Conodion Mineralogist Yol.22, pp. 16l-172(1984)

GENESISOF CHROMITITEFROM THE MITCHELLRANGE, CENTRALBRITISH COLUMBIA

PETERJ. WHITTAKER* ANDDAVID H. WATKINSON Depafiment of Geology, Corleton University, Ottawa, Ontario KIS 5/;6

ABSTRACT INTRoDUcTIoN

Allochthonous serpentinized peridotite in the Mitchell Chromite occrurences define a linear, north- Range, British Columbia, contains numerous ocqurences noftheast-trending band in allochttronous ultrama- of chromitite in which the aluminian chromite usually con- fic rocks of the Mitchell Range, 240 km northwest tains more than 5590 Cr2O3.Two groups of podiform of Prince George, British Columbia and 160 km chromitite havebeen recognized, In one group, the chromite northwest of Fort St. James(Fig. l). Allochthonous is similar in compilsition to that in layered, net- and primarily occluded-silicate-texturedchromitite; such rocks are inter- rocks of the Mitchell Range are harzbur- preted to have equilibrated with magma during initial par- gite tectonite with minor dunite, gabbro and chro- tial melting. This group originated by deformation of mitite (Whittaker 1982).Obduction in Late Triassic chromitite layers and segregations,first in the upper man- time @aterson 1971, Monger & Price ln9) brought tle and then during obduction. The secondtype of @iform rocks of the upper mantle into contact with Late chromitite occurs as massivepods without relation to pre- Pennsylvanian to Middle Permian carbonate rocks existing layers, Inclusions in chromite consist of coexisting of the lower Cache Creek Group (Armstrongl949). platinum-group minerals and Fe-Ni sulfide phases. Par- The southeast-trendingStuart Lake Belt of the Cache gasitic amphibole with abundant primary and secondary Creek Group is openly folded, with north-trending fluid-inclusions occursin the chromite grains, thls support- possibility fold axes that plunge 10 to 30o northward. This ing the that a sodium-bearing fluid phase was - present during chromite precipitation. reflects east west compression related to an east- dipping, Late Jurassic to Early Cretaceoussubduc- Keywords: Chromitite, peridotite, podiform, genesis,Mit- tion zone (Monger & Price 1979). chell Range, British Columbia. Texfuresand structurc of chromile concenlrations similar to those observedin the Mitchell Renge are SoN{raerns also seenin smaller obducted slivers of serpentinite in rocks of the CacheCreek Group at Murray Ridge, La pdridotite serpentinis6eallochtone du chalnon Mit- near Fort St. James, at Scottie Creek, 30 km north chell, en Colombie-britannique, contient plusieurs venues of Cache Creek, and in the Cameo Lake (Nicola) de chromitite d chromite aluminguse, contenant g6n6rale- deposit, 60 km northwest of Kelowna (Whittaker & ment plus de 5590 de Cr2O3. Ou y reconnait deux grou- Watkinson l98l). In the Osoyoosarea, two chromite pes de chromitite podiforme. Dans un de cesgroupes, la deposits occur in Anarchist Series rocks, possibly chromite ressembleen composition i celle des chromitites time-equivalent to the CacheCreek Group (Monger rubandes,d texture r6ticulaire ou i silicate interstitiel. On & Price 1979). These are the Anarchist deposit, interprdte cesroches comme r6sultant d'un 6quilibre avec Ie magma pendant la fusion partielle initiale. Ce groupe enclosedby recrystallizedmicritic limestone, and the resulterait de la ddformation des couches et des s6exfua- Bridon deposit, enclosedby serpentinite(James 1958, tions de chromitite, d'abord dans le manteau sup€rieur et Whittaker & Watkinson l98l). ensuiteen cours d'obduction. Le secondtype de chromi- The purpose of this paper is to present field, tite podiforme sepr€sente sous forme de lentilles massives petrographicand chemicalcharacteristics of chromite sansrelation avec les couchespr6existantes. Les inclusions occurrences in ultramafic rocks of the MitcheU dans la chromite consistenten min€raux du groupe du pla- Range. This locality is presented as a type area phases tine et de sulfures Fe-Ni. L'amphibole pargasitique, becauseofthe abundanceand variety ofthe concen- qui se prdsente dans les grains de chromite avec d'abon- dantes inclusions fluides primaires et secondaires, 6taye trations of chromite in these allochthonous l'hypothbsede la pr6senced'une phasefluide sodiqueau ultramafic rocks. cours de la prdcipitation de la chromite. (Traduit par la Rffaction) Dlsrnrsurloll oF CHRoMITEOccunnnNcss Mots-cbs: chromitite, p€ridotite, podiforme, gendse,chal- non Mitchell, Colombie-britannique. Penetrative mylonitic foliation throughout harz- burgzite of the Mitchell Rarrgehas a north-north- EPresent address: Depaftment of Geology, Univer- westedytrend, O,picalof the fabric of rhis ultramafic sity of Regina, Regina, SaskatchewanS4S 0A2. massif. Chromite occrurencesare concentratedin a t6r THE CANADIAN MINERALOGIST

