587

The Canadin x MineralogistVol. 35, pp.587-595 (1997)

ZING.RICH CHROMITE FROM PALEOPROTEROZOIC CONGLOMERATES AT TARKWAGOLD MINE, GHANA

THOROLF W. WEISERI AND WOLFGANG HIRDES

Fedzral Instiate for Geoscicnces and Natural Resources (BGR), P.O. Box 510153, D-30631 Hantnver, Germany

ABSTRACT

Zn-rich chromite is an extremely rare detrital constituent in gold-bearing meta-conglomerates -of- $e P3loryoterozoic

Tarkwaian Group of Ghana. Electron-miooprobe analyses of the cbrimite grvei mean Zn content of 13.30 wt'%o TnO,withthe

general formda (Feo .aa,Zno.ta)>t.w(Ctt.ssAlaszFqn)Oq. The highest measuredzn corrten! L9,42wt.VoZnO, yields the structural

iormula(Zno51psa.as)>1.00(Cr1.12Alo.efeo.m)x.ooO+, which coriespondsto ry edDll* zincochromite. Reflectance values for

ft1, ;dr.id;", -ii**.ir ut'"*ii 400 ;tt00 nm in air, decr6ase steadily from l7.o to l4.6vo with inoeasing Zn content'

The chemical data demontrate u f.*g" nAO of solid solution between chromiie and zincochromite, and an extended miscibility

gap in the system (ZaFe)CrzO+ -TiFezOq -Zo1Jr2O+ at least above 50 at.Vo (7nFe)CrzO+. The origin of the Zn-rich cbronite

at Tarkwa is uncertain. In the paleosourL area of the Tarkwaian conglomerates, a few examples of ultramafic rocks are known,

but these carry Zn-Poor chromite.

Keyword*: Zn-rich cbromite, zincochromite, electron microprobe analyses, reflectance data, congJomerate, Paleoproterozoic'

Tarkwa Ghana-

Sor,fi\'rARE

La chromite zincifBre est un tres rare composaff d6tritique des m6taconglondrats aurifdres du Groupe Tarkwaien, d'ige

pal6oprot6rozolque, au chana. Les analyses i la microsonde dlectronique donnen! en moyenne, 13.307o en poids fe ZnO dans

la chromite, ce qui conespond i une formule structurale @es.6azn$6b1.6(cr1.a5Al933Fe6.n)oq.la teneur maximale en Zn'

l9.427o,nEnedla formule structurale (7no51Fee.ae)1r.00(C;1.;Ab;i;;;Lq0oa,_9€ gui correspond bune zincoclromite riche

en Fe et Al. La r6flectance oe ce mat6riau,-mesur6e "otr"

aOOE Zqj;-6u* i;air, diminue de fagon monotone de 17 'o-h' 14'6To'

ks donn6es chimiques indiquent l'existence d'une solution solide 6tendue entre chromite et zincochromite' et une lacune de

miscibilit6 importante aans li systlme QnFe)CrzOa, - ZilFe2Ot - 7st/JlzOa,, dv moins pour les compositions co^ntenant plus de

50Vo (ZnFe)CrzOa Oase molaire). L'origine des grains ae *iomlt" zinciftri demeure incertaine. Dans le socle d'ot proviendrail

le d6tritu; des ;nglbm6rats tar11yaens,-lt y a quJtques exemples de roches ultramafiques, nais celles-ci contiennent la chromite

typique, b faible teneur en Zn clraduit par Ia R6daction)

Mots-cl6s: cbromite zincifdre, zincochromite, donn6es de microsonde 6lectronique, donn€es de r6flectance, conglom6ra! age

paldoprotdrozoique, Tarkwa, Ghana-

INrnolucnon

Zinc-bearing chromite was first described by Donath(1930, 1931) from Norway. Since then, the develop-ment of the electon microprobe has resulted in discov-ery of additional occurences of Zn-rich chromite theworld over (e.g., Thayet et al. t964, Weiser 1966,l967ub, Schidlowski 1970, Groves et aI. 1977,Utter1978, Bevan & Mallinson 1980, Wagner & Velde 1985,Treloar 1987, Wylie etaL 1987,BedaI' & Monchoux1991, Bjerg et aI. 1993, ChaJhs et aI. I995,lipo et aL1995. S6nchez-Vizcaino et aI. 1995). A new mineral,zincocbromite,TnCr2Oa, was propos€d by Nesterov &

