American Mineralogist, Volume 80, pages833-840, 1995

An Fe analogueof kinoshitatite from the Broken Hill massivesulfide deposit in the NamaqualandMetamorphic Complex, South Africa H,c.nrwrcE. Fnr*rnror.,DnNNts Horrnu,uN, RoN T. Wltxrxs* Department of Geological Sciences,University of Cape Town, Rondebosch7700, South Africa JoHN M. Moonn Department of Geology, Rhodes University, Grahamstovvn6140, South Africa

AssrRAcr Micas with variable Ba content in the interlayer sites occur in silicate-rich bands within the high-grade, metamorphic banded iron formation enclosing massive sulfide bodies in the Broken Hill deposit, Namaqualand Metamorphic Complex, South Africa. In those layers that contain neither potassium feldspar nor muscovite, a green, barian (up to 17 wto/oBaO), trioctahedral mica occurs that is similar to kinoshitalite. In contrast to the originally describedkinoshitalite, however, this mica is Fe-rich (averageX." : 0.65), sug- gesting the existenceof an Fe analogue of kinoshitalite not described to date. Ion chro- matographicanalysis indicates that about 8o/oof the total Fe in this mica is presentas Fe3+, and a typicalformula is (Ba'rKooXFe3jMnorMgroAlorFe3lTiorXAl3osis')O2.(OH,8F22)' The unit-cellparameters were determinedas follows:a:5.383(2), b:9.328(8), c: 10.055(8)L, B : 100.44(5)",witt^ V : 496.5A', and Z : 2. The spacegroup is C2/m- The Ba-rich mica formed at the peak of metamorphism (Z : 670 + 20 "C, P : 4.5 t I .0 kbar) at a pH below the muscovite * potassium feldspar buffer, /o, buffered by quartz * fayalite + magnetiteand f",between 10-5 and l0-7.

Ixrnooucrrox In this paper we describethe mode of occurrenceof an Fe-rich equivalent to kinoshitalite and its compositional The banded iron formation interstratified with massive characteristicsas determined by electron probe micro- sulfide layers in the Broken Hill ore bodies near Aggeneys analysis,ion chromatography,and X-ray diffraction. The in the northern CapeProvince, South Africa, contains Fe- possibleorigin of this unusual and its bearing on rich mica with up to 17 wto/oBaO. Compositionally, this the composition of metamorphic fluids in the vicinity of mica is comparableto kinoshitalite,BaMgrAlrSirO'o(OlI)r, a massive sulfide deposit are discussed. but has considerable substitution of Fe for Mg (X', = phlogopite-annite 0.3). It appearsthat, similar to the se- Gnor,ocrc,{I, SETTING ries, solid solution exists betweenkinoshitalite and an Fe The Broken Hill ore bodies near Aggeneys,South Af- analogue of kinoshitalite. Kinoshitalite was defined by rica, represent a metamorphosed stratiform Pb-Zn-Cu- Yoshii et al. (1973a)as the Ba and Mg trioctahedralbrit- Ag deposit situated in the mid-Proterozoic supra- tle mica. Nearly end-member kinoshitalite was described sulfide sequenceof the Bushmanland Subprovince in the by Dasguptaet al. (1989),who also documenteda com- crustal Metamorphic Complex (NMC)' This se- plete solid solution between the K (phlogopite) and the Namaqualand quenceoverlies a2020 m.y.-oldbasementof augengneiss Ba (kinoshitalite) end-members.Kinoshitalite containing has at its base a thick seriesofleucogneisses, which considerableamounts of K was also reported by Solie and and have been interpretedas metarhyolite (Moore, 1989).The Su (1987),and potassickinoshitalite with high Mn con- seriesis succeededby metapelites(which are locally tents was describedby Yoshii et al. (1973b),Yoshii and basal highly aluminous), massivequartzite, iron formation, and Maeda(1975), and Matsubaraet al. (1976).Recently, the massive sulfide lenses.Above these lie a suc- X-ray structure of Ba-rich trioctahedral micas has been associated cession of metaconglomerates,gray gneisses,and am- refined by Brigatti and Poppi (1993), who also summa- phibolites. The amphibolites, which are tholeiitic in char- rized the previous literature on the diferent occurrences acter, have been dated at 1650 Ma (Reid et al., 1987). of barian micas in metamorphic and igneous rocks. The Peraluminous rocks intercalated within the metapelites Fe equivalent to kinoshitalite has not been reported to (Willner et al., 1990) and associatedB-rich exhalatives date. (Willner, 1992) are thought to have formed near hydro- thermal vents during an intermediate stageof rifting that Se- * Presentaddress: School of Applied Geology, Curtin Univer- characterized the deposition of the Bushmanland sity, GPO Box U1987, Perth 6001,Western Australia. quence.This event precededthe formation of the strati-

