CanadianMineralogist Vol. 20, w. 189-202(1982)

STILPNOMELANE,IRON.RICH , K.FETDSPARAND HORNBLENDEIN BANDEDIRON.FORMATION ASSEMBLAGES OF THEDATES GORGE fuIEMBER,HAMERSLEV GROUP, WESTERN AUSTRALIA

TAKASHI MTYANO* Department of Geology, Indiana University, Bloomington, Indiana 42405, a.S.A,

AssrRAcr b, 4l% de FeO (fer total exprim6 en FeO). Des fragments angulaires de hornblende enrob6s de The banded iron-formation of the Dales Gorge sont dispers6s dans des lits riches en Member, Hamersley Group, Western Australia, stilpnom6lane et mica. Le feldspath potassique, de contains several Al-bearing silicates: stilpnomelane, composition presque id6ale, se pr&ente en sph6rolites Fe-rich mica, K-feldspar and hornblende. The mica accompagn6s surtout de stilpnom6lane et d'ank6rite. is flaky, to tabular, fine- to medium-grained, and Les textures montrent que les silicates iL K et Al, commonly coexists with stilpnomelane, riebeckite. ainsi que la riebeckite, sont des produits d'un m6ta- quartz, magnetite, hematite and ankerite. It contains morphisme d'intensit6 trbs faible (ou de diagenAse about 1 to 6 wt.Vo A12O. and 29 to 4l wt.Vo FeO tardive) et que mica et riebeckite se sont form6s (total iron as FeO). Angular fragments of horn- tard et, par endroitg aux d6pens de la stilpnom6lane. blende rimmed by riebeckite are dispersed in stilpno- Elles font aussi supposer que I'aluminium ainsi lib6r6 melano-mica bands. K-feldspar with nearly ind- aurait 6t6 pr6cipit6 sous forme de mica ferrifdre i member composition occurs in spherulites associated proximit6 des zones i riebeckite. La pr6sence de with stilpnomelane and ankerite. Textural relations riebeckite et de mica ferrifdre refl6terait I'interaction show that the K-Al silicates and riebeckite are verv de solutions alcalines avec les min6raux d6ji form6s. low-grade metamorphic (or late diagenetic) La formation des mindraux i K et Al pourrait bien products, mica and riebeckite having gen- refl6ter le potentiel cbimique 6lev6 du potassium erally formed late and locally replacing stilpno- plut6t que la temp6rature du m6tamorphisme. Ceci melane. Textures also suggest that the Al so released imp.liquerait que l'homogdn6isation de l'activit6 de may have been reprecipitated as Fe-rich mica close K+ n'a pas 6t6 atteinte dans les formations de fer to riebeckite-rich zones. The occurrence of riebeckite et que les associations stables des silicates K*Al and Fe-rich mica may reflect tlle interaction of doivent leur stabilit6 i un Equilibre local. alkali-bearing solutions wlth earlier minerals. The formation of the K-Al minerals may have been (Traduit par la R6daction) cogtrolled by the magnitude of the chemical poten- tial of K+ rather than the temperature of metamor- Mots-cl6s: m6tamorphisme de trEs faible intensit€, phism. This implies tlat homogenization of a6* formation de fer ruban6e, silicates I K et b Al, was not attained in the iron formations and that mica ferrifdre (ferriannite), membre Dales Gorge, stable associations of the K-Al silicates may have Australie occidentale, 6quilibre local. been maintained through local equilibrium. INrnouucrtoN Keywords: very low-grade metamorphism, banded iron-forrnatiou, K-Al-bearing silicates, Fe-rich The bulk chemistry of Precambrian banded mica (ferriannite), Dales Gorge Member, West- ern Australia, local equilibrium. iron-formations is dominated by iron and silica. The content of alumina is generally less than (Na,O Sorurvrernn I wt. Vo and that of the alkalis and KuO) may be - virtually zero (James & Sims La formation de fer rubann6e du membre Dales 1973, Gole & Klein 1981). Al-bearing silicates Gorge, groupe de Hamersley (Australie occiden- in low-grade metamorphic iron-formations are tale) contient plusieurs silicates alumineux, stilpno- thus relatively rare. Stilpnomelane is the most m6lane, mica ferrifire, feldspath potassique et horn- common and abundant silicate, but chamosite, blende. Le mica, de facies feuillet6 i tabulaire. de Al-, Fe-chlorite and albite have also grain fin i moyen, coexiste d'ordinaire avec stilpno- been reported from iron formations (e.g., m6lane, riebeckite, quartz, magn6tite et ank6ri6. il Dimroth & Chauvel L973, Floran & Papike contient de I i 6Vo (en pids) de AlrO" et de 29 1975, 1978, Klein & Fink 1976, Irsher 1978, Gole 1980, Klein & Gole 1981). EPresent address: Institute of Geoscience, The Uni- Banded iron-formation (BIF) of the Dales versity of Tsukuban Ibaraki 305, Japan. Gorge Member, Brockman Iron Formation in 189 190 THE CANADIAN MINERALOGIST the Hamersley Range, Western Australia" which it is intimately associated with stilpnomelane was subjected to very low-grade metamorphism laths. An occurrence similar to this assemblage (Miyano 1976b, 1978b), is somewhat different will be described here. Grubb (1971\ reported in mineralogy and petrography from other iron and chlorite from the Dales Gorge formations. Member, but his ocsturense is confirmed by This paper deals with the mineralogy and neither compositional nor X-ray data. Normal petrography of silicate-bearing bands of BIF, . muscovite and chlorite were not noted with special emphasis on the evaluation of tex- in this study. If present, they must be very tural relations and compositional variations of scarce. Figure 1 shows all Al-bearing silicates unusual assemblagessuch as stilpnomelane-Fe- found in this study. mica, stilpnomelane--K-feldspar, Fe-rich mica- riebeckite and riebeckite-hornblende. Most sam- Stilpnomelane ples were taken from drill cores of BIF of the upper Dales Gorge Member at the Colonial LaBerge (1966), Trendall &Blockley (1970), Mine in Wittenoom. Grubb (1971), Ayres (1972) and Miyano (1976a) have all given petrographic descrip- Ar-BsaRrNc STLIcATES tions of stilpnomelane from the iron formation of the Dales Gorge ltIember. Where stilpno- Only small amounts of Al-bearing silicates melane is abundant in either a micro- or meso- occur in the BIF of the Dales Gorge Member band, it has a fine grained, acicular habit, and owing to a low content of alumina, averaging the grains form massive aggxegatesor networks 0.46 wt. Vo Al"Os (Trendall & Blockley l97O). of interlocking laths (Fig. 2A). The most com- Stilpnomelane, the most abundant Al-bearing mon mineral assemblage consists principally silicate (Grubb 1971, Trendall & Blockley 1970, of stilpnomelane * ankerite -1-.iron oxide,r * Ayres 7972, Miyano 1976a), may fix almost quartz (Fie. 2B), which may be associated all aluminum and potassium in the BIF (aver- with pyrite and siderite, but rarely with Fe-rich age O.l2 w|, Vo KzO: Trendall & Blockley mica, riebeckite, pyrrhotite or K-feldspar. 197O). Other Al-bearing silicates from the Generally the association with Fe-rich mica or BIF, which are usually associated with stilpno- riebeckite is restricted to edges of adjoining melane, have been reported by several authors. riebeckite-rich bands. Quartz and hematite are Ayres (L972) described biotite, but without scarce or absent in relatively thick stilpno- compositional data, from several mesobands melane mesobands. In carbonate-rich (especial- (thickness ranging from 2 to 150 mm), where ly ankerite-rich) mesobands, stilpnomelane

