Barian Feldspar and Muscovite from the Kipush

1143

CanadianMineralogist Vol. 30"pp. | 143-1152 (1992)

BARIANFELDSPAR AND MUSCOVITEFROM THE KIPUSH!Zn-Pb-Cu DEPOSIT, SHABA.ZAIRE

MUMBACHABU Laboratoire de Mitallogdnie, Universit( de Lubumbashi, B.P. 1825, Lubwnbashi, hire

JACQTIESBOULEGUE I'aboratoire de G4ochimieet Mitallogdnie, UniversitdPierre et Marie Curie, Paris VI, Tour 16-26, 4, PlaceJussieu, 75252 Paris Cedex05, France

ABSTRAgT

Barian-bearingfeldspar and muscovite are associated with baritein the sulfidemineralization of the KipushiZn-Pb{u deposit (Zaire),hosted by LowerKundelungu dolornites and dolomitic shales, in partbrecciated, ofl,ate koterozoicage. The celsian (Cn) contentof (BaK)-feldspan,which includecelsian, hyalophane and orthoclase,varies from 1 to 84 mole 7o,but exhibitsrwo discontinuifies,at Cn5-36and Cn4j-56. The coexisting albite is mostlybarium-free. The highestBaO contentmeasured in muscovite is 7.66 w.Vo, but values around 6 wt.%oBaO are more common,even though the associatedfeldspars vary from Ba-bearing orthoclaseto celsian.The replacementof K by Ba in muscovitefollows the substitutionBaAl = (K,Na)Si andis accompaniedby increasingvacancies in octahedralpositions and by the replacementofAl by Ti. The muscovitealso is phengitic,showing the ''Al'"A1. exchange(Mg,Fe)'*Si* = Structuraland textural fabrics indicate that Kalangan regional metamorphism postdated the mineralization.It is characterizedby mineral assemblagesof the greenschistfacies. We contendthat barian muscoviteand (Ba,K)-feldsparare metamorphic in origin, andthat their baxiumcontent was inherited from barite associatedwith the Zn-Pb{u ores.

Keywords:barian feldspar, barian muscovite, barite, Kipushi Zn-Pb-Cu deposit,Lower KundelunguGroup, celsian, hyalophane, o(hoclase,substitution schemes, Katangan metamoryhism, Zaire.

SOMMATRE

Des exemplesde feldspathet de muscovitebaryfdres sont associ6si la barytine dans les minerais sulfurds du gisement Zn-Pb{u deKipushi (Zai?e), encaiss6 dans les dolomies et shalesdolomitiques, en partie br6chifi6s, du groupedu Kundelungu infdrieur, d'dge prot6rozoiquesupdrieur. La teneuren celsianedu feldspathvarie de I d,84%o(base molaire), mais elle prdsente une discontinuitdmajeure entre Cn5 et Cn30,et une autremoins marqu6eentre Cn46et Cn56.L'albite associ6erenferme trds peu ou pasde Ba. La muscovitela plus richeen Ba contientjusqu'd 7 .66Vo BaO en poids,mais les valeursautour de 67osont plus cour:rntesquelle que soit la naturedu felspathbaryfdre associ6. Le remplacementdu K par le Ba dansle r6seaude la muscovite ob6it i la substitutionBaAl = KSi accompagn6ede lacunesdans les sitesoctah6driques et par le remplacementde Al par Ti. Une substitution phengitique est 6galementobserv6e dans ce mica. Des donndes structuraleset texfurales indiquent que le m6tamorphismer6gional katanguien est post6rieuri la mise en placede la mindralisation.Nous considdronsque la muscoviteet le feldspathbaryfbres sont d'origine m6tamorphique et que le baryumqu'ils contiennenta6t6h6it9 de la barytineassoci6e au minerai. Mots-clds:feldspath bary{bre, muscovite baryfbre, barytine, gisementZn-Pb-Cu de Kipushi, Kundelunguinf6rieur, celsiane, hyalophane,orthose, modble de substitution,m6tamorphisme r6gional katanguien, Zarie.

INTRoDUcnoN goup, in bothZambia andZairq' the Zambiandeposits are stratigraphicallybelow their Zairean counterparts The Kipushideposit is oneofthree major carbonate- (Lefebvre& Tshiauka1986). hosted Zn-Pb depositsthat have been recognizedin In the Kipushideposit, the ganguecontains (BaK)- Shabaprovince, southeastern Zaire. They all occurat the feldsparsincluding celsian, hyalophane and orthoclase. snme stratigaphic level, in the Lower Kundelungu Theseoccur in associationwith barianmuscovite and Group,just as the Kabwe lead-zinc depositof 7,arnbia, barite as accessoryminerals within the ore zones,along whereasthe Cu-Co and U-(Ni-Co) depositsare found with quartz,chlorite and dolomite. in a lower stratigraphicposition, in the Roan Super- Barian silicatesare rare andmost commonlv occur in rl44 THE CANADIAN MINERALOGIST

