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Redox and nonredox reactions of magnetite and hematite in rocks

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Abstract

Redox and nonredox reactions causing pseudomorphic replacement of hematite by magnetite and magnetite by hematite are compared.Pseudomorphic replacements resulting from redox reactions are known as martitization [replacement of magnetite by hematite due to oxidation; reaction (1)] and mushketovitization [replacement of hematite by magnetite due to reduction; reaction (2)]. These two replacements cause characteristic ore textures and volume changes (reaction (1): increase of 1.66%; reaction (2): decrease of 1.64%). These small volume changes are the reason that martitization and mushketovitization are widespread in many rocks under condition, however, that oxidizing or reducing fluids (solutions) are present.The same initial and end products may also be involved in nonredox reactions. Reaction (3) is the replacement of hematite by magnetite due to simple addition of Fe2+ atoms under basic conditions. This reaction causes an increase of the volume of 47.6%. Reaction (4), causing a volume decrease of 32.2%, is the replacement of magnetite by hematite due to leaching of Fe2+ atoms under acidic conditions. From these volume changes it is concluded that reaction (4) may occur in many rock types, whereas reaction (3) is restricted to unlithified sediments only. However, ore textures caused by nonredox reactions are unknown and therefore their occurrence in rocks is hypothetical.
Redox and nonredox reactions of iron oxides in rocks
Arno Mücke and Alexandre Raphael Cabral
With 2 plates
MÜCKE, A. & CABRAL, A. R. (200x): Redox and nonredox reactions of iron oxides in
rocks.- N. Jb. Miner. Mh.
Abstract: Redox and nonredox reactions causing pseudomorphic replacement of hematite by
magnetite and magnetite by hematite were compared.
Pseudomorphic replacements resulting from redox reactions are known as martitization
(replacement of magnetite by hematite due to oxidation; reaction 1) and mushketovitization (=
replacement of hematite by magnetite due to reduction; reaction 2). These two replacements
cause characteristic ore textures and volume changes (reaction 1: increase of 1.66%; reaction
2: decrease of 1.64%). These small volume changes are the reason that martitization and
mushketovitization are widespread in many rocks under condition, however, that oxidizing or
reducing fluids (solutions) are present.
The same initial and end products may also be involved in nonredox reactions. The first
reaction is the replacement of magnetite by hematite (1) due to leaching of Fe2+ atoms under
acidic conditions. The second and reverse reaction causes the replacement of hematite by
magnetite (2) due to simple addition of Fe2+atoms under basic conditions. Reaction (1) causes
a volume decrease of 32.2% and reaction (2) an increase of 47.6%. From these volume
changes it is concluded that the first replacement may occur in many rock types, whereas the
second is restricted to unlithified sediments only. Ore textures caused by nonredox reactions
are unknown and therefore their occurrence in rocks is hypothetical.
Key-words: redox/nonredox reactions, replacement of magnetite by hematite and vice versa,
volume changes, ore textures.
Introduction
Stimulated by the recent paper of OHMOTO (2003) who discussed nonredox reactions for the
conversion of iron oxides, a critical and brief review and comparison of nonredox and redox
reactions in rocks will be undertaken. The evaluation will be based on volume changes caused
by the conversions of iron oxides and microscopic studies of the resulting ore textures.
2
In nearly all rock types where iron oxides occur, mutual replacement of magnetite and
hematite are widespread. In banded iron-formations the age relationships of hematite and
magnetite and their mutual replacement have intensively been debated. The reason for this
discussion is the existence of two concepts about the development of iron-formations. One
concept supports the idea that the iron oxides of iron-formations originated from direct
precipitation or that they are of early diagenetic origin resulting from precipitated ferric
hydroxide (designated as precipitation model by MÜCKE et al. 1996). In this model hematite
is the early mineral followed by magnetite which replaced the first (e. g. HUBER, 1959;
FLORAN & PAPIKE, 1978; MORRIS, 1980, 1993, 2002; HAN, 1982; BAUR et al. 1985;
HARMSWORTH et al., 1990; JAMES, 1992; KLEIN & BEUKES, 1993; TOMPKINS &
COWAN, 2001). The other concept favours the idea that magnetite is the early iron oxide (e.
g. HACKSPACHER, 1979; QUADE, 1988; ROSIÈRE et al., 2001) formed from mudstone-
like and iron-rich sedimentary protoliths under metamorphic conditions (MÜCKE &
ANNOR, 1993; MÜCKE et al., 1996, referred to as metamorphic model; MÜCKE, 2003).
