Petrography of the Sokoman Iron Formation in Part of the Central Labrador Trough, Quebec, Canada

ERICH DIMROTH Geological Exploration Service, Department of Natural Resources, Quebec, P.Q., Canada JEAN-JACQUES CHAUVEL Institut de Géologie, Université de Rennes, 35031 Rennes-Cedex, France

ABSTRACT calcite are early diagenetic. Minnesotaite and stilpnomelane are late diagenetic minerals. The sedimentary textures of the Sokoman Riebeckite (and perhaps talc) formed during Iron Formation are similar to those of lime- the regional metamorphism. stones; therefore the classification of textural elements in limestone (Folk) can be applied to INTRODUCTION the iron formation. The authors recognized the Precambrian iron formations are ferriferous following textural elements: (a) femicrite (a cherts. The processes that occurred during their matrix of iron silicate and carbonate) and deposition are complex, and the rocks under- matrix chert, both analogous to micrite; (b) went complex alterations after their deposition. cement chert and carbonatic cements; (c) ag- Therefore the mineralogy and petrography of gregated particles, comparable to Folk's al- iron formations are far from simple. Never- lochems: pellets, intraclasts, ooliths, and theless, pétrographie studies may, under favor- pisolites. Shard textures are derived from able conditions, yield valuable information on ooliths and intraclasts by compaction. Rock the precipitation, deposition, and epigenesis of types are defined by the combination of these important rocks. For this reason, it ap- textural elements they contain. peared worthwhile to attempt a detailed The iron formation suffered extensive epi- analysis of the petrography of the Sokoman genetic alteration. Dessiccation, shrinkage of Iron Formation of the central Labrador trough. silica-gel, compaction, and cementation are The basis of our approach has been provided early diagenetic. Primocrystallization of quartz by the observation that the textures of iron concludes the early diagenetic stage. It leads formations are similar to those of limestone through a cryptocrystalline stage to the end (Dimroth, 1968). This has enabled us to apply phase of micropolygonal quartz. Quartz re- the principles of limestone petrology, as out- crystallized further during late diagenesis lined by Folk (1959, 1962), Bathurst (1971), (burial metamorphism?) and again during a and many others, to the study of the Labrador synkinematic to postkinematic regional meta- trough iron formation. Limestone can be con- morphism. ceived as being composed of a small number of Hematite dust is the oldest iron oxide. Much textural components. Textural rock types are of the microscopic magnetite and specularite defined by the combination of the textural formed during early diagenesis. Migration of components they contain. The analysis of the iron occurred; iron has been enriched in sedimentary textures permits conclusions on magnetite- or hematite-rich layers ("metallic" the depositional processes, and the analysis of layers) during early diagenesis. Renewed cry- the textural facies permits, within limits, the stallization of iron oxides occurred during the reconstruction of the depositional environment. regional metamorphism. Iron formations, like limestones, are meta- Microgranular siderite is perhaps primary. stable chemical systems at the time of their Porphyroblasts and glomero-porphyroblastic deposition, and their textures consequently concretions of siderite, ankerite, and ferriferous show the imprint of various diagenetic, meta-

Geological Society of America Bulletin, v. 84, p. 111-134, 13 figs., January 1973 111

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morphic, or superficial alterations. Not un- "granules" in the earlbr literature. Some commonly, several stages of epigenetic altera- authors noted that these particles had been tions took place in sequence. They can be sorted and transported by water (Bergeron, recognized because the epigenetic reactions 1954; Mengel, 1966), and LaBerge (1965) pro- rarely went to completion. Relict textures, posed an interpretation identical to ours. Other representing older evolutionary stages, therefore textures observed in iron formations are of are generally present, as well as younger over- volcanic origin (LaBerge, 1966), and micro- printing textures. This fact permits the estab- fossils (Barghoorn and Tyler, 1965; Cloud, lishment of a chronology of the epigenetic 1965; LaBerge, 1967; Hofmann, 1968; Hof- processes, and their chronology in turn serves mann and Jackson, 1.969) can also be regarded as a basis for speculations on the chemical as textural elements. I. S. Zajac (1970, written reactions that took place during the epigenesis. commun.), in co-operatior; with G. A. Gross Textures of limestones and iron formations of the Geological Survey of Canada, came to are similar, but they are not identical. In par- conclusions similar to ours concerning the ticular, the occurrence of shrinkage textures, nature and significance of some textural ele- evidence for agglutination and welding of ments, particularly intraclasts. Gross (1968) textural elements, formation of oôliths around observed cross-bedding, ripple marks, and also complex nuclei, and intense syn-sedimen- (1969, oral commun.) structures related to tary compaction and deformation of some slumping and compaction. Previous paleo- textural elements, are features not commonly geographic and environmental interpretations observed in limestone in this form. They are are based on mineralogical criteria (Gastil and therefore clear indications that the iron forma- Knowles, 1960; Clarke, 1967), on the presence tions are not limestones that were replaced by of ooliths, granules, and intraclastic con- chert and iron compounds after deposition, as glomerates (Bergeron, 1954; Mengel, 1966; had been suggested by Spencer (1971). Gross, 1968) and of sedimentary structures like It should be noted that the iron formation cross-bedding. described in this paper suffered, at most, meta- This Work morphism of the pumpellyite-prehnite facies (Baragar, 1967) Highly metamorphosed iron This paper is based on material of the formations also exist in the Labrador trough Sokoman Formation in part of the central (Gross, 1962, 1968); they have not been con- Labrador trough, northern Quebec, Canada. A sidered in this paper. total of 330 thin sections and a number of polished sections were studied under the Previous Work microscope; 172 of these were 5 cm X 7 "cm or Numerous papers have been written on the larger. Of the thin sections, 33 (15 large) were geological association, mineralogy, chemistry, stained in order to determine the carbonate and economic geology of iron formations. mineralogy. Numerous mineial determinations Among the many titles we might quote, the by x-ray diffractometry, differential thermal outstanding papers are by Van Hise and Leith analysis, thermogravimetric analysis, and (1911), Gill (1927), Gruner (1946), James chemical determinations of the Ca/Mg ratio (1954, 1966), Goodwin (1956), Lepp and of carbonates complement the optical observa- Goldich (1964), LaBerge (1965), Gross (1965), tions. Baragar (1967), Gross (1968), and and Trendall (1968). Dimroth (1970) outlined the general geology of the region. Most of the observations described below and many of our conclusions are not new. Mineralogical Types of Iron Formation Oôliths and pisolites, as well as larger intraclasts, have been described from the Lake James (1954, 1966) subdivided the iron Superior region and the Labrador trough formation into hematite, magnetite, silicate, (James, 1954; Perrault, 1955; Bergeron, 1954; carbonate, and sulfide facies in the Lake Goodwin, 1956; Bérard, 1965; Mengel, 1963, Superior region, and observed that only the 1966; Gross, 1962, 1965, 1968). Cayeux (1911) following types are commonly interlayered: described oolite fragments and oôliths that had hematite •+• magnetite; magnetite + silicate + formed around oôlith fragments, and drew carbonate; and silicate + carbonate + sul- analogies to iron-bearing rocks of Mesozoic age fide. Small intraclasts have been described as We will present evidence tha: magnetite is an

