WISCONSIN SULFIDES, MOONSTONES, QUARTZ & AZURITE (AND LUTEFISK)

Life in the fast lane, Wisconsin.

I recently had the opportunity to visit my ole stomping grounds along the Mississippi River in La Crosse, Wisconsin.  I was scheduled to present three geology/minerals talks at the Coulee Rock Club Gem and Mineral Show. The Club, although small in number, has an amazing cadre of volunteers who ran a smooth Show.

 

The beautiful Mississippi River at sunset with high bluffs capped by Ordovician carbonates.  "Normal" water stage with main channel in background.

Flood stage with water covering campgrounds and boat ramps submerged.

I also had planned to get in some good fishing on the River and hoped to search the shoreline for some Lakers (Lake Superior Agates).  Unfortunately, neither of those events took place as the River continued to be above flood level during my entire 10 day stay.  Boats could not get on the water as the ramps were completely covered and of course the shoreline gravels were nowhere to be seen.  But, the rock and mineral show was fun and I nabbed a few interesting minerals for my collection.

 

The doors opened at 9:00am Friday and the crowd wandered through.  The Show was held in the Onalaska Center.

The silent auction sold hundreds of specimens each day.

 

The big seller of the Show were these pieces of Ordovician carbonate with natural bugs planted with various kinds of  low growing "ground cover."  Items were made by members of the club as a "fund Raiser."

Southcentral and southwestern Wisconsin is the heart of the lead and zinc mining Tri State District (not to be confused with the Tri State District of Kansas, Missouri and Oklahoma) and was the first area populated by persons of European descent in the upper Mississippi River Valley.  Mining actually started in the 1700s with the last Wisconsin mine closing in the late 1970s.  In the 1830s and 1840s miners arrived from Cornwall, United Kingdom, and brought with them advanced mining technology.  These “Cousin Jacks” are fondly remembered in the area today and Wisconsin hosts many Cornish Festivals.  Cousin Jacks also brought along a vegetable-meat pie called a pasty that is loved by everyone in the State, and by most visitors.

Pasty from Warrens Bakery, Cornwall, UK.  Photo courtesy of David Johnson.

Although the major mineral products were lead and zinc extracted from galena and sphalerite, copper was present in many/most mines as the sulfide minerals bornite and chalcocite.  Secondary minerals in the oxide zone included azurite and malachite and although not common as collector minerals today, I was able to pick up a piece of azurite labeled “mine near Gratiot, Wisconsin.”  I suspect it is from an old collection.

Photomicrograph showing clusters of very small (submillimeter) crystals of azurite on matrix.

The Wisconsin lead-zinc ores are a Mississippi Valley Type (MVT) of deposit and the ores are hosted by Paleozoic limestone and/or dolomite.  Geologists now believe that MVT ores arrived in these carbonate basins via heated salt-water brine traveling in very deep “channels” in the bedrock (maybe as much as 13,000 feet deep). The source for the mineral laden brine was perhaps Arkansas along the leading edge of the structural front associated with the Ouachita Orogeny (part of the Appalachian mountain building event). As waters traveled north and reached near surface rocks (perhaps 1000 feet) the brine cooled, and precipitation of ore minerals started filling vugs, cracks and fractures in the Paleozoic carbonates.  Early mines in Wisconsin were quite shallow, about 100 feet or less, and local farmers called these prospectors badgers since they were always digging shallow pits looking for ore.  It was not until the Cousin Jacks and their technological improvements arrived did underground mining start to boom.  So, the Badger State was not named after furry little creatures but after early mom and pop prospectors. 

 

The Wausau Igneous Complex is a series of overlapping igneous intrusions of Precambrian age (Middle Proterozoic ~1.5 Ga) located in east central Wisconsin near the city of Wausau.  The exposed rocks are various granites, quartzite, and syenite (like granite but deficient in quartz).  I have visited the area a few times and the hard rock stratigraphy is very confusing to a softrocker like me.  At the Show I picked up a couple of specimens as I felt sorry for the dealer trying to push some not too pretty rocks.  Not much locality information except the crazy mass (looks like pickup sticks) of stained quartz crystals with some feldspar and goethite labeled “Wausau Pluton from the collection of Al Falster.”  I throw that tidbit in since Falster has published several articles on minerals of the Wausau Complex.

The second specimen is a group of better-looking quartz crystals, some clear, and all nicely terminated collected from Rib Mountain (also part of the complex). 

