A biological model for the precipitation of Precambrian iron formations is presented. Assuming an oxygen deficient atmosphere and water column to allow sufficient Fe solubility, it is proposed that local oxidizing environments, produced biologically, led to precipitation of iron formations. It is further suggested that spheroidal structures about 30 mm in diameter, which are widespread in low grade cherty rion formations, are relict forms of the organic walled microfossil Eosphaera tylerii. The presence of these structures suggests that the organism may have had a siliceous test, which allowed sufficient rigidity for accumulation and preservation. The model involves precipitation of ferric hydrates by oxidation of iron in the photic zone by a variety of photosynthetic organisms. Silica may have formed in the frustules of silica secreting organisms, including Eosphaera tylerii. Iron formates formed, therefore, by a sediment rain of biologically produced ferric hydrates and silica and other organic material. Siderite and hematite formed diagenetically on basin floors, and subsequent metamorphism produced magnetite and iron silicates

Laberge, G. L.

English

NASA Technical Reports Server

N86-28549
>" 71r/fi
A MODEL FOR THE BIOLOGICAL PRECIPITATION OF PRECAMBRIAN
I RON-FORMATION; Gene L. LaBerge, Geology Department, University^ of Wisconsin
Oshkosh, Oshkosh, WI 54901
As the major chemical sedimentary rock formed in the Archean and Early
Proterozoic, iron-formation has been widely used as a basis for geochemical
inferences about the chemistry of the early atmosphere and oceans. However,
the present mineralogy of iron-formation probably better reflects diagenetic
and metamorphic modifications rather than chemical attributes of the early
atmosphere and oceans. On the other hand, the existence of iron-formation
sequences, which may contain thousands of cubic miles of chemically precipi-
tated iron and sil ica, probably does reflect on the nature of the early
atmosphere and oceans. In this discussion I am restricting the term iron-
formation to rocks containing roughly equal proportions of silica and iron
minerals.
Today, under an oxygenated atmosphere and consequent oxygenated ocean,
the extreme insolubility of ferric iron precludes attaining significant con-
centrations in a water column. Therefore, widespread cherty iron-formations
do not form. A relatively oxygen-deficient atmosphere seems necessary to
al low sufficient ferrous iron in solution to produce widespread iron
precipitation. However, an oxygen-deficient atmosphere, with its associated
reducing water column and load of ferrous iron, poses the problem of a
mechanism in the Proterozoic and Archean to precipitate the iron. This
would be accomplished most readily by oxidizing the relatively soluble
ferrous iron to an insoluble ferric hydrate, which textural evidence and
geochemical models suggest is the main precipitate.
Because iron-formations range in age from 3,800 Ga to about 1,900 Ga,
and were formed in a wide variety of geological settings, it is not unreason-
able to suggest that their precipitation was controlled by local conditions.
These conditions would include local oxidizing conditions developed in dif-
ferent basins at different times and local availability of iron in these
basins. The fact that the major Proterozoic iron-formations range from about
2,500 Ga for the Hamersley Range of Western Australia to about 1,900 Ga for
the Lake Superior region suggests that iron-formation deposition was con-
trolled mainly by local basin chemistry, and that atmospheric influence was
minor. Furthermore, the widespread occurrence of iron-formations in the
Archean also points to local basin control. While it is possible to pro-
duce a locally reducing environment with an oxygenated atmosphere today, I
know of no way to produce a locally oxidizing environment with an oxygen-
deficient atmosphere except by biological activity. Therefore, precipitation
of the iron would be most readily accomplished by biological activity.
The precipitation of silica, the other major ingredient in iron-
formations, is especially problemmatical, including disagreement on whether
it is primary or secondary. While most workers assume that the chert is a
primary precipitate, some (1, 2) suggest that it is a replacement of earlier
calcite. Textural relations, including syneresis cracks (3), suggest that
the silica is a primary precipitate. However, the mechanism of precipitation
is not resolved. Although most Phanerozoic cherts are considered to be
biologically precipitated (by diatoms, radiolaria and sponges), Precambrian
