American Mineralogist, Volume 65, pages 8-25, 1980
Mineralogy and petrologyof very-low-metamorphicgrade Archaeanbanded iron-formations,Weld Range,Western Australia
MaRUN J. Gore' Department of Geologt, University of llestern Australia Nedlands 6009. Australia
Abstract
There are two basicFe-rich lithologiesat Weld Range:(l) bandediron-formation contain- ing minnesotaite, siderite, quarlz,magnetite, greenalite, stilpnomelane, pyrite, and chamosite with trace amounts of pyrrhotite, arsenopyrite, chalcopyrite, apatite, and rockbridgeite; Q) Fe-shale.which occurs as laminated 2-30cm-thick bands within banded iron-formation, con- taining chamosite, stilpnomelane, siderite, greenalite, pyrite, magnetite, minnesotaite, and quartz with trace amounts of ilmenite, chalcopyrite, and apatite. Mineral assemblages,min- eral compositions, and the interpreted paragenetic sequencein both lithologies are essentially the sameas those reported from very-low-metamorphic grade Proterozoic iron-formations. In common with pubtshed greenalite analyses,those of greenalite from the Weld Range show a consistent excessof Si and a larger than corresponding deficiency in octahedral sites relative to the generallyaccepted formula of (Fe,Mg)"SiOro(OH)8.The earliestrecognizable mineral assemblagescontain Al-bearing greenalite(1.9-2.8 ttlVo AlzO), qu;artz,siderite, chamosite, magnetite, pyrite, and rarely pyrrhotite. These assemblagesare overprinted by stilpnomelane, which formed by reaction of a K-bearing fluid phasewith Al-bearing greenalite or chamosite or both. A secondaryAl- and Mg-poor greenalitedeveloped in someassemblages contempo- raneously with stilpnomelane. The presenceof quartz favored the growth of coarse-grained stilpnomelane. The relatively coarse grain size of quartz and siderite (up to 2.5mm) and the presenoeof angular fragments composedof early assemblagesin intraformational breccia in- dicates that the Fe-rich lithologies were well lithified prior to the development of stilpnome- lane. The incoming of minnesotaiteoccurred after that of stilpnomelane,and textures in- dicating reaction of earlier silicates and siderite to form minnesotaite are common. The assemblageminnesotaite-quartz+magnetite+pyrite represents the complete reaction of the earlier assemblages.The early assemblagesare preserved in bulk compositions which are Si- deficient or Al-rich or both, and under conditions of relatively lowPo, or high P.6, or both. The peak metamorphictemperature is estimatedto be 320150"C at low pressure'
Introduction per amphibolite facies (Binns et al., 1976, p. 303- 313), and sirnilar variations in metamorphic grade Banded iron-formation is a widely distributed al- occur in the greenstonebelts in the northwest of the though relatively minor (
246O E.OH. Fig. 2. Graphic log of D.D.H. 3, Weld Range. Minerals are listed in order of approximate decreasing relative abundance. Most contacts shown are gradational. Direction of the stratigraphic top is not known. Estimated thickness of the main banded iron-formation is 70-90m.
mation contains well-preserved, arrested reaction textures showing a texturally early minslal assem- blage overprinted by several generations of later minerals. The minerals in the banded iron-forma- tion, in approxirnatedecreasing order of abundance, are minnesotaite,siderite, qtJartz,magnetite, green- alite, stilpnomelane, pyrite, and chamosite. Trace amounts of pyrrhotite, arsenopyrite, chalcopyrite, and apatite are widely distributed, and rockbridgeite, first reported by Jones (1963), occurs in trace amounts in two samples.Mineral proportions, how- ever, vary widely and over large sectionsof the core someof the mineralslisted aboveare absent(Fig. 2). Fig. l. Location and geological sketch map ofthe central part Much of the banded iron-formation is mesobanded, of the Weld Range, Western Australia (after Jones, 1963). and very different mineral assemblagescommonly t0 GOLE: BANDED IRON.FORMATIONS occur in adjacent mesobands. Within the banded Analytical methods iron-formation. 2-30cm-thick bands of massive to Bulk chemicalanalyses were obtained by XRF ex- laminated Fe-rich shalesoccur, having both mineral- cept for NarO (AAS), FeO (titration against potas- ogically sharp and gradational contacts with the sium dichromate), HrO and CO' (gravimetry), and S bandediron-formation. The mineralsin the Fe-shale, (Leco furnace). Electron microprobe analyseswere in approximate decreasingorder of abundance,are performed using an automated,3-spectrometer ARL- chamosite,stilpnomelane, siderite, greenalite, pyrite, snruq electronprobe microanalyzer.For silicatesand magnetite,minnesotaite, and quartz. Trace amounts carbonates.a seriesof natural mineralswere usedfor of ilmenite, chalcopyrite, and apatite are also present. prinary and secondary standards and the data re- This study is the first detailed description of the duced by the method of Benceand Albee (1968),us- mineralogy and petrology of very-low-metamorphic ing the alpha factors of A. A. Chodos(unpublished). grade banded iron-formation of the Archaean. It will Estimatesof HrO for silicatesand CO, for carbonates be shown that the mineral assemblages,the mineral were made for purposesof the reduction procedure. compositions, and many of the textural relations in Sulphide probe analyseswere performed using pyrite the banded iron-formation at Weld Range are essen- and gallium arsenide as standards and Magic IV tially identical to thoseof the well-studiedmajor Pro- (Colby, l97l) for data reduction. terozoic iron-formations. Electron microprobe analyses with the same sample number refer to a single mesobandwithin the sample.Samples are housedin the Rock Store of the Department of Geology, University of WesternAus- bulk samples of banded iron- Table L Chemical analyses of tralia. formation and Fe-shale from D.D.H. 3, Weld Range Bulk chemicalanalyses Bulk chemical analysesof banded iron-formation sio2 49.76 14.52 2L.20 21.05 and Fe-shale (Table l, anal. I to 3, and 4, respec- Tio 0.00 o.27 2 0.01- 0.r2 tively) illustrate three features that reflect important At2o3 0 .26 0.54 0.08 6.26 aspectsof their mineralogy. (a) The CaO content, Pe203 I'7.45 r.83 16.r-5 I7.09 even of the most COr-rich sample, is negligible, FeO 24.65 44,92 34.82 which indicatesthat the carbonateis siderite.(b) The ho 0 .17 r.86 1.43 0.92 relatively low Fe'*/Fe2* ratios reflect the presenceof Mgo 2 .50 2.!3 7.91 5.47 abundant sideriteand Fe'*-silicatesand the generally CaO 0.31 0 .55 0 .69 0.09 low proportion of magnetite.No hematitewas identi- Na2O 0.03 0.02 0.04 0.03 fied as part of the unaltered,low-metamorphic grade Kzo 0.02 0.20 0.0r 0.33 assemblages,and all hematite appearsto be due to Hzo 0.07 1.95 6.00 weathering.(c) Both the banded iron-formation and 0.I7 0.95 "2" 0.06 the Fe-shalebands are relatively S-rich. 0.22 0.20 0.31 0.03
co2 0.40 32.05 t5 .50 Mineral chemistrY r.00 0 .72 0 .03 4.33 The minerals that are part of the paragenetically 99.3'7 99.'79 r00.41 r01.65 early assemblagesin the banded iron-formation from 0.r8 0.0r r.08 D.D.H. 3 are greenalite,chamosite, siderite, quartz, 99.L2 99.61 100.40 r00.57 magnetite,sulfides, and apatite. Stilpnomelane,min- F"3+ 12.20 1.28 11.30 11.95 nesotaite, and perhaps rockbridgeite, which occurs in ,. Sulfide 0 .86 0.63 0.03 3.76 veinlets, are paragenetically late. The chemistry of Other 18.30 34.29 27.04 r5.87 these minerals will be discussedin the above order in 'IOCa1 I e 31.36 36.20 38.27 3r .58 the following sections with the exception of apatite ,u3* yt"2* 0.04 o.42 0.75 and rockbridgeite, which will not be discussedfur-
7. BIF consisting of ninn, qtz, nag, sid, stifp, green, pV. ther. Sanpfe 84723. Depth 786-2-186 5n; 2. BIF consisting of sid, qtz, naq, ninn, sti7p, green, pg. sanple a4725. Depxh 198.f- Greenalite 198.4; 3. BIF consisting ot ninn, sid, qtz, naq, sti7p. Sanpfe 44730. Depth 206.0-2O6 -4n; 4. Ee-sha7e consistinq of cham, (1936), sci7p, sid, qtz, naq, p9, ninn. Sanpfe 84728. Depth 203.0- In accordancewith the work of Gruner 203.7n. 1946\, it has generally been assumedthat greenalite GOLE: BANDED IRON-FORMATIONS ll
FetMg
oo
1o- a oo o ,d oi?t.", -o-
(Fe.Mg)5Si4Oro(OH)g
\., o .].'." 1296 Mg PUBLISHEDSOURCES WELD RANGE o = Floronond Popike(1975, 1978) . =Nos.1-7 relerto toble2 a = Klein ('1974) oB = Jolliffe (1935),recolculoted Gruner (.|936) o9 = Cochroneond Edwords(1960) olo = Zoioc (1974)
Fig. 3. Compositionalvariations in greenaliteplotted within the systems(Fe+MnlMg-Si and (Fe+Mg)-Al-Si.
