Minnesota Minerals Education Workshop
Eveleth, MN
June 21-23, 2011
FIELD GUIDEBOOK
ON THE
GEOLOGY AND MINERAL DEPOSITS OF THE
EASTERN MESABI IRON RANGE
Richard Patelke, PolyMet Mining Company
Mark Severson, Natural Resources Research Institute at UMD
Jim Miller, University of Minnesota Duluth
MMEW FIELD TRIP (June 22-23, 2011)
GEOLOGY AND MINERAL DEPOSITS OF THE EASTERN MESABI IRON RANGE
Richard Patelke, PolyMet Mining Company
Mark Severson, Natural Resources Research Institute at UMD
Jim Miller, University of Minnesota Duluth
Geologic Overview of the Eastern Mesabi Iron Range
The bedrock geology of the Eastern Mesabi Iron Range contains a broad range of rock types that
formed in diverse geologic settings over a wide span of earth history (1.5 billion years). These
same rocks are also host to several types of mineral deposits. On this field trip we will examine
some the main rock types of the eastern Mesabi Iron Range and visit mineral deposits of ironformation, copper+nickel±platinum group metals, and sand and gravel.
Three geologic terranes are exposed along the Eastern Mesabi Iron Range (four counting the
glacial cover). Each represents a unique stage in the formation of the North American continent
during the Precambrian (Fig. 1).
Terrane
Age*
Formational Stage
Superior Province
~2.6
(Granite-Greenstone)
forms the core of the North American continent (Kenoraland);
created by the tectonic merging of volcanic (greenstone) terranes
and the emplacement of granite intrusions.
Penokean
represents sediments deposited on the margin of the new
continent; to the south, these sedimentary rock were deformed
and metamorphosed during the formation of the Penokean Mtns
Midcontinent Rift
* in billions of years(Ga)
~1.85
1.1
represents igneous activity related to a failed attempt by the
North American continent (Laurentia) to split apart, thus
forming the Midcontinent Rift
Figure 1. Generalized geology of northeastern Minnesota
1
The Archean-aged granite-greenstone terrane (formally known as the Superior Province) is
exposed north of the Mesabi Iron Range. It is composed of a variety of volcanic and sedimentary
rocks that have been deformed, moderately metamorphosed, and intruded by magma that
crystallized to form large bodies of granite. This assemblage of rocks is interpreted to represent
the tectonic collision of volcanic island arcs (like those that ring the western Pacific Ocean today)
and small protocontinents (represented in Minnesota by the ancient gneiss terrane of the
Minnesota River Valley). The merging of these volcanic terranes and the related intrusion of
granite occurred about 2.6 billion years ago and came to create the core of the North American
continent – a landmass we call Kenoraland.
About 2 billion years ago, a part of Kenoraland separated (rifted) from the main landmass
and the area now occupied by the Mesabi Iron Range became the approximate continental edge of
the Animikie Sea. Along this continental margin, not only were fine sediments (sand, silt and
mud) being deposited from the erosion of Kenoraland, but a unique type of "chemical" sediment
was being deposited as well – iron-formation (Fig. 2). During this time in earth history, simple
photosynthesizing organisms (cyanobacteria, or algae) had just evolved and this biological event
had a dramatic effect on the local chemistry of the oceans and the atmosphere. Prior to this
evolutionary stage, the oceans and the atmosphere were deficient in free oxygen. Under these
conditions, reduced iron could be easily dissolved and built up in seawater. With the arrival of
oxygen-producing organisms, which formed algal "reefs" along the shorelines of the world's
continents, this iron was quickly (geologically-speaking) precipitated as iron minerals and
collected on the shorelines. By this biologically-mediated process (referred to as the Great
Oxidation Event or GOE) most of the major iron-formations of the world were created. The
present position of the Mesabi Iron Range roughly approximates the northern edge of the
Animikie Sea – the body of water into which iron-formation and other sediments were deposited.
A
Offshore
(mud, turbidites)
ANIMIKIE SEA
Si,Fe
B
glacial till
S
Algal Reef
Nearshore
(iron oxide, chert)
Sediment
(sand, silt, mud)
O
Archean Crust
Natural Ore
Natural Ore
N
Virginia Fm.
Biwabik Fe-Fm.
Pokegama Qtzt.
~ 1 km
Figure 2. A. Depositional environment of the Biwabik Iron Formation and related sedimentary
rocks of the Mesabi Iron Range. B. Idealized cross section across the Mesabi Iron Range
showing the relationship of faulting to the occurrence of natural ores.
2
The nearly flat-lying sedimentary rocks of the Eastern Mesabi Range we see today indicate
that these roughly 1.85 billion year old rocks have been only mildly deformed since their
deposition at the margin of the Animikie Sea. Following these same rocks to the south, however,
we find that they become strongly folded, faulted, metamorphosed (to slates and schists), and
intruded by granite. This transformation resulted from the tectonic collision of Kenoraland with a
volcanic island arc (now exposed in central Wisconsin) about 1.8 billion years ago (Fig. 3). This
collision formed the Penokean Mountains – an impressive mountain belt that stretched
northeasterly across central Minnesota and northern Wisconsin (Fig. 4). The addition of this new
Penokean terrane to Kenoraland gave rise to a larger continental mass called Laurentia. The
Penokean Mountains did not last long, however. By 1.7 billion years ago they were eroded flat.
