IRON ORE. BOUNDARY WATERS, AND BIG FITZ

 How many roads must a person walk down

before you call them a rockhound?  Apologies to Bob Dylan

At one time in my geological past, I organized and led student field trips to the Boundary Waters Canoe Area in Northern Minnesota. Driving north from Hays, Kansas, our group observed and studied various rock units that were unavailable to see in the sedimentary section of western Kansas. For example, we took a good look (and camped) at exposures of the Proterozoic Precambrian Sioux Quartzite (~1.6-1.7 Ga) cropping out around Sioux Falls, South Dakota, and adjacent western Minnesota. These outcrops were a good eye opener for the kiddies since their previous observation of the quartzite was in a pasture in eastern Kansas where I had previously stopped during a paleontology field trip. At this cow pasture locality, the Sioux was observed as small cobbles and boulders, the result of riding down glacial ice during the Pleistocene. The glacier(s) plucked the distinctive pink quartzite from the South Dakota/ Minnesota outcrops and dropped them off in a terminal moraine in eastern Kansas. These two areas presented a great message to help students understand the power of  glaciers.

Sioux Quartzite exposed at the “falls” in the city of Sioux Falls.  Photo courtesy of Steve Dutch, University of Wisconsin.

   

Sources of identifiable glacial erratics found in northeastern Kansas. The dashed line represents the extent of Pleistocene glacier(s) in Kansas.  Map courtesy of Kansas Geological Survey.
 

Boulders and cobbles of Sioux Quartzite in terminal moraine near Wamego, Kansas. Photo courtesy of Kansas Geological Survey.

                 

 Protohistoric Catlinite pipe, probably late 17th century Ioway, from the Wanampito site in Iowa. Public Domain photo from Whittaker and Anderson, 2008, Wanampito: An Early Ioway Site: Newsletter of the Iowa Archeological Society 58(1):4-5.

Near Sioux Falls in southwestern Minnesota is Pipestone National Monument, a national treasure often overlooked by travelers zooming along on nearby I-90. At the Monument the Sioux is also exposed but contains significate layers of catlinite, AKA pipestone. Catlinite is not a formally recognized mineral, although it often appears as such in popular culture, but is a sedimentary rock named argillite. The rock at Pipestone represents tightly indurated “mudstone/claystone” that was formed between layers of the quartzite and subjected to deep burial where heat and compression lithified the clay into an argillite. At Pipestone there are specific minerals present in the argillite that give the rock a diagnostic chemical signature: kaolinite, muscovite, diaspore, hematite, and pyrophyllite. Although the sand quartzite is extremely hard at ~ 7.5 (Mohs), the catlinite is very soft at ~2.5. That softness then allows the argillite to be cut and carved into Native American pipes. The staff at Pipestone believe that “for over 3,000 years, Indigenous people have quarried the red stone at this site to make pipes used in prayer and ceremony - a tradition that continues to this day and makes this site sacred to many people.”

The town of Pipestone also offers an opportunity to observe numerous buildings constructed in the late 1800s and early 1900s of the pink quartzite. The architecture of these buildings is fantastic and well worth a walking tour. The Museum at Pipestone noted that quarried rock was used in building construction in Minneapolis, Sioux Falls, Detroit, Sioux City, Chicago, Kansas City, Omaha and other locations.

The Pipestone City Hall was constructed in 1896. Today it is home to the Pipestone Historical Society who furnished the photo.

North of the Twin Cities the group was able to observe, for the first time, the Mesoproterozoic Precambrian (~1.1 Ga) rocks of the Midcontinent Rift Zone (MRZ), the southern arm of a Triple Junction spreading center near Lake Superior. The MRZ is best known for the large-scale native copper deposits in the Keweenaw Peninsula of Michigan.  

The Midcontinent Rift System or Zone cuts across the North American Craton, the stable center of the continent, that begin to split apart (think East African Rift Zone) starting ~1.1 Ga. Around 20 Ma the rifting stopped and started to close, hence the geological term "failed rift."  The location of the rift south of Lake Superior in Minnesota (south of the Twin Cities), Michigan, Wisconsin, Iowa, Nebraska, and Kansas is inferred from gravity and magnetic data. Public Domain map and info from USGS.

