcr rieure ou montrant des pyroxBnes) texture spini- fex, soit en laves en ooussiDets. rencontre A review geological On aussi of the literature, augmented des coul6es et filons-couches ultramafiques-mafi- by recent field and laboratory studies, reveals ques stratiformes. Les ultramafiques that komatiites igneous et constitute an suite made mafiques caract6risent les ceintures de roches up of volcanic and hypabyssal rocks raoeing in vertes arch6ennes;certaines coul6es intrusions composition et from duni.te and to dunitiques renferment des gisements or magneian de sulfure de . Ultramafic komatiites (MeO nickel. On ne connait pas de komatiites ultrama- > 20 wt. % anhydrous) are rich in and fiques prot6rozoiques ou phan6rozoiques,mais les occur as spinifex-textured, massive or pillowed Iaves komatiitiques mafiques ainsi que les coul6es flows, as bedded volcaniclastic rocks, and as small et filons-couchesultramafiques-mafiques sont cou- dykes and sills. komatiites (MgO < 20 rants dans la ceinture prot6rozoique du cap Smith, wt. Vo) have. as their predominant . olivine. de plus, deslaves en coussinets,ressemblant ( Qu6bec; or pigeo,nite) and . i des komatiites mafiques en composition, se trou- They occur as massive flows with brecciated or vent pal6zoi'ques pyroxene dans les fonds oc6aniques de Terre- spinifex-textured tops, or as pillow . Neuve. AIso common are layered ultramafic-mafic flows Cfraduit par la R6daction) and sills. Ultramafic and mafic komatiites char- acteristically occur in Archean greenstone belts, where rn several areas dunitic flows and large dunitic intrusions are the hosts for nickel sulfide IrttnonucrroN deposits. Ultramafic komatiites are unknown in Although there are several early descriptions Proterozoic and Phanerozoic terranes. but mafic lavas, and layered ultramafic-mafic flows of ultramafic rocks that are now known to be and sills, are corunon in the Proterozoic Cape extrusive (e.g., Naldrett & Mason 1968), credit Smith belt of , and pillow lavas with for the recogniliel and first definitive descrip- compositions simils f6 mafic komatiites occur tions of ultramafic lavas must go to Viljoen in Paleozoic ocean-floor sequences in Nerrfoundland. & Viljoeh (1969a, b, c), who realized that many ultrarnafic units in the Barberton Land greenstone belt in South Africa had a SoMMATRE volcanic origin. They proposed the name koma- tiite f.or a suite of rocks that included these D"aprBs la litt6rature g6ologique et des travaux ultramafic r6cents sur le tenain et au laboratoire, Ies koma- lavas as well as related mafic volcanic tiites constituent une s6rie de rocbes volcaniques rocks and ultramafis dykes and sills. As evidence et hypabyssales allant de et p6ridotite e of extrusion they described chilled flow tops, basalte et and€site magn6sienne. ks komatiites pillows and the spinifex texture, formed by ultramafiques (S 2OVo en poids de MgO, base rapid crystallizauon of ultrabasic and basic sili- anhydre), enrichies en oUvine, forment (1) des cate . coul6es de lave i texhrre splnifex, massives ou en In following other descriptions of (2) coussinets. des roches volcadclastiques lit6es komatiitic lavas in Canada and were et (3) des dykes et des filons-couches de faibles published (Wilson ef al. 1969, McCall & dimensions. I-es komatiites mafiques (1 20Vo MeO) se Composent surtout d'olivine, pyroxdne (augile Leishman 1971, Nesbitt 1971, Williams L972), ou ) et plagioclase. On les trouve soit en but in most cases exposures were not suffisient coul6e$ massives br6chifi6es I la zurface sup€- for the ultramafic rocks to be positively identi. 147 t48 THE CANADIAN MINERALOGIST field as volcanic. These reports included de- no further discussion of the problem at this scriptions of spinifex textures, now found to be stage. The definition and distinguishing criteria characteristic of komatiites, but because of the of komatiites presented by Arndt et al. (L977) limihlieas in exposure, the relationship be- will be used, i.e. the komatiites will be considered tween spinifex-texfured and massive lava re- to constitute a suite that includes noncumulate mained unclear. Clarification had to await the rocks ranging from peridotite (-4OVo MgO, investigation by Pyke et al. (1973) of a large, 44Vo SiOz) to basalt (lZVo MgO, 52Vo SiOz) exceptionally well-exposed outsrop in Munro or andesite (8Vo Mep, 567o SiOs), and cumu- Township, Ontario. In their paper, Pyke et al. late rocks from dunite or peridotite {-4OVo (1973) described spectacular ultramafic koma- MgO) to (-l2Vo MgO). Individual tiite flows with spinifex-textured upper portions members are characterizedby low FeO*/ (FeOE representing rapidly cooled ultrabasic liquids * MgO), low TiOz and high MgO, Ni and Cr. and massive lower portions formed by gravita- Some, but not all, have high CaOlAlrOg ratios. tive accumulation of olivine. Since that report, Also diagnostic are volcanic structures and descriptions of komatiites elsewhere in Canada textures such as spinifex and polyhedral joint- and in other parts of the world (Australia, ing as discussedbelow. India, Rhodesia, Finland, etc.) have shown Also because the questions of nomenclature that the spinifex-textured flows are merely the are under discussion, purely descriptive ter'ms most unusual in a range of ultramafic extrusive will be used to refer to the main gtoups of rocks that also includes massive flows, pillowed komatiites. These terms arc ultramafic koma- flows and even bedded volcaniclastic rocks. tiite for olivine-rich rocks with MeO > 20 Much of this paper is a review of various wt. Vo (analyses recalculated anhydrous) and papers .dsaling with tle field characteristics and mafic komatiite f.ot olivine-rich, pyroxene-rich of Archean komatiites. Additional and plagioclase-rich rocla with MgO ( 20 information comes from resent work by the wt. /o. It is intended that these terms be used authors, including an investigation of Protero- in conjunction with the earlier terms, not that zoic komatiites from less well-known occur- they supplant them. Thus ultramafic koma- 'peridotitic rences in the Cape Smith-Wakeham Bay belt in tiite would refer to komatiite' (Vil- northern The geochemical characteris- joen & Viljoen t969b, Arndt e/ al. t977\ or Qu6bec. 'Archean,gteenstone in a separate peridotite' (Nesbitt I97l); tics of komatiites are reviewed 'basaltic paper (Nesbitt mafic komatiite would refer to koma- et al. L979). 'high-Mg tiite' (Viljoen & Viljoen 1969a), basalt' (Williams L972), or 'pyroxenitic koma- DETINrTIOT:,I AND NOMENCLATURE tiite and basaltic komatiite' (Arndt et al. 1977).

The ultrqmafic and related mafic lavas in the Barberton Mountain Land are characterized DISTRIBUTION by unusually high CaO/AlsOs ratios (1.5 to 2), a feature that was emphasized by Viljoen & Ultramafic and mafic komatiites have been Viljoen (1969a) in the original definition of recognized in almost all well-studied Archean komatiite. Si.milar rocks in other areas lypically areas (Table 1)o most commonly in have lowero more normal CaO/AlzOg ratios greenstone belts, e.g., the Abitibi belt in Can- (-1). This difference has led to considerable ada or parts of the Yilgarn Block in Western confusion about the definition and use of the Australia, where low metamorphic grade has term komatiite, and about the names that should allowed preservation of diagnostic textures and be applied to individual members of the compositions. In such areas komatiites seem suite. An attempt is currently being made to restricted to the basal stratigraphic levels of establish acceptable definitions and rock names broad volcanic-sedimentary cycles where they for komatiites: questionnaires were sent out are associated with ultramafic-mafic layered to interested people by R.A. Binns in 1976. sills and Fe- and Ti-rich tholeiitic . Replies have been compiled, and a second Overlying volcanic rocks include tholeiitic questionnaire was circulated in June 1978. A basalts, andesitesand with more normal summary paper and a conference on komatiite contents, succeeded by calc-alkaline lavas definition and nomenclafure are planned once and volcaniclastic units, and finally by sedi- replies to the second questionnaire have been mentary rocks '(Naldrett & Goodwin 1977' Jolly received. 1975, Jensen 1976, Gemuts & Theron 1975, As this project is not complete, there will be Bickle er al. 1975). FIBLD CHARACTERISTICS OF KOMATtrTBTI r49

TABIA t. @X8ro WrmUN 0F SEnm CSTImCB OF K@tATrrrrC V0LC6rtc m N rNrilrsloNs

Uet@tphlc ftade Refersm6 !e!9!!9! 4S9 lctonlc hvltomat ksoctated &cks CNM hro T@shlp' 2.165 Sltlbl Biestooe b6ltt Ferlch lhol€lltlc bslls, adealt€6' h€halte-P@tsUyrte bke (1969), hd! @rtb€st Ontarlo !0 .U2 SuFrlor hwlnce, daclt€s; lblelltlc layet€d sl1Lsi to 8r€sEchlst facl€a (1975, 1977), Mdt et al,. kadlan Shleld dlabs€ dyksi slrlcdus tuff8 ald (1977), rleet 6 [scRae (1975) k brt*vs@ .2.7 Abltlbl Cree€totre belt tulelltlc b641ts ail ardesltot; UPFr Cte6chlat (tetacbr & b€tatur dlahso aod gabbro dyhsi araFach factes (1975), G€Il@ et @2. (1977), kjote e c€lt@ (1978), IG€h (r97(, 1978)

k Cred6 area, .2,7 Norths6! Abttlbt hphlbollle tracleB S@t€Iopodou-Sepur e Freck (t979)

hlft€ 0bert Crep' a2.7 Tnlcal &chear greeo- kflc to felstc volcaolcs, llon fol_ tuphlhllte Faclea schlu (1977) Northmst Tetltorloa alone bll €lvltot]Mt @tloo, aale qertztte 1tr eaat; qutt- in ssli stable shelf z1t€B' pelltlc aad rarsly calcarsg envlro@nt ovetlylng sdl@rts lu s*t Blallc bas@nt ln csti Chulchlll Provlnce

