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Proceedings of the
NATIONAL ACADEMY OF SCIENCES
Volume 61 * Number 3 . November 15, 1968
MICROBIOTAS OF THE BANDED IRON FORMA TIONS*
BY PRESTON E. CLOUD, JR., AND GERALD R. LICARI
DEPARTMENT OF GEOLOGY, UNIVERSITY OF CALIFORNIA (SANTA BARBARA, LOS ANGELES)
Communicated August 26, 1968
Banded iron formation (BIF) is a rhythmically bedded, siliceous ("cherty")
sedimentary rock in which iron-rich layers are separated from iron-poor layers at
intervals ranging from several centimeters to less than a millimeter (Fig. 1).
Most characteristically it is siliceous in both iron-rich and iron-poor bands, but
the iron-rich layers may consist of ferrocarbonates, and subordinate facies take
the form of sulfides and complex silicates.1 The iron in typical BIF occurs in
both the ferric and the ferrous states, but the ferric oxide is commonly present or
predominant. The unenriched primary rock, containing as little as 15 per cent
iron, is often called taconite, and the more concentrated varieties of taconite are
now being mined as iron ore where beneficiation techniques permit. Because of
its economic significance, BIF has been intensively studied. It is known from all
relatively ice-free continents in rocks between about 1.8 and 3 aeons (1.8 to 3 X
109 years) old, and it seems to be particularly abundant in rocks around 2 aeons
(±- 0.2) old. Its geochemical and geological characteristics and associations sug-
gest origin as a (sometimes reworked) chemical precipitate in and toward the
margins of sizeable basins, under an anoxygenous atmosphere.
A biological origin has often been suggested for the BIF, and Cloud2 has
proposed a fluctuating dependency between the deposition of BIF characterized
by the ferric and ferro-ferric oxides (hematite and magnetite), the facies of BIF,
and the earliest oxygen-producing procaryotic microorganisms-the ferrous iron
serving as a biological oxygen acceptor in advance of oxygen-mediating enzymes.
If this is so, microorganisms should be common in or closely associated with BIF.
The now well-known microbiotas from cherty stromatolites of the Gunflint Iron
Formation of southern Ontario,3 discovered by Stanley Tyler in 1953 (ref. 4, p.
332), are of great interest in this respect, but their specific association with BIF is
a little hazy, and more (and more clearly related) occurrences are to be expected if
there is any validity to the idea of biological precipitation.
An intensive search was made for such occurrences during 1963-65, especially
in the stromatolitic parts of the Biwabik Iron Formation (a presumed Gunflint
equivalent) and the older Soudan Iron Formation of Minnesota. At that time
779
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FIG. l.-Outcrop of iron formation, Soudan mine, Soudan, Minnesota.
FIG. 2.-Polished surface of the fossiliferous stromatolitic rock associated with the Biwabik
Iron Formation, Corsica Mine, southeast of Gilbert, Minnesota. The dark laminae are hema-
titic.
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spheroidal to ellipsoidal microstructures of narrow size range were observed to be
common in the cherty phases of both formations, but, because of poor preserva-
tion, considerable hesitancy was felt about confidently interpreting these objects
as of biological origin. Then, when La Berge, in 1967, showed the prevalence
of abundant microstructures of reasonably likely biological nature (but not much
better preservation) in BIF from various localities in M\Iinnesota, M\Iichigan, Que-
bec, the Belcheit Islands, and western Australia, there seemed no cause for further
attention to this material.
Recently, however, while scanning materials collected from the southern hemi-
sphere by Cloud in 1965, we have found other probable nannofossils in the BIF of
the roughly 1.9-aeon-old Pretoria Series in South Africa; and restudy of the M\Iin-
nesota slides has revealed new structures of probable biological affinity in both
Archean and older Proterozoic BIF.
Gunflint microbiota in the stromatolites of the Biwabik Iron Formation, Corsica
Mline, Minnesota (Figs. 2-7, 10)
The Biwabik Iron Formation, known to be more than 1.7 aeons old (ref. 5, p.
5), is generally considered to be the lateral and time equivalent of the roughly 1.9-
aeon-old Gunflint Iron Formation in the M\Iesabi Range of northeastern M\inne-
sota. Being the major iron-producing formation of the region, it has been inten-
sively studied (e.g., ref. 6). It is generally divided into four members, all of
which are mined for the taconite ores that comprise the major remaining iron re-
serves of the region. Each member is also subdivided into designated "beds."
Bed I, near the top of the "upper cherty member," is recognized widely over the
.Alesabi Range as a zone of stromatolitic mounds or reeflike masses (2-10 meters
in diameter and 1-3 meters high) with an internally convoluted and digitate
structure. It is strikingly similar to and probably the equivalent of the stro-
matolitic zone that has yielded the Gunflint microbiota in Ontario. The taconite
is mined around the stromatolitic domes, which are left in place or discarded in
fragments to the dumps because of their relatively low iron content. Neverthe-
less, they are very closely associated with the iron formation itself.
