Petrology of the Low-Grade Rocks of the Gunflint Iron-Formation, Ontario-Minnesota

Home , Greenalite, Minnesotaite, Zussmanite

PAP?k^N I Department of Earth and Space Sciences, State University of New York, Stony Brook, New York 11794

ABSTRACT tent with textural and compositional data tinuation of the iron formation. Marsden supporting a primary origin for the iron and others (1968) used the term "Animikie The relatively unmetamorphosed middle silicates. Quartz, recrystallized carbonate Iron Formation" for the correlated seg- Precambrian Gunflint Iron-Formation of cements, microcrystalline siderite, hematite, ments of the Cuyuna, Mesabi, and Gunflint Ontario has undergone considerable post- and possibly magnetite are also considered Ranges of Minnesota and Ontario. depositional recrystallizarion and locally in- primary phases. Key words: mineralogy, The iron formation is structurally simple tense replacement. Although these tend to sedimentary petrology, crystal chemistry, and uncomplicated. It is nearly flat lying obscure primary textural-mineralogical re- sheet silicates. with an average southeast dip of 5°. Local lations, textural elements similar to those of folding and brecciation, often accompanied limestone can be identified and their INTRODUCTION by gravity faults, are, however, present. mineralogy defined. Two fundamentally This type of deformation was attributed by different kinds of iron formation are recog- This report deals with the mineralogy Goodwin (1956) to penecontemporaneous nized: (1) cherty iron formation, which and petrography of the relatively un- volcanic disturbances. consists of granules, ooliths, and interstitial metamorphosed Gunflint Iron-Formation The Gunflint Iron-Formation and the cements; and (2) banded or slaty iron for- of Ontario. Emphasis is placed on defining overlying Rove Formation (with which it mation, which is composed of matrices the textural relations and chemistry of forms a gradational contact) comprise the (fine-grained internally structureless silicate- and carbonate-bearing assemb- middle Precambrian Animikie Group. The laminae). Cherty iron formation corre- lages. A succeeding paper will deal with as- Rove Formation consists of interbedded ar- sponds broadly to the thick-bedded taco- pects of the contact metamorphosed por- gillite and graywacke as much as 1,000 m nite and algal chert facies of Goodwin; slaty tion of the iron range that is largely thick (Morey, 1967). The average thickness iron formation encompasses the thin- confined to northern Minnesota. of the Gunflint Iron-Formation in Ontario banded chert-carbonate and tuffaceous The Gunflint Iron-Formation of Ontario is about 120 m (Goodwin, 1956). In Min- shale facies. contains an abundance and variety of iron nesota the Animikie is divided into three Greenalite associated with cherty quartz silicate and carbonate minerals whose formations, including a basal quartzite unit and minor minnesotaite are dominant min- chemistry within iron formations is virtu- that forms the lowermost member of the eral constituents of granules; stilpnomelane ally unknown. We relate compositional var- iron formation in Ontario. Unconformities and hematite are less common. Recrystal- iation in some of the individual mineral are present at the base and top of the lized calcite, ankerite, and siderite occur phases to specific textural occurrences and Animikie strata in Ontario and Minnesota. locally as cements and as replacement min- stratigraphic position; these variations ap- On a regional scale, basic intrusions of erals. The most common cement is quartz. pear to be retained in the contact aureole. late Precambrian (middle Keweenawan) Iron silicate and siderite matrices are major Preservation of relict textures and the ap- age are intimately associated with the constituents of slaty iron formation, which parently isochemical nature of the Gunflint and Rove Formations. In Ontario also contains considerable amounts of sec- metamorphism (except for loss of H20 and these comprise the Logan intrusive rocks, ondary calcite and ankerite. Stilpnomelane COz) should enable correlation between which consist of numerous sill-like diabase and chamosite are locally abundant in slaty metamorphic mineral assemblages and sheets (Moorhouse, 1960). These are found rocks as apparent pseudomorphs after their sedimentary precursors as well as throughout the Animikie rocks, but indi- shards. quantification of some metamorphic reac- vidual sills rarely exceed 30 m in thickness. Microprobe analyses of greenalite reveal tions. In short, the Gunflint Iron-Formation little compositional variation; stilp- offers a rare opportunity to study the pro- Previous Work nomelane from slaty iron formation is ex- gressive transformation of an undeformed tremely heterogeneous. Both siderite and sedimentary rock to the pyroxene-hornfels Detailed geologic investigation of the ankerite exhibit considerable substitution facies. Gunflint Iron-Formation began with the of Fe by Mg (and Mn locally) whereas cal- studies of Tanton (1923, 1931) and Gill cite is almost pure CaC03. Geologic Setting (1924, 1927). Gill (1927) described some of Comparison of the greenalite, min- the Gunflint textures but not their detailed nesotaite, and stilpnomelane crystal struc- The main portion of the iron formation mineralogy. The iron formation was di- tures reveals many similarities. The crystal extends about 170 km northeast from the vided by Goodwin (1956) into six major chemistry of magnesium and nickel International Boundary at Gunflint Lake sedimentary facies representing four mem- analogues (serpentine, talc, and garnierite) (Fig. 1). In Minnesota the iron range forms bers. He drew attention to evidence of has been used to predict structural details of a narrow belt 20 km long and is truncated widespread volcanism associated with the iron silicate minerals. These are consis- to the southwest by the Duluth Complex. iron-formation deposition and suggested a !f Present address: Manned Spacecraft Center, TN6, Erosional remnants northeast of Thunder volcanic source for the iron and silica. The Houston, Texas 77058 Bay at Schreiber, Ontario, suggest a con- cyclic nature of the deposition is shown by

Geological Society of America Bulletin, v. 86, p. 1169-1190, 14 figs., September 1975, Doc. no. 50901.

1169

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/9/1169/3418213/i0016-7606-86-9-1169.pdf by guest on 28 September 2021 1170 FLORAN AND PAPIKE

IV^M Duluth Complex ") p-p-n , „ . f Keweenawan Group (U. Precambrian) I.:'.•! Sibley Series J I I Rove Fm •) S Animikie Group (M. Precambrian) Guntlint Fm J

Undivided (L. Precambrian)

Upper Limestone Member 100 -\

Upper Guntlint Member

Lower Guntlint Member

Basal Conglomerate Member

1 Lower* and Upper algal chert facies 4 Lower* and Upper* banded chert-carbonate facies

2 Lower and Upper tuffaceous shale facies 5 Upper limestone member

3 Lower west* and east taconite; Upper* taconite facies »abundantly sampled

Figure 1. Generalized geologic map and stratigraphic section of Gunflint Iron-Formation (after Morey, 1973, modified from Goodwin, 1956),

the repetition of facies in the Lower tablished the time of sedimentation. the sediments is shown by abundant fea- Gunflint and Upper Gunflint members (Fig. Nevertheless, these studies (Hurley and tures of shallow water (above wave base) 1). The upper tuffaceous shale unit others, 1962; Faure and Kovach, 1969; origin. In addition, at least a small part of (argillite-tuff horizon of Moorhouse, 1960) Misra and Faure, 1970) and indirect evi- the iron formation appears to have been forms a marker bed traceable throughout dence (Hanson and Malhotra, 1971) sug- deposited subaerially (Goodwin, 1956; much of the iron formation. The strati- gest the Animikie sediments were deposited Walter, 1972). graphic units defined by Goodwin (1956) in slightly less than 2,000 m.y. B.P. Ontario can be readily correlated with the Many workers have drawn attention to Analytical Procedures fourfold division used on the Mesabi the abundant sedimentation and penecon- Range (Lower Cherty, Lower Slaty, Upper temporaneous deformation features that The facies that were extensively sampled Cherty, Upper Slaty). This division was ex- occur within the Gunflint Iron-Formation are indicated in Figure 1. A large range of tended to the Gunflint Iron-Formation in (Broderick, 1920; Gill, 1927; Goodwin, rock types was collected, but detailed study Minnesota by Broderick (1920); he sug- 1956; Moorhouse, 1960; Mengel, 1963, was directed to the silicate- and (to a lesser gested that the two ranges are probably 1965; Barghoorn and Tyler, 1965; extent) carbonate-bearing rocks. Labora- continuous beneath the Duluth Complex. LaBerge, 1967a, 1967b; Gross, 1972; Wal- tory work integrated petrographic, x-ray Goodwin (1960) and Moorhouse (1960) ter, 1972). Most of these structures are diffraction, and electron microprobe published outcrop maps (scale 1:31,680) of common to clastic limestone, shale, and analytical techniques. X-ray diffractograms most of the iron formation in Ontario; sandstone and serve as indicators of the were obtained on a Picker diffractometer Morey and Papike (see Sims and others, physiochemical environment of deposition; using monochromatic CuKa radiation. 1969) presented a preliminary geologic others are unique to chemical precipitates These supplemented optical identification map of the range west of Gunflint Lake in and suggest the former existence of gels. of major and minor mineral constituents. Minnesota. The variety and abundance of these struc- Chemical analyses of silicate and carbonate Several isotopic age investigations of the tures in the Gunflint strata indicate that phases were obtained using a four- Gunflint and Rove Formations have been fluctuations in water level and turbulence spectrometer ARL-EMX-SM electron mi- carried out, but none has satisfactorily es- were common. Considerable reworking of croprobe. Operating conditions were 15 kv

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/9/1169/3418213/i0016-7606-86-9-1169.pdf by guest on 28 September 2021 PETROLOGY OF THE LOW-GRADE ROCKS OF THE GUNFLINT IRON-FORMATION, ONTARIO-MINNESOTA 1171

accelerating potential and 0.01 microamp ondary diagenetic assemblages, charac- grained monomineralic or polymineralic sample current standardized on brass. terized by the replacement of primary min- aggregates. Boundaries between granules Beam current integration rather than fixed erals, and (3) low-grade metamorphic as- and cement are usually sharp but may be time counting was used to minimize the ef- semblages, developed under moderately diffuse. Granules commonly have coarser fects of instrumental drift. The electron high temperatures and pressures. At pres- grained fillings, which are thought to rep- beam spot size was varied and depended ent, it is not possible to distinguish diage- resent shrinkage (syneresis) cracks formed primarily on the grain size of the mineral netic minerals and assemblages formed at by expulsion of water from a gel. Dimroth being analyzed. Data reduction procedures low T and P conditions from those that are and Chauvel (1973) considered all granules were those of Bence and Albee (1968). Car- truly metamorphic. Application of French's to be intraclastic in origin; Folk (1959) used bonate phases were analyzed using a car- scheme is further complicated by textural the term "intraclast" to denote reworked bonate option program to correct for the relations that may not be definitive and by fragments of penecontemporaneous sedi- matrix effects of C02. The option is a the fact that most mineral phases display ment. modification of a computer program origi- both primary and secondary characteristics. Ooliths and pisoliths occur in discrete nally designed for hydrous-silicate analysis Nevertheless, the concept of primary and beds as mixed oolith-granule layers and and, for iterative purposes, assumes that the secondary minerals is a useful one. Their within and between algal structures. Most difference between the cation-oxide sum recognition, however, is based largely or ooliths are concentrically laminated, al- and 100 percent is C02. The empirically entirely on petrographic observations that though some also have a radial structure determined C02 alpha (a) correction fac- are qualitative and, at times, unavoidably due to crystallographic orientation. Nuclei tors of Albee and Ray (1970) were utilized subjective. commonly consist of single or multiple in the modified program. It is well known that two fundamentally granulelike bodies or smaller ooliths. Some different kinds of iron formation exist nuclei contain an internal banded structure PETROGRAPHY (Wolff, 1917; Gruner, 1946; James, 1954; that is oblique to the concentric lamina- White, 1954; Mengel, 1963; French, 1968, tions; these may represent an earlier deposi- Mineralogical and textural relations 1973; Morey and others, 1972). In this re- tional history. Many of the ooliths have within the Gunflint Iron-Formation are var- port the Gunflint is similarly divided into only thin concentric coatings around nuclei iable and complex due to deposition in a two major types — cherty iron formation and are more properly termed "superficial series of spatially related but chemically and banded or slaty iron formation. Each oolites" (Illing, 1954). Shrinkage cracks are and (or) energetically different aqueous en- possesses unique textures that can be re- pronounced in both ooliths and pisoliths. vironments. Four gradational geochemical lated on a large scale to stratigraphic posi- Petrographically distinct rim and pore facies (oxide, silicate, carbonate, and tion (Morey and others, 1972). cements generally are present in cherty sulfide) can be distinguished on the basis of As defined here, cherty iron formation rocks. Rim cements consist of fine-grained the dominant iron mineral present (James, includes the taconite and algal chert facies fibrous or bladed material oriented perpen- 1954). In addition, the low-grade or rela- of Goodwin (1956); slaty iron formation dicular to the surfaces of granules and tively unmetamorphosed rocks display tex- comprises the banded chert-carbonate and ooliths. These cements are paragenetically tural evidence of pervasive recrystallization tuffaceous shale facies that appear to be earlier than pore-filling cements, which are and replacement. Although these changes closely associated with volcanism. The tuff- typically coarser grained. Shrinkage-crack have obscured some of the primary aceous shale facies represent widespread fillings are indistinguishable in composition sedimentary features, relict textures pre- disseminations of pyroclastic material in- and grain size from pore cements. serving several stages of diagenetic read- terrupting normal chemical deposition Cherty iron-formation textures have justment are common in the Gunflint and (Goodwin, 1956). been considerably modified by recrystalliza- other Superior-type iron formations. The tion and replacement. Various carbonate textures imply that a major portion of the Cherty Iron-Formation Textures minerals locally replace quartz and typi- original precipitate was selectively trans- cally occur as porphyroblasts, idioblasts, ported and deposited mechanically as a sed- Textural similarities between many iron and granoblastic aggregates. Replacement iment (Mengel, 1963, 1965; LaBerge, formations and limestone are striking. is suggested by the occurrence of multiple 1967b; Dimroth, 1968; Dimroth and Folk's (1959, 1962) petrographic ghost or relict granule outlines within car- Chauvel, 1973). In the present study we classification of limestone (based on bonate grains or carbonate laminae. Re- emphasize the mineralogy of individual tex- analogous textural types in sandstone) can crystallization is suggested by (1) coarse- tural elements and their interrelations. be applied with little modification to grained, frequently rhomb-shaped, carbon- A mineral is considered primary if it does Superior-type iron formations (Dimroth, ate porphyroblasts; (2) transection of not replace a pre-existing mineral phase 1968; Dimroth and Chauvel, 1973). cherty granules or ooliths and cement by (Ayres, 1972; French, 1973); these minerals Textural elements of cherty rocks include coarse-grained quartz; and (3) variation in probably constituted the original precipi- (1) granules, (2) ooliths and pisoliths, and grain size within granules and matrix ma- tate or, more likely, crystallized from it (3) cements. Granules greatly predominate terial. All minerals of the iron formation are soon thereafter during early diagenesis. A over ooliths and pisoliths and are generally probably recrystallized to some degree. secondary mineral is one that appears to be associated with an intergranular cement. The textures of the algal chert facies are replacing a primary or pre-existing mineral. Most granules are internally structureless rather simple, as they consist predomi- Replacement may have happened at any and lack the concentric or radial structure nantly of fine-grained cherty quartz ar- time following sedimentation, so that a sec- that is characteristic of ooliths and ranged in swirling laminae distinguishable ondary mineral may be early diagenetic, pisoliths. Measurements of relative granule by differences in grain size. Fine-grained late diagenetic, or low-grade metamorphic size by Mengel (1963) suggest that the iron oxide commonly defines or emphasizes in origin. This usage differs slightly from Gunflint granules are somewhat larger than the laminae. Ooliths and granules with in- that of French (1973) who recognized three those of other iron formations in the Lake terstitial cements are locally abundant and types of mineral associations: (1) primary Superior region. Most granules range in tend to be concentrated between individual diagenetic assemblages, formed by post- length from 0.4 mm to about 1 mm and are algal structures. Carbonate minerals may depositional crystallization (or recrystalli- irregularly rounded or ovoid in shape. The be present as disseminated rhombohedra zation) of the original precipitate, (2) sec- granules typically consist of extremely fine and anhedra. In the Gunflint Iron-

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/9/1169/3418213/i0016-7606-86-9-1169.pdf by guest on 28 September 2021 1172 FLORAN AND PAPIKE