&p6 SElth Fold < , Aphbt& o.ogenlc belt, Fr! of Tho16t1t1c baeltlc rclcdulc6 and Pred@llmtly gr6en_ nlsd at aZ. (t969), noltlEn Qu€bec <2.30, the Cllc@[ogava Seosyn- lay€red @flc-ultramflc sl1lsi Etnor *hlat facles; highdr lbore (1977). Sctsars >1.45; clhei defored ln th clastlc sdtuats, dolodtes 6nd ch€rtsi Srsd@ tn uo4h of & Fujtudd (1977), -1,6s(?) Eudenla! oioSsy rar€ calc-aLkalln€ volcaolcs reptt€d belt Freck I BF$ (L97E)

naDblet &a €r@' &rly &@-f 1@r tocb' frol€ltttc oc@n-fl@r typs Cre@chiat facl€ ere (1973) ndfoundl&d Pal@zotc upufled &d folded to bsalttc lavaa aod pyrochatlcsi fom part of the chartai f,etalc pqhytieg Appalaclao orogoolc belt

SqJTHERil ATRIS vtl.joen 6 vnjen (r969a- Uoutaln 3. 50 Barbortm ge€nstone Tholeiltlc basalte and intetuedlatc UPper Srecnichls! hrbonon 1970), blver (1975)' T!s9- 10.20 bolt, l(epvaal Craton to felglc lavas and volcoiclaglics. facl69 L6d, east€m Ituh & lumeU (1979) vael md srazllsd, layelod ult@f lc-mfic lntruslots ;

'2.4 andc- crcengchlst faclos Bld€ et dt. (1975), Bglingag gt€sstone BelinSfe greengtone Thol€tttlc bagalt snd baealtic banded Ntub€r 3, az. (1977) bolt, nhodesla belt, Plhodesleo craton slt€; qurtzlte, siltsiones, lftnst6o9, stlo@tolltlc llneston€s ffiRN A!roIA (1971), 42.7 Tholoilttc basalts and lavcred @fic_ ;re€nechist to loecr kcdl fllllrc Mt. itongot No!96@-Wllsa (1972)' greonstm€ b€lt, [email protected] alllsi clastlc and tuffa_ @phlbollle facics Itlll@ ! (1972) tuetem Godflelde c@w gdimntgi felslc lntilgions hrlb€r8 (1969), (adbalda .2.7 Nolse@-gllua h Fe, T1 (komtlltic?) basalts; Cleengchist facios lm&ll 6 tad8 (1972), g!€6tr3tone b€lt ch€ns, calbonac@G tha139 EmrB & B6oa bss t Bopk1tr6(1975)

42.7 Tholelltlc basalts, layot6d ultm- Creenechlst to loto! N€sbllt (1971), Cbrlstte S@!ia Notsele-Wilsa (1975), greenstone b6lt ufic flog od sil15, blac* shales &phibollt! factos Ne8bltr ! S@ ed€eltic pltoclastlcs (r976)

\tt. .2.1 Itolelitlc baalls: chofts Lorcr gt€€nschigt h@r E kk|lm (1971), cllfford Nors@-Wilua (L974), b6lt fdclos bres et aI. 8ro43ton6 hads (r97i)

F€-rlch thololltlc bssalts; layeaed Creonschlst facios brth t dlchurch (1975), Yalabtndie Nor9@-fllt@ (1977) goonstoo bolt slllsi clastlc ed volcdlgonlc kldrert 6!u@r

Komatiites normally occur as sequences of overlies Archean gneiseic rooks of the Superior lava florrs, tens to hundreds of metres thick, province of the (Dimroth a oI. that alternate with sequences of tholeiites of 1970, Moore L977). A variety of sills, some similar or gxeater thickness. At the interfaces differentiated from peridotite to gabbro, others between sequences, the two nagma types are largely gabbroic or largd ultramafic, have commonly interlayered. Whereas the tholeiitic intruded volcanic rocks including highly magr lavas commonly have fairly uniform composi- nesian lavas, tholeiitic and calc-alkaline volcanis tions, the komatiites characteristisally show a units and clastis sedimentary rocks. The mag- wide variability in composition in an irregular nesian lavas have many of the characteristic or cyclic ilnanner. Cycles open with relatively volsanic structures, textures and chemical fea- thin, highly mapesian ultramafic komatiites tures of Archean komatiites (Wilson er a/. and proceed upward to thicker, less 'mapesian L969, Schwam & Fujiwara 1977). Spinifex tex- mafic komatiites. Many deposits of nickel-iron ture has not, however, been observed' sulfides have as their hosts thick,.massive dunitic It is unclear whether komatiites exist in flows at the basw of such cycles (Ross & Phanerozoic terranes. No clearly nonsumulate Hopkins 1975, Barrett et al. 1977). ultramafic lavas have been rep,orted, but The Cape Smith-Wakeham Bay fold belt in aphanitic volcanic rocks with compositions northern Qu6bec is the only location in which similar to those of Archean mafic komatiites the presence of Proterozoic komatiitic lavas has but for conspicuously lower TiOg contents have been verified. The bElt is a highly folded vol- been recorded within Paleozoic osean-floor canic-sedimentary sequenc€ that unconformably basalts in Newfoundland (Gale 1973, Upadhyay 150 TEE CANADIAITTMINEMLOGIST

m scmtRtxcs grucED BSA 2. rIA.D or SNLS F rOi{AfII[C Vottrlnrc rc s [tMslos

htls Etredtu EL@ lbf,lc el@ hy€rd rI@ Yolcdcl@tlcs htrul@ &f,{nl

nPy@ttlc &aro l@htp Splntfq-@cd ed l@tltta' - k6il's nc sull ulttdlc dyl€s @lvg f1@; rar€ ouvtu or fG@Frlch de*rlpt16 {[ 3d end eMe: 1d86 laysed plll@, a@ r!.tb @tlc &outl.tt€3 pr@ ultr@flc€flc a1118 sPfuLf,q td66 edfs-lshtedr b!ec- lat€al, @aslv6 or pill@€d Aee$!e-bqcs4!e" - Ptro:@ of, pf@ep16a- lcle6 @fic k!@ruts; krdy dplnlf,d-tcturcd, cMy plllqmal o!

Ia btt6va@ Spffid-tdud, @alve Aa pr@et tu tb b€tt, not r€Frtd frhly-hdded ultra- 9@11 ulti@f,tc dylcB Ht d flU@€d fl@; c@plc- ht haee et d@- ufic affs; @sslve, d slus a@ F al161 dd obllg@ crlb€d h ry dotatl coar66 dlr@flc bydtog !! B-dtvbt@ of apl!lld-!&G6d t1loG

k &ed6 6M SDMq-t&u6d ard trot reportd Tbhly-b6dd€d, fhts l(ot reorted gr&€d, d!r@{lc Yolcdgmtc e6di@ts

ktus Albirt ApMa-ielr€d fl@ &t reportd trot ropond Not rBFrtd s@LL ultr@ftc dyts hop d s[18

&F Mth Pflol€d 6aal @ale€, he-shapsd or hterdly Fl@ top b!@cks; X'htuL (to 30(h) lats<y lold klt polyh€drally anal col|r@r- d!@le€l plll@€d or coarBe d flss pdalst6!, canpoaltg B(LL6 Jolnted f1@. no sp$ifq br@clat€d !opa; pl].lded btal@laatlt$ rltb Fildotlte, Dler pFG- t&re. tu bsl hss; col@r-lotuted @l!€ ed &bbro; ds rtoUy ollvlaa dt€s bsaalt ed p€ddotttB 1! Sbblo aad lar861y parid- htarblai tutttal mp otltlc a[k. rtth 12-162 Ugo

f,ebld he Puoed bdts ht r€pltd &t !€Frt6d trot reportd

g@llm AruCA

kb€46 b@!al,! Sptdfd-refulert fl@i gd4_@ - hrshry &rtah 16t6rd ultrs- lot roFild Tlrla, c€ra+grala€d, h @slve flm rltb pro- *gnesh (16 ro 222 @fi.c@flc slta my poehFLtlc or F }1- 8f@alv€ dmril :lacr4e [90) ptll@ed o! @Blve llt{c-td$r€d p€itdotttc t! oliYle @ntd: soe tlm. S@ ptetu sllls aad dtLei Layed p4lor€d f,lffi spdfs. Eglplgeg ult!@flc@fb alll€ lulg - c@rady crts- tal1he, cllaopFo!@e- dlcH flo€. OM11y @31v€. !gl!g!!gq lEg- 18@6 d pkglel4ebarhA, @lv€ or plLlosd, c(My vailolltlc, a@ pFo:@ sPufs

Bottig€ Cr@- Ptu.omd flffi ai6 c@; Pilllmd 1ave, s@ slth tu fls (13 b thlcL) T!f,f,s , eggl.@ret6 Perldotlttc co!f,o@bla at@ B6lt @alv€ f,l@ hry6 br4c- ryadlLd, @y dth vltb pF6@€ aphlf* d po8slble pilld sl.ft (p8slbly f1ffi)i latd topa and baa6, eDd @€llli @sslve f1@ t&!rd top derhtu blecka of @flc ky€i€d u].tr@f,lc-@jtu sl|@r-lot[t€d r.ntorloia i dth el|@r pF66e by rd@ clbopFoleG k@tllt6 c@Faltb! al[1a gpial.f,d-rdtsed flffi ei,nifu tdue tdtur€, clbpFdsne c@lats d c@bte d11t6. bths po8a- lbL€ flor (70 E thtcL) drh a q@rra gabbro

msrBmt flts'mAlrA

- t{t. Molger Spinifq-t€xtu€d ed "lUah-Mt baalts'r @slv6 Not ropofrod Not ePodod Pebeble ultr@flc dy&€s @9ir9 flds, 9@ rith flii-iittr-pfr6ime spint - ed stlle; laFod stlle a$yadaloual tops fq-tonrod ol broccl- dlffatmtiaEd ft@ hsz- atd tops. No pil1ds. bagite ti@gi oltho- Variolea ed aaygahrlee ar6 l')er@1t6, , gabbe to gtefh)'r6.