Hematite is a younger mineral and was formed at the expence of magnetite due to oxidation.
Nonredox reactions
OHMOTO,s reaction is the following: Fe3+2O3 (= hematite) + Fe2+ + H2O = Fe2+Fe3+2O4 (=
magnetite) + 2H+. The forward reaction is the conversion of hematite to magnetite by simple
addition of Fe2+ ions, whereas the reverse reaction is the conversion of magnetite to hematite
by leaching of Fe2+ in an acidic environment. From the explanation of the two reactions, it can
be concluded that they are necessarily pseudomorphic replacements. According to OHMOTO
(2003), the forward reaction (= replacement of hematite by magnetite) may occur in many (if
not all) iron-formations. Hematite, being of early precipitation origin and constituent of the
sedimentary protoliths, converted subsequently to magnetite during diagenesis using Fe2+
originated from submarine exhalations. The reverse reaction (= replacement of magnetite by
hematite) was considered by OHMOTO in the deeply burried iron- formation of the
Australian Hamersley Basin.
The reactions of OHMOTO are not volume-constant. Magnetite has a cell volume of 592.704
Å3 which contains 8 formula units Fe2+Fe3+2O4. Hematite, on the other hand, has a cell volume
of 301.279 Å3 containing 6 formula units Fe3+2O3. From these data and the above-mentioned
reverse reaction (having an iron ratio of the initial and the final products of 3 : 2), it can be
calculated that the replacement of magnetite by hematite causes a volume decrease of 32.22%.
3
This value was also discussed by OHMOTO (2003) and from this he concluded a significant
increase of the rock permeability, thus facilitating water-rock interaction. The forward
reaction (hematite to magnetite; having an iron ratio of the initial and the final products of 2 :
3), causes an increase in volume of 47.55%. Microscopic characteristics that originated from
nonredox reactions were not described by OHMOTO (2003).
Evaluating his own reactions which are indirectly based on experimental evidence, OHMOTO
(2003) stated that the replacements of hematite by magnetite, and magnetite by hematite, are
not restricted to iron-formations only. As an example, OHMOTO mentioned that many
mangnetite-skarn deposits show the conversion of early hematite to magnetite and magnetite
to late hematite; and he continues that these conversions may also have occurred without the
involvement of an oxidant or reductant. OHMOTO concluded that nonredox reactions may
have been the principle mechanism for the transformation of iron oxides in nature, especially
in hydrothermal environments. Therefore, he put forward the reactions as a model.
Redox reactions
Pseudomorphic replacements of magnetite to hematite and vice versa resulting from redox
reactions are:
1. 2Fe2+Fe3+2O4 (= magnetite) + 0.5O2 = 3Fe3+2O3 (= hematite); and
2. 3Fe3+2O3 (= hematite) + H2 = 2Fe2+Fe3+2O4 (= magnetite) + H2O
The first reaction involves the pseudomorphic replacement of magnetite to hematite due to
oxidation which is known as martitization. The oxidation reaction follows the (111)-planes of
primary magnetite and the resulting arrangement of the newly-formed hematite is known as
martite-textured hematite (Plate 1A). Incomplete martitization of magnetite leads to the same
texture, but newly-formed hematite contains relics of magnetite (Plate 1B). Martitization may
also be developed along grain boundaries of, or patchy-like within, magnetite. This
conversion is known as strain-martitization (Plate 1C; BAUMANN & LEEDER, 1991). The
above-mentioned textures are often obliterated, particularly in iron-formations, due to
subsequent metamorphism which causes the recrystallization of martite-textured hematite into
coarse-grained hematite or specularite. As an indication that they originated from early
magnetite, both coarse-grained hematite and specularite may contain relics of martite-textured
hematite (Plates 1D and 2A) and subordinately magnetite. In rare cases, magnetite may also
4
be directly replaced by hematite without martitization (Plate 2B). The second reaction is the
pseudomorphic replacement of primary hematite by magnetite under reducing conditions.
This replacement is known as mushketovitization. Incomplete reduction leads to magnetite
pseudomorphs after hematite in which remnants of hematite are preserved (Plate 2C).