Mltchell Range _ -- Jrllranatlc Allochth ra(ra L.ll x, \ir\ N \ t "r"*, bFt. Sl. Jams

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s*- tf' sf b I .o '\' o .o o it I 6/"F/ r'"-'>' 6l ,+* n Q L r' /\ -L-^{ \**' ,' a I I ,:l -\ a1'o^ .l/ _€/ l!, n ,/g\< a/ E/ )_aaaa al q -\/o- n{y\ .(' s/<,' "8/ \r \ /l. ^.Q Qo/ d/ f s"/ \ otp *.pY dio' s*/ P. olo t/. s-,3 /( -i:\ x" e. / o'lo' o5't6' 55 16. / | .P "".A''*- )'Xl I aa '' E

)%to-/ a_-l AH ,\\\/fh Ir,"T-j \(\ I /\ \;- \-( A X," y--l i I LEGEND x'' P:"^nt-t L fq{,n:+!MTTCXELLNAiOE ULTNAilAFIC ALLOCHTHON p6rnl6n

(g _ Croho Crst &oup: carbonata,lanl@tod ch€rl 6lltal@. I" -----l-^ F- 1) r-----l ahaly slttaa@. x,. fr Y F- r_l Gebbo IT TI II]TTM]]hdto: tolatod. \>..-- |llilllill Cachs Creok Group fiarzbGgltg: larlaled itn pre - omplacement gabbro lF= and edtlio - todlnglto alrkea, roaklt lollalsd rllh Fin rt cuhuLlo l6rh6. SCALE TrTI Stromly tonatod @h'lto eller hanbaglle, ,,{li+ lPc I o o,E i.o t.o { '$:i';:!Y ldlonlc brecla. S lo 20. blcka. l+r "r F-.l \ ' o o.g t.o lr. Chde occ6ren6a: nodllea bygra. txl dd

Gsl.oglcal oontact obsered, assumed.

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Ftc. l. Geology of the Mitchell Rangeultramafic allochthon. GENESIS OF CHROMITITE, CENTRAL BRITISH COLUMBIA r63 horizon that is conformableto this trend (Fig. l). process(Cassard et at. l98l) or representa primary The distribution of chromite in this horizon may magmaticchromitiferous horizon in harzburgiteof result from a tectonic concentration or orientation the upper mantle. Chromite occursprimarily in harz-

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FIc. 2' Photographsof chromite textures. A. Disseminatedchromite. B. Disseminatedchromite forming occluded-silicate texture in a chromitite layer; black dots within chromite grains are primary silicate inclusions. C. Nodular chromitite from a 0.75-m-thicklayer. D. Massivechromitite from a pod; chainsof black dots representsilicate inclusions along annealedfractures. Photos A, B and D are prints directly from polishedthin sections,in which chromite appears white and silicatesblack. t64 THE CANADIAN MINERALOGIST burgite; somehorizons are sheathedby a l-ro-3-cm- New Caledonia (Cassardet al. l98L), where dunite thick mantle of fine-graineddunite. The absenceof sheathstue common. a dunite mantle around most chromitite occurences Gabbroic dykes in harzburgite tectonile predate distinguishes the Mitchell range from other all deformation. Both the layered and podiform ophiolites, suchas at Troodos (GreenbaumL977) and chromitite occur in the gabbro-harzburgite zonethat

Frc. 3. Photographs of chromitite in outcrop. A. Planar aggregateto massive chromitite layer with thinner parallel chromitite seams,B. Chromitite layer with rounded pull-apart strucnues;pencil parallelstrace of foliation. C. Isoclinally folded chromitite pod with thickenednose of fold; limbs and pencil parallel trace of foliation. D. Compound podiform chromitite; massive core left of pencil, aggregaterim right of pencil. GENESIS OF CHROMITITE, CENTRAL BRITISH COLUMBIA t65 forms the central and southwesternpart of the jacentto chromititepods, is interpretedto be an up- massif. In the area of chromitite oqcurrencesX," per level of the mantle where segregatedpartial melt and X,, (Fig. l), as much as 25t/o of the outcrop moved into tensionalfractures. Gabbroic intrusive area consistsof gabbroic dykes. The area intruded bodiesare sparselydistributed or absentelsewhere by thesepreobduction dykes, some of which are ad- in the ultramafic massif; their absencepossibly in-