has an idealizedformula of FeuSioO,o(OH).,and that mixed layer phase composedof the end-members it is the structural analogueof antigorite. The gener- greenalite and mimesotaite. The recent X-ray and ally very fine-grained intergrowths of greenalite with electron diffraction study of greenalite by Guggen- other minerals make compositions determined on heim et al. (1977), using for the first time a single mineral separates rather unreliable, and the flne crystal of greenalite,has suggesteda structural rea- grain size of greenalite(Klein, 1974;Floran and pa- son for the deviation of the greenalite composition pike, 1975;this study) has made single-crystalstruc- from the ideal formula. They suggest(S.W. Bailey, tural determinations difficult. However, there are personal comrnunication, 1977) that greenalite con- now a sufficient number of reliable electron micro- sistsof a coherentintergrowth of a dominant trigonal probe analyseswhich, together with the first single- layer-silicate phasewith a second,minor, structurally crystal structural data, suggestthat the crystal chem- inverted monoclinic phase.They reasonthat the in- istry of greenatte is more complicated than pre- version has causeda loss of Mg and OH where the viously considered. phasesinterfere, giving an apparent excessSi con- It has only recently becomeclear from electron mi- tent. In this respect it is similar to antigorite which croprobe analyses(Klein, 1974;Floran and Papike, also has excessSi (Page,1968; Whittaker and Wicks, 1975,1978;Table 2; seeFig. 3) that most analysesof 1970),but there is no evidencefor a wave-like super- greenalite,when recalculatedon an anhydrousbasis structure in greenalite(S. W. Bailey, personal com- of 14 oxygens,show an exoessof Si over that re- munication, 1977). quired to fill the assumedtetrahedral site occupancy In published analysesand my electron microprobe of 4.00.These analyses also show a larger than corre- analyses of greenalite, AlrO, ranges from trace sponding deficiency in the octahedral site. Several amountsto a little over 3.0 wtVo(Fig.3). Coexistence reasonsfor thesefeatures have been proposed.Klein of greenalite and chamosite in the banded iron-for- suggested the presence of submicroscopic inter- mation from D.D.H. 3 suggeststhat the AlrO, value growths of chert in greenalite as the causeof the ex- of 3.0 wt%ois probably closeto a maximum. Al-rich cess Si, and Floran and Papike (1975) interpreted greenalites have lower Si contents than Al-poor their greenalite analyses as being the result of a greenalites (Table 2), which suggeststhat some Al t2 GOLE: BANDED IRON.FORMATION S
Table 2. Representative electron microprobe analyses of As noted by Klein (1974) and Floran and Papike greenalitefrom D.D.H. 3, Weld Range (1975), there is a relatively narrow range in MgO content in greenalite from low-grade iron-formations (Fig. 3). have chem- sio2 33.9 34.2 34.9 34.r 34.5 35.s The greenalites from the Weld Range that are broadly similar to pre- rio2 0.01 0.01 0.01 0,03 0.03 0.04 o.o2 ical compositions viously pubtished analyses(Fig. 3). However, they A12O3 0 . 12 0-22 0.02 r.90 2.43 2.76 2 .45 have, on average,lower Si contentsthan greenalites neol 52.a 52.2 50.9 49.3 47.3 46.0 45.4 from the Gunflint Iron Formation (see Floran and MnO 0.34 0.34 0.50 0.s0 0.50 0.48 0 .47 Papike,1975,1978). Mgo L.67 1.53 r.5'1 3.07 3.75 4.59 4.65 In the Weld Range banded iron-formation, two CaO 0.02 0.03 0.01 0.02 0.02 0,00 0.02 distinct greenalite compositions occur. Greenalites Na2o 0,01 0.04 0.02 0.04 0.0r 0.00 0 .0t- which are Al-rich (1.90-2.76't'rt%oN'O3) have moder- KzO 0.01 0.02 0.0r 0.02 0.01 0.00 0 .01 ate MgO contents (3.074.65 wtqo) whereasgreen- Total 88.88 88.59 87.94 88.98 88.56 89.37 88.63 alites that are Al-poor (0.02 0.22 wt%oAl,O,\ have
Ions on the basis of 14 oxygens relatively low MgO contents(1.53-1.67 *Vo) (Tabte correspondto si 4 .13 4.L6 4.24 4.05 4.05 4.09 4.09 2, Fig.3). The different compositions different textures in which the greenalite occurs (dis- Al 0.02 0.03 0.00 0.2't 0.34 0.37 0.34 cussedbelow). The Fel(Fe*Mg) ratio of the Al-rich Fe 5.37 5.31 5.18 4.90 4.66 4.43 greenalitevaries slightly betweenmesobands (Fig. 3), Mn 0.03 0.04 0.05 0.05 0.05 0.05 0.05 and within any one mesobandthis ratio doesnot ap- M9 0.30 0.23 0 .29 0.54 0.56 0.'19 0.81 pear to be affectedby assemblages.Greenalite con- Ca 0.00 0.00 0.00 0.00 0.00 0.00 0 .00 sistentlyhas the highest Fel(Fe+Mg) ratio of any of Na 0.00 0.01 0.0I 0.01 0.00 0.00 0 .00 the Fe-silicatesor carbonatesin a given mesoband tocT 5.72 5.67 5.53 5.17 5.71 5.64 5 .65 (Fig. a). greenalite contains submicroscopicdark dif- -ALf1 Some Fe as FeO fuse patches.In places,the patchesare so densethat plane 1. siightfg bro9|n gteen in intergrowxh with stiTp' sid and the greenalite is opaque in light. Electron qtz. N = 4. Sanple 847f5/2t Z. Siightlg btown green wixh probe analyses show no distinctive chemical differ- minor dark Aiffuse paxches (<7 Vn) inxergrown with sid and = qtz. N 5. Sanple 84715/7i Z. Pieochroic green to paie ences between greenalite containing patches and btown, fibtous green titu on sid-pg-po noiluTe in ninn-qt2- tich mesoband. N = 4. sakple 84734; 4. cfear green in sinifar intergtowth as 2 above, N = 6. sanple 84775/7; 5. Green from gteen-sid-pV-stjfp nesobanil. N = 7 . sanpTe 84776i 6 and 7. Green from nesoband containing green, sid Somple minn, sXifp, chan, pV. Sanpie 84778. No.