Formation, algal chert forms two distinct TABLE 1. MINERALOGICAL AND TEXTURAL RELATIONS Figure 2. A. Massive fine-grained stilp- IN THE GUNFLINT SAMPLES marker beds. The algal structures differ nomelane granules associated with rim and pore from those of limestone in that they are Cherty iron format-ion cements of chalcedony. Interstitial patches of siliceous and may be confused with similar Taconlte facies fine-grained siderite are texturally identical to looking inorganic "stromatolites" (Walter, Granules (allochems) Rim cements (orthochems) that found abundantly in an adjacent slaty layer. qtz qtz or chalc Note shrinkage-crack fillings of the chalcedony 1972). Hofmann (1969) interpreted some qtz + grnl ± minn ± sid within granules. Crossed nicols. Sample 724-2A. of these stromatolites to be of possible bac- hem* qtz + stilp ± minn ± B. Fibrous minnesotaite (pale) of metamor- terial origin. hem* qtz + hem Pore cements phic origin found only within greenalite-rich cale ± ank qtz and (or) chalc granules (dark). Some granules contain no visible qtz + mt ± grnl ± carb Banded Iron-Formation Textures minn minnesotaite while others consist almost entirely sid + mt of matted aggregate of minnesotaite bundles. Other Textural relations in banded or slaty cartv Sample 727-3. rocks are complex and difficult to interpret Ooliths and p1 souths Rim cements C. High-magnification photomicrograph of because of the generally fine grain size of (allochems) chalc or qtz sample 727-3. Note "bow-tie" texture of some qtz iron sil minnesotaite replacing cryptocrystalline greena- the constituent minerals and the abundance qtz + hem ± iron sil calc+ ± qtz lite. of dark organic material. The principal tex- sid Pore cements chalc ± qtz D through F. Oolith- and pisolith-bearing tural element is matrix, which is composed iron sil ± hem sample 726-2A. of varying proportions of cherty quartz, Other D. Iron-silicate (greenallte and minnesotaite?) carbonate, or iron silicate. Other less well carb + rim cement(s) surrounding and sometimes en- defined textural elements include possible Algal chert facies (algal biolithite) tirely within an earlier chalcedonic cement and volcanic shards and cements. Granules, ooliths. Matrices (orthochems) forming a continuous framework linking all pisoliths (allochems) chert ± hem ooliths. Note dark iron-silicate pore cement Matrices are fine-grained and of similar, qtz (center). Concentric laminations within ooliths although variable, grain size (<40 ¿im). qtz ± grnl ± minn ± hem consist of hematite, iron silicate, cherty quartz, They differ from cements in their presumed calct ± qtz Cements (orthochems) qtz ± chalc and recrystallized quartz. Shrinkage cracks in mode of origin — matrices were probably Banded or slaty iron formation some ooliths are filled with quartz. deposited as oozes or muds (Dimroth and Carbonate fades E. Coarse, spherulitic chalcedony pore filling Chauvel, 1973) while cements were precipi- Matrices (orthochems) between rim cement. Note straight boundaries tated in place. Cements formed in cherty chert between adjacent spherulites and local develop- sid + chert (carbonate iron formation after the deposition of femicrite) ment of coarser grained mosaic quartz between iron sil + sid (silicate and within oolites. Crossed nicols. granules and ooliths. Matrices and cements femicrite) may be texturally indistinguishable, espe- iron sil + sid ± chert F. Intergrowth of oriented lamellar stilp- (femicrite) nomelane with greenalite and minnesotaite in an cially in beds where granules are not de- calc ± ankt monstrably intraclastic in origin or where ank oolith core. Note that quartz transects earlier sid + ankt formed concentric lamination composed of stilp- Tuffaceous shale facies recrystallization has apparently been exten- nomelane. sive. In general, matrices tend to occur as Shard pseudomorphs stilp G. Pisolith composed dominantly of greena- thin, undisturbed laminae. Isolated cham lite within the lower algal chert facies. Shrinkage calc + qtz + iron sil(?) granules and granulelike bodies are not un- cracks are filled with clear quartz and later gen- Note: Ank = ankerite, calc = calcite, carb = car- common, particularly in matrices domi- bonate (ankerite ± calcite ± siderite), chalc = chal- eration of greenalite within the central core re- nated by cherty quartz. cedony, cham = chamosite, grnl = greenallte, hem = gion (white dots). The rest of the oolith consists hematite, iron sil = Iron silicate (greenallte ± min- An important difference between iron- nesotaite ± stilpnomelane), minn = minnesotaite, qtz = of fine-grained greenalite and hematite. Note ab- qartz, sid - siderite, stilp = stilpnomelane. formation and limestone matrices com- sence of hematite in area enclosed by white dots. * End-member minnesotaite + hematite (Burt, 1973) Sample 732-6A. posed of microcrystalline calcite (micrite) is is thought to represent an incompatible assemblage; however. It is included here because 1t was identified H. Columnar quartz rim cement developed on grain size. Most iron-formation matrices in several dlffractograms. iron-silicate granules composed chiefly of greena- have a grain size between 10 and 30 /u,m, t Due to postdepositional replacement. lite. Coarse-grained quartz forms a pore cement. compared to about 2 /u.m for micrite. This Crossed nicols. Sample 726-4C. could reflect grain coarsening in the iron rock types, including several samples (Figs. I. Carbonate pore filling composed predomi- formation during the long time span en- 3B through 3E) consisting of finely lami- nantly of coarse calcite. Crossed nicols. Sample compassing lithification and burial. Alter- nated cherty-slaty sequences. Textures 726-5. natively, it may be due to high chemical characteristic of the chert-carbonate facies > reactivity of certain components (G. B. (Figs. 3F through 31) are variably recrystal- line quartz (cherty quartz), chalcedony, and Morey, 1974, written commun.). A combi- lized (Fig. 3G) or exhibit replacement (Fig. coarse-grained quartz. Fine- and coarse- nation of both factors seems likely. 31) as do some carbonate occurrences in grained quartz have probably undergone The mineralogy of textural elements in cherty rocks (Figs. 4A through 4E) that partial to complete recrystallization. cherty and slaty iron formation is presented contain relict structures. Figures 4F and 4G Most granules and ooliths consist of in Table 1. Terminology used by Dimroth emphasize the diversity of the carbonate- cherty quartz in combination with other and Chauvel (1973) based on the limestone silicate association found in dark slaty minerals (silicates, iron oxides, carbonates); classification of Folk (1959, 1962) is shown rocks. A rare occurrence of siderite ooliths granules composed entirely of chert are in parentheses. Some of the typical and associated with tuffaceous material is common, however. Coarse-grained quartz more unusual textural relations are shown shown in Figure 4H. (Figs. 2F through 2G) or chalcedony (Fig. in Figures 2 through 4. In cherty rocks, 2A) are common fillings of internal shrink- granule and oolith textures of rare occur- Silicate Minerals age cracks. Granules and ooliths are gener- rence (Figs. 2A through 2G) are, neverthe- ally surrounded by a radial fringe of chal- less, important to interpretations of iron The most abundant mineral in the iron cedony or quartz that has a columnar im- silicate paragenesis. Mineralogically differ- formation is quartz, which is frequently the pingement texture (Figs. 2A, 2E, 2H, 21; see ent interstitial cements and related textures dominant constituent of granules, cements, Spry, 1969, p. 169). The latter is more (Figs. 2H, 21, 3A through 3C) illustrate the and matrices. Morphological varieties that prevalent and probably derived from chal- varied postdepositional history of many may be distinguished include microcrystal- cedony by recrystallization. Rim cements

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/9/1169/3418213/i0016-7606-86-9-1169.pdf by guest on 28 September 2021 Chq/c

0-2mm , '

'Grnf + Minn!

0'4mm

0 2mm

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/9/1169/3418213/i0016-7606-86-9-1169.pdf by guest on 28 September 2021 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/9/1169/3418213/i0016-7606-86-9-1169.pdf by guest on 28 September 2021 PETROLOGY OF THE LOW-GRADE ROCKS OF THE GUNFLINT IRON-FORMATION, ONTARIO-MINNESOTA 1175

Figure 3. A. Iron-silicate granule containing tocrystalline (<10 /am) and typically occurs to colorless felty masses or as minute acicu- two kinds of greenalite: (a) cryptocrystalline within granules. Although not an abundant lar crystals, 10 to 25 /xm long. Either type greenalite and (b) coarser grained acicular constituent, greenalite is sometimes present may be disseminated throughout granules greenalite thought to be a shrinkage-crack filling. in silicate-rich banded laminae of both the and often transects finer grained cherty B. (Top) Greenalite granules partially re- Biwabik Iron-Formation (Van Hise and quartz. Only one sample (which was locally placed by coarse sparry calcite and bladed sider- Leith, 1911; Gruner, 1946) and Gunflint ite rim cement. Coarse-grained calcite also occurs contact metamorphosed) contains a rela- as a late-stage vein mineral cutting granules Iron-Formation. tively coarse variety, 100 /um in length. (upper left) and as a pore filling (upper center). Most granules consist of aggregates of Compositionally, this acicular variety rep- (Bottom) Coarse-grained rhombohedral siderite green or brown greenalite, minor min- resents an intermediate member of the clusters associated with interstitial iron silicate. nesotaite, and cherty quartz. Less com- minnesotaite-talc series. The acicular cryst- Sample 724-5B. monly, greenalite forms apparently als are commonly grouped together as fas- C. (Top) Poorly sorted stilpnomelane monomineralic granules or occurs with cicular bundles or "bow ties" (Spry, 1969, granules partially replaced by siderite rhombs. stilpnomelane (Fig. 2F). Textural relations p. 153) scattered haphazardly within Note finer grained rounded siderite in upper right within greenalite-rich granules from the granules of cryptocrystalline greenalite part of photomicrograph. (Bottom) Iron- Gunflint Iron-Formation have been de- (Figs. 2B, 2C). Their distribution varies silicate-bearing siderite matrix. Sample 724-3A. D. (Top) Greenalite-chert granules cemented scribed in detail elsewhere (Gill, 1927); greatly within and between granules, and by quartz. Some granules consist of a single cal- other occurrences that pertain directly to their texture and similar grain size suggest cite grain. (Bottom) Chert matrix with dispersed the origin of greenalite are discussed here. that growth greatly exceeded nucleation. iron-silicate or carbonaceous dust. Sample One of these textures is characterized by a This occurrence of minnesotaite talc is un- 723-2B6. greenish-yellow variety of greenalite with a doubtedly of metamorphic origin. E. Crossed nicols. Note textural similarity nematoblastic habit, 100 to 150 ¡um in Stilpnomelane is not as abundant as with Figure 3C. length (Fig. 3A). It is found within dark greenalite nor as widespread as min- F. Siderite matrix, spheroidal siderite (~ 30 reddish-brown granules composed of cryp- nesotaite. Locally, it occurs profusely in (im in diameter) often with dark, finer grained tocrystalline greenalite and appears to rep- cores; concentric or radial structure is associated both cherty and slaty rocks where it as- resent shrinkage crack fillings formed after with rare interstitial chert. Rhombic overgrowths sumes numerous textural habits. In most on some of siderite grains are visible. Sample the granules were deposited. Similar tex- samples, stilpnomelane is cryptocrystalline 727-4. tures are present in greenalite-rich ooliths and pale brown or reddish brown in color, 3+ 2+ G. Recrystallized siderite matrix. Rhom- (Fig. 2G). Oolitic rocks with iron- indicating that Fe dominates over Fe ; bohedral siderite (50 to 100 fim in maximum silicate-bearing nuclei and cement (com- however, shardlike forms have been ob- diameter) dominates over spheroidal forms. In- posed primarily or entirely of greenalite) served that consist of greenish-brown cryp- terstitial cryptocrystalline stilpnomelane is abun- preserve successive stages of diagenetic tocrystalline intergrowths. The variation in dant. Sample 724-3A. modification (Figs. 2D through 2F). color suggests a range of Fe2+/(Fe2+ + Fe3+) H. Spheroidal siderite within single poikilo- Greenalite rim cement, sometimes visible as ratios (Hutton, 1938) on a microscopic blast of calcite. Note small patch of cherty two distinct concentric laminae (Fig. 2D), scale. quartz. Crossed nicols. Sample 723—4—6D. forms a continuous reddish-brown I. High-magnification photomicrograph Individual laths or radiating sheaves of framework deposited on an earlier layer of showing rhombohedral siderite within calcite stilpnomelane occur in subordinate poikiloblast. Many siderite cores have been re- chalcedonic rim cement. Dense greenalite amounts within cherty granules. Occasion- placed by calcite that is optically continuous and aggregates that form globular clusters not ally, massive stilpnomelane granules com- indistinguishable from calcite poikiloblast. Sam- unlike those found within greenalite prise entire laminae (Fig. 2A). Many of the ple 723-8. granules occur at the boundary between the larger granules have chalcedony-filled rim and pore cement. Apparently these shrinkage cracks, suggesting that stilp- < — served as nucleation centers for large nomelane formed early in the history of are usually distinguishable from later spherulites of later chalcedonic pore ce- these rocks. The evidence can also be inter- pore-filling cements that consist of coarse- ment. preted as indicating replacement of an ear- grained mosaic quartz or spherulitic chal- Scattered globules and irregular stringers lier mineral phase after shrinkage (E. Dim- cedony (Fig. 2E). Rim and pore cements of of greenalite also occur in beds having large roth, 1974, written commun.). We consider the same morphological type tend to be as- amounts of cherty quartz. Granule outlines this unlikely because of the lack of textural sociated. are not apparent but could have been oblit- evidence that replacement has taken place In banded iron formation, quartz occurs erated if granules were deposited in a and internal chemical homogeneity of the primarily as matrix. Silicate- or carbonate- semiconsolidated state and subsequently granules (see below). bearing chert matrix is common, and ma- deformed during compaction. Coarse- Stilpnomelane occurs rarely within the trices dominated by different minerals grained quartz resembling typical pore ce- rims and cores of ooliths where it forms occur often as interlayered laminae. With ment is occasionally identifiable. In general, fine-grained intergrowths with other phases an increase in the proportion of silicate and when granule density is very high, little or (Fig. 2F). Many of the concentric lamina- carbonate minerals (>50 percent), chert no cement is visible. tions appear to consist entirely of stilp- matrix grades into silicate and carbonate Minnesotaite is a minor but persistent nomelane. These are locally discontinuous matrices. constituent of cherty iron formation and where shrinkage cracks are present, which Irregular wisps of organic matter that ex- occurs in granules primarily with greena- strongly suggests a primary origin for this tend for short distances parallel to the lite. It is somewhat more abundant in slaty occurrence of stilpnomelane. banding are often present in chert matrix. rocks where it is often associated with Massive brown stilpnomelane granules Granules are relatively uncommon but may stilpnomelane. Optical identification is may also occur in association with siderite occur locally, scattered throughout chert difficult because of the fine grain size of (Fig. 3C). It is not clear whether siderite matrix. Rarely, coarse angular quartz minnesotaite; its presence in most cherty forms a cement or matrix. Granule margins grains of probable clastic origin may be dis- and all slaty samples was confirmed by have been partially replaced, as indicated seminated within silicate-rich laminae. x-ray diffraction only. by the preservation of original granule out- Greenalite is the most abundant and Within granules, optically identifiable lines within siderite (Fig. 3C). widespread iron silicate in the Gunflint minnesotaite occurs as pale brownish-green The textural habit of stilpnomelane in Iron-Formation. It is almost always cryp-

