Minerals of the Lake Superior Region

Rocky outcrops along Lake Superior reveal spectacular vistas. Standing on the edge of sparkling waters, it’s easy to forget that the rocks underfoot also tell a rich and varied history – a history that fascinates geologists and titillates mineral collectors around the world with what it has produced. Water is not the only immense resource of this region; there exists a mineral wealth born of long millennia.

The rocks of the Lake Superior region, some of the oldest in the world, formed over a vast expanse of geologic time and by a wide variety of geologic processes. Some are more than 2 billion years old. These are the remaining records of an exciting history of volcanic activity, slow depositions of sedimentary rocks in ancient seas and the building of mountains from huge volumes of rising molten granite that heated the rocks they invaded, re-organizing chemical ingredients to form new minerals and producing hot fluids that moved upward and deposited other minerals. Then came the time of tearing down. Lake Superior region rocks also record long periods of erosion that reduced the mountain ranges to rather flat lowlands and produced yet another generation of minerals.

So it is that billions of years went into making the wealth of minerals on or close to Lake Superior’s shores.

The region is world famous for vast deposits of iron ore and native copper. During the last 150 years, the rocks have yielded immense tonnages from those deposits and attracted early settlers to the iron and copper mines. Our regional ores were critical in helping the Allies win World Wars I and II. But iron and copper are far from the only commercial minerals around this vast lake. Gold, platinum, silver, nickel, and gemstones such as amethyst, agates and even diamonds have been found here.

The region holds fame in another geologic camp. Collectors seek many Lake Superior minerals for their beauty, aesthetic qualities or rareness. Outstanding regional minerals form private and public collections worldwide and close to home at the A.E. Seaman Mineral Museum at Michigan Technological University in Houghton.

The history is long and the diverse mineral wealth vast, but a few outstanding minerals are found here – some mined, some about to be mined, some in vast quantities and some in smaller quantities still worth noting. We introduce here a sampler, a “baker’s dozen,” of significant Lake Superior minerals, metals and gems chosen for their monetary or collector value.

Amethyst

What it is: Purple quartz, better known as amethyst, gains its attractive color from trace amounts of iron in its crystal structure. Amethyst is recovered from veins in granite and other rocks around Thunder Bay, Ontario, and to a lesser extent from Michigan’s Keweenaw Peninsula. It is an official provincial gemstone of Ontario.

How it got there: Hot hydrothermal solutions rose through fractures in rocks and partially filled the fractures with attractive crystals of amethyst, producing veins.

What it’s used for: Amethyst is used as a decorative building stone in the Thunder Bay area, as “gem rough” to make jewelry and as spectacular display specimens for mineral collectors. Its current value ranges from less than $1 per pound for decorative rock to several hundred dollars for cut gems or fine collectors’ specimens.

Agate

What it is: The banded gems called Lake Superior agates are composed of a microcrystalline variety of the mineral quartz (or silica) and are present throughout the lake basin. Agate is the official state gemstone of Minnesota.

How it got there: Agates formed when silica-rich solutions filled cavities produced by gas bubbles in basaltic lava flows more than 1 billion years ago.

What it’s used for: The attractive color banding of Lake Superior agates makes them widely prized by collectors and lapidarists. They make distinctive jewelry. Values can range from less than $1 to several thousand dollars for a single specimen, depending on its size, pattern and overall quality.

Chlorastrolite (Isle Royale greenstone)  

What it is: Chlorastrolite is a special variety of the mineral pumpellyite. It is a state gemstone for Michigan.

How it got there: Like agate, chlorastrolite formed when mineralizing fluids filled gas cavities in basaltic lava flows. It is found on Isle Royale and in some Keweenaw Peninsula copper mines. It has been illegal to remove greenstone from Isle Royale since it became a national park.

What it’s used for: As its name implies, chlorastrolite, or “green star stone,” has a silky green, radiating pattern that makes it desirable to anyone wanting a beautiful and special keepsake unique to Lake Superior. Depending on size, pattern and quality, the value ranges from a few dollars to more than $1,000 for the best larger gem-quality stones.

Platinum Group Elements (PGEs)

What it is: Platinum, palladium, osmium, rhodium, ruthenium and iridium are among the group of metals referred to as “platinum group” elements. These metals have recently been discovered in northeastern Minnesota, in the Lake Nipigon area of Ontario and in the Marquette area. They are being mined near Lac des Illes, Ontario.

How it got there: PGEs typically occur in layered igneous rocks that formed deep below the Earth’s surface. Similarity in their melting temperatures and high densities cause them to crystallize from a cooling magma at about the same time and settle into layers with other high-temperature minerals. PGEs may be associated with some massive sulfide deposits.

