Assrracr Greeiuajjte, STILPI\IOMELANE

$Assrracr Greeiuajjte, STILPI\IOMELANE$

Canadian Mineralogist Vol. 12, pp. 475-498 (1974)

GREEIUAJjTE,STILPI\IOMELANE, MINNESOTAITE CROCIDOLITEAND CARBONATES IN I\ VERY LOW.GRADE METAMORPHIC PRECA,MBRIANIRON.FORMATION

CORNELISKLEIN, JR. Department ol Geology, Indiana University, Bloomington, Indiana 47401

AssrRAcr of possible solution of earlier phases and deposition of later phases were not found. The Sokoman Formation in the Howells River area, ai the western edge of the Labrador Trough, INrnouucrrou is part of a relatively undisturbed sedimentary sequence which dips between 5' and l2o east-nortl- 16s mineral assemblagcin this study are east. The finely banded silicate-carbonate members part of the silicate-richhorizons of the Protero- of this Aon-formation were studied by detailed petrographic and electron microprobe techniques. Most of the assemblages show complex textures in which early diagenetio features are overprinted by later ones which are the result of very low-grade meta- morphio reactions. Of the three Fe-silicates, greenalite, stilpnomelane, and minnesotaite, greenalite ap pears to be of very early origin (sedimentary to diagenetic). Stilpnomelane appears to be of diagenetic origin, and minnesotaite probably began its formation in diagenetic stages as well, but is generally concluded to be of very low-grade metamorphic origin. Greenalite which occurs as oolites, gtrarules, and later cemeot, is cryptocrystalline to microcrystalline in texture, and of homogeneous chemical composition. Stilpnomelane is well-crystallized, at times medium-grained. Minnesotaite, in well-crystallized rosettes and sprays, replaces earlier greenalite and stilpnomelane as well as much of the associated carbonates. Carbonate compositions are represented by members of the dolomite-ankerite series, siderite, and calcite. Ankerite and siderite are the most common carbonate species. Man- ganoan dolomite occurs in well-crystallized oolites ffi:lINDEX MAP SHOWING tvhich show relict banding as outlined by discon- LocArloNoF LABRAD.RrRouGH tinuous concentric bands of siderite rhombohedra. Calcite occurs sporadically as cement around silicate granules and in granul:tr ocqurences of calcite-side- I t" rite pairs. Magnetite occurs in coarsely recrystallized O ulo Miles bands, oolites, granules and iregular masses. Quartz # or chert occur throughout all of the assemblages. Crocodolite is found only in higbly deformed, crenulated crocodolite-mapetite-stilpnomelane-quartz assemblages in exposures near Schefferville, P.Q., and northwest thereof, where bulk compositions are Na-rich and structural deformation is pronounced. The primary Fe-silicate materials were probably hydrous silicate gels of greenalite-type and stilpnomelane-type compositions. Greenalite and stilpnomelane appear to be the recrystallization products thereof. Minnesotaite is a reaction product of earlier silicate and carbonate-chert assemblages. All Frc. 1. Location of the Howells River area, New- carbonates and magnetite appear to be of primary foundland and Schefferville, P.Q., with respect origin but are nolq coarsely recrystallized. Although to the iron-formation horizons in the Labrador recrystallization and reaction t€xtures are present Trough (after Gross 1968). Location of biotite in all assemblages,replacement textures indicative isograd after Fahrig (1967). 475 476 THE CANADIAN I\{INERALOGIST zoic iron-formation sequence at the western Schefferville mining areas. ODe sample of Ruth edge of the Labrador geosyncline. Most of the Slate from the Schefferville mining area and samples were obtained from the Sokoman For- one crocidolite-rich assemblage from Walthier mation in the Howells River area which is 1o- Lake, 23 miles northwest of Scheffervillb, were cated approximately twelve miles west of Schef- also studied. ferville, P.Q. (Fig. 1). Several additional samples The Sokoman Formation in the Howells River were obtained from the Sokoman Formation in area consists of conformable sedimentary units tho iron mining district of Schefferville, p.e., which are relatively undistur ed and strike ap- and 23 miles northwest thereof, in the Walthier proximately 35'NW and dip about 5 to 12" Lake area. east-northeast. Faults and other evidence of The iron-formation sequencein the Howells structural deformation are extremely rare. The River area was selectedfor this study of silicate. Sokoman Formation in the Schefferville area, carbonate assemblages.b.ecause the Sokoman however, has undergone complex structural Formation in this area is unleached, and be- changes and locally strong leaching and super- cause it probably is the least metamorphooed gene enrichment. and least altered iron.formation in the Labrador The first detailed study of the geologic setting, Trough. Although the mineral assemblagesdis- mineralogy and petrology of the Sokoman For- play some very low-gtade metamorphic fea- mation in the Howells River area was made by tutes, all preserve diagenetic and possibly pri- Perrault (1955). mary textures and chemistry.

ANALYTTCALMBrnoos Gnorocrc SBrtrNc rN THE Howslrs RrvBrr. petrographic AREA Detailed studies of mineral as- semblagesand their textural relations were made The lowermost member of the proterozoic prior to electron probe microanalysis. Specific Kaniapiskau Supergroup which unconformably assemblageswere photographed at a magni- overlies the Ashuanipi Basement complex of fication similar to that of the highest power of Archean ageois the Wishart Formation, a basal the light optical system of the electron probe guartzite with an average thickness of approxi- (64OX). Chemical analysesfor nine oxide com- mately 58 feet (Fink 1972) in the Howells River ponents in the silicates, carbonates and oxides area. This is overlain by the Ruth Slate, a thin- were performed on polished thin sections using ly laminated slate consisting of carbonate, chert a three-spectrometerEtec Autoprobe. Operating and greenalite and sporadic pyrite and graphite. conditions were 15kv accelerating voltage and The Ruth Slate averages L7 feet in thickness. 0.02 microampere sample current, standardized The Sokoman Formation, with an averagethick- on quartz, using beam current integration. The ness of 373 feet, overlies the Ruth Slate. The electron beam size was adjusted to a relatively most silicate-rich members of the Sokoman For- large size of about 2 to 8 microns in diameter mation in the Howells River area are referred to because of the possibility of breakdown of the as the "Lower Iron Formation" (LIF) and the hydrous silicates and the carbonates with a "Lean Chert" (LC) by Iron Ore Company of higher electron loading per unit area. A com- Canada geologists (Fink 1972). The Lower Iron bination of well-analyzed, homogeneous syn- Formation member directly overlies the Ruth thetic silicate glasses and naturally occurring Slate, whereas the Lean Chert member forms crystalline silicates was used as standard mate- tle uppermost member of the iron-formation rials. The Etec Autoprobe spectrometers ale sequence.The Lower Iron Formation member controlled by an onJine PDP-11/05 (1.2K me- averages 27 feet in thickness and the Lean mory) computer which also performs the data Chert member averages 83 feet in thickness. handling for each analysis point. The computer The majority of the assemblagesof this study program for partial instrument control and 'oLlF" data were obtained from the and "lC" mem- handling was developed by Dr. Larry W. Finger bers of the Sokoman Formation in the Howells of the Geophysical Laboratory, Washington, River region. Samples were obtained from dia- D.C. (Finger & Hadidiacos L972; Finger 1973). mond drill cores 1032D and 1039D of the Iron Data reduction procedures were those of Bence Ore Company of Canada (Fink L97I, L972) & Albee (1965) with correction factors from and from field occurrences. A few additional Albee & Ray (1970). samples of silicate-rich members of the Soko- Complete chemical analyses of bulF samples man Formation, the silicate-carbonateiron-for- of the silicate-rich iron-formation were made mation (SCF) member and the "lower red by a combination of gravimetric, flame photo- cherty" (LRC) members were obtained in the rnetric and colorimetric techniques. LOW.GRADE METAMORPHIC PRECAMBRIAN IRON-FORMATION 477