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epigenetic mineral in the Labrador trough. TABLE 1. TEXTURAL ELEMENTS OF LIMESTONES AND IRON FORMATIONS COMPARED Consequently, we distinguish between two primary mineralogical types: (1) a ferric fades Iron formation in which hematite dust is the oldest iron mineral and (2) a ferrous facies from which Femicrite hematite dust is absent. A mixed type contains Matrix chert Cement (spar) Cement chert siderite beds and lenses alternating with beds Calcite (spar) containing hematite dust. This type shows evidence of strong physico-chemical instability. Pellets Pellets It should be noted that the distinction be- Intraclasts Intraclasts tween ferric and ferrous facies refers to the Oolites OSlites initial composition before diagenesis. Generally Pisolites Pisolites the distinction between rocks of both facies is Fossils (skeletal fragments) Fossils (organic traces) Shards (rare) Shards (diverse origin) straightforward. Some rocks of the ferric facies, however, have suffered diagenetic re- After Folk (1962). duction, and their classification is based on relict textures. Rarely, relict textures have deposited as ooze. In other cases, intraclastic been destroyed entirely; such rocks cannot be rocks have an open fabric that requires matrix classified in either facies. support (Fig. 1, D, E). Matrixes predominantly composed ot iron SEDIMENTARY TEXTURES carbonate (siderite, ankerite) and iron silicate (minnesotaite, stilpnomelane), have collec- Textural Elements tively been named femicrite (Dimroth, 1968). The sedimentary textures of the investigated Alternation of carbonate and silicate femicrites, iron formations are similar to those of lime- and vertical gradations on a millimeter or centi- stone. The iron formations can be conceived to meter scale between both, prove that at least be composed of very few textural elements. two types of particles were precipitated: an iron Just as in limestone, we distinguish between carbonate (perhaps siderite), and an iron silicate unorganized materials, either deposited as fine- (mineralogy unknown). Cross-bedding occurs grained ooze (matrix) or crystallized in the in silicate and carbonate femicrite. Its presence pore space of the rock after deposition (ce- proves that femicrites were not very cohesive ment). Both are collectively called orthochems at the time of deposition; therefore, femicrites (Folk, 1959, 1962), because they have been were likely deposited as oozes of crystalline directly precipitated. Discrete aggregated minerals, not as gels. particles that were transported before their Finely distributed hematite dust is a pigment deposition are called allochems (Folk, 1959, of matrix chert in hematite iron formation 1962). The following textural elements have (Fig. 1, A, D). Matrix chert of the silicate- been observed in the iron formations: (1) carbonate iron formation is not pigmented, but orthochems: (a) microcrystalline siderite and contains small amounts of iron carbonate and iron silicate matrixes (femicrite), (b) matrix silicate. In all likelihood, matrix chert has been chert and (c) cement chert and cementing deposited as an ooze of silica-gel containing calcite (spar). (2) allochems: (a) pellets (in minor amounts of siderite or iron silicate (or distinctly bounded ovoid particles 0.2 mm both) in the ferrous facies. Matrix chert of the long), (b) intraclasts (redeposited fragments of ferric facies probably is derived from silica-gel penecontemporaneous sediment), (c) ooliths mixed with iron-oxide hydrate. and pisolites, and (d) shards (convex-concave CEMENTS. A cement has been precipitated bodies derived from intraclasts and oolites by in the pore space of the rock after deposition. strong compaction). The textural elements of Dapples (1967) described some characteristics iron formations and limestone are compared in of cements in sandstones, and Bathurst (1971) Table 1. summarized the characteristics of cements in Orthochems. FEMICRITE, MATRIX CHERT. limestones. Similar properties permit recogni- Many beds of the iron formation are devoid of tion of cements in the iron formation. internal textures except for bedding lamina- The writers observed the following types of tions (Fig. 1, A, B). Such beds are analogous to cement: (1) Chalcedony oriented perpendic- micritic limestone and have obviously been ular to allochem boundaries (Figs. 2E, 3C) is

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rare. (2) Quartz with columnar impingement cherts; small, indistinct patches of very fine- textures is quite common (Figs. 2G, 3E). (3) grained quartz, set in the slightly coarser Very commonly, allochem grains contain grained matrix, indicate their presence. shrinkage cracks; their filling is indistinguish- The long axes of pellets are aligned parallel to able from the chert in the pore space between bedding, which indicates that these bodies the allochem grains (Fig. 3D). The chert has formed before or during the compaction. Their evidently been introduced after the shrinkage, origin is not certain. Dimroth (1968) sug- and therefore after the deposition of the gested that pellets formed by aggregation of allochems. (4) Intraclasts are surrounded by a silica-gel drops in sea water, but it is also fringe of chalcedony, whereas ferriferous calcite possible that they originated in the sediment fills the remaining pore space (Figs. 2F, 3A). by diffusive segregation. (5) Ooliths are surrounded by a fringe of INTRACLASTS. Intraclasts are transported bladed ankerite and by minnesotaite; quartz and redeposited fragments of penecontempora- with impingement textures fills the remaining neous sediment. Intraclasts of centimeter size pore space (Fig. 3B). (Fig. 1, C, D, E) commonly show laminations, In type 3, cement and allochems crystallized or contain pellets, ooliths, or intraclasts. simultaneously. The writers conclude that the Smaller intraclasts (Figs. IF, 2B, and 2E) are cement has been introduced before the cry- devoid of internal textures that would dem- stallization of quartz in the allochems. Cement onstrate their origin directly. nucleated on the allochem surface in types 1, Small intraclasts have been described as 2, 4, and 5. In these cases, the allochem grains "granules" in the earlier literature. We suggest likely were crystalline at the time of cementa- that this term be abandoned, because it also tion. The same considerations apply to the designates the grain size class between 2 and 4 textures of the filling of voids, and of veins. mm; it should only be used in the latter con- Cement chert, recognized by one of the notation. Small limestone intraclasts, devoid of criteria described above, is relatively devoid of internal textures, are commonly called ovoids hematite dust in the ferric facies, and is there- or peloids (Bathurst, 1971). These names are fore readily distinguished from matrix chert. preferable if a special term is required. On the other hand, it is rarely possible to Goodwin (1956), among others, suggested distinguish cement chert and matrix chert in that peloids ( = "granules") formed in place the ferrous facies. Recrystallization destroys the during diagenesis. There are numerous objec- characteristics of cement chert. tions against this hypothesis in the case of the Allochems. PELLETS. Pellets are elliptical peloids in the rocks studied: (1) Textural rela- or eye-shaped bodies of rather uniform size tions between peloids and cement chert prove (long axis ~0.2 mm); they commonly have that peloids were deposited as discrete par- somewhat irregular outlines. Pellets are loosely ticles. (2) Cross-bedding and graded bedding strewn in matrix chert, against which they are not uncommon in peloid-bearing rocks show gradual boundaries (Fig. 2 A). In the ferric (Bergeron, 1954; Mengel, 1963, 1966); peloids facies, pellets contain less hematite dust than are commonly well sorted and their long axes does the surrounding matrix; therefore, they are in some cases aligned, probably parallel to stand out clearly under the microscope. Pellets the current direction (Bergeron, 1954). Grain are not easily observed in ferrous matrix size analysis of graded bedding gives con- < vincing evidence of sorting (Fig. 4A): the median grain size of ooliths decreased little Figure 1. A. Laminated hematite-bearing matrix toward the top of the graded bed, because only chert. Note strong syn-sedimentary deformation. Bar a limited grain size spectrum was available. In = 1 cm. Section F 2-4, Lac le Fer. B. Laminated contrast, the median size of peloids decreased femicrite. Note pull-apart. Bar = 1 cm. Section K 10-1, drastically. The ratio between ooliths and Knob Lake. C. Cemented intraclastic conglomerate. peloids correspondingly increased upward, and Intraclasts were quite hard at time of deposition. Bar = the sorting index (inclusive graphic standard 1 cm. Section F 1-7, Lac le Fer. D. Intraclastic matrix deviation ai) decreased. Similar relations also chert. Bar = 1 cm. Section F 2-16, Lac le Fer. E. exist in graded beds devoid of ooliths (compare Intrafemicrite. Note lamination in intraclast. Bar = Fig. IF). (4) Mengel (1963, 1966) compared 1 cm. Section B 13-3, Lac de la Concession. F. Closely spaced intraclasts of sand and grit size. Note graded the mean grain sizes of peloids and sand grains bedding. Bar = 1 cm. Section G 3-4, Lac Gillespie. and found both to be hydrodynamically

