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
Fellow of the Geological Society of America (elected 2003)

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.

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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