84713 substitutesfor Si despite the apparent excessof the latter. o.90 BecauseFe3* cannot be determineddirectly by the electron microprobe, all Fe has been assumedto be =C'I FeO in microprobeanalyses. In publishedwet-chem* + g o.8o 84715 ical analysesthe amount of FerO, has been assumed 84720 84716 to be due to secondaryoxidation of FeO (e.g.Floran E 847 r8 and Papike, 1975).The assurrptionof all Fe as FeO 84719 in greenalite is probably a good frst approximation. 84737 In one retrograde assemblagein a highly metamor- o.70 4473? phosed banded iron-formation frorr the Yilgarn Block, greenalite coexists with a mineral which is 84722 probably related to cronstedtite,an Fe'*-bearing 7A o.60 phyllosilicate (Gole, in preparation). This oocurrence Green Sid' Chom SiilP Minn suggeststhat greenalite is mainly a Fe'* mineral and Fig. 4. Fel(Fe+Mg) ratios of electron microprobe analyses of that available Fe3* is preferentially housed in other minerals from D.D.H. 3 Weld Range. Tielines join minerals frorh minerals. either silicatesor oxides. the same mesoband. Greenalites plotted are Al-bear.ing. GOLE: BANDED IRON-FORMATION S l3 clear greenalite.It is unknown whetherthese patches ln iron-formations, magnetite generally has a pure are inclusionsin or an alteration of greenalite.Zajac end-membercomposition (e.g. Klein, 1974).Electron (1974,p. 116)reported similar featuresin greenalite microprobe analysesof magnetite and ilmenite in Fe- from the Sokomanlron Formation. shale assemblagesshow them also to have near end- member compositions. The compositions of pyrrho- Chamosite tite and arsenopyrite coexisting with pyrite are given Most chamositesin this study show small l4A in Table 5. Pyrrhotite and arsenopyrite from samples peaks, indicating that they are probably mixed-layer where they are not in contact with each other or do silicates. Chamositesin Fe-shale have larger l4A not coexist with pyrite have similar compositions to peaks relative to their 7A peaks than chamosites those in Table 5. fron banded iron-formation. When recalculated on a basis of 14 oxygens,chamosites in Table 3 have ex- Stilpnomelane cessoctahedral cations. Recalculation of the analyses Electron microprobe analyses of stilpnomelane on a fixed-cation basis suggeststhat small amounts of from D.D.H. 3 (Table 6) have beenrecalculated on a Fe'* may be present. The presenceof Fe3* is also consistent with the balancing of tetrahedral Al with Table 3. Representative electron microprobe analyses of octahedralAl * Fe3*in most analyses.Even assum- chamositefrom D.D.H. 3, Weld Range ing considerableFe'* is present,the octahedral site in Weld Range chamosites is filled to a greater extent than in more recent chamosites(Weaver and Pollard. 1973).This may be becausethe Weld Range chamo- sio2 22.9 23.3 24.0 23.6 sitesare mixed-layer 7-l4A phyllosilicates. Tio2 0.00 0.06 0.03 0.04 Where chamosite occurs as a minor constituent in A1203 L9.4 20.2 I8.0 19.0 banded iron-formation it is very Fe-rich. The most reol 37.5 34.9 33.5 30.2 Mg-rich chamositesare found in the Fe-shale bands Mno 0.17 0.t2 0.r1 0.15 (Table 3), indicating that their Fel(Fe + Me) ratio is Mgo 6.48 7.60 I0.58 L2.t5 largely controlled by bulk chemistry. Chamosites have low Fel(Fe + Mg) ratios relative to coexisting CaO 0.02 0.18 0.04 0.00 Fe-silicatesand carbonates(Fig. 4). Na2O 0.04 0.03 0.09 0.01 K2o 0.00 0.0r 0.00 0.00 Carbonates Total 86.5r 85.40 db.Jf, dl.l) Siderite is the only carbonate found in the banded Ions on the basis of 14 oxygens iron-formation of D.D.H. 3. It showsa narrow range in Fel(Fe + Mg) ratios within a singlemesoband but si 2.64 2.64 z.tz z.ob a considerablespread between mesobands (Table 4). AI 1.36 I.36 L.28 L.34 Siderites with the highest MgO content occur in a 'TET 4.00 4.00 4 .00 4 .00 mesoband with the unusual assemblageof siderite- A1 L.28 1.35 l.tr r.I3 minnesotaite-chamosite-pyrite-pymhotite(Table 4, Ti 0.00 0.01 0.00 0.00 anal. 3). However, in the adjacentmesoband siderite Fe 3.62 3.32 3.15 2.85 is relatively Fe-rich (Table 4, anal.4). The CaO con- tent of siderite is generallylow (0.04-1.92wt%o), and Mn o.02 0.01 0.01 0.0r the Mno content rangesfrom 0.05 to 5.65wl%o. Sec- 149 1 .II t.29 1.78 2.04 ondary siderite, occurring in a vein vdthin quafiz, 0.00 0.o2 0.0r 0.00 has a marginally diferent composition from pri- Na 0.01 0.0r 0.02 0.00 mary siderite in the samemesoband (Table 4, anal. TOCT 6.04 5.01 6.09 5.03 6 andT).
!A7J. Fe as FeO Quartz, mqgnetite,and sulfides 1. Cham coexisting wiXh gteen, sid and pg. SampLe In the banded iron-formation and Fe-shalefrom 84778; 2. Chan with gteen and pg in breccia ftagments qtartz grain cemented bg sid, SanpLe 84779; 3. Cham wixh sti7p, D.D.H. 3, sizesare easily observedun- sid, pg, minor mag and qtz in reishal.e band. SampTe der high magnifigxlisn and range from 0.3 to 2.Omm 84728; 4. Cham with ptl, stiJ.p and sid in Fe-shale band. sanple 84727. acrosswith rare grains up to 2.5mm. t4 GOLE: BANDED IRON.FORMATIONS
Table 4. Representative electron microprobe analysesof siderite of Fe2*-rich minerals in contact with stilpnomelane from D.D.H. 3, Weld Range (most commonly, minnesotaite,siderite, greenalite, and chamosite),it is probable that much of the Fe'O, content of stilpnomelane (as reflected by its coloring) reor 59.'1 52.0 46.2 54.0 46.r 45.7 46.3 is due to secondaryoxidation during weathering (see
Mno 0 .05 4.10 0.44 3.32 5.35 4.78 Eggleron,1972). iron-formations MgO 0.15 2.33 10.13 r.25 5.94 6.20 3.34 Stilpnomelane from low-grade wide range in FeO*MnO, MgO, cao 0.04 1.64 0.48 0.78 r.O2 0.77 L92 showsa relatively and Al,O, (Fig. 5) and K,O contents(0.07-3.57 vrt%o Total 60.00 60.21 51.39 59.50 58.53 5'7,57 5'1.30
Ions on the basis of 6 oxyqens
Fe 1.88 r.62 I.35 1.68 1.39 r.36 Table 6. Representative electron microprobe analyscs of Mn 0 .00 0.13 0.01 0.r0 0.r5 0.14 0 .17 stilpnomelanefrom D.D.H. 3, Weld Range Mg 0.01 0.13 0.53 0.07 0.32 0.33 0 .18
ca 0.00 0.07 0.02 0.03 0.04 0.03 0.07
Total 1 .89 1.95 1.91 1.88 I.9l 1,86 I.81 sio2 45.8 45.0 44,8 48.8 45.8 44.5 c 2.06 2.03 2.04 2.06 2.04 2.07 2 .09 0.00 0.01 0.00 0,02 0.00 0.01