manganesedeposits (Deer et al. 1962, 1963).Very few the Katangan (also known as Lufilian) Pan-African compositionshave been reported in the literature.In this orogeny(ca. 65G-500Ma) and thrust to the north. The study, we provide analyical data for (Ba,K)-feldspar Katangansequence consists of conglomerates,arkoses, andbarian muscovite resulting from the metamorphism greywackes,sandstones, shales, carbonates, and of two of a carbonate-hostedbase-metal deposit. Coupled tillitic formationstotalling nearly 10,000m in thickness. substitutionofBa for K andAl for Si is characteristicof It is subdivided,from bottom to top, into the Roan and bothmuscovite and feldspar. Kundelungu Supergroups.The base of the latter is marked by a tillite, the "Grand Conglom6rat",which GEoLocrcALSBrrrl'tc representsa lithostratigraphicmarker of regionalextent. The other tillite of the Katangansequence, the '?etit The Kipushi deposithas been studied by Intiomale& Conglom6rat",divides the KundelunguSupergroup into Oosterbosch(1974), Intiomale (1982), de Magn€e& the Lower Kundelungu and the Upper Kundelungu Frangois(1988) and Chabu(1990), who describedthe Groups(Fig. 2). In the ZaireanCopperbelt, rocks of the geology and tectonicsetting of the area.The depositis Roan Supergroupinvariably appearas huge fragments one of over 35 occurrencesof Cu{o, U-{Ni-Co) and up to severalkilometers wide, in megabrecciaslocated Zn-Pb deposi* that together constitute the Central in thecores of anticlines.Because ofthat fact.its baseis African Copperbeltof Z.are andZarnbi4 a region that unknown. containsabout l5%o of world's copperreserves. The Kipushi Zn-Pb-Cu deposit is located on the Depositsin the Copperbeltare hostedby Late ho- northemflank of the Kipushi anticline Grg. 3), nearthe terozoic Katangansediments exposed in the Lufilian contactbetween a complex brecciafield with rocks of fold belt (Fig. 1).The sedimentswere deformed during the Lower KundelunguGroup. The easternlimit of the

ilig'g"r*0il

KE'iIGERE ISHA '..-.^ ZA|RES"' \.-.s... -\.-.-.-.--.-

";' ('--'1t ZAMBIA \'--\ bo dorna ! fl KUNO€LUNGUond toqngrr r R0AN Group E PRE-KATANGAN Bo3.mrnt

o Strotitorm Cu-(Co ) drpcitt o Zr(Cu-Pb) Ycins I ttrotilorm 0 50Xn € r00 x tl-( Ni-Cu-CoI vein:

Ftc. l. Simplified geological map of the Lufilian fold belt showing the locations of major deposits. Modified after Sodimiza Company records. BARIAN FELDSPARAND MUSCOVITE. KTPUSHIDEPOSIT. ZAIRE tt45

U 2 o

2 o

KAKONIWE z : KAPONDA

F;;11 [--la E;55tr: ffila

Ftc. 2. Simplified stratigraphiccolumn of the Katangansediments in the Tairan sectorof theLufilian arc(Gecamines concession). The base is hereunknown. Abbreviations: RAT clay-talc rocks; R Roan;Ki Lower Kundelungu;Ks Upper Kundelungu,1 sandstone,2 shale,3 tillite, 4 dolomite. Modified after lntiomale (1982) and Lefebwe & Tshiauka (1986).

brecciabody hasbeen termed the Kipushi fault, and its LowerKundelungu beds. Their composition exclusively central part defines the axial fault-zone (Intiomale & reflectsstrata in contactwith the brecciaor stratigraphi- Oosterbosch1974). cally higher,suggesting that the brecciais a gravity-col- The axial fault brecciais composedofhuge blocksof lapsefeature. The breccia matrix is carbonaceousand different lithologies derived from Roan Supergroup exhibits sedimentaryfeatures (graded bedding, lamina- rocks.Blocks of a mafic greenrock alsoare found in this tion) andstructures indicative of collapseprior to folding breccia. The so-called Kipushi fault brecci4 at the of the enclosinghost-rocks (Chabu 1990). De Magnde easternmargin ofthe brecciabody,is perpendicularto & Frangois(1988) attributedthe megabrecciain the the strike and containsrock fraEmentsof tlte enclosins coresofanticlines ofthe ZaireanCopperbelt to evaporite I146 TIIE CANADIAN MINERALOGIST

oooooo OOOoOoO oooooooo aooooooo ooooooo

oooooo oooo oooo

, ffi|, flflTlfl'FFl. ffi|. =1.