What it’s used for: PGEs are widely used because of their high resistance to chemical attack. They are used as catalysts in the chemical and petroleum industry – and in catalytic converters in the exhaust systems of automobiles. They are used in high-tech electronics, surgical instruments, photography and jewelry. PGEs are among the most expensive metals with market values – in dollars per ounce – of $980 for platinum, $260 for palladium, $380 for osmium, $190 for iridium, $3,000 rhodium and $85 for ruthenium.

Diamond  

What it is: As perhaps the world’s best known gemstone, diamond hardly needs an introduction. Made of pure carbon, it is the hardest naturally occurring mineral. No commercial deposits of diamonds have yet been found near Lake Superior, but diamonds do exist in northern Michigan, Wisconsin and Ontario. Several commercial prospects are being evaluated in the Wawa, Ontario, area.

How it got there: Diamonds form deep below the Earth’s crust in the upper mantle, where heat and pressure are sufficiently high to make it the stable form of carbon. Explosive eruptions of the volcanic rock known as kimberlite carry the diamonds to the surface. Because of their hardness and chemical inertness, some diamonds weather from their kimberlite host rock, are transported great distances by streams or glaciers and are sometimes found in the resulting gravel deposits.

What it’s used for: Diamonds are an important gemstone, but most diamonds mined are not gem quality and are used as abrasives. Competition from the synthetic diamond market has affected the current value of natural diamonds, which range from less than $1 per carat for small industrial-grade diamonds to many thousands of dollars per carat for the best colored, gem-quality natural stones.

Hematite

What it is: Red iron oxide (Fe2O3) occurs as very fine, “earthy” material, in globular masses, or as fine-to-coarse crystals with a metallic luster.

How it got there: This very stable substance is formed by the weathering of iron-bearing rocks through removal of silica, thereby concentrating their iron content. Large units of rock composed mainly of hematite formed the natural ore bodies on the Mesabi, Cuyuna and Vermilion iron ranges of Minnesota, the Gogebic Iron Range of Wisconsin and Michigan, the Marquette, Menominee and Iron River-Crystal Falls iron ranges of Michigan and the Michipicoten Iron Range of Ontario.

What it’s used for: Hematite is a major ore of iron, which is used for making steel and cast iron. Steel is used extensively in the construction and automobile industries and in manufacture of an innumerable variety of machinery, utensils and cutlery and such mundane items as nails, screws and bolts. The current value ranges from cents per pound of ore-grade hematite to several hundred dollars for high-quality collector’s specimens of similar size.

Goethite  

What it is: Goethite is a brown iron oxide (FeO(OH)) that commonly occurs with hematite, but differs from it in that it contains some hydrogen. Like hematite, it occurs as fine earthy material and in globular masses. It forms large ore bodies on some iron ranges in the Lake Superior area.

How it got there: Goethite found here probably formed much in the same way as did hematite.

What it’s used for: Goethite is an important ore of iron, used to manufacture steel. Its value is comparable to hematite, though it contains slightly less iron.

Magnetite  

What it is: Magnetite, or black iron oxide (Fe3O4), is a major constituent of the rock called “iron formation,” from which the natural ore bodies of hematite and goethite formed by chemical weathering over millions of years. It is naturally magnetic; hence its name.

How it got there: It is believed that the iron formations in which magnetite occurs formed by chemical precipitation in ancient seas nearly 2 billion years ago. Oxygen required to precipitate the iron was provided by photosynthetic algae, the remains of which are preserved in these rocks as some of the world’s oldest-known fossils.

What it’s used for: Magnetite is the major ore mineral recovered in the taconite industry. The magnetite is separated from the waste rock that contains little or no iron (called “tailings”). The magnetite concentrate is converted into pellets and shipped to smelters to produce iron for the steel industry. Extensive taconite operations are on the Mesabi Range in Minnesota, and to a lesser extent on the Marquette Range in northern Michigan. Like hematite and goethite, magnetite ranges in value from less than $1 per pound for ore-grade material to several hundred dollars for fine, crystallized specimens (uncommon in this region).

Native Copper

What it is: “Native” copper is naturally occurring, pure, metallic copper. The greatest occurrence of native copper in the world  is in the basaltic volcanic rocks and interlayered sediments on Michigan’s Keweenaw Peninsula. Lesser amounts have been found in similar rocks on Isle Royale and Ontario’s Algoma District.

How it got there: Native copper was formed by hydrothermal solutions that permeated fractures and open spaces in the tops of volcanic basalt flows and associated conglomerates more than a billion years ago. Subsequently, glaciers freed and transported huge masses of copper (known as “float” copper) hundreds of miles to the south. Copper has been mined in the Keweenaw by European settlers since the mid-1800s, but it was long known by the indigenous peoples. Native Americans made implements from copper and traded it for several thousand years before Europeans arrived.