In both redox reactions the amount of the iron atoms remains constant, there is only an
addition or removal of oxygen. From the cell volumes of magnetite (592.704 Å3) and hematite
(301.279 Å3) and the amount of Fe atoms in them (24 in magnetite and 12 in hematite), it can
be calculated that the replacement of magnetite by hematite causes a volume increase of
1.66% and that of hematite by magnetite a decrease of 1.64%.
Discussion
Generally, the volumes of the initial and the final products of pseudomorphic replacements
should have nearly the same or slightly smaller volumes. However, the volumes of the final
products may also be larger, but the enlargment cannot exceed a defined value which may be
definitely lower than 5%. Theoretically, smaller volumes of the final products may have no
limitation, but practically the decrease should not be bigger than about 30%. This value was
deduced from the reaction 2FeOOH = Fe2O3 + H2O. The forward reaction concerns the
abundantly distributed pseudomorphic replacement of goethite by hematite due to dehydration
and causes a volume decrease of 27.42%. The reaction is irreversible because the reverse
reaction, the rehydration of hematite causing an increase of 37.78% is never observed in rocks
(PALACHE et al., 1944; MÜCKE, 1994, page 369-370; Plate 2D). In agreement with the
observation, LANGMUIR (1971) demonstrated the impossibility of the rehydration of
hematite on theoretical grounds.
The volume changes caused by the widely accepted redox reactions, showing only small
differences in the volumes of the initial and final products (not higher than 2 %), may explain
the abundance of martitization and the formation of mushketovite under the assumption that
the environmental conditions for the reactions 1 and 2 are fulfilled. In contrast to redox
reactions, nonredox reactions are connected with considerable volume changes. Under the
assumption that the reaction of magnetite to hematite took place, from the considerable
volume decrease of 32.2 % it is concluded that the newly-formed hematite should have a
porous appearance and/or abundant shrinkage cracks.
5
The other nonredox reaction causes the replacement of hematite by magnetite and an increase
of the volume of 47.55%. This value shows that the proposed reaction is unrealistic. In order
to explain the impossibility of this reaction, a rock that contains 50 vol% hematite should be
considered. After conversion of the preexisting hematite to magnetite, 1 m3 of this rock
increases to a volume of 1.24 m3. However, in sediments, like the protolith of iron-formations
(which was considered by OHMOTO) such a reaction may possibly occur, because these
rocks are unlithified. If such rocks increased their volumes, the excess volume has simply to
displace seawater.
However, a widely distributed pseudomorphic replacement is known in many rocks including
rocks of iron-formations, to which a nonredox reaction can be applied. This concerns the
replacement of magnetite by goethite (Plate 2D). The reaction was hitherto considered to be
the following: 2Fe2+Fe3+2O4 (= magnetite) + 3H2O + 0.5O2 = 6FeOOH (= goethite). The
postulated reaction is based on the oxidation and hydrolization of preexisting magnetite
without any removal or addition of Fe. Goethite has a cell-volume of 138.37 Å3 which
contains 4 Fe-atoms, whereas in the cell volume of magnetite, which is 592.70 Å3, 24 Fe
atoms are included. Therefore, six cell volumes of goethite must be compared to one cell
volume of magnetite, showing that the reaction causes an increase of the volume of 40.07%.
Due to this high volume increase, the reaction cannot be realized in rocks. Therefore the
correct reaction may be nonredoxic in the sense of OHMOTO as follows: Fe2+Fe3+2O4 + 2H+ =
2FeOOH + Fe2+. The reaction taking place in an acidic environment and by the leaching of
Fe2+-ions causes a decrease of the volume of 6.62%.
Conclusions
The ore textures of mutual pseudomorphic replacements of magnetite and hematite formed
under redox conditions are well-known and therefore they can easily be recognized in
reflected light. The volumes of the initial and final products are nearly of the same size
(deviations smaller than 2 vol%). The small deviations are the prerequisites for their
abundance in many rocks. Among all replacements of metallic minerals, martitization is the
most widespread one. The reducing reaction (= mushketovitization) causing the replacement
of hematite by magnetite is by far not as common as the first and occurs in contact with
intrusions, particularly if they are sulphide-bearing. In rocks of iron-formations,
6
mushketovitization is generally absent. As exceptions, in small proportions it was recently
observed in some drill hole samples of the Hamersley Basin (Dales Gorge, Joffre and Mara
Mamba Units; TOMPKINS & COWAN, 2001).