Frc. 3. Geological map of the Kipushi anticline and location of the orebody. 1 BrecciL 2 siliceous dolomite (Lower Mwashya), 3 shale (Upper Mwashya), 4 "Grand Conglom6- rat",5 dolomite (Kakontwe), 6 shale and dolomite ("Sdrie r6currente"),7 shale and sandstone,8 ore deposit. Modified after Intiomale (1982) and Gecamines Company records.

ascending diapirically under gravitational and tectonic located in massive Middle to Lower Kakontwe dolo- stresses: the later dissolution of the salt formations mites. The concordant bodies are confined to well-bed- resulted in collapsebrecciation. However, the failure of ded carbonaceous,black Upper Kakontwe dolomite and hundreds of drill holes in the surrounding areas to to the "S6rie rdcurrente", where the mineralization is intersect evaporite of sufficient thickness to explain the predominantly cupriferous. large ore-bearing breccias at Kipushi and several other Important sulfide minerals are sphalerite, chalcopy- Cu-{o deposits of the T,airean Copperbelt, lends no rite, bomite, galena, pyrite, with minor tennantite, credence to this interpretation. Nor can it explain the chalcocite, arsenopyrite, marcasite, renierite, briaflite, formation of mineralized solution-cavities, some of gallite, germanite, betekhtinite and canollite. The min- which are discordant pipes found at Kipushi in unbrec- eralization is zoned stratigraphically. Copper ores in the ciated Lower Kundelungu dolomites (see below). "Sdrie r6currente" are separated from zinc-lead ores The mineralization affected the base of the "S6rie located in Middle to Lower Kakontwe dolomite strati- r6currente", which is composedof alternating dolomites graphically below the copper ores. The transition zone and dolomitic shales. and the karst structures in the behveen the two types of ores, which is found in the underlying Kakontwe dolomites. Together with the rock Upper Kakontwe, is a mixture of both rypes matrix and gangue minerals, the ore also fills interfrag- (Zn+Pb+Cu). All ore types are present in the collapse mental voids of the collapse breccia described above. breccia. Remobilizedores also are common in veins. Gangue minerals are phyllosilicates, feldspars, car- Morphologically, the mineralization forms bodies bonates and quartz, with very rare barite. This study that are either discordant or broadly concordant with concentrates on barium-rich mica and feldspar. lithological boundaries.The discordantbodies fill voids Other gangue minerals will be addressedin later stud- in collapsebreccia and karstic structuresthat are mainly ies. BARIAN FELDSPARAND MUSCOWIE. KIPUSHI DEPOSIT.ZAIRE lt47

Muscovrre TABLBt. @MF6trrlG{ or sAnrAN MUs@vrrB'