'Al7 4.3'7 4.28 4.08 5 '2r Fe as reo. Al2o3 4.22 4.'13 32.8 32.6 33.8 31.8 7. Siil fron sid-suf nodufe with Af-poor green vein. Sanple reol 35.'7 35.r 84734;2- Sjni-lar occutrence as f above. Sanpfe 847fi, 0 '11 z K.O 1.85 L.16 r.24 1.14 1.3r .32 fixed cation basis, assuming a simplffied structural Total 90.'17 €9 .76 8'1,4'7 92 ,L5 90.3'1 89 formula of Ko."(Fe,Mg)"(Si'Al)(O,OH),'2-4HrO 2 33. 12 3'7.56 35. 34 (Eggleton and Chappell, 1978).This formula is an 38. 51 31.62 34.95 .00 .00 approximation, and the estimatedFerO, contentsof FeO 1.05 1.25 1.35 2.80 gO,97 stilpnomelane in Table 6 are, at best, rough esti- TotaI 94.63 93.53 95.47 94.03 92.86 mates.In banded iron-formation and Fe-shalefrom Ions on the basis of 15.63 cations the D.D.H. 3 locality, all stilpnomelaneis brown. 5I 8.14 8.09 8.22 5.44 8.09 7.94 Consideringthe relatively low Fe'*/Fe'* ratios of the AI 0.86 0.9r 0.78 0.56 0.85 r'05 bulk chemical analyses(Table l) and the abundance 9.00 9.00 9.00 9.00 8.94 9'00 0.02 o.o9 0.r7 0.31 0.00 0.04 Table 5. Electron microprobe analyses of arsenopyrite and Fe- 5,15 s.o9 4.83 4.31 5.00 4.'7s pyrrhotite coexistingwith pyrite from sample84715/1, D.D.H. 3, TE 0.16 o.r9 0.2L 0.41 0.00 0.00 Weld Ranee- 0.02 MII o.o2 0.03 0.o2 0.03 0.03 Arsenopyrite Pyrrhotite Mg 0.68 0,68 0.92 r.2r 1.33 1'63 5 4 0,03 o.o0 0.02 0.03 0.02 0.02 wtt SD Na o.2L O.L2 0.06 0.05 0.06 0.05 0.30 3s.sI (0.30) 60.29 (0.25) 0.36 0.42 0.41 0.27 0-26 6.70 6.8r As 42.70 (O.l-7l tocT 6.63 6.62 6.63 6.62 s 2r.49 (0.421 38.55 (0.35) 1A11 Fe as Feo 99.70 98.84 2Fe203 Ko.6(fe,MS) estinated assuminq fornuia of 6(si8A1) (o ,oH ) 27 att 7. coarse-grained stifp (200-4o0 lJn) which cats actoss of A1-poot green, sid and qtz' sanpfe 84715/2; !-e 33.71 47,37 lntergrowth 2. Sinifat texturaf occurrence as above ' Saapfe 847f5/f ' (50-l0o in A7-beating gteenafite- 30.22 i. rine-gtained sxilp Pn) lid nesoband. sanpTe 84?76r 4. stifP sheaves in Af-bearing green-chail-pg mesoband- Sanpfe 84778i 5. Stilp needfes s 36.07 52.69 which cut across contacx of chafr-green-Pg breccia ftaqnents and sial cenenx. Sanpfe 847f9, 6. stilp sheaves r00.00 I00.00 ih naxtjx of chan in Pe-shafe bana' sanple 84728' GOLE: BANDED IRON-FORMATI2NS l5 KrO). Thosefrom the Weld Range,however, show a relatively small compositional variation (Fig. 5). Within a mesoband,stilpnomelane compositions are similar regardlessof the mineralswith which they are in contact. Minnesotaite Gruner (1944) showed that minnesotaite, FerSLO,o(OH)r,has a structuresimilar to that of talc. However, Guggenheim et al. (1977) report on the basis of TEM photographsthat minnesotaitehas a new layer-like structure. Unfortunately, no details .A-l:€,:t^ d\^'+d - are yet available. ,- 4O .,/ It appearsthat a complete compositional region olc Minnesoloiie exists between minnesotaite and talc in low-grade iron-formations (Fig. 6). Theseminerals display the 80 Mg Fe widest range of Fel(Fe+Mg) ratios of all the Fe-sili- c&teminglsls in low-gradeiron-formations. Relative BIWABIKIRON FORMATION GUNFLINTIRON FORMATION to greenalite,stilpnomelane, and siderite,minnesota- ol = Gruner(1944) a = Floron ond Popike (r975, 1978) ite has a low Fel(Fe+Mg) ratio, similar to that of CUYUNAIRON FORMATION chamositein the samemesoband (Fig. a). Small but o2 = Bloke (1965) SOKOMANIRON FORMATION o6 = Zojoc (1974) significant amounts of AlrO, and KrO may be pres- BROCKMANIRON FORMATION A = Klein (1974) ent in minnesotaite(Table 7). o3 = Trendollond Blockley A = (1978) Recalculationof analyseson the basis of I I oxy- (r970) Lesher gens generally shows agreementwith the ideal for- o4 = Ayres (t972) o5 = Miyono (t976) Fig. 6. Compositionalvariation of minnesotaiteand talc from very-low-metamorphic grade banded iron-formations plotted within the system Fe-Mg-Si. mula, except that the sum of the octahedrally coordi- nated ions is consistentlyin excessof 3.00 (Table 7), " / 6 ^.?r a \ which suggeststhat a small Fe'* componentmay be a ^ ..----T-."e;:.1. a c." .y'$*..-r...'\ present. o7 TH|S STUDY . .r Textural relations o In the banded iron-formation from D.D.H. 3, in- 40 60 70 80 90 Mg Fe+ lVn terpretation of textural relations allows for the recog- BIWABIK IRON FORMATION o7 = Ayres(t972) nition of the earliest mineral assemblages,which are ol =Grout ond Theit (t924) o8 =Miyono (t976) locally well preserved although all show the effect of CRYSTAL FALLS SOUTH AFRICA a low-grade metamorphic overprinting. In some oc- o2 = Ayres (194O) o9 = Lo Berge (t966 b) currences,however, the early silicatesand carbonates CUYUNAIRON FORMATION GUNFLINT IRON FORMATION have been completely destroyed by reactions. Ar- o: = Btoke(t965) . = Floron ond ,Popike(t975, 1978) oa = (1971) restedreaction texturespermit possiblededuction of Brown SOKOMAN IRON FORMATION BROCKMANIRON FORMATIOi. e = Klein (1974) these reactions.Textural observationscoupled with o5 = Lo Berge(t966o) a =Klein ond Fink (t976) simplified phase diagrams also permit the deduction 06 = Trendottond Btocktey(t97O) a = Lesher (1978) of the conditions which the sedimentary or diage- Fig. 5. Compositional variation in stilpnomelanefrom very- netic mineral assemblagesunderwent during low- low-metamorphic grade banded iron-formations plotted within grade metamorphism. the system(Fe+MnfMg-Al. Greenallte generally occurs in magnetite-poor as- l6 GOLE: BAN DED IRON-FORMAT:IONS Table 7. Representativ€ electron microprobe analyses of (Vernon, 1976,p.135-147) and show no evidenceof minnesotaite from D.D.H. 3, Weld Range mutual replac€ment. Greenalite is too fine-grained for grain boundaries to be distinguished, so grain-to- grain relationships cannot be determined. It appears, sio2 51.5 5r .2 5I .3 50.'7 50.7 50.9 however, that the greenalite aggregatesdo not cut Tio2 0.0r 0.02 0.02 0,04 0.04 0.0 0 .01 acrossthe grain boundariesof other minslnls. $usfu AI2O3 0.75 0.83 L.24 I.02 0.85 0.21 0 -67 texturesseem to indicate that assemblagescontaining FeOI 36.1 35.5 34.5 34.4 33.s 32.6 30.1 greenalite, siderite, qvartz, sulfide, and magnetite are MnO 0.35 0.32 0.33 0.40 0,24 0.16 o .2r equilibrium assemblages.This conclusion is sup- '1 '1 Mgo 5.53 5.73 5.83 6.54 .00 .r3 I0.24 ported by the cbnsistentdistribution pattern of Fe CaO 0.01 0.o2 0.00 0.03 0.04 0.04 0 .01 and Mg between coexisting greenalite, siderite, and Na2O 0.00 0.06 0.10 0.03 0.08 0.r0 0 .00 chamosite (Fig. a). As no other mineral or mineral Kzo 0.24 0 .24 0 .23 0 .44 0.27 0 .1-5 assemblagecan be directly observed to be a pre- Total 94.49 94,91 93.55 93.60 92.72 9t.35 95.65 cursor to theseminerals, they forrn the earliestrecog- Ions on the basis of 11 oxygens nizable mineral assemblagesin the Weld Range 3.96 3.95 3.95 3.92 3.93 3.98 3.94 banded iron-formation. AI 0.04 0.05 0.05 0.08 0.07 0.02 0.06 Greenalite also occurs in the Fe-shale bands, 'TET 4.00 4.00 4.00 4.00 4.00 4.00 4.00 where textural relations are generally more equivocal 0.03 0.02 0.06 0.0I 0.0I 0.0I 0.00 than in the banded iron-formation. Chamosite, Fe 2.32 2.29 2.22 2,22 2,11 2.I4 1.89 greenalite, siderite, pyrite, and minor qufitz appear Mn 0 .02 0 .02 0.02 0 .03 0.02 0.0r 0 .01 to form the earliest assemblages.No precursor as- 14q 0.63 0.66 0.69 0.75 0.8I 0.83 1.12 semblageto these minerals can be recognizedand NA 0.00 0.01 0.02 0.00 0.0I 0.02 0.00 they show no evidence of mutual replacement. Com- K 0.02 0.02 0.02 0.04 0.03 0.0I 0.02 nonly, greenalite and very fine-grained chamosite tocr 3.02 3.02 3.03 3.05 3.05 3.02 3.04 ( -A71 irregular contacts. In rare mesobands, Fe as Feo smooth but greenalite and chamosite are intimately intermixed in 7 - Minn sheaves which cut across inxergrowth of qtz-green-stifp. sanple 84715/l; 2 - Minn sheaves which radiate into qtz grains fine-grainedaggregates. in gteen-sid-qtz assenbTage. Sanple 84716. 