Al2O3 t K- feldspor

chomosite

hornblende i{

CoO* No2O*K2O FeO*MnO* MgO mole 7" Frc. 1. Summary diaeram of Al-bearing silicate compositions. Composi- tional data for biotite and pblogopite are taken from Deer et al, (1962) and those for chamosite and Al-greenalite, from Klein & Fink (1976) and Floran & Papike (1975). All olher data are from the present study. FeO in Figures 1, 3, 5, 7, anLd9 meam total Fe recalculated as FeO. Fe-mica: Fe-rich mica. STILPNOMELANE IN BANDED IRON.FORMATION 191

commonly forms irregular aggregates interstitial to ankerite rhombs that are nearly parallel to the sedimentary banding (Fig. 2C). The X-ray powder-diffraction pattern of this stilpnomelane shows an extra peak at d = 9.41 A that may be the equivalent to doozof . Mas- sive aggregates or continuous layers of stilpno- melane are rare in iron-oxide-rich mesobands. Instead, an acicular variety occurs Dext to the grain boundaries of iron oxides. Discontinuous layers of lenticular aggregates of stilpnomelane less than O.1 mm thick are common in chert mesobands; such layers are commonly broken by intervening recrystallized quartz. The chemical composition of the stilpno- melane can be described entirely by KaO, FeO, MgO, FerOr, AlrOs and SiOz. Other components are minor and total less than lVo (Table L), The stilpnomelane formula is recalculated on the basis of 22 oxygen atoms in order to com- pare with that of Fe-rich mica. Compositional diagrams for stilpnomelane and associated min- erals are given in Figures l, 3, 7 and 9. The largest range of chemical variation is caused by variable Fe'* and Fe8+contents (Deer et al. Frc. 2. Stilpnomelaneoccurrences. A. Networks of 1962). The Fe2+-rich variety, ferrostilpno- interlocking laths of ferrostilpnomelane. Doubly melane, has pale green pleochroic colors; the polarized lieht. An: ankerite. B. Massive aggre- Fe""-rich one, ferristilpnomelane, has brown to gates of ferristilpnomelane, which lack hematite dark brown pleochroic colors. The doorof ferro- and quartz. Polarized [ght. Stilpnomelano (Stil) coexists with ankerite (An), -asretite (Mt) stilpnomelane ranges from 11.98 to 11.99 A, and pyrite (Py). C. Inegular aggregatesof fer- and th4t of ferristilpnomelane, from 12.14 to ristilpnomelane (Stil) interstitial to aolcerite 12.28 A, as determined by X-ray powder dif- rhombs (An). The opaque mineral is m.agnetite. fraction. Fet+ and Fe3+ cannot be distinguished Polarized ligh| Agereeateslie nearly parallel to by electron-microprobe analysis, but complete the sedimeglarybanding, analyses by Trendall & Blockley (1970) give 192 THE CANADIAN MINBRALOGIST