Muscovite and chlorite are the most abundantphyl- A!8lJd8 456 789 losilicateminerals in the gangue.These appear in all ore types and are associated gangue SlO2 wtg 41.t4 47:12 &rc 445 4t95 4rjr 4m 4,$ 11.6 with all minerals.Talc |nl t x39 rLlo rza tltS 32J6 30.u 9ffi 3LA 3Jsl has not been observedwithin the mineralizedzones. llq Mt LA (L33 o9 0i8 za oa7 0.t4 0.y3 r@ - 0.q, ol0 - 0.10 - o.fi otr whereasphlogopite is minor. ?ld) - o04 n@ -onut o0l o,2 0IB Muscovitecompositions include phengitic and barian rc 0a 0.n 0.4 0.@ 0.6 0.11 ur ut2 00 lfro 249 2.fl LAo 2.M 28 2,9 lJ2 2-t6 2!U varieties.Muscovite enriched in Ba was found in two co ur mineralassemblages, atthe 8l I m and992 m minelevels BrO L73 3.63 19 J.4t J.?t 630 676 7ll t41 Na2O o on 031 u42 0'36 0.t9 036 0.30 0.4 (depth):(i) Ba-bearingmuscovite - Ba-bearingortho- Keo &7J 8J6 tt6 83{ &71 6.53 t3l &r0 8J4 clase- dolomite - barite - albite (+ rare hyalophanel, F o43 0'6t (166 o3t 0J0 0& o35 0M 035 s2o 4.4 4.$ 4tB ArE 4.(b 4,n 4.lt 4n 4.$ and(ii) Ba-bearingmuscovite - hyalophane- celsian- FEO -o.18 -{129 -{uE -{Ll5 .{l2l -O5 -{ttt -o.01 -{uJ dolomite- barite- albite. T@l 99.$ r0r, 9'.21 99.(E S.66 S.@ 86 98 np Ba-poormuscovite also is presentin theseunusual k@lss b8s€d @ 24(O,Og,F) associations.It is generallyfiner grained than the Ba-rich st 631 63J 68 62t 613 69 6,6 6.19 6.16 muscovite.It shouldbe notedthat muscovitewith as lvlt, 1.66 t.6 lJl 1.79 1.At L6t 194 131 tl4 much as 3.06 wt.VoBaO has been observedin the wN 3J0 ,.O 3.ilt 3Jl 3.{l 3A 33t 331 3a absenceofbarite andof (B4K)-feldspars. rt 0.04 0.04 0.6 0.(X un 024 o.o 0.D or0 Fo 00s 0& oot - o0r 0Il1 0.0r u,l 0.6 Analysesof Ba-enrichedmuscovite were carried out rds 0J0 ojr O49 O.42 0.A O48 O48 O,|J Uql at the Universit6Pierre et Marie Curie (ParisVI), 7a o0! 0!l - 0.0t up using - - a CamebaxMicrobeam electron microprobe at an accel- lr! 0.0t 0rl eratingvoltage of 15kV, abeamcurrent of20 nA anda f 4S 4.(r 4.6 3gt 3J8 3g9 39' 335 3gl peakcounting time of 15 seconds.The spot size was 1-2 Cr - oot pm. 88 o19 021 oa 0a op 034 09 0.41 043 Standardsused were natural and syntheticminerals Na offt 0.6 0.6 0.11 0.r0 00t ol0 0s 0.6 or compounds,namely orthoclasefor Si, Al and K, K 1.47 L€ ljl 1J6 lji l.16 1.49 l.ilt 1.43 diopside for Mg and Ca albite for Na, Fe2O,for Fe, x. t'3 tJ' r&, 1.96 lgr u6 196 134 1.11 MnTiO3 for Mn andTi, sphaleritefor Zn, barite for Ba, F olE 0.D 0,n 0.r 0l,. 02, ol, 0.01 o.l.lt andLiF andfluorite for F. Resultsare given in Table l. BaO contentsrange from 3.63 to 7.66 w'r..%o,with 6 A deshEf!@ ardd !N 6io tu [email protected] wt.7oBaO being more common in muscovitefrom both 16 [email protected] ffi patu@Ia6tL Ee.tGr&roFlbode. of the abovemineral assemblages. Similar values have beenreported in muscovitefrom a strataboundBa-Zn mineralizationsubjected to amphibolite-graderegional vacanciesresulting from a 1: I replacementofK+ by Ba2+ metamorphismat Aberfeldy,Scotland (Fortey & Bed- in the substitution schemeBa + N = 2K (Guidofti 1984). doe-Stephens1982). Pan & Fleet(1991) also reported However, the lack of a negative conelation between the on muscovitewith up to 10.3wt.7o BaO in the Hemlo Ba contents and the total number of interlayer cations area, Ontario. The barium content is rarely constant, indicates that this model of substitution is not the most evenwithin individualgrains. The following discussion effective. Ba and Ti contents are positively correlated of thecompositional variations in muscoviteemphasizes (Fig. 4). Pan & Fleet (1991) also reported high Ti the natureofcoupled substitutionsinvolved toachieve contents in barian muscovite. chargebalance. There is a crude negative correlation between Ba Examinationof Table I showsthat occupancy of the contents and the total number of octahedrally coordi- octahedralsites ranges between 3.95 and 4.08. Octahe- nated cations (Fig. 5), which suggeststhat the excess drallycoordinatedAl occupies 87Vo ofthetotal occupied interlayer chargeis partly balancedby increasingvacan- positions,the remaining being mostly Mg and Fe. cies in the octahedral site, resulting in an increase in Titanium contentsare relatively high, rangingfrom 0.02 negative charge in the layer of octahedra. This relation- to 0.24 ions per formula unit, whereasMn and Zn are ship may be due to the positive correlation of Ba with Ti generallybelow the detectionlimits. shown in Figure 4; however, on a plot ofTi versus totaT Si is commonlygreater than 6 atomsper formulaunit; octahedrally coordinated cations, the data are more in comparison,the associatedBa-poor muscovite (data scattered than in the above relationship (Fig. 5). This fact not included) have still higher contentsof Si and Mg. casts doubt on the existence of the substitution 2vIR2* = qTi The total occupancyofthe interlayer site rangesfrom + N, reportedby Guidotti (1984) in both biotite and 1.56to 1.98,including l.16 to 1.56K and0.19 to 0.43 muscovitenas a possible schemefor the incorporation of Ba per formulaunit. Thesebarium contents in muscovite Ti in the muscovite analyzed, thus indicating that the are comparableto thosereported by Fortey & Beddoe- relationship between Ba and the total number of octahe- Stephens(1982) and by Dymeket al. (1984).The low dral cations is not due to Ti. occupancyof someof the interlayersites may be dueto Si displays a relatively well-defined negative correla- 1148 THE CANADIAN MINERALOGIST

6 a,, a c3 ar 6 8.c t.7 &t &e 7 7,t 7.2 X9 7/ "