Figure 3. Tectonic model for deposition and deformation of sediments deposited in the Animkie
Sea about 1.85 billion years ago.
With the formation of Laurentia, roughly two-thirds of present-day North America had been
created. The remainder would come with the addition other mountain belt terranes along the
margins (e.g. the Grenville Mtns, the Acadian Mtns, the Appalachians, the Rocky Mtns).
However, continents do not always just grow, but at various times may rift off part of themselves
(as Kenoraland did to form the Animikie Sea). About 1.1 billion years ago, Laurentia was
attempting to separate off a part of itself along a continental rupture called the Midcontinent Rift.
This rift arched from lower Michigan, through the Lake Superior region, and southwest into
eastern Kansas (Fig. 4). As the land mass inside the arching rupture migrated southward, basaltic
magma welled up from deep in the earth's mantle to erupt layer upon layer of lava flow into the
breach.
3
Midcontinent
Rift
Granite-Greenstone
Terrane
Animikie
Sedimentary
Rocks
Volcanic Arc
Figure 4. Trace of the Midcontinent Rift arcing across Penokean terrane. Although the figure
shows the extent of the Penokean Mountains which formed at 1.85 Ga, the mountain belt
was eroded to a flat plain by the time the Midcontinent Rift developed at 1.1 Ga. Arrows
show the general direction of separation of the Laurentian continent.
At its greatest separation in the Lake Superior region, the lava piled up to over 20 kilometers
in thickness – more than half the thickness of the crust. After a time, some of the magma began
to stage in sheet-like magma chambers at the base of the lava pile. Being insulated by the
overlying volcanic rocks, these magmas cooled slowly to form a coarse-grained igneous rock
called gabbro. More or less continuous bodies of gabbro can be traced from Duluth to the
Canadian border in northeastern Minnesota (Fig. 1). We call this mass of intrusive igneous rock
the Duluth Complex. In the eastern Mesabi Range, the intrusions of the Duluth Complex invaded
below the lowest volcanic rocks of the rift and therefore we now find gabbro in direct intrusive
contact with older rocks of the Mesabi Range and the Granite-Greenstone terrane (Fig. 1).
Most continental rifts progress to complete continental separation but the Midcontinent Rift
did not. The reason for this probably has to do with the fact that at this same time what is now
South America was colliding with the eastern margin of Laurentia (proto-North America) to
create the Grenville Mountains (the remnants of which stretch across upstate New York and
southern Quebec). That collision probably closed the Midcontinent Rift. With the cessation of
crustal separation and volcanic activity, the heavy basalts and gabbros filling the Midcontinent
Rift caused the crust to sag and the Rift to become infilled with sand from the surrounding
highlands. These sands are now the sandstones of east central Minnesota and the Bayfield
Peninsula. The sagging also tilted the rocks on the margins of the rift, whereupon erosion has
4
now exposed the once deeply buried base of the volcanic pile and its collection of gabbro
intrusions.
Although the area of the Eastern Mesabi has been tectonically quiet for over a billion years,
the "recent" invasion of continental glaciers over the past 2 million years has sculpted a new
landscape. With glaciers being one of the most powerful erosive agents on earth, these mighty
bulldozers of nature wore down the weak rocks in the area and left the more resistant rocks
standing as higher ground. It is no coincidence that the great watershed divides between
Hudson's Bay, Lake Superior, and Mississippi River drainage run along the granite highlands of
the Giants Range batholith just to the north of the Mesabi Range. Granite is a very erosion
resistant rock. The glaciers in their last retreat about 12,000 years ago left behind a blanket of
sediment that covers most of the crystalline bedrock. Some of this sediment is molded into
glacial landforms such as moraines, eskers, and drumlins.
Mineral Deposits of the Eastern Mesabi Range
The formation of economical mineral deposits depends on a unique combination of factors
coming together at the right time and at the right place in the earth's crust. The creation of iron
formation coincided with a singular irreversible oxygen producing evolutionary event occurring
at a time when the shoreline of the Kenoraland continent coincided with what became northern
Minnesota. The positioning of copper-nickel sulfide deposits along the base of the Duluth
Complex coincided with mixing of the right ingredients of metal-rich magma and sulfur-rich wall
rocks and with just the right amount of erosion to expose these deeply formed deposits at the
present day land surface. Indeed, one of the most important ingredients for making an economic
ore deposit is luck. And at least in terms of iron ore and perhaps copper, nickel and platinum ore,
Minnesota has been particularly lucky.