Although both, redox and nonredox reactions involve the same initial and final products, the
reactions cannot be simply compared. They are not comparable insofar as they are realized
due to an addition or removal of either oxygen or Fe2+ ions only. These exchanges occur
under oxidizing/reducing (redox reactions) or under basic/acidic (nonredox reactions)
conditions and are, respectively reponsible for the volumes remaining nearly constant or
changing drastically. As a consequence of this, the results of the two reaction types have to
cause different ore textures that, of course, must be distinguishable in reflected light.
However, textures other than those originating from redox reactions are unknown.
Acknowledgement
We are indepted to Sharon WEBB for her careful reading of the manuscript.
References
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Received:
Authors, addresses:
A. MÜCKE, Experimentelle und Angewandte Mineralogie, Göttinger Zentrum
Geowissenschaften, Georg-August-Universität, Goldschmidtstrasse 1, 37077 Göttingen,
Germany; e-mail: amuecke@gwdg.de
A. R. CABRAL, Rua Coelho, 32/704, 22231-110 Rio de Janeiro-RJ, Brazil; e-mail:
cabral70tricolor@yahoo.com.br
9
Captions
Plate 1
A: Completely martitized magnetite crystals which consist of spindle-like hematite crystals
arranged parallel to [111]-directions (three are visible) of the original magnetite crystals. –
Reflected light, oil immersion, crossed polars, longer edge 450 μm. – Kushaka iron-
formation, Nigeria.
B: Partially martitized magnetite crystals containing newly-formed and spindle-shaped
hematite crystals arranged parallel to [111]-directions of primary magnetite. - Reflected light,
oil immersion, longer edge 450 μm. – Iron-formation of Maraba Hill, Maru schist belt,
Nigeria.
C: Idiomorphic magnetite crystals being partially replaced by hematite (white arrows) due to
strain martitization along the rim. Associated minerals are goethite (black arrows) and quartz
(black). - Reflected light, oilimmersion, length of the longer edge 450 µm. – Iron-formation of
Daitari, Orissa, India.
D: Recrystallized, coarse-grained and twinned hematite crystals containing relics of martite-
textured hematite (in the center). - Reflected light, oil immersion, crossed polars, longer edge
450 μm. – Iron-formation of Tajimi, Nigeria.
10
Plate 2
A: Remnants of martited-textured hematite which is replaced by elongated and twinned
hematite (= specularite). - Reflected light, oil immersion, longer edge 450 μm. – Iron-
formation of Muro, Muro schist belt, Nigeria.
B: Replacement of slightly martitized magnetite (note the irregular grain boundaries) by
hematite. - Reflected light, oil immersion, longer edge 450 μm. – Iron-deposit of Umm Nar,
Wadi Mubarak, Western Desert, Egypt.
C: Aggregate of idiomorphic elongated hematite crystals which are partially replaced by
magnetite (arrows). - Reflected light, oil immersion, longer edge 450 μm. - Pegmatite of
Hagendorf, Bavaria, Germany.
D: Primary idiomorphic and partially martitized magnetite crystals in quartz (black).
Subsequently magnetite, which is partially preserved (mt) was pseudomorphically replaced by
goethite (go). Late goethite does not affect hematite (he) which is martite-textured. - Reflected
light, oil immersion, longer edge 450 μm. – Iron-formation of Maraba Hill, Maru schist belt,
Nigeria.
11
12
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Banded iron formations (BIFs) are iron-rich marine chemical sedimentary rocks, and their mineralogy and geochemistry can be used to gain insights into ancient ocean chemistry and biospheric evolution. Magnetite is the major iron-bearing mineral in many BIFs (particularly in the Archean) and is variably interpreted to be of primary, early diagenetic, or metamorphic origin. Different genetic interpretations for magnetite lead to divergent pictures of the Precambrian Earth system and its evolutionary models through time. The Baizhiyan Formation of the Neoarchean Wutai Group (Shanxi, North China) features magnetite-bearing, Algoma-type BIFs deposited ca. 2.52 Ga, in the lead-up to a major period of global iron formation deposition in the Paleoproterozoic. Abundant magnetite crystals found in the silica-rich bands of these BIFs show euhedral, hexagonal morphology. We suggest that this hexagonal magnetite likely represents pseudomorphs after green rust, a mixed-valence iron hydroxy-salt formed in the water column. The rare earth element composition of the BIFs shows negligible to slightly positive Ce anomalies (Ce SN /Ce SN * = 1.03 ± 0.07), which is characteristic of a dominantly anoxic water column. The presence of positive Eu anomalies (Eu SN /Eu SN * <3.9) suggests a substantial influence from proximal hydrothermal fluids. The co-occurrence of siderite layers associated with the magnetite-bearing strata may indicate iron cycling associated with ferruginous bottom seawater conditions. Geochemical signatures of the Baizhiyan BIFs are consistent with the interpretation that the magnetite was transformed from metastable green rust. This green rust could have formed via several processes, including the partial oxidation of Fe(II) by molecular oxygen/photoferrotrophs, the reaction of settling ferrihydrite with Fe(II)-rich hydrothermal fluids under anoxic conditions, or local dissimilatory iron reduction. In all cases, the contribution of primary green rust to BIF formation requires iron redox cycling, and similar pseudomorphs in the form of hexagonal magnetite may be more common in the geological record. Our findings support the models in which green rust was an important primary constituent of the Precambrian iron cycle, and the potential interactions of green rust with other elements (e.g., phosphorus) should be taken into consideration when reconstructing Precambrian biogeochemical cycles.