Ruqiantzeva (1937) and accepted bf the C91111t19non N"w Minerals and Minerals Name.s (CNMMN)'Oddly, re-exnmination of the original mate:ial ofDonait from Norway by Weiser (L966, L967a) al'dMoore (1977) did not confim the high contents of zinc'

The aim ofthis paper is to describe a new occurrenceof Zn-bearing chromite ftom Ghana containing thehighest Zn content ever rePorted.

Gsol,ocrcAr. Serrntc exn MNERALoGIcAL Aspesrs

The Paleoproterozoic Tarkwaian Group of Ghanacontains onebf tl" three known occlurences of early

t E-mail address: torry.weiser@bgr.de

588 TI{E CANADIAN MINERALOGIST

er.ecglrian quartz-pebble conglomerate currentlymined for gold, the others being the Witwatersrandgoldfields of South Africa and the Serra de Jacobinagoldfields of Brazil. The sediments of the TarkwaianGroup consist of metaconglomerate, quartzite andminor phyllites They are products of erosion ofBirimian supracrustal rocks and their coeval belt-typegranitic plutons (Fig. l), deposited under fluvial tolacustrine conditions in long, narrow intranontane gra_bens which, in Ghana occur in Birimian volcanic b-eltsca. 2185-2155 Ma in age (I*ttbe et aL 1990, Daviset aL 1994). The age of Tarkwaian sedimentation isbracketed between 2132 X 3 Ma and ca.2l}O Ma@avis et al. 1994, Hirdes & Nunoo 1994). The Tark-waian rocks and the Birimian basemenrwere deformed together during the Ebumean tectono_thermal event.

The gold-bearing conglomerates, which rarelycontain chromite, occur in a 250-km-long NE_SW_tending unit of Tarkwaian rocks in the Banket Seriesofthe Ashanti volcanic belt (Fig. l). The rocks oftheBanket Series consist of monomictic to oligomictic,relatively well-sorted quartz-pebble conglomerates thatmacroscopically show a sfriking similarity to manyWitwatersrand reefs. Two conglomerat; horizonsrvithin the Banket Series, the Main Reef and the WestReef, consistently carry economic gold values. Thechromite discussed in this paper originates fromheavy-mineral-enriched foreset beds in quartzite between theUpper Band and Lower Band of the Main Reef(11133 Overlap Main Ree$.

Unlike the Witwatersrand and Serra de Jacobinagold .deposits, which contain major heavy-minspalconstituents stable underreducing, or at least oxygen-{eficient, ahospheric conditions (e.g., pyrite, uraninitel,the ores at Tarkwa comprise a gold - magnetite _hematite-; (ilnenite paragenesii, which iJ hrgelyssmFaxable to the'"black sand paragenesis,, encoun-tered in present-day placers. Other minerals observedat Tarkwa some of which occur only sporadically,include zircon, rutile, quartz, carbonaie, sericiteocbromian muscovite (.trchsite'), cbloritre, chloritoid,lourmaline, garnet chromite, staurolite, epidote, dia_mond, pyrite, pyrrhotite, marcasite, chalcopyrite, andbomite (Whitelaw 1929, Junner et at. Lg4Z, Sestni1973, Kesse 1985, Hirdes et at. 1988, Klemd et aL1993, Hirdes & Nunoo 1994). This is certainlv a smallnumber of minerals compared to more than 80-mineralsin the ore-bearing horizons atWitwatersrand.

