BANDED IRON FORMATIONS (BIFs)
Banded iron formations, or BIFs, are unusual, dense
sedimentary rocks consisting of alternating layers of iron-rich oxides and
iron-rich silicates. Most BIFs are Proterozoic in age (although some are
Late Archean), and do not form today - they're "extinct"! Many
specific varieties of iron formation are known, and some are given special rock
names. For example, jaspilite is an attractive reddish &
silvery gray banded rock consisting of hematite, red chert
("jasper"), and specular hematite or magnetite. BIFs are known
from around the world, but some of the most famous & extensive BIF deposits
are found in the vicinity of North America's Lake Superior Basin. Many
BIFs have economic concentrations of iron and are mined. BIFs are the
most important variety of iron ore on Earth. Representative specimens
from various iron-formations in America & abroad are shown below and on the
next page.
IRONWOOD IRON-FORMATION
Jaspilite (field of view 9 cm across) from the Ironwood Iron-Formation at
Wakefield, western Gogebic County, far-western Upper Peninsula of Michigan,
USA. BIFs are relatively widespread in Michigan's Upper Peninsula
(UP). The Ironwood Fe-Fm. (upper Paleoproterozoic, ~1.874-1.878
billion years) outcrops in the western UP's Gogebic Iron Range.
Its layers have been steeply tilted by the Penokean Orogeny (~1.85 billion
years) and by mid-continent rifting (~1.10 billion years). The rock shown
here is a classic example of jaspilite. It consists of interbedded
magnetite-rich layers
(gray) (Fe3O4) and hematite-rich layers (dark
red) and hematitic red metachert (“jasper”) layers (bright red).
TEMAGAMI IRON-FORMATION
Jaspilite (field of view ~5.5 cm from top to bottom) from the Temagami
Iron-Formation at the Sherman Iron Mine, Temagami, northeast of Sudbury, north
of North Bay, southeastern Ontario, southeastern Canada. The Temagami
Fe-Fm. of Ontario is late Neoarchean in age (~2.7 billion years). It
consists of alternating bands of dark gray granular magnetite and red
jasper/metachert.
Jaspilite (above & below) - polished sides of a large block of Temagami BIF
on public display at the Cleveland Museum of Natural History (CMNH) in
Cleveland, Ohio, USA.
HAMERSLEY GROUP
Folded jaspilite (“tiger iron”) (field of view ~11 cm from top to
bottom) from Western Australia's Hamersley Range, an area famous for its
"tiger iron" BIFs. Several stratigraphic units in the Hamersley
Group contain banded iron formation (e.g., the Marra Mamba Fe-Fm. & the
Brockman Fe-Fm.). The samples here are folded jaspilites with
bands of yellowish-brown "tiger-eye" (asbestiform quartz having
chatoyancy). The Hamersley Group is an upper Neoarchean to lower
Paleoproterozoic unit (~2.47 to 2.55 billion years) that outcrops in parts of
the Hamersley Range (Pilbara Craton) of Western Australia.
Jaspilite (“tiger iron”) (field of view ~4 cm from top to bottom) from Western
Australia's Hamersley Range.
Folded & faulted jaspilite
LOWER MIDDLEBACK IRON-FORMATION
Here are some BIF samples from South Australia's Lower
Middleback Fe-Fm. (aka Lower Middleback Jaspilite, Middleback
Subgroup) (Hutchinson Group, upper Paleoproterozoic, ~1.859-1.945 billion years
or 1.81 billion years). They are dominantly composed of
silvery-gray hematite
& quartz. The
second sample is quite limonitic
(FeO(OH)·nH2O). Portions of the Lower Middleback Fe-Fm. have
economic concentrations of iron & are actively mined.
Locality:
hillslope ~3 km west of the Iron Duke Mine, southern South Middleback Range,
WSW of Whyalla, northeastern Eyre Peninsula, Cleve Subdomain of the Gawler
Craton, South Australia.
Banded Iron Formation (8.4 cm across at its widest) from South Australia's
Lower Middleback Fe-Fm. Light gray = quartz; dark gray = hematite.
Banded Iron Formation (11.5 cm across at its widest) from South Australia's
Lower Middleback Fe-Fm. Orangish-brown = quartz mixed with limonite;
silvery-gray = hematite.
NEGAUNEE IRON-FORMATION
One of the most famous BIF units in the world is the
2.11 billion year old Negaunee Fe-Fm., which outcrops in the Marquette
Iron Range of Michigan's Upper Peninsula (UP), USA. The Negaunee
Fe-Fm. has been metamorphosed to varying degrees and contains many specific
lithologies, including jaspilites, taconites, specularites, ferruginous
quartzites, ferruginous cherts, and ferruginous slates. Much of the
Negaunee Fe-Fm. has economic concentrations of iron & much of the unit in
the Marquette Iron Range has been mined away. The best remaining, easily-accessible
outcrop is Jasper Knob in the town of Ishpeming, Michigan. Earth's oldest
known macrofossils, Grypania
spiralis, occur in this unit.