Iron Ore – The Mesabi Iron Range (Fig. 5) is one of the most important mining districts in the
world. The Biwabik Iron Formation, the name given to the geological formation exposed along
the Mesabi Iron Range, has produced around 4 billion metric tons since the first shipment in
1892. There are six operating iron mines employing about 3,500 people in the region. They all
produce taconite pellets from low grade magnetic ore. Four are captive to steel companies, two
produce pellets for market, generally through long term contracts. Product shipping is largely by
rail to one of four ports on Lake Superior (Superior, Duluth, Two Harbors, and Silver Bay), then
by boat to mills on the lower lakes. About thirty eight million tons of pellets were produced in
2007. Total material movement (ore and stripping) was on the order of two hundred million tons.
The one-hundred plus year history of this world class mining district means that there is an
extensive developed infrastructure and service industry for these mines.
Mesabi Nugget in Hoyt Lakes is producing iron nuggets for use in electric arc furnaces by a
so-called direct reduction process. Essar Steel in Nashwauk is re-opening the closed Butler
Taconite mine with an aim to eventually making slab steel on site from direct reduced iron
pellets, as well as taconite pellets for Algoma Steel in Sault St. Marie Ontario.
The Biwabik Iron Formation was produced by the biologically-mediated precipitation of
iron-rich and silica-rich gels from seawater some 1.9 to 1.85 billion years ago. When these
sediments became lithified through the process of diagenesis (by burial, compaction, and
5
Figure 5. (A) Map of Mesabi Iron Range with aerial distribution of taconite pits (magenta) and
cities (blue). Trend of Biwabik Iron Formation is outlined in red (modified from Jirsa et al.,
2005). (B) Longitudinal section (looking north) of the Biwabik Iron Formation showing:
variation in thickness of the iron-formation members at each taconite operation; mined
taconite intervals at each operation represented by black bars.
dewatering) to form rock, some of the original iron minerals were transformed into the mineral
magnetite (Fe3O4), and the silica gel hardened to a microcystalline form of quartz (SiO2) called
chert. Sediments deposited in deeper water hardened to clay. Because the relative rates, and
relative sites, of sedimentation of these three materials varied, iron-formation is typically banded
between "slatey" layers (deeper water) and chert-rich layers (shallow water) that contain variable
amounts of magnetite. In fact, these rocks are often referred to as banded iron-formations, or
BIFs.
Where this BIF is particularly rich in magnetite (30% or more magnetite, about 22% iron),
the rock is referred to as taconite ore. This is the type of iron-formation being mined today. The
processing takes advantage of the fact that magnetite is, well, magnetic. As will be explained in
greater detail during our visit to the taconite plants, the taconite ore is crushed, mixed with water
and ground to a very fine powder, and then passed over magnetic drums that separate the
magnetite from the clay and chert.
During the first half of the 20th century, however, a different type of iron ore was mined one called natural ore or direct-shipping ore. The natural ore formed by the oxidation of the
iron-formation along fault zones by either descending meteoric fluids or ascending hydrothermal
fluids – two entirely different origin processes that have been debated by geologists over the last
100 years. Whatever the origin, this alteration caused the magnetite to be oxidized to other types
of iron oxides (hematite, goethite, limonite) and caused the dissolution of chert leaving behind a
rock strongly enriched in iron (>65% Fe). As can be seen in Figure 2B, the locations of these
natural ore bodies usually occurred along fault zones cutting the iron formation, suggesting that
faults provided pathways for the altering fluids.
6
Cu-Ni-PGE Ore – The magmatic intrusion of the Duluth Complex into older rocks of the Mesabi
Range and the granite-greenstone terrane has resulted in the creation of one of the largest
concentrations of copper (Cu), nickel (Ni), and platinum group element (PGE) ore in the world.
For the past 50 years, at least 22 mineral exploration companies have explored along the base of
the Duluth Complex and by drilling into the bedrock (over 2,000 holes totaling over 2.0 million
feet of drill core) they have identified 10 areas sufficiently enriched in Cu-Ni±PGE sulfide ore to
be potentially mineable (Fig. 6). Although the total tonnage of ore is larger than any other
deposit of its type, the concentration of copper, nickel, and PGE in the ore is low. With more
efficient processing techniques and demand for PGE for auto catalysts, however, these deposits
once evaluated for copper and nickel are currently being reevaluated as sources for copper,
nickel, cobalt, platinum, palladium, gold, and silver.
Figure 6. Cu-Ni-PGE and Fe-Ti deposits of the Duluth Complex (from Severson & Hauck, 2008)
These deposits formed when hot gabbroic magmas of the Duluth Complex, rich in metals,
came into contact with older rocks, locally rich in sulfur. So much sulfur was assimilated by the
gabbroic magma that it caused the sulfide to unmix from the magma as separate liquid. Given a
choice between residing in a sulfide liquid or a silicate magma, metals will strongly choose the
sulfide melt. So as the dense droplets of sulfide liquid would form, they would settle through the
magma and settle on the floor of the chamber, scavenging metals along the way. With the
cooling and crystallization of the sulfide melt and magma, the resulting rock became a gabbro
typically containing several percent sulfide minerals of iron, copper, nickel, and trace amounts of
PGE. Again, were it not for erosion which removed about 2 kilometers of lava flows overlying
the Duluth Complex these deposits would not be accessible.