The above-lisled minerals occur as allogenic miner_als, authigenic minerals, and minerals present both asfloqenic and authigenic constifirents. Hematite is byfar the most abundant opaque phase in the Tarkwaianconglomerates. Sulfides are of minimal volumetricimFortance. They are mainly confined to the selvagesof some exogenic quartz veins or dykes, where epiie-netic sulfidization of "black sand" minerals in adjadntconglomerates has led to the formation of a pyritii halo

a few desimeters thick. This resfricled epigenetic sulfi-dization bears no spatial or genetic relationship to tlegold mineralization exploited at Tarkwa.

Chromite is an extremely rare constituent in tleconglomerates at Tarkwa; in the selected forstudy, only a few grains ofmagnetile are present whichcontain a core of cbromite. The chomite forms idiomor-phic or rounded grains up to 200 [m across in the coreof larger grains of magnetite @igs. 2.U b) . T\e contactbetweenthe chromite core andthe magnetiterimis alwayssharp. A rim of felrian chromite (.Territchromit''), whichnormally is very coulmon, and was described mosrrecently by T iiFo et al. (1995) from Finlan{ is absentin all samples from Tarkwa. In some cases, a symplec-titic or eutectic-like intergrowth with magnetite ispre-sent (Fig. 2b). Figure 2b also shows a core of cbromitethat has been broken and displaced by magnetite.

Cowosmou or rrm Cm.oi!ffrE

fuwlytical metlads

For optical and electon-microprobe examination,six specimens containing chromite from the Maiu Reefof the Tarkwa gold mine were mounted in epoxy resinand polished with dianond powder on a Duerenerpolisher with lead laps. Measurements of reflectancewere made using aleitz Ortholux microscope equippedwith a Hamamatsu photomultiplier. The stanaarO uieOwas SiC (Zeiss). Electron-microprobe analyses werecarried out with a CAMEBAX Mcrobeam analvzer(Cameca Instruments). Measurement of 10 secondsduration were taken for Ko lines at an acceleratingvoltage of 20 kV at 10 nA beam currenl The raw datawere corrected using the PAP program- Cbromite (Cr,Fe), gahnite (A1, Zn), synthetic MgO (Mg, syntheticMnTiO3 (Mn, Tt, and vanadinite (V) were used asstandards. The proportion ofFeO and Fe2O3 were cal-culatred to fit the stoichiometic composition of spinel-group minerals. For the calculation of proportions ofthe end members, MnO and TiO2 were added to FeO,and V2O3 to Fe2O3.

Optical propenies

Under reflected ligh! the Zn-ich chromite is greywith a bluish ttnge and isoffopic, both in air and underoil immersion. The reflectance in air is between17.0 and 1.4.67o n the 400-700 nm nuge (Table l).There is no significant difference in reflectance amongindividuat grains in a sample. The reflectance of theZn-ich cbromite from Tarkwa is distinctly higher thanthat of zinc-free cbromite from the Stillwater Complex,Montana (Criddle & Stanley 1993) @ig. 3). This is theeffect of the substitution of Fe by Z-n.There is, for annnknown reoson, a discrepancy with the data for zinco-cbromite of Nesterov & Run{antzeva (1987), whoproposed much lower values than those for the Zn-rich

589Zn-RICH CHROMITE. TARKWA GOLD MINE, GHANA

R K I N A F A-.-T -.a-- +.-.-+.-=- -.+-.i -j:s-+--.{r-Thr

\ \.+ + + +' - . i \+ + +

i!

/ + + +i ' + + tl + + +Lt + .&r3 ,

GEOLOGY OF

CENTML AND WESTERN GHANA

'- voLTAtAN Flou"rrrn*"hats8tkose,

ilco senes ffi ou"rrara.halo, phytltto

F;;;;:] congloneret., qarttt€'l::: :: :: I phyltte, grlt

Uppsr greanochlsuamPhlbol,te|-_ melamorchlcsdlmantS.T-Zt m'eidmorphlc ssdlmsnts,la ) t' )l gdtrlly melasmatlzad

ernrur,m [-l g!fljgjil&?&".li"dr"n -s ru,".lffi]Voruntc taclae':E-t-yJ Basltlc flows and subvolernlc

rocks, volcanlclaatlaS

DAHoMEYAN \$!! eorar" *a baslc snalso and

O rlnrwl MINE

w

+ + \+ + +

+ \ - / - - + + + ++ { + + + + + +

+ + + + + + + + ++ + + + + + a ' ) +

+ + + + ++ + + + + + t {xT ! /+

+ + + + +

fi- ' )*fi:'H't""Flc. 1. Geology of central andwestem Ghana and positionof the Tarkwa gold mine(after Hirdes & Nunoo 194).