equivalent. (5) The sorting index of peloids is a function of the mean grain size, as in allo- chemical limestones and in sandstones (Fig. 4B). There can be no doubt that the peloids of the Sokoman Formation were deposited as discrete particles, and that they were transported and sorted before their deposition. On the other hand, it is more difficult to prove that the peloids formed by fragmentation of penecontemporaneous sediment. They could perhaps have formed by accretive processes, al- though this seems to be unlikely. However, a complete spectrum exists from small peloids to intraclasts of centimeter size; no basis exists for an objective separation of both. Rocks containing intraclasts of centimeter size also contain peloids, and grade laterally into peloid- bearing rocks without centimeter-sized intraclasts. Therefore, the writers feel that peloids and large intraclasts belong to one population of objects, that they have a common origin, Figure 3. Cement textures. A. Cementation first by and that their separation would be artificial. fibrous quartz, followed by precipitation of calcite. B. Relict texture showing bladed ankerite and min- LaBerge (1965), was the first to suggest that nesotaite tables at the surface of ooliths, followed by peloids ( = granules) originated by fragmenta- precipitation of quartz with columnar impingement tion of penecontemporaneous sediment. texture. Ooliths replaced by ankerite, hematite, and Intraclasts generally have not been deformed stilpnomelane. C. Cementation by fibrous chalcedony. much during the compaction of the rock; there- D. Microcrystalline quartz cement. E. Quartz with fore they must have been relatively hard at the columnar impingement texture. time of their deposition. Occasionally we have observed intraclasts that contained stylolites; OOLITHS AND PISOLITES. Ooliths (Fig. 2D) such intraclasts are derived from sediment that are composed of a nucleus and a concentrically had been lithified. Only in exceptional cases laminated skin. Pisolites are large ooliths with have intraclasts been compacted to an extreme exactly the same internal structure. degree, and form now what we call accom- Any object may serve as nucleus of the modation shards (Fig. 2C). ooliths: an intraclast, a fragment of an oolith, or a fragment of oolitic or intraclastic iron formation, or even clastic grains of zircon. Intraclasts are by far the most common. The Figure 2. A. Pellets in hematite matrix chert. 700 X. intraclastic nuclei contain numerous irregular Section C2-S1, Lac de la Concession. B. Shrinkage shrinkage cracks. A thick concentric shrinkage cracks in sand-sized intraclast. Bar = 0.2 mm. Section crack generally separates nucleus and skin, and H 10-6, Lac Helluva. C. Accommodation shards. Note indicates that shrinkage of the nucleus ex- transition in intraclastic texture (top) from which the shards are derived by compaction. 50 X. Section R ceeded shrinkage of the skin. Shrinkage cracks 2-5, Ritchie Lake. D. Ooliths and small intraclasts. in the skin form radial and concentric systems; Bar = 0.2 mm. Section C2-S1, Lac de la Concession. the radial cracks generally widen toward the E. Chalcedony cementing small intraclasts. Crossed nucleus. nicols. 140 x. Section He 1-2, Lac Luche. F. Cementa- Double and triple ooliths are not uncom- tion of intraclasts begun by precipitation of fibrous mon. They apparently formed by agglutina- chalcedony (rim around intraclasts) followed by calcite. tion of several ooliths. Ooliths with complex 140 X. Section K 5-2, Squaw Lake. G. Cement quartz growing with columnar impingement texture perpen- internal structure formed around fragments of dicular to allochem boundaries. Crossed nicols. 50 x. oolitic or intraclastic iron formation. Section D 26-20, Lac Hematite. H. Void of unknown The distribution of oolitic rocks suggests that origin filled by chalcedony (rim) and coarse-grained ooliths formed in shallow, agitated water by quartz (center). Bar = 0.2 mm. Section C 2-11, Lac de accretion of silica-gel and iron-oxide hydrate at la Concession. the surface of any object that was kept in

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Figure 4. A. Left: composition of a cemented oolitic intraclastic iron formation with graded bedding. Per- centage of (].) intraclasts, (2) oolith fragments, (.!) complex ooliths, (4) 2.0- B simple oolitl.s, and (5) total allo- chemii. Right: corresponding median + grain sizes M (in millimeters) and sorting (inclusive graphic standard deviation) a\ (in 0-units). (1) intraclasts, (2) all ooliths, (3) total allochems. 1.0- B. Relations between sorting ("inclusive graphic standard deviation"

suspension. Ooliths and pisolites are composed tions. Most shard textures are poorly pre- of cherty quartz and hematite dust. Specular served. However, in some thin sections, shard hematite, magnetite, iron carbonates, and iron textures grade vertically into well-preserved silicates have replacement textures relative to intraclastic or oolitic textuies (Fig. 2C). Such the delicate laminations of the oolith skins observations prove the derivation of the shards (Fig. 1 IE). Consequently, we infer that ooliths in our rocks from ooliths or intraclasts by com- preserved by these minerals are derived from paction. We have never observed volcanic chert-hematite ooliths. Primary silicate or car- shards, as described by LaBerge (1966). bonate ooliths are absent from the Sokoman CLASTIC COMPONENTS. Clastic components, Formation, and the ferrous facies is devoid of notably quartz, alkali feldspar, tourmaline, oolitic rocks. zircon, and chlorite have been observed in SHARDS. We use the term "shard" des- many thin sections. Some beds of the iron criptively for more or less closely welded con- formation, in parts of the area studied, are fer- vex-concave bodies, without genetic connota- riferous sandstones or siltstones. Clastic grains

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of quartz and feldspar never show the slightest all hematite iron formations therefore fall on trace of etching, in contrast to the clastic the face matrix chert-cement chert-allochems. grains in calcareous rocks. In silicate-carbonate iron formations, on the other hand, cement chert and matrix chert Textural Rock Types are indistinguishable in all but exceptional Folk (1959, 1962) proposed a classification of cases and have therefore to be treated as one textural limestone types analogous to the component for practical reasons. Consequently quantitative classification of clastic rocks. A the silicate-carbonate iron formations have to first-order subdivision is based on the propor- be shown in a diagram with femicrite, allo- tion of matrix, cement, and allochem grains, a chems, and combined cement chert + matrix second-order subdivision on the nature of the chert as end members. Proposed subdivisions allochems. Iron formations are much more are shown in Figure 5 (in analogy to the lime- complex systems; Dimroth (1968) therefore stone classification of Folk, 1959, 1962). A proposed a first-order classification based on the second-order classification may be based on the proportion of the following four end members: nature of the allochem, and a third-order femicrite, matrix chert, cement chert, and classification can be based on their mechanical allochems—pictured in a tetrahedron. Only properties (soft, firm), on provenance (mono- two faces of the tetrahedron are occupied: mictic, polymictic), and on bedding properties. femicrite is absent in hematite iron formations; Shard textures should be classified with the

SAND GRAINS ALLOCHEM GRAINS

MATRIX (MICRITE)

CLASTIC ROCKS LIMESTONES

ALLOCHEM GRAINS ALLOCHEM GRAINS

M1CR0CRYSTALLINE MINNESOTAITE- MATRIX CHERT SIDERITE MATRIX (FEMICRITE) Figure 5. Proposed classification of the iron forma- clastic rocks. In analogy to Folk (1959, 1962). tion compared with the classification of limestones and

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intraclastic or oolitic categories, wherever their depos tion of allochem grains

origin can be established. {¡ntraclasts and ooliths) To designate all possible variations, Folk introduced a very practical system of abbreviated names. We use only the analogous terms "femicrite" and "intrafemicrite" (Dimroth, dcissiccation cracks 1968). The first- and second-order subdivisions of the iron formations are listed in Table 2.

