<p>Fluoritized fossil echinoderms from Illinois, USA.
</p><p>Left = Platycrinites sp. crinoid head
Right = Pentremites pulchellus blastoid head
Purple = fluorite (CaF2 - calcium fluoride)
</p><p>The fluorite-replaced fossils shown above are from a Mississippi Valley-type deposit in southern Illinois.  Commonly abbreviated "MVT", Mississippi Valley-type deposits are named for a series of mineral deposits that occur in non-deformed platform sedimentary rocks along the Upper Mississippi River Valley, USA.  Many specific minerals occur in MVT deposits, but are dominated by galena, sphalerite, barite, and fluorite.  These minerals occur in caves and karst, paleokarst structures, in collapse fabrics, in pull-apart structures, etc.  MVT deposits in America are mined as important, large sources of lead ore and zinc ore.  The classic areas for MVT deposits are southern Illinois, the tristate area of Oklahoma-Missouri-Kansas, northern Kentucky, southwestern Wisconsin, and southeastern Missouri.  The minerals are hydrothermal in origin and were precipitated from basinal brines that were flushed out to the edges of large sedimentary basins (e.g., the Illinois Basin and the Black Warrior Basin).  In basin edge areas, the brines came into contact with Mississippian-aged carbonate rocks (limestone and dolostone), which caused mineralization.  The brines were 15% to 25% salinity with temperatures of 50 to 200 degrees Celsius (commonly 100 to 150 degrees C).  MVT mineralization usually occurs in limestone and dolostone but can also be hosted in shales, siltstones, sandstones, and conglomerates.  Gangue minerals include pyrite, marcasite, calcite, aragonite, dolomite, siderite, and quartz.  Up to 40 or 50 pulses of brine fluids are recorded in banding of mineral suites in MVT deposits (for example, sphalerite coatings in veins have a stratigraphy - each layer represents a pulse event).  Each pulse of water was probably expelled rapidly - overpressurization and friction likely caused the water to heat up.  Some bitumen (crystallized organic matter) can occur, which is an indication of the basinal origin of the brines.  The presence of asphalt-bitumen indicates some hydrocarbon migration occurred.  Some petroleum inclusions are found within fluorite crystals and petroleum scum occurs on fluorite crystals.  MVT deposits are associated with oil fields and the temperature of mineral precipitation matches the petroleum window.  The brines may simply have accompanied hydrocarbon fluids as they migrated updip.
</p><p>The high temperatures of these basin periphery deposits wasn't necessarily influenced by igneous hydrothermal activity.  Hot fluids can occur in basins that are deep enough for the geothermal gradient to be ~100 to 150 degrees Celsius.  If a permeable conduit horizon is present in a succession of interbedded siliciclastic sedimentary rocks, migration of hot, deep basinal brines may be quick enough to get MVT deposit conditions at basin margins.
</p><p>MVT deposits occur in the Upper Mississippi Valley of America as well as in northern Africa, Scandinavia, northwestern Canada, at scattered sites in Europe, and at some sites in the American Cordillera.  Some of these occurrences are in deformed host rocks.  MVT deposits have little to no precious metals - maybe a little copper (Cu).  Mineralization is usually associated with limestone or dolostone in fracture fillings and vugs.  Little host rock alteration has occurred - usually only dolomitization of limestones.
</p><p>The age of the host rocks in the Mississippi Valley area varies - it ranges from Cambrian to Mississippian.  Dating of mineralization has been difficult, but published ages indicate a near-latest Paleozoic to Mesozoic timing.
</p><p>MVT deposits in the Upper Mississippi River area are often divided into three subtypes based on the dominant mineral: 1) lead-rich (galena dominated); 2) zinc-rich (sphalerite dominated); and 3) fluorite rich.
</p><p>The fluoritized fossils shown above are from the Illinois-Kentucky Fluorspar District ("fluorspar" is a very old name for fluorite), which is an MVT fluoritic subtype.  Fluorite and fluorite-rich rocks are mined for the fluorine, which is principally used by the chemical industry to make HF - hydrofluoric acid.