Basalt flows associated with the MFZ exposed along the St. Croix River at Interstate State Park, near metro St. Paul.

Hull-Rust-Mahoning Open Pit Mine near Hibbing is a National Historic Landmark. Photo is Public Domain courtesy of  McGhiever.
 

Further north in Minnesota we took a gander at the giant Mahoning-Hull-Rust Iron Mine near Hibbing (home of Bob Dylan born Robert Zimmerman).  The Mine is located in  the Mesabi Range, one of four iron ranges in Minnesota, the others being the Cuyuna, Vermillion, and Gunflint. The term range refers to a linear feature rather than a topographic high. Rocks of the ranges are Proterozoic Precambrian in age and are the result of the erosion of older Precambrian rocks that were a part of the geologically complex Churchill Craton. Marine waters occupied a large, passive, continental margin of the Churchill  known as the Animikie Basin that accepted (2.5-1.8 Ga) erosional debris consisting of large amounts of silica (quartz etc.) and iron-rich minerals and happened to coincide with a massive change in the composition of the atmosphere known as the Great Oxidation Event (GOE).  The earth’s early atmosphere was a reducing atmosphere with little oxygen and consisting mostly of nitrogen and carbon dioxide that were probably derived from volcanic events. Somewhere in the Proterozoic, questionably as early as 3.5 Ga, photosynthetic cyanobacteria, with chlorophyl, begin replacing the anoxygenic life forms of the reducing environment. The GOE refers to the massive oxygenic time around 2.4 Ga when the chlorophyll-based photosynthesis of cyanobacteria released oxygen as a byproduct. At this same time massive amounts of silica and ferrous iron (Fe2++)was being transported into the Animikie Basin and hitting the oxygenated oceanic waters that oxidized the iron into insoluble ferric (Fe3+++) iron that combined with the silica to form the famous banded iron formations (BIF). Around 1.88 Ga supracrustal BIF rocks of the Animikie were thrust northward to their current localities in the Iron Ranges.The BIF were the sources of the ores fueling the massive iron mining industry. In the Mesabi Range the BIF were close to the surface and hence the concentration of large open pit mines.       

 

The Iron Ranges of Lake Superior. Photo Public Domain courtesy of W.F. Cannon (USGS).

As the high-grade BIF iron ores became depleted in the mid-20^th century mining engineers developed the “taconite process” whereby  low grade ore (termed taconite)   was crushed to a fine grain and the magnetite was removed by magnets, mixed with a bonding agent, usually bentonite, and then wetted, rolled and concentrated into marble size “balls” which were then hardened by subjection to high heat. At that point the taconite contains about 70% iron and heads to one of four taconite shipping ports in Minnesota (Duluth-Superior, Taconite Harbor, Two Harbors, Silver Bay) where it is shipped to the steel mills of Indiana and Ohio. Currently there are only about a half dozen iron ore mines operating in Minnesota; all are in the Mesabi Range. The taconite process is fascinating to observe, and I was pleased to get the class into a personal tour.

The most famous taconite freighter on the Lake was christened in 1958 as the Edmund Fitzgerald and was a monster: 729 feet long and weighing ~13,600 tons. On November 9, 1975, the Big Fitz, with a full load of taconite pellets and a crew of 29, left the Port of Superior, Wisconsin, headed to the steel mills near Detroit, Michigan. Unfortunately, the Big Fitz steamed into a stormy Lake Superior and on November 10 broke apart and capsized in winds at least 90 mph and wave swells of 25-30 feet. All aboard perished and today the ship and crew rest in ~530 feet of water not far from Whitefish Bay. The 1976 disaster was immortalized by Gordon Lightfoot’s recording of the immensely popular folk ballad, The Wreck of the Edmund Fitzgerald.