Ftc. 4. Photomicrographsof fluid and sulfide inclusions.A. Subparallelplanar swarmsof sphericalto tubular fluid inclusions.B. Larger fluid inclusionswith dark endsand bright cores.-C.Inclusion train of Fe - Ni sulfide filling an annealedfracture in chromite. D. Euhedral primary Fe - Ni sulfide with dark areasof exsolvedNiS. Scalebar in all photos is 25 pm. 166 THE CANADIAN MINERALOGIST dicatesa deeperlevel in obductedmantle, wherethere tures. Layeredand podiform structuresexhibit net' was lesspartial melting. nodular @igs. 2B, C) and massive(Fig. 2D) textures (Thayer 1969,Greenbaum 1977), Cnnourrr, OccunnsNces Disseminatedchromite is an accessoryphase in tec- tonized harzburgite,forming up to 390ofthe rock. Chromite occurrences are classified as Individual grainsare subhedraland very fine grain- disseminated(Fie. 2A), layered and podiform struc- ed. Few layersor podsof disseminatedchromite oc-

Frc. 5. Photomicrographsof platinum-groupinclusions in chromite.Scale bar in all photographsis 25 pm. A. Euhedral laurite, B, C, D. Anhedral to subhedrallaurite. GENESIS OF CHROMITITE. CENTRAL BRITISH COLUMBIA t67 cur; they consist of 35 to 4590 fine- to medium- silicate-texturedand massivechromitite in the harz- grained chromite in a l-to-2-cm dunitic envelope, burgite tectonite are usually fractured. Alteration with the layer or pod itself contained in harzburgite. rims enclosingchromite and zonesalong fractures Layered chromitite consistsof occluded-silicate- consistof ferritchromit produced during postserpen- textured, net-texturedor massivechromitite. The tinization metamorphism (Spangenburg 1943). term sggregate chromitite is used in this study to Disseminated chromite commonly exhibits ferrit- designatethe two former textural types, with more chromit rims, whereasmassive chromitite develops than 75c/odisseminated chromite, whereasmassive ferritchromit along fractures. chromitite has greaterthan 95q0chromite. A hack- Solid and fluid inclusionsoccur in chomite in net- ly fracture is typical of massive chromitite. textured, occluded-silicate-texturedand massive Chromitite layers @gs. 3A', B) range from planar chromitite layersand pods. Fluid inclusionsare rarely undeformedsegments to layersthat havepull-apart observedin accessorydisseminated chromite, and are structures.Layers may occlu singly or as parallel, most commonly observedin chromitite layersand thinly separatedpairs or triplets. Layer contactsare pods that exhibit fine- to medium-grained net and generallysharp and, in the caseof aggregatelayers, occluded-silicatetextures. grain sizeis constant. Layers are not continuous but Fluid inclusions (Figs. 4A, B) are sphericalto are segmentsas long as2 m of initially longer layers tubular and commonly exhibit a thinned or necked separatedby shearing.Deformation has produced appearance.Isolated fluid-inclusionsare lesscom- earlytensional pull-apart structures@g. 3B) and was mon than those occurring in patchesor in swarms followed by brittle deformation, which produced (Whittaker & Watkinson l98l). The latter are planar angular fragments.The most intensefolding of layers and may intersector be subparallel(FiC. 4.