590 T}IE CANADTAN MINERAI-OGIST

chromite from Tarkwa and the Zn-free chromite fromthe Stillwater Complex (Fig. 3).

The Tarkwa cbromite is invariably sunounded bymagnetite, which is grey with a brownish tint inreflected light in air and oil. The reflectance of themagnetite is typical, ranging from 21.5 to 20.l,Vo(4@-700 nm), and clearly higher than that of cbromite.

Chemical composition

Electron-microprobe analyses of the cbromite grainsClable 2) reveal a large compositional variation amongthe six samples analyzed and also within the samesample. However, single grains of chromite have arelatively homogeneous composition, as demonstrated

FIc.2. Back-scattered electron images @EI) of spinel grains in the Tarkwaian conglom-erate. a. Idiomorphic chromite (dark) rinmed by magnetite. b. Chromite (dark) in asymplectite-like intergrowth with magnetite.

Zn-RICH CHROMITE TARKWA GOID MINE GHANA 591

5605806006206q660680700

40042044m/.tn

5005?n540

TABLE l. REFLECTAI.ICE VALIJES oF(FqAI)"RICH UNCOCIIROIvIITB*

FROMTARKWA GIIANA

trnm R7o (air) l r - RVo (air)

17,00t6,84t6,7216,471,62816,1816,0615.83

The calculated proportion of FeO lies between15.73 and 24.34 vrt.Vo, with a mean of 19.78 wL-clo. Allchromite grains are free of Mg, and show a nearlyconstant and low concentration of Mn (0.84 wt.7oMnO). Vanadium and titanium were detected at lowvalues, only in a few grains Clable 2).

The most interesting feature of the chromite grainsinvestigated is their unusually lrrgh 7-n content. Themean aoncentation of Zn in the analyzed grains is13.33 vt.Vo ZnO. The highest measured value is19.42,xt.Vo ZnO Clable 2, garn 9) and leads to theformula (7no.51Fe6.ae)>r.6(Cr1.pA1e.6sFen .2s)v.ssOa. Asthe proportion of Zn is higher than Fe in the formula,the composition corresponds to (Fe,Al)-rich zinco-chromite. A similar zinc content (L9.09 wt.Vo ZnO) isreported in zoned chromite from the Sykesville districtMaryland, which occur inultramafic rocks and metape-liles (Wylie et aI. L987). These, however, contain muchmore Al (26.74 wt.Vo AlzOl) than the chromite fromTarkwa (16.21 rttt.Vo Alzos).

Chromite from the Witwatersrand deposited in anenvironment comparable to the one in which the Tark-waian conglomerates formed, contains distinctly lowerconcenffations of 7-n (ca. 5.8 wt.Vo ZnO) (e.9., Stupp1984). To our knowledge, tle highegl reporled valuefrom the Witwatersrand is 10.89 wt.Vo ZnO for deftitalgrains of chromite from the South Reef, We'st RandConsolidated mine @ales & Reynolds 1983).

1.5,7115,48t5,2915,07t4,91r4J41.4,7014,55

i Grainombor9.

by back-scattered electon images @igs. 2a"b), pointanalyses, and line scans.

The Cr content of the chromite lies between37.17 arrd 6O.54 wt.Vo CrzO:, with a mean of50.O2wt.Vo. There is much greater variation in Al con-tent, between 2.70 and 16.2L wt.Vo Al2O3 (mean:7.64 wt.Vo). Calculated Fe2O3 contents vary from3.58 to 16,72 wt.Vo, with a mean of 7.77 tttt.Vo. T\ehighest Al content corresponds to the lowest Cr value,whereas the relationship betwee,n the Fel and othertrivalent elements is not so evident.

