『Abstract
Banded iron-formations (BIFs) occur in the Precambrian geologic
record over a wide time span. Beginning at 3.8 Ga (Isua, West
Greenland), they are part of Archean cratons and range in age
from about 3.5 until 2.5 Ga. Their overall volume reaches a maximum
at about 2.5 Ga (iron-formations in the Hamersley Basin of Western
Australia) and they disappear from the geologic record at about
1.8 Ga, only to reappear between 0.8 and 0.6 Ga.
The stratigraphic sequences in which BIFs occur are highly variable.
Most Archean iron-formations are part of greenstone belts that
have been deformed, metamorphosed, and dismembered. This makes
reconstruction of the basinal setting of such BIFs very difficult.
The general lack of metamorphism and deformation of extensive
BIFs of the Hamersley Range of Western Australia and the Transvaal
Supergroup of South Africa allow for much better evaluations of
original basinal settings. Most Archean iron-formations show fine
laminations and/or microbanding. Such microbanding is especially
well developed in the Brockman Iron Formation of Western Australia,
where it has been interpreted as chemical varves, or annual layers
of sedimentation. BIFs ranging in age from 2.2 Ga to about 1.8
Ga (e.g., those of the Lake Superior region,U.S.A., Labrador Trough,
Canada, and the Nabberu Basin of Western Australia) commonly exhibit
granular textures and lack microbanding.
The mineralogy of the least metamorphosed BIFs consists of combinations
of the following minerals: chert, magnetite, hematite, carbonates
(most commonly siderite and members of the dolomite-ankerite series),
greenalite, stilpnomelane, and riebeckite, and locally pyrite.
Minnesotaite is a common, very low-grade metamorphic reaction
product. The Eh-pH stability fields of the above minerals (and/or
their precursors) indicate anoxic conditions for the original
depositional environment.
The average bulk chemistry of BIFs, from 3.8 through 1.8 Ga in
age, is very similar. They are rich in total Fe (ranging from
about 20 to 40 wt％) and SiO2 (ranging from
43 to 56 wt％). CaO and MgO contents range from 1.75 to 9.0 and
from 1.20 to 6.7 wt％, respectively. Al2O3 contents are very low, ranging from 0.09 to
1.8 wｔ％. These chemical values show that they are clean chemical
sediments devoid of detrital input. Only the Neoproterozoic iron-formations
(of 0.8 to 0.6 Ga in age) have very different mineralogical and
chemical make-ups. They consist mainly of chert and hematite,
with minor carbonates.
The rare-earth element profiles of almost all BIFs, with generally
pronounced positive Eu anomalies, indicate that the source of
Fe and Si was the result of deep ocean hydrothermal activity admixed
with sea water.
The prograde metamorphism of iron-formations produces sequentially
Fe-amphiboles, then Fe-pyroxenes, and finally (at highest grade)
Fe-olivine-containing assemblages. Such metamorphic reactions
are isochemical except for decarbonation and dehydration.
The common fine lamination (and/or microbanding) as well as the
lack of detrital components in most BIFs suggest that such are
the result of deposition, below wave base, in the deeper parts
of ocean basins. Those with granular textures are regarded as
the result of deposition in shallow water, platformal areas. Carbon
isotope data suggest that for a long period of time (from Archean
to Early Proterozoic) the ocean basins were stratified with respect
to δ13C (in carbonates) as well as organic carbon content.
In Middle Proterozoic time (when granular BIFs appear) this stratification
diminishes and is lost.
The Neoproterozoic BIFs occur in stratigraphic sequences with
glaciomarine deposits. These BIFs are the result of anoxic conditions
that resulted from the stagnation in the oceans beneath a near-global
ice cover, referred to as “Snowball Earth.”
All of the most “primary” mineral assemblages appear to be the
result of chemical precipitation under anoxic conditions. There
are, as yet, no data to support that BIF precipitation was linked
directly to microbial activity. The relative abundance of BIF
throughout the Precambrian record is correlated with a possible
curve for the evolution of the O2 content
in the Precambrian atmosphere.』
Introduction
Distribution of iron-formations throughout Precambrian time
Stratigraphic setting
Sedimentary structures
Diagenetic to very low-grade metamorphic assemblages
Physical and chemical conditions of iron-formation diagenesis
and very low-grade metamorphism
Metamorphism of iron-formations
Theoretical evaluations of some of the conditions of prograde
metamorphism of iron-formation
Rare earth element chemistry of selected iron formations
Organic carbon content; carbon, sulfur, and iron isotopes
Basins of iron-formation deposition
Concluding remarks
Acknowledgments
References cited