23 January 2013
To: The Wisconsin State Assembly Committee on Jobs, Economy
and Mining
From: Marcia Bjornerud, Professor of Geology, Lawrence
University,
PhD, 1987, University of Wisconsin-Madison, Structural
Geology
As a geologist who has studied the rocks of the Gogebic
Range for more than
ten years, I would like to share some basic information
about the geologic context of
the iron formation there and its relevance to the mining
bill being considered. I have
just two essential points to convey: 1) that the geometry
and orientation of the iron
formation make it comparatively inaccessible and would
necessitate the removal of
immense volumes of ‘overburden’ rock in an open pit mine (a
problem not faced by
mines in Minnesota and Michigan); and 2) that the waste rock
has significant enough
levels of sulfide minerals that acidic solutions would
likely be generated as precipitation
interacts the fragmented rock, which has much larger surface
area than intact rock.
Point 1: Inaccessibility of iron formation and the volume of
waste rock.
Unlike the Precambrian iron formations mined in Minnesota
and Michigan, the
Ironwood Iron Formation in the Gogebic Range is rather thin
(150 m) and steeply tilted
(ca. 60° to the NNW), and thus has limited natural exposure
at the surface. A 4-mile
long, 300 m (1000 ft) deep open pit mine in the Ironwood
would therefore have a very
large surface area and would require the removal of an
immense volume (more than
330 million m3) of waste rock from the overlying Tyler
Formation, a shale or low-grade
slate (please see accompanying cross-section). This is a
minimum estimate of waste
rock because: 1) it is for the pre-blasted volume of the
rock (fragmented rock takes up
significantly more volume); 2) it assumes unrealistically
steep walls, which would actually
have to be benched; and 3) it does not include waste from
the Ironwood formation
itself – much of which has too little iron to be
economically useful. Responsible
monitoring and managing such a volume of waste rock, and a
pit of such depth, over the
long term would pose significant engineering challenges and
costs. Open pit iron mines
in Minnesota and Michigan do not have much overburden rock
to dispose of; the waste
rock they generate is iron-poor rock from within the iron
formation itself.
Most of the old mines in the Gogebic Range, from late 19th
and early 20th centuries,
were underground shafts or shallow open pits that followed
particularly rich ores down
the dip of the layers. They avoided the Tyler Fm because it
was prone to collapse, and
the waste rock they produced was from the unusable parts of
the Ironwood Fm.
The inaccessibility of the Ironwood, and the impracticality
and expense of dealing
with so much waste rock, are among the main reasons that
companies that have been
involved in iron mining in Minnesota and Michigan – MinnTac,
US Steel, Cliffs
Corporation – have never shown any interest in taconite
mining in the Gogebic Range.
Many of us who are familiar with the geology of the Lake
Superior region are simply
puzzled that GTac or any other company is seriously
interested in the deposit, which is
not of particularly high grade and poses such challenges for
extraction. In my view,
there is nothing in current state mining laws that would
prevent a company from
developing an iron mine in the Gogebic Range, other than the
expense of disposal of
such great quantities of waste rock. It is simply unlikely
to be economically viable. The
proposed mining bill greatly expands the definition of
suitable sites for waste rock
disposal, allowing some public lands to be used for waste
rock piles. This effectively
transfers the cost of the disposal of waste rock from the
mining company to the public.
Point 2: Presence of sulfides in the Tyler and Ironwood
Formations
Although the economic target mineral in the Ironwood
Formation is an iron
oxide (magnetite), the overburden rock that would be
stockpiled as waste (the Tyler
Formation) contains significant amounts of reduced iron as
sulfide (pyrite, pyrrhotite and
related minerals), which could react with water and oxygen
to generate acid mine
drainage. This is has been known for many years.
A 2008 USGS Professional Paper (#1730), “The Gogebic Iron
Range: A sample of
the Northern Margin of the Penokean Fold and Thrust Belt”,
by William Cannon and
others, describes the Tyler Fm as a ‘black pyritic shale and
slate’. Moreover, various
reports on the Ironwood Fm, dating from the 1920s through
1970s, describe the
internal layers within that unit in detail, based on
information from underground mines
and also from cores taken by US Steel in the 1950s. Several
of these reports describe
one 3-m thick layer within the Ironwood that consists
largely of pyrite.
The US Steel cores are now in the hands of RGGS, the company
from which
GTac has purchased mineral rights. Ideally, those cores
would be made available for
scientific analysis. When attempts to get access to the
cores were unsuccessful, a
colleague and I at Lawrence University assembled 18 samples
of the Tyler and Ironwood
formations from surface outcrops and one drill core held by
the Wisconsin Geological
and Natural History Survey and carried out geochemical and
mineralogical analyses on
these rocks using several instrumental approaches (XRD, XRF
and optical microscopy).
We found sulfide minerals in finely disseminated form
throughout the Tyler
formation, and as a major mineral in one particular layer in
the Ironwood formation, as
other studies have found. For the average sulfur content that
we found in these rocks
(about 0.15 % by mass), and the estimate of waste rock
mentioned above, there would
be a total of at least 1.1 billion kg of sulfur in broken
and pulverized rock sitting in waste
piles and easily mobilized through the percolation of rain
and melting snow. When this
rock is undisturbed underground, there is little opportunity
for water to interact with it,
but when it is blasted and pulverized, the net surface area
for interaction with
precipitation is exponentially increased and could lead to
acid drainage. This is a serious
concern because the proposed mining bill would allow such
waste rock piles to placed
in areas where they are currently prohibited.
Waste rock piles can be lined and engineered (e.g,
interlayered with buffering
rock like limestone) to reduce the likelihood of acid
drainage into natural waters. This
is very expensive, however, and it is hard to imagine that
any company would be able to
manage the large volume of waste rock that would have to be
dealt with and then
monitor it responsibly over the long term.
In summary, the scale of a modern open pit mine in the
Gogebic Range would be
completely different from historic mines in the region,
which were localized, primarily
underground mines that targeted high grade ore and did not
involve excavation of the
Tyler Formation. Such a mine would also generate a far
greater volume of waste rock
than open pit mines in Minnesota and Michigan. The waste
piles created by a mine of
this kind will be with us for centuries, long after the
company that created them is gone.
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Editor's note: We now have access to Tim Myers' "rebuttal" and "debunking" data from last Wednesday's hearing. Check this blog next Tuesday or so for our "arm chair scientist's" analysis. It should be fun.
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Editor's note: We now have access to Tim Myers' "rebuttal" and "debunking" data from last Wednesday's hearing. Check this blog next Tuesday or so for our "arm chair scientist's" analysis. It should be fun.
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