0003-o04x/95/070848 33$02.00 833 834 FRIMMEL ET AL.: Fe ANALOGUE OF KINOSHITALITE

form base-metalsulfide horizons, which have beenequat- Kanai (1990) for the determination of Fe2* and Fe3+, ed with Sedex-typedeposits (Ryan et al., 1986). with the Fe2+/Fe3+ratio determined by comparison with The ore bodies contain pyrrhotite, galena, sphalerite, standard rock powders of known Fe-oxidation state (le chalcopyrite, and pyrite in decreasing order of abun- Roex and Watkins, 1995).Fifty mg of mica, finely pow- dance. They are enclosed by alternating magnetite-rich dered under acetone,was dissolvedby gentleheating with and manganiferoussilicate-rich bands, with garnet- and a HF:HrSO. mix in a covered Pt crucible. The dissolved amphibole-rich layers occurring within the latter. The sil- sample was diluted with de-oxygenated,de-ionized water icate-rich bands contain the Ba-rich micas describedhere. and 50 pL injected onto a Dionex CS-5 separatorcolumn Steep geochemical gradients are preserved on a milli- coupled to CG-5 guard column. The sample was eluted meter scalebetween the iron formation and sulfide bands, with 6 mM pyridine-2 6-dicarboxylic acid (PDCA), and indicating that only very limited element transfer took the iron specieswere detected using a UV-VIS variable place acrossthe lithological boundaries during metamor- wavelengthdetector at 520 nm after postcolumn reaction phism (Frimmel et al., 1993).Accordingly, the geochem- with a broad metal complexing agent 4-(2-pyridylazo) istry of the iron formation bands is believed to be rep- resorcinol (PAR). Individual mica concentrateswere an- resentativeof the original depositional environment. The alyzed a minimum of three times until highly consistent silicate-rich bands have compositions similar to chemical results were obtained, thereby eliminating occasionalab- precipitates from modern submarine hydrothermal flu- errant values that may result from imperfect sample ids, and the garnet- and amphibole-rich bands are inter- preparation. The Fe2+/Fe3+ratio was measuredby direct preted as evolved and immature hydrothermal sedi- comparison of the ratio of the individual peak heights ments, respectively. The latter originated through rapid with those obtained from similar analysisof certified rock precipitation after exhalation, whereasthe former resided standardW-2 (le Roex and Watkins, 1995). longer in the seawaterbefore being deposited in more Powdered concentratesof the micas were analyzed at oxidizing environments distant from the hydrothermal the University of Cape Town, using standard X-ray dif- vents (Frimmel et al., 1993;Hoffmann, 1994). fractometry techniqueswith a CuKa radiation of 40 kV The Bushmanland Sequencewas subjected to upper and 30 mA. An automatic divergenceslit was used and amphibolite to granulite faciesmetamorphism during the countswere recorded for individual stepsof0.02.20 size l.l Ga Namaqualand metamorphic event (Waters, 1989). and a counting time of 2 s per step. Single crystals were In the Aggeneysarea, metapelites contain the equilibrium studied at the Institute of Mineralogy and Crystallogra- assemblagemuscovite * biotite * sillimanite * quartz phy, University of Vienna, using both single-crystalpho- + garnet + magnetite + potassium feldspar + graphite tography and a Stoe four-circle diffractometer AED2 and + sulfides. Coexistenceof muscovite + biotite + silli- graphite-monochromatic MoKa radiation. The operating manite * potassium feldspar + quartz, together with the conditions were as follows: 40 steps per reflection, in- absenceof migmatites, constrains the peak metamorphic creasedfor a, - a, splitting;0.03'and 0.5-2.0 s per step; P-Zconditions at 4.5 + 1.0 kbar and 670 + 20 "C. Gar- 2 x 4 steps for background measuremenl 3 standard net + biotite and garnet + cordierite thermometry yields reflectionseach 120 min; and 20^*:60o. Structurere- temperaturesthat are in good agreementwith these esti- finement was obtained with the SHELXL-93 packageof mates(Waters, 1989). programs.

ANlr.yrrc,Al pRocEDuREs PlntcnNnsrs Compositionsof the micas and associatedminerals were The mineral assemblagesin the iron formation at Ag- determined using a Cameca (Camebax) electron micro- geneys containing the Ba-rich mica are quartz + mag- probe at 15 kV acceleratingvoltage and 2 pm electron- netite + garnet + apatite + sillimanite + gahnite + sul- beam diameter. Mica analyseswere double-checkedwith fides, with amphibole * olivine + the electron beam defocusedto 5 pm. The counting time additionally present in amphibole-rich layers. The Ba- was l0 s, except for Ba, Zn,F, and Cl, which were count- rich micas occur in textural equilibrium with quartz, ed for 30 s. The standards used in the analysis of the magnetite,garnet, and amphibole. The garnetsare essen- micas were natural fluorite (F), scapolite (Cl), chromite tially solid solutions between almandine and . (Cr), hornblende (Na, K, Si, Al, Fe, Mg, Ca), synthetic Considerablevariation in X.o.exists between different rock (Mn), rutile (Ti), and Ba-Si glass(Ba). Con- types and also within individual iron formation bands. centrations were corrected by ZAF, except for Fe-rich There is a steepgradient from high X.o" (up to 62 molo/o) analyses,which were corrected following the method of in the iron formation bands close to the massive sulfide Bence and Albee (1968). Total Mn and total Fe were bodies to low X.o" (down to 9 molo/o)in the femrginous computed as MnO and FeO, respectively. quartzites and pelites further away from the ore lenses. Mineral concentrateswere obtained by use of a mag- Similar variations were found in the compositions of the netic separator and final hand-picking under the micro- amphiboles, most of which are grunerites with variable scope. Fe2+/Fe3+ratios were determined on pure mica amounts of Mn and Mg. Those in the amphibole-rich concentratesusing a Dionex 4000i ion chromatograph. iron formation bands have the highest Mn contents,with The method used was a modification of that describedby an average formula of Nao,rMno ,oFeo2-fMg, oFeflj,(Alor,- FRIMMEL ET AL.: FC ANALOGUE OF KINOSHITALITE 83s

