Early Proterozoic strata in the Emily district of the
Cuyuna iron range of east-central Minnesota unconformably
overlie older folded rocks of the North range (North range
group). They are correlative with strata of the Animikie
Group on the Mesabi iron range, which consist of a lower
quartz arenitic sequence (Pokegama Quartzite), an
intermediate iron-rich sequence (Biwabik Iron Formation),
and an upper feldspar-rich, graywacke-shale sequence
(Virginia Formation). In the Emily district, however, the
stratigraphic position of the Biwabik Iron Formation is
occupied by three units of iron-formation separated by
intervening sequences of black shale. Manganese occurs in
the lowest iron-rich unit (informally termed Unit A of the
Ruth Lake area).
Unit A can be divided into six lithotopes. They are: (1)
an epicIastic lithotope of quartz-rich siltstone and shale; (2) a
mixed epiclastic jaspery chert litho tope; (3) an oolitic and
pisolitic lithotope; (4) a thick-bedded lithotope of cherty or
granular iron-formation; (5) a mixed thick- and thin-bedded
lithotope characterized by thick intervals of slaty or nongranular
iron-formation; and (6) a ferruginous chert
lithotope. In general, lithotopes 1, 2, and 3 have shallowwater
attributes, whereas lithotope 6 was deposited in
quieter, presumably deeper water. Lithotopes 4 and 5
interfinger, and thus were deposited in generally similar
sedimentological regimes in water of intermediate depth that
was variably affected by currents. Unit A was deposited
during two trangressive-regressive cycles in a basin that
deepened to the north. Well-rounded grains of terrigenous
quartz, which persist throughout lithotopes 1-5, imply that
much of the sedimentation occurred relatively close to
Although Unit A in the Ruth Lake area has many
mineralogical and chemical attributes typical of "ordinary"
iron-formation, it contains manganese at levels that are one or two orders of magnitude larger than the norm.
Manganese oxides are distributed throughout lithotopes 1-5
as disseminated grains, as thin pods or lenses, and as layers
as thick as 1.5 meters that typically contain about 10
percent Mn, but some contain as much as 20-30 percent. In
addition, Unit A contains two major, laterally persistent
zones about 15 to 18 meters thick, in which the manganese
tenor has been enriched to the 10-50 percent range by
secondary processes. Both enriched zones more or less
coincide with stratigraphic positions occupied by the ooliticpisolitic
lithotope. They contain various proportions of
psilomelane, cryptomelane, hematite, and quartz. Goethite
and manganite may be locally abundant, and where they
occur they are secondary phases that formed during a period
of intense chemical weathering in Late Jurassic or Early
Primary stratigraphic, textural, and mineralogic
attributes of the Ruth Lake strata correspond to those used
by James (1955) to define the oxide (hematite) facies of ironformation.
The hematite and chert are syngenetic, but the
manganese oxides are more likely epigenetic. It is inferred
that the manganese oxides were deposited in porous and
permeable parts of the Ruth Lake sequence by a reflux
process involving reducing solutions that leached manganese
from older rocks of the North range group. The principal
mechanisms for manganese concentration are inferred to have
been early diagenetic, and therefore to have operated in Early
Proterozoic time. Mesozoic weathering phenomena have
been imposed on the rocks and have caused some
redistribution of manganese. The abundance of manganese
makes the Ruth Lake area in the Emily district a potential
target for in situ mining techniques currently being
developed by the U.S. Bureau of Mines and the Mineral
Resources Research Center of the University of Minnesota.
Morey, G.B., Southwick, D.L. and Schottler, S.P., 1991, Manganiferous Zones in Early Proterozoic Iron-Formation in the Emily District, Cuyuna Range, East-Central Minnesota, Minnesota: Minnesota Geological Survey Report of Investigations 39, 42 p.
Morey, G.B.; Southwick, D.L.; Schottler, Shawn P..
RI-39 Manganiferous Zones in Early Proterozoic Iron-Formation in the Emily District, Cuyuna Range, East-Central Minnesota.
Minnesota Geological Survey.
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