Mississippi Valley Type (MVT) ore deposits are part of a spectrum of deposits that form during the evolution of a sedimentary basin. They are defined as ores of the minerals galena and sphalerite with associated barite and fluorite.

Mississippi Valley-type (MVT) lead-zinc deposits are a varied family of epigenetic ore deposits that form predominantly in dolostone and in which lead and zinc are the major commodities

The ores are stratabound

Most are found in rocks of Cambrian and Ordovician, Devonian and Carboniferous, and Triassic ages.

Deposits within a district have similar geological and geochemical attributes and ore controls; whereas individual districts can be quite distinct from one another

Ore controls typically are district-specific; examples include shale edges (depositional margins of shale units), limestone-dolostone transitions, reef complexes, solution collapse breccias, faults, and basement topography. Most MVT ore districts are the product of regional or sub-continental scale hydrological processes

Some of the better known and studied districts are:

a) Tri-state district-southwest Missouri-Oklahoma-Kansas b) Central and eastern Tennessee c) Wisconsin area of the Upper Mississippi d) Pine Point and Polaris-NWT e) English Pennies, Ireland, and the Eastern Alps.

Deposit geology

The most important characteristics of MVT lead-zinc deposits (Leach and Sangster, 1993) are:

(1) most deposits are in dolostone, less commonly in limestone or sandstone,

(2) ore is epigenetic and stratabound,

(3) deposits are not associated with igneous activity,

(4) deposits are at shallow depths at flanks of basins,

(5) deposits are in platform carbonate sequences, located either in relatively undeformed rocks bordering foredeeps or in foreland thrust belts,

(6) most deposits are in districts that cover hundreds of square kilometers; a number of districts may even form metallogenic provinces,

(7) deposits form districts that are localized by geologic features, including breccias, depositional margins of shale units (shale edges), facies tracts, faults, and basement highs that permit upward migration of ore fluids,

(8) ore deposition temperatures are low (50oC to 200oC), but typically higher than those attributable to local basement-controlled thermal gradients; districts are commonly in thermal equilibrium with respect to surrounding host rocks,

(9) deposits are mineralogically simple; dominant minerals are sphalerite, galena, pyrite, marcasite, dolomite, calcite, and quartz,

(10) associated alteration consists mainly of dolomitization, brecciation, host-rock dissolution, and dissolution/crystallization of feldspar and clay,

(11) evidence of carbonate host rock dissolution, expressed as slumping, collapse, brecciation, or some combination of these, is common

(12) ore fluids were dense basinal brines, typically containing 10 to 30 weight percent salts,

(13) isotopic data indicate crustal sources for both metal and reduced sulfur,

(14) sulfide mineral textures are extremely varied; ore ranges from coarsely crystalline to fine-grained, massive to disseminated.

Host rocks

Most MVT deposits are hosted by dolostone; less important hosts are limestone and sandstone.

Exploration geophysics

Geophysical exploration has been used successfully in some districts to map known geologic ore controls (Guinnessand others, 1983). For example, airborne magnetic surveys have been used in Southeast Missouri to define buried Precambrian topography, an important control on the localization of some ore (Allingham, 1966; Cordell, 1979;Cordell and Knepper, 1987)

Surrounding geologic terrane

MVT deposits commonly are at shallow depths along basin flanks. They form in platform carbonate sequences located either in relatively undeformed rocks bordering foredeeps or in foreland thrust belts. Some deposits are associated with salt diapir

Alteration:

Hydrothermal alteration associated with the carbonate rocks that host the MVT deposits is typically represented by:

1) Dissolution of carbonate rocks which included hydrothermal brecciation and dissolving of the rocks by acidic fluids.

2) Dolotimization-hydrothermal dolomite, which replaces the host limestone or occurs as cement in open space areas such as breccia zones or karst areas. Dolotimization increases the rocks porosity.

3) Silicification-this is a minor alteration type and its intensity is dependent on the temperature of the hydrothermal fluids along with the amount of cooling that occurred during ore formation. Silicification is greater the hotter the system (>2000C)

Nature of ore

MVT ore is extremely varied in character and form. Ore bodies range from massive replacement zones to open space fillings of fractures and breccias to disseminated clusters of crystals that occupy intergranular pore space.

Most MVT deposits show clear evidence that open space deposition was accompanied by dissolution and replacement of host carbonate rocks.

Deposit trace element geochemistry

Trace and minor element suites and abundances vary between districts; the trace element content of minerals fromdifferent paragenetic stages within a district often display significant variation

Ore and gangue mineralogy and zonation

Most MVT deposits have simple mineral assemblages that consist of galena, sphalerite, pyrite, and marcasite. The most important environmental consideration for MVT deposits may be their iron sulfide content.

Chalcopyrite, bornite, and other copper sulfide minerals are abundant in some districts.

The most abundant gangue mineral is hydrothermal dolomite, which may form alteration halos around MVT ore. Barite and fluorite are abundant gangue minerals in some districts. Other common gangue minerals include calcite and quartz.

Mineral characteristics

Sulfide mineral textures are extremely varied; ore ranges from coarsely crystalline to fine-grained, massive to disseminated.

Topography, physiography

Most MVT deposits are located in flat-lying carbonate sequences. However, some deposits are located in thrust andfold belts

Hydrology

The formation of MVT deposits requires enormous quantities of fluid; therefore, most districts have some spatialconnection to major aquifers, karst systems, and faulted ground with high fluid transmissivity.

Mining and milling methods

Present mining is generally by underground room-and pillar-or by longwall methods. Ore is processed by pulverizingand flotation, concentrates are now generally shipped to smelters outside mining districts.

Origin:

Putting all the above together it turns out the formation of MVT deposits is not much different than that of the movement and localization of oil; that is the source material for oil formation occurs in off-shore shale and sand beds and, with compaction of these, oil, gas and water migrate up dip and into a variety of traps important among which are permeable reef complexes.

The idea for the origin of MVT deposits is similar. It is thought that the base metal ions are initially fixed in off shore sediments in foreland basins by absorbtion onto clays and organic matter, and also by precipitation directly as sulfides in black shales. Upon burial and compaction the metals are leached into, or dissolved by, the pore space brines (connate sea water) and start to migrate up dip and/or along suitable structures (normal compressive faults).

Brines of the oil-field type are known to develop high salinities similar to those found in the MVT fluid inclusions. Oil field brines can also be rich is Zn and Pb; their ph is between 4 and 6 and metal content can reach several hundred parts per million.

During diagenesis and tectonism the large volume of connate pore fluid is heated up and migrates up dip along major faults . Where the hydrothermal fluid encounters local traps (reefs, karst, facies changes, etc.) it precipitates out metals as sulfides

CONCLUSIONS

Within the class of hydrothermal ore deposits, MVT deposits are paradoxical in that they are at the same time the simplest and the most complex.

The following list of facts and ideas, based on the working hypothesis, contains implications for exploration strategy

1. Large, deep basins: the sorts of processes discussed can only take place in rather large sedimentary basins and MVT deposits generally occur towards the edge of these

2. Carbonates: within the basins, it is a fact that carbonates usually form the host rocks for MVT deposits

3. Evaporites: the known presence of evaporite beds somewhere in the basin must be regarded as a favourable sign

4. Cavities: not enough stress has been laid on the importance of the existence of cavities prior to ore deposition. In many districts, it can be shown that the cavities existed before the ore came in

5. Unconformities: in carbonate units, unconformities create the possibility for Karst structures, solution breccias and so on, which are ways of creating openings and cavities

6. Dolomitization: this is so often associated with MVT ore that it must be regarded as a promising if not a very diagnostic sign