40 aoa 4r M oa u2 qr ota ou 9,4 a2 0g o,%

Ftc.4.Relationship between Ti andBa in barianmuscovite.

tion with Al (Fig. 6.4').This is an expressionof the common phengitic substitution of normal muscovite (Mg,Fe)2+Si= vlAllvAl. If the muscoviteis a binary ,3 a1 aa

Frc. 6. A. Relationshipbetween Al and Si in muscovite (phengiticsubstitution). B. Relationshipbetween Mg+Fe andSi expressingthe Tschermak or phengiticsubstitution.

solid-solutioncomposed of end-membermuscovite and celadonite(Guidotti 1984, Miyashiro & Shido1985) by Tschermaksubstitution, it mustplot on the straightline shownin the diagrams(Fig. 6). Actually,most compo- sitionsplot considerably below the straight line in Figure 64' andconsiderably above the ideal slope in Figure68. The Al content(too low) and the (Mg+Fe) content(too high) resultfrom the fact thatAl alsois involved in other substitutions,such as Ti = vrAl (Guidofti1984) and BaAl = (Na,K)Si(Fortey & Beddoe-Stephens1982, Guidotti 1984),as depictedin Figure7. Despitea corrsiderable scatler in the dat4 Figure 7 shows a broad positive 3to 3.95 1,OO 1A6 1to IvAl. Total nambar of octah.dr.l catlont correlationbetween Ba and The excessinterlayer chargeresulting from the replacementof K by Ba thus Ftc. 5. Variation of the occupancy of octahedrally coordinated seemsalso partly balancedby substitutionof Al for Si sites with Ba content in muscovite. in tetrahedraloositions. BARIAN FELDSPARAN'D MUSCOVITE. KIPUSTil DEPOSIT.ZAIRE lt49 Locally, the orthoclaseencloses relict barite(Fig. 8) and is commonlyconoded by dolomite. Results of electron-microprobeanalyses of the (Ba,K)-feldspargrains, at instrumentalconditions described above, are given in Table 2. In terms of a Cn-Or-Ab diagram(Fig. 9), the (BaK)-feldsparsfrom Kipushi can be divided into three types: (i) Cn-rich specimenswith compositionsCn5e-eaOrt,-3eAb3_4, (ii) Cn-poor, Or-rich feldspars with the compositions Cn1_5Ore1eeAb6-,,which show very low contentsof albite, and (iii) specimenswith intermediatecomposi- tions:Cn31- aOrar-srAbs-t r. One feldspar of thistype has an inclusion of an albite-rich phase,probably repre- senting an unmixed crystal with the composition Ftc. 7. Correlation of Ba with IvAl in muscovite.The two Cn OrroAbru. c.ompositionsof Ba-rich muscoviteshowing the highest It is noteworthy that the albite content of these "Al contentsdeviate from a muscovitecomposition in that feldspars increasesfrom the orthoclaseend-member they have lessthan 6 Si atomsper formula unit (Guidotti toward Ba-poorhyalophane, whereas it decreasesfrom 1984).These could then be neglectedin interpretationsof Ba-poor hyalophaneto Ba-rich hyalophane(Cnso<). this diagram. Ba-rich hyalophaneand celsianhave the sameamount (3-4 mole 7o Ab) of albite content.There is a greater (Ba,K)-Fsluspans spreadin the albitecontent (l-57o) ofcelsianfrom the zoneof Ba-Zn mineralizationin the ScottishDalradian (Fortey In the Kipushi deposit,the (BaK)-feldsparsinclude & Beddoe-Stephens1982). Viswanathan & Kielhorn ( 1983)reported decrease orthoclase,hyalophane and celsian. These represent a in the albite content with about l7o in volume of someores and are confined to an increasein Cn contentin Ba-rich feldsparfrom Otjosondu, the same mineral assemblagesas barian muscovite. Namibia.In their study,the albite contentof barian containing Thesefeldspars mostly occur as optically inhomogene- a feldspar 30407o celsian is higher (more ous untwinned crystalsin which cleavageis generally than20 mole 7oAb) thanin feldsparwith 46-557o poorly developed.Celsian and hyalophanecommonly display sectorzoning and complex patlernsof extinc- tion. They showwhite-grey, turbid grey and dark zones TABLB 2 GBMICAL OOll|PsrTIOt Or (B€'KFFEIDSAR' in crossednicols. These zones" of variablewidth and habit, bear different chemical compositions,the clear Aalydt I 789 zonesbeing richer in barium than darker zones.These S!o2 et & 6.9 a,m &,.M 6/,35 53.18 40.75 49 40fi n6 probablyrepresent unmixed crystals. Orthoclase grains Alfr tE9t 19.42 19.6 1836 nfi u.tx x,g 3t nJ' also show undulatoryextinction, Tfir- 0rl u@ - o,M 0.6 0rr but they have a con- }lO - 0.t2 - (L(E stantcomposition. The nearly euhedralrhombic grains z& oot (utt 0.11 olt o04 0.u o.gl n€O ofi - 0.4 0.(b - 0.6 of orthoclaseare enclosedin dolomite and sphalerite. tdp - 0.01 - 0.02 0'01 CaO (L(x u)9 0(B - 0.01 BO 0.&| lJ6 2.8 238 1467 t&72 2607 3199 33fi Ne2O 0.ll o10 0IB 0.06 09 Lm 0.41 0,9 OA r2o r.m rt,4t Ea Afi 866 6'81 4.t4 2.4 rc8 Tdl rm.oa rm9 99t9 101.6 9.n tm.6l [email protected] t0t93 t@J8