BuLr CnsrursrRy oF SrLrcerE-RtcH 1 to 5. Analysis 7, which was made on a minne- InoN-FonvrerroN sotaite-rich band of the silicate-carbonate iron- formation member (SCIF) in the Schefferville Complete chemical analyses of eight bulk mining area, is almost equivalent to that of pure samplesof silicate-rich members of the Sokoman minnesotaite (see Table 4 for comparison). Formation and Ruth Slate are given in Table 1. Analysis 8, made on a highly oxidized part.of Analyses 1 to 5 represent samples from the un- the Ruth Slate, consists of hematite-limonite- leached and unaltered iron-formation in the chert and lesser, brown oxidized minnesotaite. Howells River area. In each of these, FeO ex- ceedsFezOa, reflecting the presenceof abundant siderite and Fe-silicates (which contain large MINERAL OnnrvrrstnY amounts of FeO), and lesser amounts of magne- Many of the minerals in this only very slightly tite. Oxide percentages in these five analyses are metamorphosed iron-formation are very fine- simil4l to those given by James (1966) for tle grained and finely intergrown with each other. silicate-carbonate iron-formation samples from Although such assemblagesare difficult to study the Mesabi district, Mimesota and the Gogebic by methods which require mineral separation district, Michigan. Analysis 6, from the Wal- and concentration, a combination of detailed thier Lake area, represents a strusturally de- petrography and quantitative electron probe mi- formed and sheared assemblage of riebeckite, croanalysis allows for chemical and texfural stillpnomelane, magnetite and quartz; FezOgand cbaracterization of all of the coexisting phases. NasO contents are much higher than in analyses The minerals which make up this silicate-rich iron-formation are, in approximate decreasing order of abundance: greenalite, stilpnomelane, TABLEI. CHEI4ICALAMLYSES OF BIJLKSAI.IPLES OF SILICAIE-RICHSOrcfiAN minnesotaite, magnetite, chert (or quartz), car- rcRMTION bonates (members of the dolomite-ankerite se- 1234 5678 calcite) and locally small si02 33.5 2..7 24,0 43.5 26.1 39.9 54.33 44.3t ries, siderite and 't10 0.066 0,075 0.041 0.11 0.20 0 .083 trace trace amounts of crocidolite (riebeckite). The chem' 2 'I Al Z0S 0,77 0.60 0.23 0.66 .56 0,74 0.BB 0.08 istry of these are treated in the above order in Fe203 11.7 8.94 23.2 n.d.* 7.23 32.1 2,57 44.75 the subsequentsection. Fe0 4.6 32.3 40.6 32.6 36.8 16.1 32.06 0.n $rn 0,86 1.78 0.64 1.53 0.94 0.15 0.52 0.02 Greenalite Mso 3.42 4.54 1.32 4.33 4.54 4.56 1.22 0.03 'f.9B Ca0 9,21 0.90 5.46 0.69 n.d. 0.06 0.05 Greenalite, which may be chemically charac- Naf 0.030 0.056 0.023 0.023 0.093 2.95 0.04 0.03 terned as (Fe,Mg)uSieOro(OIDe,is structurally Kf 0.31 0.31 0.17 0.17 0.36 0,12 0.05 0.03 '1.19 tle Fe-rich analogueof antigorite (Gruner 1936). H20(a) l.l7 2.97 3.19 4.38 2,45 5.08 6.03 Published wet chemical analyses (e.s. Leith HN(.) 0.17 0.38 0.t3 0.07 0.51 0,n 0.02 0.05 t936, 1,946:,and other references P*s 0.050 0.055 0.0&1 0.006 0.12 0.055 tmce 0.04 1903; Gruner '16.4 co2 16.6 5.4 7.2 19.8 0.03 0.02 0.05 in Table 2) show large amounts of Fe2Os, rang- s n.d. n.d. n.d. 0.005 0.030 0.010 0.041 0.074 ing from 8.40 to a maximum of 34.85 ttrt. Vo a's c n.d. n.d. n.d. n.d. n.d. n.d. 0.09 <0.02 given by Gruner (1936). 'Whetherreported in an analysis Total 100.03 100.52 99.83 100.04100.16 99.52 99.98 99.84 all or some of this ferric iron sompo- n.!. - rcre oel{w. Analyso I @ b Dy mynarc !. Lotter, uepl nent is part of the original structure or whether of G@logy,lndlam Uniyerelq, Bl@nington, Irdlana. Analys6 7 it resu,lts from oxidation during burial, or subse- and 8 by Hlrcshl Harmura,Tokyo l6titute of T(hmlogy, Tokyo. l. Hmells Rlver ars, D.D.H.1032D at 183', frm the L(er ircn- quent weathering, is still unknown. Alaos ranges fomtlon mqbef (LIF) comlsting of nagnetlte, grenallte, stllp- only from trace amounts to a maximum of 3.03 rcnelare, chert, nlnresotaite, arkerlte and slderlte. (Cochrane 2. HowellsRlver area, D.D.H.10390 at 130', lrcn tie L@n Chertmfl- wt. Vo & Edwards 1960). Except for ber (LC), co6lstlng of greemllte, stllprcnelane, nlm6otaite, an- a considerable range i,o MgO, other oxide com- kslte, magnetlte.siderlte, and trace of chert. 3. ttorells Rlver area, D.D.H.1039D at 154', frcm the L@n orertmfl- ponent$ are generally present in amounts less ber (Lc), conslsting of greemlite, nagmtite, mlmsotalte, cal- no. 3 (fable quartz. than 1 wt. Vo, although analysis cite. siderlte and gives per 4. Horells Rlver area. D.D.ll. 10390at 186r, frcm the L@n 0rertns- 2) an NazO content of 1.4 weight cent. ber (LC), onslstlng of mlnngoialte, greenallte, dolmlte, The high Na"O (1.4 wt. Vo) in analysis 3 of Ta- slderlte, stllpmnelane, mgnetJte, quartz, and nircr calclte. 5. Horells Rlver area, D.D,H.1039D at 459', lrco the Lowerlrcn- ble 2 might be due to a 5 to 6 7o admixture of fomtlon mmber (LIF) conslstlngof magnetite,stllpmmelane, an unidentified mineral 1972). quartz. Q,ajac nimsotalte, slderlte and complete analysesin Table 2 have been 6. l{althler Lakearea, 23 nll6 mrthuGt of Scheffenllle, Quebs, The fM bmds of si licat*carbonat*lmn-fomtlon (sCIF)consisting oJ recalculated on the basis of 18(O,OH) per for- (rl pmmelane quartz. mgnetlte, cmcldolI te Sckl te), stll and mula unit and on an anhydrous basis, for ease of 7. scheffewtl l e nl nl ng area, sll lcat+carbomte I rcn-fomation Dq- ber (sclF), corelstlng of mlmGotalte (95-98s), and tracs of comparison with the electron probe data. The quartz and mgnetita. microprobe analyses have been recalculated on 8. Scheffenllle n'lnlng ard, RuthSlate, coEisting of chert, hsa- tlte, llmnlte, andbmn, oxidizedmlmesotaite. an anhydrous basis of 14 oxygens in which ttrc 478 THE CANADIAN MINERALOGIST

TABLE2. REPRESII\TATIVECOMPLETE ANALYSES A},ID ILICTROI{ I.IICROPROBE AMLYSES OF CI,:EIIALITE l 2 3 5 6 7 8 Si02 32.02 32.27 .Ii02 35.87 32.91 32.89 0.c5 4.25 n,d 0.07 0.00 0,02 0.09 At z0: 1.01 3.03 0.51 0.1 r .90 1,52 1.52 l.6l Fe203 22.9a 14,83 14.55 50.32j 51.27r 52.5ir 50.9Cr t€0 2!.15 35.00 35,1B ,Hd 0,s7 1.76 0.t 0.20 0.34 0.43 0 .59 Hso 5.34 '1.43 3.65 4.4 3.tl 1.29 2. Ca0 trace 0.03 0.00 0.03 0.07 0.17 hzo .-- 0.t4 1.4 n.d. 0.15 0.06 0.00 0.08 t'/) 0.10 0.00 0.0 0.05 0.00 0.00 0.01 Hzo(+) 9 -52 e.71 6.66 'll.13it l0.9ti ll.29li 12.7671 il.90ri H/(-) :- 2.37 0.04 Tohl 99.99 t00.48 99.90*1 (r00.00) (too.0o) (too.o0) (l0o.o0) (100.00) I|

a@Ldhd on b@b of r8(0,0H) ta oxygensI te(o,ou) 14orygens I ts(o,oH) 14 0xyqe6 I si 3.6eel: o, 0.13ij -'- 3, 3.ffi1,.*3.ilil,.* 3'3111,.*3'#314 Xl#lq.r: 2'lllia.zz ;'6so AI 0.000 3'333i,.* 14'oo I 0.027I 0.3021 0.351I 0.0721 0.0691 I 0.0141 Ti 0.000| 0.000 0.219l 0.r82I | 0.004 0.005 0.022 0.022 0.000 0.002 r.995{ s.os:l | I 0.009 l ;;2+ 2.816| l:|i!l s.orrrl l:il? 4.6441 4.7574 5.3467 s.zosj.l HnZ* 0.000f5.73 0.00016.04 0.088| 5.77 0.089 0.tS0 0.172?5.t5 ng 0.9t9 15.89 lf.t2 o.oto i5.55 0.044 15.98 0.t€l I 0.965 | 0.377 0.382 0.655| 0.626t 0.769 15.93 0.c00 0.260 | 0.111 I 0.000 | 0.000 0.000 0.004 o.oo4 o.oool Na 0.000 0.000 I 0.009 c.t22 I 0.032 0.031 t\.:'17 0.31:, 0.000 0.0t0 I c,00rI 0.000 0.016J j 0.ll'l I 0.015I 0.i.J0 0.000I 0.000l 0. vCCl 0.r02 j 0t 7.336 ---@r seEmrne; n.d.: mne detetd * 0.06 rt. r FaTa(W tt H20 valts obtained by subtracting €letrcn n.icmpebe total iroir i6rit 1' altabik lrcn Fomation. (Gruner 'lite ltllnnesob 1946). 2. orcenalile grdrules, RoperRlv€r lrcn Foetion, Austfalia (Cochrdne& rd/lrds 1960). 3. of medlm size' sokomn "i. 6reend- srdJn Fomatlon,.canioatzaiai-iiiiij.- e"ii"arrie gra*t", 0uniii,,i'i;; 6;;ii;", 0nrnr.to, caeda (nofan & papJr.el9/3). 7;"9;D'f' l9iil'.$"ps.183'i ereflite-sranutes,ln ctioi, witir-i*erii"-iiiar. s, ialr! sj, ;r; itiiimiii!-iJ,-naenerre, nrnnesoraite, and otDr,.iderlb li'tii"iiriili"3'i"r.ii"'i!;i;.'il'i;i:oli lil-;,:ir*';:"0:;:i:"6fBy,:sril;:rnll#If;,!i L?iliil-"il'fl:l.ili coarse-gralnd @ngamanslderlie @tfix (Fel.z7Ms0.l9\.4ecuo.oo). :1j""1if:,3:'i;,,:':;"""-fl::i" 8.'D.D.H. 1039i,-depthtoe'i rmti g"olniii:o granuie inctosec'tr ia,:,ui iartoi.iu. sr- t" ftl.h ."".i.r. *{n]Jof@4a@ando]o ompi i tion ( Ldt reo. __f .00 1ol&3.47In6. 34 ) .

8(OH) groups in the formulation of Fe0Si4O10light (Gruner 1946). All greenalitesof this study (OII)" are considered equivalent to 4 oxygens. occur in felted massesand show a grain size of As no information can be obtained in electron about one micron or less for individual crystal- probe techniques on the oxidation state of the lites; they exhibit bluish-grey interference col- elements, all iron has been recalculated as FeO. ours. Zajac (1972) reports one occurrence of S_omeof the analyses in Table 2 (nos. 3 and 4) tabular greenalite porphyroblasts (200 to 8,00 show atomic Si contents which are considerably microns long) in a finer-grained greenalite ma- larger than 4.00. During electron probe analysis trix. This coarse greenalite, which is pleochroic of greenalite granules it is not uncommoj ro pale yellow to green, was used for analysis obtain !r9m SiOg contents that exceed stoichiometric 3 in Table 2. This greenalite occurs in the Soko- requirements. Floran (personal communication) man Formation adjacent to a diabase dvke. Za- high Si values in analyses of green- jac (1972) interprets 'alite$-ds $milar the increase in grain size from the Gunflint Iron Formation. ,{s the to be due to contact metamorphic effects. minglal This is commonly exhemely finely inter- occr[rence is exceptional because almost all grown with cherl it seems probable ihat the known greenalites ar€ extremely fine-grained eroneously high Si values are caused by submi- even though the ass.ociatedminerals show well- croscopic intergrowths with chert. Although the -developed medium- to coarse-grained recrystal- ?nalys9. in Table 2 were made on greenalites lization textures. This general tendency of green- from highly variable assemblagestheir composi- alite not to recrystallize to a coarser grain size tional range is remarkably small. Electron probe may be due to possible stacking mismatches and analyses were made on green greenalite gra- misfits between the atomic layers in the struc- nulesnon surrounding light green greenalite-ce- ture. In the Mg-analogue of greenalite, antigo. ment as well as dark brownish-green greenalite rite, -'J ese layers are undulating and may show oolites and granules, but no distinctive changes considerablemisfit (Kunze 1956). in composition were found. It is concluded that tle changes in colour and intensitv of colora- Stilpnomelane tion are due to changesin the Fesi/Fe2+ ratio. The earliest and most complete chemical, In thin section the colour of greenalite varies physical and structurai data on stilpnomelane ftom olive green to dark greenish brown. Be- were dbtained by Gruner (1937) on material cause of its generally extremely fine grain size, from- the Mesabi Range and by Hutton (1933) it may appear isotropic under doubly polarized on stilpnomelane from metamorphic assemblages LOW-GRADE METAMORPHIC PRECAMBRIAN IRON-FORMATION 479