3. Minn sheaves which cut across intergrowtb of green-sid-chad-stilp assenbTage. Greenalite in the early assemblages,both in Sanpfe 84778; 4- Minn which cuts ac.oss ragged stilp aggreqate in ninr-qtz-rich nesoband. Sanpfe 84734; 5. Minn in greeA- banded iron-formation and Fe-shale bands, is in- chan-stifp-sid assenblage. Sanple 84719t 6- Minn sheaves cuXting sid-qtz grajn boundaries. SanpTe 847 37 , 7 . Minn in M9-rich s id-chan-pq-po nesoband. sanple 84722. variably an Al-bearing variety (Table 2). One meso- band (mesobandl, sample 84715)contains two dis- tinct greenalite types: an Al-bearing greenalite in the semblages,the most common of which are green- early assemblage of greenalite-siderite-quartz-py- alite-siderite-quartz=pyrite, greenalite-siderite, and rite, and an Al-poor greenalite which appears to be greenatte-quartz. Most greenalite-bearing assem- secondary(Table 2, Figs. 7C and D). The latter, with blages show an overprinting by stilpnomelane or a grain size similar to the Al-bearing greenalite, is the minnesotaiteor both (Figs. 7A and B). Greenalitein dominant greenalite type in parts of the mesoband.It these intergrowths is extremely fine-grained (sub- is readily distinguished optically by a slightly brown micron) and appearsalmost isotropic under crossed coloring and the presence of variable amounts of 'clean' nicols. Greenalite aggregateshave smooth, dark diffuse patches, similar to those occurring in boundarieswith both siderite and quartz. Of the as- some Al-bearing greenalites. The contact between sociated sulfides pyrite, very minor pyrrhotite, and the two greenalite types is generally diffuse but may arsenopyrite, generally only pyrite is in contact with be sharp (Figs. 7C and D). The contact of siderite greenalite, siderite, or quartz, and it occurs as eu- and quartz with Al-poor greenalite is at high magni- hedral grains,rarely up to l.Omm in diameter. Mag- fication diffuse, with some greenalite occurring netite is uncornmon in these mesobands,but where within qvartz near the contact. Stilpnomelanenee- present is euhedral and relatively coarse-grained dles cut across both Al-bearing and Al-poor green- (0.1-0.8mm), and contains small pyrrhotite in- alite (Fig. 7D). Another mesoband(mesoband 2 of clusions near sulfide aggregates.Grains of all these sample 84715) contains only Al-poor greenalite in a minerals, with the exception of greenalite, have matrix of siderite, minor qvartz, stilpnomelane, py- shapesindicative of minimum interfacial free energy rite, and very minor magnetite. Stilpnomelane nee- GOLE: BANDED IRON-FORMATIONS dles and a mesh of clear greenalitelocally cut across theselow-grade banded iron-formationsbecause it is or otherwisedisrupt siderite-quartz grain boundaries never observedin reactiontextures. Magnetite grains (Fig. 7E). near aggregatesofeuhedral pyrite comrnonlycontain The secondaryAl-poor greenalite in both meso- small blebs of pyrrhotite. Equilibration between bands appearsto have formed at the same time as magnetite(or a magnetiteprecursor) and the original stilpnomelane.The possiblereactions for the forma- sedimentarysulphide [perhapsmackinswils, Fe,*"S tion of these two minerals will be discussedin the (Berner, 1970)l may have left pyrrhotite grains iso- section on stilpnomelane.Other Al-poor greenalites lated in magnetitegrains during recrystallization. in the Weld Range banded iron-formation (Table 2, Sulfidesgenerally display euhedraloutlinss, except Fig. 3, and discussedbelow) also appear to have for pyrrhotite inclusions in magnetite.Pyrite gener- formed contemporaneouslywith stilpnomelane,al- ally forms coarse-grained(0.1-l.00mm) aggregates though generallythe critical evidence,thought com- which are concentratedin somemesobands. In some pelling in sample 84715,is lacking. greenalite-pyrite-richassemblages, Irace amounts of Chamositeoccurs in Fe-shalebands and as a minor pyrrhotite and arsenopyrite occur in contact with the constituent in banded iron-formation assemblages.silicatesand carbonates.In Fe-shalebands, trains of Rocks with signfficantproportions of both chamosite pyrite grains define a fine lamination. In some min- and greenalite are spatially associatedwith the Fe- nesotaite-richmesobands, the assemblagepyrite-pyr- shalebands. In the Fe-shales,chamosite occurs typi- rhotite-magnetite-arsenopyrite-siderite occurs in cally as a fine-grainedmesh betweenmagnetite and nodules (seebelow). pyrite grains. Generally it is so fine-grained ( The importance of the availability of Si during The secondary siderite is slightly enriched in Ca rela- stilpnomelaneformation is also seenin the Fe-shale tive to primary siderite (Table 4). bands, which are generally quartz-poor relative to In mesobands containing greenalite-siderite- the banded iron-formation. Where stilpnonelane re- quartz-rsulfideststilpnomelane, minnesotaite is com- places chamosite alone (Fig. 7G), Si and K must mon in rosettes and sprays of fine needles, cutting have beenintroduced into the mesobandand Al lost. across grain boundaries between the silicates and Where qvartz is present in chamosite-rich assem- siderite. In greenalite-rich mesobands, minnesotaite blages,stilpnomelane is relatively coarse-grainedand occurs as isolated, extremely fine-grained sheaves (5- is concentrated around and projects into qtartz lOpm), whereas on the contact between greenalite- grains, isolating them from further reaction. rich and quartz mesobands, massive large sheaves of Although the stilpnomelane-forming reactions minnesotaite (200-300pm long) rim and appear to ef- suggestedabove indicate considerablemobility of fectively isolate quartz grains from further reaction elementsduring such reactions,the Fel(Fe+Mg) ra- with greenalite and siderite (Fig. 8A). tio of stilpnomelanelargely reflectsthe bulk compo- ln chlorite- and stilpnomelane-rich assemblages, sition of the host mesoband(Fig. a). minnesotaite is uncommon except near qvafiz grains, Minnesotaitecomprises over 60-70 volVoof many and even there it forms only a small proportion of massivemesobands in the Weld Range banded iron- the mineral assemblage. In less Al-rich mesobands, formation, particularly in magnetite-richparts. These stilpnomelane is commonly replaced by minnesotaite mesobands,which consist of minnesotaite-quartz- (Fig. 8B). magnetite+pyrite, alternate with thick, essentially Although mimesotaite is ubiquitous in the banded monomineralic magnetite mesobands.In less mag- iron-formation in D.D.H. 3, it generally forms a netite-rich parts, textures indicating partial