TABLEI. REPRESENIATIVE14ICROPROBE ANALYSES OF AL.BEARINGSILICATES IN SANDEOIROII-FORMATION OF TI{E OALESOORGE I4EI'IBER

A t o 7 9 l0

wt. g FC7 J294 8402 2262 DE2 zL28 I't3?5 280 tL22 ES5 slo, 45.71 44.52 48.40 47,6L 38.39 38.69 35.09 47.85 48.62 65.43 Nz6s s.35 4.62 4.44 ,r.19 4.99 5.52 1.83 7.12 6.7L 18.14 ri02 0.06 0.01 0.01 0.02 0.00 0.08 0.00 1.41 1.51 0.00 cr2o3 n.al. n.al. q.00 0.00 n.dl. 0.00 n.at. 0.01 0.02 n.ct. FeO-r 33.05 31.86 25.65 26.45 30.85 29.47 40.99 L2.06 L3,21 0.18 r.fno 0.12 0.00 0.03 0.10 0.09 0.04 0.01. 0.35 0.49 0.02 Nto n.d. n.d. 0.06 0.00 n.d. 0.00 n.al. 0.00 0.00 n.d. !4eO 3.82 4.94 9.55 9.43 L2,25 11.84 7.13 15.37 I'1.43 0.00 cao 0.04 0.02 0.04 0.01 0.02 0.00 0.00 10.78 10.88 0.02 Na2O 0.32 0.01 0.23 0.04 0.03 0.03 0.00 1.69 1.80 0.07 Kzo 1.83 2,96 2.89 L.?5 8,76 8.33 7.85 0.36 0.56 L6.26 rotal 90.30 89.24 91.30 89.60 9s.39 94.00 93.50 97.61 98.29 t00.12 Fe./(Fe+ugl 0,829 o.77s 0.c05 0.611 0.s86 q.583 o.z5t 0.306 0.340 ------Nu[ber of lons on th€ ba8ls o! 22 oxygeng 23 o:(ygss 8 oxygens 7.398 7.372 7.477 6. 238 6.308 6.2L4 5.956 a n1 a 0.602 0,628 0.530 0.523 u.yto r.05t 0. 382 1. 044 0.930 0 8.000 8.000 8.000 8.000 ?.194 ?.369 6.596 9.000 8;000 3.016 0.419 0.268 0.278 0.253 0.000 0.000 0.000 0.280 0.220 0.986 0.007 0.00t 0. 00r. 0.002 0.010 0.000 0. 154 0.L65 0.000 0.000 ltlt 0. 000 0.00r 0.002 :::: FeZt 4.474 4.383 3.311 3.474 4.r94 4.019 6.071 1.466 l. 614 0.007 MN 0.016 0.000 0.004 0.0L3 0.006 0.002 0.044 0.060 Ni 0.007 0.000 !:9'l 0.000 0. 000 0.000 v:: Ug 0.921 r.211 2.L97 2.207 2.967 2.877 2.040 t t 2n 3.L27 !:-9.99. t 7.173 6.9L2 8.113 ilrE 5.18I 0.994 Ca 0.007 0.004 0.007 0.002 0.003 0.000 0.000 r.6?9 1.695 0.001 NA 0.100 0.003 0.069 0.012 0.009 0.009 0.000 0.476 0.50? 0.006 K ga!.29. 0.621 0.569 0.3s1 1. 816 l. ?33 L.714 0.067 0.104 0.956 t 6.322 5.491 6.443 6.314 r.828 L.7.42 L.774 2.222 2.306 0.963

L) Aolcular to f,tblous ferlostllpnonelme assoclated rlth pyrlte anal pyrrhotLt€ ln spharlcal aggr€gates of, K-f,sldspar (s€e Fig. 8). 2) FelrostLLpnonelane Bsocj.atad nlth pyrlte and nagletite, fornLng net- worka of lnterlockl.ng laths (see I'tg. 2A ). 3) Felrostllpnoaelane aaaosl,atsd rrlth henattte ild quartz, Iocally roagnetltse, ankerlter anal ELdlerlte, fomlng a thln lentioula! and dlscontinuous band. 4) Brosn f,srllstilpnoaeLans aEaoctateal wi.th Fe-rlch nlcal dlepersed lagnetite ed upblboleg (Fee 319. 6 ). 5l .Ffaky graln of rerlch nica aggoclated nith hmatl,t€, nagnetlte, ankcl,te, qurlz, and rlebecklte ln a chert resobud. 5) rlaky graln of Fe-rlch rdoa f,oruLng thla balril6 whlcb altetnat€ wlth heroatite+ mgnetitFbearlng t0iclobatrd,s. 7) Ma6slv€ aggregate of Fe-rLch r0ica aasociated wltb henatlte, quartz, anll snkellte, locally rlsbeckLte prlses md mgnetite, f,ornl.ng a Lentloulat and dLscontinuoue band (see Flg. {B). 8) BornbLende aulrounded by riebecklte (see Fig. 6). 9) San€ as 8. 101 SpherlcaL agg!€gate of E-fel.dspar (see Fig. 8). rA11 Fe leported as F€O.

FegOscontents of f0rro- and ferristilpnomelane origin and that all primary stilpnomelane origi- as 2.90 and 18.49 wt. Vo, respectively.flowever, nated as ferrostilpnomelane. The possibility of feqostilpnomelane is easily altered to ferristilp- a secondary origin for the high Fe3+ contents nomelaneby oxidation (Miyano 1976a). Ayres in stilpnomelane has been proposed by many (1972) noted from his microprobe analyses workers (e.g., Klein L974, Flomn & Papike that ferristilpnomelane has a higber Fe content 1975, Gole 1980). It appears that the wide than ferrostilpnomelane. The chemical analyses range of doot of ferristilpnomelane is a reflec- obtained in this study do not confirm this tion of different degrees of oxidation. It is finding. The pale green variety has a similar evident that brown ferristilpnomelane has a total FeO content to Ayres's ferristilpnomelane lower iron content (about 23 to 27 wt. % (36.63 wt. Vo F&), and the brown variety has total FeO) than the dark brown variety, which a similar total FeO content to Ayres's fer- has more than 30 wt % total FeO (Table 1). rostilpnomelane (21.94 wt. lo FeO). Owing to It may be that very iron-rich stilpnomelane is the ease of oxidation of Fez+ to Fe"+ in the more easily oxidized to ferristilpnomelane than relatively open structure of stilpnomelane a relatively iron-poor variety because of the (Eggleton 1972), it is very likely that the brown inherently higher content of ferrous iron The to dark brown stilpnomelane is of secondary KsO content of stilpnomelane in iron-formation STILPNOMELANE IN BANDED IRON-FORMATION t93

Al203 t

hornblende slilpnomelone 7 \*- dste Fe-mico{gj .. Fe-micor 50 FeO*MnO mole 7"

. hm -mt ossembloge CoO* KaO a mt-Ft ossembloge B A o py-pyrrhoossemblogs I + ml-horn-rk ossembldgo dolomite-onkerile

Mso FeO* MnO mole 7"