in Otago, New Zealand. The chemistry of stil- (1969) has shown that green ferro-stilpnomelane pnomelane is considerably more complex than in metamorphic assemblagesfrom Otago, New that of the other two associated Fe-silicates, Tnaland, may weather to brown ferri-stilpno- greenalite and minnesotaite. melane on the outer surface. Znn (1960) has The crystal structure of stilpnomelane has suggested that most primary stilpnomelane is been determined recently by Eggleton (1972) ferrous in composition. In one of the occurrences who suggeststhe following average formula: in this study @.D.H. 1039, depth 459') light green stilpnomelane in chert-rich bands with (Ca"Na,K)a (SiaaAle) CIio.rAla.gFeas.sMns."Mge.") minor magnetite alternates with dark bro.wn (O,OH)zrs'rHzO stilpnomelane in siderite-magnetite bands. As If stilpnomelaneis consideredto be mainly a hy- the mineralogically distinct bands are finely in- drous Fe-Mg-Al silicate, this formulation can terlayered, and as outcrop weathering is not a be simplified to (Fe,Mg,Al)r.'(Si,Al)n (O,OH)rz. factor in this occurrence, it seems probable that .rH,O. This formulation facilitates comparison tle two types of stilpnomelane have dissimilar with the chemistry of greenalite, FeeSiaOro(OH)e Fe"+/Fez+ ratios as a result of variations in the and minnesotaite, FeaSLOro(OH)r.The structure original, sedimentary or diagenetic conditions. of stilpnomelane has features which are present Analysis 10 (Ia le 3) was made on a green stil- in talc (or minnssslaite), in trioctahedral micas. pnomelane; an analysis of tle adjoining brown and in antigorite (or greenalite), as well as large stilpnomelane showed no distinctive chemical cavities for Na, K, Ca, and H:O as in zeolitic differences. structures (Eggleton 1972). H:0 in published analyses of stilpnomelane A large variation in Fe:Oa qontent is reflected shows a large range of values. Eggleton's (1972, in published analyses (Table 3) with a range recalculation of 48 complete analyses on the from a low of 2.90 wt. Vo (trendall. & Blockley basis of (Si+A1+Ti+Fe*Mn*Mg) = 120 re- 1970) to a high of. 33.24 vtt. 7o FeaOa(Ayres sults in OH values ranging from 10.7 to 48.7. 1940). How much of this ferris comFonent is HzO values in the same analyses range from part of the original, unaltered and unoxidized O.2 ta 72.0. T:he average (OII) in Eggleton'o stilpnomelane structure is uncertain. Robinson table 4a for 37 representative analyses is 28.7,

IABLE 3. REPRESENTATIVE@I{PLTTT AMLYSES AND ELECTRON MTCROPROBE AMTYSES OF STILPNOI4ELAI{E 'l 2 3 4 5 6 7 8 9 l0 sio2 45.54 42.42 45,24 44.2 50.00 45.70 45.38 47.0s 46.85 45.99 Tt02 0.26 0.33 n.d. 0.00 0.07 0.00 0.08. 0.00 0.04 rl d: 4.75 6.71 4.5 4.25 5.30 4.48 4.22 4.20 4.23 Fe203 2.90 25.34 35.2i 26.mr 27.nt 33.50r 36,281 36.70i 33.30f feo 25.38 0.85 l'lno 0.13 2.27 0.60 0.1 o.l5' 0.10 0 .23 0.22 0.08 l,fgo 7.75 5.m 7.67 3.0 9.03 4.40 3.86 2.60 2.62 4.t6 Cao 0.04 n.d. I.91 0.4 0.20 0.21 0.05 0,35 .l{ad 0.82 0.03 n.d. 0.08 0,02 0.t5 0.42 0.42 0.28 (20 1.96 n.d. 1.67 2,0 0.07 0.69 1 -41 I .06 2.15 120(r) 8.46 8.33 6.72 .t.56 'I I 0.6rt I 0.021. 10.96f+ I 0.53. 7.66tf 7.88rt 9.41rf fl20(-) .45 Total 99.55 100.47 100.45 troooo1 troolot troffit troo-ool trooool troo-r troolol *ii"#{i,i-;i#-;;'*ii[li7'"*t.itr:;- it i 3ffl40o j.itil,* i:iitln,o i.isiii:-3'li3l..*3'.Lg1l'.* 0.rs7l 0.133 0.000I 0.002 2.4641rr? 2.258 0.0rsl-'" 0.0053.20 r,rfrl,,i,i;il,-l,ill},,' 0.3131 0.s03 ff"l,ilil,,' i,ir{|., ;,3$1,,'iifrl,',;'lii},', 0.004| 0.030 0.065I 0.044 i, i:!;ili:ltil i:iill trifrli:il?l i:ifl ililil i:iiil 0.r09 I 0.223 n.d.'rcnedet*ted;dadd1tlonale-Isents.]isted:n2050.44 Itotal Fe fcalc. as Feo only ftH20 obbined by subtract{n9 electrcn m{crcprobe total fio; lO0%.

I' FerrcstilpnomelaE frcm the Brocknanlrcn.Fomation, Hmersley Grcup, Australla (Trendall & Blockley l97O). 2. Sti'lp@melanefron Jfon-fomauon near - Crystal Falls, Michlgan (Ayr6 1940). ,3. Stilpnooelane fmm albtte-stitpnomelare-acdnolite schist, Otago, Nq Zealard (Hutton lg4O). 4. Stilpnonelane granule frcn Gunfllnt lrc[Fomauonr 0ntarlo (Floran & Papike 1973). 5. uslthier Lake araa, 23 nit6 mrihwest of sciefferville, quebec; stllpnomelane-riebckite (cEcidollte){agnetite-quaftz assdblage (see Table I, m. 5 tof rlebeckite amlysjs). 6, Ffanclscan Fomatlon in LaytonvJlle Quafy, Mqdeclno County' califoxnia; crcssltd (or ferrcglau;ophane dependingon Fe2+/Fe3+)-stllpEmeline-quartz schlst (metachert of ChGteman1966). A bulk analysjsof thls schlst ls as foliors: stot-sz.4, Ti0z-0.15, Ai203-4.40,Fe203-1.85, -spectrcgraphicailyFeO-3.Oti, Mm-o.34, Mi0-1.60,CaO 1.35, Ndzo-0.75,Kr-o.82, Hd(+) r.03, Hr(-) 0.04, p205-0.079,cO2:0.37, S-0.040, totat-96,30 wt. Z: 6timabd-ili-0.05, Co 0.3 and Cu-o.3Ht.-2. l'{.E.eoller, Indla@ UnlveElty; inalyst (seETabte 8, m. 6, for crcssite aDlysii). 7' o.D.H. 10320, depth 183'; brNn' {ell-crystalllzed stllpnorelane in chert-ankerJte-sJderite bands (s@ Table 5, m. 5 fof ankerlte ard Table 6, rc. t for.siderlte a6lysls). B. D.D.H.1039D, depti l30'; brcwn, uell-crystallized stllpnonelane adJacent to very flm-grained greenallte, dhedral @g- netlte and ankerite. 9. D.0.H. 1039D,depth 186'; brown, well-crystalllzed stllpnonelane ln mlm6oialte, grmmlite, dotooite, siderlte, mgm- tlte and quartz assobld9€. 10. D.D.H. I039D. depth 459'; Hell-crystnlllzed grefl gtl'tpnomelane,plschrcic im al@st colorless to medJm gres. AdJacentt! bmn stilpnorelane, quartz, mgnetite, 6lnnsotalte and slderlte. lhe analysl! of bnn stilp@re'lane is almst ldentical to tie above, the colour varlat'lon b€ing mlnly a reflstlon of varlable ferrcus-feffic ratlo. 480 THE CANADIAN MINERALOGIST out of a total of. 2L6 (O,OH). In the above- in smaller amounts than K:O. It is of interest mentioned, simFlified formulation which was to note that analyses 5 and 6 show very low obtained by dividing by J.8, the 216 (O,OH) Na:O contents. In these relatively NarO-rich reduce to 12 (O,OH) of which approximately L rocks the NazO component is concentrated in to 1.5 OH are part of the total 12 (O,OH). All the coexisting Na-amphibole. analyses in Table 3 have been recalculated on Most of the stilpnomelane occurrences in the the approximate and anyhdrous basis of 11 oxy- Sokoman Formation of tle Howells Riyer area gens.The additional oxygenscontributed by HzO are pleochroic from light golden yellow to dark molecules in Eggleton's formulation were ignored brown, suggesting a considerable ferric iron because of tle difficulty in distinguishing be- content. Some assemblagescontain a colourless tween absorbed and structural water bv chem- to light green pleochroic variety of stilpnome- ical analytical methods. lane as well, and in one occurrence (Fig. 6C) ALO" is clearly a major constituent in stilpno- brown and green stilpnomelane grains adjoin melane, as can be seen from the selected anal- each other, reflecting variations in the oxidation yses in Table 3. MCO is highly varia,ble, wit} state of the primary (depositional or diagenetic) the most Mg-rich stilpnomelane in this study assemblages. The texture of stilpnomelane is being from a stilpnomelane-crocidotte-magnetite- strikingly different from tlat o,f greenalite. Stil- chert assemblage (analysis 5) and the least Mg- pnomelane occurs in well-recrystallized sheaves, rich varieties being from greenalite-stilpno- sprays and bundles which frequently cross-cut melane-minnesotaite-carbonate-magnetite assem- very fine-grained greenalite granules and bands blages (analyses 8 and 9). MnO contents range (Fig. 6D). In carbonate-chert-magnetite-stilpno- from very small to several per cent. The highest melane assemblages,the stilpnomelane sheaves MnO value (5.23 wt. Vo) n this study is found are an integral part of the mosaic textures formed in analysis 6 for a stilpnomelane from the Fran- by recrystallization (Fig. 68). ciscan Formation. KsO as well as NazO are relatively major constituents in moet analyses. Minnesotaite Analysis 10 Clable 3) shows a K content of As shown by Gruner (L944), minnesotaite has O.223 pet 11.orygens. NaO is generally present a structure similar to that of talc MgsSiaOro(OH)r"

IABLE4. REPRESENTA1IVEcoMpLErE ANALysEs ANo ELECTR0N l{IcmpR0BE AMLysEs 0F tilNNEsoTAtTE

Sl02 51.29 51.38 50.36 51.78 49.40 Tl02 0,04 n.d.r 0.00 0 .04 0.00 0.00 Al d3 0.61 n.d.* 0.00 0.89 0.57 0.6B 0.54 Fed3 2.00 23.4r 40.82f 42,621 39.45t Fe0 33.66 20.06 MrO 0.12 trace 0.18 0.25 0.37 0.40 Mgo 6,26 16.Bl I 5.8 2.80 2.07 3.07 Cao 0.00 0.03 0.0 0.01 0.02 0.01 0 .20 Na, 0.08 n.d.* 0.00 0 .00 0.06 0 .09 K20 0.03 n:d.t 0.0 0.49 0.31 0.25 H20(+) 5.54 3.83 3.43i1 3.72t+ 4.04ii 6.60ri Hr(-) 0.24 0.05 T0ta l 99.87 99.91 (r00.00) .(r 00 .00) (roo.oo) (loo.00) (loo.00)