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This study demonstrates the great value of iron oxide chemistry in low-temperature systems. Hematite records elemental signatures of structural control, partial inheritance of pre-existing alteration, and allows chemical fingerprinting of multistage BIF-hosted ore. Mt. Whaleback and Mt. Tom Price are Western Australia’s largest banded iron formation (BIF) hosted hematite ore deposits. The mechanisms and timing of iron enrichment from BIF (with ∼20-40 wt.% Fe) to high-grade iron ore (with up to 69 wt.% Fe) are still controversially debated. In-situ iron oxide chemistry support post-metamorphic, structurally controlled, hydrothermal hematite ore formation via multiple stages of hydrothermally altered BIF. Iron oxides in two hydrothermal alteration zones in BIF at Mt. Tom Price (distal alteration assemblages: magnetite-siderite-stilpnomelane±pyrite and intermediate alteration assemblages: hematite-magnetite-ankerite-chlorite) show similar trace element patterns that are distinct from metamorphic magnetite in least-altered magnetite-quartz±hematite±Fe-carbonate±Fe-silicates BIF. At Mt. Whaleback, the hydrothermal alteration is completely eradicated by subsequent microplaty-martite hematite ore formation, and solely recorded in hematite composition by the inheritance of various conservative element abundances. Petrographically indistinct zones within a given orebody are chemically distinct and demonstrate formation via contrasting structurally controlled hydrothermal fluid flow: At Mt. Whaleback, specifically in the vicinity of Central Fault and Whaleback Fault, systematic trace element group abundance trends are recorded: major controls are fO2, pH, and acquired country rock signatures, i.e., evolved (Al, Mn, Mo, Pb, U) at Central Fault, and primitive (Al, Cr, Ti, V, Cu, Ni, Co) at Whaleback fault. Complex zoned growth textures and element distribution in microplaty hematite support precipitation during protracted hydrothermal fluid/rock interaction. In combination, our results falsify the importance of alternative hematite crystallisation models, including goethite dehydration and coalescent nanoparticles.
