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Carbonate-hosted lead-zinc ore deposits are important and highly valuable concentrations of lead and zinc sulfide ores hosted within carbonate (limestone, marl, dolomite) formations and which share a common genetic origin. An ore is a volume of rock containing components or Minerals in a mode of occurrence that renders it valuable for mining Carbonate rocks are a class of Sedimentary rocks composed primarily of Carbonate Minerals The two major types are Limestone and Dolomite Limestone is a Sedimentary rock composed largely of the Mineral Calcite ( Calcium carbonate: CaCO3 Marl or Marlstone is a Calcium carbonate or lime -rich mud or Mudstone which contains variable amounts of Clays and Aragonite Dolomite (ˈdɒləmaɪt is the name of a Sedimentary Carbonate rock and a Mineral, both composed The various theories of ore genesis explain how the various types of mineral deposits form within the Earth's crust.

These ore bodies range from 0. 5 million tonnes of contained ore, to 20 million tonnes or more, and have a grade of between 4% combined lead and zinc to over 14% combined lead and zinc. These ore bodies tend to be compact, fairly uniform plug-like or pipe-like replacements of their host carbonate sequences and as such can be extremely profitable mines.

This classification of ore deposits is also known as Mississippi Valley Type or MVT ore deposits, after a number of such deposts along the Mississippi River in the United States, where such ores were first recognised; these include the famed Lead Belt and Viburnum trend of southeastern Missouri, and deposits in southeast Iowa, southwest Wisconsin, and northwest Illinois. The Mississippi River is the second longest River in the United States, with a length of from its source in Lake Itasca in Minnesota to The Lead Belt is a Lead mining district in the southeastern part of Missouri. Missouri ( or) is a state in the Midwestern region of the United States bordered by Iowa, Illinois, Kentucky, Tennessee The State of Iowa ( is a state in the Midwestern region of the United States of America. Wisconsin ( or wɪˈskɑnsɨn (French Ouisconsin) is one of the fifty United States of America, located in the north central part of the United States The State of Illinois ( roughly ill-i-NOY is a state of the United States of America, the 21st to be admitted to the Union.

Similarly Irish-type carbonate lead-zinc ores, exemplified by Lisheen, Co. Tipperary, are formed in similar ways, although significantly larger in tonnage.

Contents

Source

The source of sulfur and metals within carbonate-hosted lead-zinc ore deposits is the host carbonate sequence where diagenetic recystallisation of low-temperature calcite into a higher-temperature aragonite form prompts adsorbed zinc to be metamorphically ejected from the crystal lattice of the carbonate minerals. In Geology and Oceanography, diagenesis is any chemical physical or biological change undergone by a Sediment after its initial deposition and during Calcite is a carbonate mineral and the most stable polymorph of Calcium carbonate ( Ca[[carbon C]] O 3 Aragonite is a Carbonate mineral, one of the two common naturally occurring polymorphs of Calcium carbonate, Ca[[carbon C]] O 3 The zinc then enters the formational solutions and typically forms a chelated bond with hydrocarbons which typically contain sulfur. In Organic chemistry, a hydrocarbon is an Organic compound consisting entirely of Hydrogen and Carbon. Sulfur or sulphur (ˈsʌlfɚ see spelling below) is the Chemical element that has the Atomic number 16

Transport

The low-temperature hydrothermal solution formed by diagenetic recrystallization of the carbonates then migrates into stratigraphic highs, such as folds, and typically also concentrates in fault zones at the margins of basement grabens. See also Folding The term fold is used in Geology when one or a stack of originally flat and planar surfaces such as Sedimentary A graben is a depressed block of land bordered by parallel faults Graben is German for ditch.

If the solutions are not trapped successfully within the carbonate host, it is possible for the hydrothermal fluids to leak out into the ocean basin and form SEDEX lead-zinc deposits. Sedimentary exhalative deposits (abbreviated as SEDEX from SED imentary EX halative are Ore deposits which are interpreted to have been formed by release

Trap

The trap for carbonate-hosted lead-zinc sulfides is a chemical reaction which occurs as a consequence of concentration of sulfur, often hydrocarbons, and zinc and lead which are adsorbed by the hydrocarbons. The hydrocarbons can either leak out of the fault zone or fold hinge, leaving a stockwork of weakly mineralized carbonate-sulfide veins, or can degrade via pyrolysis in place to form bitumens. Pyrolysis is the Chemical decomposition of organic materials by heating in the absence of Oxygen or any other reagents except possibly Steam Bitumen is a mixture of organic Liquids that are highly Viscous, black sticky entirely soluble in Carbon disulfide, and composed primarily