'Thtck faobalita fl@6" - l0-:O 0 Uasslv€, ptlloed o! Not rolonod Uafic hyaloclstitog tu @elvo polldotltic ttLcl, rtth b@cldtod b@I6tod baaltg ed flor b&cci.4 @lt3 @y b6 lntruj.vo tol)g, thttr spinifor- €lled @athol6litog tenred laysn (6 E) bG rttb loE Pa ed Tl dd a p!o!@cod rt@ftsrd 110!16r to Eaflc lae@E ln oltgtng @ntd k@tlltoe Ov6tlein by rtitn flmn (0.3 to 7 o) rhtch @lt @ sptltfor-t*usd

scdla Eltdf*-t€na€d ed Lor Tl @tatho1.1it6 Iot !epo4!d lxt@tve fraS@tal Soee @sivo dtbttgr @y gls @elvo @1$ of verr.ablo bB intrulv6. thtclnsg ilfildr-Uc It. Cltf6ord SpiDl,f6r-t6trE9d fldg bsalBr' - fld Not ropoted fu8@td ulta- Porldotttic alllsi dd @tlY3 fls rtth mrpholoE M ropof,€d @flc s€dlMtaly larSo deLtlc ,.Dtrul@ dmhld i8q@o t! eits c@taintng olivlno @t@t olmgatod &d flat- tdod ult@flc fsg@ts Ia a b€dd6d a1blt6 @d p)'!it6:rl.ch utri:

gptltf€r-torGed nPytux@ltoBf. lsbbtrilt6 ed rl||amosl4 liot ropofted Not rslbft€d Slall po.tdotitlc ri.ll8i @31fr fl@ h$lt6n ed nbseltsir ldgo &altlc lntroi@ iG-Iffirrshed d-66- b4r,! of 0ln6n1o$. and tl80 @ds. Io tn!o!- @tl6 6 glq @rnholoof FIELD CHARACTERISTICS OF KOMATIITBS t5l

TABIJ 3. DESCIIPTTOtrS OF OMET (O}'ATIITE '!VS ASD ITTBUSIONS lorti A@rt@r (Na!.dtett &ado@ld T@b1p, (ht.r ulri@f,tc @d @f1c L@tltt€ la€ fto$, 1ay6Ed @llc-s1t!@f1c fLos ed sllls & ]'a@ f968) lnbd.no a!@' O!t.t dra@f,tc ed @f,lc [email protected] La@ (ryka L97O, L975, Slrllard Ia&&IaLe Abttlbi Eav6se, Otrt.r ultraeaftc ed Mllc &o@t11te8, c@dt ptl1@d (Jem6 1976) ltl.ch!,pt@t@ bet-t' Uam' ont.! a si.!814 pill@d @flc Lo@tlLb (Bieks ! Eatt 1972) Criffitr IrLs, ltori:brgt lerlbrleBt [email protected] L@tLLts fI@, al@t co@l€tely sptdfq-tqtuled (8a11 1978) (t Botts cdo, ifla, r qtt" ko@tltt1c plll@ lava dd aheoted dylis, e@ rfcir fs trippit o1ivln6' ,! a l@! ordovtcle oPh:lollt€ Pedtryay 1976) South@ Atrl@r J@M 6cht6t b€1t, Bdb€rt@ [email protected] led: hlgbl-y dtlded @f,1c 8d ultr@f!3 ko@tlltes coaelatlE tlth those ln the 1@! Oave'@cht erolp (ADha€GB€t l9?2) S€d6L!e; ultrofl.c coropLs, Kruggrsdorp alLstll.ct: Eatlc sd [email protected] k@tllte levs (A!h66osser 1976) (VlU@ L976) SbdganL nlns at@, Sbod@!4. ntut;X sulfids delosit ltr a psridottttc tattut@i d!i@fl3 ard @flc ko@ttt@ bv4 et dl. Alatralta g@tta d.tre ar@, Eastom coldfieldsr d.E@fLc ed @ftc ko@tllt€ la@ sd s@ vol@lclastlc rocks oEilylsg E4sl9e Petldotlte cotrt.lning tho BtcLsl ssLftiB tieposlt (!6sbitt 1971' Ctlrlstie 1975) feuch u€lIs. EasEaE Goldttoldsl a thic&, @le ulr@f1c la% fld (Isia & ttlllr@ 1973) (csoqHl 1977) jilffi;;;k* ili.i-6rair"ia", a prorslorcic rayerea perraotriepyiueJegattro lntruio of Probable Loutllte afflaltv Iillla Kold gold ftEld! p1ll@6d @f,lc [email protected]@ (vls@tlan 1974) Dbami greotom-b€lBr ht8b1y @6rpbos6d [email protected] eal @f,ic rocks, possl.bly of Lo@llitlc oitgld (Iaqvl 1976) Rtplald V6rl.@ g!@!@e b€t-b ta @ten lisled: ult@f,lc ed @flc &@tlites (falr:s'M 1976) Ce€d6d (llcct€go! 6 codthab d€, uLtr@ftc @al Ef,tc @cl4res fu islsic g@18@ ele ltr@ipretod e ht8bl.y @@rpbosed L@tlltes ed tholelltes ltar6 1977) Cetral Sshara ?ioE ozolc ap{qltetstu€d sltr@fic [email protected]@ (ghat&a 1977) trola FoEl!€uLar II.S.S.B.

ultra@Jlc ed @flc Lo@tLLt@ ol s ldge of, tt@r @d titLild.Le Plec€lbrtu ag@ (s@lova 1977) 'ltllq la@ X! th6 VetElljlotr dl.strLct, ltls@oB (et6o ! Scbulz L977) ed so@ of the Vedtelsdorp lavs' South Aftl€ (hleel 1975) he@ trF ed ll-rlch coiqositi@ ies€Eblltrg those of tbol€llt€ .athei th6 Lo@tiits (Aedt & Brcoks 1978).

1976) and from other drverse locations (Brooks pletely replaced pseudomorphically by serpentine & Hart L974. Other volcanic rocks described ind chlorite. In relatively fresh samples the as picritic in older literature (e.9., the upper pyroxene is unaltered, but in more highly_meta' pillow lavas at Troodos, Cyprus: Searle & Vokes morphosed examples it is replaced by chlorite L969) lays similar compositions. Uttle has or tremolite. $imil6ly, may be unal- been published on tle field characteristics and tered or partly or completely replaced by mag- petrology of these lavas and it is not known netite. how closely they resemble Archean komatiitis In areas of high* meta,morphic grade, espe- lavas. Whether or not'such Phanerozoic mag- cially where COg is introduced during the nesian lavas qualify as komatiites hinges on the alteration process, the ultramafic lavas bcome questions of definition and nomenclature that completely recrystaltized and tle pdmary phasee are currently under review. are iephCed by a variety of secondary minerals TABLE 4. COUPoSrlrol{S 03 COU}iotrUTXEnAI.S rN Kol.lArIrrIC IAVAS

Ur,rnervrerrc Kortrernrr Vor-ceNrc Rocrs s102 41.8 50.98 - 55.2 57.1 51.6 54.4 - 0.1L 0.04 0.26 0.35 0.03 the following sec- r1o2 0.45 In Tables t and 2, and in d2o3 5.04 13.9 r'.O2 0.91 2.48 3.10 27.6 - tions, the mineralogy, textures and field char- Cr203 O.27 - 46.8 0.63 0.50 0.00 F 7'03 acteristiss, firstly of the ultramafic komatiites, 2o3 9.gt 8.52+ 9.4t 7.91 7.21 9.48t 1.O6i tlen of tle mafic komatrites, are summarized. FeO 27.9 - o.52 0.26 0.21 0.16 0,2r 0.00 to original papers containing de- ho References Ugo 44.2 17.33 4.20 24.9 32.0 L1.5 L7.7 0.06 tailed descriptions of these rocks may be found Cao 0.27 \6.37 - 4.O2 1.18 19.1 t7.2 11.6 in Tables 1 and 3. Mzo 0.04 0.01 0.11 0.13 5.L2