Recrystallization destroys the sedimentary cementation by textures. However, some relict textures are generally preserved even in highly altered silicagel rocks, and permit at least approximate classifi- J / cation. Recrystallized matrix cherts, femicrites, primociystallization of quartz and intrafemicrites generally stand out clearly. Recrystallized conglomeratic rocks are also distinct in most cases. Cement textures, on the

other hand, are very sensitive to recrystalliza- cementation tion. Therefore, the types 3 and 5 of Table 2 are indistinguishable in many cases, parti- by chalcedony cularly in the ferrous facies. Rocks of class 7 and columnar quartz appear to be derived for the most part from fine- to medium-grained intraclastic rocks. cemented and crystalline cherts EPIGENESIS Figure 6. Lithifaction of the iron formation. Epigenetic textures overprint the sedimentary textures of the iron formation. The sequence of crystallization of the various epigenetic textures can be described as due to minerals. lithifaction (dessiccation, compaction, and Lithifaction cementation), and to crystallization and recrystallization (including reactions between Hematite-bearing cherts show remarkably crystalline phases). The processes occurring well-preserved textures that developed during during lithifaction are schematically repre- the earliest diagenetic stages. Early diagenetic sented in Figure 6, and Figure 12 illustrates the textures are less conspicaous in silicate-car-

TABLE 2. TEXTURAl ROCK TYPES OF THE S0K0MAN IRON FORMATION

Type Textura! components Ferrous subtypes Ferric subtypes

1. Matrix chert Matrix chert, Silicatic and carbonatic matrix Laminated or ribboned matrix chert pellets cherts. Always ribboned 2. Femicrite Femicrite Silicate and carbonate femicrite Absent 3. Intraclastic or Intraclasts, ooliths, Generally indistinguishable from Dolitic *nd intraclastic types. oolitic matrix pisolites, matrix type 5. Ooliths absent Further subdivision possible accord- chert chert ing to size of intraclasts and ooliths->isol1tes, mechanical properties of allochems, bedding properties, and so forth 4. Intrafemicrite Intraclasts, femicrite Silicate and carbonate intra- Absent femicrites. Further subdivision according to grain size 5. Cemented intra- Intraclasts, ooliths, Silicate and carbonate facies. Subtypes as in type 3 clastic or pisolites, cement Further subdivision according to oolitic cherts grain size. Generally indistinguishable from type 3. Ooliths absent 6. Shard-bearing Shards (compacted ? Should be included with types 3 or cherts ooliths and/or 5 wherevor possible intraclasts) 7. Recrystallized ? Massive silicate-carbonate-bearing Massive cherts without internal cherts cherts. Generally derived from textures. Patches with hematite types 3 or 5 dust coraionly present. Carbonate and some silicate commonly present. Hay be confounded with ferrous -acies ii; hematite dust completely destroyed. Generally derived from "ypes 3 or 5

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bonate iron formation. The early diagenetic chalcedony (called "quartzine" in Europe) oc- textures were produced by the following curs in intraclasts and is probably a variety of processes: desiccation of silica-gel leads to primocrystalline quartz. shrinkage and to the opening of shrinkage Primocrystalline quartz consists of irregu- cracks (Fig. 2B). Compaction causes flattening larly shaped, interpenetrating amoeboid cry- of components (Fig. 2C), and differential com- stals generally less than 0.01 mm across (Fig. paction causes distortion of bedding planes 7A). Its grain size is fairly constant within (Figs. 8, A and B; 10). During this stage the small domains but varies considerably within rocks were cemented. Occasionally voids and between beds. According to Voll (1960) formed and were later filled with a cement. and Spry (1969), the irregular grain boundaries Desiccation cracks intersect intraclasts and of primocrystalline quartz are not stable. oóliths, but not the cement—chert indis- Consequently, this quartz generation is thought tinguishable from the cement fills them. Vari- to represent a metastable state that has been ous processes, such as desiccation(P), differ- preserved because grain growth has been ar- ential compaction, and tectonic deformation, rested before stable grain shapes were attained. gave rise to later generations of cracks. They Replacement of chalcedony by primocrys- are filled by chert, quartz with columnar talline quartz is also common. We have ob- textures, ankerite or calcite, or magnetite. served all gradations between well-preserved Voids formed in some rocks. Some voids are chalcedony (Fig. 2E), through a primocrystal- probably derived from gas bubbles, others rep- line quartz fabric in which radial optical ori- resent dissolved intraclasts, some are of un- entation is clearly displayed (Fig. 2H) to known origin (Fig. 2H). They were filled from primocrystalline quartz fabrics with barely the outside by quartz, magnetite, iron silicate, perceptible radial optical preferred orientation. or iron carbonate, or by a combination of Length-slow chalcedony ("quartzine") (Fig. these minerals. 7B) forms spherulites 0.04 mm across, com- Desiccation cracks occur also in femicrite and monly grouped as cauliflower- or grape-shaped in intraclasts of the silicate-carbonate iron aggregates. The spherulites commonly nu- formation. They are difficult to observe in cleated at a crystal of iron ore. According to silicatic and carbonatic cherts, because these Pittman and Folk (1971), length-slow chal- rocks lack the pigmentation that traces delicate cedony forms by replacement of gypsum, or textures in the ferric facies. by crystallization of quartz in an evaporitic en- Flattening of textural elements (Fig. 2C), vironment. Therefore, its presence is of great and deformation of quartz veins (Fig. 7G) ecological importance. permit estimation of the degree to which iron Micropolygonal quartz (Fig. 7C) consists of formations have been compacted. Strongly well-shaped polygons of very constant size compacted beds not uncommonly grade (~0.03 mm), with straight boundaries meeting laterally into material that lacks compaction in triple points. According to Voll (1960) and (Figs. 8A and 10); they also contain "con- Spry (1969), this represents an equilibrium cretionary" patches that lack compaction, and shape. Therefore, it is likely that micropoly- that contain perfectly preserved sedimentary gonal quartz represents the mature end stage of textures (Fig. 8B). The "concretions" and the the primocrystallization of quartz. Hematite uncompacted parts of the beds shown in dust has characteristically been dissolved at the Figures 8A and 11 probably were cemented be- boundaries of micropolygonal quartz. This fact fore the compaction process began. Such ob- permits recognition of the former presence of servations prove that cementation began in micropolygonal quartz in some recrystallized patches of the rock, and that the processes of cherts. cementation and compaction overlap to a large Recrystallized quartz (Fig. 7D) is char- degree. acterized by strongly variable grain size rang- ing up to 1 mm. The grains are generally Epigenesis of Quartz irregularly polygonal; grain boundaries may be Three generations of quartz have been dis- slightly curved and somewhat irregular but can tinguished in the investigated rocks: primo- also be papillary. crystalline quartz, micropolygonal quartz, and Sizes and shapes of primocrystalline and of recrystallized quartz. Normal chalcedony has recrystallized quartz depend in part on the been observed as a cement. Length-slow presence of inclusions, in particular of hematite