Upon leaving the iron ranges the class started experiencing a high level of excitement as we headed to Ely, home of the canoe outfitter. My mind begins to fade a little here and unfortunately my maps are still buried “somewhere” after my move. I do remember that we put in at Lake One heading counterclockwise, canoed to the Canadian border near Ensign Lake, canoed down the big Moose Lake, and finished after six days of paddling. Little did the kiddies, on that first trip, know that I had never been in the Boundary Waters previously and was sort of flying by the seat of my pants, my Brunton compass, and my maps. We did miss a few portages the first time, managed to escape most bears, brewed morning coffee that tasted fantastic, and had the time of our lives. Forty years later I periodically run into a student who canoed with me and has stories for their grandchildren. And the kiddies learned much about igneous and metamorphic rocks, how faults may define some lakes, and how other lakes are the result of Pleistocene glaciers scooping out large, and small, basins.

Boundary Waters: quiet and loud.

Now for the mineral, I have a specimen from the Thunderbird Mine in the Mesabi Range near the mining town of Eveleth. Today the property is owned by Cleveland-Cliffs Inc. and is named the United Taconite Mine and includes the former Thunderbird Mine North (TBN) and the Thunderbird Mine South (TBS), collectively the Thunderbird Mine of earlier literature. The Thunderbird is one of the few large open pits still operating in Minnesota. According to Cleveland Cliffs, magnetite-bearing taconite is currently the principal iron-bearing rock of economic interest on the property. In line with other Superior-type iron formations, magnetite-bearing intervals within the Biwabik Iron Formation occur as laterally extensive, stratiform intervals. Economically mineable magnetite occurs exclusively within granular iron-formation (cherty) units of the Biwabik. The ore is sent approximately 10 miles by rail to the concentrator at the Fairlane processing facility in Forbes, Minnesota, to produce a magnetite concentrate, which is then delivered to the on-site pellet plant. From the plant site, pellets are transported by rail to a ship loading port at Duluth, Minnesota.

The specimen I have for this posting is minnesotaite, an iron silicate and really not much to look at. I spent a very few bucks for the specimen due to the facts that: 1) the mineral was named for the State of Minnesota and that is not a common naming practice; and 2) J.W. Gruner (1944), the naming author, first described it as an “iron talc” Fe²⁺₃Si₄O₁₀(OH)_2. That tidbit intrigued me so I took it home and then noted that talc [Mg₃Si₄O₁₀(OH)₂] is a hydrated magnesium silicate while minnesotaite is a hydrated iron silicate and therefore they are isostructural with each other. As best I can tell, there is no substitution between minnesotaite and talc due to the differences in the ionic radii in the two cations (I "think" but don’t always trust my thinking).

Steel gray masses of amorphous Minnesotaite in matrix

Top width FOV ~9.0 mm; bottom FOV ~ 3.0 mm.

Minnesotaite is tough to physically describe since it often is a fine grained, greenish-gray (may appear almost black), massive “blob”. It really does not have much of a luster or shine although MinDat calls it resinous, waxy, or greasy. Some specimens evidently have tiny radiating crystals or platy forms. I might have expected an iron mineral to be quite hard; however, minnesotaite is very soft at maybe ~1.5 (Mohs), about the same as it’s isostructural relative, talc. It is always associated with the Banded Iron Formations.

In retrospect, canoeing the Boundary Waters was much more interesting than examining the iron minerals of northern Minnesota. My memories are quite valuable to me as they are lifelong mementos, something like treasured souvenirs of an interesting journey.

REFERENCE CITED

Gruner, John W., 1944, The composition and structure of minnesotaite, a common iron silicate in iron formations: American Mineralogist, vol. 29, nos. 9-10. Pgs. 363-372. 

RIP Big Fitz

In a musty old hall in Detroit they prayed
In the Maritime Sailors' Cathedral
The church bell chimed 'til it rang twenty-nine times
For each man on the Edmund Fitzgerald

 

The legend lives on from the Chippewa on down
Of the big lake they call, 'Gitche Gumee'
Superior, they said, never gives up her dead
When the gales of November come early

`                               Gordon Lightfoot.