TrEle 1, Representativeelectron microprobe analyses of biotite Tlele 2. Representativeelectron microprobe analyses of mus- from variousrock typesaround the massivesulfide covitefrom variousrock typesassociated with mas- bodies sivesulfide bodies

Samole HFN40 HFN41 HFN44 HFN45 HFN47 TS12 TS3 Sample HFN41 HFN44 HFN45 HFN47 HFN46 sio, 34.78 37.90 32.38 34.94 36.16 40.74 40.23 sio, 43.92 44.23 45.20 43.71 44.31 Tio, 4.01 2.03 3.43 2.09 0.84 0.17 0.00 Tio, 1.68 0.99 1.23 1.54 1.53 Alros 19.24 19.85 17.24 20.26 16.37 12.66 15.12 Al203 32.75 33.79 34.08 34.68 34.15 0.00 0.00 0.00 C12Og 0.00 0.00 0 00 0.00 0.00 0.00 0 00 Cr.O. 0.00 0.00 't.74 FeO' 24.29 13.19 29.82 2481 14.82 11.67 14.20 FeO' 1.41 5.32 2.50 1.88 MnO o.24 1.40 0.62 0.00 0.27 0.48 0.71 MnO 0.00 0.00 0.00 0.12 0.13 Mgo 5.61 13.18 4 55 5.77 15.62 19.25 15.94 Mgo 1.70 1.04 0.66 1.98 2.02 CaO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 CaO 0.00 000 0.00 0.00 0.00 Naro 0.16 0.13 0.00 0 20 0.26 0.29 0.28 Na2O 0.32 0.00 0.33 0.19 0.19 K.o 9.23 6.57 7 30 9.24 8.08 8.92 8.00 KrO 9.86 6.14 10.34 4.62 4.61 F 0.00 2.32 0.00 0.00 3.72 5.92 4.90 F_ 0.00 0.00 0.00 1.65 1.80 0.13 0.00 0.00 0 00 0.00 0.00 0.00 0.03 000 0.00 0.00 0.00 BaO 0.29 0.97 2.25 0.45 3.72 0.00 0.00 BaO 1.96 3.76 o.54 5.56 4.89 SrO 0.18 0.20 0 18 0.19 0.17 0.00 0.00 SrO 0.26 0.25 0.25 0.19 0.26 Total 98.13 96.76 97.77 97.95 98.46 97.61 97.32 Total 93.88 95.52 95.13 95.29 95.01 Si 5.29 5.54 5.16 5 32 5.48 6.02 5.95 JI 6.06 6.04 6.11 5.98 6.05 rltAl 2.71 2.46 2.84 2.68 2.52 1.98 2.05 rrtAl 1.94 1.96 1.89 202 1.95 16lAl 0.74 0.96 0.39 0.96 0.40 0.22 0.58 r6tAl 3.39 3.48 3.54 3.57 3.55 r6tTi 0 46 0.22 0.41 0 24 0.10 0.o2 0.00 rorTi 0.17 0.10 013 0.16 0.16 te'. 3.09 1.61 2 07 e 1A 1.88 1.44 1.76 F€F+ 0.16 0.61 o.28 0.20 0.2'l Mn2+ 0.03 0.17 0 08 0.00 0.03 0.06 0.09 Mn2+ 0.00 0.00 0.00 0.01 0.02 Mg 1.27 2.87 1.08 1.31 3.53 4.24 3.51 Mg 0.35 o.21 0.13 0.40 0.41 K 1.79 1.23 1.48 1 80 1.56 1.68 1.51 K 1.74 1.O7 1.78 0.81 0.80 Na 0.05 0.04 0 00 0.06 0.08 0.08 0.08 Na 0.09 0.00 0.09 0.05 0.05 Ba 0.02 0.06 0.14 0.03 0.22 0.00 0.00 Ba 0.11 o.20 0.03 0.30 0.26 Sr 0.02 0.o2 0.02 0.02 0.02 0.00 0.00 Sr o.02 o.o2 0.02 0.02 0.02 F 0.00 1 07 0.00 0.00 1.78 2.77 2 29 F 0.00 0.00 0.00 0.71 0.78 CI 0.03 0.00 0.00 0.00 0.00 0.00 0.00 cl 0.01 0.00 0.00 0.00 0.00 Nofe.' HFN40and HFN45: pelite,HFN41 and TS12 : massivesulfide Note: for rock types see Table 1. HFN46 : massivesulfide rock. Totals ore, HFN44 and HFN47 : garnet-quartzite,TS3 : banded iron formation correctedfor F, Cl: O equivalency. 'All Normalizationbased on 22 Oqu*r*,1totals corrected for F, Cl : o equiv- Fe reportedas FeO. atency. 'All Fe reoorted as FeO.