480 520

TARKWA

STITLWATER

560 600 640 680 \nm

Frc. 3. Reflectance spectrum (air) of (FeAl)-rich zincocbromite from Tarkwa' Ghana Grain9) compared with the spectrum of Zn-free chromite from Stillwater Complex, Montana(Criddle & Stanley fgS:), anO of zincochromite from Karelia Russia (Nesterov &Ruqjantzeva 1987).

592 TIIE CANADIAN MINERALOGIST

TABLE 2. RESIJLTS OF ELECIRON-MICROPROBE AITALYSES OFZ!-RICH CHROMITE AND (FqAI)-RICE ZI{COCIIROI{nE

FROMTARKWA GIIANA

TABLE 3. RESULTS OF ELEC{RON-MICROPROBB ANALYSES OFMAGf{ETTTE SURROIJNDING Zn-RICH CIIROMTE

FROMTARKWA GIIANA

r(2') 214' 3Q' 4(4 5(t q4, 7(3) r(2) e(2)

gnin no.8 1 Q \weight pet €nt

2(1) 3 (3) 4Q) 5 (2) 6 @ )

Cr,o"

Al.qvrq

Fqot

FeO

M8o

MIO

7^O

rqTotal

(t

Al

Mg

Ma

78

55J5

s:74

rd

5,69

u34

nd

1,05

E32!d

r&,69

46,83

6A

nd

t3,0rtE32

rd

0,81

t4,6

03rrm,l4

6992

o2s

nd

31,t4

100,41

68,78

0J6

od

3129

100,63

68,9

0J6

nd

3121

r0016

68,96

0,65

nd

3132

rmB3!d

0,60

t9,42nd

926

I,l16

0,683

o,ol

0.470

37,t7

lorJ

030

16J2

ls,E9

nd

0,48

1E,09nd

9893

1952

ad

0,80

t4,440g

9E,75

60J4 $,m230 591nd nd

3JE 6JI

2198 2193

nd nd

l3l 096

9Jt 1036

nd 0,19

9999 99.6

eeight Fr @t

56J3 5538 45,75

393 3,87 13,89

n d n d d

s,& 6,85 4,15

?,32 $pnd nd

0,69 0n1221 r2$r

trd nd

9,t4 99,47

39,,18 F"rQ

162l aro,d

vrq

tSJ3 FeO

6E,95 67J8

nd 121

u,l) m

31,06 31,@

t00,06 100,08

lJ9E rtSl0246 O,rr9

0,156 0,1@oJ44 0,624

0,026 0,0t5 0,01to,M 0,494 05t20,0@

2230 34.t5

23,60 10,053J0 7,00

50,60 48,m

. TlembqdealyHisshmitp6€dhffi: nd rct<trtectert Ff thecal@tsti@ofthsEdMbqs,V ru #d"dto Ff, edllh @d Tirc addrdto pf.

All analyzed grains of cbromite from Tarkwa arefree of Mg. The data show that in Zn-rich chromite, Zntakes the place of Mg (e.g., Weiser 1966, Bevan &Mallinson 1980, Wagner & Velde 1985, pan & Fleet1989, Bezl;at & Monchoux 1991, Sdnchez-yncajnoet al. L995). Owing to the inverse relationship betweenFe and Zn in the Tarkwa samples, a similar substitutionof Zn forFeS*, described by Challis et al. (1995), is alsopossible.