Frc. 3 A. Compositional variation of hornblende, Fe-rich mica, and stilp- nomelane in banded iron-formation of the Dales Gorge Member. Fe-mica: Fe-rich mica. One group of Fe-rich mica (Table 1, columns 5, 6) has a similar Algos content to that of stilpnomelane (fable I, columns 1 to 4) but contains more alumina and less iron than another group Cfable l, column 7). The compositional variation shown by stilpnomelane reflects the differences in the mineral assemblages (see texl also Fig. 3B). B. Variations in stilpnomelane compositions as a function of differing assemblages. Generally, assemblages with hematite and magneiite have a higher Mg/(Mg*Fe) ratio than those with magnetite and pyrite or with pyrite and pyrrhotite,

rocks generally ranges from 1.0 to 2,A wt. lo Fe-rtch mica (e.9., Klein 1974, Klein & Fink 1976, Floran & Papike 1,975, t978). However, in the Dales Iron-rich and Al-poor mica, as first de- Gorge Member, the KrO content oflen exceeds scribed from this Precambrian iron-formation, 2 wt. Vo (Table 1, columns 2,3), especially in refers to mica with an Alsos content that is stilpnomelane that occurs near or within lower by 5 to 10 wt. Vo than that of average riebeckite-rich 2qqss. plsliminary analytical data biotite or phlogopite (Table 1, Fig. 1). The door show that the KzO content of such stilpno- value in such mica, as obtained by X-ray melane ranges from 1 to 6 wt. /o. powder diffraction, ranges from 10.14 to 10.16 194 THE CANADIAN MINERALOGIST ffi ffii

A, and is slightly larger than that of biotite or phlogopite. The textural appearance of this Fe-rich mica is similar to that of stilpnomelane, but it i8 generally more coarsely crystallized tban stilpno' melane. Flaky and tabular grains range from 10 to 40 pm in diameter and are dispersed in chert-rich mesobands (Fig. 4A)' associated with ankerite, hematite, magnetite, stilpno- melane, quartz and riebeckite. In most occur- rencss, the mica is closely associated with riebeckite-rich bands; it is usually concentrated in those that parallel the overall falding. Thin (0.2 mm across) Fe-rich-mica layen alternate with iron-oxide microbands or form discon' tinuous lenses with elongated hematite grains and are crossed by prismatic riebeckite grains (Fig. 4B). It appears that such bands may originally have been stilpnomelane bands, be- cause stilpnomelane is replaced by Fe'rich mica in many occurrences where they coexist. The Fe-rich mica also occurs as thin veinlets devel- oped from stilpnomelane bands. The occur- rence is similar to a veinlet of biotite-like material reported by Klein & Gole (1981) from the Marra Mamba Iron Formation, Hamersley Group, 3@ m lower than the Dales Frc. 4. Fe-rich mica occurrences. A. Dispersed Gorge Member. However, that material is more grains of Fe-rich mica (Fe-mica) in a chert Al-rich and Fe'poor than the mica in this study. mesoband. Stilpnomelane (Stil) grains are sub- Fe-rich mica and riebeckite generally appear parallel to the banding. Polarized lieht. The to be later than any of the other minerals, scale in A is the same as in B. B. Massive but the mica itself is in places cut by riebeckite aggregates of Fe-rich mica lenticular bands, form prisms cut acicular or fibrous one of which is traversed by a prismatic rie- that also asross bands. It may be that fibrous beckite crain (Rk). Elongate grains of hematite riebeckite-rich (black, Hm) are enclosed within the bands. riebeckite is essentially contemporaneous with Polarized light. Mica has a lower AlzOs content the mica. than that shown in A (Table I, column 7). The chemical composition of the Fe-rich STILPNOMELANB IN BANDED IRON.FOA,MATION 195

CoO* NqzO* KzO t * hemotiteI mognetiter

Mso 50 FeO* MnO mole "/o FIc, 5. Compositional variation of the assemblage ferrodolomite + rie- beckite { Fe-rich mica (Fe-mica). The assemblage also contains hematite magnetite and quartz.

mica is similar to that of stilpnomelane, but Results of chemical analyses of stilpno- with a higher IGO and a lower SiO: content melaire, ankerite (ferrodolomite) and riebeckite than stilpnomelane. Results of electron-micro- coexisting with the mica are given in Tables 1, probe analysesare given in Table l, and com- 2 and 3, The compositional variations of fer- positional diagrams are shown in Figures 1, rodolomite * Fe-rich mica * riebeckite and 3A, 5 and 7. Tll'e mica occurs in two colored of stilpnomelane * Fe-rich mica * riebeckite varieties, one with light reddish brown (X) to * hornblende are shown in Figures 5 and 7, pale yellow green (Y,Z) pleochroism and the other with brownish red (X) to pale greenish TABLE 2. REPRESET{TATIVEIIICROPROBE AIIALISES OF CAREOIIATES brown (Y,Z) pleochroic solors. Table I shows III 8AI{DEDIROTI-FORfiIATION OF THE DALSS GORGEIiIEI{8ER that the latter variety (column 7) has a higher FeO content by about 10 wt. lo and a lower xt. g DE29 4460 J319 FE34 4466 AlzOs content by about 3 wt. Vo than the former sro2 o.l8 0.16 0.18 0.31 0.14 (columns 5, 6). It is likely that the reddish EzO: o.o0 0.oo o.o! 0.00 o.oo brown (X) geen (Y,Z) Tto2 0.00 0.00 0.02 0.04 0.02 to deep variety is a cr,o' a.al. 0.00 n.tl. a.dl. 0.03 prodqct of secondary oxidation of the former re5'- 10.32 L4.72 20.29 19.51 49.23 ho 0.58 0.00 1.31 0.59 0.2a type because both varieties give almost identical Nto !.a1. 0.0o !.a1. !.a1. 0.06 uso 15.06 12.15 7.56 7.59 t.66 electron'probdtdnalyses. The number of ions cao 2a.49 24.33 27.50 27.sL 0.X3 in the mica, recalculated'sn the basis of 22 tra2o O.O2 0.00 0.00 0.00 0.07 RZO 0.00 0.00 0.00 0.09 0.oo oxygen atomsn shows a cation deficiency in the roral 55.70 55.36 56.87 56.04 59.52 tetrahedral sites and a cation excess in the F6l(re+ugt 0.266 0.405 -0.601 0.59L 0.740 octahedral sites. Assuming that the deficiency &nber of iore oaihe basLs of 2 oxygeG Fe2+ 0.273 0.405 O.s1? 0.563 1.472 is filled with Fe3+ for 8 cations. the number of l,lD 0.015 0.000 0.038 0-020 0.008 Ug 0.752 0.596 0.383 0.390 0.515 octahedral cations becomes6.05, 6.03, and 6.16 Ca 0.960 0.999 1,.002 L-O27 0.00s for specimens 5, 6 and 7 in Table 1, respectively. tcati@s 2.000 2.OOO 2.000 2.000 2.000 Such values, larger than 6, can be caused by a U t{aitl@9!atn€d' eu}Edttal f€rodol@lc€. aa@Latsa eitb - Iower KaO content than in ideal mica. It is g*2, h@tLte' rgEotite, !I#ckLt€, eal FFllch nLq Ln a che* @6obdtl. 2) kdlw- to @ua€-gEahed, euhedral possible that the tetrahedral Al-position is in forroatoloElto, asBocLatedl eLth qrElz, he@ttte, al.dlelt te, forrcstlllDoElare. ild l@alty @stite ID a qudz-lrcn part substituted for by ferric iron. Synthetic oride resbata. 3, nbdirgratn€d, euhedtal anketlb' associ.at€a Fitb ali.sFleoal !ryrLt€ @al @gaatito Ln a @!r!: Al-free mica ("ferri-annitd') KrFe'+.Fee+, of Lntdlocklng laths of, ferestllptr@lee (sea Flg. 2 l. 4) hal@graLneil, subhaabal ar&erLtar ee@tateil sttb glth€rlcal Si.O.(OH)o (door1O.16 - 10.18 A) tlat b""o ryrlte, pyrrhotlte, anal ferrutLlp@elaBe lD a aqgEesab of K-felalslE! Iseo FLg. 8t. 5) PtDe b Etllle (1963) glth described by Wones and Donnay et aJ. Ga1n6d, subh€dral sLderLt€, ssocLateal h@tib, CerrostllpnoEleo, ual l@a1ly ugtrettE LD a gualtFlron (1964), This may be considered an end member o:tdo Eesobetl. iAlI Fe aa F@. for this Fe-rich mica. 196 THE CANADIAN MINERALOGIST