covite, given in Table 2, are of samples coexisting with FefllrSi,u,OrrXOH)r. Olivine is restricted to amphibole- the biotite listed in Table l. IncreasedBa, Sr, Ti, and F rich iron formation bands and is of the knebelite variety, contents in the biotite are similarly reflected in elevated with an averageof 82.8 molo/ofayalite, 16.3 molo/ote- concentrations of the same elements in the coexisting phroite, and 2.9 molo/oforsterite. Orthopyroxene is man- muscovite. The latter contains up to 5.6 wto/oBaO, 0.3 ganoan ferrosilite with an average formula of wt0/oSrO, and up to 1.7 wto/oTiOr. Few samplesare rich Fe,ouMnorrMgo26Al00rsir nnOuoo. It occurs rarely in the in Ba and contain both muscovite and biotite but no amphibole-rich bands, where pyroxferroite of average potassium feldspar.In thesefew samples,the distribution composition Feo.nrMnorrMgoilSi2ooo6oois more com- of Ba betweenmuscovite and coexistingbiotite is regular mon. The gahnite is ferroan and its composition depends (Fig. l). Partition coemcientsdescribing the fractionation on the host rock: Xo o: 0.46-0.69,Xs.: 0.19-0.49, of Ba and K between muscovite and biotite were calcu- and Xser: 0.02-0.05. The Zn content of the spinel de- lated: Ko [:@a/K)B,/@a/K)'.)] ranges between 0.38 in creasesabruptly away from the ore bodies. garnet quartzite and 0.77 in massive sulfide ore. F con- Biotite with light- to dark-brown pleochroism and col- tents are below the detection limit (0.16 wto/o)in most orless muscovite may be distinguished within the mas- samples,but up to 1.8 wto/oF was found in muscovite sive sulfide bodies and enclosingbanded iron formation, from a massive sulfide lens that contains no biotite. quartzites, and pelites. Representativeanalyses of biotite A third, greenvariety of mica occurs exclusively in the from the various rock types are given in Table l. The silicate-rich bands within the iron formation, where it is biotite contains up to 3.7 wto/oBaO and 0.2 wto/oSrO. localized particularly in association with garnet-rich bands. The F content rangesfrom below the detection limit (0.16 This mica forms subhedrallaths up to 0.2 mm long. It is wto/o)to 5.9 wto/0.Cl contents are below the detection characterizedby a high BaO content of up to 17.0 wto/o limit (0.025 wto/o)except in sample HFN40, a pelite, with and is describedin detail below. 0. 13 wto/oCl in the biotite. Variation in the intensity of oF THE Ba-RrcH MrcAs color and pleochroism is largely becauseofa variation in CrHnl,crnmsrrcs TiO, contents,which range from below the detection lim- The Ba-rich mica variety displays, like all other micas, it (0.04 wto/o)to 4.0 wto/o. a perfect {001} cleavage.The grains are strongly pleo- Muscovite is restricted to the massive sulfide bodies, chroic (X: grassgreen, I: dark brown green,Z: dark pelites, and quartzites. Representativeanalyses of mus- greenishgray-brown); absorption is X << Z < Y; and the 836 FRIMMEL ET AL.: Fe ANALOGUE OF KINOSHITALITE