The general formula of the chromite at Tarkwa cor-responds to @en.6aZn6)s1.6(Cr1j5A1s3 s Feocz)v,.mO q.The proportions of spinel etrd-members are as follows:64.0 aLVo FeCr2Oa (chromite), 16.5 at.Vo ZnAbO4(gabnite), lL.o aLVo ZnFe2Oa (franklinite), and g.5 aLVoZaCr;2Qa (zincocbromite). In this suite, tle spinel con-tains between 6.0 and34.2 at%o gahnrts,5.0 aad"23.6%ofrarklinite, 2.3 and22.6%o zincocbromite, and 4g.g and77.6Vo chromite (Iable 2). This spread illustrates alarge field of solid solutiou between cbromite and zin-cochromite.

Compositions of individual grains from Tarkwa arecompared with those of chromite (>35 tttt.Vo Cr2O3; seeWyEe et al, 1987) $elected from the Uterature (Fig. 4).

rJs4

0259

0,187

0,680

Mic prop6d@

r,674 t,636 1300 1J68 I,OA20,t74 0,171 0Jt8 0,270 0,446

0.@90,152 0,193 oJn 0362 0,630,638 0,627 0J&7 0J66 0.491

r Tlembadml5mis shomilpscdhffi;lrt ldde0ed AI,lttgl\ro,NiedTi@ belw tbg linit ofdcterti@

The spread of value.s for chromite for Tarkwa is similarto that for detrital Zn-rich chromite from the Witwaters-rand conglomerate, from the Pongola Group of SouthAfric4 and from various other environments. The dataalso demonstrate an extended miscibility in the(7a,Fe)Ct2Oa - ZnFe2Oa - ZnNzOq system, at leastabove 50 at.Vo (ZnFe)CrzOa.

Electron-microprobe analyses of mapetite rimmingchromite contain up to 1.2 wlVo Cr2O3 flable 3, grain6), and tp to 0.l5Vo V2O3 (Iable 3, grain 5). All otherelements are below the detection limit. Magnetitegrains without a core of chromite arc free of Cr. Thisfinding suggests that the mapetite rim is a product ofalteration of the cbromite core.

D$CUSSIoN

Zinc-bearing cbromite (>{5 at.Vo Zn) is mostlyMg-poor and associated with sulfide mineralization inulnamafic sequences (Groves et aL L977). Czamanskeet al. (1976) explained the origin of such chromite inAlaska as a product of crystallization from a sulfidemelt segregated from gabbroic magrna. The same modeof formation is assumed for the Zn-bearing chromite ofWestem Austalia (Groves et aL 1977,1983).

Schidlowski (1970) and Stupp (1984) suggesred areaction between Zn-beuing circulating fluids anddefital chromite to explain the origin of zoned chromitrevdth a Zn-rich rim and Za-poor core in the Witwaters-rand conglomemte and the Pongola Supergroup inSouth Africa. Frey (1988) explained zoning in the'lVitwatersrand

cbromite by a process involving diffrr-sion of Zn from core to rim of the grains during meta-morphism.

Metasomatism by Zn-rich fluids is invoked toexplain the high Zn content of the zoned chromite fromthe Sykesville district, Maryland Qvylre et al. 1987),where Zn enrichment is associated with massive sulfi.demineralization. Similar processes may have played arole in the formation of zonedZn- and V-rich cbromiteat Outokumpu (Weiser 1966, 1967b). According to

0,032 0,041 0,030 0,022 0,o2s o,t)4o,24 0335 0ll4 0340 q348 0383

0'm6 o.m5

gphliE 1230 595

Adnilib 7,t0 5,m

iryh@iE 40 2255

ch@ib 7/,60 6610

e[d-tumbq Foponiost29S t,70 tJ5 ,,40 13J0935 7,@ 9,65 5,60 l8,ro610 l7,m 16,60 330 8,40

71,60 66.00 65r! 6r,70 60,m

Zn-RICH CHROMITE TARKWA GOLD MIN4 GHANA 593

Bjer:g et aI. (1993), the zonation and Zn-content of thechromite fromthe Cordillera Frontal Range, Argentina,are the product of alteration processes during serpenti-nization of ulramafic rocks. Associated sphalerite-bearing sulfide mineralization is considered as apossible source of the zinc in the chromite.