 

Mass (jumble) of hematite stained quartz crystals.  Width FOV ~4.3 cm.

Cluster of terminated quartz crystals.  Width FOV ~4.0 cm.

Perhaps the most famous rocks/minerals coming from the complex (Stettin Pluton) is Wisconsin Moonstone whose specimens are “hot” in the local mineral markets.  The specimens are a type of sodium-potassium silicate feldspar called anorthoclase [(Na,K)AlSi₃O₈].  It is an intermediate member of the albite (mostly sodium)—orthoclase (mostly potassium) solid solution series. According to Bill Cordua (Wisconsin’s go-to mineral man), homogenous anorthoclase splits on a fine scale into intergrown potassium feldspar and albite. Sometimes the bands of alternating minerals are coarse enough to see. Other times they are microscopic. If they are just the right size and spacing, they scatter the light that penetrates the various layers in the mineral – producing the moonstone effect, or schiller.

 

Tumbled Wisconsin Moonstone without schiller effect.

So, “moonstone” is not really a defined mineral but a combination of feldspar minerals.  Some, like anorthoclase, produce a “moonstone” schiller effect.  Most gemmy moonstone, as that from Myanmar, is more transparent with a strong blue sheen and schiller effect.  The specimens from Wisconsin are essentially opaque and take a very special skill in cutting and polishing to bring out the schiller effect.  This lapidary achievement has been perfected by the owner of Bill’s Moonstone and Crystals in Wausau, Wisconsin.

 

Unpolished/cut Wisconsin Moonstone showing a slight schiller effect (vertical near center of sample.  Width FOV ~4.1 cm.

I first saw Wisconsin Moonstone on the market several years ago when it was advertised and sold as a gemstone.  Today the marketing strategy has changed, and it now seems to be some sort of a magic rock.  According to information pushed at the show “Wisconsin Moonstone is very powerful…It is super calming and brings emotional balance…[and] provides protection while dreaming…[It] clears and opens all chakras…etc., etc., etc.”  And life goes on pretty well if you carry around a piece of Wisconsin Moonstone.  Try it! 

Epidote is a common low-grade metamorphic mineral found in the Precambrian metamorphosed basalts of the Mid Continent Rift System. In these rocks it usually replaces plagioclase, pyroxene and olivine during metamorphism. Most of the Keweenawan volcanic rocks in the northwestern part of the state in Polk, Ashland, Bayfield, Douglas, and Iron counties contain abundant epidote (Wisconsin Geological and Natural History Survey).  The volcanic basalt poured out of vents associated with the triple rifting of the late Precambrian continent about 1.1 Ma.  The Triple Junction of the three rifting arms was located about where Lake Superior is today, and the Lake lies in a basin created by the rift.  After about 20 million years of rifting and tearing apart the Precambrian stable craton, the rift failed, perhaps due to compressive forces to the east (in today’s directions) and started to heal.  Today the old rift system is filled with both volcanic rocks (from deep seated magma) and sedimentary rocks washed in from the surrounding highlands.  Rift system rocks in the western arm are only exposed as far south as Interstate State Park near St. Paul, Minnesota. However, subsurface high gravity anomalies (more dense basalt) have been traced to Kansas.  Probably the mineral best related to the Rift rocks are the native copper deposits of the Keweenawan Peninsula of upper Michigan (Yooper Country). 

 

Mass of green epidote crystals.  Width FOV ~3.4 cm.

Photomicrograph of radiating epidote crystals from above specimen.  Width FOV ~1.4 cm.

My specimen of epidote, a complex hydrated silicate with calcite, aluminum and iron, is essentially a mass of green, elongated crystals.  Small portions of the original basalt are present along with small amounts of quartz and unknown sub millimeter shiny black crystals along with dark prismatic crystals.  My level of skill does not allow me to identify these tiny crystals.  The location label simply said, “Epidote crystals, Wisconsin, Keweenawan volcanics.”

 

Pine River Pegmatite with numerous small crystals of elbaite, many colorless, some "green", others with a pink/red hue.  Width of specimen ~2.2 cm.

Photomicrograph of above specimen.  Arrow indicating prismatic crystal of elbaite with small areas of green coloring. Length of crystal ~ 8 mm.

Photomicrograph of above specimen.  Arrow indicating prismatic crystal of elbaite. Length of crystal ~4 mm. Faint red tint.