1 I Fe analogueof kinoshitalite i 0.2 0.8 j kinoshitalite lal I o 0.6 i^ o '^t (g lf$-lt d) i^- r 0.1 X' a 0.4

biotite 0.2 r= 0.996 irl 0 0 0.1 0.2 0.3 oo' ^ ^.f g"muscovtte 0.2 0.4 0.6 0.8 1

Fig. l. Atomic contentof Ba in muscowiteplotted against XTuz* thatin biotite.The data points represent average valuesobtained Fig. 2. Xsuvs. Xr",* diagramfor biotiteand the bariantrioc- for samplesthat containboth micasin texturalequilibrium. tahedralmicas analyzed.Note that most of the latter have X".z*valu€s >0.5 and aretherefore described as Fe analogueof refractive index B : I .680. The strong characteristiccolor kinoshitalite. of this mineral inhibits the determination of the birefing- ence in a thin section, but it appears similar to biotite. of Bal(Ba The optic characteris biaxial negative.Dispersion is very the + K + Na) ratio, which is as high as 0.73, this mica weak, and 2 Z" is around 20'. Representative composi- approachesthe composition of kinoshitalite. It contrasts, however, tions of this mineral are given in Table 3. On the basis with originally described kinoshital- ite (Yoshii etal., 1973a),a Mg-phase,by having X." [:Fe/ (Fe + Mg)l values of up to 0.72 (Fig. 2). Thus rhe mica TABLE3. Representativeelectron microprobe analyses of Ba- can be describedas a solid solution betweenkinoshitalite richbiotite and Fe analogueof kinoshitalite and an Fe analogue of kinoshitalite. Most of the green Sample HFN42 HFN42 HFN43 HFN43 189113 TS8 micas describedhere have Xr. > 0.5 and are therefore Fe analoguesof kinoshitalite. sio, 28.54 28.04 28.72 29.98 28.49 30.43 Tio, 1.72 2.74 2.43 The Fe analogueof kinoshitalite from Aggeneysis not 't4 2.34 1.40 1.79 Al2oe 32 14.05 15.32 15.24 15.71 15.93 only more ferrous but also less aluminous than kinoshi- Cr2O3 0.00 0.00 0.00 0.00 0.00 O.OO Fe,O" 2.14 2.21 2.32 2.31 2.12 2j5 talite, with Al:Si of approximately 0.6 rather than 1.0 in FeO 22.20 22.84 23.96 29.86 21.98 22.28 kinoshitalite. The Al-free end-member of the correspond- MnO 1.14 1.53 1.09 1.16 1.33 0.41 ing solid-solution series is anandite, BaFe.(Si,Fe)o- MgO 7.42 5.77 6.34 6.59 7.81 8.37 CaO 0.00 0.00 0.00 0.00 0.OO O.O0 (O,OH).0(OH,S,CI).An additional substitution found in Na"O 0.20 0.18 0.23 0.20 0.06 O.3O the Fe analogueof kinoshitalite from Aggeneysis that of K.O 3.20 3.08 3.45 3.87 1.94 3.20 F for OH. With F concentrations of up to 4.1 wt0/0, F- 3.70 2.91 2.19 2.45 4.02 3.38 the o- 0 00 0.00 0.00 0.00 0.oo 0.oo F/(F + OH) ratio is as high as 0.6. The F contenr can BaO 13.90 14.25 13.73 12.66 16.95 12.99 vary considerablybetween I and2.6 F atoms per formula SrO 0.14 0.00 0.00 0.00 0.OO O.OO unit. The range Total 97.06 96.37 98.86 99.63 100.09 99.79 in F content in the barian biotite from si 5.09 5.07 5.00 5.12 4.98 5.13 the surrounding rock types is even greater(between 0 and rrrAl 2.91 2.93 3.00 2.88 3.02 2.87 3 F atoms per formula unit) with no apparent correlation rrTi 0.00 0.00 0.00 0.00 0.00 o.oo r6rAl 0.10 0.06 0.14 0.19 0.21 0.29 betweenF and Ba (Fig. 3). Poor negative correlations are rcrTi o.23 0.37 0.32 0.30 0.18 0.23 evident between F and Xr", and F and I6lAl (Fig. 4). The Fe3+ 0.10 0.11 0.11 0.11 O.1O O.1O former accordswith the well-establishedF-Fe avoidance Fe2+ 3.31 3.45 3.49 3.41 3.21 3.14 Mn2+ 0.17 0.23 0.16 O.17 0.20 0.06 rule (Valley et al., 19821,Guidotti, 1984), and the latter Mg 1.97 1.55 1.64 1.68 2.09 2.1O confirms a similar F-t6tAlavoidance already suspectedby K 0.73 0.71 0.77 0.84 0.43 0.69 Guidotti (1984). Na 0.07 0.06 0.08 0.07 0.02 0.10 The low degree of correlation can be Ba 0.97 1.01 0.94 0.85 1.16 0.86 explained by the influence ofother variables, notably the Sr 0.01 0.00 0.00 0.00 O.0O 0.00 X.. of the whole rock, and which varied con- F 2.09 1.66 1.21 1.32 2.22 1.80 .fo, f",, J;F, siderably between the different rock types (Frimmel et Note: Fe3+lFe* ratio as derived from ion chromatography.Normaliza_ al., 1993). tion based on 22 O*,,,*; totals corrected tor F, Cl : O equivalencv. A series of exchangecomponents can be constructed FRIMMEL ET AL.: Fe ANALOGUE OF KINOSHITALITE 837

1.5 1 r = 0.371 r = '0.644 0.8 |' l al ^a.$ a ^l a tl^a- ^ {+, 0.6 rlA f1r^^^ A ^ rA (6 4 lll'

d) l I". X* ^4a

0.4 A .t 0.5 A tt.t'a 0.2 a r^A AAA 0 A 0 2 3 2 3 1.5 F r = -0.549 Fig. 3. Atomic contentof F plottedagainst Ba in Ba-poor and in Ba-richbiotite and an Fe analoeueofkinoshitalite from the BrokenHill deposit.