rArLE 3. REPRESEIITATMtltCRoPRoEE AtlALlSEs 0F IIEBECTITEIN 8AIOEDlR0t{-F0B!lArl0[ 0F THEoALES GoRGE TiEUBER HOfnblende

I Angular fragments of hornblende occur in mesobands of the iron for- rt. I 278 7.t2L DE2 stilpnomelane-rich mation along contacts with hematite-magnetite- sro2 51.89 52.25 54.46 a2o3 0.45 0.33 0.05 Fe-rich-mica bands. Hornblende grains rimmed EtO2 0.65 1.00 0.0r Cr2O3 0.oo 0.00 a.d. by riebeckite are dispersed in a matrix of aci- ?eOr 33.7,1 33.39 27.L2 stilpnomelane and Fe-rich mica, asso- l4ro 0.00 0.03 0.04 cular Nlo 0.01 0.01 n.d. ciated with euhedral magnetite grains and in- u90 2.49 2.73 7.76 quartz (Fig. The cao 0. 15 0.19 0.14 terstitial patches of 6). Na2o 5.72 5.87 6.41 hornblende plays the part of a seed for riebeckite Rzo o.22 0.r8 0.08 overgrowths. The pleochroism of the horn- Eotal 95.72 95.98 96.0? blende is difficult to determine because of 0.868 0.873 0.662 Fe,/(FelUg) blue to yellowish green pleochroic colors of NrE!€r of Lda o! ti6 baaLs of 23 oxygds the rimming riebeckite. A similar textural oc- ii f'lf$e.za!'l$fia.'r 3:33fi'.,' currence of these two minerals is described by AI 0.08{1 0.0621 0.0091 r1 0.078| 0.120| 0.001| Milton & Eugster (1959) in the Green River e 0.000t o.ooot -----l Fe2* 4.4791s.334.{4rls.2s 3.4s4ls.28 Formation. There, light blue, secondary rie- h 0.0001 0.0041 0.00s1 Nt 0.001| 0. o0r I ---- | beckite has formed as rims around detrital, t{s 0.684J 0.64P L.777r ca 0.02q 0.034 0.023'l brown hornblende grains. Ayres (1972) also Na 2.06812.L1 1.8I01L88 1.90911.9s K 0.0451 0.037. 0- 016J noted that some angular fragments of am- phibole from the B'IF of the Dales Gorge lrRt€beskite arloulalLng & egqLd frag@nt of hornblentle (se€ Fl.g. 6r. 2, S@ a6 1. 3) Flbrcu Member appear to be partly replaced by blue riebackl.te assoclateil gLtb F6-rLch eL€, f,ete doloElte, h€@tlte, ad @96etlte ln a chd riebeckite. n€sobd. 'All F€ as Fd. Results of electron-probe analyses of the central hornblende and the rimming riebeckite respectively. Figure 7 shows that the ratios of are given in Tables I (columns 8'9) and 3, Fel(Fe*Mg) of the Fe-rich mica and the asso- and they are shown graphically in Figures 3A ciated stilpnomelane are almost identical be- and 7. Figure 7 gives the compositional range cause the Mn contents of both minerals are of minerals in the assemblage hornblende * virtually zero. riebeckite f stilpnomelane + Fe-rich mica.

::F:S$I]i Frc. 6. I{ornble,nde occurrences in a stilpnomelane-rich band. Angular grain of hornblende (IIo) surrounded by riebeckite (Rk). Euhedral gra'ins of magnetite (black, Mt) are dispersed through the stilpnomelane and Fe-rich mica matrix. Polarized light. STILPNOMELANE IN BANDED INON-FORMATION r97

CoO * mognelile t

hornblende

SompleNo Z-lO tFe-mlco riebeckite slilpnomelone

Mgo FeO*MnO mole 7o Frc. 7. Compositional variations of hornblende cores and riebeckite rims. The- compositions of associated stilpnomelane and Fe-rich mica (solid circle) are also shown.

Irrc. 8. K-feldspar occurrences in a stilpnomelane-rich band. Spherical aggregates of K-feldspar (K-fel) around a fibrous to acicular stilpnome- lane (Stil) core and surrounded by ankerite rhombs (An). polarizA light.