Figure 4. A through E. Carbonate replace- Chamosite is present principally in the poikiloblasts (Figs. 3H, 31). In some of the ment and recrystallization textures in cherty iron tuffaceous shale facies and is especially laminae, the cores have been replaced by formation. abundant in the vicinity of Kakabeka Falls calcite. A. Calcite porphyroblasts associated with (Fig. 1). Pale-green chamosite typically Recrystallization of siderite in a silicate- cherty granules. Relict granule outlines are visi- forms irregular wisps that are interpreted as bearing carbonate matrix (Fig. 3G) results ble within calcite and are partially defined by shard pseudomorphs. These often occur in a mixture of fine-grained silicate and fine-grained hematite. Dark (oxidized) siderite clusters of rhombohedral or anhedral siderite rhombs cut across granules boundaries. Crossed within a silicate-bearing carbonate matrix nicols. Sample 727-2F. (Fig. 4G). Chamosite also occurs as a ce- (100 /urn) with a granoblastic texture. In B. Carbonate-rich zone between slaty layer ment in volcanic breccias from Kakabeka many dark slaty beds, spheroidal siderite and chert pod. Cherty granules have been re- Falls. Cross fibers of chamosite perpendicu- occurs with stilpnomelane (Fig. 4F) or placed by coarse, zoned rhombohedra of siderite lar to a central zone of discontinuity resem- chamosite (Fig. 4G). A rare occurrence of and ankerite. Relict granule outlines have been ble the axiolitic structure found in siderite in slaty iron formation is shown in partially outlined in black. Note rounded areas in devitrified material (Ross and Smith, 1960, Figure 4H. These structures are of uncer- center right of photomicrograph (arrows). These p. 37). tain origin but appear to be ooliths. are believed to represent spherulitic chalcedony Calcite occurs as a coarse-grained cement replaced during in situ growth. Crossed nicols. Sample 724-6A. (Inset) Same sample (thin sec- Carbonate Minerals and as a replacement mineral. Within tion). Note (1) stilpnomelane-siderite slaty layer, cherty rocks it forms a sparry pore cement (2) siderite-ankerite "reaction" zone, and (3) Most samples from cherty and slaty iron (Figs. 21, 3B) or fills shrinkage cracks within chert pod consisting of cherty granules in chal- formation contain two or three coexisting greenalite granules. It is also a principal cedonic rim and pore cements. carbonate minerals (siderite, ankerite, cal- carbonate phase of mottled taconite. Mot- C. Coarse anhedral calcite granules cemented cite). Both replacement and recrystalliza- tles of replacement origin occur in cherty by anhedral to subhedral ankerite. Area shown tion have influenced the present morphol- iron formation where calcite porphyro- grades into zone consisting of greenalite granules ogy and distribution of carbonate in the blasts have replaced ooliths and cement and quartz cement. Sample 723-2B2. iron formation. Replacement is recognized (Figs. 4A, 4D). It should be emphasized that D. Coarse sparry calcite apparently replacing by the retention of relict structures within many of the carbonate replacement textures cherty ooliths and chalcedonic cement. Calcite are of local origin and due, in part, to re- consists of large, optically continuous grains. In individual rhombohedra or anhedra. In some ooliths replacement has been confined to cherty rocks, carbonate replacement fre- crystallization. Although the textures indi- the concentric laminations that tend to be ex- quently results in the transection of cate replacement, little or no evidence exists tremely fine grained. Crossed nicols. Sample granules and quartz cement. Recrystalliza- in many rocks to suggest that carbonate 723-2C2. tion is assumed (although not proved) when was introduced metasomatically. In some E. Relict granule outlines in a matrix of coarse such criteria are lacking and when carbon- areas, however, such as Kakabeka Falls, ankerite. Dark material is of unknown mineral- ate occurs as coarse single crystals. carbonate appears to have been introduced ogy. White areas at lower left are holes in the sec- into the system after deposition. tion possibly caused by volume shrinkage ac- Siderite occurs in cherty iron formation companying ankeritization. Sample 723-2B3. principally as coarse rhombohedra that are Calcite is closely associated with pyro- F. Alternating siderite and stilpnomelane-rich sometimes zoned. Twinning is not uncom- clastic rocks of the tuffaceous shale facies. laminae in slaty iron formation. Coarse siderite mon, and most siderite is brownish in color In the argillite-tuff unit at Kakabeka Falls, rhombs float in very fine grained chert matrix. due to subsequent oxidation. Disseminated calcite poikiloblasts, some >2 cm in length, Abundant inclusions and dark streaks pass rhombs occur throughout quartz cement comprise entire laminae 1 to 2 mm thick through siderite undisturbed, suggesting that the but may replace granules and cement alike. (Fig. 3H). Adjacent laminae consist of siderite is paragenetically late. Sample 724 -4A. Fine-grained anhedral siderite is commonly either single anhedral calcite grains having G. Slaty sample composed primarily of zoned associated with other carbonates in small slightly different optic orientation or chert- siderite rhombohedra with darkened cores in a mottles or concretions. rich laminae in which coarse calcite exhibits matrix material of unknown composition. Wispy a patchy distribution. In other areas, calcite white areas consist of chamosite thought to be Bladed siderite rim cement is rarely pres- pseudomorphs after shards. Groundmass con- ent on the surfaces of greenalite granules. occurs as possible shard pseudomorphs and tains abundance of felty prismatic crystals re- Relict granule outlines within siderite sug- volcanic-breccia cements. In addition, sembling microlites. Sample 727-6C. gest that partial replacement of the granules fine-grained calcite is the major constituent H. Rare siderite ooliths with concentric and has taken place (Fig. 3B). Between and of the upper limestone member that extends radial fibrous structure partially replaced by within these granules are rounded siderite the length of the formation and has been coarse-grained calcite. Sample 727-6C. grains with radial and concentric structure. shown by Goodwin (1956, Fig. 13) to con- These occur profusely with rhombic sider- tain volcanic shards. < : : : ite, and together, they form a major portion In both cherty and slaty iron formation, slaty rocks is not easily discernible because of the cement. Clusters of rounded siderite ankerite is present typically as coarse, of the semi-opaqueness of many slaty locally form entire granules and fill shrink- zoned rhombs generally containing abun- layers. In places, it occurs with min- age cracks. dant inclusions. Most ankerite is coarsely nesotaite and a 7 A mineral (chamosite or Siderite from slaty iron formation is typi- recrystallized, although one slaty • sample greenalite) as silicate matrix. Elongated cally present as small spheres (10 to 40 ju,m) was found to contain thin monomineralic shardlike forms associated with siderite that frequently display rhombic over- laminae of cryptocrystalline ankerite. The matrix are common and generally consist of growths (Figs. 3F through 31). Commonly, latter occurrence is probably primary, al- cryptocrystalline aggregates oriented per- the spheroids have cryptocrystalline brow- though a variable grain size suggests some pendicular to grain boundaries. This tex- nish cores and may exhibit radial fibrous recrystallization has taken place. tural feature may be related to the axiolitic extinction due to crystallographic orienta- Within cherty rocks, ankerite occurs as structure reported to occur in stilp- tion. Many of the cores appear to be isolated rhombohedra or forms mottles nomelane of volcanic derivation (LaBerge, organic-rich but microprobe analyses of a with other carbonates. Some ankerite-rich 1966a, 1966b). Similar "shards" that ap- few of them indicate the presence of Si, Fe, zones selectively replace quartz cement, pear to have been flattened and compacted and Al. At Kakabeka Falls, thin laminae of which results in the partial to complete ob- have been observed in association with the chert-carbonate facies consist of literation of older textures (Fig. 4B). Above rounded granules of stilpnomelane floating spheroidal siderite with rhombic over- the argillite-tuff unit at Kakabeka Falls, in a turbid cryptocrystalline matrix of growths disseminated throughout calcite ankerite is an important constituent of cherty quartz and iron silicate (Fig. 4F).

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/9/1169/3418213/i0016-7606-86-9-1169.pdf by guest on 28 September 2021 1178 FLORAN AND PAPIKE

some thin-bedded carbonate laminae. In be made, however, for minerals that consis- abundant iron silicate in the Gunflint Iron- one particular sample (Fig. 4E), alternating tently form certain textural elements. Formation, (2) few accurate chemical gray, green, and yellow bands, distinguish- The earliest recognizable textural ele- analyses of this mineral exist, and (3) able by their relative abundances of anker- ments in cherty rocks are those that formed numerous analyses from different samples ite, calcite, and cherty quartz, appear to prior to interstitial cements. These include would establish the extent of homogeneity have been affected by ankeritization. primarily granules and locally abundant of greenalite granules. In addition, Morey Within the thicker beds, relict granules sug- ooliths and pisoliths. Minerals recognized and others (1972) concluded that variations gest that these laminae have, for the most as primary or texturally earliest in granules in mineral compositions and assemblages part, been converted to ankerite after dep- and ooliths include cherty quartz, crypto- are largely retained during metamorphism osition. Ankerite also occurs as vein fillings crystalline hematite, greenalite, stilp- and can be related to textural occurrence. associated with soft sediment deformation. nomelane, and possible calcite, magnetite, Granule-bearing rocks occur frequently and minnesotaite. Recrystallization has within the stratigraphie column of the Oxide and Sulfide Mineral's considerably coarsened some of these min- Gunflint Iron-Formation and their textures erals in many areas. Cements composed of are especially well preserved within the con- Hematite is widely distributed through- quartz, iron silicate, siderite, calcite, and tact aureole. The association of greenalite out cherty rocks and is especially concen- possibly ankerite are also recognized as and granules could thus prove useful in de- trated in jasper beds and stratigraphically primary. Most carbonate is, however, ex- termining whether metamorphism of these related algal chert units. In jaspery iron tensively recrystallized and some occur- particular units was, indeed, isochemical. formation, a gradation exists between rences are of replacement origin. Chal- Comparatively little new information re- cherty granules with dispersed hematite cedonic, sideritic, and iron silicate rim ce- garding the crystal chemistry of greenalite dust and massive hematite granules. Lo- ments formed after deposition of granules and minnesotaite has been published since cally, cryptocrystalline hematite is concen- and ooliths. Recrystallization of chalcedony the work of Gruner (1936, 1944a, 1946). A trated near the rims of cherty or iron silicate may have resulted in columnar quartz, al- recent interpretation of the stilpnomelane granules. Hematite, cherty quartz, and though the latter could be a primary tex- structure (Eggleton, 1972) is a modified silicate-rich granules commonly are found ture. In the later stages of lithification, version of the structure proposed by Gruner together and generally display identical in- quartz and minor carbonate cements filled (1937, 1944b). Except for stilpnomelane, ternal textures. Coarse recrystallized rhom- available pore spaces. only powder x-ray diffraction techniques bohedral hematite as much as 300 /am in In slaty rocks, common matrix minerals have been successfully employed in struc- maximum diameter is occasionally present — cherty quartz, siderite, and iron silicate tural determinations of these extremely fine in granules. In ooliths, fine-grained hema- (stilpnomelane, minnesotaite?) — are con- grained minerals. Chamosite has been the tite occurs as concentric laminations and sidered primary as are such less abundant subject of several powder diffraction globular clusters associated with iron sili- minerals as greenalite and rare cryptocrys- studies (Brindley, 1951; Brindley and cate. The latter occur at the boundary be- talline ankerite. In general, the iron silicates Youell, 1953; Youell, 1955). tween rim and pore cements (Fig. 2D). escaped recrystallization while carbonates In the following sections, the iron silicate Abundant magnetite was found only in did not. Recrystallization produced rhom- structures are reviewed, and similarities one locality — at the base of the iron for- bohedral siderite and rhombic overgrowths among them and related silicates are ex- mation near the lower algal chert facies. on spheroidal siderite. In some siderite-rich plored. Concepts and theories relevant to Recrystallization is conspicuous in samples samples, several stages of postdepositional the origin of minerals other than the iron from this area — many granules and ooliths growth or recrystallization are apparent. In silicates have been summarized by Ayres consist of coarse recrystallized quartz. In the tuffaceous shale facies, devitrification of (1972). Unlike the iron silicates, carbonate ooliths, quartz grains transect delicate con- what appear to be shards produced stilp- structures are all similar and well known. centric laminations that have been pre- nomelane and chamosite as primary The formation of carbonates appears to be served by a dust of unknown composition. minerals. strictly a function of bulk composition and In other samples, magnetite is sparsely dis- Several apparently transitional rock types the activity of C02. Crystal chemical differ- tributed within greenalite-rich granules. have characteristics of both cherty and slaty ences, such as cation ordering in ankerite, Pyrite is a minor constituent of the tuff- textures. They consist of granules floating do not appear to have significantly con- aceous shale facies forming pyritiferous, in carbonate (siderite) matrix and may be trolled which carbonate phase crystallized. often nodular, shaly units. Disseminated analogous to Folk's (1959) intramicrite. euhedral pyrite occurs within calcite-rich These could have formed if granules pro- Silicate Minerals laminae and very rarely in granule-bearing duced in a high-energy nearshore envi- units of cherty rocks. It is not an abundant ronment were transported to deep and Greenalite and Minnesotaite: Chemistry. phase of the iron formation. calmer areas (for example, during storms) The intimate association of greenalite with where siderite mud was being deposited. minnesotaite and quartz makes an accurate Paragenesis The distinction between cement and matrix chemical determination of greenalite then becomes very difficult to recognize, difficult. Previous attempts have led to er- The recognition of distinct textural ele- especially if recrystallization has been con- roneous conclusions regarding its true ments permits a paragenetic mineral se- spicuous. composition (Leith, 1903; Jolliffe, 1935). quence to be developed for most of the In the present study, all x-ray diffracto- samples examined. Relict textures are par- MINERALOGY grams from relatively unmetamorphosed ticularly valuable in elucidating the history greenalite-rich samples that were analyzed of a particular rock, although a universal A detailed microprobe investigation of by microprobe (except one) reveal the pres- sequence of mineral development is un- iron silicate and carbonate minerals was ence of small amounts of minnesotaite. likely because local bulk compositional dif- undertaken to determine the extent of To a first approximation, the composi- ferences within different rock types proba- chemical variation within these phases. The tion of greenalite lies entirely within the sys- bly led to mineral growth at various times chemistry of greenalite was a primary ob- tem Fe0-Fe203-MgÔ-Si02-H20; all other and at different rates. Generalizations can jective because (1) greenalite is the most components are of minor importance. The

amount of Fe203 varies considerably from sample to sample (Van Hise and Leith, 1911; Gruner, 1946; Cochrane and Ed- wards, 1960), but probably none or very little of this component is primary. Many workers have assumed that the presence of Fe3+ is a result of weathering, although Steadman and Youell (1958) reported that greenalite oxidizes upon slight heating. Any Fe3+ that may be present is considered by us to be oxidized Fe2+ and hence of secondary G*Greenalite origin. This approach simplifies interpretation of microprobe data where all iron is M=Minnesotai*e reported as FeO. If the above assumption is T=Talc valid, then the iron in 'FeO' reported in the probe analyses represents the amount of Fe2+ present prior to oxidation. In other words, no ferric iron correction of an Figure 5. Compositional variation of greenalite in weight'percent from the Gunflint Iron- analysis is necessary (see Appendix 1). We Formation plotted within the system FeO (=FeO + Fe203)-Mg0-Si02. Length of error bars = la. 1 believe that the oxidation-dehydration Note that samples are from widely separated parts of the iron formation (Appendix 2). reaction proposed by Brindley and Youell tion data, suggests that the microprobe placement of Fe and Si by small quantities (1953) for the conversion of ferrous analyses may represent a submicroscopic of Mg and Al. In minnesotaite, substantial 2+ chamosite to ferric chamosite (Fe + Oh" mixture of greenalite and minnesotaite. substitution of Fe by Mg has been reported 3+ 2- —» Fe + O + Vi H2) occurs in greena- Further support for this conclusion is given (Trendall and Blockley, 1970; Ayres, lite, minnesotaite, and stilpnomelane. below. 1972). Complete solid solution between Variation in the composition of greena- Published analyses and new data pre- minnesotaite and talc appears possible al- lite from the Gunflint Iron-Formation is sented here indicate that both greenalite 1 shown graphically in Figure 5 and pre- and minnesotaite have a rather simple Copies of GSA supplementary material 75-25 may be ordered from Documents Secretary, Geological Soci- sented in Table 2. Most analyses are from chemistry (Tables 2 and 3). Substitutions in ety of America, 3300 Penrose Place, Boulder, Colorado granules. Only two analyses are from greenalite are minor and usually involve re- 80301. ooliths (samples 726-2A and 732-9A) and, although the data are limited, these appear to be atypical. It is evident that chemical TABLE 2. CHEMICAL ANALYSES AND STRUCTURAL FORMULAE OF GREENALITE variation of greenalite within granules is quite restricted both within and between 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 samples. Two sigma (2a) error bars based