^ for the barian biotite and for kinoshitalite (Tracy, l99l). I ^^^^ The exchangevectors include the interlayer, coupled with tetrahedral,substitutions (1) BaAIK_rSi_r, (2) BaFe3+- -<(O ^f K-,Si-,, (3) NaK ,, (4) SrAIK-,Si-,; octahedralsubsti- ^{ tutions (5) FeMg-,, (6) TiAlrMg_,Si_r, (7) Rl+trMg ,, 0.5 (8) Rl+Mg ,Si ,; tetrahedralsubstitutions (9) Fe3+Al_,, ^^A i ^^ and the anion substitution (10) F(OH)-r. The exchange A vectors (l) and (3) are supportedby the sympatheticvari- , .l^ ation of (K + Na + Si) with (Ba + t4rAl)(Fig. 5). The , i^^ A rl barian micas from Aggeneysdo not show good correla- ^i tion betweenBa and A1,.,(Fig. 64,) as describedby Tracy 0 (1991) for barian micas from the Franklin Marble, New 0 Jersey. The micas in this study, however, show strong F correlationbetween [K+ + 3(Mg,Fe)2++ 3Si4+]and (Bar+ Fig.4. Atomic contentof F plottedagainst X." /(Fez+ -0.987) l:Fe2+ + 2Ti4+ + 3Al3+)(r: (Fig. 68), which supports + Mn + Me)l (top) and t6rAlOottom) of the biotite from the the combination of the exchange vectors (l), (6), and BrokenHill deposit.The poor but noticeablecorrelations suggest Titr(Fe,Mg) , as proposedby Mansker et al. (1979).These not only an Fe-Fbut alsoan I6lAl-Favoidance. combined substitutions introducing Ba should increase A1,",,but this is not supported by our data (Fig. 6A). An additional substitution, such as Fe'z+Fel+SiMg-rAl ,, re- site allocation, there is no necessityfor any Fe to be pres- flecting the kinoshitalite-ananditejoin, is required to ex- ent as Fe3+.We attempted to quantify the Fe3+/Fe2*ratio plain our data. in representativebiotite and samplesof the Fe-analogue Using Mdssbauer spectroscopy Guidotti and Dyar of kinoshitalite using ion chromatographic analysis. Re- (1991) determined that biotite from different silicate min- peated analysis ofessentially Ba-free biotite from pelites eral assemblagesand metamorphic gradeshas 8 t 30/oof (samplesHFN40 and HFN45, Table l) revealed that 6 Fe., as talFe3+,and that in biotite coexisting with mag- and 4o/o,respectively, of Fe,o,occurs as Fe3+.The results netite l0-130/oof Fe,o,occurs as t6lfs3+, whereas in biotite for the Fe analogueof kinoshitalite (sample HFN42, Ta- coexistingwith graphite 40loof Fe,o,occurs as I6lFe3+.New ble 3) indicate that, on average,8o/o of Fe,o,is present as data reported by Swope et al. (1994) indicate, however, Fe3*. This reflectsa degreeofFe-oxidation that is in good that t4lFe3+contents are negligible, and thus quoted Fe3+ agreementwith the results recently obtained on synthetic contents are essentiallyI6lFe3+ contents. Magnetite is sta- annite (Rancourt et al.,1994). Taking this Fe3+/Fe2+ra- ble over a wide range of fo,. The micas of the present tio as representativefor the Fe analogueofkinoshitalite, study locally coexist not only with magnetite but also a typical formula for this mica would be (Ba' r$.o)- with sulfides.On the basis of charge-balancecriteria and (FeljMno rMg, oAlo,Fefr| Tio r)(Al3 0Si5 0)O,o (OH' EFr,r). 838 FRIMMEL ET AL.: FE ANALOGUE OF KINOSHITALITE

8 I r= -0.975 A A t^ l 4 @ 7 6 + -' o I (d z i ri{i^ ^ + 3 4 \< 6 r = 0.042 0.5 1.5 5 Ba 2 3 4 5 talAl 14 Ba + B r4rAl + Fig. 5. Ba + plottedagainsr K + Na + Si of the Ba- I rich and Ba-poormicas from the BrokenHill deposit. 'E 12 5 X-ray diffraction of a powder comprising 990/opure Fe analogueof kinoshitalite yielded six well-defined reflec- ;10 tions and six poorly defined reflections. The main peaks ^,\r represent 001 basal reflections becauseof the preferred tr $r orientation of the individual particles in the sample. The +g rl 00/ peak positions and relative intensitiesmatch very well (E {^ polytype positions (n with those of the 2L[, of anandite. The -0.987 ofthe weak Jkl, k0l, and hkl reflections,however, are not r= { in agreement with those of anandite and match better 6 those of kinoshitalite. Single-crystalphotography of five 2I 30 32 34 36 38 selectedgrains yielded very consistent results, which in- dicate a lM structure for this mica. The refined unit-cell K+3(Mg,Fe2*)+3Si parametersfrom a total of 3075 measuredreflections are Fig. 6. (A) Al., vs. Ba diagram,(B) tK + 3(Mg,Fe'?+) + 3Sil as follows:a: : 5.383(2),b: 9.328(8),c 10.055(8)A, plottedagainst [Ba + 2Ti + 3(Al,Fe3+)]of the Ba-richand Ba- P : 100.44(5)",wirh v: 496.5A,, and Z : 2.The space poor micasfrom the BrokenHill deposit. group choice is C2/m.