F@ls bqed @ 8 am8 of qyEa

si 29t 2X 294 2.9E rn 2g 2.4 z8t 2"t6 AI 1.(B 1.6 1.6 1.6 t32, ln L&, 1.78 lE7 TT - 00t - o.mr o@ o.qD & ouB - 01ff omz - om4 MS - o.mlu.ar - (lmr oq)l izt oqD - (Iml 0.m4 uv, up offi 0.cB rr! - ilr! OII2 Cr fiI! oqz--olDt- Ea oM 0.6 "*0.6 0,(E o8 0J8 0.:r 0.71 Ltg Na 0I)1 oot om om5 ol0 0.r0 0.6 0.or 0rB K 0.y3 0.91 092 0J6 ut6 0,4t oa 0.19 o.a

ltdslEFqottc (%)

Cn Ll 32 $J 4{r9 61.E 75J E4.0 Ab 1.0 t.t 03 0.6 roj ff 56 43 32 o 96.9 vt:l 94.6 I J9.0 4&4 32.6 m2 12E Fto. 8. Photomicrographshowing relict of barite(white) $ains A dad regtseo a rcom ls h tb (ffio Iffi dtu el8{ar dsqlobo d . qerre-"rnoFtbo in orthoclase.Scale bar: 50 um. hs 6e olDl Ms !q ft@lr Et d&. I r50

aa\ , r]'

Ftc. 9. Variationsin chemicalcomposition of the (Ba,K)-feldsparsfrom Kipushi, Zaire. I Cn-poorspecimens, 2 feldsparwith intermediatecomposition, 3 Cn-richfeldspar. celsian(10-12Vo Ab). In the presentstudy, there is no crease in the albite content does appear in Ba-rich changein the albite contentof feldspars(8-ll7o Ab) feldsparcontaining at least56Vo celsian. over this compositionalrange. However, a strong de- Examinationof Figure9 suggeststhe presenceof t'rvo compositionaldiscontinuities, at Cn5-30 and Cn40.s0. Pan & Fleet(1991) also reported a compositionaldisconti- nuity at Cnr125and two more at Cn3040 and CnaT_65, whereasGay & Roy (1968)postulated a compositional gap befrveenCnu5 and Cnro,also reportedby Fortey & Beddoe-Stephens(1982); in the presentstudy, the compositionof (Ba,K)-feldsparsis continuousthrough this compositionalrange. The feldsparcompositions plot closeto theideal line BaAl = (Na,K)Si (Fig. l0) in the seriesKAlSi3Os - BaAl2Si2O8,reflecting the low abundanceof other cationsin the feldsparstrucfure as comparedto muscovite. But mostof our dataplot slightly belowthe line, especially in those cases representingfeldspar with relatively high albite contents.