on microprobe counting statistics represent S102 29.31 32.02 40.13 35.91 37.61 33.58 32.27 35.4 35.9 35.6 35.8 35.6 35.4 35.5 36.4 0.00 N. D.* N. D. N. D. N. D. N. D. N. D. 0.02 N. D. the precision of individual analyses. If sam- T102 0.00 0.00 0.00 0.00 0.00 0.05 ple 727—3 is ignored, the variation in 'FeO' A1203 0.81 1.02 0.68 1.45 0.00 0.00 3.03 0.81 0.06 0.18 0.26 0.00 0.00 0.34 1.92 Fe2Û3 32.69 22.95 7.80 6.66 10.26 11.16 14.83 ' is entirely within the error. Variation in Si02 W 50.0^ 49.7+ 48.3+ 49.5+ 49.61 49.1+ 47.0f 1 is similarly restricted, but MgO fluctuates FeO 22.41 29.15 37.48 40.40 37.84 45.19 36.00 • N. D. N. D. 0.22 0.00 0.04 0.06 0.04 0.04 0.02 0.16 0.28 0.04 from sample to sample and cannot be ac- MnO N. D. N. D. 0.87 MgO 5.08 5.34 6.48 7.00 6.00 0.00 2.11 3.99 3.83 3.62 4.37 3.47 2.97 2.10 1.96 counted for by counting statistics alone. CaO 0.61 0.00 0.00 0.05 0.23 0.00 Tr. 0.08 0.00 0.18 0.05 0.00 0.06 0.11 0.47 These differences could reflect minor Na?0 0.00 0.00 0.00 0.00 0.00 0.00 0.14 N. D. N. D. N. D. N. 0. 0.00 N. 0. 0.00 N. D. 0.00 0.00 heterogeneities in the composition of the in- K20 0.00 0.00 0.00 0.00 0.10 0.01 0.00 0.01 0.01 0.00 0.00 0.04 0.53 + 9.09 9.52 . 7.43 8.53 7.84 10.07 8.71 N. D. N. 0. N. D. N. D. N. D. N. D. N. 0. N. D. itial precipitate. Similarly, some of the vari- HZ0 H20- - - - - — 0.00 2.37 N. D. N. D. N. D. N. D. N. D. N. D. N. D. N. D. ations in 'FeO' and Si02 are probably real, Total 100.00 100.00 100.00 100.00 100.00 100.00 100.48 88.9 89.8 89.3 88.8 88.7 88.2 87.5 88.3 even though they are within the counting statistics. Greenalite from sample 727—3 Numbers of ions on the basis of 14 oxygens may not be representative because it has St 3.33 3.61 4.28 4.21 4.13 4.00 3.75 4.17 4.21 4.21 4.21 4.23 4.24 4.28 4.30 Al 0.11 0.13 0.09 0.20 0.00 0.00 0.62 0.11 0.01 0.03 0.03 0.00 0.00 0.05 0.27 been contact metamorphosed. The presence 3+ Fe 2.79 1.95 0.63 0.59 0.85 1.00 1.30 1 of secondary iron talc replacing greenalite V 4.79 4.90 4.91 4.75 4.92 4.97 4.95 4.64 Fe2t 2.13 2.75 3.34 3.96 3.47 4.50 3.49 J (Figs. 2B, 2C; Table 3) and the absence of Mn - - - - 0.02 0.00 0.09 0.00 0.01 0.00 0.00 0.00 0.02 0.03 0.00 iron oxides suggest that magnesium was in- Mg 0.86 0.90 1.03 1.22 0.98 0.00 0.36 0.70 0.67 0.64 0.77 0.61 0.53 0.38 0.34 troduced into the system during meta- EOctS 5.94 5.78 5.13 6.07 5.32 5.50 5.65 5.59 5.59 5.56 5.53 5.52 5.43 5.38 morphism; coexisting greenalite has the "4 -0.16 -0.06 0.25 0.04 0.16 0.00 - 0.11 0.13 0.13 0.13 0.15 0.15 0.18 0.20 highest Mg/Mg+Fe ratio of any sample

(Fig. 5). Note: Data 1n columns as follows: 1 through 4, Recalculation of Leith's analyses {Van H1se and Leith, 1911, p. 167); Hz0" has been neglected; Blwabik Iron-Formation, Minnesota. 5, Recalculation of JolUffe's analysis (1935); H20- has been neglected; C0Z was combined with a corresponding amount of FeO and deducted; Blwabik Iron-Formation, Minnesota (see Gruner, 1936). The inset of Figure 5 illustrates the rela- 6, Gruner (1936); composition of a theoretical Iron serpentine with Fe2+/Fe2+ + Fe3+ s 1.22. 7, Cochrane and Edwards (I960); Roper Bar Iron Formation, Australia. 8, This study, sample 726-5; granule (Lower West Taconite fades). 9, This study, sample tion between greenalite microprobe 724-1A; granule (Upper Taconite fades). 10, This study, sample 726-3CL; granule (Lower West Taconite fades). 11, This study, analyses and the end-member compositions sample 7Z7-3; granule (Upper Taconite fades). 12, This study, sample 724-1A; late aclcular crystals within granule (Upper Taconite fades). 13, This study, sample 725-6; granule (Lower West Taconite fades). 14, This study, sample 732-6A; oollth of greenalite, minnesotaite, and talc. Al- (Lower Algal Chert fades). 15, This study, sample 726-2A; oollth (Upper Algal Chert fades); X may be a rough estimate of a stilpnomelane component rather than a 9A (minnesotaite) component-all three Iron silicates are present in this sample (F1q. though the analyses lie close to the ideal 2D through 2F). greenalite composition, they plot between * N.D. = no data. t All Fe reported as FeO. the two end-member iron silicates. This ob- S lOct • EFe + Mn + Mg + 3/2 Al. servation, coupled with the x-ray diffrac- # xm - proportion of 9A-type layers (minnesotaite), calculated from (Si/ZOct).

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/9/1169/3418213/i0016-7606-86-9-1169.pdf by guest on 28 September 2021 1180 FLORAN AND PAPIKE

TABLE 3. CHEMICAL ANALYSES AND STRUCTURAL FORMULAE OF MINNESOTAITE AND IRON TALC (7A-type): 6FeO • (4 + 4xJ Si02 • (4 - 1 2 3 4 5 6 7 8 2xm) H20 (xm = proportion of minnesotaite layers); minnesotaite (9A-type): 60.14 54.4 S102 51.29 54.6 51.26 56.73 67.38 55.69 N.D. 3FeO • (4 - 2xg) Si02 • (1 +xg) H20 (*„ = T102 0.04 N.D. 0.00 N.D. 0.02 0.04 N.D. 1.52 0.00 0.04 0.13 N.D. 0.00 AI2O3 0.61 N.D. proportion of greenalite layers). The calcu- 0.52 2.39 10.90 ) Fe203 2.00 0.5 20.74* 16.78* 24.0* FeO 33.66 35.5 37.23 20.06 16.52 1 lation of x is as follows: xm = 1.5 MnO 0.12 N.D. '0.77 N.D. <0.01 N.D. 0.14 0.01 3.4 3.38 16.81 2.16 16.80 19.41 15.4 [Si/XOct]-l and xg = 2 - 1.5 [Si/XOct] MgO 6.26 VI CaO 0.00 N.D. 0.00 0.03 <0.01 0.05 N.D. 0.01 where SOct = Fe + Mn + Mg + 3/2 A1 ; N.D. Na20 0.08 N.D. 0.00 N.D. <0.01 0.02 N.D. 3+ N.D. 0.00 Fe , if present, is assumed to be oxidized K20 0.03 N.D. 0.00 N.D. 0.31 0.03 + 5.29 3.83 2.39 1 2+ H2O 5.54 N.D. 7.32+ 5.53+ N.D. Fe . H2O- 0.24 N.D. 0.03 0.06 0.11 1 - Total 507 905 100.0Í 59791" 100.30" 100.82 102.0 SO Values for x m and x g are listed in Tables 2 Numbers of Ions on the basis of 11 oxygens and 3. Negative values probably indicate SI 3.93 tt 3.95 3.95 tt 3.98 4.05 3.94 inaccurate analyses. These inaccuracies are Al 0.06 0.14 0.00 0.01 0.00 (£Tetr)SS ••(3.99) ••(4.00) 3.95 ••(3.99) 4.05 3.94 magnified in greenalite if any of the Al (as- s+ Fe 0.11 0.03 0.13 1 1.24 0.94 1.45 sumed to be in octahedral coordination) is Fe2+ 2.16 2.40 1.17 1 Mn 0.01 0.05 0.01 0.00 located in the tetrahedral sites. Because of Mg 0.72 " 0.39 1.74 1.79 1.95 1.66 E0ct(E0ct)## 3.09(3.00) 3.08(3.00) 3.04 3.04(3.03) 2.90 3.11 this uncertainty, xm has not been calculated 0.09(0.00) 0.08(0.00) 0.05 0.04(0.02) -0.09 0.10 %*** for analysis 7 of Table 2. The small but significant proportion of Note: Data 1n columns as follows: 1, Gruner (I944b)-B1wabik Iron-Formation, Minnesota; 2, Gruner (1946)-B1wab1k Iron-Formation, Minnesota, partial analysis; 3, Blake (1965)-Cuyuna iron formation, Minnesota; 4, Trendall and xm in greenalite analyses from the Gunflint Blockley (1970)-Brockman Iron Formation, Australia; 5, Trendall and Blockley (1970)-Marra Mamba Iron Formation, Australia; 6, Ayres (1972)-Brockman Iron Formation, Australia; 7, Ayres (1972)-firockman Iron Formation, Australia; Iron-Formation is striking. On the other 8, this study, sample 727-3-granule, contact-metamorphosed sample (Upper Taconite fades). N.D. = no data. hand, analyses 3 through 5 (Table 2) from * All Fe reported as FeO. the Biwabik Iron-Formation appear to rep- t Obtained by difference 1n computer program, i Includes -2% excess SIO2 occurring as quartz. resent variable proportions of greenalite # Adjusted; uncorrected analysis contained 0.08 T102, 0.10 CaO, 0.06 Na20, and 0.44 K20 considered as Impurities by the author. and minnesotaite (xm = 0.26, 0.06, 0.16, ** Includes excess S10a occurring as quartz, 0.36 P205 + 0.04 FeS2 + 0.07 C02. respectively). Interpretation of the ft Not determined because of quartz contamination. II (ETetr) = Si + AlIV. minnesotaite-talc analyses in terms of a #< EOct » EFe + Mn + Mg + 3/2 Al.(IOct) = EFe + Mn + Mg + 3/2 Vlfll. mixed-layer arrangement depends on how *** xq = proportion of 7A-type layers {greenalite) calculated from [S1/E0ct]. the cations are allocated. If only Si occupies the tetrahedral sites, an apparent deficiency though experimental studies by Forbes of 3/4. Similar relations were observed by occurs (Table 3). Alternatively, no tet- (1969, 1971) suggested that iron substitu- Brindley and Hang (1973) in the structur- rahedral deficiency may actually exist be- tion in talc is limited and controlled by oxy- ally related Mg-Ni silicate garnierite. They cause of the presence of Al in tetrahedral gen fugacity and temperature. Apparently, distinguished two structural types within coordination. In this view, xg would be very very little replacement of Si by Al or Fe3+ the garnierite group — a 7A variety re- small and perhaps not real (Table 3, values occurs in the tetrahedral sites. Trace sembling the serpentine group minerals, of xg in parentheses). Note that if the STetr amounts of Ca, Na, and K reported in and a 10A variety possessing a talclike = Si+Al (with excess Al in octahedral analyses of talc and minnesotaite are either structure. The iron analogues of these are coordination), the O/T ratios are nearly 3/4; 3+ present in lattice defects or possibly as inter- greenalite and minnesotaite, respectively. addition of a small tetrahedral Fe compo- layer ions. The latter situation could repres- The obvious similarities between these nent would result in virtually ideal site oc- ent a transition to the stilpnomelane struc- phases justify closer scrutiny. cupancies with xg ~ 0. ture. Brindley and Hang (1973) attributed de- Table 4 emphasizes the close structural Analysis 5 of Table 3 is of interest be- viations in the O/T ratios to deficiencies of and chemical affinities among greenalite, cause of its high Fe3+ content. This is the octahedral cations in 7k -type gamierites minnesotaite talc, and 7 A and 10 A garnier- first reported occurrence of ferric-rich min- and to deficiencies of tetrahedral cations in ite. An average "greenalite" composition, nesotaite, although its existence was antici- lOA-type garnierites. The same reasoning based on 47 microprobe analyses from the pated by Gruner (1944b). Oxidized min- can be extended to greenalite and min- Gunflint Iron-Formation, consists of 86 nesotaite resembles ferristilpnomelane in its nesotaite. Analyses 1 and 2 of Table 2 (Van percent xg and 14 percent xm. Such a large optical properties (Perrault, 1955) and Hise and Leith, 1911) and analysis 7 of and consistent xm component suggests that chemical composition (Floran and Papike, Table 3 (Ayres, 1972) do not conform to it is a definite quantity whereas the validity in prep.). the predicted pattern (see mineral for- of xg in the minnesotiate-talc analyses is Structural formulae for greenalite and mulae). The excess tetrahedral component questionable. minnesotaite talc have been calculated on in Ayres's analysis could be due to a small One important difference apparently ex- the basis of 14 and 11 oxygen anions, re- admixture of quartz or to an inaccurate ists between the iron silicate structures and spectively. The octahedral positions are as- analysis. The latter explanation may also those of garnierite. Brindley and Hang sumed to be occupied by Al, Fe, Mn, and apply to Leith's analyses. (1973) observed a 10A basal layering in the Mg. When calculated in this manner, all Alternatively, the O/T ratios that lie be- talclike garnierites and not 9.4A, which is greenalite analyses from the Gunflint Iron- tween 6/4 and 3A may reflect a mixed-layer typical of normal talc. In addition, the + Formation and several published analyses phase composed of the two end-member H20 content of the 10A garnierites was have more than 4 Si and less than 6 oc- silicates (Brindley and Hang, 1973). The found to be much higher than theoretically tahedral cations (Table 2). The octa- proportions of each pure phase can be rep- predicted. Brindley and Hang explained hedral/tetrahedral (silicon) or O/T ratio resented by the parameter x as follows: in these discrepancies by proposing a "talc is distinctly less than the ideal ratio (6/4). In "greenalite," (l-xm) is the proportion of monohydrate" component with the com- contrast, several analyses of minnesotaite greenalite layers and xm is the proportion of position H20 (Mg, Ni . . .) Si4O10 (OH)* and iron talc from various iron formations, minnesotaite layers; in "minnesotaite," the The additional H20 molecule replaces the including the Gunflint (Table 3), have O/T opposite is assumed. The composite iron K+ ion in the phlogopite structure to yield a ratios somewhat greater than the ideal ratio silicate formulae are then greenalite basal spacing similar to that of mica (about

TABLE 4. COMPARISON AMONG GREENALITE, MINNESOTAITE, AND GARNIERITE greenalite. The two chlorite structural types between layer components is achieved in seem to be related by a simple polymorphic several ways (Deer and others, 1962): (1) transition that requires appreciable sub- substitution of various cations in either

SiOj 35.7 35.0 55.1 52.3 stitution of Al in both octahedral and tet- layer (that is, larger ions for Si, smaller ions A1203 0.26 0.47 0.00 N.D. rahedral sites (Nelson and Roy, 1958). The for Mg), (2) distortion of one or both of the Fe 0, 0.10 N.D. 2 j 49.1* { 23.5» FeO N.D. N.D. very limited Al substitution in greenalite ideal layer components, and (3) curvature MnO 0.08 N.D. 0.01 N.D. of the composite sheet with the tetrahedral MgO 3.55 2.70 15.8 16.3 precludes such a transition taking place. NiO N.D. 49.3 N.D. 20.8 Comparison between greenalite and the layer on the concave side. It is instructive to CaO 0.06 <0.05 0.01 N.D. Na20 N.D. 0.14 N.D. N.D. serpentine group minerals (antigorite, apply these possibilities to greenalite. The K20 0.01 0.08 0.00 0.08 + lizardite, and chrysotile) is instructive be- oxidation-dehydration reaction discussed H2O N.D. 11.4 N.D. 8.6 H2O- N.D. N.D. N.D. N.D. cause little is known about the detailed above can be considered to be a special case Total 8878 557? 5C4 S57T structure and crystal chemistry of greena- of (1). Tetrahedral substitutions in greena- A C lite. Both have a trioctahedral kaolinite- lite are limited but can involve the replace- SI 4.00 4.22 4.00 4.28 4.00 3.95 4.00 3.95 type structure consisting of a pseudohex- ment of Si by very small amounts of Al. Al 0.04 0.04 0.06 0.06 0.00 0.00 3+ Therefore, the tetrahedral layer cannot be Fe 0.00 0.00 I agonal array of Si04 tetrahedra linked to a Fe2+ trioctahedral brucite layer. All tetrahedra readily expanded. The most important oc- Mn 0.01 0.01 0.00 0.00 Mg 0.59 0.63 0.46 0.50 1.71 1.69 1.86 1.84 point in the same direction with two out of tahedral substitution is the replacement of Ni 4.54 4.86 1.28 1.26 Fe2+ by Fe3+, but this is considered secon- £0ctt 5.09 5.45 3.14 3.10 every three OH groups ideally replaced by H20+ 4.36 3.62 2.20 2.17 oxygen (Fig. 6). Appreciable mismatch be- dary. If all octahedral sites were occupied XI 3+ 0.14 0.18 0.09 0.09 tween the larger octahedral and smaller tet- by the smaller Fe cation, greenalite would rahedral layers must exist in greenalite as it still possess a curved sheet structure. On the Note: Data in columns as follows: 1, this study, 2+ average of 47 microprobe analyses of greenalite; 2, does in serpentine, resulting in a warped other hand, substitution of Fe by Al Brindley and Hang (1973), 7A-type garnierite; 3, this study, average of 7 microprobe analyses of iron talc; structure considerably removed from the (radius = 0.53A) is also possible. In platy 4, Brindley and Hang (1973), 10A-type garnierite. A = ideal representation. If most of the iron in serpentine (Fig. 6B), a "wave"-type of numbers of cations based on 4 silicons; B = numbers of cations based on 14 oxygens; C = numbers of cations greenalite is in the ferrous state, this misfit superstructure is created by tetrahedral in- based on 11 oxygens. N.D. = no data. should be even more severe because of the version (that is, where the apices of adja- * All Fe reported as FeO. 2+ t TOct = Fe + Mn + Mg + Ni + 3/2 Al. larger ionic radius of Fe compared to Mg cent tetrahedra point in opposite direc- § For analyses 1 and 2, X = proportion of 9A-type (0.77k versus 0.72 A, respectively; Shannon tions). Most greenalite appears to have a layers; for analyses 3 and 4, X = proportion of 7A-type layers. and Prewitt, 1969). The replacement of platy habit, and on the basis of the close Fe2+ by Fe3+ (radius = 0.65A) or Mg causes structural identity with serpentine, we pre- lOA). In contrast, minnesotaite from the a net contraction of the octahedral sheet dict that the greenalite analogue of antigo- Gunflint Iron-Formation possesses a 9.6k and partially offsets this distortion. Oc- rite has a similar superstructure controlled basal spacing, and chemical analyses yield tahedral vacancies, if present, result in by tetrahedral inversion with various anhydrous sums close to the predicted further contraction. This suggests a struc- periodicities. value. tural explanation for oxidation- The extremely fine grain size of greenalite Greenalite: Structure. Greenalite has dehydration reactions that may have occur- may be due to the mismatch between layers. been categorized structurally as an iron ser- red during burial or weathering. Strain on Relatively coarse varieties of greenalite the structure would be partially alleviated pentine (Gruner, 1936) and as a septechlo- 2+ 3+ exist (Gruner, 1936) but are rare. The rite (Nelson and Roy, 1958). The former in- by oxidation of Fe to Fe . This would acicular greenalite shown in Figure 3A sug- stabilize greenalite to higher temperatures terpretation is preferred here, because the 3+ gests a tubular variety analogous to term "septechlorite" (or 7 A chlorite) im- than might otherwise be expected if Fe chrysotile. Recently, a coarse-grained tabu- plies an analogous 14A structure not were not present. lar variety (-100 to 150 fim X 40 to 50 known to exist for either serpentine or In the serpentine minerals, a better match /am) has been identified within the contact