Minerology lota1 100.3 98.3 100.8 L00.6 99.8 98.9 99.9 99.8

komatiites is ko 0.78 0.88 0.81 o.77 The mineralogy of ultramafic @ia- ''- simple, with principal primary phases being l. OJ.J.vtse fron spl61fe8-tdtsed ultt@tlc k@tllte (Atndt et aL, olivine (Fo'o-o), aluminous augile and basaltic 1977, 16b1e 2). 2. AletnoE aq!.te froo sPtutfd-tdtued t@tlLt€ (nsbl.tt 1971, Table IV). 3. Chtodlte fr@ aptaifd-t*tueal k@tl- glass. Chromite is usually present in minor ite (Aildr at aL. L977, labls 4). 4. Mag!61e PLgsrtte tu pyroxde gplDl.fes t*td6, fred's f1@ (Andt t Flest 1979). 5. B!@LE f,i@ amounts. Representative analyses are given in c@f,ate 1ayer, lsyor€d ko@lllte fl@ (Andt t F16et 1979). 6. Auglte glass to a froE c@1ar6 layer, Fredrs flw (Addt 1977, Iabl€ 1). 7. AtrCt6 fM Table 4. Invariably the is devitrified @fl.c ko@tilre taE flm (Atudt et aL, L977, rabLe 3). 8. lrad@laa submicroscopic intergrowth of hydrous mine- (Arso) fron gabbro' leyerad f1@ (@pubushed sa1ystu). i loEL 1r@ @reBed as tr6o. rals, and usually the ofivine is partly or eom- ts2 THE CANADIAN MINERALG}IST including ahtigorite (instead of chrysotile or or granular, usually O.2 to O,7 m.m in average lizardite in less metamorphosed rocks), talc, dimensions (Donaldsons (1976) terminology magnesiter dolomite, , and second- is used to describe the olivine habits). Chromite ary and olivine (Binns et al. L976, forms euhedral octahedra (0.05 to g./ mm) and Eckstrand 1975, Williams 1972, Oliver et aI. augite occun as someu/hat skeletal, prismatic 1972), grains (0.1 to 0.3 mm). In thick, layered ultra- mafic-mafis florys and sills (discussed below) Tertures all grains are coarser and even the In the relatively slowly cooled central have nonskeletal habits. parts of flows, the minslnl grains tend to be By contrast, in the quenched ,margins of relatively large and nonskeletal, and have flows or in pillmrs, all minerals are finer equant or prismatic habits. Olivine is polyhedral grained and usually are skeletal. Olivine occurs

qI!IED AIO POLYf,EORALLT T CHTLLED ILOI IOP 0nrl0 Ftof loP ffH FllE POL!XaORlL JolxtlN0 t FrxE aAtoot gPtxttEx

coaRst onENtEo SPtNtaEt

un PAiT of tof ' nlltED gtEL:r L rEorur l0 flNe onal{Eo I oUvttE PEntootrt8 frlr coaRSt &rHEDiA! JOrlllxS I cutullrE souD ouvn:

J (ro8tY PEntoonlt I T i curutaTE 90LrD oltvne

A. 3PINIIEX TEXTUFSD FLOT 8. ras9tvE FLow

- : TiIXLY BEODEDTIFTS

].% ctcLlc ALiERXAI|OI rr cn0| gtzE ato ffi TIilERAGT

PILLO'S frlt : '-T-',, fiO$ SEDDEO POLYIEOiAL ruF JOtttil€ == S l:i;ii!,#;ii+:;;ir:li:;l,tj.ilii jif ciaoEo StoDtxo r:ia's.::i;f.::i:i.i ;.,i:ilii.. {il;}iitji I i :!.i:..r:::;+...irt,iT;}i:i,i l:.1,;;,:{'ir:grirtrii:gf .' jg!: g="^=ci;o:ett- t*-4 coARsEBREccrA ot2 -m---; g r Zi?r^--.?*o- -- .o*ilili*'n'lffJl_1" IfTERS u--_: rnrcr:rn rrrn spriiiex -@.*o-;JlJ--ja-. { onrlsive rexrunts -e"* -o4@

C. PILLOTEDLAVA 0. volcafircLAgTtcs FIo. 1. Diagrammatic representationsof comnon types of ultramafic lavas ard volcaniclastic rocks. FIBLD CHARACTERISTICS OF KOMATIITES 153

as delisate chains, plates or dendrites (0.1 to 0.5 mm); chromite forms intricate highly Nl,E 5. mDAL AMLYsm or &ohnlmt skeletal, cruciform grains, and the p)rroxene forms feathery dendrites (Fleet 1975, G6linas 84.1 54.7 I5.7 - - - 59.4 & Brooks 1974). oltvLoe 41.3 40.8 35.2 58.6 13.0 In the -free upper sections of lava cllnopFoxso 48.6 8.5 35.3 61.8 72,7 3L,7 flows, where temperature and perhaps compo- orthopyro:eue--30.75.7 sition gradients are steep,spinifgl textures form Plgeontte 8.3 by rapid growth of mafic minerals from chilled &phtbole o,2 - - flow-tops down into supercooled ultrabasis Chlorite 9.8 6.8 8.6 - liquids (Nesbitt L971, Donaldson 1976, Arndt Class 9.4 6.8 9.6 20.5 r9.3 - 21.6 -59.4 - et al. 1977). In this texture, large, platy, skeletal PlaSlochse 40.0 45,3 grains of olivine 3 cm or greater in length are r.1 L4 L.l

arranged in parallel groups or radiating clusters. chroElte 0.8 0.5 tr 2.0 - 1.0

They lie in a of fine dendritic augite, Fe-Tl oxXdea - tr 1.5 tr L.4 2.1 -

skeletal chromite and devitrified glass. 1. oltvlne spi.llf,q-tdtced dtramixc k@tllte laB (P+168)2. 2. c@late zoBe, ultt€@fl.c k@lxtto fld (P9-190) ' 3. hs6i€ ultt@f,lc La€ 09-194). Field chmacteristics 4. bflc k@tllb la€ vtth olivlae pberocry€ts (P9-U6) . 5. hflc ko@tttte 1aE slth redosly o!16ted pylox@ Besdles (P9-227). 6, bflc t@t11te lave r{th ptrc@ ed plagtoclase (p}'110). Ultramafic lava forms a variety of types of rstrl.ng:beaf' 7. pyro!@ spfulfq-tdtGed la!a, fred's fld (P9_107)' lava flow and florr . 8. cabbro, rred's fl@ (PF173). 9. Auglt*blotrz1te c@].ate, Fledrs fl@ (PF203). (a') Spinitex-textured Lava Flows. The most 10. 01i.vtn6 c@late, hed's f1@ (P9-199). of I mere altered' propordons of prtury pbes ar€ eattuted. spectacular type ultramafic komatiite flow, 2 u1 malyeee ref,er to rocb frm Mmro Towhtp (amdt 1975). the type tlat has received most attention in the geological literature, is that with a spinifsl- grains of olivine. The uppermost part (Br) textured upper section and a cumulate lower contains hopper grains oriented rougbly parallel section 1Fig. 1a). Flows of this type have been to the flow top. In some flows,- a knobby- described in detail (Pyke et aI. 1973, Barnes zone (B') that contains irregular et al. L974, Arndt et al. 1977, Imreh 1978, patches of pyroxene*glass matrix material oc- Lajoie & G6linas 1978), and will be discussed curs part way through the olivine cumulate here only briefly. zone, dividing it into two similarly textured Individual flows range in thickness from less divisions (Bz and Ba), each of whioh contains than a metre to greater than 20 m. The thickest polyhedral or granular olivine in a spane augite- flows are the dunitic units which form the glass matrix. The base of each flow is depleted bases of ultramafic-mafic flqw sequences.The in olivine and is chilled to aphanitic rock con- flows may extend hundreds of metres along taining fine skeletal olivine grains. Model anal- strike with little variation in thickness, or they yses of various parts of ultramafic lava flows may end abruptly in bulbous terminiations. are given in Table 5, and chemical analyses of Spinifex-textured flows (Fig. la) have a primary phases are listed in Table 4. chilled flow-top (Ihe Ar zone of Pyke et aI. Spinifex-textured flows are believed to form 1,973) containing a small proportion of solid by extrusion of ultrabasic lava containing a olivine and a larger proportion of small number of olivine phenocrysts. These skeletal hopper olivine grains in an augite-glass settle towards the base of the flow, growing matrix. The flow top exhibits unusual polyhe- and being joined by additional olivine grains dral jointing. Underlying the chilled zone is that crystallize from the melt, to form the basal spinifex-textured lava. Pyke et aL (1973) re- cumulate zone. Spinifex texture forms by rapid cognized a single spinifex-textured layer (Az growth of olivine downward from the zone) in which the grain size progressively in- chilled flow-top into the underlying phenocryst- creases from top to base. Lajoie & G6linas free, supercooledliquid (Pyke et aI. 1973, Andt (1978), Ross & Hopkins (1975) and Barnes et al. 1977, Donaldson 1976). et al, (1974) recognizedtwo divisions, an upper A variant of the typical spinifex-textured division (4) containing relatively small ran- flow is described by Lajoie & G6linas (1978). domly oriented chain-like crystals, and a lower In examples from LaMotte Township, Qu6bec' division (Ar) n which larger platy crystals are the between the spinifex-textured and arranged in intersecting, upward-pointing cones. cumulate zones is interpreted as erosional. The lower cumulate section (B zone) of spihi- The lowermost spinifex crystals are cut and fex-textured flows is enriched in polyhedral or fragments from them are trapped in con- rounded or less commonly, elongate skeletal eave depressions in the contact. The un- t54 TIIB CANADIAN MINERALOGIST