Elements of the Gunflint microbiota are now recorded from specimens of this
discarded stromatolitic rock on the dumps of the old Corsica MWine, southeast of
Gilbert, M\Iinnesota (Figs. 3-7) Cloud's locality 3 of 5 October, 1963. Although
alteration here has caused loss of the finer structures in most of this rock, some
patches preserve the microstructures in detail almost equivalent to that of the
Gunflint. The main difference is that the former cell walls, carbonaceous in the
Gunflint, are replaced by hematite. Structures observed include filaments,
spheroids, and perhaps radiate forms one to several microns in diameter that show
blue-green algal and perhaps bacterial affinities and are attributable to Barg-
hoorn's genera Gunflintia, Huroniospora, and possibly Eoastrion.3 The Huroni-
ospora-like spheroidal bodies vary in degree of preservation from well-defined
structures, convincingly compared with the Gunflint microbiota, to degraded
spheroidal bodies more typical of structures observed in other and older BIF of less
favorable preservation (Fig. 10). As in the Gunflint itself, these nannofossils are
confined to the stromatolitic structures and are most abundant along particular
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unequally spaced, upwardly convex laminae of the individual stromatolitic
fingers or ridges.
Spheroids of presumptive biological origin in BIF of older Proterozoic and Archean
ages
Biwabik Iron Formation, greater than 1.7 aeons old, Hoyt Lakes Mine, Minnesota
(Figs. 8 and 9).-The Biwabik has also yielded spheroids that may be of biologic
origin in black argillaceous and ferruginous rocks of the "lower slaty member" at
the Hoyt Lakes Mine of the Erie Mining Company, about 25 kilometers east of the
Corsica 1\ine (Cloud's locality 1 of 27 August, 1968). We would never propose
these as fossils if they were all we had to go on, but the transition seen in Figure 10
and the presence of more convincingly biological structures of this size range in the
Biwabik and other iron formations described in this paper lead us to believe that
they may well be fossils. These bodies are very abundant locally, cluster in the
size range of 10-15 a, in part show suggestions of double walls and reticulate
surfaces, and crudely resemble Huroniospora. A biological origin for these
spheres, therefore, is consistent with the evidence available-even though the
locality is not far from the intrusive contact with the Duluth gabbro, and the con-
taining rocks have been affected by moderate thermal metamorphism.
Stromatolitic chert is characteristic of Bed I in the east pit of the mine, above a
zone of fragmented, lensy, and cross-bedded cherty and ferruginous rock indicat-
ing shallow-water turbulence and a local direction of transport from the northeast,
but it is recrystallized to a sugary texture and has revealed nothing of biologic
interest to our examination.
Pretoria Series, roughly 1.9 aeons old, northern Cape Province, South Africa (Fig.
11) .-The Pretoria Series, at the top of the Transvaal System, is the main source
of iron in South Africa and the youngest BIF there, being in age' not far from the
Gunflint and Biwabik iron formations in North America. A collection from BIF
in the basal 15 meters of the Pretoria Series about 10 kilometers north of Daniel-
skuil, on Farm Gladstone, in northern Cape Province (Cloud's locality 4 of
7 September, 1965), has yielded extremely abundant reticulate spheroids ranging
between 5 and 10 , in diameter and resembling reticulated specimens of "Huroni-
ospora" (Fig. 11). These spheroids are present in such large numbers in some
laminations that they stand out as darker bands in the generally lighter-colored
chert. Their great abundance, narrow size range, and observable structure imply
a biological origin.
Soudan Iron Formation, greater than 2.7 aeons old, Soudan, Minnesota (Figs. 12
and 13).-The Soudan Iron Formation of northeastern Minnesota8 is intruded by
FIG. 3.-Gunflintia filaments and Huroniospora spheroids replaced by hematite are abundant
in some of the dark laminations in stromatolitic rocks at the Corsica Mine.
FIG. 4.-Huroniospora, Corsica Mine.
FIG. 5.-Wide filament, Corsica Mine.
FIGS. 6 AND 7.-Gunflintia filaments, Corsica Mine.
FIGS. 8 AND 9.-Spheroids from the "lower slaty member" of the Biwabik Iron Formation,
Hoyt Lakes Mine, Minnesota.