Figure 6. A. Platy serpentine (antigorite) crystal structure (from Zoltai, 1965). Ideal greenalite is inferred to have similar type of structure. B. View along b axis of antigorite serpentine showing periodic inversion of the tetrahedral layer and warping of the octahedral-tetrahedral couple due to layer mismatch. The curved layers reverse polarity at PP', RR', and near QQ'. Note that the T-O (serpentine) configuration changes to a T-O-T (talc) configuration at PP', RR', and QQ' (from Deer and others, 1962, based on Kunze, 1956).

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/9/1169/3418213/i0016-7606-86-9-1169.pdf by guest on 28 September 2021 1182 FLORAN AND PAPIKE

aureole in Minnesota (Floran and Papike, (9.18A) is almost identical to an undis- in prep.). The various textural types indi- torted silica tetrahedral sheet. The larger b cate that the term "greenalite" probably in- axis of minnesotaite obtained by Gruner cludes a group of structurally related min- (9.40A, 1944b) and Blake (9.38A, 1965) erals rather than a single phase. For con- suggests that the tetrahedra may be dis- venience, we use "greenalite" as a general torted or tilted. Outward tilting results in term that includes all textural and struc- expansion of the tetrahedral sheet (Fig. 8) tural varieties. whereas rotation or twisting leads to com- Minnesotaite: Structure. The min- pression. Octahedral vacancies (if present) /V\/V\ AAAA Aj\Aj\ nesotaite structure was determined by could lead to inward tilting around the va- Gruner (1944b) who concluded that it was cant sites. Slightly warped surfaces might ABC the same as talc. Single crystal x-ray studies also be produced by a small amount of primary Fe3+ in the tetrahedral sites Figure 8. A. [001] projection of the ideal of talc reveal much new information that 6-member tetrahedral ring configuration in layer may be extended to minnesotaite. It is thus (Forbes, 1969). silicates. B. Tetrahedral rotation or twisting in ab advantageous to again carefully examine Stilpnomelane: Chemistry. The term stilp- plane resulting in contraction of tetrahedral the Mg-rich analogue. nomelane (singular or plural) refers to a layer. Arrows indicate relative direction of rota- Minnesotaite and talc have a trioc- group of closely related minerals (Deer tion. C. Tetrahedral rotation combined with tilt- tahedral structure consisting of a brucite and others, 1962). Three major varieties ing. Tilting causes the tetrahedral sheet to ex- 3+ pand. component sandwiched between two sheets exist: ferristilpnomelane (Fe end 2+ of linked Si04 tetrahedra (Fig. 7). In min- member), ferrostilpnomelane (Fe end nesotaite, all octahedral sites are ideally oc- member), and parsettensite (Mn end Fe203)-Mg0-Al203-Si02 tetrahedron. In cupied by Fe2+. Crystals of minnesotaite member). Their occurrence in diverse cherty samples from the Gunflint Iron-For- suitable for x-ray precession work have not geologic environments was well docu- mation (Fig. 9), chemical variation is large- been reported, but single crystal x-ray mented prior to 1921 when stilpnomelane ly constrained by random errors generated studies of talc by Hendricks (1938), Rayner was first discovered in iron formations by by counting statistics. Significant variation and Brown (1966, 1973), and Ross and Gruner (1937). Only iron-rich varieties between samples emphasizes the impor- others (1968) have revealed a partially dis- have been identified from Lake Superior- tance of local bulk compositional controls. ordered structure in which the stacking se- type iron formations. Figure 10 illustrates the chemical quence is irregular. The unit_cell of talc is Graphical representation of stilp- heterogeneity of stilpnomelane encountered triclinic with space group CI. Rotation or nomelane analyses is difficult because of the within a single thin section (sample twisting of tetrahedra about the c* axis and large number of components present. It is 724-2A). Variation in 'FeO,' Si02, and flattening of octahedra are common in possible, however, to adequately represent A1203 exceeds the 2 a error bars by a wide many layer silicates (Brown, 1965) includ- the phase chemistry by considering two margin. This sample is composed, in part, ing talc, and most likely, minnesotaite. adjacent faces of an FeO (= FeO + of thin slaty laminae with varying propor- The tetrahedral twist in the talc examined by Rayner and Brown (1973) was very small (3.4°) because the ¿-axis length

KEY

o • sample 724-2A(3) • • sample 725-3BU x = sample 724-4BL A • sample 726-2A} Oolitic • = sample 724-6B v « sample 727-2F Figure 9. Compositional variation of stilpnomelane-bearing granules and ooliths from Gunflint

Iron-Formation (in weight percent). A. Plotted system FeO (= FeO + Fe203)-Si02-Mg0. B. Plotted in system FeO (= FeO + Fe203)-Si02-Al203. Length of error bars = 2a.

Al203 ture and chemistry. The smaller O/OH ratio of stilpnomelane results in an excess of - 2- 12/ OH and a deficiency of O relative to minnesotaite. This is true, especially, when the iron in stilpnomelane is primary or entirely Fe2+. Stilpnomelane: Structure. The recently determined stilpnomelane structure (Eggle- ton and Bailey, 1966; Eggleton, 1972) confirms earlier views (Gruner, 1937; 1944a) of a talclike structure having close X affinities to the trioctahedral micas. Addi- tional features characteristic of platy serpentine (antigorite) are also present. Figures llAand 1 IB represent two views " FeO"- •Si Ogof the stilpnomelane structure as defined by 54 50 46 42 38 Eggleton (1972). Sandwiched between two Figure 10. Variation in stilpnomelane compositions from a single thin section (sample 724-2A) smaller tetrahedral layers is a warped plotted in system FeO (= FeO + Fe203)-Si02-Al203. Triangles and closed circles are from different octahedral layer shaped like a saucer. The areas within slaty laminae; open circles are analyses from adjacent cherty layer (5 analyses of 6 majority of tetrahedra point toward the oc- granules). Length of error bars = 2 a. tahedral layer but do not form a continuous hexagonal sheet as in talc; instead, isolated tions of siderite, stilpnomelane, and car- nomelane prior to any secondary oxidation units or "islands" of 24 tetrahedra are bonaceous material. The slaty bands are in that may have taken place. Numerous linked laterally by 6-member rings of in- sharp contact with a thicker cherty layer workers (Hutton, 1938; Zen, 1960; Eggle- verted tetrahedra (Fig. 11B). Tetrahedral containing numerous massive stilp- ton and Bailey, 1966; Brown, 1971) have inversion occurs only where the islands and nomelane granules (Fig. 2A). Cherty stilp- offered compelling textural, chemical, and rings meet. Its major purpose is to permit nomelane is characterized by simple tex- experimental evidence that suggests a sec- expansion of the tetrahedral sheet. Adjust- tural relations and a relatively invariant ondary origin for all or most of the Fe3+ in ment of the island tetrahedra to the larger composition. Analyses from granules (open stilpnomelane. octahedral layer is achieved also by out- circles, Fig. 10) show little chemical varia- Several important observations may be ward tilting of all tetrahedra. This is possi- tion. In contrast, slaty stilpnomelane is noted from Table 5B: (1) The amount of es- ble because the tetrahedral islands are not compositionally heterogeneous, and tex- timated primary Fe3+ required to fill the tet- linked directly to one another. Similar tural relations are difficult to observe be- rahedral sites is either zero (column A) or structural relations are found in zussmanite cause most bands are nearly opaque; how- near zero (column B). (2) The O/OH ratio is (Lopes-Vieira and Zussman, 1969), a re- ever, some stilpnomelane appears to be less than the ideal ratio in minnesotaite lated iron silicate and possible stilp- pseudomorphous after shards. Both ferri- (10/2). (3) The sum of the octahedral ca- nomelane polymorph. stilpnomelane and ferrostilpnomelane are tions when normalized to 11 oxygens is var- Some of the interrelationships between present in the slaty bands. These can be iable but near 3.00 (as in minnesotaite). Al- the composition and structure of stilp- distinguished from each other by their dif- though the amount of Fe3+ could not be de- nomelane were discussed by Eggleton ferent optical properties (Hutton, 1938). termined directly, it is apparent that only a (1972) but many remain obscure. For ex- Table 5A is a compilation of published trace amount is required to fill the tet- ample, his refined structural model does not stilpnomelane analyses from iron forma- rahedral sites. The differences in the O/OH include the positions of Al or K. These cations. Structural formulae have been nor- ratio between minnesotaite and stilp- tions (including minor amounts of other malized to 120/18 or 6.66 cations following nomelane reflect real differences in struc- species that substitute for K) distinguish the method proposed by Eggleton (1972). If the unit cell content is divided by 18 and zeolitic water is ignored, the mineral formula is simplified considerably:

2+ 3+ 3+ (A)<„.3(R , R )2.«r(Si, R )4(0,0H)12, where R2+ = Fe2+, Mg, Mn; R3+ = Fe3+, Al; + + 2+ + + and A = K , Na , Ca , H30 , NH4 , and so on. Stilpnomelane analyses from the Gunflint Iron-Formation are shown in Table 5B. Structural formulae have been calculated by two methods: column A is normalized to 11 oxygen anions while column B is normalized to 6.66 cations. The first method is convenient because it allows comparison with minnesotaite analyses that are calculated in the same manner. Both formulae 2+ Figure 11. Crystal structure of stilpnomelane (after Eggleton, 1972). A. C*-axis projection of an assume that all iron is present as Fe — an undistorted ferristilpnomclane sheet. "Island" tetrahedra (solid triangles) are linked laterally by obviously incorrect assumption for stilp- 6-member rings (open triangles) of inverted tetrahedra. Unit cell is outlined. B. [110] projection nomelane. Nevertheless, they yield an indi- through the ring and island tetrahedra. Large monovalent cations (not shown) partially fill the large cation of the original compositions of stilp- holes formed as a result of tetrahedral inversion. Dark circles are octahedral cations.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/9/1169/3418213/i0016-7606-86-9-1169.pdf by guest on 28 September 2021 1184 FLORAN AND PAPIKE

TABLE 5A. CHEMICAL ANALYSES OF STILPNOMELANE FROM IRON FORMATIONS OTHER THAN THE GUNFLINT

1 2 3 4 5 6 7 8 9 10 11 12

Si02 46.85 42.42 46.55 47.54 46.90 44.0 40.4 45.54 44.04 47.3 46.41 51.08 T102 0.15 N.B. 0.00 0.00 0.00 N.D. 0.1 0.26 0.03 0.03 0.08 0.15 A1203 4.64 6.71 5.38 5.18 5.12 6.2 9.9 4.75 2.10 5.30 3.68 2.64 Fe203 11.60 33.24 8.00 8.61 17.90 3.2 3.9 2.90 18.49 T r33.8 36.63* 21.94* FeO 20.00 0.85 24.78 25.03 16.49 23.6 26.9 25.38 18.55 J MnO 0.33 2.27 0.58 0.85 0.45 0.04 0.40 0.13 0.01 1.52 0.04 N.D. MgO 5.75 5.20 3.90 3.58 3.73 7.60 7.8 7.75 6.52 2.74 5.65 12.40 CaO 0.94 N.D. 0.46 0.33 0.29 N.D. 0.1 0.04 < 0.01 0.34 0.07 0.03 Na20 0.27 N.D. 0.14 0.00 0.06 0.56 0.1 0.82 0.26 0.21 0.41 0.71 K26 2.07 N.D. 1.84 1.32 1.48 3.3 7.20 1.96 + 5.63 1.88 1.61 4.49 H2O 5.84 5.79 5.71 5.77 8.33 6.15 4.68 8.46 3.66 N.D. ) + > 5.68 7.33' H2O- 1.80 1.45 2.53 1.77 2.86 3.0 0.46 1.56 0.41 N.n. ) § # Total 100.17 100.47 100.00 100.00 100.99 97.65 101.97 100.27 100.02** 93.1 100.63 100.80

Numbers of ions on the basis of 6.,6 6 cations

Si 3.60 3.30 3.61 3.64 3.63 3.46 3.01 3.51 3.43 3.62 3.44 3.74 Al 0.40 0.62 0.39 0.36 0.37 0.54 + 0.87 0.43 0.19 0.38 0.32 0.23 Fe3 0.00 0.08 0.00 0.00 0.00 0.00 0.12 0.06 0.38 0.00 0.24 0.03 ETetrt+ 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 AlVI 0.02 0.01 0.11 0.11 0.09 0.03 0.00 0.00 0.00 0.10 0.00 0.00 Ti 0.01 0.00 0.00 0.00 0.00 0.00 0.02 0.00 0.00 0.00 0.01 Fe3+ 0.67 1.86 0.47 0.50 1.04 0.19 0.10 0.11 0.70 1 }• 2.16 2.03 1.31 Fe2+ 1.28 0.05 1.61 1.60 1.07 1.55 1.68 1.64 1.21 J Mn 0.02 0.15 0.04 0.05 0.03 0.00 0.02 0.01 0.00 0.10 0.00 0.66 0.60 0.45 0.41 0.43 0.89 0.87 0.89 0.76 "g tt++ 0.31 0.63 1.35 t0ct 2.66 2.67 2.68 2.67 2.66 2.66 2.67 2.67 2.67 2.67 2.66 2.67 Ca 0.08 0.04 0.03 0.02 0.00 0.01 0.00 0.00 0.03 0.01 0.00 Na 0.04 0.02 0.00 0.01 0.08 0.01 0.12 0.04 0.03 0.06 0.10 K 0.20 -- 0.18 0.13 0.14 0.33 0.68 0.19 0.56 0.18 0.15 0.42 0H- 1.07 0.80 2.09 2.07 1.53 2.44 2.56 2.54 2.00 2.57 2.66 + 1.61 H O.nO 0.70 0.11 0.18 0.17 0.13 0.28 0.18 0.00 0.11 0.33 0.00

