MVT minerals of the Midwestern United States
Terry E. Huizing
Mississippi Valley-Type (MVT) Deposits
What are MVT deposits? They are mineral deposits with common characteristics that were firstrecognized as such in the United States. Midwestern MVT deposits are hosted in shallow-watercarbonate rocks of Paleozoic age, mostly in dolomitized areas; each mining district is located ona geological dome or arch where mineralization is typically confined to certain stratigraphic rockunits. The host rocks of MVT deposits contain fresh formation water, whereas saline waters arepresent in adjacent basins. The mineralogy is simple, consisting of sphalerite, galena, fluorite
and barite with quartz and carbonates, such as calcite, plus traces of asphaltic material.
Deposition took place in open spaces at low (100°-200° C) temperature. Geologists have nowclassified other deposits and mining districts in other parts of the world as belonging to this type.
MVT deposits in the midwestern United States have long been the source of many finecrystallized calcite specimens.
Other minerais common to these deposits are galena,sphalerite, fluorite and barite and minor amounts of chalcopyrite, pyrite, marcasite and quartz.
The association of sphalerite or galena produces a preservative environment, because whenthese two sulfide minerals oxidize, they form sulfates without production of sulfuric acid
(Eriksson, 1995). However, when iron sulfides predominate, post-mining oxidation may damage
calcite and other minerals.
There are five classic MVT locations in the midcontinent region of the United States.

Upper Mississippi Valley Zinc-Lead District
This formerly important district is located in the southwestern corner of Wisconsin, and extends
to Galena, Illinois, and Dubuque, Iowa. Operations for lead were recorded from as early as1658, and a map from 1687 shows lead mines near the town of Galena. This district has thelongest recorded production history of any mining area in the United States (Lasmanis, 1989).
Following a decline in lead production, a zinc mining boom began, peaking in 1916 because ofWorld War I demand. During the life of the district, more than 280 mines were developed in
orebodies located in limestones of Middle Ordovician age; most were in shallow deposits. By
1979 the last remaining mine at Shullsburg, Wisconsin was closed and allowed to flood.
Many beautiful carbonate and sulfide crystal groups were recovered from these mines, but
unfortunately matrix specimens from the district are difficult to preserve because over time theassociated marcasite breaks down, releasing sulfuric acid.

Illinois-Kentucky Fluorspar Districts
The MVT deposits located in southern Illinois and northwestern Kentucky have been one of themost important sources for fluorine and minor amounts of lead, zinc and barium. It has beenreported that as early as 1819 galena was mined near Cave in Rock, Illinois, and in 1875 formerPresident Andrew Jackson began mining galena across the Ohio River in Kentucky (Lasmanis,1989). The early development of lead mining in the Rosiclare district led to the discovery oflarge vein systems that later became the nation's major source for fluorite.
Mineralization is localized in stratiform orebodies and in veins, or fissure fillings, of faults that cut
through the Mississippian-age limestones in the Rosiclare district, the Cave in Rock district andII

the adjacent Harris Creek district. The primary ore is fluorspar. In this deposit world-classfluorite crystals occur in association with beautiful carbonate, sulfide and sulfate minerals.
Alas, in 1998 for economic reasons, the mines were closed with millions of tons of proven ore inreserve. The shafts have been capped and structures removed; it is unlikely that these mines
will open again in my lifetime.

Tri·State District
This district is named for its location at the junction of three states. In Missouri, the district
contains the Granby, Joplin and Webb City fields. The extensive Picher field is located inOklahoma and Kansas. Mining began in 1848, and in the mid-1860s during the U.S. Civil War
period, both Union and Confederate armies mined the deposits for lead. Zinc mining began at
Granby in 1870, and production peaked in 1916 during World War I. In 1957 all operations
were closed. Specimen collecting continued for years thereafter, until water reached even thehighest mine levels.
Nearly all the ore deposits were localized in limestones of Mississippian age that were heavily
faulted and altered by dolomitization and by the introduction of silica, which formed chert. The
openings in which mineralization took place 'were enormous in size, and it was not uncommon
to find simple scalenohedra of calcite more than a meter in length. Although nearly every
museum and many private collections contain specimens of galena, dolomite and calcite fromthis district, most of the crystals have been affected by corrosive fluids and, in later years, by theoxidation of iron sulfides.

Viburnum Trend District
The largest MVT mining districts in the world are in southeastern Missouri, where the Old Lead
Belt district near Bonne Terre was exploited as early as 1719 from near-surface galena
deposits, followed by development of underground workings in 1864. By 1945 reserves in thismining district were seriously depleted, and the last operation was closed in 1972. Open spacewithin the rock was limited, and few specimens of note were preserved.
In 1955, to the west of the Old Lead Belt district and on the other side of the Ozark Dome, ablind drilling program in the Viburnum area resulted in the discovery of the northern extent of theViburnum Trend district. Soon thereafter, at a place called Sweetwater, the southern end of thedistrict was located, more than 50 miles away. The huge ore deposits of this district are locatedin dolomites and limestones of Upper Cretaceous age. Mineralization is concentrated in reef
complexes and collapse breccias; the best crystals are found in the open-space fillings inbreccia zones. Unlike the Old Lead Belt deposits, every mine in the Viburnum Trend has
encountered zones where minerals have space to crystallize (Lasmanis, 1989). Viburnum Trend
mines have produced world-class lustrous, twinned galena crystals and beautiful calcite
crystals, some of which are twinned. Barite, fluorite, sphalerite and iron sulfides occur in minor
quantities in this district.

Central Tennessee District
This district is located east of Nashville in Smith County, Tennessee, and extends to thenortheast and southwest along the axis of the Cincinnati Arch. At the surface, more than thirty
shallow prospects are known from the time of early settlement (Lasmanis, 1989). In 1967, adrilling program near Elmwood discovered the orebody in a dolomite of Middle Ordovician age.
Mineralization in this district is localized in collapse breccias related to the paleokarst surface ofthe dolomite. In open spaces within the breccia, the principle ore mineral, sphalerite,
crystallizes with fluorite, barite and stunning transparent to golden-orange scalenohedra of12

calcite. Calcite crystals to 50 cm in length are known, although most average 20 to 25 cm.
These superb crystals are often twinned on {0001} and could be the best from any mining
district in the world.

References
ERIKSSON, S. C. (1995): Only a Calcite. Rocks & Minerals 70,217·230.
FRANCIS, C. A. (1982): Fluorite and Associated Minerals from South Illinois. Rocks & Minerals

57, 63-71.
HUIZING, T. E. (2003): A Bountiful Harvest of Calcite from the American Heartland. extraLapis
English 4, 76-80.
LASMANIS, R. (1989): Galena from Mississippi Valley-Type Deposits. Rocks & Minerals 64,10-34.
LILLIE, R. C. (1988): Minerals of the Harris Creek Fluorspar District, Hardin County, Illinois.
Rocks & Minerals 63,210-226.
PRESMYK, L. (1997): Brushy Creek Mine, Reynolds County, Missouri. Rocks & Minerals 72,370-377.

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