7
Sand and Gravel – Many people are surprised to learn that the mining and processing of sand
and gravel (aggregate) resources is the largest mining industry in the State of Minnesota. And
while thanks to the glaciers, we have a fair abundance of unconsolidated sediment spread over the
surface of the state, much of this sediment contains too much clay to be used. Indeed, just like
other mineral deposits, some special factors had to come together to produce a "washed" sand and
gravel deposit. Usually washing required the sorting and winnowing action of glacial meltwaters
as it carried away the glacial sediment. Consequently, well-washed sediment can be found in
meltwater channels, on outwash plains that spread out from the margins of the receding glaciers,
and in eskers, which represent sediments deposited in subglacial meltwater valleys.
Figure 7. Major glacial ice lobes that advanced over Minnesota around 14,000 years ago. In
their retreat around 12,000 years ago, the glaciers left behind deposits of glacial till and
outwash .
8
Field Trip Stops
DAY 1
POLYMET PLANT SITE AND BIWABIK IRON FORMATION
AT THE INACTIVE LTV MINE, AND WETLEGS CU-NI DEPOSIT
At the inactive workings of the LTV mine are eleven separate open pits from which the
Biwabik Iron Formation was mined. Initially operated by the Erie Mining Company, this was the
second taconite mine to be established on the Mesabi Iron Range with production starting in
1957. Most of the original LTV facilities have been purchased by PolyMet Mining Company and
will be used to process Cu-Ni-PGE ore from their NorthMet deposit in the nearby Duluth
Complex (Figs. 8 and 9).
The Erie plant was the largest project ever built when done in the 1950’s at a cost of $330
million. Construction included the plants, mines, 70 mile railroad, enough shops to be selfsufficient, the Town of Hoyt Lakes, a power station, and docks on Lake Superior.
PolyMet has a resource of about 1 billion tons. Within that resource there is a reserve of
about 275 million tons of ore. The current plan (June 2011) is to mine about 225 million tons over
twenty years at a grade of 0.28% copper, 0.08% nickel, 70 ppm cobalt, plus platinum, palladium,
and gold.
The Process Plant design is based on key parameters determined by the characteristics of the
deposit to be mined and the beneficiation and hydrometallurgical processes that will extract the
metals from the ore. On average, 32,000 short tons of ore will be processed each day. This
results in annual production of ________tons of copper concentrate, ______tons of nickel and
cobalt concentrates, about 22,000 ounces of platinum, 87,000 ounces of palladium and 13,000
ounces of gold.
The process plant is located at the LTVSMC plant site. The entire Process Plant is in an
area that was previously disturbed by mining operations. The Beneficiation Plant will use the
Coarse Crusher Building, Fine Crusher Building and Concentrator Building that were part of the
LTVSMC taconite plant.
The flotation process has been designed to recover virtually all of the sulfide minerals to the
concentrate and minimize the amount of sulfide minerals remaining with the tailings. This
process has been tested with ore samples representing the NorthMet ore in a pilot plant set up to
represent the proposed grinding and flotation process. The flotation tailings generated by this
work has been subjected to rigorous waste characterization. Collectively this has demonstrated
that the flotation tailings can be placed in the LTVSMC Tailings Basin. The result is that there is
no surface discharge of process water at the Plant Site and that the requirement for make-up water
via water appropriation from Colby Lake is minimized.
9
Figure 8. PolyMet’s NorthMet project site.
10
Figure 9. Detail of Erie Plant site showing existing facility and proposed construction.
11
Stop 1-1: PolyMet offices – bathroom stops and introduction to PolyMet staff.
Stop 1-2: Coarse Crusher
Stop 1-3: Rod and Ball Mill
Stop 1-4: Tailings Basin
Stop 1-5: Lower Cherty and Lower Slaty members of the Biwabik Iron Formation
Location: Pit 5E, Cliffs-Erie site, T. 60 N., R. 13 W., sec. 31, SW, SW
Allen quadrangle; UTM: 571,040E/5,275,876N (NAD-83)
Description: At this inactive mine pit the entire stratigraphic section of the Lower Cherty and
Lower Slaty members can be viewed. Also present at this site are localized exposures of the
underlying Pokegama Formation (quartzite) and granitic rocks of the Archean Giants Range
Batholith. Both are exposed in the floor of the mine where they are present as several small
domal features
The Lower Cherty member at this site is about 80 feet thick. Most of the Lower Cherty
constitutes taconite ore. At the bottom of the iron-formation are several thin units that were each
formed in different energy environments due to deposition at variable depths below wave and
storm action. These units consist of: conglomerate (high energy), algal stromatolite horizons
(moderate energy), thin-bedded iron-formation (low energy) and pale gray chalcedonic chert/flint
bands with quartz-filled syneresis cracks (indicating that the chert was deposited as a silica gel).