Note: Data in columns as follows: 1, Grout and Theil (1924); Biwabik Iron-Formation, Minnesota. 2, Ayres (1940); Crystal Falls, Michigan. 3, Blake (1965); Cuyuna iron formation, Minnesota. 4, Blake (1965); Cuyuna iron formation, Minnesota. 5, Blake (1965); Cuyuna iron formation, Minnesota; total incorrectly reported as 100.00. 6, LaBerge (1966a); Brockman Iron Formation, Australia. 7, LaBerge (1966b); South Africa. 8, Trendall and Blockley (1970); Brockman Iron Formation, Australia. 9, Trendall and Blockley (1970); Brockman Iron Formation, Australia. TO, Brown (1971); microprobe analysis of ferristilpnomelane; Cuyuna iron formation, Minnesota. 11, Ayres (1972); microprobe analysis of ferristilpnomelane; Brockman Iron Formation, Australia. 12, Ayres (1972); microprobe analysis of ferrostilpnomelane; Brockman Iron Formation, Australia. N.D. = no data. * All Fe reported as FeO. t Obtained by difference in computer program. § Includes 0.03P205. # Includes 0.23C0Z + 0.44P205. ** Includes 0.32C02. ft £Tetr and EOct calculations from method by Eggleton (1972).

stilpnomelane from the other iron silicates sheet cannot be sufficiently expanded. Con- lationship between stilpnomelane and the and are probably necessary to stabilize the sequently, four tetrahedra in the hexagonal nontronite-montmorillonite group should structure. Tetrahedral inversion creates array are replaced by six inverted tet- be investigated. Stilpnomelane and mem- holes large enough to easily accommodate rahedra creating the large holes in the struc- bers of this group have been shown to be in- water or alkalies. These ions or molecules ture. Potassium occupies some of these timately associated with each other (Per- are either mechanically trapped or loosely cavities instead of an interlayer position as rault, 1955; LaBerge, 1966a, 1966b) and bonded, a situation not unlike that in zeo- it does in micas. Interruptions in the hex- both occur as alteration products in pyro- lites. Potassium is always present in small agonal sheet caused by tetrahedral inver- clastic rocks. amounts but occupies few of these cavities; sion are most likely responsible for the Chamosite: Chemistry. The chemistry others may be filled with water or large less-than-perfect (001) cleavage of stilp- and crystal structure of chamosite will be + + monovalent ions such as Na , NH4 , and nomelane. described only briefly because it does not + H30 that are able to replace water Eggleton (1972) recognized four different appear to have a close paragenetic relation- molecules without disturbing a layer struc- octahedral environments that he considered ship with the other iron silicates. It is not an ture (Brown, 1965). to be variably susceptible to oxidation. Var- abundant phase and is restricted to the tuff- Eggleton's (1972) average of 37 analyses iation in the Fe2+/Fe3+ ratio is allowed by aceous shale facies. suggested that most A1 replaces Si with the structural expansion (by tetrahedral tilting) An average microprobe analysis (Table excess occurring in octahedral sites. The or contraction (by tetrahedral rotation). All 6) is similar in composition to chamosite presence of A1 in either the inverted or is- the features cited above suggest a highly from Paleozoic ironstones (Brindley, 1951). land tetrahedra would appear to alleviate flexible structure capable of accommodat- The most significant compositional feature some of the strain on the structure. Thus ing tremendous chemical variation and sta- of the Gunflint chamosite is its high A1 and the amount of A1 present appears to control ble under a wide range of geologic condi- Mg content. None of the other iron silicates whether the stilpnomelane structure or that tions. can accommodate as much Al, which sug- of a trioctahedral mica will form. A con- Although the structure proposed by Eg- gests that formation of chamosite may well tinuous hexagonal sheet of tetrahedra is gleton (1972) is probably substantially cor- have been controlled by the bulk composi- possible in the micas because of substantial rect, the large structure factor (R ~ 21 per- tion of the tuffaceous shale facies. The lat- A1 substitution in the tetrahedral sites. The cent) obtained in his study suggests that ter is roughly equivalent to the intermediate comparatively small amount of A1 in stilp- other interpretations of the stilpnomelane slate of the Mesabi Range, which averages nomelane indicates that the tetrahedral structure are possible. In particular, the re- approximately 6 percent more Al and 1

TABLE 5B. CHEMICAL ANALYSES OF STILPNOMELANE FROM THE GUNFLINT IRON-FORMATION 0 1* 2* 3* 4+ 5* 6 7 9+ 10+ 11 + 12+

Si02 39.2 45.6 40.5 44.7 47.7 40.3 42.2 45.4 45.5 45.6 47.0 44.0 AI0O3 7.59 4.41 6.77 4.49 4.60 5.39 3.97 3.41 4.02 2.51 4.07 4.66 TiO? N.D. N.D. N.O. N.D. 0.03 0.07 0.02 0.03 N.D. 0.00 N.D. 0.03 Fe203 "I r 37.1s 37.1s 37.0s 35.5s 30.7s 36.6s 39.9s 39.6s 35.9s 36.0s 34.3s 36.3s FeO 1 MnO 0.08 0.09 0.14 0.15 0.00 0.04 0.02 0.03 0.03 0.16 0.04 0.07 MgO 4.40 3.11 4.22 3.13 2.85 3.30 2.77 1.86 1.97 3.43 3.15 3.11 CaO 0.53 0.35 0.40 0.34 0.50 0.57 0.13 0.32 0.07 0.18 0.34 0.34 Na20 N.D. N.D. N.D. N.D. 0.00 0.00 0.00 0.00 0.00 0.00 N.D. 0.00 k25 0.75 1.43 1.14 1.79 1.12 0.81 1.03 1.55 1.89 1.13 1.14 1.16

Total 89.6 92.1 90.2 90.1 87.5 87.1 90.0 92.2 89.4 89.0 90.0 89.7

Numbers of cations on the basis of 11 oxygen anions and 6.66 cations

A B A B A B A B A B A

Si 3.30 3.05 3.69 3.51 3.39 3.15 3.69 3.53 3.87 3.83 3.50 3.27 3.58 3.33 3.73 3.56 3.78 3.65 3.80 3.62 3.81 3.68 3.65 3.47 Al 0.70 0.70 0.31 0.40 0.61 0.62. 0.31 0.42. 0.13 0.17 0.50 0.52 0.40. 0.37. 0.27 0.31 0.22 0.35 0.20 0.24 0.19 0.32 0.35 0.43 Fe3+ — 0.25» — 0.09» — 0.23» — 0.05» — — -- 0.21* 0.02» 0.30» - 0.13 0.14» — — — 0.10» zTetr 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 AlVI 0.05 0.11 -- 0.06 — 0.13 — 0.31 0.27 0.05 0.00 — 0.06 — 0.17 0.03 0.05 — 0.20 0.06 0.11 -- Fe 2.62 2.16 2.51 2.30 2.59 2.17 2.45 2.29 2.09 2.06 2.65 2.27 2.81 2.34 2.71 2.46 2.49 2.41 2.51 2.25 2.32 2.24 2.53 2.30 Mn 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.00 0.00 0.00 0.00 Mg 0.55 0.51 0.38 0.36 0.53 0.49 0.38 0.37 0.35 0.34 0.43 0.40 0.35 0.33 0.23 0.22 0.24 0.24 0.43 0.41 0.38 0.37 0.39 0.37 zOct 3.23 2.68 3.01 2.67 3.19 2.67 2.97 2.67 2.75 2.67 3.13 2.67 3.16 2.67 3.00 2.68 2.90 2.68 3.00 2.67 2.90 2.67 3.03 2.67 Ca 0.05 0.04 0.03 0.03 0.04 0.03 0.03 0.03 0.04 0.04 0.05 0.05 0.01 0.01 0.03 0.03 0.01 0.01 0.02 0.02 0.03 0.03 0.03 0.03 K 0.08 0.07 0.15 0.14 0.12 0.11 0.19 0.18 0.12 0.12 0.09 0.09 0.11 0.10 0.16 0.15 0.20 0.19 0.12 0.11 0.12 0.12 0.16 0.12 0H- 3.18 2.68 2.90 2.67 3.13 2.67 2.84 2.67 2.51 2.43 3.08 2.67 3.16 2.67 2.94 2.68 2.73 2.65 2.95 2.67 2.70 2.61 2.92 2.67 H+ 0.52 0.80 0.10 0.29 0.41 0.68 0.06 0.23 0.00 0.00 0.31 0.54 0.29 0.55 0.05 0.23 0.00 0.14 0.04 0.23 0.01 0.14 0.14 0.35

Note: Data in columns as follows: 1, sample 724-2A (1). 2, sample 724-2A (1). 3, sample 724-2A (2). 4, sample 724-2A (3); granule. 5, sample 724-3A (1A); matrix. 6, sample 724-3A (2); late vein material. 7, sample 724-3A (2); late vein material. 8, sample 724-4BL; granule. 9, sample 724-6B; granule. 10, sample 725-3BU; granule. 11, sample 726-2A; oolith core. 12, sample 727-2F; granule. All stllpnomelanes examined (except analysis 2) are ferristilpnomelanes. N.D. = no data. * Banded or slaty iron formation, t Cherty iron formation. i All Fe reported as FeO. # Estimated ferric iron content of tetrahedral sites.

percent more Mg than the cherty members placed by Fe3+. Brindley and Youell (1953) Microprobe analyses of siderite reveal (Gruner, 1946; Lepp, 1966). demonstrated that partial oxidation of Fe2+ considerable solid solution. Substitution by Chamosite: Structure. Chamosite from to Fe3+ is accompanied by dehydration Mg is usually considerable, and locally, Mn the Gunflint Iron-Formation is the 7A vari- upon heating to 300°C. As in the other iron is an abundant constituent (Table 7, Fig. ety (septechamosite), which has a structure silicates, the bulk of the iron is believed to 13). Spheroidal siderite from the chert- similar to greenalite. The tetrahedral- have been originally Fe2+. carbonate facies often exhibits composi- octahedral misfit should not be as severe tional zoning with respect to Fe and Mg. because of A1 substitution in both tet- Carbonate Minerals Fine-grained siderite cores are usually more rahedral and octahedral sites. Ferrous iron Mg-rich than rhombic overgrowths, but the is the dominant cation in the octahedral Compositions of carbonate minerals reverse has also been encountered. The sheet but may be partially or wholly re- from both slaty and cherty rocks are shown cores frequently contain an unidentified in Figure 12; selected analyses of siderite, dark-brown material rich in Si, Fe, and Al. TABLE 6. CHEMICAL ANALYSIS OF CHAMOSITE FROM THE GUNFLINT IRON-FORMATION calcite, and ankerite are presented in Table In sample 724—2A, Mn-rich and Mn-poor 7. Pétrographie, x-ray diffraction, and siderites coexist with stilpnomelane. Fine- Sample grained siderite interstitial to stilpnomelane 726-Cu* chemical data are in qualitative agreement with the experimental studies of Rosenberg granules (Fig. 2A) is both compositionally SIO2 26.9 and texturally similar to siderite associated A1203 18.4 (1963a, 1963b, 1967), who defined one Fe^a 1 36.8+ three-phase, two two-phase, and three with slaty stilpnomelane (Fig. 10). Thirty- FeO I 0.00 M11O one-phase fields for synthetic carbonates in nine microprobe analyses of siderite from MgO 8.15 the Gunflint Iron-Formation yield the fol- CaO 0.05 the system CaC03-MgC03-FeC03. The ex- K20 0.12 tent of the ankerite and siderite composi- lowing average composition: (Feo.79Mg013 Total 5574" tional .fields portrayed in Figure 12 is not Ca0.05Mn0 03)CO3. This is somewhat more Numbers of ions based on 14 oxygens well known. Tie lines represent coexisting iron-rich than the average composition ob- Si 2.96 A1 1.04 carbonate assemblages that are believed to tained by French (1968) for the Biwabik 4.00 Iron-Formation. Si«"' 1.26 be in equilibrium. At present, the data are Fe 3.33 insufficient to accurately define the three- The few analyses of ankerite that are Mg 1.32 EOct* 5.91 phase field. The exact position of this field available show extensive replacement of Fe * Average of two analyses; pale-green shard pseudo- depends on temperature, thus making it a by Mg. Ankerite-rich beds above the morph, banded or slaty Iron formation (argllUte tuff). potential geothermometer. Small amounts argillite-tuff unit at Kakabeka Falls (Fig. + All Fe reported as FeO. § ITetr = Sf + AlIV. of Mn probably complicate the system con- 4C) contain considerable amounts of Mn # SOct = XFe + Mg + Al"I. siderably (Rosenberg, 1967). (Table 7). In contrast, calcite from all tex-

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/9/1169/3418213/i0016-7606-86-9-1169.pdf by guest on 28 September 2021 1186 FLORAN AND PAPIKE

TABLE 7. CHEMICAL ANALYSIS OF CARBONATES FROM THE GUNFLINT IRON-FORMATION granules have been observed by us, but their abundance and distribution are un- 1* 2+ 3+ 4 5+ 6+ 7* 8+ 9 10+ 11 + 12+ 13 known. Goodwin (1956) noted the close association between granule texture and Si02 • 0.00 0.10 0.00 0.52 N.D. N.D. 0.04 0.57 0.00 0.00 0.00 0.00 0.34 FeO 49.6 42.6 53.7 50.1 55.1 0.42 1.38 1.28 1.41 9.16 16.61 19.5 50.0 wavy banded structure that forms the bulk MnO 0.94 10.7 1.21 1 .48 0.36 0.66 0.72 0.00 0.25 2.14 4.47 0.28 1.71 of the iron formation to the southwest. MgO 6.06 3.98 2.33 5.76 3.69 0.05 0.17 0.00 0.12 11.4 7.13 6.36 4.73 CaO _2.06 2.12 3.30 Jj40 0.71 53.6 53.8 54.7 54.2 34.4 30.6_ 29.0b _2^20 These beds cannot be reconciled with a simple particulate origin according to Total 58.7 59.5 60.5 59.3 59.9 54.7 56.1 56.6 56.0 57.1 58.8 55.2 59.0 Goodwin. The problem of granule origin is thus complex, and it is not unlikely that the Molecular proportions of carbonates granules formed by several independent FeC03 77.5 67.4 84.8 78.8 87.4 0.60 1.93 1.78 1.98 12.10 22.8 28.5 79.4 mechanisms. Most of the granules we have MnC03 1.49 17.1 1.93 2.33 0.57 0.97 1.02 0.00 0.36 2.86 6.20 0.41 2.75 MgC03 16.9 11.2 6.57 16.0 10.5 0.06 0.43 0.00 0.03 26.8 17.4 16.6 13.4 observed, however, appear to be intraclas- CaC03 4.13 4.29 6.67 2.80 1.55 98.4 96.6 98.2 97.6 58.2 53.6 54.5 4.47 tic in origin.