dust (Fig. 7D). Therefore chert patches (for called "specularite" in the following pages, in example, intraclasts) rich in hematite dust are order to distinguish it clearly from hematite generally finer grained than are the chert dust); (3) magnetite that has formed octahedra patches which are poor in hematite dust. The about 0.003 mm across; (4) skeletal crystals of inclusions apparently inhibit grain growth of "reticulated" magnetite and/or hematite quartz, perhaps as a result of surface tension which are interstitial to quartz. effects. Such effects play a great role in the re- Hematite dust traces delicate sedimentary crystallization of quartzites (Voll, 1960). and early diagenetic textures, whereas specular Textures of cementing chert suggest that the hematite (Fig. 11E) and magnetite (Fig. 8C) primocrystallization of quartz began before the show replacement textures relative to the cementation was complete. Primocrystalline delicate sedimentary and shrinkage textures of and micropolygonal quartz occur in flat-lying, ooliths, intraclasts, and matrix cherts. There- unmetamorphosed iron formations, which sug- fore, hematite dust is unequivocally the oldest gests that both quartz generations formed be- iron oxide; it probably crystallized from iron- fore the folding and the regional meta- oxide hydrate that was mixed with silica-gel morphism. Recrystallized quartz has also been in the matrix cherts and allochem grains of the observed in flat-lying, unmetamorphosed iron ferric facies. Specular hematite and magnetite, formations. Consequently, some recrystalliza- on the other hand, are evidently epigenetic tion must have occurred at a late diagenetic minerals. stage, perhaps related to slight increase of Unambiguous evidence proves that hematite temperature during burial (burial metamor- dust has in part been removed from matrix phism). On the other hand, iron formations cherts and from allochem grains during dia- commonly show significant recrystallization genesis. Very commonly, hematite dust is con- close to zones of strong folding, imbrication, centrated in the skins of allochem grains, at the and brecciation. Therefore it is concluded that surface of shrinkage cracks, and at the surface renewed recrystallization of quartz took place of matrix chert laminae (Fig. 9). Relict textures during the deformation and the regional meta- leave no doubt that these materials were morphism. originally homogeneous. These observations suggest that iron emigrated in part from the Epigenesis of Iron Oxides interiors of allochem grains and also from the The investigated rocks contain four "pri- interior of some laminae of matrix chert, at a mary" forms of iron oxide: (1) hematite dust time when these were still relatively hydrated. (diameter <0.001 mm) that has imparted the Their skins, on the other hand, must have red color to jasper; (2) microcrystalline suffered dehydration at a relatively early time, hematite as idiomorphic crystals generally and this may be the reason why they remained about 0.03 mm long but that may reach 1 mm comparatively unaffected. in exceptional cases (this type of hematite is Hematite dust has in part also been removed during later epigenetic stages. It is absent in seams at the grain boundaries of micropolyg- Figure 7. A. Primocrystalline quartz in matrix onal quartz (Fig. 7C). These seams may be chert growing perpendicular to magnetite crystal. quite wide, leaving only a rounded patch of 350 X. Section 13-3-8, Myrtle Lake. B. Globular aggregates of length-slow chalcedony. Note crystals of hematite dust in the center of the micro- iron oxide in center of aggregates. Bar = 0.1 mm. polygonal grains. There is no doubt therefore Section He 1-5, Lac Luche. C. Micropolygonal quartz. that hematite dust has in part been destroyed Bar = 0.1 mm. Section C2-25, Lac de la Concession. during the growth of micropolygonal quartz. D. Recrystallized quartz. Note that grain sizes and Specular hematite and magnetite occur in shapes are partly determined by the presence of hematite four associations described below in sequence. dust. Crossed nicols. Bar = 0.1 mm. Section H 2-U, 1. Small porphyroblasts of magnetite and Lac Helluva. E. Stylolites bounding intraclasts in minnesotaite intrafemicrite. Bar = 1 cm. Section C2- hematite are dispersed in matrix cherts of the 17C, Lac de la Concession. F. Minnesotaite nucleating ferric facies. Isolated dispersed magnetite at a stylolite. Bar = 0.2 mm. Section H 2-11, Lac porphyroblasts are common in matrix cherts Helluva. G. Chert veins in laminated matrix chert and in femicrites of the ferrous facies. Primo- folded during compaction. 140 X. SectionR 2-5, Ritchie crystalline quartz (Fig. 7A) and length-slow Lake. H. Lozangic pseudomorphs (after gypsum?). chalcedony (Fig. 7B) nucleated at the surface 700 x. Section H 2-13, Lac Helluva. of these crystals, around which they form an

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/1/111/3443119/i0016-7606-84-1-111.pdf by guest on 03 October 2021 Figure 8. A. Differential compaction related to the aliochem grains. Bar = 0.2 mm. Section B 13-10, Lac evolution of a metallic layer. Compare with Figure 10. de la Concession. D. Microgratndar siderite embedded Bar = 1 cm. Section C2-S1, Lac de la Concession. B. in chert. Bar = 0.01 mm. Section A 16-4, Lac de la Differential compaction around a concretionary spot Concession. E. Outlines of aliochem grains in an that suffered early lithifaction. Bar = 1 cm. Section ankerite concretion. Bar =• 0.2 min. Section A 1-8, Lac C2-S2, Lac de la Concession. C. Magnetite replacing Apollon.

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G 3-4 occurs (for example, Fig. 8B ; but note that the matrix chert laminae shown in Fig. 8A have not been replaced by iron oxides). Compacted oôlith textures (Fig. 10, details), and differential compaction phenomena observed at lateral gradations from "metallic" laminae into normal hematite iron formation (Figs. 8, A and B; 10) prove that the metallic laminae were compacted to about 45 percent of their depositional thickness or less. "Concretionary" spots of the normal iron formation in metallic layers (Figs. 8, A and B; 10) contain well- preserved, uncompacted, oôlith textures (compare Fig. 2D, showing detail from the oolitic part of the thin section presented as Fig. 8A) ; it appears likely that these "concretionary" spots suffered early cementation and dehydration. The writers' observations therefore sub- stantiate the conclusions of Duff and others (1967) and of Trendall and Blockley (1970). Figure 9. Leaching of iron from intraclasts in the We furthermore conclude that iron was pre- hematite iron formation. Note that hematite dust is con- dominantly concentrated in intraclastic and centrated at the skins of intraclasts and at the borders of oolitic beds before their cementation. shrinkage cracks. At base: iron has been leached from the center of a bed of pellet-bearing matrix chert. The magnetite laminae and layers of the silicate-carbonate iron formation are lenticular, oriented overgrowth. Therefore we conclude and also appear to show replacement relations that this generation of iron oxides formed relative to delicate sedimentary laminations. before the primocrystallization of quartz. They are associated with carbonate laminae, 2. Specular hematite or magnetite (or both) not with intraclastic beds; this fact suggests are concentrated in so-called "metallic" that they formed by partial oxidation of laminae and layers of the ferric facies. "Metal- bivalent iron in the carbonates, as inferred by lic" laminae and layers of the ferrous facies LaBerge (1965). Little is known about their consist of magnetite. The "metallic" laminae age, except that they behaved as competent and layers are as much as 5 cm thick, and gen- laminae during the deformation; therefore, erally contain more than 50 percent iron oxides. they are not metamorphic as seems to be the Duff and others (1967) suggested that they case in the Lake Superior iron formation (La- form by diagenetic unmixing because they con- Berge, 1965), but diagenetic. tain oolitic relict textures. Trendall and 3. Specularite (Fig. 11E) and magnetite Blockley (1970) described differential com- (Fig. 8C) replace oôliths and intraclasts. paction phenomena that suggest early dia- Reticulated magnetite and hematite is as- genetic iron concentration in "metallic" sociated with this form of replacement. Not mesobands. uncommonly, the replacement affects diffuse The Labrador trough iron formation contains irregularly bounded spots and zones, often compelling evidence that the "metallic" related to cracks, to cross-cutting veins filled laminae and layers of the ferric facies formed by with magnetite or hematite (or both), and to early diagenetic replacement. The layers and solution sutures. Compaction phenomena are laminae grade vertically and horizontally (Figs. not associated with this type of replacement. 8A, 10) into the normal iron formation with Such textures are common in flat-lying, un- perfectly preserved relict textures. Poorly pre- metamorphosed iron formations which proves served shard textures, derived from compacted that they are at least in part late diagenetic. It is oôliths and intraclasts are very common in possible that renewed crystallization of iron "metallic" layers (for example, Fig. 10) and oxides took place during the deformation and leave no doubt that the metallic layers pre- the regional metamorphism. ferentially replaced oolitic and intraclastic 4. Isolated porphyroblasts of specularite and materials; replacement of matrix chert also magnetite and isolated patches of both minerals

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1 mm

Figure 10. An oolitic and intraclastic bed (intra- "metallic" material (dark) are largely destroyed. Relict clasts = i) has been partly replaced by specularite and textures (enlarged detail drawings) prove its derivation magnetite. Sediment textures are well preserved and from oolitic and intraclastic chert, and strong com- lack evidence of compaction in the unreplaced material paction (o = relict oolith). Section C2-S1, Lac de la (light, compare Fig. 2D). Sediment textures of the Concession.

