File:Oncolitic dolostone with galena (Bonneterre Dolomite, Upper Cambrian; St. Joseph's Lead Mine, Flat Rock, Missouri, USA) 1 (41418720931).jpg

Oncolitic dolostone with galena from the Cambrian of Missouri, USA. (public display, Geology Department, Wittenberg University, Springfield, Ohio, USA)

This gray rock is a dolostone, composed of dolomite (CaMg(CO3)2 - calcium magnesium carbonate). The rounded to irregularly rounded structures in the rock are mobile stromatolites called oncolites. Stromatolites are large, layered structures built up by mats of cyanobacteria. Stromatolites vary in appearance, ranging from slightly wrinkled horizontal laminations in sedimentary rocks to low mounds to prominent mounds to columnar structures and other forms. Unattached, free-rolling stromatolite masses on ancient seafloors are called oncolites ("algal balls"). The clasts in some limestones or dolostones are dominated by oncolites.

The dark silvery-grayish colored areas at left are composed of the mineral galena, which is PbS - lead sulfide - the number one lead ore mineral. Galena has a metallic luster, a silvery-gray color, cubic crystals, cubic cleavage, is moderately soft, and has a high specific gravity.

This rock comes from a Mississippi Valley-type deposit in Missouri. 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.

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.

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.

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.

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.

Stratigraphy: Bonneterre Dolomite, Upper Cambrian