replace- small proportion (<5-15 vol%o)of those mesobands ment of quartz-siderite, greenalite-quartz, and that contain the assemblage greenalite-stilpnome- greenalite-quartz-sideriteassemblages (Figs. 7A and lane-siderite+quartz+pyritearare pyrrhotite. Most 8A) by minnesotaiteare present.Reactions based on minnesotaite-rich assemblages(>60-7 0 v ol%o)consist electron probe analysesof minerals in quartz-side- of minnesotaite-quartz-magnetiteipyrite. rite-minnesotaiteintergrowths indicate that Mg and Minor oxide components in minnesotaite range Al must have been added as reactantsand that Mn widely and reflect the compositions of the reactant and Ca were releasedinto solution. Near one siderite minerals. For example, minnesotaite formed by the + qnartz-+ minnesotaitereaction site (Fig. 8A), sec- reaction of quartz and siderite has low AlrO, and ondary siderite occurs in veins cutting relict quartz. KrO contents (e.9. anal. 6, Table 7), whereas minne- Fig. 7. Greenalite, chamosite, and stilpnomelane occurrences at Weld Range. (A) Extremely fine-grained Al-bearing greenalite (G) and coarse-grained siderite (S) contains subround quartz aggregates (Q). Quartz shows high reliefdue to minute inclusions. Dark brown, highly pleochroic stilpnomelane (St) cuts across greenalite-siderite-quartz grain boundaries. Sample 84715/1. Scale bar represents 0.lmm. (B) Crude microbanding defined by alternating laminae of Al-bearing greenalite (G) and quartz-mimesotaite (Q,Mi) assemblages. Greenalite laminae contain small quartz grains. Minnesotaite needles cut laminae boundaries. Sample 84720. Scale bar represents 0.lmm. (C) Al-bearing greenalite (G), quartz (Q), siderite (S), and pyrite (Py) assemblage. Al-poor greenalite (aG), which contains fine dark inclusions, has diffuse contacts with Al-rich greenalite. Stilpnomelane (St) cuts across contacts between greenalite, quartz, and siderite. Sample 84715/1. Scale bar represents 0.lmm. (D) Al-bearing greenalite (G) shows sharp contacts with Al-poor greenalite (aG), which contains fine dark inclusions. Siderite (S) and 'clean' quartz (Q) show sharp, boundaries with Al-bearing greenalite but diffuse contacts with Al-poor greenalite. Coarse-grained stilpnomelane sheaves qoss-cut greenalite, siderite, and quartz contacts. Sample 84715/1. Scale bar represents 0.lmm. (E) Siderite (S), quartz (Q), Al-poor greenalite (aG), and stilpnomelane intergrowth (St). Mottled area around greenalite is an extremely fine-grained intergrowth of siderite and greenalite. Minor minnesotaite is present. Sample 84715,/2. Scale bar represents 0.1mm. (F) Fine-grained brown stilpnomelane (St) in stringers in very fine-grained Al-greenalite-siderite (S) mesoband. Fine needles of minnesotaite occur in greenalite. Sample 84716. Scale bar represents 0.2mm. (G) Stilpnomelane (St) in veins and aggregates in chamosite (C)-siderite (S)-pyrite (Py) assemblage in Fe-shale band. Sample 84734. Scale bar represents 0.lmm. (H) Fine-grained, dark brown stilpnomelane (St) occurs in greenalite (G) but not chamosite (C) in greenalite-chamosite assemblage in Fe-shale band. Sample 84719. Scale bar represents 0.lmm. 20 GOLE: BANDED IRON-FORMATIONS Fig. 8. Minnesotaitetextural relationsand siderite-sulfidenodules at Weld Range. (A) Al-bearing greenalite(G), very coarse-grainedquartz (Q; grain, not all showruis 2.5 mm across),and sid€rite (S) cut by coarse- grainedsheaves of minnesotaite(Mi). Minnesotaiteforms rim around quartz grain. Extremelyfine-grained needles of minnesotaiteoccur in greenaliteaway from quartz. Secondarysiderite (sS) occurs inside quartz. Crossedpolars. Sample 84716. Scale bar represents0.lmm. (B) Sheavesand spraysof mimesotaite cross-cuttingstilpnornelane. Opaque is magnetite.Sample 84734. Scale bar represents0.05mm. (C) Aggregates(Py) of coarse-grainedpyrite, minor pyrrhotite, and magnetite surrounded by siderite (S) in nodules rimmed by Al-poor greenalite (aG). Greenalite has sharp contactswith minnesotaite (Mi) and quartz (Q) assemblage.Fine-grained opaque in mimesotaite- quartz assemblageis magnetite. Stilpnomelane(St) is ernbayedby minnesotaite(see Fig. 8B). Sample 84734.Scale bar represents 0.lmm. (D) Contact between siderite-sulfide nodule and minnesotaite-quartz+magnetite assemblageshown in reflected light. Nodule consists of an aggregateof coarse-grainedpyrite (Py), siderite (S), magnetite(M), and pyrrhotite (Po). Black area consistsof siderite (against pyrite aggregateand in contact with large magnetite grain), Al-poor greenalite (rim, 0.03mm wide between siderite and minnesotaite- quartz), and minnesotaite-quartz assemblage(right side of figure). Pyrrhotite and arsenopyrite (A) occur in siderite. Magnetite in mimesotaite-quartz assemblagecontains a small pyrrhotite inclusion. Sample84713. Scale bar r€presents0.025mm. (E) Large siderite (s)-sulfide (Py) nodule with rim of Al-poor greenalite (aG) which shows optical alignment of fibrous grains. Magnetite (M) forms rim between greenalite and minnesotaite-quartz-richassemblage. Crossed polars. Sample 84713. Scale bar represents0.lmm. (F) Ragged and embayed Al-poor greenalite (aG) in mimesotaite (Mi)-quartz (Q)-rich assemblage.Al-poor greenalite in rim of siderite (S)-sulfide (Py) nodule is in contact with minnesotaiteand quartz but is not embayed by minnesotaite.Sample 84713. Scale bar represents0.05mm GOLE: BANDED IRON-FORMATIONS 2l sotaitewhich replacesstilpnomelane shows relatively high AlrO, and KrO contents(anal. l, 3,4, Table 7). The variable Al,O, (0.27-0.69wtVo) and, K,O (0.07- 0.22 wtVo)in minnesotaitein one speci-ficmesoband -ro may reflect its formation by a combination of reac- tions within the mesoband. Siderite-sulfide nodules are present in several -30 mesobandsdominated by the assemblageminnesota- ite-quartz-minor magnetite-minor pyrite. Nodules are up to 1.3 mm acrossand show well defined 60- (9 l00pm rims of clear greenalite(Fig. 8C), which oc- o -so curs in extremely fine-grained fibrous aggregates J showingbulk pleochroismfrom greento pale brown. Greenalite on the outsideedge of the vein has sharp 'clean' contacts with minlesotaite, qtartz, and mag- Mog i netite, whereas greenatte ( tion of the reactionsquartz + siderite-+ minnesota_ Actinolite in the metabasitesbecomes progres- -+ ite and greenalite + quartz minnesotaite, with sively more aluminous to the south, acrossthe Weld abundant minnesotaite replacing all three earller Range, and in D.D.H. 14 (Fig. l) moderately alu- minerals.At Weld Range,the mineralogy of silicate- minous actinolitesoccur with metamorphiclabrado- rich mesobandstends to be dominated either by the rite. Theseoccurrences indicate metamorphiccondi- early assemblages,which are preservedlargely be- tions of upper greenschistto lowermost amphibolite causeof specifc conditionssuch as low a",o,,low po., faciesat