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139 samples of iron-formations from all the five continents belonging to 16 countries and 45 localities were investigated. The investigation of the rocks is based on ore-microscopic studies, electron-microprobe and XRF analyses. The results of the whole-rock analyses reveal that the SiO2 + iron (expressed as Fe2O3) contents are mostly higher than 90 wt %. This indicates that iron-formations are enriched in magnetite, martite and hematite rarely goethite and SiO2 mainly in the form of quartz. These mineral assemblages represent iron-formations that belong to the oxide facies comprising the magnetite and hematite subfacies. In the magnetite-free silicate facies, the magnetite-silicate facies and the carbonate facies, the (Fe2O3 + SiO2) concentrations are lower (75.5 to 86.1 wt %) and the Al2O3 or the CO2 concentrations are correspondingly higher. In general, the iron-formations are characterized by their low concentrations of Na2O, K2O and P2O5 (mainly below 0.1 wt %). The first Fe-oxide occurring in iron-formations is magnetite in the form of porphyroblasts, whereas hematite is mainly of secondary origin due to the formation of martite (= pseudomorphic oxidation of magnetite) and its subsequent recrystallization. Goethite is the youngest Fe-mineral which originated either from the replacement of magnetite (not of hematite) or from Fe-rich descending solutions. The analytical points of the iron-formations within the [(FeO + MnO) - Fe2O3 - SiO2 = 100 %]-diagram occur in two fields (I = silicate and carbonate facies; 11 = magnetite-silicate and oxide facies; names according to the facies defined by JAMES 1954 and 1992) which are clearly seperated from one another. The seperation of the two fields is based on two differences. 1. The chemical composition of the protoliths: It is inferred that the protolith of field I represents chemical precipitates of volcanic exhalations, whereas that of field 11 originated predominantly by the same source, but was contaminated from continental-derived material. 2. The environmental conditions during subsequent metamorphism: The silicate and carbonate facies of field I were formed under conditions of low oxygen fugacity due to the relatively high Fe2+-concentrations. The minerals occurring are aluminous Fe2+-silicates and siderite and rarely quartz. In the magnetite-silicate and the oxide facies of field 11 the minerals were formed under higher oxygen fugacity indicated by the occurrence of porphyroblastic magnetite Fe2+Fe23+O4, minor aluminous Fe2+-silicates and varying quantities of quartz. Due to martitization (= oxidation of magnetite) and the recrystallization of martite into hematite, field IIa (magnetite-silicate facies) grades transitionally into that of field IIb (magnetite subfacies) consisting of magnetite, martite and quartz and finally into field IIc (hematite subfacies) which is dominated by hematite and quartz (of varying proportions). Hematite of field IIc may contain relics of martite and/or magnetite. Some of the investigated iron deposits, known as iron-formations contain minerals or relics of minerals within later and replacing quartz which cast doubt upon their classification as iron-formations. However, these deposits cannot be differentiated from true iron-formations within the [(FeO + MnO) - Fe2O3 - SiO2 = 100 %]-diagram. Their doubtful origin concerns in particular the occurrence of amphiboles (magnesiohasting-site, magnesiohornblende, actinolite, tschermakite, wincheite and richterite) with Mg-numbers distinctly higher than 0.5. In iron-formations amphiboles (ferrohornblende, ferroactinolite and grunerite) have Mg-numbers markedly lower than 0.5. Other minerals of the deposits of doubtful origin are unknown in iron-formations (e.g. fluorite, allanite) or have compositions that are not known in iron-formations. These are: magnetite (with TiO2-contents up to 6.6 wt %); pyroxenes (diopside and aegirine-augite); chlorite (32.2 to 59.2 mol % clinochlore, 24.1 to 43.4 mol % chamosite and 14.7 to 26.6 mol % pennantite); garnet (andradite and grossularite); and mica (mainly phlogopite). In iron-formations these minerals are: pure magnetite; Fe-rich augite and clinoferrosilite; chlorite (59.9 to 63.1 mol % chamosite, 21.1 to 24.4 mol % permantite and 14.9 to 16.5 mol % clinochlore); almandine-spessartite solid solutions; and annite. Additionally, the deposits of doubtful origin contain high concentations Of P2O5 (UP to 0.7 wt % in the form of microscopic apatite). These deposits are classified as Itakpe type. The iron-rich rocks of Itakpe Hill, Nigeria (containing up to 0.5 wt % P2O5) are simply phenomenolocically similar to iron-formations, but are of magmatic origin and are metamorphosed and silicified.
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Numerous specimens of metamorphosed Proterozoic iron formations in North America were studied microscopically. Textural relations exhibited by magnetite and hematite are described and interpreted. Hematite-magnetite-quartz is known to be an equilibrium assemblage in oxide iron formations of all metamorphic grades. Most investigators interpret the persistence of this assemblage to indicate the lack of movement of oxygen between layers of iron formation during metamorphism. Microscopic evidence presented here reveals extensive replacement of hematite by magnetite and vice versa. This suggests that oxide iron formations were indeed more open to the movement of oxygen between layers during metamorphism than has often been realized. The study also shows that textures exhibited by hematite-magnetite depend on the type and metamorphic grade of iron formations.