Note: Data in columns as follows: 1, Sample 723-8; spheroidal siderite in calcite poikiloblast (argillite tuff). 2, Sample 724-2A; fine anhedral siderite interstitial to stilpnomelane granules. 3, Sample 724-2A; same Phyllosilicate Minerals grain as 2. 4, Sample 724-5B; bladed siderite rim cement. 5, Sample 727-2F; rhombic siderite apparently replacing cherty granules. 6, Sample 723-2B6; coarse calcite occurring as a single granule. 7, Sample 723-4-6U; sparry poikiloblast in thin limestone laminae (argillite tuff). 8, Sample 727-2F; porphyroblast replacing cherty One of the greatest gaps in our knowl- granules and cement. 9, Sample 724-5B; sparry pore-filling cement. 10, Sample 723-2B6; ferroan dolomite replacing chert cement; associated with calcite. 11, Sample 723-2B6; rhombic ankerite replacing chert cement; associ- edge of iron formations is our poor under- ated with calcite. 12, Sample 726-5; rhombic ankerite occurring within chert cement; associated with greenalite calcite, and siderite. 13, Average siderite composition, based on 39 microprobe analyses. N.D. = no data. standing of the phyllosilicates — in particu- * Banded or slaty iron formation, lar, how, why, and when they formed and t Cherty iron formation. their relationship to one another. Eh-pH diagrams (Garrels and Christ, 1965) illus- tural occurrences deviates little from the Goodwin (1956) did not consider most trate that all major minerals in sedimentary ideal end-member composition. This is true granules to be transported material. He fa- iron formations, including iron silicate, of calcite in general, although manganoan vored instead an in situ development during have a stability field at 25°C and 1 atm total varieties are known to exist (Deer and lithification of a hydrous silica-ferrous iron pressure, provided that the concentrations others, 1962). gel. Yet, many oolith- and granule-bearing of certain components are fixed. Although units are unquestionably of intraclastic these theoretical diagrams are extremely PETROGENESIS origin, especially those of the upper taco- useful, they are severely limited in their ap- nite facies near Thunder Bay where the evi- plication to real geologic systems (Huber, Although iron formations have been dence for shallow-water wave action in- 1958). The silicate stability field provides studied intensely since the early part of this cludes cross-bedded granules, abundant an extreme example. Iron silicate of com- century, many basic questions concerning ooliths, hematitic algal structures, and in- position FeSi03 (used by Garrels and their origin remain unclear. The source, traformational breccias (Goodwin, 1974, Christ, 1965) is only stable at high pressure transport, and deposition of iron and silica written commun.). Dimroth and Chauvel (Lindsley and others, 1964) and has never are just a few of the major problems (1973) voiced numerous objections to the been found forming at or near the Earth's (James, 1966). In the Gunflint Iron- in situ crystallization model. Although most surface at low temperatures. Perhaps the Formation, the genetic association between workers favor a particulate source, some most important limitation in the use of deposition and volcanism has been em- granules may have had a different origin. Eh-pH diagrams is the assumption of true phasized by Goodwin (1956). Volcanic ac- For example, cherty granules with indis- thermodynamic equilibrium. Rate proc- tivity was considered riot only as a source of tinct boundaries and lacking shrinkage esses, which may have been an important iron and silica but also to have induced cyc- cracks could have formed in place by recrys- factor in mineral paragenesis, cannot be lic sedimentation resulting in iron- and tallization (analogous to the pseudoal- evaluated on such diagrams. silica-rich banded rocks. lochems of Folk, 1959). These kinds of The data presented here do not prove

MgC03 MnC03

MnC03

(F«»Mg)C03

(Fe*Mn)C03 C0CO3

Figure 12. Ranges in carbonate compositions from Gunflint Iron- CaCC>3 (Fe+Mg)C03 Formation plotted in the system (Fe + Mn)C03-CaC03-MgC03. Solid circles are plotted analyses while tie lines represent coexisting carbonate Figure 13. Siderite compositions from a single thin section (sample phases. Single-phase boundaries where well known are outlined in solid; 724-2A) plotted in the system (Fe + MG) C03-MnC03-CaC03. Closed where less well known, dashed; and where poorly known, dotted. C = cal- circles are from cherty iron formation; open circles from slaty iron forma- cite, A = ankerite, S = siderite. tion.

which, if any, of the minerals in iron forma- structure, and these are a consequence of tallize more readily than talc (Deer and tions are true chemical precipitates. Tex- tetrahedral inversion. others, 1962). Free-energy data are not yet tural relations only suggest criteria that can Blake (1965) presented textural and available for the structurally similar iron be used to distinguish between so-called chemical data for the transformation of silicates, but analogous low-temperature primary and secondary minerals. We favor minnesotaite to stilpnomelane. Unlike stability fields may well exist. Garrels the view held by Gruner (1946) that all of greenalite, the curling tendency due to layer (1959) noted that, in general, the first pre- the iron silicates formed either within the mismatch should be substantially reduced cipitate to form from an aqueous solution original precipitate or during early in the ideal minnesotaite structure where represents a metastable equilibrium. In a diagenesis. We believe that a detailed con- the constraining influence of two opposing solution supersaturated with respect to sev- sideration of their structures and chemistry tetrahedral layers should result in a flat eral solid phases, the first solid to appear is leads to the conclusion that bulk composi- sheet configuration. This generalization "the one that relieves supersaturation with tion was of prime importance in determin- may not be valid for poorly crystallized respect to its components at a greater rate ing which iron silicate mineral formed. minnesotaite where tetrahedral tilting, dis- than the other solids do" (Garrels, 1959, p. Although the idealized structures of tortion, or even inversion may prevent 28). In other words, reaction rates (nuclea- greenalite and minnesotaite are quite differ- coarse crystals from forming. tion and growth) greatly influence which ent, the real structures may be transitional Idealized compositions of the three major mineral will precipitate and be preserved. into one another. Microprobe analyses are iron silicates normalized to 4 tetrahedral Unfortunately, criteria are not available consistent with this view (Table 2). The sites are greenalite, Fe6Si4O10(OH) g; min- to differentiate minerals of the original pre- wave-structure model proposed by Kunze nesotaite, Fe3Si4O10(OH)2; and stilp- cipitate (if indeed such minerals ever ex- (1956, 1961) for antigorite can probably be nomelane, Fe2.67(Si,Al)4(0,0H) 12, (modi- isted) from primary minerals. We agree applied to greenalite. Gruner (1936) and fied from Eggleton, 1972). with French (1973, p. 1064) that "a given Steadman and Youell (1958) considered the The relatively small amounts of alkalies mineral may have a wide enough stability greenalite structure to be essentially identi- and zeolitic H20 in stilpnomelane have field to form stably under conditions rang- cal to that of antigorite. Kunze presented been ignored, and all octahedral positions ing from diagenesis to low-grade evidence that the ideal formula of antigorite in the iron silicates are assumed to be filled metamorphism." This helps to explain why varies systematically between the theoreti- by Fe2+. This comparison reveals additional minerals such as stilpnomelane display both cal limits, Mg3Si4O10(OH)2 (talc) and similarities between minnesotaite and stilp- primary and secondary textures. Mg6Si4O10(OH)8 (serpentine), although nomelane and suggests that the formation most observed compositions occur be- of the latter was controlled primarily by the DISCUSSION tween Mg5 4oSi40 io(OH) 6.80 and MG5.68- availability of Al; presence of K (or in some Si4O10(OH)7.26 (Iishi and Saito, 1973). The instances, Na) may have been an additional Textural and mineralogical data for the wide range in possible compositions was at- controlling factor. Note that the smaller Gunflint assemblages are summarized in tributed to a superlattice along the a axis number of octahedral sites in stilpnomelane Figure 14. Some of the textural types (Fig. having varyingperiodicities controlled by tet- (2.67) relative to minnesotaite is a conse- 14B) are remarkably well preserved at rahedral inversion. The implications of this quence of tetrahedral inversion. higher grades of metamorphism and can be model for greenalite and minnesotaite can At present, tetrahedral inversion has been traced into the contact aureole of the best be visualized by referring to Figure 6B. demonstrated to occur only in stilp- Duluth Complex in Minnesota. The succes- Here, tetrahedral inversion changes the nomelane, but it is considered here to be the sion of textural elements in the sedimentary structure from a two-layer (T-O) to a three- structural link among the iron silicates. Tet- rocks is demonstrated by the sequence layer (T-O-T) configuration. These extend rahedral inversion in minnesotaite should granules —» rim cements —» pore cements —* as three-dimensional sheets through the result in fewer octahedral sites. Mineral replacement. structure (parallel to PP', QQ' and RR' in formulae calculated for minnesotaite by Components required to illustrate the Fig. 6B). One of the most perplexing prob- Gruner (1944b) and Blake (1965) reveal primary mineral chemistry of the iron for- lems concerning greenalite is the discrep- deficiencies in the octahedral sum (2.79 and mation are FeO, Fe 0 , MgO, CaO, Si0 , ancy between the predicted density (3.25 2 3 2 3 2.96, respectively). Both workers used A1203, K20, MnO, HzO, and C02; MgO, cm /g, Gruner, 1936) and observed values + H20 to determine the amount of OH in MnO, and K O are minor constituents, and (2.75 cm3/g, Leith, 1903; 3.00 cm3/g, Jol- s the mineral formula. This method differs CaO is a major component of ankerite and liffe, 1935; 2.93 through 2.95 cm3/g, Per- from the approach used here that is based calcite only. Most primary assemblages, rault, 1955). The smaller observed density on 10 oxygens and 2 OH groups per unit therefore, can be represented in a tetrahe- is probably due to a combination of factors: + cell. If the HaO determination is disre- dron with FeO, Fe203, Si02, and A1203 as

(1) the presence of adsorbed H20 between garded, the octahedral sums are quite dif- apices. Projection onto the Fe0-Fe20.TSi02 some of the layers in the crystal structure, ferent (Table 3). It is uncertain which of the plane is feasible because all minerals except (2) ionic substitutions such as Mg for Fe, two methods is correct. If the deficiency is stilpnomelane lie essentially on it. The most (3) deviations from the ideal structure, (4) real, however, it could be a result of tet- common primary assemblages in the inaccurate density determinations. rahedral inversion rather than of vacancies Gunflint Iron-Formation are shown Stilpnomelane appears to possess a struc- within the minnesotaite structure. schematically in Figure 14A, which is a tural type intermediate between that of Bricker and others (1973) inferred stabil- highly modified version of a similar dia- greenalite and minnesotaite. Parts of the ity fields for talc and chrysotile serpentine gram presented by Klein (1973). stilpnomelane structure consist of periodi- at 25° and 90°C under 1 atm total pressure The wide variation in the Fe2+/Fe2+ + cally repeated talclike units. At the bound- for the system MgO-SiO^H^. They sug- Fe3+ ratio of the iron silicates allows a more ary of these units, the T-O-T stacking gested that a large field of metastability ex- lucid representation of the assemblages, changes to a T-O couple (Fig. 11B), very ists where both chrysotile and talc crystal- even though most, if not all, Fe3+ is of sec- much like that found in serpentine (and lize together. Available evidence indicates ondary origin. Addition of A1203 to the probably greenalite) where the tetrahedral that both minerals form in natural envi- iron-silica system apparently stabilizes sheet occurs on the concave portion of the ronments at temperatures <100°C (Hos- stilpnomelane and suggests that a three- composite sheet (Fig. 6B). Only the zeolitic tetler and Christ, 1968; Bricker and others, phase silicate field is compositionally cavities are unique to the stilpnomelane 1973) although serpentine appears to crys- possible. Because stilpnomelane always

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/9/1169/3418213/i0016-7606-86-9-1169.pdf by guest on 28 September 2021 1188 20 FLORAN AND PAPIKE

A. MINERALOGICA!-

UNMETAMORPHOSED B. TEXTURAL

Carbono!« faciei Stilpnomelane (projected from AI2O3)

SiO Qtz (Sid) Figure 14. Summary diagram of mineralogical and textural relationships in Gunflint Iron-Formation. A. Schematic diagram illustrating the most common primary assemblages, plotted in the system Fe0-Fe203-Si02-Al203. All mineral phases lie close to the plane Fe0-Fe203-Si02 except stilpnomelane, which has been projected from A1203, and siderite, which has been plotted at FeO for convenience. Consideration of the Fe203 component permits a clearer representation of the iron-silicate-bearing assemblages. Magnetite is not shown because of its rarity. If Ca is introduced into the system at a high aCo„ calcite or ankerite may precipitate as primary phases. Variation in the aH20/aco, ratio results in the assemblage quartz + minnesotaite or quartz + siderite (not shown on diagram). B. Textural relations of taconite and chert-carbonate facies showing the sequential development of rim cements, pore cements, and carbonate replacement.

contains Al, tie lines connecting it with sufficiently high enough to permit its for- with a relatively homogeneous aqueous en- other phases lie within the tetrahedron of mation. Replacement of quartz by carbon- vironment. Microprobe analyses of Figure 14A. When A1203 (and K20, which ates was observed in samples collected near "greenalite" granules reveal a small but seems to follow it geochemically) was not or within the tuffaceous shale facies and significant minnesotaite component (10 to available, stilpnomelane apparently could shaly laminae associated with cherty rocks. 20 percent), but it is not certain whether not form. A similar association exists in the Biwabik minnesotaite is a reaction product growing Carbonates in cherty rocks occur locally Iron-Formation where ankerite-bearing at the expense of greenalite or whether the as a replacement of chert, although much mottled cherty iron formation occurs below structures of the two phases aire transitional carbonate is probably original cement the intermediate slate of the Lower Slaty into one another. The latter interpretation modified only by recrystallization. The member (French, 1968, p. 34). Massive an- is supported by a similar structural rela- source of carbonate is unclear; either it was keritic chert beds have been found both tionship in serpentine (antigorite). introduced metasomatically or it was pres- above and below pyroclastic units in the The iron-silicate minerals possess dis- ent in the primary precipitate and crystal- Hamersley Range of Australia (LaBerge, torted crystal structures, which explains lized as the gelatinous mass began to expell 1966a); the thickness of the ankeritic cherts some of the peculiarities in their phase H20. In the process, the crystallization of is directly correlated with the thickness of chemistry and their fine-grained nature. carbonate may have taken place partly in the pyroclastic beds now largely altered to The stilpnomelane structure may be con- open spaces of the shrinking gel and partly stilpnomelane. The close association in Ar- sidered intermediate to that of greenalite perhaps as a replacement of fine-grained chean iron formations between the carbo- and minnesotaite. Both greenalite and quartz. nate facies and volcanism has been stressed stilpnomelane possess remarkably flexible As pointed out by Blatt and others by Goodwin (1964, 1973). A similar as- structures that allow much of the original (1972), the solubility of silica is not only a sociation appears to exist in the Gunflint Fe2+ to oxidize to Fe3+. Structurally related function of pH and temperature (Kraus- Iron-Formation. minerals, such as serpentine, garnierite (7A kopf, 1956, 1959, 1967) but also of grain and 10A varieties), and talc offer valuable size. Extremely fine grained aggregates tend CONCLUSIONS insights in the interpretation of the iron- to be highly soluble because of a large sur- silicate structures. face energy component. Colloidal-size Textural relations in the Gunflint Iron- Primary crystallization of stilpnomelane cherty quartz (<0.1 /xm) or an amorphous Formation indicate that quartz, greenalite, was controlled by the availability of Al (and precursor becomes unstable with increasing stilpnomelane, chamosite, hematite, and possibly K). Stilpnomelane and chamosite temperature or when the pH rises above 9. siderite are primary minerals. Magnetite, in banded or slaty iron formation probably Changes in these parameters would be ex- minnesotaite, ankerite, and calcite may be formed in part by the alteration of volcanic pected during burial, although a rise in pH primary but are commonly secondary. shards. to >9 is considered geologically rare. Dis- Lack of compositional variation in Iron-formation textures have been solution of silica would be accompanied by greenalite from widely separated areas pos- modified considerably by postdepositional carbonate replacement if the ac0z was sibly reflects an approach to equilibrium recrystallization and replacement. In spite