ney edificeson seafloorbasalts. In addition to the precip- GnNnsrs oF Ba-RrcH MrcAs itation as barite, Ba may be concentratedby substitution The geochemistry of the silicate-rich iron formation into clays and zeolitesand by adsorption into Fe-Mn ox- bands hosting the Ba-rich micas is consistent with a hy- ide particles.Additional terrigenousBa could be contrib- drothermal to hydrogenousorigin (Frimmel et al., 1993; uted by detrital Ba-bearing feldspars. Hoffmann, 1994). The most likely depositional environ- During diagenesisand subsequentmetamorphism, de- ment for the banded iron formation and the associated trital feldspars, clays, and zeolites become unstable. Ba Sedex-typemassive sulfide bodies is a rift-related basin. presentin thesephases is releasedand re-incorporated in Evidence for this comes from the relative thinness of the metamorphic , such as micas and feldspars.The supracrustal sequence(<3 km), the preservation of un- most important Ba precursor mineral, particularly in a disturbed millimeter scale banding indicative of a quiet hydrothermal environment, is believed to be barite. Bar- depositional environment, and the geochemistry of the ite can remain stable up to granulite facies metamorphic mafic rocks in the sequence(Reid et al., 1987).Ba en- conditions (Grew et al., l99l), dependingupon the pre- - richment in metalliferous sediments on the seafloor has vailing fo, .f",conditions. been documented in numerous areasin proximity to hy- The alternation ofFe-oxide- and Fe-sulfide-rich layers drothermal activity, such as oceanicridges. Haymon and documents the preservation of geochemical gradients with Kastner (1981), for instance, reported particulate barite respect to fo, - i,. Systematic variations in the com- togetherwith amorphous silica from white smoker chim- position of garnet, amphibole, olivine, and micas across FRIMMEL ET AL,: Fe ANALOGUE OF KINOSHITALITE 839 the massive sulfide bodies and their surroundings also T = 670 oC,P: 4.5 kbar indicate that the exchangeof elements such as Fe, Mg, and Mn across lithological boundaries was minimal de- spite the high grade of metamorphism (Frimmel et al., 1993). Green, coarse-grained,Pb-bearing amazonite is present only locally, along contact zones a few centime- ters thick between metapelite and massive sulfide hori- zons. This indicates a strong gradient in /,, between sul- fide and silicate layers. -5 Steep gradients in /," are documented by the highly aC! variable F contents in micas and apatite. Following Gun- ow et al. (1980), logfi",J was calculated o) f"r) on the basis o of the F intercept value for biotite. Similarly, log(f"*/ fi.) was calculatedfrom apatite using the thermodynam- _10 ic data of Zhu and Sverjensky( I 99 I ). For the surround- ing metapelites,these values are in the rangebetween 3.5 and 4.3, which is in agreement\rrith the findings in am- phibolite-granulite transition zones elsewhere(Yardley, fayalite 1985;Nijland et al., 1993).Within the bandediron for- mation, dramatic variations in ffi,o/ f.r) were found with -25 -20 -15 -10 /"" being increasedby up to four orders ofmagnitude. The occurrenceofthe Fe analogueofkinoshitalite de- log/9, scribed here is confined to iron formation bands for which Fig. 7. Log - log diagram showing the stability of anomaloushigh valueswere obtained.The samebands fo, "G, "G, pyrite, pyrrhotite, fayalite, magnetite, and hematite (+ quartz) lack potassium feldspar,which can be explained by a low in the Fe-O-S-SiO, system at the peak metamorphic conditions pH in the peak metamorphic fluid, as indicated by the estimated for the Broken Hill deposit. The hatched area indi- high L.. In the absenceof potassium feldspar, the micas cates the most likely conditions for the formation of iron ki- became the dominant sinks for Ba. Thus it appearsthat noshitalite. Thermodynamic standard-statedata used are from the major factors controlling the formation of the Ba-rich Johnson et al. (1992\. micas include original whole rock composition, pressure, temperature, gH, .fo,, and f",. The pH must be below the muscovite * potassium feldspar bufer, which is around CoNcr,usroNs 5.8 at the given P-Zconditions (4.5 kbar and 670'C) and The compositional characteristics of the micas from ar- : 0.1. Within the horizon containing the Fe analogue the Broken Hill deposit suggestthat the group of trioc- of kinoshitalite, log fo, is buffered at - 17.3 by quartz * tahedral brittle Ba-bearing micas comprises not only ki- fayalite + magnetite (Fig. 7). The predominance of pyr- noshitalite and anandite but also an Fe-analogueof ki- rhotite in the associatedmassive sulfide bodies indicates noshitalite.Complete solid solution seemsto exist between log /1, < - I in the sulfide horizons. Only locally was the kinoshitalite and the Fe-analogueofkinoshitalite and pyrrhotite found together with pyrite in the massive sul- their respective F-bearing equivalents. The gap between fide bodies. The breakdown of barite in the silicate layers the data fields for the less barian and more barian micas may be approximated by the reaction 2baiIe in Figures 3 and 6,4, suggeststhe possible presenceof 2BtJ^,* + 52 + 40, for which log K was calculated as immiscibility between the phlogopite-annite series and -64.8 at the given P-Z conditions,on the basis of the the join between kinoshitalite and its Fe analogue. thermodynamic data of Johnson et al. (1992). The Ba2+ Ba-rich micas from both igneous and metamorphic activity is directly proportional to jl,, and a minimum rocks have been described.These include manganoanva- log /!, of -7 can be calculated for an assumed a*,* of rieties in Mn-oxide-rich rocks from the Noda-Tamagawa 0. l. Thus the ambient log /., in the silicate layers con- mine (Yoshii etal.,1973b),from Hokkejino, Japan(Mat- taining the Fe analogueof kinoshitalite must have been subaraet al., 1976),and from the SausarGroup at Netra, - -7 between 4 and at Ihe /o, conditions outlined above India (Dasguptaet al., 1989).In the latter example,ki- (Fie. 7). noshitalite occurs in rocks that have experiencedmeta- Ba partitions into silicate phasesin the following se- morphism (650 "C, 6 kbar) similar to that of the rocks quence:potassium feldspar > muscovite > biotite. Pro- described here, but Dasgupta et al. (1989) ascribed the vided the pH is below the muscovite * potassium feld- formation of kinoshitalite to the infiltration of postme- spar buffer, a further requirement for the formation of tamorphic pegmatitic and CO'-rich hydrothermal fluids. the Fe analogue of kinoshitalite is the absenceof mus- Solie and Su (1987) describedMn-poor kinoshitalite from covite. Becauseofthe preferential partitioning ofBa into contact metamorphosed calcareousmetasedimentary muscovite, Ba contents in any coexisting biotite would rocks in the Alaska Range and also concluded that ki- be lower, probably too low for the formation of a kinosh- noshitalite formed at high temperatures (>600 "C) but italite. not necessarilyat high pressure.It thus appearsfrom these 840 FRIMMEL ET AL.: Fe ANALOGUE OF KINOSHITALITE

