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 I o Pr. Geotgo +I BRITISHCOLUMBIA I n\> o 100 200 I Un 'rd o Kamloooe I I /, \ffintiV I /3 q /to \'.'. l. /a C^---- =:-' /6 --.t 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. -,_.- Fault llneamnl- Ooolost !, P. J. Whll!6(€i' leAl. 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- &" '..9;-- ,i. 3am 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