of this, preservation of textural elements al- oxygen/total iron ratio remains the same during 130, p. 247-274. lows a paragenetic mineral sequence to be the postulated oxidation-dehydration reactions Dimroth, E., and Chauvel, J. J., 1973, Petrog- established for many of the rocks. (neglecting the change in H content, which is triv- raphy of the Sokoman Iron Formation in part of the central Labrador trough, Chemographic representation of mineral ial). Note that a correction for ferric iron must be made for such minerals as hornblende, in Quebec, Canada: Geol. Soc. America Bull., assemblages is consistent with a bulk com- which the Fe3+ is a primary constituent present at v. 84, p. 111-134. positional control although kinetics may the time of crystallization. Eggleton, R. A., 1972, The crystal structure of have greatly influenced mineral formation. stilpnomelane; Pt. II. The full cell: The existence of one, two, and three carbo- REFERENCES CITED Mineralog. Mag., v. 38, p. 693-711. nate assemblages is in qualitative agreement Eggleton, R. A., and Bailey, S. W., 1966, The with the experimental work of Rosenberg Albee, A. L., and Ray, L., 1970, Correction fac- crystal structure of stilpnomelane. Pt. I. The (1967) and also suggests that local bulk tors for electron probe microanalysis of sili- subcell, in Clays and Clay Minerals Natl. composition was the dominant control in cates, oxides, carbonates, phosphates, and Conf., 13th, Madison, Wise., 1964, Proc.: the formation of carbonate minerals. sulfates: Anal. Chemistry, v. 42, p. New York, Pergamon Press, p. 49—63. 1408-1414. Faure, G., and Kovach, J., 1969, The age of the Local replacement of cherty quartz (or Ayres, D. E., 1972, Genesis of iron-bearing min- Gunflint Iron Formation of the Animikie amorphous silica) by carbonate minerals erals in banded iron-formation mesobands Series in Ontario, Canada: Geol. Soc. was possibly facilitated by an unstable in the Dales Gorge Member, Hamersley America Bull., v. 80, p. 1725-1736. grain size when silica was poorly crystal- Group, Western Australia: Econ. Geology, Folk, R. L., 1959, Practical petrographic lized and still in a gellike state. Dissolution v. 67, p. 1214-1233. classification of limestones: Am. Assoc. Pe- of silica was aided by an increase in tem- Ayres, V. L., 1940, Mineral notes from the troleum Geologists Bull., v. 43, p. 1-38. perature accompanying burial or by a rise Michigan iron country: Am. Mineralogist, 1962, Spectral subdivision of limestone in pH of the interstitial fluid to >9. v. 25, p. 432-434. types, in Classification of carbonate rocks Barghoorn, E. S., and Tyler, S. A., 1965, Mi- — A symposium: Am. Assoc. Petroleum croorganisms from the Gunflint chert: Sci- Geologists Mem. 1, p. 62-84. ACKNOWLEDGMENTS ence, v. 147, p. 563-577. Forbes, W. C., 1969, Unit-cell parameters and Bence, A. E., and Albee, A. L., 1968, Empirical optical properties of talc on the join We thank A. E. Bence for helpful sugges- correction factors for the electron micro- Mg3Si4O10(OH)2-Fe3Si4O10(OH)2: Am. tions related to quantitative microprobe probe analysis of silicates and oxides: Jour. Mineralogist, v. 54, p. 1399-1408. Geology, v. 76, p. 382-403. analysis and D. H. Lindsley for agreeing to 1971, Iron content of talc in the system Blake, R. L., 1965, Iron phyllosilicates of the Mg Si 0 „(OH) -Fe Si 0 ,„(OH) : Jour. run samples for x-ray diffraction analysis. 3 4 2 3 4 2 Cuyuna district in Minnesota: Am. Geology, v. 79, p. 63-74. We also thank G. B. Morey for stimulating Mineralogist, v. 50, p. 148-169. French, B. M., 1968, Progressive contact discussions in the field. A. E. Bence, E. Dim- Blatt, H., Middleton, G., and Murray, R., 1972, metamorphism of the Biwabik Iron- roth, A. M. Goodwin, C. Klein, Jr., G. B. Origin of sedimentary rocks: Prentice-Hall, Formation, Mesabi Range, Minnesota: Morey, and W. Meyers critically reviewed 634 p. Minnesota Geol. Survey Bull., v. 45, 103 p. the manuscript. R. M. Noxin provided in- Bricker, O. P., Nesbitt, H. W., and Gunter, W. 1973, Mineral assemblages in diagenetic spirational guidance. This research was a D., 1973, The stability of talc: Am. and low-grade metamorphic iron- portion of Floran's Ph.D. thesis and was Mineralogist, v. 58, p. 64-72. formation: Econ. Geology, v. 68, p. supported by National Science Foundation Brindley, G. W., 1951, The crystal structure of 1063-1074. Grant No. A-012973. some chamosite minerals: Mineralog. Garrels, R. M., 1959, Rates of geochemical reac- Mag., v. 29, p. 502-522. tions at low temperatures and pressures, in Brindley, G. W., and Hang, P. T., 1973, The na- Abelson, P. H., ed., Researches in APPENDIX 1. IRON-SILICATE ture of garnierites; I, Structures, chemical geochemistry, Vol. 1: New York, John MICROPROBE ANALYSES compositions and color characteristics: Wiley & Sons, Inc., p. 25-37. Clays and Clay Minerals, v. 21, p. 27—40. Garrels, R. M., and Christ, C. L., 1965, Solu- In the ferrous-ferric iron silicates of iron for- Brindley, G. W., and Youell, R., 1953, Ferrous tions, minerals, and equilibria: New York, mations, no correction for ferric iron is required chamosite and ferric chamosite: Mineralog. Harper and Row, 450 p. 3+ of a microprobe analysis if all of the Fe was ox- Mag., v. 30, p. 57-70. Gill, J. E., 1924, Gunflint iron-bearing idized subsequent to mineral formation and no Broderick, T. M., 1920, Economic geology and formation, Ontario: Canada Geol. Survey iron was introduced from outside the system. stratigraphy of the Gunflint iron district, Summary Rept., pt. C, p. 28-88. Oxidation is believed to accompany dehydration Minnesota: Econ. Geology, v. 15, p. 1927, Origin of the Gunflint iron-bearing 2+ 3+ 2 of the type Fe + OH" -> Fe + 0 ~ + Vi H2. 422-452. formation: Econ. Geology, v. 22, p. Example: (a) greenalite (ideal unoxidized com- Brown, E. H., 1971, Phase relations of biotite 687-728. 2+ position): Fe6 Si 4010(OH) 8 —» partially oxidized and stilpnomelane in the greenschist facies: Goodwin, A. M., 1956, Facies relations in the 2+ 3+ greenalite with intermediate Fe /Fe and O/OH Contr. Mineralogy and Petrology, v. 31, p. Gunflint iron formation: Econ. Geology, v. 2+ ratios —> (b) "oxygreenalite" (all Fe converted 275-299. 51, p. 565-595. 3+ 3+ to Fe ): Fe6 Si4Ol6(OH)2. In (a) and (b) and all Brown, G., 1965, Significance of recent structure 1960, Gunflint iron formation of the intermediate compositions, mineral formulae of determinations of layer silicates for clay Whitefish Lake area: Ontario Dept. Mines greenalite probe analyses should be calculated on studies: Clay Minerals, v. 6, p. 73—82. [Ann. Rept.], v. 69, pt. 7, p. 41-63. the basis of 14 oxygen anions. This yields the Burt, D. M., 1J73, The system Fe-Si-C-O-H: A 1964, Geochemical studies at the Helen iron 2+ original Fe content. Normalization to 17 ox- model for metamorphosed iron formations: range: Econ. Geology, v. 59, p. 684-718. ygen anions in (b) is incorrect. In other minerals Carnegie Inst. Washington Year Book 71, 1973, Archean iron formations and tectonic such as magnetite and hematite, a ferric correc- p. 435-443. basins of the Canadian Shield: Econ. Geol- tion is necessary because of the extra oxygen(s) Cochrane, G. W., and Edwards, A. B., 1960, The ogy, v. 68, p. 915-933. required for charge balance. Microprobe Roper River oolitic ironstone formations: Gross, G. A., 1972, Primary features in cherty analyses of a hypothetical mineral of composi- Australia, Commonwealth Sei. and Indus. iron-formations: Sedimentary Geol., v. 7, p. tion FeO should theoretically be reported as Resources Organization, Mineragraphic. 241-261. 100.0 percent FeO, whereas analyses of magne- Inv. Tech. Paper no. 1, 28 p. Grout, F. F., and Theil, G. A., 1924, Notes on tite (FeO • Fe203) and hematite (Fe-O^ would Deer, W. A., Howie, R. A., and Zussman, J., stilpnomelane: Am. Mineralogist, v. 9, p. yield, respectively, 93.1 percent and 90.0 percent 1962, Rock-forming minerals. Vol. 3. Sheet 228-229. "FeO" using the Bence-Albee (1968) method. silicates: London, Longmans, 270 p. Gruner, J. W., 1936, The structure and chemical The apparent decrease in weight percent from Dimroth, E., 1968, Sedimentary textures, composition of greenalite: Am. Miner- FeO —> Fe304—» Fe203 is due to an increase in the diagenesis, and sedimentary environment of alogist, v. 21, p. 449-455. oxygen/total iron ratio. In end-member greena- certain Precambrian ironstones: Neues 1937, Composition and structure of lite, as well as other oxidized iron silicates, the Jahrb. Geologie u. Paläontologie Abh., v. stilpnomelane: Am. Mineralogist, v. 22, p.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/9/1169/3418213/i0016-7606-86-9-1169.pdf by guest on 28 September 2021 1190 FLORAN AND PAPIKE

912-925. Mineralogie, v. 39, p. 206-324. v. 261, p. 683-690. Gruner, J. W., 1944a, The structure of stilp- LaBerge, G. L., 1966a, Altered pyroclastic rocks 1963b, Synthetic solid solutions in the sys- nomelane re-examined: Am. Mineralogist, in iron-formation in the Hamersley range, tems MgC03-FeC03 and MnC03-FeC03: v. 29, p. 291-298. Western Australia: Econ. Geology, v. 61, p. Am. Mineralogist, v. 48, p. 1396-1400. 1944b, The composition and structure of 147-161. 1967, Subsolidus relations in the system minnesotaite, a common iron silicate in 1966b, Altered pyroclastic rocks in South CaC03-MgC03-FeC03 between 350° and iron-formations: Am. Mineralogist, v. 29, African iron-formation: Econ. Geology, v. 550°C: Am. Mineralogist, v. 52, p. p. 363-372. 61, p. 572-581. 787-796. 1946, The mineralogy and geology of the 1967a, Microfossils and Precambrian iron- Ross, C. S., and Smith, R. L., 1960, Ash-flow Mesabi Range: St. Paul, Minn., Office of formations: Geol. Soc. America Bull., v. 78, tuffs: Their origin, geologic relations, and the Commissioner of the Iron Range Re- p. 331-342. identifications: U.S. Geol. Survey Prof. sources and Rehabilitation, 127 p. 1967b, Evidence on the physical environ- Paper 366, 81 p. Hanson, G. N., and Malhotra, R., 1971, K-Ar ment of iron-formation deposition: East Ross, M., Smith, W. L., and Ashton, W. H., ages of mafic dikes and evidence for low- Lansing, Mich., 13th Ann. Inst, on Lake 1968, Triclinic talc and associated am- grade metamorphism in northeastern Min- Superior Geology Proc., p. 25. phiboles from Gouverneur mining district, nesota: Geol. Soc. America Bull., v. 82, p. Leith, C. K., 1903, The Mesabi iron-bearing dis- New York: Am. Mineralogist, v. 53, p. 1107-1114. trict of Minnesota: U.S. Geol. Survey Mon. 751-769. Hendricks, S. B., 1938, On the crystal structure 43, 316 p. Shannon, R. D., and Prewitt, C. T., 1969, Effec- of talc and pyrophyllite: Zeitschr. Kristal- Lepp, H., 1966, Chemical composition of the tive ionic radii in oxides and fluorides: Acta lographie, v. 99, p. 264-274. Biwabik Iron Formation, Minnesota: Econ. Cryst., B 25, p. 925-946. Hofmann, H. J., 1969, Stromatolites from the Geology, v. 61, p. 243-250. Sims, P. K., Morey, G. B., and Green, J. C., 1969, Proterozoic Animikie and Sibley Groups, Lindsley, D. H., Davis, B.T.C., and MacGregor, The potential for new mineral discoveries in Ontario: Canada Geol. Survey Paper I.D., 1964, Ferrosilite (FeSi03) synthesis at Minnesota, 30th Ann. Mining Symposium: 68-69, 77 p. high pressures and temperatures: Science, v. Minneapolis, Univ. Minnesota, p. 84—86. Hoste tier, P. B., and Christ, C. L., 1968, Studies 144, p. 73-74. Spry, A., 1969, Metamorphic textures: New in the system Mg0-Si02-C02-H20(l): The Lopes-Vieira, A., and Zussman, J., 1969, Further York, Pergamon Press, 350 p. activity-product constant of chrysotile: detail on the crystal structure of zussman- Steadman, R., and Youell, R. F., 1958, Mineral- Geochim. et Cosmochim. Acta, v. 32, p. ite: Mineralog. Mag., v. 37, p. 49-60. ogy and crystal structure of greenalite: Na- 485-497. Marsden, R. W., Emanuelson, J. W., Owens, J. ture, v. 181, p. 45. Huber, N. K., 1958, The environmental control S., Walker, N. E., and Werner, R. F., 1968, Tanton, T. L., 1923, Iron Formation at Gravel of sedimentary iron minerals: Econ. Geol- The Mesabi Iron Range, Minnesota, in Ore Lake, Thunder Bay district, Ontario: ogy, v. 53, p. 123-140. deposits in the United States, 1933-1967 Canada Geol. Survey Summary Rept., Pt. Hurley, P. M., Fairbairn, H. W., Pinson, W. H., (Graton-Sales volume) Vol. 1: New York, CI, p. 1-5. Hower, J., 1962, Unmetamorphosed min- Am. Inst. Mining, Metall. and Petroleum 1931, Fort William and Port Arthur, and erals in the Gunflint Formation used to test Engineers, p. 518-537. Thunder Cape map areas, Thunder Bay dis- the age of the Animikie: Jour. Geology, v. Mengel, J. T., 1963, The cherts of the Lake trict, Ontario: Canada Geol. Survey Mem. 70, p. 489-492. Superior iron-bearing formations [Ph.D. 167, 222 p. Hutton, C. O., 1938, The stilpnomelane group of thesis]: Oshkosh, Wisconsin Univ., 140 p. Trendall, A. F., and Blockley, J. G., 1970, The minerals: Mineralog. Mag., v. 25, p. 1965, Precambrian taconite iron-formation: iron formation of the Precambrian Hamers- 172-206. A special type of sandstone: Geol. Soc. ley Group, Western Australia, with special Iishi, K., and Saito, M., 1973, Synthesis of an- America Abs. for 1965, Spec. Paper 87, p. reference to the associated crocidolite: tigorite: Am. Mineralogist, v. 58, p. 106-107. Western Australia Geol. Survey Bull., v. 915-919. Misra, A., and Faure, G., 1970, Restudy of the 119, 336 p. Illing, L. V., 1954, Bahama calcareous sands: age of the Gunflint Formation of Ontario, Van Hise, C. R., and Leith, C. K., 1911, Geology Am. Assoc. Petroleum Geologists Bull., v. Canada: Geol. Soc. America Abs. for 1970, of the Lake Superior region: U.S. Geol. Sur- 38, p. 1-95. v. 2, p. 398. vey Mon. 52, 641 p. James, H. L., 1954, Sedimentary facies of iron- Moorhouse, W. W., 1960, Gunflint Iron Range Walter, M. R., 1972, A hot spring analog for the formation: Econ. Geology, v. 49, p. in the vicinity of Port Arthur: Ontario Dept. depositional environment of Precambrian 235-293. Mines [Ann. Rept.], v. 69, pt. 7, p. 1-40. iron formations of the Lake Superior re- 1966, Chemistry of the iron-rich sedimen- Morey, G. B., 1967, Stratigraphy and sedimen- gion: Econ. Geology, v. 67, p. 965-980. tary rocks: U.S. Geol. Survey Prof. Paper tology of the middle Precambrian Rove White, D. A., 1954, The stratigraphy and struc- 440-W, 61 p. Formation in northeastern Minnesota: ture of the Mesabi Range, Minnesota: Jolliffe, F., 1935, A study of greenalite: Am. Jour. Sed. Petrology, v. 37, p. 1154-1162. Minnesota Geol. Survey Bull., v. 38, 92 p. Mineralogist, v. 20, p. 405-425. 1973, Mesabi, Gunflint and Cuyuna ranges, Wolff, J. F., 1917, Recent geologic developments Klein, C., 1973, Changes in mineral assemblages Minnesota, in Proc. of the Kiev Symposium, on the Mesabi iron range, Minnesota: Am. with metamorphism of some banded Pre- 1970: Earth Sciences, v. 9, p. 193-208. Inst. Mining Engineers Trans., v. 56, p. cambrian iron-formations: Econ. Geology, Morey, G. B., Papike, J. J., Smith, R. W., and 142-169. v. 68, p. 1075-1088. Weiblen, P. W., 1972, Observations on the Youell, R. F., 1955, Mineralogy and crystal Krauskopf, K. B., 1956, Dissolution and precipi- contact metamorphism of the Biwabik structure of chamosite: Nature, v. 176, p. tation of silica at low temperatures: Iron-Formation, East Mesabi district, Min- 560-561. Geochim. et Cosmochim. Acta, v. 10, p. nesota: Geol. Soc. America Mem. 135, p. Zen, E-an, 1960, Metamorphism of the lower 1-26. 225-264. Paleozoic rocks in the vicinity of the 1959, The geochemistry of silica in sedimen- Nelson, B. W., and Roy, R., 1958, Synthesis of Taconic Range in west-central Vermont: tary environments, in Ireland, H. A., ed., the chlorites and their structural and chemi- Am. Mineralogist, v. 45, p. 129-175. Silica in sediments — A symposium: Soc. cal constitution: Am. Mineralogist, v. 43, p. Zoltai, T., 1965, Stereoscopic drawings of Econ. Paleontologists and Mineralogists 707-725. polyhedral mineral-structure models: Min- Spec. Pub., no. 7, p. 4-19. Perrault, G., 1955, Geology of the western mar- neapolis, Univ. Minnesota, 58 p. 1967, Introduction to geochemistry: New gin of the Labrador trough [Ph.D. thesis]: York, McGraw-Hill Book Co., 721 p. Toronto, Ont., Toronto Univ., 302 p. Kunze, G., 1956, Die gewellte Struktur des An- Rayner, J. H., and Brown, G., 1966, Triclinic tigorits, I: Zeitschr. Kristallographie, v. form of talc: Nature, v. 212, p. 1352-1353. MANUSCRIPT RECEIVED BY THE SOCIETY 108, p. 82-107. 1973, The crystal structure of talc: Clays DECEMBER 12, 1973. 1961, Antigorit, strukturtheoretische and Clay Minerals, v. 21, p. 103-114. REVISED MANUSCRIPT RECEIVED AUGUST 5, Grundlagen und ihre praktische Bedeutung Rosenberg, P. E., 1963a, Subsolidus relations in 1974 für die weitere Serpentinforschung: Forschr. the system CaC03-FeC03: Am. Jour. Sci., MANUSCRIPT ACCEPTED SEPTEMBER 10, 1974

Printed in U.S.A.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/9/1169/3418213/i0016-7606-86-9-1169.pdf by guest on 28 September 2021