The stromatolites are recognized by bright red jasper columns. The conglomerate contains a
variety of iron-formation clasts with a sprinkling of detrital quartz grains from the underlying
Pokegama Formation.
Most of the rock in the pit wall at this stop exhibits what is called wavy-bedding in that the
dark-colored magnetite-rich bands show pinching, swelling, terminations, and splitting in a semirandom fashion. Cross-bedded features are also present in the wavy-bedded iron-formation
indicating that they were deposited in a relatively high-energy environment (shallow water on the
edge of a shoreline). Also present within the wavy-bedded iron-formation are pink-colored
spheres, or mottles less than 1 cm across, that are composed of ankerite (iron-carbonate).
The Lower Slaty member of the Biwabik Iron Formation is exposed along the southern edge
of this pit. It is a weakly-magnetic rock and is not taconite ore. The rock is characterized by thinbedded rock indicative of deep water deposition (low energy environment). At the very base of
the Lower Slaty member is the "Intermediate Slate" which is a thin-bedded, black, organic-rich
mudstone that may contain volcanic tuffaceous materials. The "Intermediate Slate" is extremely
fissile (readily breaks into small chips) and locally exhibits bright, shiny graphitic surfaces with
bedding-parallel slickensides (grooved surfaces that indicate one rock face moved past another
rock face).
12
Stop 1-6: Biwabik Iron formation at the Cliffs-Erie site
Location: Pit 3, Cliffs-Erie site, T. 59 N., R. 14 W., secs. 14 and 15, Allen quadrangle
Description: While continuing to stop 7 the bus will go through Pit 3 where ore was mined from
the Lower Cherty member. Note that the undulating floor of this pit was exposed during striping
by following the bedding trends of one or two bed forms. Use your imagination and peer back
through time (1.9 by) to reconstruct a scene where these roll-and-swell bedding planes represent
iron-rich sediments that are accumulating along a continental shelf on gently rolling surfaces.
Stop 1-7: Archean/Paleoproterozoic unconformity at the Cliffs-Erie site
Location: Cliffs-Erie site, T. 59 N., R. 14 W., sec. 14, NE, NW, NE
Allen quadrangle
Description: After a short walk from the bus, this stop is an excellent exposure of a 700 million
year break in the geological record. Such breaks are called unconformities. At this location,
steeply-dipping Archean (2.6 by) sedimentary rocks are unconformably overlain by the
Pokegama Formation (1.9 by). Again, imagine yourself to have traveled back in time (1.85 by) to
a sandy beach on the northern shore of an inland sea. As you look to the south all you see is a
vast sea, but if you could look underwater you would notice that iron-rich sediments of the
Biwabik Iron-Formation are accumulating on the continental shelf, and further out, deep water
sediments of the Virginia Formation are accumulating. As you turn and look to the north you
would see a low range of hills, consisting of the ancient Archean rocks, from which streams
would meander down to the inland sea. Now, back in the present, it is possible to actually put
your finger on the unconformity line that separates rocks that are 2.6 by old from rocks that are
1.8 by old. This gap in time represents approximately a 700 million year time span - a span that
is 13 times greater than the span of time that has passed since the dinosaurs last roamed this
planet. Try to imagine the immense passage of time represented by the sharp contact between
these two different-aged rocks and think about what could have taken place in that time span.
Stop 1-8: Algal submember near the top of the Upper Cherty member, Biwabik Iron
Formation
Location: Pit 2E, Cliffs-Erie site, T. 59 N., R. 14 W., sec. 14, N 1/2, NW
Access to this site is via Dunka Road, which is a private mining company road.
Allen quadrangle; UTM: 568,202E/5,270,625N (NAD 83)
Description: Algal structures were first described by Leith (1903) as "contorted bedding." Grout
and Broderick (1919) were the first who assigned an organic origin to them. This locality is an
excellent place to view a nearly horizontal portion of the iron-formation that contains abundant
individual mounds of algal stromatolites. Internally, the mounds are characterized by many
individual, columnar, finger-like structures that are convex upward. Recent studies have shown
that the bacteria that produced these algal mounds were able to directly precipitate iron (an
intermediary oxygen-producing stage was not necessary to precipitate the iron). Between the
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mounds are conglomeratic units that were deposited as storm debris. Again, use your imagination
to peer back through time to reconstruct a scene where shallow waters along the shoreline of the
Animikie sea are covered with close-spaced algal mounds that are responsible for the deposition
of the iron-formation. A remarkably similar scene of algal mounds could be viewed today at
Shark Bay in Australia.