replacing intraclasts are common in many re- phase of crystallization is indicated by the crystallized iron formations. Some of these are fact that strongly folded, imbricated, and associated with sedimentary carbonate lentils. brecciated rocks commonly are also strongly re- Recrystallized rocks of this type also occur crystallized. stratigraphically below members composed of The rocks investigated by the writers con- carbonate iron formation. There is little doubt tain irrefutable evidence of iron migration. In that the crystalline iron oxides of these rocks hematite iron formations, iron has preferenti- formed after the consolidation of the rocks. aLy been enriched in some oolitic and intra- Such rocks are locally flat-lying and unmeta- clastic beds before their cementation. Iron has morphosed; therefore, this generation of iron preferentially been removed from the cores of oxides is in part late diagenetic. intraclasts, and from certain beds of matrix In summary, our observations prove that chert. Our observations suggest (hat the move- magnetite and specularite formed repeatedly. ment of iron is related to deferential per- Much of both minerals is very early diagenetic. meability; iron has apparently been concen- Renewed crystallization of iron oxides took trated in the most permeable portions of the place during late diagenesis; presence of a third rock; it has been removed from relatively hy-

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1cm

0.5mm 1mm Figure 11. A. Irregular ankeritization (ank) of Concession. C. Ankerite rim (a) around quartz-filled siderite femicrite (fm). Laminations in the siderite (qz) patch and crack in siderite femicrite (stippled). femicrite have been destroyed in the coarse-grained Section A 16-4, Lac de la Concession. D. A calcite (cc) ankeritized material. Note that ankeritization occurred concretion with relict oolith textures is rimmed by before the syn-sedimentary micro-faulting. Section He ankerite (a). Section H 2-6, Hematite Lake. E. Stilpno- 1-8, Lac Luche. B. Ankerite (a, left) and siderite (s, melane (s), ankerite (a), and hematite replaced an right) porphyroblasts set in siderite femicrite (stippled). oolith. Section C2-13, Lac de la Concession. Note quartz (qz) in ankerite. Section A 16-4, Lac de la drated (that is, still relatively permeable) writers note the absence of microgranular matrix cherts; materials that suffered early ankerite or calcite with reservations, consider- dehydration and cementation, and which ing the small number of thin sections in which therefore were quite impermeable, were little the mineralogy of fine-grained carbonates has affected. been studied. Its grain size is less than 0.005 mm. The grains are more or less spherical with Epigenesis of Carbonates possible indications of crystal facets (Fig. 8D). Siderite, ankerite, and a calcite with high The microgranular grains are embedded in magnesia and variable iron contents were cherty quartz. Microgranular siderite has been determined by staining, x-ray diffraction, dif- observed in femicrites and in the intraclasts of ferential thermal analysis, thermogravimetric intrafemicrites. It could represent an original analysis, and chemical analysis. Carbonate min- grain size, as noted by LaBerge (1965). erals occur in the three forms described by Recrystallized siderite, ankerite, and calcite LaBerge (1965): minute grains (microgranular), form idioblastic and granoblastic crystals of idioblastic and granoblastic crystals (recrys- strongly variable size (~0.04 to 10 mm). tallized), and glomeroporphyroblastic con- Siderite and ankerite porphyroblasts replace cretionary aggregates. the microgranular siderite fabric (Fig. 1 IB). Siderite is the only carbonate preserved in Evidence for ankeritization of microgranular microgranular form in our thin sections; the siderite is common, and there is also some

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evidence for ankeritization of siderite por- (1954), and Gross (1968) described greenalite phyroblasts. Not uncommonly, fine laminations from the central Labrador trough. One of the in microgranular siderite disappear at the writers (Dimroth, 1968; found extremely fine contact against a coarsely crystalline ankerite grains of a green mineril in one thin section, fabric (Fig. 11 A). Ankerite rims around small and believed it to be greenalite; their quantity quartz patches set in a fabric of microgranular is too small to permit identification by x-ray siderite (Fig. 11C) also suggest ankeritization. diffractometry. None of aur other thin sections Some siderite porphyroblasts have been re- contains greenalite; apparently this mineral placed by ankerite and hematite. Ankerite is has a limited distribution. also common as filling of small cracks, which Minnesotaite forms sheaflike or radiating suggests that it is relatively late compared to aggregates; it is not oriented parallel to bed- the other carbonate minerals. Porphyroblastic ding. Its grain is relatively fine in femicrites carbonates have been dissolved at stylolites. (table length <0.01. mm), whereas minnesotaite This observation proves that porphyroblastic in matrix chert is relatively coarse grained carbonates are relatively early diagenetic. (length of tables up to 0.2 mm). In intra- Glomeroporphyritic concretionary carbon- femicrites, minnesotaite is coarse grained in the ate aggregates as much as up to several centi- matrix, fine grained in int raclasts. meters across are common. They consist of Stilpnomelane generally has crystal shapes calcite, siderite, or ankerite. Calcite and and sizes similar to minnesotaite, but thick siderite concretions have ankerite rims (Fig. tabular crystals also occut. Riebeckite needles 11D). Intraclastic and oolitic textures have form a felt on shear planes and cleavages, and been preserved in the concretions even where also occur distributed through the rock. the surrounding material has been recrystal- Riebeckite asbestos (crocvdolite) fills tension lized. It follows that the concretions formed at joints. Talc, occurs as minute tablets generally an early diagenetic stage. oriented subparallel to a cleavage. Minnesotaite and stilpnomelane are present Epigenesis of Silicates in flat-lying, completely undeformed and un- Minnesotaite, stilpnomelane, iron chlorite metamorphosed iron formations. This indi- (chamosite, 5001 = 14A), riebeckite, and talc cates that both minerals formed during the have been determined by x-ray diffraction diagenesis. Minnesotaite nucleated at stylolites methods (Table 3). Perrault (1955), Bergeron (Fig. 7F), suggesting a relatively late diagenetic

TABLE 3. SOME IDENTIFICATIONS OF SILICATE MINERALS

Minnesotaite Green chlorite Stilpnomelane Talc

"Magnesian chamosite" H2-13 ASTM card H2-13 ASTM card He2-3 ASTM card R2-8 ASTM card

di dl d A dl dl dl dl dl

9.50 9.53 14.1 14 13.3 9.32 9.34 4.756 4.77 7.1 7.1 12.03 11.90 4.66 4.66 3.183 3.18 4.716 4.70 6.06 4.57 4.55 2.75 3.558 3.53 4.74 3.511 3.510 2.642 2.65 2.837 2.83 4.¿5 3.411 3.43 2.523 2.52 2.720 2.69 4.036 4.04 3.110 3.116 2.40 2.612 2.61 3.56 2.892 2.31 3.34 2.629 2.199 2.21 3.025 3.03 2.597 2.595 2.837 2.82 2.479 2.476 2.72 2.70 2.336 2.335 2.572 2.55 2.214 2.212 2.51 2.196 2.196 2.427 2.42 2.122 2.348 2.34 2.101 2.103 2.19 1.930 2.11 1.868 1.870 1.9E 1.725 1.886 1.89 1.682 1.682 1.558 1.557 1.526 1.527 1.509 1.509 1.460 1.406 1.336 1.336 1.318 1.318

Samples decarbonated with diluted HC1. philips diffractometer, Nl-flltered cuKa radiation.