Rqal@lateA ot dta boia of l2(0,0H) 1l oxyger l l orygens. e ll oxygens-*i$,, 3.949t. ^^ 3.930)- ^^ ^rli u.urUi'Pl],.* u.uaz)l'?191+.oo "'-' ;';;; | 3.s5 *'" 0.000J o.osrI o.o5zJ'''o 3:8ii),." at o.oool o.oo3l o,oooI o.oool o.o3ol o.oool o.o43l o.oooI Tr^ 0.002| 0.002| 0.000 0.000 0.000 0.002 0.000 0.000 i | | | I [:ii g'Hll z.zatl t.301rl I .404i1 2.6241 2.78111 2.55411 2.64r z.sz 3.03 r'os I i.riz* o.ooO'f o.ooef o.ooof3'06 o.6ooI o.orzf3'04o.otz i3'08 o.ozrf3'ol o.o27f3'oB Ms 0.691| 0.i18| 1.754 | 1.6B9 0.34 I 0.241I 0.347| 0.364| ca 0.000I 0.000| 0.002I 0.0001 o.oor o.ooz I 0.001I 0.0r7| Na 0.012 o.otzl 0.000 0.000i 0.001 0.000l 0.009| 0.011I K 0.003J 0.003J. 0.000J 0.000J 0.048J c.035J 0.030_J 0.025) 0H 2.736 l. ElwabtkIrcn Forutlon, ltilnncota (6rumr 1944). 2, BrcckmanIrcn Fomat.lon,Hamenlry orcup, AustralJa (Trenda]j& B.lock.tey1970). '1039D, 3. Gunfllnt Ircn Fomttlon,ontado (noran & Paplke1973). 4. D.D.H. depth 130'; mlnnesotlltesprays fomd frcm very fim-graired greenalite. 5. D.D.H.10390, deptn 154'; ninnesotaltesprays cuttlng acrcsscoa6e-gra1red slderite (Fe1.73t{6.141i1ng.69Cag.64). 6. D.D.H.1039D' depti 136ri nimesotalte rcsettc fomed lmlde gredalite bandsand grarul6. 7. D.o.H. t0390,depth 1g6,i mimesotaite 'sheaves cuttlng acmss edgeof carbonateoollte conslstlng of dolomlte(Ca1.g2ilq.5gFe0.z9flno.ll) ad s.lderite (Fel.22MgO.*lrlnp.45Ca6,1g)- LOW.GRADB METAMORPHIC PRECAMBRIAN IRON-FORMATION 481

Chemical analyses of minnesotaite show that size measure was used to differentiate chert from the only major substitution occurs between Mg quartz. When the grains appear as very fine. and Fe, with almost all of the iron present as grained under high magnification it is referred Fe'+ Cfable 4, nos. L ard 2), Other oxide com- to as chert; when coarser grain sizes are noted ponents are generally considerably less than 1 it is stated to be quartz. wt. Vo each. Minnesotaite is ch'emically the Quartz or chert are found as thin bands in- simplest'and tle least hydrous of the three iron- terlayered with magnetite, Fe-silicates and car- silicates found in very low-grade metamorphic bonates, as granules and oolites, as regions in iron-formations. The complete analyses in Ta- complex silicate-carbonate-chertgranules, and as ble 4 have been recalculated on the basis of cements about granules and oolites. elec- 12(O,OH) as well as Ll oxygens, and the Members ol the dolomite-ankerite series tron probe analyses have been recalsulated on the same anhydrous basis of 11 oxygens. As Members of this carbonate seriesare about as the Fe'+ component is small, the two methods abundant as siderite in these iron-rich rocks. give essentially identical results. Minnesotaite Ankerite-type compositions are more common analyses 1 to 3 (Table 4) are considerablymore in these occrurences than the generally Mn- Mg-rich than those obtained in this study. rich dolomites. Representativeanalyses of mem- In thin section minnesotaite is transparent, bers of the dolomite-ankerite series are given in clear and non-pleochroic. It is much coamer Table 5. The most magnesian dolomite found than greenalite,but generally not as tabular and contains L4.9O wt. Vo Mgp and 8.67 wt. Vo chunky as the associatedstilpnomelane sheaves, MnO. MnO in dolomite ranges from 2.51 to Minnesotaite sprays and rosettes have beeu 13.37 wt. %. Ankerites range up to a maximum found to cut across greenalite, stilpnomelane and of.22.77 wt. Vo F{ and show a range in MnO carbonates. No distinct difference in composi- from 1.25 to 4.63 wL % . Many of the dolomites minnesotaiteswhich tex- and ankerites coexist with siderite, and such tion was found among ^t\e turally appear to have formed at the expense pairs are shown in Figure 2. element frac- of earlier silicates (greenalite s1 st'lFnomelane) tionation between the carbonates (Fig. 3), and or car,bonatesand chert. therefore the tieline slope between coexisting pairs, is very similar to that presented by French Magnetite (1968) for the unmetamorphosed and essen- Magnetite is present in all of the unleached tially unaifected @rench's Zones 1 and 2) parts assemblagesof this study. Hematite was found of the Biwabik Iron Formation. It should be only in a sample of Ruth Slate from the Schef- noted that the bulk chemical analyses (Iable 1) ferville mining area, where the rocks are locally of unaltered Sokoman Formation show a range highly oxidized and leached (analysis 8, Table from only 0.15 to 1.78 wt, Vo MnO. All Mn must 1). With electron microprobe techniques the variation in very small amounts of elements such TABLE5. REPRESETIATIYEELECTNON ilICNOPNOBE AMLYsES OF IiEAERS OF as Mg, Al, Ti and Mn could not be established 7234557 witb confidence. In magnetite-manganoan car- 'l bonate (dolomite, ankerite or siderite) assem- Feo 5.03 5.90 6.07 l.l9 22.77 15.6] l6.m l|Io 'll10.30 8.00 9.08 2.51 1.25 3.57 3.25 blages content of magnetite was found lg0 .18 I t .88 12.9 12.24 6.33 7.27 6.85 the MnO Cao 29.01 31.8 33.15 8.73 26.80 6.79 6.8 to be only 0.04 wt. %. MgO, AlzOr and TiO: @/r u.48 42.93 39.31 t14.33 42.85 46.76 46.81 were found to be 0.02, 0.02 and O.O3 wt. % rotal (100.00) (100.00) (100.00) 000.00) (lqt.00) [100.00) (1(x1.d)) respectively. Magnetite, therefore, appears to naaal@l,ated @ lhe b@i,s of 2(Ee']h'W'ca) bo essentiallypure FeaOain these iron-formation 0.139 0.l'/ 0.152 0.304 0.654 0.459 0.490 0.288 0.215 0.231 0.069 0.036 0.109 0.l(X) assemblages. 0.!t9 0.563 0.553 0.592 0.324 0.390 0.370 Iq thin section, all magnetite occurrencesex- L035 0.986 1.032 hibit feahrres of recrystallization, ranging from 1,2,3. D.D.H.103D, depti t86'. lb, I ls certnl' fiE-gnlned, somhat cloudy part of mngmil doloalte oolite. ib. 2 is very fine-grained to coarse-grained octahedral clear, outer edge of sm mrqa@n dolffilte oolite. Amll6ls of crystals well-developed crystal outlines siderlte pathes rltiln tt|ls oo]'lte ls glven ln Tabla 6, aml. 6. with lb. 3 ls clear, outer edgeof @Bplq doloElte @llte rhlch en- against coexisting minerals such as greenalite and clos6 firgnlned stllpmlane ad grgalite. 4. D.D.H.10390, deptfi l$', clear, euhsdnl, Edi@ to c@EF recrystallized sarbonates. gralrEd dolslte coslstlng rlti grmalite ad mgEtite. 5. D.D.tl. 10320, depth ltB'. qler, bat|sparmt' mdlegmlrEd, EnEtal l ized anlsl te, cogxistirq ulth stll pn@elane, siderl te, Chert or qua?tz grmllte ald mgretite. (stllFtorelam ahalysls in Table 3, m. 7: slderlte aEll6ls ln Table5, D. li gremllte mlFls The grain size of the SiOz phase ranges from in Tlble 2, m. 5). 6.7.- D.D.lt. lO39D.deDth 130'. llo. 5 ls cl6r' flF t! Edl@ submicroscopic in some cements around gra- orElEd Elxt!re ol qlarular snterlte 8rd sldsrlts' c@ntltE &uoslte ankedte ;nd sr€mllte ollt6 ald grarulE (sa nules to medium- to coarse-grained in firlly re- talie o. rc. 2 for slderlte rmlysls). lb. 7 is ailslte crystallized SiOorich bands. No specific grain granule. 482 THE CANADIAN MINERALOGIST

B a ro32D-183 tn?on-r?n

mqg- groen-sfilp- mog- green-slilp - rlinn- onk- sid- ch minn- onk- sid- ch

rlt00oj FeO+ MnO AA ,8 t.o FeO+MnO Feo+MnO Mole Percenl SIDERITE

D E F lo39D-t54 lo39D-t86 lo39D-459 - - - mog green minn mog - green- slilp - mog - slilp - minn- sid -qlz sid-col-qtz minn- dol- sid- col - qiz

FqO+MnO Feo+MnO FeO+ MnO

Frc. 2. Compositions of carbonates in silicate-rich horizons of the Sokoman Formation in the Howells $iv-9r ar91 Labrador, Newfoundland, Canada. A. Composite dragram of all carbonate qompositions in five ditferent silicate-rich. assemblages. Tielines conneat coexisGg ghysically touching) carbonates. B. to F. Carbonate compositions of individual assemblages. TielineJ connect physically tJuching carbonate pairs. Abbreviations are as folows: mag - magnetite; green-greenalite; stilp - stilpnJmelane; minn - minnesotaite; ank - ankerite; sid - siderite; cal - calcite-; cn - cnertl qtz - quafiz.