Stop 1-9: Virginia Formation near Siphon fault
Location: Cliffs-Erie site, T. 59 N., R. 14 W., sec. 14, SE, SE, NE, Allen quadrangle
Description: This area contains the only natural exposures of the Virginia Formation on the
Mesabi Iron Range. Unfortunately, the formation is only a few 10s of feet thick at this site. The
Virginia Formation overlies the Biwabik Iron Formation and a total of 1,443 feet of the formation
is present in drill cores from holes drilled south of the Mesabi Iron Range. At this stop, note the
sedimentary features such as: graded beds, mud chips, concretions, thin cross-bedded lenses, and
loading at the base of some beds. These sedimentary features are indicative of deposition by
turbidity currents which are generated when loose sediment near the shoreline is jostled by storm
activity (or seismic activity) and the resultant muddy slurry flows downward along the bottom of
ocean. Note that the bedding is near vertical in this location due to proximity to the north-trending
Siphon fault. The sedimentary features indicate which way the beds top – which is to the east.
Also present at this site is a large glacial erratic of marble derived from a horizon at the top
of the Biwabik Iron Formation. This marble horizon is unique in that it records evidence of a
major catastrophic event that occurred over 500 miles to the east when a meteorite struck the
earth near the present town of Sudbury, Ontario. This Sudbury Impact Event, produced by a
meteorite over 10 miles in diameter, is the second largest such impact on the planet and took
place 1.85 b.y. ago. The table below shows the effects that might have been expected in this area
(calculated for the Gunflint Lake area of Minnesota by Mark Jirsa). Some scientists have
suggested that the impact drastically changed the environment, wiped out the bacteria in the
Animikie Sea, and subsequently stopped deposition of the iron-formation.
Arrival Time
~13 seconds
~2-3 minutes
Effect
Fireball
Earthquakes
~5-10 minutes
Airborne ejecta arrives
~40 minutes
Air Blast
~1-2 hours
Tsunami
14
Modern Analog
3rd degree burns, trees ignite
Richter scale 10.2 at Sudbury,
seismic shaking at Siphon
Fault site
Layer at base of marble
contains
airborne
microtektites and shocked quartz
Maximum wind speeds ~1,400
mph
Tokyo 2011?
Stop 1-10: Wetlegs Copper-Nickel deposit (railroad cut off Dunka Road)
Location: Rail tracks just south of Dunka Road T. 59 N., R. 13 W., sec. 17, NE, SE, SE
Allen quadrangle; UTM: 572,72E/5,271,178N (NAD 83)
Description: This stop is a chance to see and sample gabbroic material from the Duluth Complex
with copper-nickel (Cu-Ni) mineralization. Drilling in this area by three exploration companies
(International Nickel Co., Bear Creek Mining, and Exxon Minerals) during the late1950s through
the 1970s outlined a small Cu-Ni deposit of about 38,000,000 tons of material grading 0.57%
Cu+Ni. Sulfide-bearing rocks exposed in the railroad cut consist of heterogeneous ophitic augite
troctolite to olivine gabbro with patches and lenses of augite-rich pegmatite. Note the rusty ironstaining that is characteristic of the rock. This type of staining is normally looked for when
exploring for Cu-Ni deposits and is due to weathering and oxidation of the sulfides in the rock.
The sulfides that are present include pyrrhotite (iron sulfide), pentlandite (nickel sulfide), and
chalcopyrite (copper sulfide). One of the gabbroic exposures on the tracks exhibits variably-sized
anorthosite inclusions that range from 2 inches across to 10x15 foot blocks. The anorthosite
crystallized earlier from a plagioclase-rich magma only to be broken-up, surrounded and carried
upward by later gabbroic magmas.
Two other major base metal deposits occur further to the northeast in the Duluth
Complex: the NorthMet (Dunka Road) Cu-Ni deposit and the Mesaba (Babbitt) Cu-Ni deposit.
The NorthMet deposit, initially drilled by United States Steel Corp. (1969-1974) is currently
undergoing environmental review to be permitted as a possible mine by PolyMet Mining Corp.
PolyMet estimates that the deposit contains about 1 billion tons of material grading 0.43% Cu and
0.11% Ni with localized concentrations of 1-2 ppm Pd (around 0.03 oz/ton). Probably the largest
of the known Cu-Ni deposits is the Mesaba deposit which was drilled and defined by Bear Creek
Mining (1959-1971) and by AMAX (1974-1978). The deposit is currently controlled by Teck
American., Vancouver, B.C., who are also trying to establish a non-ferrous mine in the state of
Minnesota. Teck estimates that the Mesaba deposit, contains about 1 billion tons of material that
grades about 0.46% Cu and 0.12% Ni.
DAY 2
UNITED TACONITE MINE AND FAIRLANE PLANT, IDEA DRILLING,
AND MINE VIEW IN THE SKY
Stop 2-1: United Taconite Mine/Thunderbird North Mine (Eveleth, MN)
Description – At this locality, magnetic taconite is mined from the Lower Cherty, Lower Slaty,
and Upper Cherty members of the Biwabik Iron Formation. Direct-shipping ore, also referred to
a natural ore, was originally mined in the immediate vicinity. Exploration for taconite at this site
began in earnest in 1960, after the opening of pioneering taconite operations at the Reserve
Mining Company (now Northshore) and Erie Mining Company (now Cliffs-Erie site) in the mid1950s. Drilling by Oglebay Norton Company identified several magnetite deposits in the area
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and the property was jointly developed by the Ford Motor Company – groundbreaking occurred
in June, 1964.