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the Saganaga granite of Algoman age, which gives concordant uranium-lead and
potassium-argon ages of about 2.7 aeons.9 Thus the Soudan is older than 2.7
aeons and may approach or exceed 3 aeons-certainly one of the oldest sedimen-
tary rocks yet known anywvhere, and, together with the Ely greenstone beneath it,
reminiscent of the similarly very ancient lower Swaziland System of the eastern
Transvaal.10 In a previous report, although structures of possible biologic origin
were described from pyrite balls in the Soudan,8 other objects from associated
BIF were not considered sufficiently suggestive of organisms to be so attributed.
On going back to the same slides, however, we now find structures in them that
are more persuasive than ones observed on earlier study. Spheroids within a size
range of 4-10 1 are locally abundant, and some of them show reticulate surfaces
and even suggest division into internal compartments (Fig. 13). This is very sug-
gestive of the structures of some of the bodies assigned to "Huroniospora."11
The rock in which they occur is BIF from an outcrop (Fig. 1) east of the road that
runs north from Soudan past the old Soudan -Mine, northeastern M\Iinnesota
(Cloud's locality 4 of 14 August, 1963). It is a part of the rock that was formerly
mined for iron at this locality.
Conclusions
Our observations and those of La Berge4 indicate that mierobiotas were
abundant and widespread in and associated with BIF in its characteristic range
of ages from about 1.8 to 3 aeons ago.
The cherty stromatolitic masses within the Gunflint and Biwabik iron forma-
tions contain a variety of microstructures, some of which (such as the Gunflintia
filaments) were probably directly associated with the growing algal reefs, and
others of which may have been planktonic forms that simply adhered to and
became preserved within the growing gelatinous mass. In rock having the
characteristic banded structure of BIF we seem to find only spheroidal forms that
were probably floaters. They might have lived in a water layer of intermediate
density near the bottom of the photic zone as Weyl12 has so imaginatively sug-
gested. Here they could react with ferrous oxygen acceptors in solution without
ordinarily being circulated into the surface few meters penetrated by high-energy
ultraviolet radiation. The ferric or ferro-ferric iron precipitated as a result would
have settled to the depositional interface below, where it became incorporated
with a rain of dead and sometimes iron-impregnated phytoplankton from above
(or carried laterally to the growing stromatolitic masses or related BIF of shal-
lower waters). All components became imbedded in a silica gel of chemical
origin that was possibly also biologically mediated. Subordinate facies of
iron formation are visualized as oxygen-deprived variants of the same theme.
FIG. 1O.-Hiironiospora-like spheroids from stromatolites of the metamorphosed Biwabik Iron
Formation may undergo transition to irregular subspheroidal blobs, Corsica Mine.
FIG. 11.-Reticulate-surfaced spheroidal structures, Pretoria Series, the northern Cape Prov-
ince, South Africa.
FIGS. 12 AND 13.-Spheroidal structures from the Soudan Iron Formation, Minnesota.
The two spheroids in Fig. 13 show reticulate surfaces and possible internal compartments
similar to those of Fig. 11, from the Pretoria Series.
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* This research was supported by grant no. NGR-05-007-169 from the National Aeronautics
and Space Administration.
' James, H. L., Econ. Geol., 49, 235 (1954); Gross, G. A., Geological Survey of Canada, Econ.
Geol. Rept. No. 22 (1965), vol. 1, 181 pp.
2Cloud, P. E., Jr., Science, 148, 27 (1965); Science, 160, 729 (1968); in Evolution and
Environment, ed. E. T. Drake (New Haven: Yale Univ. Press, in press).
3 E.g., Barghoorn, E. S., and S. A. Tyler, Science, 147, 563 (1965).
4 La Berge, G. L., Bull. Geol. Soc. Am., 78, 331 (1967).
5Goldich, S. S., A. 0. Nier, and others, Minnesota Geol. Surv. Bull. 41 (1961), 193 pp.
6Gruner, J. W., Minnesota Geol. Surv. Bull. 19 (1924), 71 pp; Gundersen, J. N., and G. M.
Schwartz, Minnesota Geol. Surv. Bull. 43 (1962), 139 pp.
7Nicolaysen, L. O., in Petrologic Studies, ed. A. E. J. Engel et al., (Geol. Soc. America, Bud-
dington Volume, 1962), p. 569.
8 Cloud, P. E., Jr., J. W. Gruner, and Hannelore Hagen, Science, 148, 1713 (1965).
9 Hanson, G. N., Minnesota Geol. Surv., Rept. Invest. 8 (1968), 20 pp.
10 Engel, A. E. J., Univ. Witwatersrand, Econ. Geol. Research Unit, Info. Circ. 27 (1966), 17
pp.; Anhaeusser, C. R., et al., Univ. Witwatersrand, Econ. Geol. Research Unit, Info. Circ. 38
(1967), 31 pp.
1 Cloud, P. E., Jr., and Hannelore Hagen, these PROCEEDINGS, 54, 1 (1965).
1Weyl, Peter, Science, 161, 158 (1968).
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