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Although many Proterozoic iron-formations have undergone various degrees of metamorphism, most of the interpretations in this overview are based on data derived from iron-formations that have undergone only low-grade metamorphic (lower greenschist) facies conditions. These relatively unmetamorphosed iron-formations consist mainly of chert, magnetite, various carbonates (siderite, members of the dolomite-ankerite series, and calcite), hematite, and silicates, such as greenalite, stilpnomelane, minnesotaite, and riebeckite. The mineralogy of iron-formations as a function of diagenesis and metamorphism is reviewed in the chapter. Several well-known Proterozoic iron-formations have undergone only lowgrade (greenschist) metamorphism, whereas others show a considerable range in metamorphic grade. Very-low-grade metamorphic mineral assemblages make up most of the iron-formation lithologies of the Hamersley Basin. Medium-grade metamorphic assemblages of iron-formation are characterized by the common development of amphiboles, mainly members of the cummingtonite–grunerite series. High-grade metamorphic iron-formations consist of essentially anhydrous assemblages in which variable amounts of ortho- and clinopyroxene predominate. Fayalite may be present, as well as carbonates and garnet, and lesser amounts of amphiboles; quartz, magnetite, and/or hematite are the major constituents of oxide-rich iron-formations.
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Precambrian iron-formations are the source of the bulk of the world's iron ore reserves. Three-quarters of the known iron-formations are found in eight major districts, distributed over five continents, within shelf-type sequences of early Proterozoic age. These deposits, as much as 1000 m thick, are considered to be products of a reaction between upwelling anaerobic deep ocean waters, in which iron and silica had been accumulating, and oxidic surface waters of continental shelves. Initial deposits, depending upon local conditions, consisted variously of iron oxide hydrates, iron-rich carbonate, and iron-rich silicate mud, all interbedded with silica gel. These materials were converted by sea-bottom reactions and ensuing diagenesis to stable assemblages of iron oxides, siderite, iron silicates, and chert. Under extreme reducing conditions, reactions between organic material, entrapped sulfate-bearing seawater, and initial iron precipitates yielded pyritic iron-formation. The metamorphic imprints on these deposits are many and varied. -from Editors
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The transformation of magnetite to hematite, or hematite to magnetite, in nature has generally been considered a redox reaction and linked to a specific redox state of fluid; however, a nonredox reaction, Fe2O3(hm) + Fe2+ H2O = Fe3O4(mt) + 2H(+), may have been the principal mechanism for the transformations of iron oxides in nature, especially in hydrothermal environments. For example, the transformation of goethite and/or hematite (primary precipitates) to magnetite in banded iron-formations (BIFs) probably occurred through nonredox reactions with Fe2+-bearing hydrothermal fluids during the accumulation of a BIF sequence, rather than through redox reactions involving organic matter during and/or after the BIF deposition. The proposed mechanisms for the transformation of magnetite to hematite provides new exploration strategies for hematite-rich secondary ores, extending the target for orebodies to much deeper zones below the paleosurface. Another important implication of the proposed mechanism is that the presence or absence of magnetite and/or hematite in geologic formations may or may not provide meaningful information on the redox state of fluid.
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The ores derived from banded iron-formation in situ in the Hamersley Iron Province represent a series of events probably related to the time of emergence of their parent rocks by the slow process of erosion of the overlying cover. The simplest and least mature ores consist essentially of residual oxides in a matrix of goethite, the latter derived from the supergene replacement of part of the chert and other components of the original banded iron-formation. The iron necessary for this enrichment logically comes from the now-eroded extension of banded iron-formation outcrop.
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The Ironwood iron-formation of the Gogebic Range of Michigan and Wisconsin is made up of several rock types, each of which is characterized by a different iron-rich mineral: hematite, magnetite, pyrite, iron carbonate, or iron silicate (minnesotaite, stilpnomelane). Where the Ironwood iron-formation is relatively unaltered the Plymouth, Norrie, and Anvil members consist of wavy-bedded magnetite and silicate-rich rocks, whereas the Yale and Pence members consist of even-bedded carbonate, silicate, magnetite, and pyrite-rich rocks. These rock types represent primary facies of the iron-formation that were deposited under differing physical and chemical conditions during a period of continuous iron-rich sedimentation. Animikie sedimentation in the Gogebic district began with the deposition of sandstone and dolomitic limestone in a shallow sea advancing over a low-lying land mass of lower Precambrian granite and greenstone. Continued advance of the sea, with effective separation of clastic material near shore, permitted the dominantly chemical sedimentation of the iron-formation in somewhat deeper water. The development of an off-shore basin with partially restricted circulation would have facilitated such deposition. Minor fluctuations in the physical and chemical conditions within the depositional environment are reflected in the differing facies of the iron-formation. Deposition of the iron and silica-rich chemical sediments was terminated by increased tectonism and the deposition of the thick sequence of slates and graywackes of the Tyler formation.