The Thunderbird North mine and Fairlane plant began producing in November, 1965,
with an initial rated capacity of 1.6 million tons of iron ore pellets per year. In 1977, addition of a
second concentrating and pelletizing line, and the opening of the adjacent Thunderbird South
mine, increased rated capacity to 6.0 million tons of pellets. The Thunderbird South mine closed
in 1992, and in 1996, ownership of the operation was transferred to Eveleth Mines LLC. Eveleth
Mines closed the concentrating and pelletizing Line 1 in May, 1999, reducing capacity to 4.2
million tons of pellets. The remaining operation was idled in May, 2003.
The idled facility was purchased and re-opened by United Taconite LLC in December,
2003 (now owned 100% by Cliffs Natural Resources). They subsequently refurbished and
reactivated Line 1 in December, 2004, which increased the annual rated capacity to 5.2 million
tons of pellets.
Field trip stops at this site will include a general view of the mine operations and other
geologic sites to be announced on the day of the field trip.
Stop 2-2: Fairlane Processing Plant (Forbes, MN)
Description – Taconite is an economic term for iron-formation from which iron can be profitably
extracted after fine-grinding the rock followed by magnetic separation and pelletizing. Because
taconite is incredibly hard there are several steps that are needed to grind it to a face powder
consistency. Figure 10 is a generalized flow sheet that illustrates the basic steps to produce
taconite pellets. This chart should be taken with you while taking the tour through the plant to
help you understand what you will be viewing (the plant is noisy and it is difficult to hear what
the guides are saying all of the time).
A few items that are not displayed in this figure are:
 It takes 3 tons of mined iron-formation to produce 1 ton of pellets;
 Most of the cost of producing pellets is in the first crushing phase – it takes a lot of
energy to crush the extremely hard rock to a consistency of face powder;
 Water is added to the process in the rod and ball mills – at least 20 rods (out of over 100
rods) are replaced in the rod mill every other day due to intense wear-and-tear in
crushing the hard rock;
 Magnetite is separated from the rock. Bentonite clay (or an organic binder) is added to
this magnetite powder in the balling drum to hold the material together as spherical
pellets that are made by a rolling action; and
 Several temperatures are needed in the furnace as follows:
o Pre-oxidation temperature of 800º F is initially needed to convert the magnetite
pellet to a hematite pellet through oxidation (90% oxidized);
o A maximum of 2200-2400º F is reached to indurate the pellet (a temperature of
>2500º F starts to melt the furnace); and
o A final heating stage of 600º F converts any remaining magnetite to hematite –
this final oxidation step is recognized when all of the free oxygen in the furnace
is consumed by burning.
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Figure 10: Simple flow sheet of steps to produce taconite pellets from magnetic iron-formation
(from 1959 USS training manual).
Stop 2-3: Idea Drilling Office (Virginia, MN)
Here we will see how the exploration core drilling process works. IDEA is one of the main
service companies to the local exploration community. Weather permitting they will set up a drill
rig in the parking lot and help us explain the components of the process.
Mechanically, the drill rig consists of a power unit that is capable of rotating a series of
interconnected steel rods (called a core barrel). A tubular steel bit, in which industrial-grade
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diamonds are set, is screwed onto the bottom of the core barrel. This bit, and core barrel above it,
are rotated at speed under controlled pressure. Water is pumped down through the core barrel to
cool the bit and to remove rock cuttings that are generated as the diamonds in the bit abrade the
rock. With advance of the bit, a cylindrical core of rock (called drill core) passes up into the core
barrel. Retrieval of the drill core used to be by withdrawing all of the steel rods to get at the
bottom-most rod. This tactic has since changed with the invention of a wire line which is a
“grabbing” tool that is sent down within the string of rods, slips over the drill core, and the entire
assembly (along with the drill core) can then be pulled back to the surface. Ask the drillers to
show you how the wire line assembly works. Other questions should become obvious as you are
shown the basics of how drilling rigs operate.
Stop 2-4: Giant’s Range Granite at the Laurentian Divide wayside rest, Hwy. 53.
Location: T.58N., R.17W., sec. 19, SE, SE; wayside off Highway 53: Virginia Quadrangle;
UTM: 534,337E/5,269,458N.
Description: At this stop, we see outcrops of the Giant’s Range batholith, the “granitic” portion
of the Archean “granite-greenstone” terrane. The Giant’s Range granite crops out from Babbitt,
on the east Range, to west of Hibbing, a distance of about 60 miles. Rising 200 to 400 feet above
the surrounding countryside, it forms the hills that we see in the distance to the north of the
Rouchleau mine overlook (Stop 2) and at various points along the trip. These hills form the
Laurentian Drainage Divide in this area – water on the northern side of the divide flows to
Hudson Bay; on the southern side, water flows to the Atlantic ocean via the Great Lakes and the
St. Lawrence Seaway.