K-feldspar rhombs. Small amounts of pyrite, pyrrhotite and, rarely, magnetite are dispersed throughout K-feldspar occurs in spherulites with a the spherulites. diameter of up to 3 mm; these are dispened The main chemical components, as sho\rn in a 3-cm-thick stilpnomelane-rich band. The by microprobe analyses, are trGO, AlrOa and spherulitesconsist of three distinct parts: a core, SiOr. Other components total less than LVo middle, and outer rim. K-feldspar is present in (Table 1, column 10). Results of representative the middle as a concentric aggregate of radiating electron-probe analyses of ankerite and stilpno- sprays and mosaic forms (Fig. 8). The core melane coexisting with the feldspar are given in consists mainly of aggregates of stilpnomelane Tables t and 2. Compositional relations of the with fibrous or acicular spray texture. The rims assemblage K-feldspar * ankerite f stilpno- of the spherules are made up of ankerite melane are shown in Figure 9. 198 THE CANADIAN MINERALOGIST

K-feldspor

* pyrileI pyrrhotlte

slilpnomelone

Mgo 50 qnkerile FeO*MnO mole 7"

Frc. 9. Compositional variation of the assemblage K-feldspar a ankerite f ferrostilpnomelane. The assemblage also contains pyrite and pyr- rhotite.

Dtscusslott variable through the entire iron'formation member. In further discussion, the Fe/ (Fe* Fe/ (Fe*MS) ratio Mg) ratio can be regarded as a function of l(Or) onty, becausel(Sir) is directly dependent The possible relationship between the Fel on l(Or) in the mineral assenblages of the (FefMg) ratio of a specific mineral and the Dales Gorge Member (Miyano 1976b). A variety of assemblages in whish it occurs was similar conclusion has been reached fe1 min- evaluated for all silicates and carbonates re- nesotaite in the Dales Gorge Member by Miyano ported here. The significance of the relation- (1978a). However, the Fe/(Fe*Mg) ratio of ship between l(O) and Fe/(Fe*Mg) in iron minnesotaite seems to be more senstive to the silicates has been noted by many authors (e.9., magnitude of l(Or) than that of stilpnomelane. Mueller 1960, Burt L972, Miyano 1976b, Frost If reduction has taken place during meta- 1979), There are remarkable differences iu morphism (Miyano 1976b), stilpnomelane with such relationships for stilpnomelane, riebeckite a high Fe/ (Fe*Mg) ratio may have recrystal- and ankerite. Stilpnomelane associated with lized from one with a lower ratio by enrich- both hematite and magnetite (but hematite ment in ferrous iron. dominant) contains ?3 to 27 wt. Vo total FeO; The Fe/(Fe+Mg) ratio of riebeckite in a note that the total iron expressed as FeO would hematite-ferrodolomite association (Table 3, reflect mainly ferrous iron as already discussed. column 3; also see Fig. 5) is lower than the Stilpnomelane soexigring with both magnetite ratio in an association including neither hematite and pyrite or both pyrite and pyrrhotite has nor carbonates (Table 3, columns 1, 2; also 31 to 36 wt % total FeO. This implies that see Fig. 7). The presence of hematite, however, the Fel (Fe+Mg) ratio of stilpnomelane tends does not appear to affect the Fe/ (Fe+Mg) to increase with decreasing oxygen (or sulfur) ratio much, judging from these preliminary fugacity at constant P(@r) and P(H,O), and compositional data. that the magnitude of l(Or) or l(Ss) is locally Members of tle dolomite-ankerite series STILPNOMELANB IN BANDED IRON-FORMATION I99

CaMg(CO")z-CaFe(COr)s have variable Fe/ Oz, 2) stilpnomelane * 3.36(Fe) * 1.65K+ = (Fe*Me) ratios depending on the variety of l.2z Fe-tich mica * tn(z)Hro + 1.65H+ + mineral assemblagesin which they occur, as n(2)oz and 3) stilpnomelaue + 0.3gK* = o.93 shown in Figurg 38. In some oscurences of KAlSigoa + 4.65sio, * 5.45(Fe) + m(3)H"o the same assemblage,however, the Fe/(Fe+ + 0.38tr{+ * n(3)Oz, whdre (Fe) implies iion Mg) ratios of the carbonates are apparently oxides (or sulfides). The stoicfriometric co- different, which can be_ seen by comparing efficients of water (zz) and oxygen (or sulfur) Figures 38 and 5. Generally, the Fe/(Fe*Mg) (n) are dependentupon the unkn-ownnumber of ratios of carSonates hgye a wider range than HzO in and stilpnomelanen and tlqse of coexisting silicates (Miyano 1976a, upon the variety of (Fe), resp6ctively. 1978a, Klein & Gole 1981). Each reaction described above pioceeds to the right with an increasing activity ratio ar+/ Possible reactions and stability relations as+ a.t constant l(Or) [or l(Sr)], I(Hzo) and T. Becauseor a lackor experimental audther- il3"o'?11i"':rnff3,?'uTtlff*yff'rfli mochemical data, the stability relations of Al- as+ at conirant l(ot tor l(sb)], l@ro) ind r: bearing silicates reported here cannot be eval- nontronite-+stilpnomelane-+f-feidspar and non- uated quantiQtively. The minelal assemblages tronite->stilpnomelane-+Fe-rich mica (Fig. 10). can be used, however, to provide sqme qua,li- Such a sequencehas been determined in tf,e sys- tative stability-relations for these minerals by tem KzG-Al,OrSiO,-HrO by Hemley (Lgig) considering an additional Al-bearing silicate and Helgeson et al. (Lg6g).' tney showeOtne assemblagereported by other authors. LaBerge s"quett"ei of -+K-mica-+K-feldspar ( 1966, p. 159) noted a possiblemontmorillonite and kaolinite-+K-+muscoviti+ (no_ntronite?)-stilpnomelaneafsemblage in the microcline with increasing ar+/aa+, keeping rocks of the Dales Gorge Member. I{e con- other parameters fixed. cluded that stilpnomelane might have formed The ar.+/an+ ratio may be one of tle most from nontronite. Trendall & Blockley (1970, effective parameters for determining such re- p-. 115) also. reported montmorillonoid from a action seqir"oc"s of Al-bearing silicaiis, because shale__ macroband, -suggesting that the mont- differencej in the magnitude-s of l(iOr) anC morillonite was earlier misidentified as chlorite l(HO) during low-giade metamorphism (at under the microscope. Qualitative phase-relations of Al-bearing silicates may be evaluated by proposing possible KaO-Alz Os - SiOz- H2O lron- Formcfion chemical reactions among them. Simplified Syslem System chemical formulae for such silicaGs are as follows: nontronite Fe'+".rrAL.nSiz.goOro(OH)n' koolinite ??? mHzO, stilpnomelane Ko.ssFe2+s.a#{lo."gSiz.