derlying .B division shows layering resulting (c) Pillowed Flows. Pill,ows in ultramafic lavas from grain-size variations, either parallel have shapes,dimensions and, presumabln modes or cblique to the flow contacts. Also reported of origin similar to pillows in other types of are nonnal and reverse grading within 16" subaqueous . Dimensions range layers. These features are interpreted to be the from about 30 cm to several metres (Nisbet result of flowage of lava beneath a cnrst of et al. t977, Pyke et al. 1973, Imreh 1978) previously formed spinifex-textured lava. Characteristically they are polyhedrally jointed The proportion of spinifex-textured to'cumu- throughout, with the polyhedra finer at the late lava varies considerably. Normally between margins than at tle centres. Textures seem one ttrird and two thirds of the flow is spinifex- similar to those of massive lava flows. Arndt textured, but in flows from Griffin Lake, North- et al. (1,977) described pillows (referred to as west Territories (Hatl L978), almost all of each lava lobes: p. 333), in which a 'shell' of flow h3s this texture, with only a rhin rubbly spinifex-textured lava wraps around an olivine zone (less than I0% of the flow,s thickness) at cumula.te sore at the bottom of each pillow. the base. At the other extreme, the thick dunitic (d) Volcaniclastic rocks. Fragmental ultramafic flows that host nickel sulfide deposits (Ross volcanic rocks have recently been described by & Ilopkins 1975) and the thinfer flows in G6linas et al. (1977) and Stamatelopoulou- which olivine in the cumulate zone is looselv Seymour & Francis (L979). The volcaniclastic packed (Arndt et al. tW7) may have spinifei rocks are intimately associated with ultramafic zones that are only a small fraction of tne komatiite lava flows. Gihnas et al. (1977) flow's total thickness. described two fires in LaMotte Township, (b) Massive Flows. Most ultramafic lava flows Qu6bec: type A consists of fine-grained, thinly are not spinifex-textured but massive throughout bedded tuffs that have accumulated in channels (Fig. 1b). The lava in these flows coitains between ultramafic pillows; type B, a massive rounded or polyhedral olivine Crains in an augite- breccia containing angular fragments from all glass matrir; a texture similar to that of the parts of ultramafic lava flows, occupies an cu,mulate zones of spinifex-textured flows al- erosional channel in a sequenceof flows. Stama- tglgn the olivine grains are less closely packed telopoulou.Seymour & Francis (L979) describe (Table 3). Occasionally t}'e majority ot-grains a thinly bedded ultramafic sediment character- are skeletal, as in the thick flow dascribed by ized by a cyclic variation in fra.gment size and Lewis & Williams (L973). In some flows thl mineralogy with climbing ripples, graded bed- olivine grains are concentrated towards the ding and soft-sediment-deformation structures. centre, leaving the margins depleted in olivine, In neither area are primary minerals pre- presumably a result of flowage differentiation. served. G6linas et aI. (1977) interpreted the In others there is a progressive down'rard in- secondary mineral assemblagesto indicate that crease in the proportion of olivine. the clasts are chloritized shards, granules and A characteristic feature of massive ultramafic globules, or larger angular fragments, either flo1s is polyhedral jointing of the type restricted once glassy or with textures identical to the to flow tops of spinifex-textured flows. These lavas in associated flows. Dimensions of the joints are relatively fine and angular near the fragments range from less than 0.5 mm for the margins of flows and produce distinct polyhedra shards to several tens of centimetres for the rangug in average dimensions from a6out 3 to angular fragments. 1.0 cm. At the centres of flows they are longer, Both volcanic and sedimentary processes thigker, more widely spaced and gintly contribute to the formation and deposition of and outline crudely elliptical volumes of"oJ"a, lava these fragmental rocks. A cautious discussion 1O to 50 cm across. These joints presu,mably of the relative importance of sedimentary re- form during cooling of the lava, perhaps in a working during deposition, and of detrital, pyro- manner similar to columnar joints. clastic or hyaloclastic processes in the origins Although not common, 3-1O mm of the fragrnents, is given by G€linas et al across filled with chlorite or serpentine have (1e77). been reported in some areas (Lewis & Williams 1973, Barnes et ol. 1974; in the Carnilya Hill Merrc KoMATtrrEs ar€aa Western Australia: C.D. Arndt, pers. comm.). Generally the a,mygdules,which re- Mafic komatiites include lavas with a wide present infilled vesicles formed by the release range of cornpositions and an even more diverse of volatiles from the lavas, are confined to the range of mineralogies and textures. Concentra- tops of massive ultramafic fl,o!ys. tions of MgO range from 2OVo (the lower limit FIELD CHARACTERISTICS OF KOMATUTES 155 of ultramafic komatiites) to less than 7Vo, and to those previously described. Rounded, poly- SiOz contents range from about 48Vo to morc hedral or various types of skeletal grains may than 55Vo, be present, with the specific habit depending on cooling conditions. Mineralogp Augrte and grains that crystallize The most magnesian mafic komatiites, those in tle centres of flows are prismatic and solid. with more than about LSVo MgA, usually have These grains may settle to form cumulate layers olivine (and minor chromite) as liquidus phases. or else they crystallize in place, in the company Between aboat t2 and LSVoMgO, the dominant of plagioclase, minor qvarE, and mineral in some flows is olivine and textures opaque phases in less mafic komatiites, to prod- and overall mineralogy resemble those of ultra- uce fine- to medium-grained subophitic textures. mafic komatiites. In other flows with roughly In chilled margins of flows and in pillow lavas, the same compositi,ons, olivine is absent and fine-grained augite adopts a range of skeletal, the abundant minerals are calcic pyroxenes, acicular, dendritic or whiskery spherulitic habits i.e., pigeonite and augite. In this composition (Fleet 1975). Pyroxene spinifex is a macro' range the minsplegy seems to depend on the scopic texture that has as its dominant textural conditions under which ttre lava crystallized. element skeletal composed of For example, in the central parts of flows and clusters of composite needles with pigeonite in sills, where liquids crystallize under condi- cores and augite mantles (Arndt & Fleet L979). tions approaching equilibrium, itrs minerals At the upper margins of flows these needles olivine, bronzite and augite might appear from are oriented perpendicular to the flow contacts 'string a with t5Vo MgO. The same to produce unusual beef (Arndt 1977) solidifying rapidly at the top of a flow would or 'columnar' (Nisbet et al. 1977) pyroxene crystallize magnesian pigeonite and augte, and spinifex texture. Pigeonite-augitd needles in the olivine would not appear. The effects of - central parts of flows are fine grained apd lization conditions on the mineralogy and tex- commonly randomly oriented. Although the tures of komatiite lavas are discussed by matter is under review, we believe that only Arndt & Fleet (1979). macroscopic texfures in which the pyroxene In less mafic komatiites (MgO between 12 grains show preferred orientation should be and 9Vo), plagioclase is present, pyroxene is called spinifex. The matrix of the pyroxene abundant, and olivine occurs only as rare, needles consists of finer augite grains and glass skeletal phenocrysts. In the least mafic koma- in more magnesian lavas, and augite, plagto- tiites, quartz is present as well as augite and clase and quartz in less magnesian lavas. plagioclase, and olivine is absent. Spherical volumes of relatively lava, Accessory phases in mafic komatiites are called varioles or ocelli, are coflunon in mafic glass in all rapidly cooled lavas, - komatiites from many areas (Viljoen & Viljoen rich in Mg-rich fires, and 1969a, Williams 1972, Nisbet et al. 1977). and iron- oxides in less mafic types. Ferguson & Cunie (1972) suggestedthat they As with the ultramafic komatiites, there al- migbt be droplets of immiscible liquid. ways is some alteration of the primary phases. Field ckqracteristics Glass invariably is devitrified, and as yet no fresh olivine has been reported (except in (a) Olivine-bearing mafic komadites. These cumulate zone of thickn layered mafic-ultra- lavas have textures and volcanic structures very mafic flows). Of the pyroxenes, augite most similil to ultramafic komatiites (Fig. 2a). commonly escapes alteration; pigeonite and Olivine spinifex texture overlying olivine cumu- bronzite almost always have been replaced by late has been reported, but seems not to be chlorite or serpentine. Plagioclase usually, but cornmon. More commonly the flows are marssive not always, is albitized or saussuritized. With throughout or pillowed. Polyhedral jointing is highsl metamorphic grade the primary mine- conspicuous and characteristic of this type of ralogy is replaced by assemblages that include flow. antigorite, vario,us chlorites, tremolite, astino- (b) Pyroxene-rich malic komaiifes. Illustrated lite or hornblende, , clinozoisite and in Figure 2b is a c"omplex 11o* ssalaiaing a minerals (Viljoen & Viljoen L969b, variety of volcnnic structures and textures. Some Williams 1972,Banett et al. 1977, Jolly 19741. flows contain all these features: a brecciated flow top, a layer of pyroxene spinifex texture, Textureg a ,massive central zone made up of randomly TVhere present, olivine adopts habi.ts similar oriented pyroxene needles, and a basal pyroxene- r56 THE CANADIAN MINERALOGIST

FIXE rcLYHEDNAL JOIITIiO AT 'LOY IOP FLOWrcP BRECAA

PYROXEiESPIXIFEX TEXTURE

FrxE, RAt{oorlt -ontExTEo @ARsER POLYNEORAL PYROXEI{EXEEOLB JOIXTIilO Iil FLO| CEXTER