These granitic rocks intruded, or forced their way into, the pre-existing volcanic and
sedimentary rocks (outcrops ¼ mile to the south), metamorphosing and deforming them about 2.6
billion years ago. Subsequent erosion exposed these deep-seated rocks at the surface by the time
the younger Biwabik Iron formation was deposited on top of them (about 1.9 billion years ago),
as we now observe just to the south of here. The lighter colored rock is called tonalite while the
darker is a diorite; which intruded which? Other dark patches may be inclusions of
metamorphosed volcanic or sedimentary rocks which the “granite” intruded. The complex
intrusive relationships of one rock into another, and vice versa in other areas, have led to the
naming of this exposure as “Confusion Hill.”
Stop 2-5 (optional): Gravel pit near Britt, MN
If time permits we will make a stop at a gravel pit. The sand, gravel, and boulders in this
pit were left behind by glaciers as they retreated from this area around 12,000 years ago (Fig. 7).
The materials in this pit are typical of the Rainy Lobe - they were quarried out of the granitegreenstone terraine to the north and then carried southward by the glaciers to their present site.
Every boulder in this pit has a story to tell - in what rock did they originate, when did they form,
and how did they get here. Find a geologist/guide in your group and test their knowledge by
having them “interpret” the stories that the rocks tell, and then see if you too can hear the stories.
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Stop 2-6: Mineview in the Sky Overlook (Virginia, MN)
Location: T.58N., R.17W., sec. 17, NW, SE; north of the junction of highways 53 and 135;
Virginia Quadrangle; UTM: 535,710E/5,261,650N.
Description: This stop presents an excellent view of a typical “natural ore” mine within the
Biwabik Iron-Formation of the Mesabi Range. The term “iron-formation” refers to bedded
sedimentary rock that contains 15 percent or more iron. Natural ore, also referred to as direct ore
or high-grade ore, was the first type of ore mined from the Mesabi Range at the turn of the 20th
century. This ore type was composed mainly of the minerals hematite and goethite and was
easily dug out of the ground due to its soft/weathered nature. Natural ores were formed by
oxidation and leaching of the Biwabik Iron Formation by circulating water, in the vicinity of
faulted and/or fractured zones, to produce a soft iron-enriched material (Fig. 2B). Almost the
entire section of the Biwabik Iron Formation (over 700 feet thick) can be viewed on the sides of
the pit at this location. The Rouchleau Mine, and a series of adjoining smaller mines, extends for
a distance of over 3 miles to the north along a NNW-trending fault (see Mesabi Range map).
Note the high hills on the northern horizon, formed by the Giant’s Range granite. Also on the
northern horizon, and a little to the west, is United States Steel Corporation’s Minntac taconite
mine and plant. At this locality, magnetic taconite is mined from the lower portion of the
Biwabik Iron Formation. Taconite is an economic term for iron-formation from which iron can
be profitably extracted after fine-grinding the rock followed by magnetic separation and
pelletizing. Currently, there are six operating taconite mines on the easternmost 2/3rds of the
Mesabi Range.
Stop 2-7: Archean pillowed basalt lavas at Gilbert Junior High School
Location: T.58N., R.17W., sec. 23, NW, SW, SW; north edge of athletic fields, Gilbert Junior
High School; Gilbert Quadrangle, UTM: 539,820E/5,259,750N.
Description: At this stop are beautifully preserved pillow structures, formed over 2.6 billion
years ago, as lava flowed into a marine sea-floor environment and cooled relatively rapidly. The
pillows are defined by the darker grey-green “rinds” forming lobate shapes as the outer portion of
the lava quenched on contact with the water. The pillows pile up and drape over one another on
the sea floor (Fig. 11). Deformation has tilted these volcanic rocks to a near vertical position and
thus giving us a cross sectional view. Can you tell by the draping of the pillows which way was
up (toward the water)? Some of this tilting may have been caused by the intrusion of the Giants
Range batholith (Stop 1), which is exposed just to the north of here.
Note the local occurrence of fractures filled with a reddish glassy material. This is chert, a
microcrystalline quartz mineral formed by precipitation from seawater. Red chert is called
jasper; black chert is called flint. Chert occurs with iron oxide as the major constituent of banded
iron formation. The Biwabik Iron Formation crops out just to the south of this exposure . How
would you explain the presence of this chert here?
Note also the smooth, glacially scoured nature of the outcrop. Grooves, or striations, some
several centimeters across and ½ to 1 cm. deep, are evident and the result of abrasion by the
sediment load carried within the glacial ice mass. These smooth, rounded outcrops are often
referred to as whalebacks.
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Cooling against Seawater
Pillow Basalts
Vesicular
Crust
Inflation
Figure 11. Formation of pillowed structures in submarine basalt flows.
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