Jaspilite (6.1 cm tall) from the Negaunee Fe-Fm. at Jasper Knob, Ishpeming,
central Marquette County, UP of Michigan, USA.
Interbedded specularite-quartzite (7.6 cm across at its widest) from the Negaunee
Fe-Fm. at the Republic Mine, western Marquette County, UP of Michigan,
USA. Silvery-gray = specularite, composed of specular hematite (micaceous
hematite) (Fe2O3); dark purplish = quartzite (SiO2).
GOLDMAN MEADOWS FORMATION
Magnetite banded iron formation (10.0 cm tall) from the Goldman Meadows Formation at
the Atlantic City Iron Mine, north of Atlantic City, southwestern Fremont
County, west-central Wyoming, USA. Some iron mines in west-central
Wyoming exploit BIFs in the Goldman Meadows Formation, a lower
Neoarchean unit (2.87 billion years) exposed in the Wind River Range.
These rocks have been multiply metamorphosed, principally at ~2.8 b.y. and at
~1.4 b.y. The result of this metamorphism is highly contorted folding
& fracturing. The rock shown here is a magnetite-rich banded iron
formation with some quartz and serpentine.
VULCAN IRON-FORMATION
Jaspilite (banded magnetite-jasper) from the Vulcan Iron-Formation
(Paleoproterozoic, ~2.1 billion years) in the Menominee Iron Range, Upper
Peninsula of Michigan, USA. Vulcan Fe-Fm. BIFs are mined in the vicinity
of the towns of Vulcan, West Vulcan, and Iron Mountain. The rock shown
above is from the banded magnetite-jasper facies. CMNH 12715, Cleveland
Museum of Natural History (Cleveland, Ohio, USA).
BIWABIK IRON-FORMATION
Northern Minnesota's Mesabi Iron Range has
numerous iron mines that exploit the Biwabik Fe-Fm. (upper
Paleoproterozoic, >1.85 billion years). The Biwabik contains numerous
iron-rich lithologies. The most visually intriguing lithology is
stromatolitic ferruginous chert (stromatolitic jasper) (aka "Mary Ellen
Jasper"). The convex-upward, layered, columnal structures in the
rock shown below are the stromatolites. Stromatolites are built up by
mats of cyanobacteria living in (typically) very shallow marine
settings. The layering of stromatolites is principally the result of
tidally rhythmic deposition of sediments atop the cyanobacterial mats.
The stromatolites in the four photos below have been
assigned to Collenia undosa.
Stromatolitic ferruginous chert (above & below) - cut & polished surface of
stromatolitic jasper from the Biwabik Fe-Fm. (upper Paleoproterozoic) of
Minnesota's Mesabi Iron Range; CMNH P-26290 (Cleveland Museum of Natural
History, Cleveland, Ohio, USA).
What's especially intriguing about these pieces of
“Mary Ellen Jasper” (above & below) is the sinuosity of the
stromatolite columns (see especially the middle stromatolite indicated above by
white arrows in above photo & the right stromatolite column in the photo
below). This has been considered to represent tracking of the sun over
many seasons. Cyanobacteria are photosynthetic and, like sunflowers, they
“want to face the Sun”, wherever it is. Summer-winter seasonal changes in
the sun's position in the sky will be expected to result in stromatolite
sinuosity.
Precambrian sinuous stromatolites from elsewhere have
been used to show that the number of days per year has changed through
time. Earth used to be rotating much more quickly than at present.
The duration of the year hasn’t changed through time, but the rotation rate
has. The early Earth had more than 1000 days per year!
“Mary Ellen Jasper” (field of view ~5.5 cm from
top to bottom) having slightly sinuous stromatolites from near Mary Ellen Mine,
near Biwabik, central St. Louis County, northeastern Minnesota, USA.
“Mary Ellen Jasper” (field of view ~4.9 cm
across) with stromatolites from near Mary Ellen Mine, near Biwabik, central St.
Louis County, northeastern Minnesota, USA.
PROHIBITION IRON-FORMATION
“Starry Jasper” (field of view 9.0 cm across) -
ferruginous banded chert that's been metamorphosed, folded, faulted, and
veined. This rock is from (apparently) the Prohibition Iron-Formation
(Neoarchean, ~2.7 billion years) near Meekatharra, western Western Australia.