3ot o o e o e Y o o "8 &o o o'. O'3 E a

e ot >R = + R oi o t = o; o ON lf e fl .o Lo : o 20 40160 80 l@ l@ o n 40l60 go too SIDERITE DOLOMITE-ANKERITECALCITE SIDERITE CALCITE SIDERIIE DOLOMITE-ANKERITECALCITE CoO (moleperceni) CoO (mob percenl) CoO(molepercenf)

Flc. 3; Fe, ldn antl Mg fractionation among the carbonatss shown in Figure 2A. Tielines connect coexisting Ghysically touching) carbonate paim. LOW-GRADB METAMORPHIC PRECAMBRIAN IRON-FORMATION 483

TABLE6. REPRESENTATIVEELECTNON MICROPROBE AMLYSE5 OF SIDERITE t2 3 Feo 54.24 50.48 54.53 M,m $.54 38.59 37.74 36.03 n.62 50.04 I'lno 0.23 3.27 3.76 9.38 3.75 1s.48 13.61 17.58 1,28 1.s7 l'lso 3.80 2.5B '1.652.15 3.06 2.09 3.14 3.98 3.50 6.m s.48 Cao 0.45 1.75 1.53 0.56 1.09 2.46 2.33 0.42 0.65 c!21 41.2s 41.92 37.91 41.83 40.06 41.70 42.21 40.56 40.88 41.86 Total (100.00)(100.00) (100.00)(100.00) (100..00) (100.00) (100.00) (l0o.o0) (loo.0o) (.100.00) Feca'Iaulatedon lAe b@i8 af 2(Ee,Itr,tdg,Ca) Fe 1.755 I .665 1 ,697 1.436 1.733 1.260 1.222 1.143 1.s6B | .598 l.h 0.008 0. I 09 0.'ll9 0.323 0.123 0.512 0.446 0.565 0.040 0.064 Mg 0 .219 0..l52 0.ilg 0,177 0.r2r 0.t83 0.230 0.198 0.376 0.312 Ca 0.019 0.074 0.066 0.064 0.023 0.046 0.102 0.095 0.017 0.027

1. D.D.d. 10320,depth 183'i coa6e-9ralned, recrystalllzed siderlte jn slderite-ankerlte-stilDnonelane- grenalite{agnetlte-chert asssnblage(see Table 3, m.7 for stilprcnelane analyslsi Table 5, m. 5 for ankerite analysisi Table 2, no. 5 for qreenalite aBlysls). 2,3. D.D.H..1039D,depth 130'. No. 2 is r6rystalllzed, euhkrai siderlte csenting ankerlte grarul6 and oolltes (se Table 5, F.6 for ankerlte analysJs). No. 3 is rsrystallized; nedlun_qr;lnedeu_ hedral siderlte-csentlng cmplq g.aeles conslsting of greenallte, ankerite, airdmlmes6talte (for ankerlte amlysls se Table 5, no. 7, and see Fiq. llB). 4'5. 0.D.H. 1039D'deptn 154'. No. 4 ls slderite li coareelv r4rystalllzed siderite-calclte asss- blage (see Table 7, no. 2 for calcite analyslsi see also ilq. liA). No. 5 ls recrystalllzed slder- Ite.on outer edgeof greenalit! granules, aqented by slmll;r slderite (see Table 2: m. 7 for gree- nal ite analysls). 6,7,8, D.D.H. 1039D,depth 186', No. 6 ls'flne-gralred, euhedralslderite regioc ln l,tn-richdolonlte 0ollt6 (for dolomlte amlyss see Table 5, rcs. 1,2,3, and see Fig. IOC). Nos. 7 & 8 are simllar slderite segregationsl6lde dolonite @lltes. 9,10. 0.D,rl. 1039D,deptn 459'. No. 9 ls medim-gralned, rcrystailized slderlte tn finelv banded stilpnmelaG-slder'lte-nagnetlte assEnbldges(see Table 3, m. lo fof stllpnonelane analvsls). No. l0 is nedlm-gralned slderlte in tJlln sldarite-nagnetlte-mlmr stilprcnela;e bands, be concentlated in dolomite, ankerite and side- patterns in dolomite oolites as well as in medium- rite as none of the other coexisting phasescontain to coarse-grained mosaic intergrowths with an- significant MnO. Perrault (1955) concluded kerite in finely banded magnetite- carbonate-stil- that ferroan dolomite * siderite compatibilities pnomelane-greenalite assemblages. are common in many members of tle Sokoman Formation. Calcite ^Polomite generally makes up the major part of the well-rounded carbonate oolites fbe, Calcite in these iron-formation assemblages Gi!s. is 10B). central, somewhat cloudy paits have by far the least abundant carbonate. Repre- -T,he sentative given more Mn and somewhat less Mg tb.an the more analyses are in Table 7. FeO in transparent calcite ranges from 4.33 to a maximum of 5.46 clear outer edges of the same ool.ites .wt. (analyses I and 2, Table 5). These seme oolites Vo in a calcite coexisting with siderite. MnO ranges show discontinuous, unevenly distributed regions from 0.82 to 4.07 wt. % and MgO of siderite which are subpaiallel to the coicen- xangesfrom 0.37 to 0,72 wt. %. In the (1039D-154 tric banding of rhe ooHtes (FiC. 10C). Ankerite only two assemblages and 1039D-186) occurs in medium- to coarse-grained recrystal- in which calcite was noted, it occun as sporadic, lized, subhedral grains often in a mosaic inter- fine- to medium-grained cement interlayered greenalite growth with medium-grained siderite (Fig. 1lA). with cement between granules (Fig. Such occurrences are especially common in thin- and oolites 10D) or as medium- ly banded magnetite-stilpnomelane-ankerite-side-to coarse-grained recrystallized equigranular rite-chert assemblages. grains coexising with siderite in carbonate-greenalite-minnesotaite bands. Perrault (1955) reports Siderttu that calcite is a minor constituent of the silicate- Siderite is common in the silicate-rich members of the Sokoman Formation. It is about as IABLE 7. REPRESEI{TATI,VEELECIRON MI,CROPROBE AMLYSES OF CATCITE abundant as members of the dolomite-ankerire seriesin most of the samplesof this study. Rep- 1 2 3 aqalqlated.mthebuia F€0. S.5l 5.46 5.46 of z(FeJ,eLW,ca) reoentative analysesof siderite are given in Ta- Mno 1,38 1.80 0.90 1 2 3 MsO 0.35 0.37 0.4'l ble 6. MnO in siderite ranges 0.147 from O.23 to Cao 52.51 53.19 53. Fe 0.144 0.,t45 17.58 wt. 7o, MgO from 2.09 to 6.80 wt. % mi* io'.rs !.t:i6 i;.ri f:9.111g.gfg g.gi4 and CaO from 0.45 to 3.85 wt. %. As in the Totar(r00.00) ooo.oo) (roo.oo) H l:9lJ ?:9li ?:31? case of members of the dolomite-ankerite se- ..tCoZobtalned by subtracting micrcprcbe total frcm 1001 ries, siderite houses large amounts of le2. D.D.H.lO39D, depth I54'. l{o. I is medim-gralnedcalclte MnO covering greenallte, both csenting greenalite, minnesotaite, whereas the other coexisting phases show in- and mgnetlte granulc (se Fiq. IoD). No, 2 is coarse-grained significant calcite coexistlngwlul slderite (see Fig. 1lA, and analysls 4, MnO contents. Table6). Siderite occurs as fine- to medium-grained 3. D.D.H-10390, depth 186'. CoaEe-grained,recrystallized calclte csentlng (witi greenalite) oolit6 composedof dolomite euhedral regions distributed along concentric and slderite (see Tables 5 and 6 for analyses) 484 THE CANADIAN MINERALOGIST

carbonate iron-formation in the Howells River 75' thick and 40O' wide are found in the si- area, but 7-ajac (1972) does not report calcite in licate-carbonate iron-formation member (SCIF) his study of tle Sokoman Formation in the of the Sokoman Formation (Richard E. Arndt, Schefferville area. It clearly is an uncommon personal cornmunication). One specimen from sarbonate species in these iron-formation as- this ocsurrence was analyzed (no. 6, Table 1) semblages. French (1968), in his detailed op- and studied in detail. In it, fibrous, blue riebeck- tical and r-ray diffraction study of tle carbon- ite (cross-fiber) occurs in bands up to 1 sm thick. ates of the Biwabik Iron Formation, concludes interlayered between quartz-magnetite bands. In that calcite is absent in unmetamorphosed as- thin section the riebeckite is found to be finely semblages (French's Zone t). Very minel aa6 interbanded with stilpnomelane-rich bands. The sporadic amounts as observed here by electron riebeckite is pleochroic from almost colourless probe analysis can, however, be missed easily to greenish-blue and the stilpnomelane varies by optical and .r-ray techniques. from very light yellow to medium brown. Analyses of the two minerals are given in Table Riebeckite 8 (no. 5) and Table 3 (no. 5). Representativecro- No riebeckite, or its fibrow variety, crocido- cidolite analyses from Australian and South lite, has bsen found in the Sokoman Formation African iron-formation occr[Tences are given in the Howells River area. Crocidolite-magne- in Table 8, as well as an analysis for crossite tite-quartz schists, however, are sporadically (or ferroglaucophane, depending on the Fe'+./ present in the Schefferville region in the lower Fe'* ratio) from a crossite-stilpnomelaneassem- red cherty (LRC) and pink grey cherty (PGC) blage in the Franciscan Formation, California. horizons (Zaiac 1972). In contrast, crocidolite Crocidolite analyses from iron-formations Cfa- can be a major constituent of the Precambrian ble 8, nos. 1-5) are remarkably similar except iron-formations in Australia (e.g. Trendall & for some variations in MgO content. The cros- Blockley 197O) and South Africa (e.9. Peacock site analysis from the Franciscan Formation 1928). In the Walthier Lake area, bands of in- (Table 8, no. 6) is much higher in AlzOg, and terlayered magnetite-croci'dolite-quartz up to lower in total Fe. It is noteworthv that the bulk

TABLE8. REPRESEMATIVECo,,IPLETE ANALYSES AND ELECTn0N ilICR0PR0BE AMLYSES 0F CR0CImLITEAXD RIEBECKITE t2 3 4 156 Si02 53.66 53.52 5t .94 54.14 54.64 53.73 Tl02 0.02 n.d.* 0.07 0.11 0.02 Alf: 0.08 0.41 o.20 0.6 0.46 8,91 FeZ03 17.73 17.72 18.64 21.07 29 Feo 14.67 16.?6 9.27 3st zt.lol MrO 0.07 0.01 0.06 0.05 0.34 il90 4.54 3.63 ,-.i, 7.06 4.68 Cao 0,23 0.08 0.19 0.tl 0.t2 0.14 NaC 5.0t 5.s7 6.07 5.1 7.56 K20 0.07 0.05 0.04 0,00 0.01 0.01 HZ0(r) 2,a1 2.63 2.58 2,02 Hr(, 0.33 0.32 0.31 0.02 (2.29)1r (3.5r)fi F-.- . 0.09 'total 99.2n 100.208 99.60 troo.ool troo.oot