Unlike these examples, the grains of detrital Zn-richchromite from Tarkwa are not zoned. Thercfore, theirformation in relation to circulating Z*rich fluids inchromite-bearing conglomerates, as described bySchidlowski (1970), Stupp (1984), and Frey (1988)from South Africa, is unlikely. If chromite had crystal-lized from a sulfide melt at Tarkwa as Groves et aJ.(L977, 1983) suggested for Zn-bearing cbromite asso-ciated with iron-nickel sulfide ores from WesternAusaalia, it would have required a magFa endched in7n

Hence, on a theoretical basis, the detrital Zn-richchromite from Tarkwa could serve as an indicator ofmineralized source-rocks, e.g., ulfiamafic-rock-hostedsulfide bodies. However, the evidence for ulframaficsource-rocks is scanty in the Birimian of Ghana- Theonly noteworthy occurrence of ultamafic rocks in thesouthernmost Ashanti belt contains chromite with aninconspicuous Zn content. A massive sulfide depositsaid to contain gahnite, but no cbromite, occurs in roclsof the Birimian Supergroup in Burkina Faso (Napon &Ouedraogo 1990). Minor volcanogenic massive sulfideore consisting of disseminated sphalerite, pyrite, gers-dorffite, ullnannite, and alabandite has been describedby Hirdes & Leube (1989) for the Nsuta manganesemine (4 km SE of Tarkwa town), which is located in theenvisaged hinterland of the paleoplacers at Tarkwa.Additional detailed mineralogical and sedimentological

(Zn,Fe)Cr2Qa

ZnAl2O4

Tarkwa, Ghana

Wltwatersrand & PongolaSupergroup, South Af rlca

Varlous locallues

Zlncochromlte, Karella

o

A

X

Ffc. 4. Plot of the composition of samples of Zn-rich chromite ftom Tarkwa, Ghana (filled circles), compared with other samplx

taken from the literature, in t 1ms of. (/nfle)CrzOq - ZnFqOa - 7n1.J.2O4, Qrcn circles: Witwatersrand conglonerate and

Pongola Supergroup, SouthAfrica [1,2, and 5: Frey (1988), 3,6,7,g,and 10: Stupp (1984),4: Eales &Reynolds (1983)'

8: Utter (19i8i1. Triangles: Zn-rich chromite from various environments [1: Val d'Aoste, Italy (Wagner & Velde 1985)'

2: Satamanca districg Argentina @jerg et al. 1993), 3: Kambafd4 Australia (Groves et aL 1977),4: Outokumpq Finland(Weiser 1966), 5: Trojan-mine, Zinbanwe (Groves et al. L983),6: Mashaba mine,Timbahwe @evan & Mallinson 1980)'7: nortlwestern Nelson. New Zealand (C\dlis et al, 195), 8: Hemlo, Ontario @an & Fleet 1989), 9: Naftiiniemi' Finland(I;i1p et at.1995), 10: MontagneNoire, France @6ziat & Monchoux 1991), 1l: Outokumpu' Finland (Weiserf967b, Treloar

iggi), tZ Sykesville, Maryland (Wylie er aL 1987)1. Square: Zincocbromite from Kmelia Russia (Nesterov & Rumjantzeva1987).

ZnFe2Oa

o a B 5 A

s94 TIIE CANADIAN MINERAL@IST

studies of the Tarkwaian conglomerate, as well as fieldwork in its Birimian hinterland, will be necessarv roidenti& the source rocks of the Zn-rich chromite'andthe possible base-metal mineralization associated withthem.

ACKNOWLEDGEMENTS

We thank Je. Lodziak for carrying out the electron-microprobe analyseso H.-J. Bernhardt from the Minera-logical Institute of the Ruhr Universitilt Bochum formaking the measurements of reflectance. We are grate-ful to referees D. Blztat and R. Grapes, to AssociateEditor Y. Modlo, and to Intedm Editor W.H. Macleanfor their helpfrrl comments and suggestions.

RmsRB{cEs

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Received August 17, 1996, revised mnnascript acceptedMarch 12. 1997.