What it’s used for: More than 11 billion pounds of native copper have been recovered from the large and small mines along the Keweenaw. Copper occurs here in a wide variety of crystal forms, producing spectacular display specimens. The copper also has been extensively used in electrical wire and instruments and in alloys, such as brass. Copper pipes replaced lead pipes in home water systems many years ago. The value of refined copper is about $1.80 per pound, but a fine, one-pound crystallized specimen may be worth more than 1,000 times that to a serious collector.

Native Silver

What it is: The Lake Superior copper district is also a world-class source of native silver, the mineral form of the metal silver. Most native copper mines produced some native silver. The close association of these two metals was somewhat distinctive to the region. Native silver was also mined from Silver Islet as well as onshore mines in the Thunder Bay District.

How it got there: Like the region’s native copper, silver was formed from hydrothermal solutions that followed openings in the rocks, precipitating silver and other minerals as they cooled, mixed with other solutions, or reacted with the rock itself.

What it’s used for: In the past, silver was used in coins and mirrors, but tarnished quickly. About one-third of all current silver production is used in the manufacture of photographic film and paper, although its use in the photo industry has been diminished by the introduction of digital cameras. Because of its high conductivity, silver is still widely used in electronics. It is also used in fine tableware and jewelry. Like copper, native silver occurs in many crystal forms, yielding exquisite display pieces for collectors. The current value ranges from around $8 per ounce for refined silver metal into the low thousands of dollars for the best-quality collector’s specimens of comparable size.

Native Gold

What it is: Like copper and silver, native gold is simply naturally occurring, pure, elemental gold.

How it got there: Typically gold is associated with hydrothermal quartz veins in older volcanic rocks in the region. The universal attraction of gold has lured prospectors to Lake Superior, and some have been lucky and successful. Over the past few centuries, gold has been discovered in Ontario, for example, the Red Lake area, near Geraldton and more recently at Hemlo, near Wawa. Gold also has been mined near Ishpeming, Michigan.

What it’s used for: In addition to its widespread use in jewelry, gold is extensively used in high-tech electronics because of its superior electrical conductivity. The current value of gold is about $485 per ounce, but as with most minerals, fine collector’s specimens (especially if well crystallized) can far exceed that value.

Datolite

What it is: Datolite is a hydrous calcium boron silicate that occurs in a wide range of colors. Within the region it is unique to the Keweenaw native copper deposits.

How it got there: Like the native copper with which it occurs, datolite formed when hydrothermal solutions invaded open fractures in the basaltic rocks of the Keweenaw Peninsula in Michigan more than a billion years ago. It occurs both in veins and has roughly spherical nodules of compact, fine-grained material that takes polish excellently.

What it’s used for: With its many colors, lake region datolite makes beautiful displays. Most is cut and polished for collectors, jewelry or scrimshaw art. The current values range from a few dollars for small, white nodules to several thousand dollars for larger, fracture-free, colored varieties.

Nickel

What it is: Nickel does not occur as a native element in the rocks of the Lake Superior region, but rather in certain sulfide minerals such as millerite (NiS) and pentlandite (Fe,Ni)9S8. The greatest nickel deposits in the world occur in the Sudbury, Ontario, area about 180 miles east of Sault Ste. Marie. A nickel deposit was mined recently near Lake Shebandowan west of Thunder Bay. A rich sulfide ore deposit containing economic concentrations of nickel and other metals has been discovered near the Huron Mountains in northern Michigan.

How it got there: Nickel deposits are typically associated with copper- and iron-sulfide minerals that form in large masses of magnesium- and iron-rich molten rocks that rise through the Earth’s crust to form “gabbro.”

What it’s used for: The vast majority of nickel mined today is used to make stainless-steel and other corrosion-resistant alloys, as well as nickel-cadmium batteries. It has a current market value of around $6 per pound, making it an extremely attractive metal to mine.

This “baker’s dozen” represents a fraction of the minerals and metals harbored in the rocks of Lake Superior.

The world is hungry for the geologic fruits of the earth – the Mineral Information Institute estimates that the average U.S. baby born today will “use” nearly 3.6 million pounds of minerals, metals and fuels in a 77.3-year lifetime. That is for basic food, clothing and shelter as well as resources to build roads traveled and recreational facilities enjoyed. With its rich history of providing mineral wealth, the Lake Superior region will no doubt continue to be a rock-solid resource to the world.

A great place to see minerals: The A.E. Seaman Mineral Museum at Michigan Technological University in Houghton, Michigan, displays 20,000 of its 60,000-mineral collection. Open 9 a.m. to 5 p.m. Monday-Saturday.

Love minerals and our region’s history? Check out Michigan Gold & Silver, Dan Fountain’s book about the search for precious metals in the Upper Peninsula.

Dr. LaBerge is the author of Geology of the Lake Superior Region and is professor emeritus of geology at the University of Wisconsin-Oshkosh. Dr. Robinson is the former curator of the A.E. Seaman Mineral Museum and professor of mineralogy at Michigan Technological University, Houghton.