R%atel,atad. an the b@is of

24(0,0H) 24(0,0H) 24(0,0H) 24(0,0H,r) 23 oxyge6 23 oxygeb 23 oxygere -'-- 8.3rola?r ii 3:33?),.*0.024) 0.039 "' 0.0441"'"" 0.000I "'-" 0.000J"'"' 3:333i'* Ar o.oo7l 0.04e1 o.oot^l 0.0471 0.0821 r .4e2I Ti 0.002 0.0001 0.0001 0.0081 0.008 0.01 3.] 0.002. '1.987 _l re3+ 1.987 2.1271 2.330| 3.65711 3.7331l 2.608r re?: 1.B28 2.023 2.460) r.r39l I I'lnz' 0.009 | 6.34 0.001f6.m 0.000f6.72 0.007f6,57 0.008f6.97 0.006t7.20 0.043fi.36 Mg 1.008 0.806 1.619 I 1.705 1.600 1.03I I Ca 0.037 0.012 0.031 0.0171 0.018 0.0ml 0.022| Na 1.446 r.611 1.786 1.453 1.530 1.742 1 2.r66| K 0.0 14.7 0.010.J 0.008J 0.000j 0.000l 0.002| 0.042) 0H 2.793 2.614 2.619 I .978 - ^^ F

obialned by subtractlng micmprcbe iotal fm 100 , *addltloml oDponenis Jn 6lmn-l:_Pr5 0.36, CoZ0,57, FeS20.02 wt. %, maklngtotal 100.17. In colunn 2, Pr5 0.20 Ht. ,, @klng bbl 100.40: l. Cmcldollte, ircn-fomatlon of the HameEleyGrcup, Austral16 (Trendall & Blockley 1970). 2. llasslve rl+ b4klte, lrcn-forutlon of tle H@6lg cmup, Australla (Tfendall & BIocklry 1970). 3, Ctuldollte ln c@ldollb@gretlt€-quartz ass@blagefm lrcn-fomatlon near Kllphuls, Souti Afrlca (Peacock1928). 4. Ctucldollte fm crccldollte4gEtlte-chert nssgrblageln the Sokomn to@tlon, KDb lake ar@, Qteb* ad Ilsfoundland (ZoJac 1972). 5. ls'ltJrler Lake !ro, A oll6 M of scheffenllle, Q!eb6; stllm@laE rlebcklie (crccidollt6)egmtlte-qllrtz Esrqblage (see Table 3, rc. 5 for o!1. of coqlstlng stllprco.) 6. Fnrclscan Fomtlon in LeytoBllle qlarry, ierdslm Cally, Cal lfo.nlai crcssite (or fqrcglalcophare dopendlng on ferrcus-ferrlc ratlo) ln crcssite-stllpn@sl@-quartz schlst (detach€rt of Ch6t€man 1966). S4 llb16 3, 6. 6 for amlys15 of cgqlstlng stllp|@laE. LOW-GRADE METAMORPHIC PRECAMBRIAN IRON-FORMATION 485

chemical analysesfor the crocidolite- and cros- curs as massive thin bands up ro 0.5 cm thick. site-bearing host rocks show NazO contents of The bands parallel the very fine, well-preserved 2.95 and 0.75 wt. % respectively (Iable L, no. banding common in some greenalite-magnetite- 5 and footnote no. 6 of Table 3). This NazO is chert-carbonate assem,blages(Fig. 4D). Leas concentrated almost exclusivelv in the Na-am- commonly, greenalite is found as angular and phibole as the NasO values fbr the coexisting broken fragments concentrated in bands parallel stilpnomelane are only 0.08 and O.O2 wt. Vo to the chert-magnetite banding of the host rock (Table 3, nos. 5 and 6). Stilpnomelane analyses (Fig. 5B). Large composite carbonate granules (Table 7 to L0 3), however, show Na:O values often enclose greenalite granules @ig. 5C). The ranging from 0.15 to 0.42 wt. Vo. greenalite matrix of some larger granules ap- The complete chemical analyses quoted from pears to have cracked, possibly due to shrinkage the literature in Table 8 were recalculatedon the caused by dehydration of a precursor of green- basis of z4(O,Olt, except for analysis no. 4 alite-type composition (Fig. 5A). which was alss recalculated on the basis of 23 In many occrurences greenalite is so fine- oxygens as was done for the electron micro- grained tlat, in places, it appears almost iso- probe data. Comparison of the two recalculation tropic under doubly polarized light. In some schemes for analysis no. 4 shows that the as- thicker greenalite bands, almost isotropic bands sumption of total Fe as Fe2+ only, can lead to alternate with greenalite bands having a slightly large errors in cation numbers and sation site larger grain size (about one micron). These tex- assignments when Fe3+ is high. The cation re- tural observations lead to the conclusion tlat calculations of analyses 5 and 6 are, therefore, greenali0e is probably one of the earliest and approximate at best. still only very slightly recrystallized constituelrts The crocidolite occurrences in the Sokoman of the iron-formation. In many occulrences, Formation have textures which reflect strong however, greenalite has not survived thc low- shearing,crenulation, and deformation. Such de- grade metamorphic conditions and has reacted formational textures are absent in the rocks of to form stilpnomelane'or minnesotaite (Fig. 5D) the Howells River area. Although strong structur- as discussedbelow. al metamorphic effects are clearly visible in the Stilpnomelane is not as common as greenalite Schefferville region and are virtually absent in in the rocks of this study. In contrast io gren- the Howells River area, there is no evidence to alite, stilpnomelane is generally medium-grained suggest that the low-temperature, tlermal re- and occurs in well-crystallized sheaves, rosettes gimes were much different in the two regions. and irregular patches. It is not a major constituent of oolites or granules, but occurs frequently as irregular, well-crystallized patches TExnRAL RsrerroNs within greenalite granules or bands (Fig. 6A). It commonly occurs in close association and mosaic The Sokoman Formation in the Howells River textural intergrowth with magnetite and siderite area is especially well-suited for a study pos- of tl finely banded iron-formation (FiC. 68). By sible primary, as well as diagenetic and low- far the most common stilpnomelane is highly grade metamorphic textures. Although sedimen- pleochroic from light yellow to dark brown, in- tary feafures such as very fine banding, granu- dicating a considerable Fe8+ component. How- lar and oolitic texhrres are very well preserved, ever, in a few assemblagesa colourless to light all of the assemblagesshow the effect of a very green and a dark brown pleochroic variety of stil- low-grade metamorphic overprinting, This al- pnomelane adjoin each other. As no difference in lows for the possible deduction of reaction se- composition between tlese two varieties was quences among the carbonates, Fe-silicates and found by electron microprobe methods, their chert. colour differences probably reflect differences in Greenalite occurs typically in very fine grain Fe2+/Fes+ ratios @ig.6C). tt is concluded that size in somewhat ellipsoidal granules and more these Fe'+/Fe'+ variations reflect the oxidation rarely in thinly banded oolites @igs. 4A, B). The state of original sedimentary precipitates, or dia- layering in the oolites h shown by finely alter- genetic products, and that they are not the prod- nating dark green and brownish-green banding. uct of later oxidation. Stilpnomelane has never Greenalite is found also as a cement arounil been found as a cement, ashas greenalite, around granules and oolites (Fig. C). These greenalite granules and oolites. Much stilpnomelane has cemenb are generally somewhat lighter green reacted to form 141s1minnssstaite, as shown by than the greenalite of the granules and oolites, the common cross-cutting relations sf minngss- but no distinctive difference in composition was taite sprays and roseftes in medium-grained stil- found between them. In addition, greenalite oc- pnomelane (Fig. 6D). Thme textures suggesttha.t 486 THE CANADIAN MINERALOG]ST

Frc. 4. Greenalite occurrenc€s. A. Massive greenalite granules surrounded by chert (colourless) and minnesotaite (M). B. Greenalite oolite with fine banding; lighter greenalite cement at left. White patch in oolite is minnesotaite. C. Greenalite cementing greenalite granules. Cement interior is chert (C). White sprays are minnesotaite (M). D. Thinly banded greenalite; white bands at left are minnesotaite. Bar length represents 0.2 mm. ':"t ;:: ::l:.,:

Frc. 5. Greenaliteoccurrences.-A. C_racks,_possiblvdue to shrinkage, in massive greenalite $anule. Sp-raysare minnesotaite..B. shapedfragnrents of greena'iite(dark in coloir) in (clear, .lrreqularlv chert-matrix colourless). Sideriie (S) rhombohedron at Io.werrightlC. Small-greenalitegranule inside larger manganoan dolomite oolite. Greenalite cement at left. D. Relict outliie of gree"naliteg.uofts anO oolites which now in large part consist of felted massesof minnesotaite (light coloured). Relict edges aro--commonlyaccentuated by fine stringers of magnetite at the outer edge of the granuie. fn"r" well -uy be the-result of greenalitereacting to fomr minnesotaiteand maen;tite tr"" ritr. i-ol.bar tene& xepresents0.2 mm. Frc. 6. Stilpnomelane occurrenc€s. A. Dark brown highly pleochroic stilpnomelane (St) sprays inside greenalite (G). Siderite (S) at top. 1v[innssotaite(IVI) cross-cutting stilpnomelane. Opaque.is. magnetite. B. Stilpnomelane sprays-with (dark) in 6ands of magnetite-siderite-quartz. C. Coexistence of dark brown stilpno- -eluo" (br) at iop light green stilpnomelane (et) at bottom. Opaque is magnetite. -Colourless, low relief mineral is chert (C). D. Well-crystallized dark brown sheaves of stilpnomelaae (St) cross-cutting very fine-grained greenalite (G). The itilpnomelane in turn is cut by minnesotaite (M)' Bar length represents 0.2 mm. LOW-GRADE METAIVIORP.HIC PRECAMBRIAN IRON-FORMATION 489 stilpnomelane is an early recrystallization and al interpreiations of assemblagesin low-grade reaction product greenalite-type in matrices and iron-formations), the author concludes that the in carbonate-magnetite-chert assemblages. euhedral outline of magnetite is the result of its Minnesotaite is common in all silicate and recrystallization from earlier, undoubtedlv less carbonate assemblagesof this study. It occurs well-organized FesO4 or hydrous p"zi-p"s+ as fine- to medium-grained sprays and rosettes oxides. Euhedral outline against coexisting min- and as massive bands (Figs. 7A, B). In some erals suggestsa strong force of crystalliiation, occurrences, as in the silicate-carbonatemem- but is by no means unambiguous evidence for ber (SCIF) of the Sokoman Formation (Table replacement. l, no.7), it makesup 95% or more of the whole Qua.rtz or chert are present in all assemblages rock @ig. 7C). Here the original fine sedimen- as oolites or granules, as cements,and as finely tary banding, and possible granular or oolitic granular, recrystallized bands and patches. Fig- textures have been completely obliterated. In ure 9A shows the very common occurrence of carbonate-rich occurrences minnesotaite shows gtanular quartz around relict granules and cross-cuttingr"lullenshiFs acrosscarbonate grain oolites,. and Figure 98 shows chert cementing boundaries and oolites (Fig. 7E). greenalite and chert granules. In some assem_ From the cross-cutting relations of minneso- blages quartz veinlets are noted inside fracfured taite in greenalite,stilpnomelane, and carbonates granules and oolites, indicating sems ssmsfilizn- it is concluded that minnesotaite is not primary tion of SiOr, probably during the continuous but the result of a combination of diagenetic procell from diagenesisto very low-grade meta_ and very low-grade metamorphic reactions. morphism. Magnetite, Although magnetite ranges from Carbonatesare present in all of the u,rleached fine- to coarse-grained,its form is always eu- Fe-silicate-rich assemblages of this study. Cal- hedral (octahedral). Magnetite occurs moit fre- cite, the least common carbonate, occurs jn two quently in tlin bands, interlayered with chert, rather distinct textural modes. It is found as carbonates and Fe-silicates(Figs. 8A, B). It also medium-grained cements around granules and occurs in strongly recrystallized assemblages oolites @ig. 10D) and in medium- to coarse_ which show the relict outline of magnetite gia- qti"gd intergranular mosaic intergrowths with nules (Fig. 8C). In many assemblagesit forms siderite (Fig. 11A). As its occurrenie is relative- recrystallized rims around greenalite granules, ly. sporadic it is not easily detected under the in which the magnetite borders are medium- to microscope. coarse-grained, greenalite but the center shows Members of the dolomite-ankerite seriesoccur its customary extremely fine grain size (Fig in well-crystallized medium- to coarse-grained 8D). Magnetite is the only Fe-oxide in the as- mosaic textures as a matrix for relict gianules sernblagesstudied except -in for the hematite in a of chert and dolomite or ankerite, and mas- sample of the Ruth Slate. sive bands in silicate-carbonate assemblages. Because magnetite always displays well-de- Manganiferous dolomites are the main consiitu- veloped octahedral faces against the coexisting ent. of recrystallized oolites (Figs. 10A to C) in minerals, it is concluded product to be the of which euhedral siderite patchei outline the re_ recrystallization during diagenesis and subse- lict. concentric banding of the oolitqs. These quent very low-grade metamor.phism. The eu- ooute occurrences are texturally very informa_ hedral form is not regarded as the product of tive. The central parts of the oolites are sen_ replacement granules of or granule edges, or finer-grained and somewhat cloidy, thin bands in the "1utly iron-formation. French (1968, whereasthe outer edgesare coarser-grainedand 1973) shows photomicrographs of occrurences transparent. The outer parts of the oolites also very similar to the ones described here (e. g. show the siderite rhombohedra and patchesmost French 1968, Fig. -around 11; reproduced in French abundantly. Cementing materials the 7973, as Fig. 9) but inierprets them to be the oolrtes are greenalite and calsite. These oolites result of magnetite replacing earlier silicate gra- show only features of recrystallization, none of nules. Dimroth & (L973) Chauvel show photo- replacement. They are con-cluded to be the re_ micrographs and black-line drawings (their Figs. crystallized products of a Ca-Fe-Mn_carbonate 8C and 11E) which are essentially identical to ooze, originally probably very fine-grained and the photographs shown in Figure 8. They also poorly ordered. During diagenesis and subse- interpret these textures as replacement quent of earlier _very,low-grade metamorphism the dis_ materials by magnetite. Contrary to such tex- ordered carbonate material ordired itself anA tural interpretationsn *ia-the which are common causedthe,phase sepaafion (exsolution) of side_ literature on low-grade iron-formation (see rite from the Mn-dolomite &atrix. Ankerites are French 1973 lot a listing of authors and textur- medium- to coarse-grained, granular, and often THE CANADIAN MINERALOGIST