{). Ellip- is found at their extremities, where 3-to-5-cm tical and tubular fluid-inclusions in somecases have fragmentsmay occur. Pinch-and-swellstructures are dark endsand a light core @ig. 4B), suggestingthe exhibitedby somelayers, and pull-apart structures presenceof coexisting fluid and vapor. Euhedral isolate individual boudins of chromitite. Whefe negative-crystalcavities also occur and may have boudins were formed before the end of ductile shear- hosted fluid inclusions. ing, they werethemselves folded into pods(Fig. 3C). Unserpentinizedsolid inclusions usually consistof Chromitite pods exhibit the sameoccluded-silicate, subhedralto euhedralolivine and, lesscommonly, or- net and massivetextures as the chromitite layers. thopyroxeneand clinopyroxene.Most silicateinclu- Chromitite pods occur up to I m in maximum sions have been altered to talc or lizardite, which dimension,are usually elliptical and may exhibit a form an interlocking texture (Wicks & Whittaker sigmoidal form. Rounding and reduction in pod size 1977).Pargasitic amphibole with as much as 3.30 haveoccurred in responseto ductileshearing. Many w.9o Na2O occrus as primary inclusions and has deformed pods have associated trains of smooth boundarieswith enclosingchromite. 0.5-to-1.5-cmsubangular fragments occurring up to Inclusionsof Fe - Ni sulfide, confirmed by energy- I m from the parent pod. dispersionanalysis, are most abundant in net- and The range of structural types and the similarity of occluded-silicate-tefiuredchromitite layersand pods. texturesobserved in thesestructures suggest that duc- In massiveand aggregatechromitite, sulfidessuch tile shearingcould form many of the observedpods as pentlandite are secondary, filling existing or an- of chromitefrom initial layers.Deformation of pre- nealedfractures in chromite. This resultsin a train exisling layers of chromitite generated layer ofsubhedral to anhedralgrains ofsulfide (Fig. 4C). segments,some of which remainedplanar, whereas Individual inclusionsof Fe - Ni sulfide unrelatedto others were folded into deformed pods. existing or annealedfractures are rare. They are Compound pods of chromitite exhibit both ag- euhedraland may be primary inclusions(Fie. 4D) gregateand massivetextures. Aggregate chromitite associatedwith a secondexsolved phase, possibly NiS is leastextensive and forms patchesup to 4 cm across or Ni-Fe alloy (Talkington et ql. 1982).Secondary in massive chromitite. Alternately, aggregate sulfides occur in groundmass silicates and with chromititeforms l-to-5-cm rims on chromititepods. silicate inclusionsin chromite. Bordersbetween massive and aggregatechromitite Platinum-group-mineral (PGM) inclusions in are locally sharp (Fie. 3D), although patchesof ag- chromite (Frg. 5A) havebeen identified using energy- gregatechromitite are irregular in shape. dispersionmicroprobe analysisand are similar to thoserecently recognized in other ultramafic suites CHROMTTEPEtnocrApny of ophiolitic affinity @richard et ol. 1981, Talk- inglon et al. 1982).These inclusions are euhedralto Chromite is pale reddishbrown to pale amber in anhedral, occur as isolated grains not related to frac- plane-polarizedlight. It is darker and locally opa- tures and are thought to be primary inclusions. que along fractures and on somegrain boundaries Laurite RuS2is the predominant PGM and, where owing to oxidation of iron. Disseminated,occluded- euhedral,has a white reflectanceand high relief (Fig. 168 THE CANADIAN MINERALOGIST