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A. OLIVINE.RICH MAFIC KOMATIITE B. PYROXENE-RICHMAFIC KOMATIITE

PILLOWEOBAgALT I|-' L" UASgIVEBASALI

C. PLAGIOCLASE-BEARINGMAFIC KOMATIITE Flo. 2. Diagrammatic representationsof common types of mafic koma- tiite lava flows. Textures are exaggeratedfor clarity. or olivine-rich cumulate layer. Most flows, are medium grained even in relatively thin however, contain only some of tlese features. flows. Pillows, usually massive, but in some cases Some composite lava flows 2 to 5 m thick thin o411 rims of pyroxene spinifex texture, are in tle Cape Smith fold belt have upper sections not _uncommon. ,Polyhedral jointing once again with basaltic mineralogy and textures, and lower is characteristic. sections enriched in mafic.minerals. The differen- (c) Plagioclqse komatiites. The least mafic tiation in these flows resembles but is not so ko'matiites, which are rish in plagioclase and pronounced as in the much thicker, conspi- contain modal quartz, only rarely have unusual cuously layered flows desc.ribed below. volcanic structures or textures (Fig. 2c). They are usually massive or columnar-jointed with brecciated or ropy flow tqls, or they are pil- LeyeRBo Merrc-Ur,rnervrerrc Flows lowed. Only in rare examples do they contain thin bands of pyroxene spinifex texiure. The In some areas, thick lava flows with peridotitic chilled margins of flows and pillows contain fine lower portions and gabbroic upper portions are s!919tal pyroxene needles and sparse skeletal interlayered with normal komatiitic flows. Ar- o]ivine microphenocrysts in a glassy matrix. chean layered flows in Munro Township (Arndt centres are,massive with subophitic textures, and 1977, Arndt et al. t977) and in the Belingwe FIELD CHARACTERISTICS OF KOIVTATIITES 157 greenstone .belt, Rhodesia (Nisbet et al. 1977) contain only pyroxene spinifex texture. Gab- have been described in detail and will be dis- broic sections are medium grained, ryith tex- cussed here only briefly. They will be"com- tures like those of intrusive sills. Pyroxene pared with recently recognized thick, layered cumulate layers, which contain augite or flotrs from the Proterozoic Cape Smith fold bronzite or both, are relatively thin. They are belt. underlain by a much thicker interval of olivine Layered flows are sharacterized by pro- cumulate. A layer of pyroxene-rich equigrauular nounced differentiation, which has produced medium-grained rock forms the basal several truly ultramafic cumulate layers, and quartz- metres of each flow. normative gabbroic upper sections. They also are The Proterozoic layered flows share with distinguished by their unusually great thick- their Archean counterparts their gabbroic upper nesses: Fred's flow in Munro Township is about sections, the thin cumulate pyroxene layer, the 120 m thick; one flow in the Belingv*e belt is thicker layer of olivine oumulate and the 13 m thiok and another possible flow is 70 m equigranular or pyroxene-rich quench-textured thick; the florvs in the Cape Smith belt range basal layer. However, the upper and lower flow from 30 to at least 8O m thick. Fred's flow contacts are different. The upper third to trvo can be traced for several kilometres along strike thirds of the gabbroic layer is fine-grained and with little diminution in thickness. Some of the columnar-jointed (as are patrs of the olivine flows in the Cape Smith belt are similarly per- cumulate layer). In some sases a thin breccia sistent along strike, but others are roughly lenti- forms the top of the flow; in others the upper cular in cross-section and appear to be the several metres are pillowed. A layer of pillows, result of ponding on the underlying topography, 11 m thick, occurs at the base of many flows. or of differentiation in situ of large lava sacks Thick, layered flows are believed to have or fubes analogous to, but much larger than, formed by differentiation following eruption of typical pillows. magnesian komatiite lavas with t2 b 2AVo The principal features of the Archean layered MgO. The olivine cumulate layers form by flows are illustrated in Figure 3a. Underlying a grivitative settling of olivine phenocrysts and flow-top breccia is a spinifex-textured layer. of olivine that crystallized following eruption. In Fred's flow, olivine spinifex oyerlies 'string Pyroxene and accumulation beef pyroxene spinifex. The Belingwe flows follow. In the Archean flows the spinifex-tex-

FLOW TOP BRECCIA OLIVINE SPINIFEXTEXTURE TOP PYNOXENESPINIFEX TEXTURE PILLOUED FLOW

COLUII{AR. JOINTED BASALT

GABARO

E F PYROXENECUIULATE 3 IYP,GF-o-e SHARPCOXIACT PYROXENECUUULATE I "o %g,"gu" oooo ooo o F A o u o o o o -oo^o o ooo o o o^o o o ao o ^- ov o OLIVINE CUIIULATE oo o oo ooo o o o OLIVINE CUIULATE OO ooo oo o o oo o o IOO ^ o O^ o o o ooo oooo oooo o o ooo ooo ,.@!^o- BASAL no a-l adt a' .-{"'= PILLOI'ED BASE ;et€@ BASAL PYROXEI{ITE

A. ARCHEAN LAYEREO FLOTV B. PROTEROZOICLAYERED FLOW FrG. 3. Diagrammatic ropresentationof layered ulhamafio-mafic lava flows. Textures are exaggerated for clarity. 158 THB CANADIAN MINERALOGIST tured layers form by $owth of mafic minslals Large dunitic intrusions downward from the of flow-top breccia. In the Proterozoic flows, the tops become Much larger intrusions with dunitic cores and pillowed, or solidify to give a fine-grained rock peridotitic margins have been reported in Ar- in which columns later develop. Finally, in chean areas in Canada and Australia. What is both types, the residual, fractionated liquid interpreted as a single intrusion, now broken solidifies to produce gabbro. by fold;ng and faulting into a number of sepa- rate lenses, extends from Wiluna to Mt. Clif- ford in Western Australia, a dhtance of several hundred kilometres (Naldrett & Turner 1977, KoMATtrTrc INrnustoxs Barrett et al. 1.977).Individual lenseshave thick- nesses ranging from 300 to 15OO m. Other Certain types of in areas of Australian intrusions (in the Forrestania area) komatiitic lavas have mineralogical, chemical and Canadian intrusions (the Dumont com- and certain textural features similar to those of pler; in Qu6bec) are somewhat smaller, but the lavas, and are believed to be genetically of the same magrritude. The intrusions are related. There are several types, which have concordant on a regional scale, but locally been divided somewhat arbitrarily for the pur- discordant. poses of description into three groups: small Individual lenses have dunitic cores com- ultramafic dykes and sills, large dunitic intru- posed entirely of coarse (3 mm), interlocking sions, and layered peridotite-gabbro sills. In- grains of olivine (Foer-sr) (usually serpentinized trusions composed solely of gabbro of koma- or replaced by talc and magnesite), minor tiitic affinity probably exist, but have not yet chromite and, in some locations, interstitial been described. nickel-iron sulfides. Margins of the lenses are peridotitic grains Small ultramafic dykes and sills and contain anhedral of augite and orthopyroxene (altered to tremolite, Concordant and discordant ultramafic in- serpentine and talc) in addition to olivine. .mafis trusions invade and ultramafic komatiite Naldrett & Turner (1977) interpreted a crude lava flows in sevenal locations (Barberton cyclic variation in MgO, CaO, AlaOs, Ni and Mountain Land: Viljoen & Viljoen 1970, Cu in the marginal to be a result of Williams & Furnell 1979; Munro Township: sequential influxes of magma into the intrusion. Arndt et al. 1,977; Belingwe greenstone belt: The dunitic intrusions are economically im- Nisbet et aI. 1977). The examples described portant because tley contain very large de- range in thickness from less than a metre to posits of low-grade disseminated nickel sul- 20 or 30 m, and some have been traced along fides (e,g., Mt. Keith, Western Australia; strike for up to 250 m, in many cases to the Dumont, or rich deposits of massive limits Qu6bec), of outcrop. sulfides (Perswerance and the deposits of The ultramafic sills and dykes have rnine- the Forrestania areao Western Australia). ralogy similar to ultramafic lava flows: olivine grains are set within a matrix of augite and Layered mafic-ultramafic intrusions devitrified glass. Usually the olivine griins are Many of the layered peridotite-gabbro in- concentrated towards the centre, leaving the trusions that are commonly found in areas of rnargins of the in'trusion relatively rich in pyrox- komatiitic are unrelated to the koma- ene. Textures. in the intrusions resemble those tiite lavas but have magmatic affinities with of ultramafic lavas, except that textures resul- iron-rich tholeiitic volcanic rocks. Examples of ting from extremely rapid cooling are absent. this tytre of are described' by Naldrett & Whereas pyroxene in lavas is fine grained and Mason (1968), MacRae (1969), Williams & acicular, pyroxene in the intrusions tends to Hallberg (1973) and Viljoen & Viljoen (1970). be coarse and to poikilitically enslose olivine Other layered sills are comagmatic with the grains, which also are coarser (O.5 rnm to 1 komatiites. The tryo types are best distinguished cm) than in lavas. In several dvkes in Munro by chernical criteria (the tholeiitic intrusions are Township, bands of pyroxene spinitex texture richer in Fe and Ti: Arndt et al. 1977) but (10-50 cm wide) occur along both margins, the komatiitic sills may also be identified by 1O-20 cm from the contacts with adiacent lavas. petrological characteristics shared with the Small intrusions of this type may represent komatiitis layered flows described previously. feeders to the ultramafic lava flows, or they The komatiitic sills have very thin layers of may be incidental intrusions into the lava pile. pyroxene cumulate and a preponderance of FIELD CHARACTERISTICS OF KOIVIATIITBS 159 olivine cumulate. Estimated initial maema com- Both the Archean and Froterozoic layered position are highly mafic, with MgO contents sills probably form by essentially in situ differ- between 13 and 259o. Tke tholeiitic ,sills have entiatlon of highly basic komatiitic . thick layers of pyroxene cumulate, and their In the case of the Cape Smith sills, however, high ps contents are reflected in an abundance the presence of isolated wholly gabbroic or of iron oxides, features that they share with largely ultramafic sills and the erosional con- tholeiitic layered flows (Arndt et al. 1977, tacts between gabbro and ultramafic rocks in Arndt 1977). layered sills suggest continued movement of Layered intrusions probably of komatiitic af- ma,gmas during and following eruption. finity have been identified in Proterozoic areas. The Jimberlana dyke in Western Australia, dated at 2.4 Ga, has similar petrological fea- DrscussroN tures aud compositions to the Archean layered sills (Camfbell L977\. Diagnostic of komaiites Laterally extensive layered sills with gab- features broic upper sections and cumulate lower In the original definition (Viljoen & Viljoen sections are a conspicuous feature of the mafic L969a) and in subsequent modifications (e.9. igneous in the Cape Smith fold belt. Brooks & Hart L974, Arndt et al. 1977), koma- Above a fine-grained, chilled sontact zone is tiites have been considered to form a series a 1G-20 m thiok transitional zone with am- of rocks with compositions ranging from peri- phibolitized pyroxenite at the base grading up dotite to basalt or andesite. The implication into pyroxene-rich peridotite and then passing has been that these roeks represent magmas gradually into the overlying main peridotite. with a similar range in compositions. Of these The latter, typically colu'mnar jointed, is com- magrnas, only those with ultrabasic compo- monly 3G-40 m thick, and consists of euhedral sitions, and the rocks formed from them, are olivine (80Vo) and oikocrystic pyroxene (20%). unique; the less mafic rocks have some unuzual A relatively thin (10 m) cumulate pyroxenite characteristics but do not differ sufficiently layer or a zone of rhythmic layering, with alter- from other types of picrite or basalt to justify nating pyroxene-rich and olivine-rich bands, separate names. Definition of komatiites there- overlies the main peridotite zone. ap- fore relies heavily on identification of rocks pears immediately above this unit and the formed by crystallization of ultrabasic liqaids. rock grades rapidly into pyroxene gabbro. In The characteristic features of 'these rock$ then some casesthe contact between gabbro and the form a foundation of any set of diagnostic underlying ultramafic rocks is sharp, and an criteria. Other rocks that are said to constititute erosional origin is suggested by truncation of part of the series owe their status to features, the rhYthmiq laYers by tle gabbro. either morphological, textural, mineralogical The overlying gabbroic complex consists of or chemical, that they share directly or by extra- three units. The lorrermost is a relatively mas- polation with the rocks derived from ultrabasic sive pyroxene gabbro which contains olivine liquids. in its lower portion. This is succeeded by a To identify the rocks formed frorn ultrabasic crudely layered gabbro unit characterized by liquids, ,both textural and conpositional criteria fluctuating solor index and schlieren enriched must be considered. Olivine spinilex textures, in blue opalescent quartz, leucoxene and . skeletal habits of mtneral grains, and an abund- Finally, a nu'mber of sills are capped by a dark ance ol g/a,rs usually are sufficient to ,indicate grey quartz . that a rock is derived from a silicate liquid The fine-grained basal chilled margins of with little accumulation of mafic minerals. some of sills indicate a komatiitic initial liquid Mineral composition (high Mg/Fe in mafic running l2-L6 wt. %oMgO. Although the lower minerals, high CrlAl in chromite, etc.), high are also komatiitic in composition, the rnodal abundances ol mafic minerals, and ultr uppermost gabbros and are enriched in basic whole-rock compositions may then be iron and titanium, indicating possible tholeiitic used to show that the liqui