</p><p>The fossils are derived from either the Fredonia Limestone Member of the Ste. Genevieve Limestone (Middle Mississippian) or the Rosiclare Sandstone (Middle Mississippian) (possibly a misattribution of the Spar Mountain Sandstone Member of the Ste. Genevieve Limestone).  Fluorite mineralization occurred at about 277 Ma, during the Early Permian, according to one published study (Chesley et al., 1994).  Another study concluded that fluorite mineralization was much later, during the Late Jurassic (see Symons, 1994).
</p><p>The fossil on the left is a fluoritized Platycrinites sp. crinoid head.  Crinoids (sea lilies) are sessile, benthic, filter-feeding, stalked echinoderms that are relatively common in the marine fossil record.  Crinoids are also a living group, but are relatively uncommon in modern oceans.  A crinoid is essentially a starfish-on-a-stick.  The stick, or stem, is composed of numerous stacked columnals, like small poker chips.  Stems and individual columnals are the most commonly encountered crinoid fossils in the field.  Intact, fossilized crinoid heads (crowns, calices, cups) are unusual.  Why?  Upon death, the crinoid body starts disintegrating very rapidly.  The soft tissues holding the skeletal pieces together decay and the skeleton falls apart.
</p><p>The fossil on the right is a Pentremites pulchellus blastoid head.  Blastoids are an extinct group of echinoderms.  They resemble crinoids in having a head, or theca, perched atop a long stem of stacked columnals composed of calcite.  Crinoids and blastoids are pelmatozoans - stalked echinoderms.  Both groups are sessile, benthic, filter-feeding organisms.  The overall structure and morphology of crinoid heads and blastoid heads are quite different.  Blastoid heads have pentaradial symmetry and somewhat resemble closed flower buds or nuts.  They have 5 ambulacral grooves extending outward and downward from the summit of the theca.  During life, many thin, delicate brachioles extended from the ambulacral grooves - these structures captured tiny particles of food from seawater as the blastoid engaged in filter feeding.
</p><p>Both crinoid and blastoid appear to have been silicified (= replaced by quartz - SiO2) as well as being partially fluoritized (= dark purple).
</p>
<hr><p>From exhibit info.
Fluorite (CaF2) was mined from the Illinois-Kentucky fluorspar district from about 1880 to 1995.  In many locations, fluorite occurs in veins, but near Cave-in-Rock, the mineral deposit replaces limestone strata.  Near the edges of major mineralization, the fluorite can selectively replace the calcite (CaCO3) in fossils without affecting the surrounding limestone or sandstone.
</p><p>The fossils are mostly from the Upper Misssissippian - Rosiclare Sandstone and Renault Formation limestone beds.  These layers are rich in fossils.  Some were replaced by quartz (silicified) and coated with fluorite later.
</p><p>This display of self-collected specimens shows some rare and unique specimens, preserved because they were in waste rock.  The collecting areas have been mined away due to on-going limestone quarry operations.
</p>
<hr><p>Locality: unrecorded/undisclosed mine northwest of the town of Cave-in-Rock, southeastern Hardin County, far-southern Illinois, USA (possibly from the Cleveland Mine)
</p>
<hr><p>Some info. on Mississippi Valley-type deposits was synthesized from:
</p><p>Chesley et al. (1994) - Direct dating of Mississippi Valley-type mineralization: use of Sm-Nd in fluorite.  Economic Geology 89: 1192-1199.
</p><p>Symons (1994) - Paleomagnetism and the Late Jurassic genesis of the Illinois-Kentucky fluorspar deposits.  Economic Geology 89: 438-449.
</p><p>Rakovan (2006) - Mississippi Valley-type deposits.  Rocks &amp; Minerals 81(January/February 2006): 69-71.
</p>
Fisher et al. (2013) - Fluorite in Mississippi Valley-type deposits.  Rocks &amp; Minerals 88(January/February 2013): 20-47.
<a rel="nofollow" class="external text" href="https://www.flickr.com/people/47445767@N05">James St. John</a>
CC BY 2.0
<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/jsjgeology/35974990995/">Platycrinites sp. &amp; Pentremites pulchellus (fluoritized crinoid &amp; blastoid fossils) (Mississippian; near Cave-in-Rock, Illinois, USA)</a>