Drscussror.l

The Katangan episode of regional metamorphism (650-500 Ma) hasaffected the Kipushi deposit(Chabu 1990).It is of lower faciesand is charac- ',6 1,7 '.E ',9 20 Al $eenschist , ,.t t.2 ,.3 t,1 ,,6 terizedby the following mineralassemblages: (i) dolo- Frc. 10. Correlationof Ba with Al in (BaK)-feldspar.Line mite- albite- muscovite- quartz- biotite(phlogopite) representstheoretical substitution. I Data from this study, - chloritein ores,and (ii) dolomite- quaxtz- albite- 2 additionaldata from Fortey& Beddoe-Stephens(1982). talc - magnesite- phlogopitein gypsum-and anhydrite- BARIAN FELDSPARAND MUSCOVITE. KIPUSHI DEPOSIT.ZAIRE I151 bearing dolomite in the "S6rie r6currente",above the barite, a reducing agent is necessary.This has been mineralization. demonstratedin experimentsin ceramicsystems (Segnit Thesemineral assemblagesgive supportto the sug- & Gelb 1970).Heating of kaolinite-barite mixtures up gestionby Lefebvre& Patterson(1988) thatthe Kipushi to 1000'C railed to produceany bariansilicates, but deposit is situatedin the biotite zone of the regionally when carbonwas addedto the mixture, the sulfatewas metamorphosedsequence. reducedaccording to the following reaction: Available experimentaldata (Winkler 1,965,1967, BaSOor 4C -+ BaS+ 4CO Puhan& Johannes1974) and studies ofmetamorphosed 3BaSOa+ BaS-r 4BaO+ 4SO2 carbonaterocks (Ferry 1976,Rice 1977,Bowman & and celsianwas formed at 800oC.To form celsianfrom Essene1982) are consistentwith the first development barium carbonateand kaolinite, a reducing agent was of biotite under a total pressureof a few kilobars, at naturally not necessary.Although these experiments temperaturesbetween 300' and 400'C. were carriedout at a much higher temperaturethan that The biotite - chlorite isogradicreaction muscovite + which can be expectedto apply to assemblagesof dolomite + quartz+ water = biotite + chlorite + calcite metamorphicminerals found at Kipushi, they showthat + carbondioxide, which seemsto apply to the mineral in the absenceof a reducingagent, barite is stableat a assemblagesfoundin the Kipushi deposit, was estimated temperatureat which barian feldspar may form in its to occur at 370'C in the metamorphismof impure presence. dolomite(Ferry 1976). Becauseof the very low solubilityof barite(Blount Experiments(Fawcett & Yoder 1966)indicate that 1977) and considering the fact that this mineral is Mg-rich chlorite is stable over a wider range of P-T unstableunder sulfate-reducing conditions or conditions conditionsthan Fe-rich chlorite. The stability field of the that favor the formation of witherite, BaCO, (Segnit& latter is drasticallylimited underreducing conditions to Gelb 1970,Blount 1977),we assumethat for barian a temperaturebelow 350'C undera pressureof 1 kbar. muscoviteand (Ba,K)-feldsparto haveformed in the SinceFe-rich chlorite is presentin the reducingenviron- Kipushi deposit,barite was lrst decomposedby reaction ment of the Kipushi ores (Chabu 1990), it seems with either CO producedduring the metamorphismof reasonableto assumethat the temperaturethat affected impuredolomite or with carbonaceousmatter. The latter thedeposit during the Katanganregional metamorphism is abundantin the Kakontwedolomite (seeabove) and was below 350'C. This is in agreementwith Cluzel's in karsticstructures (Chabu 1990). The releasedBaO (1986)estimate of the P-T conditionsof the Katangan reactedwith quartz,detritic ordiagenetic chlorite orclay regionalmetamorphism in the area. minerals to form barian silicatesduring the Katangan regionalmetamorphism. CONCLUSIONS ACKNOWLEDGEMENTS Although barianfeldspar is most commonin manganesedeposits (Denr et al. 1962,1963),celsian also has The authorsexpress their deepgratitude to D. Velde, beenreporied in casesof exhalativebase-metal miner- Universit6Paris VI. and B. Velde" Ecole Normale alization (Fortey & Beddoe-Stephens1982). These Sup6rieure"Paris, for helpful discussionsduring the autlors suggestedthat Ba-rich feldspar and barian preparationof the manuscript.Constructive comments muscovitein the schistsat the Aberfeldy Ba-Zn deposit, from two reviewers and R.F. Martin have helped to Scotland,originated either by replacementof barite or improve the quality of this paper. by the dehydrationof cymrite [BaAl2Si2(O,OH)8.H2o] (1991) during metamorphism.Pan & Fleet also con- REFERENcES cludedthat barianmuscovite and (BaK)-feldspar in the - late Archean Hemlo Hero Bay greenstonebelt, BLouN"r,C.W. (1977):Barite solubilities and thermodynamic Ontario, were formed by regional thermal metamor- quantitiesup to 300oCand 1400 bars.Am. Mirural. 62, phism of a barite-richunit. 942-957. The closeassociation ofbarian silicateswith barite- bearing metamorphic mineral assemblagesand the Bowrrrar,J.R. & EssENE,E.J. (1982): P-T-X(CO2) conditions presenceof relict barite in someBa-bearing orthoclase of contact metamorphismin the Black Butte aureole, grainsgive supportto the suggestionthat barianmusco- Elkhom.Montana. Am. J. Sci.?32,31 l-340. vite and(BaK)-feldspar in theKipushi deposit derived theirbariumby thebreakdown of bariteduring Katangan CHABU,M., (1990):Metamorphism of the Kipushicarbonate (Shaba metamorphism. hosted Zn-Pb{u deposit Zaire). /n Regional Metamorphismof Ore Depositsand GeneticImplications Underoxidizing conditions, barite is stableup to very (P.c. Spry& L.T. Bryndzia eds.).VSP, Utrecht, Holland high temperature(e.g., 1000'C: Segnit & Gelb 1970). (n47. This fact accountsfor its survival in somehigh-grade amphibolite-faciesmetamorphic rocks (Fortey & Bed- (Lnzst, D. (1986):Contribution e l'6tudedu mdtamorphisme doe-Stephens1982). For bariansilicates to form from desgisements cupro-cobaltifdres stratiformes du Sud-Sha- tt52 THE CANADIAN MINERALOGIST

ba,kiire. Le district minier de Lwisha.J. Ay'. Earth Sci.5, -& Tsstnure,T. (1986):Legroupe des mines dLubembe 557-574. (ShabaZare). Soc.G6ol. Belg., Annales 109,557-571.