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origin. However, we also observed rocks in mation, except for a little hematite or mag- which minnesotaite is oriented parallel to a netite. cleavage; therefore, no doubt exists that min- Stylolites formed after the primocrystalliza- nesotaite was stable, and could perhaps grow, tion of quartz. Their development is con- during the deformation and metamorphism of temporaneous with or prior to the crystalliza- the rocks. Stilpnomelane not infrequently oc- tion of minnesotaite (Fig. 7F) and follows the curs in spots, several millimeters or centi- growth of carbonate porphyroblasts. In some meters across, in minnesotaite-bearing rocks. In cases, stylolitization is related to unequal com- this case, it apparently replaced minnesotaite. paction. It is likely that stylolites formed in Riebeckite is commonly associated with car- rocks that had already crystallized at a time bonate in hematite iron formations that have when compaction and cementation still went been sheared, breccia ted, and recrystallized; on in domains a few centimeters or decimeters therefore it is clearly a metamorphic mineral, away. At this time, porosity remained and as noted by Gross (1968). Grubb (1971) ob- fluid escape was still possible when the stylolites served replacement of stilpnomelane by rie- formed; pressure solution consequently could beckite in Australian iron formations; both be the source of much cementing silica. Park minerals occur in different facies in the in- and Schot (1968) came to similar conclusions vestigated rocks. Talc is generally oriented regarding stylolites in limestones. parallel to a cleavage and therefore^also is a metamorphic mineral. Chlorite (14 À chamo- Supergene Alterations site) is associated with clastic components. The iron formation suffered considerable alteration by descending meteoric waters in the Other Minerals vicinity of soft iron-ore bodies of Mesozoic Irregularly bounded and prismatic crystals age. The alterations have been discussed by of apatite occur in a number of thin sections. Stubbins and others (1961) and by Gross They are unrelated to clastic components. (1968) from whom the following descriptions Lozenge-shaped pseudomorphs of microcrys- are in part abstracted. Three stages of intensity talline quartz (Fig. 7H) are present in one thin of the supergene alterations can be distin- section. The pseudomorphs have the habit of guished: oxidation of minerals, leaching of gypsum. Length-slow chalcedony, also an in- silica, and the evolution of enriched iron ores. dicator of evaporitic conditions, occurs at the Minerals containing bivalent iron are oxi- same locality, but not in the same thin section. dized at many localities in the south-central Finally, we note the presence of graphite in Labrador trough. Oxidation reactions convert some sections of the silicate-carbonate iron siderite to goethite, iron silicates to powdery formation. hematite and goethite, and magnetite to maghemite and martite. Silica is dissolved in Stylolites highly altered rocks. Complete or nearly com- Stylolitization is common in the ferrous and plete dissolution of silica leaves vuggy and ferric facies of the iron formation. James (1954) porous leached iron ore. Crystallization of described stylolites in silicate-carbonate iron powdery hematite of spherulitic hematite and formations. particularly of goethite in the pores of leached Stylolites are of the rectangular, pointed, rocks results in the evolution of enriched iron seismogram, and sutured types in the geo- ores. metric classification of Park and Schot (1968). Gross (1968) suggested that the supergene Most stylolites are subparallel to bedding, but alterations occurred during the Mesozoic in a transverse stylolites have also been observed. tropical climate. Occasionally, we have observed stylolite net- works approximately parallel to bedding. Chronology of the Epigenetic Alterations Stylolites have been observed at the contacts In a broad sense, one can distinguish between between chert and femicrite (siderite and min- five stages of epigenetic alteration (Fig. 12): nesotaite femicrite), as well as within beds com- (1) pre-cementation; (2) older than the primo- posed predominantly of chert, siderite, or min- crystallization of quartz; (3) younger than the nesotaite. We have observed some stylolitic primocrystallization of quartz (late diagenetic); seams between femicrite intraclasts. Seams of (4) syntectonic or post-tectonic metamorphic; insolubles do not form at stylolites in iron for- and (5) supergene (Mesozoic). Cementation

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silicagel precursor silicate

I • speculante

quartzine crystalline chalcedony

micropolygonal quartz reaction with Al-silicates

recrystallized stilpnomelane quartz /

recrystallized A F recrystallized recrystallized specula ri te magnetite ,iebe,:ki,e quartz siderite silicates

recrystallized specularite maghenite magnetite quartz and \ I martite "limoni te" Figure 12. Epigenetic processes in the Sokoman Iron Formation. and primocrystallization of quartz did not following mineral combinations, apparently proceed uniformly; both processes appear to in stable coexistence: hematite + magnetite + overlap. ankerite + calcite; magnetite + siderite + Most of the migration of iron and of the ankerite. The stability relations between the crystallization of coarse-grained iron oxides ap- silicate minerals are unknown, except that parently occurred before and during cementa- riebeckite is absent frcm flat-lying, unmeta- tion. Some of the coarse-grained iron oxides, morphosed iron formation. however, appear to postdate cementation, and The reactions that gave rise to the growth of are possibly metamorphic. Much of the seriate magnetite and that caused migration of iron are recrystallization of quartz took place before probably complex. Certain stratigraphic units the deformation, but local recrystallization at of the Sokoman Formation were originally later stages occurred as well. Siderite, ankerite, composed of hematite iron formation alter- and calcite apparently crystallized and re- nating with beds and lenses of siderite femi- crystallized for the most part during early crite. These members are strongly recrystal- diagenetic stages (pre-primocrystalline); min- lized and consist now mainly of magnetite and nesotaite and stilpnomelane, however, appear iron carbonate (mainly ankerite). It appears to be essentially late diagenetic. Some recrys- likely that magnetite formed by reaction be- tallization of these minerals occurred obvi- tween siderite and hematite (or iron-oxide ously during later diagenesis and during tne hydrate) according to metamorphism. Riebeckite and talc are metamorphic minerals. Fe203 + FeCOs — Fe304 + C02. (1) It should be noted that :.ron has not been con- Stability Relations and Epigenetic Reactions centrated in bedding-parallel "metallic" layers The stability relations between the iron during this reaction. oxides and carbonates appear to be clear in the Magnetite probably formed by oxidation of investigated rocks. The writers observed the siderite according to reaction 2 in silicate-

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carbonate iron formations, as noted by La- Little is known about reactions involving the Berge (1965): silicates. Silicate femicrites have bedding properties different from those of matrix cherts. It 6FeC02 + 02 -> 2Fe304 + 6C02 . (2) is therefore likely that minnesotaite of femi- Infiltration of slightly oxygenated ground water crites and of intrafemicrites formed from a into the sediment shortly after deposition crystalline iron silicate of unknown mineralogy would lead to the evolution of metallic layers and did not form by reaction between iron along the most permeable bedding planes. compounds and silica-gel. Stilpnomelane not More complex oxidation reactions involve uncommonly appears to be pseudomorphous ankeritization of siderite and can yield hematite after minnesotaite, a relation suggesting that it as a product. may have formed by reactions between min- Reduction of hematite dust to magnetite in nesotaite and aluminous silicates (clay min- the ferric facies and migration of iron are less erals?). Hematite ooliths have occasionally readily explained. Perry and Tan (1973) sug- been replaced by minnesotaite and stilpno- gested that part of the magnetite may have melane; in this case, both minerals developed formed by reactions between trivalent iron and perhaps by reduction of ferric iron and sub- organic matter. This concept is a valid one. sequent reaction with silica. Minnesotaite and High carbon contents in shales are good evi- stilpnomelane porphyroblasts in the hematite dence that organic life was abundant at the iron formation may have originated in the time of the deposition of the Sokoman Forma- same way. Talc and riebeckite occur in car- tion. Some organic matter should necessarily bonate-bearing rocks, which suggests that they have been present at the time of deposition. may have formed by reactions between mag- Furthermore, indirect oxidation of organic nesian and ferriferous carbonates with silica. matter distributed throughout the rock by On the whole, reactions between iron oxides or oxygen supplied into the most permeable beds carbonates and silica do not appear to be very could provide energy for the migration of iron important during the epigenesis of the Soko- described above. man Formation. According to this hypothesis, iron has been reduced in the relatively impermeable beds CONCLUSIONS after reaction 3: Precipitation and Deposition 4Fe(OH) + C + 8H+ 4Fe++ (3) 3 It has been noted above that at least four + co2 + 10H20 . types of particles have been precipitated from Following the concentration gradient, bi- sea water: siderite, an iron silicate of unknown valent iron migrated into the relatively mineralogy ("precursor silicate"), silica-gel, permeable beds, where it has been oxidized and silica-gel with adsorbed iron-oxide hy- according to reactions 4 or 5: drate. According to Castano and Garrels (1950), Krumbein and Garrels (1952), and 4Fe++ + 80H~ + 02 2Fe203 + 4H20 . (4) Huber (1958), iron-oxide hydrate is precipitated under oxidizing conditions. Siderite is 6Fe++ + 120H- + 0 -> 2Fe 0 + 6H 0 . (5) 2 3 4 2 precipitated if the electron potential is below These reactions would necessarily lead to the the "hematite-siderite fence." Precipitation of enrichment of iron in the most permeable parts siderite furthermore depends on the carbon- of the rock. Small traces of carbon (~0.2 per- dioxide content of sea water. The physico- cent) would be sufficient to cause the transport chemical conditions of the precipitation of the of significant amounts of iron. The reactions "precursor silicate" are of course unknown. would proceed until the supply of either However, silicate femicrite is closely associated oxygen, carbon, or trivalent iron was ex- with siderite femicrite; both must be assumed hausted. In case trivalent iron and carbon to have formed under very similar conditions. remained in the donor region when the Iron and silica were co-precipitated as mixed oxygen supply was exhausted, reactions analo- gels under oxidizing conditions. However, they gous to reaction 3 could give rise to the forma- were in part precipitated in form of separate tion of siderite or ankerite concretions or particles under reducing conditions. porphyroblasts. Occasionally these have been The precipitated particles accumulated as oxidized again at a later time. femicrites and as matrix cherts. Pellets are in-