cherts are generally considered to be inorganically precipitated because
Precambrian silica-secreting organisms have not been recognized. The
typical, ubiquitous association of iron and silica in iron-formations sug-
gests that the precipitation of the two must be related, even though the
https://ntrs.nasa.gov/search.jsp?R=19860019077 2020-03-20T14:47:03+00:00Z
72
BIOLOGICAL PRECIPITATION
LaBerge, Gene L. ORIGINAL PAGE IS
OF POOR QUALITY
chemistry of silica is very different from that of iron. I propose to show
evidence that the si l ica as well as the iron in iron-formations was
precipitated biologically.
One of the characteristic features of iron-formation cherts is the wide-
spread occurrence of spheroidal structures about 30 urn in diameter that
consist of a central 20 urn sphere surrounded by a variable number of 5 urn
spheroids (4). In most cases, the spheroids are revealed by a fine hematite
"dust" that outlines the structures. Although the spheroids are not present
in every layer, they are present in every Proterozoic and Archean iron-
formation which has not been excessively metamorphosed and sheared that I
have examined. As pointed out by LaBerge (4), the structures are remarkably
similar in size and morphology to the organic-walled microfossil Eosphaera
tyleri, and I suggest that the spheroids are best interpreted as relict
forms of that organism. If so, the details of these structures may have
Sketch of hypothetical situation of what an accumulation of
Eosphaera tyleri might look like.
Photomicrograph of jasper with Eosphaera-like
structures showing their abundance.
BIOLOGICAL PRECIPITATION ORIGINAL PAGE 13
LaBerge, Gene L. OF POOR QUALITY 73
significant implications on the precipitation of iron-formation. First, the
fact that these structures are abundantly preserved as spheroids in the
cherty layers suggests that they had some rigidity, and were not simply an
organic sheath, because a vast majority of organic matter in iron-formation
does not retain its organic structure. Many jasper layers are virtually
composed of spheroidal structures. Microscopically, the textural pattern of
the Eosphaera-like structures suggests that the layers formed by accumulation
of the spheroids. The fact that the spheroids appear to have had rigidity
and to have formed the siliceous layers suggests that Eosphaera may have had
a siliceous test. Similarly, the presence of similar structures within
siliceous granules seems to attest to the durability of the spheroids. In
addition, all Eosphaera-like structures are typically preserved by a fine
hematite "dust" that colors the jasper red. The dust, then, may represent
fine ferric hydrate that adhered abiotically to the organic wal ls of
Eosphaera, or alternatively, it may indicate that Eosphaera was photosynthe-
tic and the ferric hydrate accumulated on the membrane where oxygen was
expelled (such as in modern Vo lvox) , or it may be similar to the way the
chemotrophic bacterium Leptothrix encases itself in a sheath of ferric
hydrate. These two observations in the jaspers lead to the tentative inter-
pretation that Eosphaera may have been a photosynthetic organism with
siliceous frustules. If this interpretation is correct, it is significant
regarding iron-formation and early life because these spheroids are present
in Archean iron-formations including some from the 3,500 Ga old Pilbara
block of Western Austral ia. Furthermore, the abundance of Eosphaera-like
structures in laminated (deep water) jaspers may indicate that it was plank-
tonic rather than benthic in life-style.
Several authors (4, 5, 6, 7) have reported that siderite in relatively
unmetamorphosed iron-formations occurs as prominent spherical grains about
30 urn in diameter. Microprobe examination of spherical siderites in
Proterozoic iron-formation from the Gunflint district of Ontario reveals that
they are double-walled structures remarkably similar to Eosphaera tyleri.
Comparison of double-walled spherical siderite ( left)
and Eosphaera tyleri (right). Siderite is white in
the electron micrograph. Photos are approximately the
same scale.
74
BIOLOGICAL PRECIPITATION
LaBerge, Gene L.
These double-walled, spherical siderite grains are interpreted to have
formed by diagenetic alteration of primary hematite-coated varieties.
Bacterial degradation of organic matter in the anoxic zone on the basin floor
may have produced C02 and utilized ferric hydrate as an electron donor which
reduced the iron to the ferrous state. The ferrous iron and C0£ combined to
form siderite "pseudomorphs" after hematite-pigmented forms involving pro-