DEER,W.A., HowrE, R.A. & Zussrr,nN,J. (1962): Rock-For- oe MacMr, I. & Fnengols,A. ( I 988):The origin of the Kipushi rningMinerals.3. ShzetSilicates. Longmans, London. (CuZn,Pb) depositin direct relationwith a Proterozoicsalt diapir, Copperbeltof Central Africa Shaba,Republic of & - (1963): Rock-FormingMinzrals. 4. Zaie. In Base Metal Sulfide Deposia (G.H. Friedrich & FrarneworkSilicates. l,ongmans. London. P.M. Herzig,eds.). Springer Verlag, Berlin (7+93).

Dvuer, R.F.,BoAK, J.L. & KERR,M.J. (1983):Green micas in Mrvesuno, A. & SltrDo,F. (1985):Tschermak substitution in the ArcheanIsua and Malenesupracrustal rocks, southern low- and middle-gradepelitic schists.J. Petrol. 26,449- westemGeenland, and the occurrenceofbarian--chromian 487. muscovite.Rapp. Gr@nlands Geol. Unders. L12,71-82.

Fewcsrr, J. & Yonrn, H.S.,JR. (1966): Phase relationships of PAN,YUANMTNG & Flerr, M.E. (1991):Barian feldspar and chloritesin thesystem MgO-AI2O3-SiO2-H2O. Am. Mine- barian-chromianmuscovite from the Hemlo area,Ontario. ral.51.353-380. Can.M ineral. 29. 48 1498.

FeRRv,J.M. (1976):Metamorphism of calcareoussediments in PuHau,D. & JoHANNES,W. (1974):Experimentelle Untersu- the Waterville-Vassalboroarea south central Maine: mi- chung der Reaktion Dolomit + Kalifeldspat + H2O = neral reactionsand graphical analysis.Attt J. J,ci.216, Phlogopit+ Calcit + CO2:ein Methodenvergleich. Contrib. 841-882. Mineral.Petrol. 8, 23-31.

Fonrry, N.J.& Benuos-SrupuENs,B. (1982): Barium silicates in strataboundBa-Znmineralization in the ScottishDalra- RrcE, J.M. (1977): hogressive metamorphismof impure dian.Mineral. Mag. 46,63-12. dolomitic limestonein the Marvsvilleaureole. Montana. Am.J. Sci.m,124. GAY,P. &Roy, N.N. (1968):The mineralogyof the potassium- bariumfeldspar series. III. Subsolidusrelationships. Mire- SEcNn,E.R. & GrLs, T. (1970):Reactions of kaolinitewitl ral. Mag.36,914F932. bariumcarbonate and barium sulphate. Aust. J. Ceram.Soc. 6,12-t8. Gutoorrt, C.V. (1984):Micas in metamorphicrocks. 1rl Micas (S.W.Bailey,el.). Rev.Mineral. 13, 357467. VrswANA'mAN,K. & KrrlsonN, H.M. (1983):Variations in the chemical compositions and lattice dimensions of INrtouat-r,M.M. (1982):Iz gisementZn-Pb-Cu de Kipushi. (Ba,K,Na)-feldsparsfrom Otjosondu,Namibia, and their Etude gdologiqueet mltallogdnique. T\dse de doctorat, significance.Am. Mineral. ffi, 112-121. Univ.Catholique de Louvain, Louvainla-Neuve, Belgrque.

- & OosrtnsoscH,R. (1974):Geologie et g6ochimiedu WTNKLER,H.G.F. (1965): Die Gencseder metamorphenGes- gisementde Kipushi, kifte. In Gisementsstratiformes et reize.Springer-Verlag, Berlin. provincescuprifhres (P. Bartholom6,ed.). Soci6t6 G6ologi- quede Belgique, Libge, Belgique (123-l&). -(1967): Petrogenesisof MetarmorphicRoclts (2nd ed.). Springer-Verlag,Berlin. LEFEBvRE,J.J. & Per-rrnsoN, L.E. (1982): Hydrothermal assemblagesof aluminium serpentine,florencite and kya- nite in the Zairean Copperbelt.Soc. G€ol. Belg., Awnles Received October II, 1991, revised manuscript accepted 105.sr-71. FebruaryI,1992.