Land, South Africa. Geol. Soc. S. Alr. Trans. Dnnorn, E, BA&ecaR, W.R.A., BrncenoN, R. & 75, 225-255. Jecrsox, c.D. (1970): The filling of tle Cir- (1976): Geologioal and geochemical investi- cum-Ungava geosyncline. ln Precambriatr Basins gations of the Roodekrans ultramafic complex and Geosynclines of the Canadian Shield (A.E. and surrounding Archaean volcanic rocks, Kru- Baer, ed.), Geol. Surv. Can. Pap, 7040, 45- gersdorp District. Inform. Circ. Econ. Geol. Res. 142. anil, Univ. Witwatersrand L03. DoNALDSoN,C.H. (1976): An experimental investi- ARNDT, N.T. (1975): Ultramafic Rocks of Munro gation of olivine morphology. Contr. Mineral. Towwhip and their Volcanic Setting, Ph.D. thesis, PetrologX 57,187-2L3. Univ. Toronto. EcsrnaNo, O.R. (1975): The Dumont serpenfinite: (1977) : Thick, layered peridotite-gabbro a model for control of nickeliferous qpaque lava flows in Munro Township. Can, I. mineral assemblages by alteration reactions in Sci. L4,2620-2637. ultramafic rocks. Econ, Geol. 70, 183-201. & Bnoors, C. (1978): Iron-rich basaltic Ewrns, W.E. & HuosoN, D.R. (1972): An inter- komatiites rn the early Precambrian Vermilion pretive study of a nickel-iron sulfide inter- district: discussion. Can. J, Earth Sci. 15, 856- section, Lunnon Shoot, Kambalda, Western Aus- 857. tralia. Econ. Geol.67, lM5-1092. & Ft-Bsr, M.E. (1979): Stable and meta- Fnncusott, J. & Cunnrs, K.L. (L972): Silicate im- stable pyroxenes in thick, layered komatiite lava miscibility in the ancient "basalts' of the Bar- flows. Amer. Mineral. 64, (in press). berton Mountain Iand, Transvaal. Nature Phys. -, NALDRETT, A.J. & PvKE, D.R. (L977): ,Sci. 235, 86-89. Komatiilic and iron-rich tholeiitic lavas of Munro Fr.rnr, M.E. (1975): Growth habits of clinopy- Township, northeast Odario. l. Petrology 18, roxene, Can. Mineral. 13, 336-341, 3t9-369. & MecRer, N.D. (L975): A spinifex BARrrrEs,R.G., Lswrs, J.D. & Grn, R.D. (D7a): rock from Munro Township, Ontario, Can. l. Archean ultramafic lavas from Mount Clifford. Earth Sci. L2,928-939, W. Aust. Geol. Surv, Ann. Rep. Lg7g, 59-70. FnaNcrs, D.M. & Hn[Es, A.J. (1978): Are there BARRBTT,F.M., BrNNs, R.4., GRovEs, D.I., Mans- komatiite-derived tholeiites in northern Ungava? rory R.J. & McQuneN, K.G. (1977): Structural Geol. Soc. Amer. Program Abstr. 10, 403. history and metamorphic modification of Ar- Gar-r, G.H. (1973): Paleozoic basaltic komatiite chean volcanic-type nickel deposits, Yilgarn and ocean-floor basalts from northeastern New- Block, Western Australia. Econ, Geol, 72, lL95- foundland. Earth Planet. Sci. Lett. 18' 22-28. 1223. G6r,ruas, L. & Bnoors, C. (1974): Archean quench- Brcrc-r, M.J., Menrnr, A. & Nrsnnr, E.G. (1975): texture tholeiites. Can. I. Earth Sci, LL, 32+ Basaltic and peridotitic komatiites and stroma- 340. tolites above a basal in the Beling- La..rorr, J. & Bnoors, C. (1977): Ttre we gre€nstone belt, Rhodesia. Earth Planet. Sci. origin antl significance of Archean ultramafic Lett. 27, 155-762, volcaniclastics from Spinifex Ridge, La Motte Township, /z Volcanic Regimes in Can- Bnws, R.A., GnNTHoRpq RJ. & GRovEs, D.I. Quebec. (W.R.A. Baragar, L.C. Coleman & J.M. (L976): Metamorphic patterns and development ada Geol. Assoc. Can. Spec. Pap. 18, of greenstone belts in the eastern Yilgarn Block, Hall, eds.), 'Westero 297-309. Australia. In T\e Early History of the I. TtrsRol.{, A. (1975): The Archedn Earth (B.F. Windley, ed.), John Wiley & Sons, Gervrurs, & New York. between Coolgardie and Norseman-stratigraphy and mineralization, In of BRooKs, (1972): C. & Henr,,S.R. An extrusive Australia and Papua New Guinea. 1. Metals basaltic komatiite from a Canadian Archean (c.L. Knieht, ed.), AustraL Inst. Mining Met. metavolcanic belt. Can. l. Earth Sci. 9, 1250- Mon. 5, 66-74. 7253. GnrnN, J.C. & Sqnn-2, l<1. (1977): Iron-rich & _ (1974): On the significance of basaltic komatiites in the early Precambrian Ver- komatiite. Geol. 2, 107-110. milion district, Minnesota. Can. J. Earth Sci. CAraronr.r, lJJ. (1977): A study of macro-rhytlmic l4,2l8l-2t92. layering and cumulate processes in the Jimberlana Herr, R.S. (1978): A Study ol Komatiitic Flows intrusion, Western Australia. tr. The Upper Laye- in the Henik Group, District ol Keewatin, red Series. I. Petrology 18, 183-215. Northwest Territories. B.Sc, tlesis, Lakehead GAYKA, V.M. (L977): Proterozoic komatiite of Univ., Thunder Bay, Ont. the Central Sahara. Dokl. Acad. Jci. U,S,SR, IMnss, L. (1974): L'utilisation des coul€es ultra- Earth Sci. Sect.23l, 155-157. basiques dans la recherche minidre: esquisse ClrRrsflr, D. (1975): Scotia nickel sulphide de- structurale et lithrctratigraphique de La Motte psit. /z Economic Geology of Australia and (Abitibi-Est, Qu6bec, Canada). Bull. Yolc. 38, Papua New Guinea. 1. Metals (C.L. Knieht, 291-3t4. ed.), Aastrol. Inst. Mining Met. Mon. 5, lZL- (1978): Photographic album of submarino 125. Archean meta-ultramafic flows in the La Motts- 162 THB CANADIAN MINERALOGIST

