The Ely miner. [volume] (Ely, Minn.) 1895-1986, 1902-03-28
How Mesaba ore was Formed.

on the late research made by the
United States geological survey, pre-
senting new and interesting features
regarding the geology OF the Mesaba
district:
n The Keewatin series of the Mime
sofa survey has been sub-divided by
the survey into lower igneus base
metit, corresponding TO the archaen
or basement complex of other parts of
the Lake Superior region, and an
upper well defined sedementary series
resting uncomfortably upon the lower
rocks and corresponding to the lower
Huronian of other parts of the Lake
Superior country. The upper of the
two sedementary series of the Mesaba
district containing the iron ore 1.-THE
Animikie of the Minnesota survey;
the upper Huronian of the United
States geological survey comprises
three Theological units-a qnartzite
below, an iron formation in the mid
die horizon, and a slate abOve-all
previously supposed to be conform
able. 1t is now found that structur-
ally the series is dual one-a quart
ute below (Pokegama) separated by
slight unconformity from an iron and
slate formation above (Biwabik and
Virginia formations The Biwabik
iron formation contains quartZite and
conglOmerate at its base and both
grades upward and literally into the
Virginia slate. In the fullness of it's
succession and in the clear-cut ma
ttlre of the unconformities, the Me-
saba district may stand as Il type pre-
Cambrian district Of the Lake Super
ior region The most interesting of
tile late developments concern the
origin of the iron ores. They have
resulted from the alteration of GER
tain rocks containing green granules,
which. on analysis, prove to be essen
italy ferrous silicate. They lack
potash, and cannot be glauconite and
of organic origin as supposed by
Spur The irOn-bearing formation
bears clear evidence of sedimentary
origin, and it is believed that the
origin of the green granules making
up the original rock is closely similar
o.thatof the cherry iron carbonates
wHich are the original rocks of the
irenJbearing formationsso of other psr-
Lipns of the Lake Superior region.
The iron, probably derived from the
teaching of the archean basic rocks.
was carried in solution. mainly in the
form OF. carbonate, though perhaps in
part as sulphate, To the ocean. in
which iron formation mate: ial was
being deposited. There the carbon
ate of iron was thrown down by oxi-
dation and hydration as iimonile.
The limonite settled and became
mingled with organic material, the
presence of which is shown by the
association with carbOnaceOus states,
and was reduced to the prOtoxide
form. Simultaneous decomposition
OF the organic material freed carbon
diexide. Silica also precipitated
lchert is known to develop under such
conditions) probably through the
agency of organisms. Both of these
subsia nces could combine readily
with the iron protoXide, but in the
case of the Mesaba rocks the main
combination was the protoxide and
silica, giving the ferrous silicate
which we now find. The ferrous sill
cate took the form of granules in the
same way that oOites take their
form. When the iron formation was
brought above the ocean, the ordi-
nary weathering agencies began to
alter the ferrous silicate of the iron
formation. The iron was oxidized
and hydrated, and was concentrated
aud separated from the silica, through
the agency of underground waters.
The deposits are ow found in gently
pitching trOughs in the iron fOrma-
ton, bottomed by thin slaty layers.
new largely altered to paint rocks,
which serve, and have in the past
served. as channels for the outside
circulation of underground water.
The channels have the irregularity
ard complexity of ordinary surface
drainage channels. A conspicuous
feature is their general fiatness, as u
of which the areal eX-
'tent of the ore bodies is very great as
compared to their thickness. Seldom
is the depth greater than 300 feet,
willie the horizontal dimension is US
suaAy many times that figure. In
tffis the Mesaba differs from the old
ranges, where the ore deposits have
much greater or vertical dimensions
and correspondingly small horizontal
The difference in areal
extent is shown by the fact that the
ore deposits liz the Mesaba district
occupy perhaps four per cent of the
total area of the iron-bearing form
~. Ia I'll ,.ay fhn