Section of "red" elbaite crystal. Width of crystal ~3 mm.

I also have a small specimen (thumbnail) labeled “Elbaite var. rubellite, Pine River Pegmatite, Florence County, Wisconsin.” Elbaite is a member of the tourmaline group of minerals and rubellite is a pink to red variety (usually).  The specimen I acquired has tourmaline crystals that are mostly colorless but also have a pink tint.  My color blindness did not allow me to pick out a very good sample.  Florence County is home of the Animikie Red Granite (red granite, several locations, is the State Rock of Wisconsin) and has a number of associated complex pegmatites.  The elbaite is rather uncommon but has produced some nice crystals according to photographs (mine is not one of them).  MinDat.org believes the pegmatite dikes are related to the 1.76 Ga Bush Lake Granite intrusion.  Florence County is in northeastern Wisconsin and is mostly covered with glacial drift.

When I moved to Wisconsin in 1998 the “big news” among geologists and environmentalists was the closing of the open pit Flambeau Mine near Ladysmith in the north central part of the state.  Kennecott Minerals Company had “discovered” copper in the area 30 years before.  In those days Wisconsinites, before the recent evisceration of the DNR, were proud and protective of their environment, especially of the woodlands and recreational waters of the northland.   This respect for nature led to a quarter century of lawsuits and permitting delays and planning documents before mining started in 1993.  The deposit was extremely rich, and ore was shipped directly to a smelter therefore avoiding any sort of a concentrator. The surface mine of 32 acres and ~220 feet in depth produced 181,000 tons of copper, 334,000 ounces of gold, and 3.3 million ounces of silver in a four-year period.  The mine closed in 1997 and was reclaimed by 1999.

MinDat.org described the deposit as an exhalative massive sulfide deposit deformed during the 1.80-1.85 billion-year-old Penokean orogeny [~1.86 to 1.81 Ga]. It was subjected to significant oxidation and supergene enrichment producing a zoned cap of, from the surface downward, gossan, chalcocite, bornite, and chalcopyrite. What this statement actually says is that during plate collision metal bearing solutions erupted via hot springs on the ocean seafloor, reacted with cold seawater, and “dropped their load.”  In todays parlance these springs are known as Black Smokers and they are responsible for some of the earth’s richest mineral deposits.

A Black Smoker on the ocean floor.  Photo courtesy of D. Kelley, University of Washington, Seattle, USA

The specimens I picked up at the show are: massive bornite with scattered crystals of chalcopyrite and perhaps some very tiny chalcocite crystals; and 2) some beautiful azurite crystals on the oxidized gossan (iron oxides goethite/limonite).  According to the Wisconsin Geological and Natural History Survey The major ore minerals at Flambeau are massive sulfides bornite [copper iron sulfide Cu₅FeS₄], chalcocite [copper sulfide Cu₂S], and chalcopyrite [also a copper iron sulfide CuFeS₂].  Both the bornite and chalcocite are sort of dark and gray and ugly while the chalcopyrite forms brassy colored crystals.  What made Flambeau famous (sort of like the beer that made Milwaukee famous) is the very high enrichment of the supergene strata where these minerals were mined.  The azurite [hydrated copper carbonate Cu₂(CO₃)₂(OH)₂] came from the highly oxidized zone very near surface.  According to MinDat.org the Flambeau Mine has yielded 67 valid mineral species.  Unfortunately, very few of these minerals appear on the market with the exception of chalcocite and bornite.  It appears that collectors were not allowed on site until later in the mining process and immediately after mining ceased reclamation started and the pit was filled and covered—no mine dump to sort through.

 

Massive bornite with scattered brassy chalcopyrite crystals.  Width FOV ~4.4 cm.

Azurite on goethite/limonite.  Width FOV ~2.5 cm.

Photomicrograph of azurite section from above specimen.  Width FOV ~1.0 cm.

In summary, I really have only a few minerals from Wisconsin considering I lived there for eight years.  But that was in my days of traveling across the country promoting undergraduate research, keeping a college of faculty members, staff and students reasonably “happy” and trying to locate sunnies and walleye in the mighty Mississippi River.  And don’t forget the pasties, smelt fries, lutefisk, pickled herring, 9,999 kinds of cheese, and smoked carp washed down with liquid from breweries such as the Lazy Monk, Dead Bird, Grumpy Troll and Pabst Milwaukee.