5A). SubhedralPGM havelower reflectanceand ex- microprobe was operated at 15 kV accelerating hibit slightly irregular borders with chromite (Fig. voltage, with a specimencurrent of 0.05 nA deter- 5B, C, D). mined on a pure iron standard.The counting time Severalsubhedral PGM haveinclusions or an ex- was 40 seconds.Data were reducedby the program solvedor entrapped,highly reflectiveyellowish white of Rucklidge & Gasparrini (1969), and Fd" and sulfide phase.The grains are small, one-tenththe Fe3* werecalculated from the reduceddata assum- host grain-sizeor less,and occur individually or in ing spinel stoichiometry. clusters.The larger pale grey grain oflaurite in Figure The analytical data pertain to ageregate(highly 5D contains several bright inclusions of nickel disseminated), podiform and layered chromite. sulfarsenide. Chromite from various types ofpods and layers,net- textures, occluded-silicate-texturedor massive-tex- CHEMICALCouposttroN oF CHRoMITE tured, showslittle chemicalvariation. Consequently, for the purposesof plotting, the various types of The compositionaldata presentedin Figure 6 are chromite have been grouped as podiform and based on 123 new determinationsby wavelength- layered. dispersionelectron-microprobe analysis; some com- The composition of olivine occurring as inclusions positions are given in Table l. A CambridgeMkV in chromite is similar to that in the groundmass

- o r3 E t: L a oooa o@ oo a -Q aaaa. o a a o- .o. %

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o r.oo fo .co Msrug.rf Frc. 6, Compositionalvariation of chromite. A. Variation of Fel(Fe + Cr + Al) rerszs Mg,z(Mg+ Fe) (atomic), show- ing a high-Fe grouping of podiform chromitite (solid circles) and an overlap of podiform and layered chromitite (open circles).B. Xs, yersusX1,4n. showing constant Xq, for both low- and high-Fe chromitite pods and layers.C. TiO2 versusXy1*, showing a similbr rangeof compositionfor layeredand podiform chromitite. D. Cr - Al - Fe3+ diagram, showing similar compositionalranges for layeredand podiform chromitite. GENESIS OF CHROMITITE, CENTRAL BRITISH COLUMBIA t69