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variably present in matrix chert of the ferric and pisolites were transported and deposited to facies, and perhaps also in the ferrous facies. It form second-cycle rocks. Repetition of these follows that pellets either aggregated neces- processes leads to the evolution of intraclasts sarily during the precipitation and deposition and ooliths with complex internal textures. of silica-gel, or formed consistently during the Schematically, one can visualize the processes earliest stage of the diagenesis of matrix chert during the precipitation and deposition of the by diffusive separation. Sokoman Iron Forrration as in Figure 13. The first-cycle rocks (femicrite and matrix chert) were fragmented to form intraclasts. Epigenesis Ooliths and pisolites accreted at the surface of It appears chat the larger part of the epi- objects that were kept in suspension in a genetic alterations occurred shortly after turbulent environment. The intraclasts, oôliths, deposition. Reactions that occurred after the

Silicate-Carbonate Iron Formations Hematite Iron Formations SEA WATER

PRECIPITATION PRECIPITATION

(REDUCING ENVIRONMENT) (OXIDIZING ENVIRONNEMENT}

silicagel silicagel drops

drops adsorbed iron oxide hydrate

aggregation

pellets

First Matrix chert - Matrix chert cycle rocks

IN TRABASIN EROSION

femicrite intraclasts intraclasts

(silicate and carbonate) (chert) aggregation

silicagel

oolith1 s

Second Intrafemicrite Intracfastic matrix chert IntracJastic and oolitic matric chert cycle Cemented intraclastic chert Cemented intraclastic end oolitic chert rocks

INTRABASIN ERC SION

Complex

intraclasts - ' aggregation of

silicagel \ complex

ooliths Third Cherts with complex ' cycle Figure 13. Deposition of the Sokoman Iron Formation. allochem textuns rocks

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complete primocrystallization of quartz played Bathurst, R.G.C., 1971, Carbonate sediments and on the whole a subordinate role. their diagenesis; Developments in sedi- Usually, the iron has been enriched and mentology, Vol. 12: Amsterdam, London, New leached in alternating patches and veins one or York, Elsevier, 620 p. a few centimeters across. This observation may Bérard, J., 1965, Géologie de la région du lac Bérard, Nouveau-Québec: Quebec Dept. indicate that the early diagenetic reactions took Natural Resources Geology Rept. Ill, 172 p. place in a multiplicity of small interlocking Bergeron, R., 1954, A study of the Quebec- subsystems which were essentially closed to the Labrador iron belt between Derry Lake and exchange of iron, but were open to exchange of Larch River [D.Sc. thesis]: Quebec, Laval - O2, CO2, H+, OH , and, to some degree, of Univ., 230 p. silica. Castano, J. R., and Garrels, R. M., 1950, Experi- The epigenetic reactions lead on the whole to ments on the deposition of iron with special a reduction of the hematite iron formation and reference to the Clinton iron ore deposits: to an oxidation of the silicate-carbonate iron Econ. Geology, v. 45, p. 755-770. Cayeux, L., 1911, Comparaison entre les minerais de formation. In transitional types, reaction be- fer huroniens des Etats-Unis et les minerais tween siderite and hematite may have been de fer oolithiques de France: Acad. Sei. the leading process. In general, however, it ap- Comptes Rendus, v. 153, p. 1188-1190. pears likely that the redox reactions and migra- Clarke, P. J., 1967, Gras Lake-Félix area, Saguenay tions are complex and involve reduction of County: Quebec Dept. Nat. Resources trivalent iron by organic matter in some parts Geology Rept. 129, 66 p. of the rock, oxidation of bivalent iron by Cloud, P. E., Jr., 1965, Significance of the Gun- atmospheric oxygen in others, and diffusive flint (Precambrian) microflora: Science, v. 149, transfer of iron between both domains. p. 27-35. Dapples, E. C., 1967, Diagenesis of sandstones, in ACKNOWLEDGMENTS Larsen, G., and Chilingar, G. V., eds., Dia- genesis in sediments: Amsterdam, Elsevier, p. Gert Ott and Bruno Sarrat ably assisted the 91-126. writers in the field. I. S. Zajac, of the Hanna Dimroth, E., 1968, Sedimentary textures, dia- Mining Company, Ironton, Minnesota, kindly genesis, and sedimentary environment of cer- gave advance information on the part of his tain Precambrian ironstones: Neues Jahrb. work concerning the lowermost member of the Geologie u. Paläontologie Abh., v. 130, p. 247- Sokoman Formation northwest of Scheffer- 274. ville. Our work at Schefferville would have 1970, Evolution of the Labrador geosyncline: been impossible without the kind hospitality of Geol. Soc. America Bull., v. 81, p. 2717-2742. Iron Ore Company of Canada, Schefferville, Duff, P. M„ Hallam, A., and Walton, E. K„ 1967, Cyclic sedimentation, in Developments in P.Q. Travel expenses were supported by a sedimentology, Vol. 10: Amsterdam, Elsevier, grant of the National Research Council of 280 p. Canada (to Chauvel), by funds of the Depart- Folk, R. L., 1959, Practical petrographical classifi- ment of Geology, University of Rennes, and cation of limestones: Am. Assoc. Petroleum by funds of the Governments of France and Geologists Bull., v. 43, p. 1-38. Quebec. 1962, Spectral subdivision of limestone types, A. M. Goodwin and F. W. Beales reviewed in Classification of carbonate rocks: Am. Assoc. Petroleum Geologists Mem. 1, p. 62-84. the manuscript, and suggested numerous im- Gastil, G., and Knowles, D. M„ 1960, Geology of provements. The authors accept responsibility the Wabush Lake area, southwestern Labrador for any shortcomings that remain. We wish to and eastern Quebec, Canada: Geol. Soc. acknowledge our debt of gratitude to Robert America Bull., v. 71, p. 1243-1254. Bergeron who helped throughout this project; Gill, J. E., 1927, Origin of the Gunflint iron-bear- doubtlessly, the work would not have been ing formation: Econ. Geology, v. 22, p. 687- concluded without his encouragement. 728. Goodwin, A. M., 1956, Facies relations in the Gun- flint iron formation: Econ. Geology, v. 51, p. REFERENCES CITED 565-595. Baragar, W.R.A., 1967, Wakuach Lake area, Gross, G. A., 1962, Iron deposits near Ungava Bay, Quebec-Labrador: Canada Geol. Survey Mem. Quebec: Canada Geol. Survey Bull. 82, 48 p. 344, 174 p 1965, Geology of iron deposits in Canada. Barghoorn, E. S., and Tyler, S. A., 1965, Micro- Vol. I. General geology and evaluation of iron organisms from the Gunflint chert: Science, v. deposits: Canada Geol. Survey Econ. Geology 147, p. 563-577. Rept. no. 22, 165 p.

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