!*--r'. -*"----r

Fro. ?. Minnesotaite ocsrurences' A' Fine-grainecl minnesotaite sprays in matrix of fine-erained quarE' B. Minnesotaite masses in what originally had been greenalite granules. Relict edges of granules, and some -aUgreenalite cement (at right side) still remain. C, Felted masses of minnesotaite which have obliterated orig'tnat textuie, d. Minnesotaite sprays cross-cutting the outer edge of a manganoan dolomite oorte. The central part of the oolite (Dl is at the upper left' Bar lengfh represents 0'2 mm' LOW-GRADE METAMORP.HIC PRECAMBRIAN IRON.FORMATION

Flc. 8. Magnetite occurences. A. Finely banded magnetite (black) - siderite-stilpnomelane iron-formation. B. Similar ocqurerrce as in A, but in reflected light, showing euhedral outline of all magnetite grains. C. Coarsely recrystallized magnetite granules in siderite-quartz-minnesotaite matrix. Doubly polarLed light. D. Greenalite granules with outer edges of magnetite. The greenalite is very fine-grained but the magnetite is recrystallized and euhedral, Bar lcngth represents 0,2 mm. 492 THE CANADIAN MINERALOGIST

Frc, 9. Quartz and chert occurrences. A Fine-grained quartz matrix around relict siderire (S) granule. The siderite is medium-grained and well-crystallized. Opaque is magnetite. Doubly polarized light. B. Chert (C) and greenalite (G) cements surrounding gteenalite and chert granules. Transparent sprays are minnesotaite. Bar length represents 0.2 mm. much coarser than coexisting stilpnomelane and able in finely crenulated and sheared Na-rich chert. The coarcenessof ankerite is concluded to rocks, providing conclusive evidence that croci- be the result of recrystallizati'on, and associated dolite is a very low-grade metamorphic product diffusion of cations, during diagenesisand sub- oI Na-rich bulk compositions. Dimroth & Chau- sequent very low-grade metamorphism. Perrault vel (1973) and Zajac (7972) reached similar (1955) suggests that these ferroan dolomites conclusions on the origin of the crocidolite in represent replacement of earter materials. The the Sokoman Formation. The coexistence of present author concludes that the ankerites are s'tilpnomelaneand crocidollte (Fig. 11D) is evi- the recrystallization products of earlier much dencefor an overlap of the very low-grade meta- finer-grained ankerite compositions. morpbi^ stability fields of the two minerals. Siderite is an abundant constituent of well- banded stilpnomelane-quartz-magnetite-siderite PsrRocsNnsrs AND CoNcLUsroNs occurrences (Fig. 11C). Grain sizes in such oc- cturences can alternate abruptly from very fine The above textural observations enable in- to soarse granular siderite bands. The composi- terpretation of the possible primary, diagenetic tions of both types of siderite are essentially and low-grade metamorphic iron-formation as- identical. As in the case of the other carbonate semblag,es.The products of sedimentation were occurrences,the author considers siderite to be probably hydrous Fe-silicate gels of grcenalite- the medium- to coarse-grained recrystallization type composition, locally hydrous Na-, K-, A1- product of earlier much finer-grained material rich iron-silicate gels approximating stilpnome- of siderite composition. lane compositions,SiOz gels (or rnagadiite, or a Crocidolite is not found in the Howells River sodium silicate gel, as suggestedby Eugster & area, but occurs in the structurally highly de- Chou 1973), Fe(OH)r and Fe(OH)g precipitates, formed area of Schefferville, and northwest and very fine-grained carbonate oozes of vari- thereof. Crocidolite occurrences are most not- able composition. The possible products of sedi- Frc. 10. Carbonate occurences. A. Manganoan dolomite oolites, in doubly polarized light, showing radial arrangement of dolomite crystallites. Black area is very fine-grained greenalite (G). B. Close-up of a dolomite oolite. Felted massesat upper right are minnesotaite (M). Greenalite at lower left (G). C. Close- up of siderite rhombohedra (S) inside oolite shown in B, The rhombohedra are concentrically arranged. D. Calcite (Ca) cement inside greenalite (G) cement around dolomitic oolites. Minnesotaite (M) at upper right. Bar length represents 0.2 mm. Frc. 11. Carbonate and riebeckite occurrences. A, Coexistence of medium-gtained, euhedral siderite (S, with high relief) and subhedral calcite (Ca). B. Conrposite granule with greenalite (G) center, and ankerite edge (A), cemented by more ankerite (A) and siderite (S). C. Medium-grained, transparent siderite at lefl, and fine-grained siderite of similar composition at right, in finely banded magnetite-siderite assemblage.Sheaves in the right hand band are stilpncmelane. Opaque is magnetite. D. Coexistence of crocidolite (Cr, cross-fiber variety of riebeckite), stilpncmelane (St) and magnetite (opaque). Bar length represents 0.2 mm. LOW-GRADE METAMORPHIC PRECAMBRTAN IRON-FORMATION 495

mentation are listed in Table 9 and a possible TABLE9. INFERREDII{ITIAL COMPOSITIONSAIID TIIEIR 5EDII,IENTARYPRODUCTS IN glmury assemblage diagram is given ir Figure PRII'[r.RylRCl']-F0RI'AIl0ll A5SEltBLActS 12A. During collojdal Sl02 + chert diagenesisHzo is lost from the arcrphous dissolved Sio. + fe2-.liq - > Fe-S1-0-0H3el of more open,gel-type compositionsdue to compac- greenali te type tion. Furthermore, locally Na{, K+, Al3+ + ilPrplou!.feSi-0-0H-(Na'K'Al) the amoqphous silicate gels gel of stllpnonelane type probably become somewhat less disordered dissolved Fe3+ + Fe(0H)l (brcones hqatite) structures. For example, rmo{phous greenalite- 1 dissolved Fe2+ + Fe(0H)2 + Fe(0H)3 (becones magnefite) type compositions may tend toward very fine- disiolved Or, grained, somewhat ordered greenalite ana ny- Fe2t, Mg, Ca+ very fine-gralned,'nlxed,,cartonats drous Na-, K-, Al-rich iron-silicate gels may tend glert.my- fglfl fr@,nagadli te, NaSJTol3(0H)3. 3HZO, duri ng d.tagen6.ts (Eugst€r& Chou1973). to -acquire the strucfural arrangement of a possible precursor to stilpnomelane. The very fine- grained carbonate oozes would tend to iecrvt- vasively at the expense of earlier silicates (green- t47lize, as would very fine-grained magnetr'-te- alite and stilpnomelane), and by the reaction of tylre compositions. A possible set of diagenetic chert and Fe-carbonates.Reactions which mav assomblagesis given in Figure 128, showing the be responsible for the formation of minneso- possible onset of minnesotaite formation. With taite, as deduced from texfural evidence, are a continuous increase in temperature, in going glven in Table 10. An assemblagediagram for from diagenetic to very low-grade metamorphii tho very low-grade metamorphic products of conditions, stilpnomelane would tend to coarsen iron-formation is given in Figure 72C, T\e in grain size and chert. would recrystallize. The trend, as expressedby the assemblagegroupings carbonateswould also coarsen in grain size and in Figures l2A, B, and C, is one of decreasing produco distinct phase separations across misci- HzO content as would be expected in going from bility gaps (e.9. siderite-ankerite and calcite-side- sedimentaryto low-grade metamorphic products. rite coexistences).Minnssstaite would form per- A chemically somewhat more realistic set of

DIAGENETIC ASSEMBLAGES

C- chert il LOWGRADE G- greenolite -quortz METAMORPHIC Q ASSEMBLAGES S- siderite Minn- minnesotoite +Mognetite Stilp- stilpnomelone