TASUr. AVERA@@tlposrlroils 0F ctR0trlr,.li|lTcHEt-LMXGE gabbro - harzburgitezone of the ultramafic massif. !3:A!!9:7.! The spatialcorrelation between bodies of segregated !x.02 0.1 0.3 0.4 0.2 0.1 0.{ 0.{ 0.4 Nr3 8.2 8.1 I7.8 8.2 L4.2 15.2 15.5 16.5 partial melt and chromite with high levelsof TiO, cr203 62.2 60.7 s.2 53.2 54.5 52.8 it.1 5t.O F€203 2.7 3.5 3,9 5.2 4.7 3.8 4.0 1.2 supports the hypothesis that the central and FS [.9 I].1 U.0 8.6 t2.6 14.3 tS.6 19.5 lJr, 0.6 0.5 0.6 O.2 0.2 0.5 l.O 1.0 southwesternpart of the massif was higherJevel !r@ 1:}.5 12.8 l{.6 10.2 L1.2 l:t.l l0.t s.7 mantle than the remainder of the allochthon. @ ,9.2 99.1 99.5 100.S 100.5 r00.M.0 101.3 - One major trend is apparentin Figure 6D and in- ], 2 Podi fom ch@ttlb f@ glbbrc harzburglb z@e, 3 msslve chrcolil& layor' 5' 6 69gregatr (hlghly dlsseFlnabd) dMlb ]wr, 7, I @3slve. volvesconstant Fe3* with variableAl. This podlfom chMtltt. All c@posltlohs am arerl$s ob&tn€d by the electmn- trend is dcDprch dalysls- of fiw gral6, ex@pt for 3, rhlch Epreaenb !n rvemge of tour g.alns. P. lhlthk€r, drlyst. shown most strongly by layered chromitite. Layered chromitite consistsof fine- to medium-grainedor nodular chromite (Fig. 28, C) with as much as 3090 (Fosr_sr, suggesting equilibration of the two silicate inclusions,most of which are olivine com- minerals.This is in contrastto the data of Johan & positionally similar to that in the groundmass Lebel (1978)and Watkinson & Mainwarine (1980) (Fogo-sr.The trend involving essentiallyconstant Fe taken from other localities, which indicate that and variable Al could result from equilibrium of olivine inclusions are more highly magnesian@or., chromite with trapped interstitial partial melt. A and Fo*.g) than groundmassolivine (Foe0.7);they similar trend involving variation in the Cr,/Al ratio inferred rapid, nonequilibrium precipitation to ex- is also seenat Rhum, where equilibration among plain their data. Chromite precipitation in rocks cumulus chromite, cumulus olivine and a liquid with from the Mitchell Range may have been contem- a plagioclasecomponent was invoked by Henderson poraneouswith that of olivine and sufficiently rapid (r97s). to incorporateolivine crystalsas inclusions. Smooth Individual chromite grains from both podiform contactsbetween olivine inclusionsand chromite and and layeredchromitite show a high degreeof com- the absenceof a reaction rim in chromite adjacent positional homogeneity.Samples from separateloca- to an olivine inclusion support the hypothesisthat tions in layer segmentsand within chromitite pods olivine compositionsare primary. similarly show homogeneity.Four points from a Chemicalcompositions of chromite from the loca- single chromitite layer and five points from a single tions shown in Figure I indicate two groups of grain of chromite range from 50.76to 51.35wt.9o podiform chromitite. One group has chromite of ap- CrtO, and 16.17 to 16.99 wt.slo AlrOr. This proximately the samecompositional range as that restrictedrange in compositionand the lack of zon- in layeredchromitite, whereasthe secondgroup has ing in individual grains of chromite suggest a lower Xr, = Mg/(Mg + Fd*) (Figs.64, B, C, D). equilibrium with a liquid of constantcomposition. For podiform chromitite, Xyu rangesfrom 0.20 to Partial melting of primitive mantle peridotite could 0.70 and for layered chromitite, it is in the range 0.50 achievesuch a buffering effect and has been sug- to 0.70 @gs. 64', B, D). Ferric iron is most variable gestedby Arai (1980),on a regionalscale, to explain in podiform chromirire, 0.02 - 0.13 (Fig. 64,), bur the compositionalhomogeneity of chromite within is restrictedto 0.01 - 0.08 in layered chromitite. single litlologies in alpine ultramafic terranes.In the Thee types of chromitite from the Mitchell Range Mitchell Range,this homogeneityis representedon a exhibit nearly constantCrl(Cr+Al) (X".) ratios, in local scaleamong individual layers and pods. the range0.65 - 0.88 (Fie. 68), and the chromitehas a normal content of Cr for that in ophiolitic rocks; Suvvany oF FEATURESSHowN By the compositionsproject in the aluminian chromite CHRoMITE FROM THE MTTcHSLI- ReNcT field defined by Stevens(1944). This field is shown in Figure 6D as partly filled by the black shadedarea. 1. Chromite occursin a linear NNE-trendingzone Titanium is variable, and clusteringof the data is parallel to the foliation of the harzburgitehost-rocks. apparent on Figure 6C. Approximately one-third of 2. Chromitite occursas layersand pods showing the data points for layered and podiform chromitite aggregate,(net and occludedsilicate), nodular and project in the field defined by Dickey (1975) for massivetextures. chromite from stratiform deposits (where TiO, is 3. Chromitite is hostedregionally by harzburgite, equalto or greaterthan 0.30 wt.Vo).This would im- and dunite locally forms thin selvages. ply that someof the high-iron podiform and layered 4. Chromite exhibits minor alteration to ferrit- chromitite with greater than 0.30 fi.90 TiO, may chromit rims and contains inclusions of silicate. have equilibrated with a gabbroic magma. However, sulfide and fluid. the lower Xr, values expectedto conelate with 5. Silicateinclusions are usually olivine of similar higher TiO, do not occur. ConstantXr" could in- Fo contentto olivine in the groundmass.Sulfide in- dicate Mg metasomatism during serpenlinization. clusionsare Fe, Fe - Ni and PGM varieties. Compositionsof layered and podiform chromitite 6, The chromiteis aluminian, usually with greater that plot in the "stratiform" field are locatedin the than 5590 Cr2O3. 170 THE CANADIAN MINERALOGIST