1OO to 15O'C) are very small (see fugacity AlsOs content (below 1.0 wt. Vo: Table 3) and data of Ryzhenko & Volkov 1971, Burnham the coexisting stilpnomelane is not enriched in et al. 1969, Miyano 1978c). The l(Oz) may Al. However, Fe-rich mica commonly occurs also influence the stability ranges of K-Al- near or within a riebeckite band or zone. This bearing silicates. Stilpnomelane-K-feldspar as- suggests tha! when stilpnomelane is replaced semblages are not observed in the presence of by riebeckite, the Al released has been repreci- hematite (*maguetite-rpyrite). In contrast, pitated as Fe-rich mica close to the riebeckite stilpnomelane-Fe-rish mica assemblagesdo not ione. Accordingly, Al may not have traveled occur under reducing conditions where pyr- farther than about 5 mm from such a band. rhotite ( * pyrite-rmagnetite) is stable. However, On account of the deficiency of alkalis in iron the l(O,) may not have as influential an effect formations and the above suggested behavior on the stability sequencesag the at+/a.+ ratio of Al, additional alkalis for the local crystalliza- because, in reactions 2) and 3), an increase tion of riebeckite and mica may have been or decreasein l(Or) can be compensatedby a derived from elsewhere in the bulk-rock system, decreaseor increasein Fe/ (Fe*Mg) of stihno- probably through alkali-bearing soludons. Be- melane, respectively. iause there are effective impermeable barriers The sequence shown in Figure l0 cannot against the migration of aqueous solutions, such be observed at one locality. The respective as- as shale macrobands alternating with the BIF, semblages appear to be a function of the mag- it is possible that differences in a"+ (also aor"+) nitude of 4r+, or potassium availability. Dif- may have been caused by the difficulty of trans- ferences in ar+ may have had a greater effect port or mobility of alkali-bearing solutions. on the mineral stabilities than temperature, Much of the crystallization of riebeckite and which may not have differed by more than 10oC Fe-rich mica, however, may be related to alkali- across the member. This means that homo- bearing solutions derived from deforrnation as genization of a*+, the chemical potential of suggestedby Trendall & Blockley (1970). K+, was incomplete in the iron formation at temperatures of 11O to 150'C (Miyano 1976b, CoNcr-uslous 1978b). Therefore, stable associationsof K-Al- bearing minerals are probably maintained Textural relations show that the majority of through local equilibrium. Considering the Al-bearing minerals are of late diagenetic to intimate association of Fe-rich mica and rie- very low-grade metamorphic origin. The origin beckite and the replacement of stilpnomelane of the hornblende is uncertain; it may be by riebeckite, as will be discussed below, parti- authigenic or it may possibly represent vol- cipation of both potassium and sodium is canic detritus. Its texture and chemical com- essential in such associations. However, very position are not diagnostic of either mode of few Na-bearing minerals other than riebeckite origin. The stability relations of Al-bearing are found in the BIF. Miyano (1976a) noted minerals are very qualitatively discussed on authigenic orthoclase in a shale macroband of the basis of possible reactions predicted from the Dales Gorge Member. Klein & Fink (1976) mineral assemblages.The formation of stilpno- reported a small amount of authigenic albite melane, Fe-rich mica and K-feldspar appearsto with nearly end-member composition from the have been controlled by differences in ar+/an+ Sokoman Iron Formation, which underwent ratios during very low-grade metamorphism. very low gxade metamorphism. This suggests The Al-bearing silicates observed in the very that differences in arv.+ similar to those of ar+ weakly metamorphosed iron-formation rocks may be responsible for the variety of Na-bearing are not direct indicaton of the depositional mineral assemblagesin rocks of the iron forma- environments of chemical sediments such ast tion. the BIF. However, investigation of the Al- bearing mineral assemblagesmay provide infor- during Stilpnomelan'e-riebeckite association madon on the'environmental conditions late diagenesis or very low-grade metamor- The relationship between stilpnomelane and phism of the iron-formation rocks. riebeckite is complex. Grubb (1971) reported that riebeckite completely replaces stilpnomelane ACKNoWLEDGEMENTS as a result of interaction with Na-bearing solutions, but he did not evaluate the alumina I thank Hancock and Wright Prospecting Pty. budget in such a replacament process. Fe'rich Ltd. for permission to collect diamond drill mica may be a possible source or sink (or both) core from the Colonial mine area- Part of this for aluminum. Riebeckite generally has a low study was supported by Grant-in-Aid for En- STILPNOMTLANE IN BANDED IRON.FORMATION 2Al couragement of a Young Scientist (5743?5) ot Fnosr, B.R. (1979): Metamorphism of iron-forma- the Ministry of Education, Science and Cultwe tion: pa.ragenesis in the system F€Ji-C-O-H. of Japan. Further funding was provided by NSF Econ, Geol. 74, 775-785. grant EAR 8O-2O377to C. Klein. I used elec- (1980): ptrology tron microprobes at the Analytical Center of Golr, M.J. Mineralogy and of very-low-metamorphic gade Archaean Tsukuba University and the Geological banded at Sur- iron-formations, Weld Range, vey Western Austra. of Japan. I am much indebted to A. Sasaki lia. Amer. Mineral. 65, 8-25. and K. Okumura at the Geological Survey of Japan and N. Nishida of Tsukuba Univer- & Kr,uN, C. (1981): Banded iron-forma- sity for their help in obtaining microprobe data, tions through much of Precambrian tlme. L Geol. to S. Sueno and S. Miyano for obtaining X-ray 89, 169-183. powder data, to Mrs. Thea Brown for the typing of the manuscript, and to C. Klein for Gnurr, P.L.C. (1971): Silicates and their para- genesis his critical review and constructive suggestions, in the Brockman Iron Formation of Witte- which considerably noom Gorge, Western Australia. Econ. Geol. 66, improved its content. 28t-292.

