Monday, June 16, 2014


Looking east, from Minnesota, across the Mississippi River toward LaCrosse, Wisconsin.

I lived in southwestern Wisconsin for several years and my rockhounding was sort of limited to sedimentary rocks, fossils, and agates. In fact, when I think about the Midwest, the states around the Great Lakes and the upper Mississippi River, I generally pass it off as Paleozoic sedimentary rocks covered by glacial drift, the latter obscuring much of the surficial geology.  Good collectable minerals seemed scarce, except calcite, but fossils were abundant in limestone quarries (assuming you could enter such).  However, that is really a very unfair characterization and nice mineral specimens are available for the rockhound—with a little travel.  For example, Upper Michigan has the great copper deposits, and a variety of copper minerals, in rocks associated with the Mid-Continent Rift System (see Blog posting April 7, 2013).  Wisconsin not only has iron deposits but nice copper minerals in the Ladysmith area.  Illinois and Kentucky have some of the world’s best fluorite crystals. Minnesota has a banded iron formation (BIF) that collectors cut and polish and highly value. Ohio has celestine crystals of a delicate shade of blue.  And then there is Missouri, a state rich in metallic resources and minerals; the now-defunct Tristate lead and zinc area and the Mississippi Valley-type Deposits such as the Viburnum Trend producing lead and zinc, copper and silver.  All of the Midwest produces agates of one sort of another (i.e. Lake Superior), beautiful geodes containing quartz and calcite (i.e. Keokuk), and some spectacular calcite crystals.

I have always had several questions about mineralization, mostly the sulfides, in the central and southern reaches of the Midwest.  Mineralization in the northern parts, Wisconsin, Minnesota and Michigan, is related to the Precambrian rocks and the ore genesis is easier to understand.  But why do the sulfides appear in the limestones and geodes to the south essentially from Michigan to Kentucky?  Where did the iron, zinc, copper and even the sulfur originate? 

I am still not certain that a definite answer exists but Wenz and others (2012) believed the ores of the Mississippi Valley-type Deposits (such as the Viburnum Trend, the Old Lead Belt, and the New Lead Belt in the Missouri Ozarks) are the result of the introduction of sulfides into lead- and zinc-rich ore fluids that in turn were derived from the 1460 Ma Precambrian basement rocks.  The sulfides may have been derived from local organic- and sulfur-rich carbonate rocks.  Galena (lead) and sphalerite (zinc) are non-complex sulfides and are the stable forms in a low temperature environment (the depositional environment of the Mississippi Valley-type Deposits).  Marcasite and pyrite, common simple sulfides found in the carbonate rocks of the Midwest, may be the result of precipitation from marine waters (I think).

Another thought seems to center around fluid movement from the deep basins present in the Midwest to the rims of the basins during later orogenic events.  In this model the source of the metals is leaching from sedimentary and other rocks (Precambrian basement?) at higher temperatures in the deep basins, and then transport to shallower levels where they can combine with free sulfide ions, probably resulting from the action of sulfide-reducing bacteria in the shallow rocks (model from Stefano, 2014, discussion on 

Millerite single crystals collected Halls Gap Road Cut, Kentucky.  Crystals ~1 mm. or less in length.

Arrow points to single millerite crystals.  Large pyrite cube ~ 1 mm.  Part of a geode collected Brummett Creek, Brown County, Indiana.

The least common sulfide in the Midwest carbonates is millerite (nickel sulfide, NiS).  Although fairly rare, millerite has a widespread distribution throughout the Midwest from Michigan and Wisconsin through Iowa, Illinois, Ohio, Indiana, and Missouri to Halls Gap, Kentucky.  Stefano (MinDat discussion) believed the source and depositional model is similar to galena and sphalerite—from deep-seated basin rocks to migration along basin rims.  In fact, I asked the MinDat group if the surficial distribution of millerite had been delineated.  Stefano noted that it seems to generally correlate with the basin rims. 
A new specimen of millerite picked up at the Denver Show in September.  Collected from halls Gap, Kentucky.

Radiating millerite crystals on crystalline calcite.  Collected “near Iowa City, Iowa.”  Length of “whiskers” ~4 mm

Millerite is a fascinating mineral, at least to an old paleontologist like me.  The crystals are clusters, or individuals, of shiny metallic acicular crystals that are pale brass-yellow when fresh but tarnish to an iridescent “black.”  The clusters of millerite often appear, at least to me, to be a “cat’s whisker.”  In most examples the crystals are quite small, sometimes on the order of a millimeter, and are often missed when hand samples are examined in the field.  For example, I have some “Keokuk Geodes” collected on a field trip during my tenure in Missouri.  I recently examined them under high power and noted tiny millerite crystals.  Neat.

Individual crystals of ?polydymite encased in calcite, each ~1mm in length.  Collected Grays Quarry, Hancock, County, Illinois.

But wait!  I thought perhaps I had acquired a basic understanding of millerite when “Grays Quarry, Hancock, County, Illinois” appeared on the radar screen.  While sorting through my few specimens the Illinois description appeared and so I decided to check it out.  Much to my surprise, Grays Quarry contains tiny acicular crystals of polydymite, a nickel sulfide, Ni++Ni+++2S4, encased in calcite crystals.  Most crystals of polydymite seems to be octahedrons; however, some are finely acicular.  As best that I can tell, polydymite forms from the oxidation of other nickel minerals.  The photos on MinDat are a ‘dead ringer” (at least in my mind) to a specimen in my collection labeled (by a seller)) as millerite.  So, without a pocket XRD or Microprobe I will label these tiny crystals as polydymite!

In summary, I sometimes find daydreaming a worthwhile endeavor!  I had examined some sulfide specimens from Missouri and had appreciated the beautiful crystals of sphalerite, dolomite, galena and chalcopyrite.  This was a taxing experience J so I retired out to my hammock for a bit of sunshine and relaxation.  This activity then lead to an old question—where did the sulfides dotting the Midwest carbonates originate?  That was a puzzle to me!  I could speculate on the pyrite and marcasite but what about the lead and zinc?   And then it hit me that I have a few specimens of these acicular crystals of nickel sulfide, millerite.  Now nickel is not a rare element on earth but why would it be found in these Paleozoic carbonates of the Midwest?  So, I posed the question to a Mindat Discussion and much to my surprise a vigorous discourse ensued.  Wow.  That was great and I learned much.  If only I had taken a course in geochemistry or ore genesis!

I want to thank Chris Stefano of Michigan Technological University for his informative discussions on MinDat as well as his personal emails.


Wenz, Z.J., M.S. Appold, K.L. Shelton and S. Tesfaye, 2012, Geochemistry of Mississippi Valley–type Mineralizing Fluids of the Ozark Plateau: A Regional Synthesis: American Journal of Science, v.312, no. 1 22-80.