7. Aggregate chromitite layers have higher Al conditions of unhindered growth as found in a where silicate compositional inclusions are most magma,and might then be trappedby rapidly grow- numerous. ing chromite. In contrast, massivechromitite and 8. Similar compositional fields are occupied by disseminatedaccessory chromite have few inclusions most layered and podiform chromite. of silicate and sulfide. 9. Chromitite pods in the gabbro-harzburgite zone Microprobe data support petrographic interpreta- have higher Fe and Ti, suggestingequilibrium with tions in that two groups of chromite are distin- an early gabbroic partial melt. guished.These are normal and high-Fe-Ti podiform chromite, which would representprecipitation in dif- Drscussrox ferent environments. Podiform and layered- aggregatechromitite sharethe samecompositional Chromite in the Mitchell Rangerepresents a possi- rangesand havea similar Fe content. The abundance ble chromitiferouszone in refractory harzburgiteof of silicateinclusions in this type of chromitite is com- upper-mantleorigin, that has beenelongated by duc- patible with a magmatic origin involving rapid tile shearing. Partial melting of upper-mantle growth of chromite.In contrast,massive chromitite peridotite, to the point whereliquid segregationcould with few inclusionsis richer in Fe and may repre- occur, takes place in the l2-to-25-km depth range sentprimary fine-grainedchromitite pods, possibly (Dickey 1975).This may suggestthat chromitefrom in depressionson a magma-chamberfloor, that have the Mitchell Range formed in this depth interval. undergonesubsolidus recrystallization to a coarser Deformation in the upper mantle and later, dur- grain-size.These would have formed by accumula- ing obduction, has led to the development of tion of chromite from an early, more Fe-rich par- isoclinally folded pods of chromitite, folded layers, tial melt. In the field they correspondto isolatedpods and later brittle fracturing, to form angular of massivechromitite. Two of thesepods (Fig. 6D, fragments of layers and pods. Deformation of solid circles) occur in sharp contact with harzburgite chromititelayers was observed to rangefrom planar and haveno associatedtrains of fragments,in con- undeformed segmentsto isoclinally folded pods trast to other pods formed from boudinagedlayers thought to have originated from the breakup of of chromite(Fig. 6D, solid trianglesand openstars). layers. In thin section, chromite commonly shows Chromite with hieh TiO2 (greater than 0.30 signsof brittle fracturing and older annealedfrac- wt.9o) plots in the field definedby Dickey (1975)as tures, reflected by linear and planar trains and representativeof stratiform environments.Chromite swarmsof inclusions.These annealed fractures may occurrencesfrom the Mitchell Rangethat plot in this be relatedto high-temperaturefracturing, with subse- field are groupedin the central and southwesternpart quent annealingin the upper mantle. A rising diapir of the massif. This is also the area occupiedby as of mantle material containing approximately l0 to much as 2590ofpre-obduction gabbro dykes.This 20 0/opartial melt (Maaloe & Printzlau 1979, Bot- areamight representa highJevelzone in the upper tinga & Alldgre 1978,Malpas 1978)would allow, by mantle in which magmasegregated and crystallized partial melting, the formation of small magma- as dykes. Harzburgite tectonite elsewherein the Mit- chambers in which refractory chromite could chellRange hosts chromite with lessthan 0.30wt.qo accumulate. Ti02, typical of chromite from refractory Textural variation of chromite suggestsdifferent peridotite, and is interpretedto underliethe gabbro environments of chromite growth. Disseminated - harzburgitezone. Movement in the plane of the chromite, that forms net- and occluded-silicate- existingfoliation would result in the observedposi- textured chromitite layers and pods, is commonly tion of the gabbro - harzburgitezoire with respect subhedralto euhedral,with smooth planar edges. to the rest of the massif. This suggestsprecipitation and accumulationfrom The solid silicateinclusions have mantle-type com- a magma.Accessory disseminated chromite in harz- positions. Olivine (Foso- e) is the most common burgite tectonitecould form euhedraby growth vla silicate inclusion; minor orthopyroxene and solid-statediffusion (Leblanc1980). Massive coruse- clinopyroxenealso are found. Pargasiticamphibole grained chromitite may have resulted from with 3.30 wt.Vo Na2Oforms primary subhedralin- recrystallization of pods of very fine-grained clusions in several occurrencesof chromite in chromitite. Recrystallizationin a subsolidusstate has aggregateJayeredchromitite. Pargasiticamphibole led to a massivetexture with serrated,irregularly and other Na-bearinginclusions have been reported shapeddomains of interlocking grainsof chromite. from ultramafic rocks of ocean-crust affinity The distribution of silicateand sulfide inclusions (Watkinson & Mainwaring 1980).The appearance supports this interpretation. Net- and oc- of sodic phases in chromite from alpine-type cluded-silicate-texturedlayers and pods contain the ultramafic rocks requires an extraneoussource. highestproportion of silicatesand sulfide inclusions. Seawater,known to penetratethe oceanfloor and Thesesubhedral to euhedralinclusions would require to disturb oxygen-isotoperatios in upper-mantle GENESIS OF CHROMITITE, CENTRAL BRITISH COLUMBIA 17l rocks, including harzburgite tectonite (Gregory & HeiloBnsou,P. (1975): Reaction trends shown by Taylor l98l), may be sucha source. chrome-spinelsof the Rhum layered intrusion. Net- and occluded-silicate-texturedchromitites Geochirn,Cosmochim. Acta 39, 1035-1044. commonly host linear and planar trains and swarms of fluid inclusions,whereas massive chromitite has Jer'rss,A.R.C. (1958):Belchrome @elair Mining Cor- few fluid inclusions.Compositions of fluid inclusions poration Ltd.). In British Columbia Minister of in such chromite are yet to be determined.These Mines,Annual Report for 1957,35-36. featuresare compatible with our working hypothesis that during extensionof the seafloor, seawatercould Jorur, Z. & LsBer, L. (1978):Origin of chromitite have gained accessto a rising mantle diapir, pro- layers in rocks of ophiolite suite. Int. Mineral. moting partial melting, with resultant local stabiliza- Assoc.,XI Gen,Meet. (Novosibirsk)Abstr. Vol. I, tion of pargasiticamphibole and, on continuedten- 5l-52. sional episodes,chromite precipitation. Lesr-ANc,M. (1980):Chromite growth, dissolution and deformationfrom a morphologicalviewpoint: SEM AcKNowLEDGEMENTS investigations.Mineral. Deposita15, 201-210.