ReruRENcss HELcrsoN, H.C., Bnowu, T.H. & LespER, R.H. (1969): Handbook of Theoretical Activity Dia- Aynns, D.E. (1972)t Genesisof iron-bearing min- grams Depicting Chemical Equilibia in Geologic erals in banded iron fo,nnation mesobandsin the Systems Involving an Aqaeous Phase at One Atm Dales Gorge Member, Hamersley Group, West- and 0" to 300"C. Freeman, Cooper & Company, ern Australia.Econ. Geol.67, 1214-1233. San Francisco,

BunNua,u,C.W., Hor-lowev, J.R. & Devrs, N.F. Hrur-Bv, J.J. (1959): Some mineralogical equilibria (f969): Thermodynamicproperties of water to in the system IGO-AlrOs"-SiOs-H2O. Amer. J. 1,00O'C lnd 10,000 bars. Geol. Soc. Amer. Sci. 257, 241-270, Spec.Pap. 182. (1973): Buer, (1972)t Jeprcs, H.L. & $us, P.K. Precambrian D.M. The system Fe--Si--C-O-H: a iron-formations of the world. Introduction. model for metamorphosediron formations. Car- Econ. Geol. 68, 913-914. negie Inst. Wash. Year Book 7L, 436-443. Denn,,W.A., Howrr, R.A. & ZussMaN,I. (1962): KrerN, C., Jn. (1974): Greenalite, stilpnomelane, Rock-Forming Minerals. 3, Sheet Silicates:.Long- minnesotaite, crocidotte and carbonates in a mans,London very low-grade metamorphic Precambrian hon- formation. Can. Mineral. 12, 475498. Drrrnorn, E. & Cneuvsr,,J,-J.(1973): petrography of the Sokoman Iron Formation in part of &n- & FrNK, R.P. (1976): Petrology of the tral Labrador Trougb, euebec, Canada, Geol. Sokoman Iron Formation in the Howells River Soc.Amer, Bull. 84, lll-I3'4. area, at the western edge of the Labrador trough. Econ. Geol. 71, 453487. DoNNAy, G., MoRrMo[g N., Tereoe, H. & Dox- NAy, (1964)z J.H.D. Trioctahedrat one-Iayer & GoLe, M.J. (f981): Mineralogy and . I. of a synthetic iion petrology of parts of tle Marra Mamba Iron- mica. Acta Cryst. 17, 1369-1373. Formation, Hamersley Basin, Western Australia. Amer. Mineral. 66, 507-525. EccLEToN,R.A. (1972): The crystal structure of stilpnomelane. II. The full cell. Mineral. Mas. 38,693-711. LABERcE, G.L. (1966): Altered pyroclastic rocks in iron-formation in the Hamersley Range, ---- - & Creurr.r,, B.W. (1978): The crystal Western Australia. Econ. Geol. 61, 147-161. structure of stilp'nomelane.III. Chemistrv and physical properties. Mineral. Mag. 42, 36i-36S. LESHER,C.M. (1978): Minera,logy and petrology of the Sokoman Iron Formations near Ardua FroneN, R.J. & PAprKE,JJ. (1975): petrology of Lake, Quebec. Can. I. Earth Sci, 15, 480-500. the low-grade rocks of the Gunflint Iron-Foima- tion. Ontario-Minnesota.Geol. Soc.Amer. Bull. Mu,roN, C. & Eucsrrn, H.P. (1959): Mineral 86,1169-1190. assemblages of the Green River Formation. fn Researches in Geochemistry (PJI. Abelson, ed.). * -- (1978): Mineralogyand petrol- John Wiley & Sons, New York. ogy of the Gunflint Iron Formation, Minnesota- Ontario: corelation of compositional and as- MrreNo, T. (1976a): Mineral assemblages of the semblage variations at low to moderate grade. Proterozoic baDded iron formation in the Hamers- l. Petrologlt 19, 2lj-288. Iey area. Mining Geol. tapan 26, n3-?AB. 202 THE CANADIAN MINERALOGIST

(1976b) : Physicochemical environments RyzsEr{Ko, B.N. & Vor,rov, V.P. (1971): Fugacity during burial metamorphismof the Dales Gorge coefficients of some gases in a broad range of Member, Hamersley Group, Western Australia. temlrratures and pressures, Geochem. Int. 8, 468- Mining Geol. tapan 2,6,311-325. 48t.

(1978a): Effect of CO2 on mineralogical Srno, T. (L974)z Clay Minzralogy, Iwanami Book differences in some low-grade metanrorphic iron Co., Tokyo. formations. Geochem, l. Iapa,n 12, ?nl-Zll. (1978b): Phaserelations in the systemFe- TIENDALL, A.F. & Br.ocn sy, I.G. ( 1970) : The hoql Mc--Si-GH and environmentsduriog low-grade formations of the Precambrian Hamersley Group, metamorphismof some Precambrianiron forma- Western Australia, with special reference to the tions. J. Geol. Soc. tapan U, 679-690. associated crocidolite. West. Awt, Geol. Surv. Bull. ll9. (1978c): Stability relationsof iron-bearing minerals in the H2O4O2 mixed-volatile region at Wowes, D.R. (1963): Phase equilibria of Terri-an- lower temperatures.t. GeoI. Soc.Japan U,7lL- nite' KFe+2sFe+8Si.O1o(OH)2.Amer. J. Sci. Nil., 719. 58t-596. Murr-r.rn, R.F. (1960): Compositionalcharracter- istics and equilibrium relations in mineral as- semblagesof a metamorphosediron formation. Received August 19E1, revised manuscript accepted Amer, t..lci. 258, 4y';9497. February 1982,