studiesand the presentwork that the formation of barian in nephelinitesfrom Oahu, Hawaii. American Mineralogist, 64, 156- micas in metamorphic rocks is favored by high temper- I 59. K., Matsuo, (1976) The atures(>600 The role of pressureremains unknown. Matsubara, S., Kato, A., Nagoshima, and G. "C). occurrenceof kinoshitalite from Hokkejino, Kyoto Prefecture,Japan. Grapes(1993) explaineda Ba-enrichmentof muscovite Bulletin of the Natural (National) ScienceMuseum, SeriesC (Geology), in Ba-rich metacherts by higft /i,, as indicated by the 2, 7 t-78 presenceof sulfidesand a correspondinglylow oxidation Moore, J.M. (1989)A comparative study of metamorphosedsupracrustal state. Our results confirm this trend also for barian bio- rocks from the westernNamaqualand Metamorphic Complex. Bulletin ofthe Precambrian ResearchUnit, 37, 370 p. University of Cape Town, tite, kinoshitalite, and its Fe analogue. Cape Town, South Africa. Nijland, T.G., Jansen,J.B.H., and Maijer, C (1993) Halogen geochem- AcxNowr.BocMENTS istry of fluid during amphibolite-granulite metamorphism as indicated The results presentedhere emergedfrom a project that was funded by by apatite and hydrous silicatesin basic rocks from the Bamble Sector, Gold Fields of South Africa. The authors are grateful to the following for SouthNorway. Lithos, 30, 167-189. their cooperation and support in this study: F. Pertlik, University ofVi- Rancourt, D.G., Christie, I.A.D., Royer, M., Kodama, H., Robert J.-L., (1994) l'lFer*, enna, for single-crystalphotography and diffractometry; R S. Rickard for Lalonde, A.E., and Murad, E. Determination of accurate t6lFe3+, t6lFe2+ populations microprobe assistance;and S. Horwood for help with the ion chromato- and site in synthetic annite by Mrissbauer graphic analysis.Constructive reviews by R.F. Dymek and E. Grew great- spectroscopy.American Mineralogist, 79, 5 l-62. ly improved the manuscript. 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