low pressure(Liou et al.,1974). There is no or high P.o,, or by the higher-temperatuieminne- direct evidence of metamorphic pressure at the sotaite-quartz coexistence.This suggeststhat reac- D.D.H. 3 locality, but comparisonwith other areas tion of the earlier assemblagesto form minng56hils suggestsit was probably lessthar- 2-3 kbar (Binns et may have beena rapid process,and is consistentwith al.,1976). metamorphicconditions in minnesotaite-quartz-rich It should be stressedthat 300+50oC is the esti- mesobandshaving passedthrough the invariant mated maximum temperature of metanrorphismat point to Figure 10. An alternative explanation, Weld RangeD.D.H. 3 locality. There is no direct evi- equally consistentwith the observations,is that the dence to assessthe minimum temperaturesof the peak metamorphictemperature was well abovethose various reactions in the low-metamorphic-grade of the minnesotaite-forming reactions, allowing a banded iron-formations. Minnesotaite can probably wide temperatureinterval for complete breakdown form at temperaturesas low as 1@-l50oC. These of all reactant minerals in those mesobandswhere temperaturesare suggestedby observationsin low- the fluid composition favored minnesotaite-bearing grade Proterozoiciron-formations (French, 1973: assemblages. Klein, 1974;Klein and Fink, 1976;Floran and Pa- That Figure l0 is applicable only to the higher- pike, 1975, 1978),by the relative stabilities of the temperature side of the greenalite-siderite_quartz minerals in low-grade iron-formations in theoretical stability field is suggestedby (a) the presenceof mi- phase diagrams (Klein and Bricker, 1977),and by nor amounts of minnesotaitein much of the essen_ comparisonwith the thermal stability of talc (Bricker tially unmetamorphosedGunflint Iron Formation. et al.,1973). Thus at Weld Rangeminnesotaite prob- well away from the Duluth Intrusive Complex (Flo- ably formed in responseto an increasein temper- ran and Papike, 1975);and (b) the relative stabilities ature related to the metamorphic gradient now pre- of low-temperature Fe-minerals calculated from served. thermodynamicdata (Klein and Bricker, lg77\. Summaryof mineral relations Peak metamorphictemperature Basedon textural interpretations,the earliest rec- The arsenopyrite geothermometer (Kretschman ognizable assemblagesin the Weld Range iron-for- and Scott, 1976), which correlatesthe atomic per- mation include Al-bearing greenalite, siderite, centageof As in arsenopyritecoexisting with pyrite qvartz, magnetite,and various sulfide minerals, with and pyrrhotite with temperature of formation, in- chamositein the more aluminousbulk compositions. dicates a ternperature of 320.C for sample g6715 Pyrite is the dominant sulfide. Pyrrhotite is rarely (Table 5). The analytical error in the electronmicro- part of the assemblagebecause it generallyoccurs as probe analysisand the extent of extrapolation from inclusionsin pyrite or magnetite,and as such is iso- the experirnental data points of Kretschnan and lated from the remainder of the assemblage.Trace Scott (1976) suggeststhis estimate is probably amounts of arsenopyrite and chalcopyrite are also =50"c. part of the early assemblage.Both the formation of This temperatureis consistentwith the meramor- breccia composedof angular fragmgnl5and the ap- phic mineral assemblagesin metabasitesfrom near parent recrystallization and development of rela- the collar of D.D.H. 3. These contain relict igneous tively coarse-grainedquaftz and siderite appear to augite and labradorite which are partially replaced have occurredprior to the growth of stilpnomelane, by low-Al actinolite, albite, epidote, and chlorite. which suggeststhat the banded iron-formation was Available experimental data do not allow an accu- well lithified before this mineral developed. rate estimationof the minimum temperaturefor this Stilpnomelane overprints the early textures and metamorphic assemblage,although it appearsthat a mineral assemblagesand is formed by reaction be- temperature of 300+50oC is probable (Zen, 1974: tween K-bearing pore solutionsand the pre-existing Coombset al.,1976). aluminous minerals, Al-bearing greenalite and GOLE: BANDED IRON.FORMATIONS chamosite.In somemesobands, a secondaryAl-poor per. I thank Mrs. Charles L. Brown for typing. This study was un- a Commonwealth Postgraduate Re- greenalite is formed during the stilpnomelane reac- dertaken during tenure of Award at ttre University of Western Australia' Support been favored by rela- search tion and its growth may have from NSF grant EAR-76- I 1740(Klein) during preparation of this tively low Po,. Reactionsbased on electron micro- paper is gratefully acknowledged. probe analysisofreactants and products suggestthat there was considerablemobility of components,al- bulk mesoband composition was the though the References dominant influence in determining the Fel(Fe+Mg) of iron-bearing minerals in banded of stilpnomelane.It is clear that the presenceof Ayres, D. E. (1972) Genesis ratio mesobandsin the Dales Gorge Member, Ham- high c",o,, favored stilpnomelane iron-formation qluartz,providing a ersleyGroup, WesternAustralia. Econ. Geol',67' 1214-1233' grain growth. In some stilpnomelane-rich mesobands Ayres,V. L. (1940)Mineral notesfrom the Michigan iron country' there is no evidence as to earlier mineral assem- Am. Mineral., 25, 432414- blages,and it is possiblethat all earlier textureshave Bence,A. E. and A. L. Albee (1968)Empirical correction factors microanalysis of silicates and oxides' f' Geol" been destroyed during recrystallization. for th€ electron forms by reac- 76.382403. Texturesindicate that minnesotaite Berner, R. A. (1970) Sedirnentarypyrite formatiot' Am' f' Sci', tion of greenalnte,qvattz, siderite, and stilpnomelane. 268,l-23. Mesobandscomposed of minnesotaite-quartz-mag- Binns, R. A., R. J. Gunthorpe and D. I. Groves (1976)Metamor- netite+pyrite representthe completereplacement of phic patterns and development of greenstonebelts in the East- Block, Western Australia. In B. F' Windley' Ed" the precursorassemblages. irn Yilgarn Early History of the Earth. Wiley, London' euhedral and do The Magnetite and pyrite are always Blake, R. L. (1965)Iron phyllosilicatesof the Cuyuna district in not exhibit reactiontextures. Both mineralsare stable Minnesota.Am. Mineral., 50' 148-169. in the earliestassemblages as well as the later minne- Bricker. O. P., H. W. Nesbitt and W. D. Gunter (1973)The stabil- sotaite-bearingassemblages. ity of talc. Am. Mineral., 58,6+72. relations of biotite and stilpnomelane mineral assemblagesand textural ob- Brown, E. H. (19?l) Phase The above greenschistfacies. Contrib. Mineral- Parol', 31,275-299' iron-formation in the servationsfor the Archaean banded Burt, D. M. (19'1.2)The systemFe-Si-C-O-H: a model for meta- at Weld Range are essentiallyidentical to those de- morphosed iron formations. Carnegie Inst. Wash' Year Book, scribedfor Proterozoiciron-formations (Klein, 19'14; 71,435-443. River oolitic Klein and Fink, 1976; Floran and Papike, 1975, Cochrane,G. W. and A. B. Edwards(196O) The Roper cSIRo Min. Inv. Tech' Pap' l' seeFrench, 1973for review of earlier studies). ironstoneformarionr. 