Frc. 12. Graphical representation of iron-formation assemblagesin terms of FeO, SiOr and H2O. For this representation all Fe in the Fe-silicates is assumed to be FeO only. COg is considered as a perfectly mobile component. A. Primary assemblages of hydrous Fe-silicate gels, SiOr gel, mapetite*type precipitates and very fine-grained carbonate (siderite in the diagrams). B. Diagenetic assemblages result- ing from the recrystallization of the original gelg and fine-grained precipitates, and the onset of some minnesotaite formation. C. Low-grade metamorphic assemblages showing the disappearance of gf,een- alite and stilpnomelane with the production of minnesotaite. 496 THE CANADIAN MINERALOGIST assemblage representations is shown in Figure TABLEIO. REACTIOI{SFOR I4INNESOTAITE FORMTION A5 SHCUI]TEXIUMLLY IN F 13 in which Fez+ as well as Fe"+ are considered Fe6Sl40t0(0H)B+ 4sj02 - 2FtSi40lO(0H)2+ 2H, as components. The sequence of diagrams re- fri@otuite flects the continuous change from primary to 2Fe6si4010(0H18+ 0Z = 2Fe3St401g(0H)2+ 2F%04 + 3H, very low-grade metamorphis conditions. The greetuL.ite nltusoteite nogretitu complex reactions which are porhayed in this 3FeC03+4Si02+H20 ' Fe3si40to(0H)2r 3C02 figure involve chert and siderite to form minne- fri@otuib ..Fe2.7si4(o,oH)t2.dr2o+ 0.33Fe2+ = Fe,st401s(0H)2+ H20+(il:fi+,cas) atiLp@el@ frir@$otaite

sotaite, the reaction of chert and greenalite (f some Na, K, Al) to form stilpnomelaneand the disappearance of greenalite and stilpnomelane to form minnesotaite.Textural evidence for many of the reactions has been presentedabove. phases Increosinggrode The author concludes that all of the presently observedin the assemblageshave sedimentary precursors in terms of gels, precipitates or oozes (see Table 10). During diagenesis,re- organization and recrystallization of these materials began, concurrent with diffusion of cation components to crystal structures which would provide a good fit. For example' Na and K cannot be housed in greenalitebut can be incorpor- ated in the more open stilpnomelane structure. lnct€Amssroai Similarly, the disordered carbonate oozes began separation of FeCO"i CaCO'- and CaFe(CO)o regions by means of diffusion of the various cations (equivalent to exsolution). Diffusion probably took place over short distances as there is no textural evidence for large distance re- mobilization of materials. Sporadic, fine fras- ture zones and chert fracture fillings in some granules are generally no longer than I mm. There is no evidence, tlerofore, for replace- lncreosinggrode ment textures in the sense of later solutions having dissolved and replaced earlier materials. The marked differences in grain size of some of tho phaseshas often been considered as evidence for replacement processes.The extremely C-chert G-greenolile fine grain size of greenalite and therefore its H - hemolile M* mogneiiie Q -ouortz S- siderite lack of recrystallization, is probably controlled Ank - onkerite by misfits and mismatches in its crystal struc- Minn- minrcsoloite The euhedral form - ture, as described earlier. Stilp stilpnonelone and coarse grain size of magnetite, and the coarse lncreoslnslrode* grain size of ankerite, are concluded to reflect tho strong forse of recrystallization of these FIc. 1.3. Graphical rcprosentation of changes in minerals in a very hydrous, low-temperature iron-formation assemblages in terms of FeO, (diagenetic) environment. FerOs and SiOr. HrO and CO: are considered as There is much evidencein the assemblages,as perfectly mobile components. This sequence of presently observed, of approach to chemical diagrams portrays the possible reactions which equilibrium among the phases. Greenalite, al- primary occur in changing from to low-grade though always exn€mely fine-grained, is homo- metamorphic conditions. Hematite was not found geneous in composition. Similarly, coarser- as an oxide in the assemblagesof this study, Co- show existences in the primary assemblages are Yery gr'ained stilpnomelane and minnesotaite similar to data from the literature on several only very small ranges of composition. The car- Precambrian iron-formations (Klein 1973). bonates show a very slear and consistent ele' LOW.GRADE METAMORPTIIC PRECAMBRIAN IRON-FORMATION 497

ment frectionation pattern as shown in Figures CHESTERMAN,C. W. (1966): Mineralogy of the Lay- 2 and 3. It is concluded, therefore, that the touville Quarry, Mendecino County, California. samples studied represent equilibrium assem- In Geology of Northern California. CaL Div, blages of very low-grade metamorphic origin. Mines Geol. Bull. L90, 503-508. (1960): Although specific temperature and pressure Cocnexr, G. W. & Eowenos, A. B. The Roper River oolitic ironstone formations. Austra- values cannot be obtained for such low-grade lia, Commonwealth Sci, Indust. Org. Tech, metamorphic Res. iron-formation assemblages,French Paper 1. (1973) suggestsa range of temperature of 150o DrMRorH, E. & Cnewrr, J. L (1973): Petrography to 350"C and a pressurerange from 2 to 5 kb of the Sokoman Iron Formation in part of the for such diagenetic to low-grade metamorphic central Labrador Trough, Quebec, Canada. Geol. conditions. Clearln the assemblages represent Soc. Amer. Bull. 84, llL-134. conditions in the very low-temperature range EcclntoN, R. A. (1972): The crystal structure of of the greenschistfacies. stilpnomelane. Part II. The full cell. Mineral. Mae. 38, 693-711. EucstEn, H. P. & Cnou, I-vrNc (1973): The de- AcrNowr.roctvIENTs positional environments of Precambrian banded I am very grateful to the Iron Ore Company iron-formations. Econ. Geol. 69, Ll44-t169. FAHRIc, (1967): Lake map-area, of Canadafor giving me, W. F. Shabogamo in the summer of. L973, Newfoundland-Labrador Geol. mine and Quebec. Surv, free access to the areas in Schefferville, Canada Mem. 354. P.Q., to the Howells River area, and to the FINcER, L. W. (1973): Geowhiz, Canberra Indus- diamond drill cores and for making geological ffies Automated Microprobe Program for Geo- drill records and reports available for study. I logical Applications. Cartberra Inds., Meriden, am also grateful to the Iron Ore Company for Conn. its hospitality while I was in the Schefferville & Heomrecos, C. G. (L972): Electron area. I would like to express my thanks to Lee microprobo automation. Ann. Rept. Director Geo- Nichols, Richard P. Fink, Richard E. Arndt and phys. Lab., Carnegie Inst. Wash. 71, 598-600. FrNr, R. (1971): Taconite drill hole records for fim Orth, all of the Iron Ore Company of Can- holes no. 1032D and 1039D. Iron Ore Co. Can- ada, for their invaluable and spontaneoushelp ada Rept. and advice during my stay in Schefferville, (1972): Summary of development work P.Q. The crocidolite-stilpnomelane assemblage on the Howells River Project witl special refer- was obtained for me from the Walthier Lake ence to the geology of tl.re iron-formatior. Iron area by Richard E. Arndt. I wish to thank May- Ore Co. Canada Rept. 1972-!9, nard E. Coller for the majority of the bulk FLoRAN, R. J. & Perrrn, l. I. (L973): Silicate and chemical analyses, W. H. Moran and Richard carbonate relationships in unmetamorphosed iron- T. Hill, of the Indiana Geological Survey, for formation. Program Abst, Ann, Mtg, Geol. Soc. the drafting of the final illustrations, N. T. Amet., Dallas, Texas, 623, FnrNcn, B. M. (1968): Progressive contact meta- Immega for his aid in computer programming, morphism of the Biwabik Iron Formation, Mesabi and Mrs. T. F. Brown and Mrs. Bonnie A. Range, Minnesota. Minnesota Geol. Surv. Bull. Burks for the typing. I am grateful to C. V. 45. Guidotti, Bevan M. French and Guv Perrault for (1973): Mineral assemblagesin diagene- their criticat reviews which coniiderably im- tic and low-grade metamorphic iron-formation. proved the manuscript. Researchon iron-forma- Econ. Geol. 68. 1063-1075. tions was made possible by NSF grant GA- Gnoss, G. A. (1968): Geology of iron depositsin 36L86. The electron microprobe us,ed in this Canada. Iron Ranges of the Labrador Geosyn- study was obtained by joint funding from NSF clrne. Geol. Surv. Canada Econ. Geol. Rept. 22, grant GA-37109 and the Indiana University v. 3. Foundation. GRUNER,J. W. (1936): The structure and chemical composition of greenalite. Amer. Milteral. 21, 449-455. RrFsRENcrs (1937): Composition and structure of stilpnomelane. Amer. Mineral. 22, 912-924. ALBEE,A. L. & Rev, L. (1970): Correctionfactors for electronprobe microanalysisof silicates,ox- (1944): The composition and structure of ides, carbonates,phosphates and sulfates.Anal. minnesotaite, a common iron silicate in iron- Chem. 42, 1408-1414. formations: Amer, Mineral. 29, 363-372, Avnrs, V. L. (1940): Mineral notesfrom the Mich- (L946): A4i,neralogl and Geology of the igan iron country. Amer. Mineral. 25, 432-434. Mesabi Range. St. Paul, Minn., Office of Com- BENcE,A. E. & Arnee, A. L. (1968): Empirical missioner of Iron Range Resources and Rehabili- correction factors for the electron microprobe tation, 127 p. analysesof silicatesand oxides.J. Geo\.76,382- Hurrom, C. O. (1938): The stilpnomelane group of 403. minerals. Mineral. Ittag. 27, 172-206. 498

Jeurs, H. L. (1966): Chemistry of the iron-rich east Otago, New Zealand. Program Abstr. Geol, sedimentary rocks. U.^S.Geol. Surv. Prof. Paper Soc. Amer. Ann. Mtg., Atlantic City, N.J., 288- 440-W. 289. KrerN, C., Jn. (1973): Changes in mineral assem- TRENDALL,A. F., & Br-ocrlr1 J. G. (1970): The blages with metamorphism of some banded Pre- iron-formations of the Precambrian Hamersely cambrian iron-formations. Econ. Geol. 68, 1075- groupn western Australia. Geol. Surv, Western 1088. Australia Bull. 119. KuNzr, G. (1956): Die gewellte Struktur des Anti- Ze.tsc, I. S. (1972): The Stratigraphy and Mineral- gorits, I: Zeit. Krist. 108, 82-107. ogy of the Sokoman lron Formation in the Knob LsrrH, C. K. (1903): The Mesabi iron-bearing dis- Lake Area, Quebec and Newfoundland. Ph.D. trict of Minnesota. U.S. Geol, Surv. Mon. 43. thesis, Univ. Michigan. PEAcocK, M. A. (1928): The nature and origin of Zru, E-aN (1960): Metamorphism of the lower the amphibole-asbestos of South Afnca. Amer. Paleozoic rocks in the vicinity of the Taconic Mineral. 13, 241-285. range in west-central Vermont. Amer. Mineral. Prnnlurl G. (1955): Geology ol the Western Mar- 45. 129-175. gin of the Labrador Trough. Ph.D. thesis, Univ. Toronto. RoBrNsoN, P. (1969): Weathering of ferro-stilpno- Manuscri,pt received July 1974, emended August melane to ferri-stilpnomelane in the Otago Schist, t974.