Microbiotas of the Banded Iron Formations

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 suggest 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 Downloaded by guest on September 27, 2021 780 GEOLOGY: CLOUD AND LICARPPROC. N. A. S.

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. Downloaded by guest on September 27, 2021 VOL. 61, 1968 GEOLOGY: CLOUD AND LICARI 781

spheroidal to ellipsoidal microstructures of narrow size range were observed to be common in the cherty phases of both formations, but, because of poor preservation, 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 intensively 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 stromatolitic 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 Downloaded by guest on September 27, 2021 31.O[L _ Aess

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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 containing 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. Downloaded by guest on September 27, 2021 4.

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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 sedimentary 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 suggestive 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 formations 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 suggested. 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. Downloaded by guest on September 27, 2021 786 GEOLOGY: CLOUD AND LICARI PROC. N. A. S.

* 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). Downloaded by guest on September 27, 2021