1978; W. (1971)Magic IV Progran. Bell TelephoneLaborato- similarities Colby, J. This clearly establishesthe mineralogical ries Inc., PennsYlvania. betweenArchaean and Proterozoicbanded-iron for- Coombs, D. S., Y. Nakamura and M. Vuagnat (1976) Pump- mations. Similarities also exist in bulk composition ellyite-actinolite facies schistsofthe Taveyanne Formation near and in sedimentary or diagenetic structures (Gole, in Lo6che,Valais, Switzerlatrd.J. Petrol., 17,440-4.71' Eggleton,R. A. (1972)The crystalstructure of stilpnomelane:Part preparation). Mag., 38,693-'lll' peak II. The full c*ll. Mineral- The early assemblagesare preservedat the - and B. W. Chappell (1978)The crystal structureof stilp- metamorphictemperature of 320+50"C as estimated nomelane. Part III. Chemistry and physical properties' Mineral' for the D.D.H. 3 locality only under certain specific Mas., 42,361-368. Igneousand metamor- conditions: (a) if the bulk composition was in- Eugster,H. P. and G. B. Shippen (1967) gas equilibria. In P. H' Abelson' Ed'' for minnesotaite formation, either be- fhic reactionsinvolving appropriate in Geochemistry,Vol. 2' p' 492-520' Wiley' New Fe-shale Researches cause of high Al or K contents as in the York. bandsor in Si-deficientassemblages such as in green- Floran, R. J. and J. J. Papike (1975)Petrology of the low-grade alite-siderite mesobands;or (b) if Po, was relatively rocks of the Gunflint Iron-Formation' Ontario-Mirmesota' low or P.o, relatively high, or both. The latter condi- Geol. Soc.Am. Bull.,86' l169-1190. - - (1978)Mineralogy and petrologyof the Gun- would causegreenalite-quartz-siderite assem- a1d tions flint Iron Formation, Minnesota-Ontario: correlation of compo- blages to be stable relative to minnesotaite-bearing sitional and assemblagevariations at losi to moderate gtade' J' assemblages. Petrol.,19,215-288. French, B. M. (1964) Graphitization of organic material in a pro- gressively metamorphosed Precambrian iron formation' Sci- Acknowledgments ence,147,917-918. grade I thank the Director of the Geological Survey of Western Aus- - (1973) Mineral assemblagesin diageneticand low tralia for permission to sample the core from D.D.H. 3' Weld metamorphiciron-formation. Econ. Geol-,68' lM3-1074' and Range. I am also grateful to Cornelis Klein for his support during Garrels, R. M. and C. L' Christ (1965) Solutions' Minerals the latter part of this study and his constructive review of this pa- Equitibria. Harper and Row, New York. GOLE: BANDED IRON.FORMATI)NS 25 Greenwood,H. J. (1967)Mineral equilibria in the systemMgO_ Lesher,C. M. (1978)Mineralogy and petrology of the Sokoman SiO2-H2O-COr.In P. H. Abelson, Ed, Researchesin Geochem_ Iron Formation near Ardua Lake, euebec. Can. J. Earth Sci., istry, Yol. 2, p. 542-547.Wiley, New york. 15,480-500. - (1975)Butrering ofpore fluids by metamorphicreactions. Liou, J G., S. Kuniyoshi and K. Ito (1974)Experimental studies Am. J. Sci.,275,57i-593. on the phaserelations between greenschist and amphibolitein a Grout, F. F. and G. A. Theil (1924)Notes on stilpnomelate.Am. basalticsystem. lzr. J. Sci.,274,613-6j2. Mineral., 9, 228-229. Miyano, T. (1976)Mineral assemblagesof the proterozoicbanded Gruner, J. W. (1936)The structureand chemicalcomposition of iron formation h the Hamersley area, Western Australia. greenalite. (in Am. Mineral., 21, 449455. Japanese)Mining Geol.,26, 273-288. - (1944) The composition and structure of mimesotaite, a Ohmoto, H. and D. Kerrick (1977)Devolatilization equilibria in common iron silicate in iron-formations.Am. Mineral., 29,363_ graphitic schists.lrn. J. Sci.,277, l0l3-1044. 3'12. Page, N. J (1968) Chemical differencesamong the serpentine - (1946) The Mineralogt and Geologyof the Taconitesand "polymorphs." Am. Mineral., 53, 201-215. Iron Ores of the Mesabi Range, paul Minnesota. St. Office of the Robie, R. A. and D. R. Waldbaum (1968)Thermodynamic prop- Commissionerof the Iron Range Resourcesand Rehabilitation. erties of minerals and related substancesat 298.15.K (25.C) Guggenheim,S., P. Wilkes and S. W. Bailey (1977) X_ray and and one atmosphere(1.013 bars) pr€ssure and at higher temper_ electrondiffraction study ofgreenaliteand minnesotaite(abstr.). atures. U. S. Geol.Surv. Bull. 1259. EOS, s8,52s. Scott, S. D. (1974)Experimental methods in sulfide synthesis.In Jolliffe, F. (1935)A study of greenaltte. Am. Mineral., 20,405425. P. H. Ribb€, Ed., SulfideMineralogy, p. Sl-S38. Mineral. Soc. Jones,W. R. (1963) The iron ore depositsof the Weld Range, Am. Short CourseNotes l. Murchison Goldfield. Ann. Rep. Geol. Surv. I4test.Ausr., 1962, Trendall, A. F. and J. G. Blockley (1970)The iron formations of 54-60. the Precambrian Hamersley Group, Western Atstralia. Bull. Kerrick, D. M. (1974) Review of metamorphic mixed_volatile Geol. Sum. West Aust. lI9. (HrO-CO2) equilibria. Am. M ineral., 59,7 29_762. Vernon, R. H. (1976)Metamorphic processes,Reactions and Mi_ Klein, C., Jr. (1974)Greenalite, stilpnomelane,minnesotaite. cro_ crostructureDevelopment. Allen and Unwin, London, cidolite and carbonatesin a very pre_ low-grademetamorphic Weaver, C. E. and L. D. Pollard (1973) The Chemistryof Clay cambrian iron-formation. Can. M ineral., t 2, 4i 54gg. Minerals. Elsevier, New York. - and O. P. Bricker (1927) Someaspects of the sedimentary Whittaker, E. J. W. and F. J. Wicks (1970)Chemical di_fferences and diageneticenvironrnent proterozoic 'polymorphs": of bandediron_forma_ among the serp€ntine a discussion.Am. Min_ tion. Econ.Geol., 72,1457-1470. eral., 55, 1025-1047. -and R. P. Fink (1976)petrology ofthe SokomanIron For_ Zajac, I. S. (1974)The stratigraphyand mineralogy of rhe Soko- mation in the Howells River area, at the western edge of the man Formation in the Knob Lake Area, euebec and New_ Labrador Trough. Econ. Geol.,71,41?.4gg. foundland. Geol.Sum. Can. Bull. 220. Kretschman,V. and S. D. Scott (1926)phase relations involvine Zen, E-ala (1974) Prehnite- and pumpellfte-bearing mineral as- arsenopyritein the system Fe-As-S and their applications.Cazl semblages,west side of Appalachian metamorphicbelt, penn- Mineral., 14,36+386. sylvania to Newfoundland,.f. Petrol., 15, 197-242. LaBerge,G. L. (1966a)Altered pyroclasticrocks in iron_formation in the Hamersley Range, Western Australia. Econ. Geol., 61, t47-t6t. - (1966b) Pyroclasticrocks in South African iron_forma_ Manuscript received, June 13, 1979: tions. Econ. Geol.,61, 572-581. acceptedfor publication, September lg, IgZg.