Thanks are extendedto Dr. R. Talkington for Maaroe, S. & Pnrrvrztau,l. (1979):Natural partial many discussionson chromiteand ophiolitesand to meltingof spinellherzolite. J, Petrology?n,7n:741. Mr. P. Jonesfor help with the microprobe work. Dr. M. Duke provided welcomecriticism of an ear- Marpas, J. (1978):Magma generationin the upper ly draft. Funding from the Geological Survey of mantle, field evidencefrom ophiolite suitesand ap- Canada, Natural Sciencesand EngineeringResearch plicationto the generationof oceaniclithosphere. Council of Canada (grant A7874)and the British Roy. Soc.Lond. Phil. Trans.4.288, 5n-546. Columbia Department of Mines and Petroleum Resourcesis acknowledgedwith appreciation. MoNcsn,J.W.H. & Pnrcr,R.A. (1979):Geodynamic evolutionof the CanadianQssdillsla - progressand problems.Can. J. Earth Sci. 16,770-791. RBrEnsNces PatBnsoN,l.A. (1977\:The geologyand evolutionof Aner, S. (1980):Dunite - harzburgite- chrornitite the Pinchi fault zoneat Pinchi Lake, centralBritish complexesas refractory residuein the Sangun- Columbia. Can. J. Earth Sci. t4, l3U-1342. Yamaguchi zone, western Japan. J. Petrology 21, l4t-165. Prucrranp,H.M., Porrs,P.J. & NsARv,C.R. (1981): Platinum group element minerals in the Unst AnustnoNc,J.E. (1949):Fort St. Jamesmap-area, chromite,Shetland Isles. /zsl. Mining Metall, Trans. Cassiarand Coastdistricts, British Columbia. Geol. 890. 186-188. Surv. Can.Mem.252, Rucxuocr, Gaspennrur,E.L. (1969):Specifica- BornNca,Y. & ArlEcnE,C.J. (1978):Partial melting J. & tions of a computerprogram for processingelectron under spreadingridges. Roy. Soc. Lond. Phil. Trans,4..?,f,8, 501-525. microprobeanalytical data. EMPADR VII. Dep. Geology,Univ. Toronto. Casseno,D., Nrcoles, A., ReurNovttcH,M., Mourrr, J., Lrnrauc, M. & PnrNznorBn,A. (1981): Struc- SpeNcrNsunc,K. (1943): Die Chromerdagerstatte von tural classification of chromite pods in southern New Tampadel am Zobten. Z, Prokt. Geol. 5l' 13-35. Caledonia. Econ. Geol. 76. 805-831. SrrvBNs,R.E. (1944):Composition of somechromites Drcrey, J.S., Jn. (1975): An hypothesis of origin for of the WesternHemisphere. Amer. Mineral. 29' podiform chromite deposits.Geochim. Cosmochim. t-34. Acta 39, 106l-1074. (1982): GnsBNBaurr4,D. (1977): The chromitiferousrocks of T'cLKrNctoNr'R', wnrrnsor' D'H' & JoNns'P' and other solid inclusions the Troodosophiolite complex, "-''-' cyprus. acon. ceii. 1J",,*llE"-ygnerals '2,,is-rsi. T"::::H:tfXI",;:i';t%:::Hf:F;E:;k Gnrcony, R.T. & Tayr-on,H.P., Jn. (1981):An oxygen Abstr' 7, 83' isotope profile in a section of Cretaceous oceanic crust, Samail Ophiolite, Oman: evidence for 6180 Tnaysn, T.P., (1969): Gravity differentiation and buffering of the oceansby deep( > 5 km) seawater- magmatic re-emplacement of podiform chromite hydrothermal (H.D.B. circulation at mid-ocean ridges. "/. deposits. In Magmatic Ore Deposits Geophys. Res. 86, 2737-n55. Wilson, ed.) Econ. Geol. Mon. 4, 132-146. t72 THE CANADIAN MINERALOGIST

Wexrnsou, D.H. & MerNwanrNc,P.R. (1980): & WarrrxsoN, D.H. (1981):Chromite in some Chromitein Ontario:geology and chromitezones, ultramafic rocks of the CacheCreek Group, British Puddy Lake - ChromeLake area, and chromite Columbia.,IzCurrent Research, Part A. Geol.Sum. chemistry..IleGeosci. Res. Grant Prog., Summ. Res. Can. Pap. t1-1A, 349-355. 1979- [email protected]. Pye,ed.). Ont. Geol.Sum. Misc. Pap. 93,220-234. Wrcrs,F.J. & WHrrreren,E.J.W. (1977):Serpentine textures and serpentinizalion. Can. Mineral. 15, WurrrarBn, P.J. (1982): Chromite occurrencesin 459-488. ultramafic rocks in the Mitchell Range, central British Columbia. In Current Research,Part A. ReceivedNovember 26, 1982, revbed manuscript ac- Geol. Sum. Can. Pap. E2-1A, 239-245. ceptedAugust I, 1983.