Vassantclt. Qu4becMinistdre Rich. Nat. Rep. & TunNrn, A.R. (1972): The geology and v-6. petrogenesis of a greenstone belt and related JEI{sE}I,L.S. (1976): A new cation plot for clas- nictel sulphide minsl6lization at yakabindie, sifytng zubalkdine volcanic rocks. Ontafia Div. .Western Austrata. Precambrian Res, 5, 43.-103. Mines, Misc. Pap. 66. Neevr, S.M. (1976): Physico-chemical conditions foLL& W.T. (1974): Regional metamorphic zona- during the Archean as indicated by Dharwar geo- tion as an aid in study of Archean terranes, chemistry. In T:he Early History of the Earth Abitibi region, Ontario. Can. Mineral. LZ, 499- (8.F. Windley, ed.), John Wiley & Sons, New 508. York. (1975): Subdivision of Archean lavas NEsnnr, R.W. (1971): Skeletal crystal forms in tle of the Abitibi area, Canada, from Fe-Mg- ultramafic rocks of the Yilgarn Block, Western Ni{r relations. Earth Planet, Sci. I*tt, 27, Australia; evidence for an Archean ultramafic 2no-210. liqurd. Geol. Soc. Aust. Spec. Publ. g, 331-348. I(RarscaMel, U. & Kr,nrscnMAR,D. (1975): Geol------& SuN, $rBN-Su (1976): ogy and ultramafic flows of the Malartic Group, of Archaean spinifex-textured peridotites and N.W. Quebec.Geol. Soc. Amer. Program Abstr. magnesian and low-magnesian tholeiites. Earth ?, 901. Planet. Sci. Lett, 8L, 433-453. Le.rore, f. & Gfr,nvls, L. (1978): Emplacement -, & Purus, A.C. (1979): Koma- of Archean peridotitic komatiites in La Motte tiites: geochemistry and genesis. Can. Mineral. Township, Quebec. Can. t. Earth Sci. 15, 672- L7, 765-786. 677. NrsBET, E.G., Brcrr,r, MJ. & MAnm.l, A. (1977): LEwrs, J.D. & IVnruus, I.R. (1973): The p+ The mafic and ultramafic lavas of the Belingwe trology of an ultramafic lava near Murphy Well, greenstone belt, Rhodesia. t. Petrologl 18, 521- Eastern Goldfields, Western Australia. W, Aust. 566. Geol, Surv. Ann. Rep. 1972, 60-68. OLrw& R.L., Nnsntrr, R.W., Hlusrs, D.M. & MlcRer, N.D. (1969): Ultramafic intrusions of FRANZEN, N. (1972): Metamorphic olivine in the Abitibi area, Ontario Can. l. Earth Sci. 6, ultramafic rocks from Western Australia. Contr. 281-303. Mineral. Petrology !6, 335-342. per- Manrnt, t.E & Ar.lcruncn, P.D. (1975): Pvrr, D.R. (1970): Geology of Langmuir and s€verance nickel deposit Agnew. /n Economic Blackstock Townships. Ontario Dep. Mines Geol. Geology of Australia and Papua New Guinea. l. Rep. &5, Metals, (C.L. Knight, ed.), Austral. Inst, Mining (1975): On the relationship of gold mine- Met. Mon.5, 149-155. ralization and ultramafic volcanic rocks in the McCarr,, G.J.H. (1971): Some ultrabasic and Timmins area. Ontario Div. Mines Misc. Pap. basic igneousrock occurrencesin the Archean of 62. Western Australia. Geol. Soc, Aust. Spec. Publ. --, NALDRETT,A.J. & EcrsrnaNp, O.R. (1973): 3, 429-U2. Archean ultramafic flows in Munro Township, & Lusnnrml, I. (L97L): Cluesto the origin Ontario. Geol. Soc. Amer. Bull. 84, 955-918, of Archean peridotitic komatiites in La Motte Ross, J.R. & Horrms, G.M.F. (1975): Kambalda uature of serpentinisation.Geol. Soc.Aust. Spec. nickel sulfide deposits. /n Economic Geology of Publ, 3, 281-299. Australia and Papua New Guinea. 1. Metals McGnrcon, V.R. & Mlsox, B. (L977): Petro- (C,L. Knight, ed.), Ausffal. Inst. Mining Met. genesis and geochemistry of metabasaltic and Mon. 5, 100-121. metasedimentaryenclaves in the Amitsoq , Scneu, M. (1977): "Komatiites" and quartzites West Greenland,.Amer. Mineral. SZ, 8t7-9O4. in the Archean Prince Albert Group. In Yol- Mclvrn, J.R. (1975): Aspectsof some high mag- canic Regimes in Canada (W.R.A. Baragar, L.C. nesia eruptives in southern Africa. Contr. Mine- Coleman & J.M. Hall, eds,), Geol. Assoc. Can. ral.-Petrology 51, 99-118. Spec. Publ. L6, 341-354. Mooxr, J.M., Jn. (1977): Orogenic volcanism Scnwanz, E.J. & Fu.rrwnr,.l, Y. (1977). Komatiitic in the Proteroznic of Canada. In Volcanic Re- basalts from the Proterozoic Cape Smith range gimes in Canada (W.R-A. Baragar, L.C. Cote- in northern Qugbec, Canada. /z Volcanic Re- gran & J.M. Hall, eds.),Geol, Assoc, Can. Spec. gimes in Canada (W.R*A. Baragar, L.C. Cole- Pap.18,t27-748. man & I.M. Hall, eds.), Geol. Assoc. Can. Spec. MureNrN, T. (L976): Komatiites and komatiite Pap.16, 193-201. provincesin Finland. Geologi 28, 49-56, SEeRr.E, D.L. & Vorss, F.M. (1959): Layered NeloRErr, A.J. & GooowrN, A.M. (1977): Vol- ultrabasic lavas from Cyprus. Geol. Mag. t0$, canic rocks of the Blake River Group, Abitibi 515-530. greenstone belt" Ontario, and their sulfur con- Sretvrersr.opour.ou-SEyMouR, K. & FhANcrs, D.IVI. tenL Can. t. Earth Sci. I4, 539-550. (1979): An ultramafic turbidite, James Bay, & Ma,soN,G.D. (1968): ContrastingAr. Quebec. Geol. Assoc. Can., MineruI. Assoc. Can. chean ultramafic igneous bodies in Dundonald Program Abstr. 4,80. and Clergue Townships, Ontario. Can. I. Earth Susr.ove, S.N. (1977): Komatiite rn Lower Pre- Sci.5, lt7-143. cambrian metavolcanic units of the Kola Penin- FIBLD CIIARACTERITITICS OF KOMATIITES t63

sula. Do&L Acad. Sci. USSR, Earth Sci. Sect, ogy and geochemistryof the layered ultramafic 228, t62-165. bodies of the Kaapmuiden areao Barbertoo T!,Avrs, G.A. (1975): Mount Clifford nickel de- Mountain l-and. In Symposium on the Bushveld posit. /n Economic Geology of Australia and Igneous Complex and otler Layered Intrusions Papua New Guinea. 1. Metals (C.L. Knight, (DJ.L. Visser & G. von Gruenewaldt,eds.), ed,), Austral. Inst. Mining. Met. Mon. 5, 14+ Geol. Soc, S. Afr, Spec.Publ.1, 661-688. t46. VtsweNArtreN, S. (1974): Basaltic komatiite os- Upaurrev, H-D. (1976): Komatiites from Betts crurencesin the Kolar gol<1field of India? GeoI. Cove, Ne$rfoundland.Geol. Soc. Amer. Program Mag. LIj., 353-354. Abstr.8, tt50. Wnr.rlMs, D"{.C. (1972)t Archaean ultranafig VTLJoEN,M.J., BrnNascoM, A., vAN CoLLER,N.' mafic and associatedrocks, Mt. Monger, West- Knwocx, E. & VruoxN, R.P. (1976): The ern Australia. t. GeoL Soc. Aust. 19' 163-188. geology of the Shanganinickel deposit, Rhodesia & FunNsr.L,R.G. (1979): Reassessmentof Econ. Geol.7l, 76-95, part of the Barberlon ty[,e area, South Africa. & Vn-.rorr.r,R.P. (1969a) The geotogyand PrecambrianRes. 8 (in Press). geochemistryof the lower ultramafic unit of the & Har-r.uraa,JA. (t973): Archean layered Onverwacht Group and a proposed new class intrusions of the Eastern Goldfields region, West' of igneous rocks. fz Upper Proiect enr Australia. Contr. Mineral. Petrology 98, Geol. Soc.S. Alr. Spec.Publ.2' 55-85. 45-70. & - (1969b): Evidencefor the exis- Wnsory H.D.B., KILBURN,L.C., Gtaunvr, A.R. & tence of a mobile extrusive peridotitic magna Rerwlr., K. (1969): Geochemistry of some from the Komati Formation of the Onverwacht Canadian nickeliferous ultrabasic intrusions. Ia Group. In Upper Mantle Project" Geol, Soc. Magmadc Ore Deposits (H.D.B. Wilson, ed.), S. Afr. Spec.Pabl. 2,87-tt2, Econ, Geol. Mon. 4,294-309. WooDALL,R. & Tnevrs, G3'. (1969): The Kam- geoohemicalsignificance of the upper formations balda nickel deposits, Western Australia. Proc. of tle Onverwacht Group. In Upper lVIantle gth Commonw.Mining Met. Congr. 2, 5t7-533. Project, Geol. Soc.S. Afr. Spec. Publ.z, tt3' 151. ReceivedAugast 1978, revised manuscript accepted Vnropr.r, R.P. & Vu,.rorx, MJ. (1970): The geol- December 1978.