gressive replacement of the silica. Siderite is a major mineral in some
iron-formations, however, it appears to be primarily of early diagenetic
origin, because siderite is part of the sedimentary fabric of the rock. The
conversion of primary ferric hydrate to siderite is often incomplete,
because hematite and siderite may co-exist in iron-formations. Perhaps this
may be due to the amount of associated organic matter that was available for
bacterial degradation on the basin floor. The net result seems to have been
a primary precipitate containing a mixture of ferric and ferrous minerals
in a metastable association.
Magnetite is the most abundant iron mineral in metamorphosed iron-
formations, and I suggest that the hematite and siderite react to produce
magnetite, which appears to be the equilibrium phase at elevated tempera-
tures. In contrast to hematite and siderite, which were involved in the
depositional fabric, magnetite typically forms crystal aggregates and veins
that tend to obliterate primary features (6). Furthermore, Han (8) showed
that much of the magnetite forms as overgrowths on earlier, rhombohedral
hematite crystals. It may be appropriate to note here that these diagenetic
(?) hematite crystals, which resemble monoclinic selenite crystals, may have
been misidentified as pseudomorphs after gypsum (2, 9). If so, this may
lead to erroneous interpretations about the oxygen content of the Precambrian
atmosphere and oceans.
I suggest that the iron in iron-formations was precipitated as a ferric
hydrate product by photosynthetic oxidation of iron in the photic zone.
Presumably a variety of organisms were involved in the process, including
planktonic iron-stripping bacteria such as Leptothrix and Metallogenium,
which precipitated ferric hydrate in their sheaths from ferrous iron in the
water. Whi le some of the iron accumulated on the Eosphaera-like structures,
most of it evidently formed as minute ferric hydrate particles that settled
in the water column. The sil ica may have been precipitated largely as
frustules of silica-secreting organisms, particularly Eosphaera tyleri.
Thus, the "sediment rain" consisted of Eosphaera, ferric hydrate, and other
organic materials, which accumulated on the basin floor. Incipient layering
of iron and silica may have been produced by a relatively constant "rain" of
ferric hydrate punctuated by periodic "blooms" of siliceous organisms. In
the organic-rich ooze, anaerobic decay and chemoautotrophic bacteria may have
flourished, giving off waste C02 and ferrous iron. These products reacted
to form siderite, which may have been nucleated by the ferric hydrate on
the Eosphaera or elsewhere. The notorious lack of sulfide minerals in cherty
iron-formations suggests that sulfate was scarce in the basins, and that it
did not serve as an oxygen source for organic delay. Continued accumulation
of iron on the upper part of the siliceous layer, coupled with diagenetic
formation of siderite would produce a marked density contrast between sil ica
and ferruginous "grains". The layering would then be enhanced by currents
separating the ferruginous and silica "grains". During periods of higher
energy bottom currents, this laminated material might have been broken up
to form granule-bearing units in shallower water as discussed by LaBerge (3 ) .
Subsequent metamorphism of the metastable siderite-hematite-chert rock would
BIOLOGICAL PRECIPITATION
LaBerge, Gene L.
75
result in widespread development of magnetite and/or iron-silicates
OXYGEN-DEFICIENT ATMOSPHERE
Oxygen from photosynthetic organisms
converts Fe*2 to insoluble ferric
hydrate, which rains down with
organic matter and siliceous tests.
Depth of light penetration
Anaerobic bacteria in bottom mud
decompose organic tissue utilizing
oxygen from ferric hydrate. This
produces Fe and C02 which combine
to form siderite.
Sketch of the proposed model for the biological precipitation
of Precambrian iron-formations.
REFERENCES CITED
1. Lepp, H., and Goldich, S.S., 1964, Econ. Geol., p. 1025-1060.
2. Lougheed, M.S., 1983, Geol. Soc. Amer. Bull., v. 94, p. 325-340,
3. LaBerge, G.L., (in press), Jour. Geol. Soc. India.
4. LaBerge, G.L., 1973, Econ. Geol.. v. 68, p. 1098-1109.
5. James, H.L., 1954, Econ. Geol., v. 49, p. 235-293.
6. LaBerge, G.L., 1964, Econ. Geol., v. 59, p. 1313-1342.
7. LaBerge, G.L., 1967, Geol. Soc. Amer. Bull.. v. 78, p. 331-342.
8. Han, T.M., 1978, Fortsch.Min.. v. 56, p. 102-142.
9. Dimroth, E. and Chauvel, J.J., 1973, Geol. Soc. Amer. Bull..
v. 84, p. 111-134.


A model for the biological precipitation of Precambrian iron-formation

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server