Once hydrocarbons are converted to bitumen, their ability to chelate metal ions and sulfur is reduced and results in these elements being expelled into the fluid, which becomes saturated in zinc, lead, iron and sulfur. Sulfide minerals such as galena, sphalerite, marcasite and pyrite thus form. Galena is the natural mineral form of Lead sulfide. It is the most important Lead Ore mineral Sphalerite (( Zn, Fe) S) is a Mineral that is the chief Ore of Zinc. The Mineral marcasite, sometimes called white iron pyrite, is Iron Sulfide (FeS2

Commonly MVT deposits form by the combination of hydrocarbon pyrolysis liberating zinc-lead ions and sulfur to form an acidic solution which dissolves the host carbonate formation and replaces it with massive sulfide accumulations. This may also take the morphology of fault-hosted stockworks, massive tabular replacements and so forth.

Porous limestones may form disseminated ores, however most MVT deposits are massive sulfides with knife-like margins between carbonate and sulfide mineralogies.

Mineralogy and Alteration

Ore minerals in carbonate replacement deposits are typically lead sulfide, galena, and zinc sulfide sphalerite. Galena is the natural mineral form of Lead sulfide. It is the most important Lead Ore mineral Sphalerite (( Zn, Fe) S) is a Mineral that is the chief Ore of Zinc. Weathered equivalents form anglesite, cerussite, hydrozincite and secondary galena and sphalerite within the supergene zone. Anglesite is a Lead Sulfate Mineral, PbSO4 It occurs as an oxidation product of primary lead sulfide ore Galena. Cerussite (also known as lead carbonate or White lead ore) is a Mineral consisting of Lead Carbonate (PbCO3 Hydrozincite, also known as zinc bloom, is a white carbonate mineral consisting of Zn 5( CO3)2( OH)6 In Ore deposit Geology, supergene processes or enrichment occur relatively near the surface

MVT and Irish type deposits are commonly associated with a 'dolomite front' alteration, which manifests as a yellow-cream wash of dolomite (calcium-magnesium carbonate) within calcite-aragonite assemblages of unaltered carbonate formations.

Most ore bodies are quite sulfidic, and most are very low-iron, with pyrite-marcasite contents typically below 30% of the mass of sulfides. The Mineral marcasite, sometimes called white iron pyrite, is Iron Sulfide (FeS2 This makes MVT lead-zinc deposits particularly easy to treat from a metallurgical view. Metallurgy is a domain of Materials science that studies the physical and chemical behavior of metallic elements, their intermetallic compounds, and their Some MVT deposits can, however, be very iron-rich and some sulfide replacement and alteration zones are associated with no lead-zinc at all, resulting in massive accumulations of pyrite-marcasite, which are essentially worthless.

There is sometimes an association with quartz veining and colloform silica, however silicate gangue minerals are often rare. Quartz (from German) is the most abundant Mineral in the Earth 's Continental crust (although Feldspar is more common in In Geology, a vein is a finite volume within a rock, having a distinct shape filled with Crystals of one or more Minerals which were precipitated

Oil synergies

The importance and synergies between hydrocarbon source-transport-trap 'fairways' and MVT and Irish Type lead-zinc deposits has been known for several decades. Often the prospectivity of particular carbonate formations for lead-zinc deposits of this nature is first identified by core drilling by oil explorers.

This concept of a cogeneration of hydrocarbons and precursor brines by the same process allows many lead-zinc explorers to use hydrocarbon basin models to predict if a carbonate sequence is likely to host MVT or Irish Type mineralization.

Exploration

Exploration for MVT deposits is reatively complex in theory and straightforward in practise. During the area selection phase, attention must be paid to the nature of the carbonate sequences, especially if there is a 'dolomite front' alteration identified within oil exploration wells, which is commonly associated with lead-zinc mineralisation.

Thereafter, attention must be paid to picking floral facies of any reef carbonates formed from coral reef accumulations. Coral reefs are Aragonite structures produced by living organisms found in marine waters with little to no nutrients in the water The facies of the carbonate sequence is critical, as this is controlled mostly by faults which are the ultimate target of exploration. A fore-reef/back-reef transition is the 'sweet spot', and thus depending on the age of the carbonate sequence, familiarity with coral palaeontology is considered essential. Palaeontology redirects here For the Scientific journal, see Palaeontology (journal.

Finally, once a basin model of the carbonate sequence is formulated, and the primary basin-margin faults are roughly identified, a gravity survey is often carried out, which is the only geophysical technique which can directly detect MVT deposits. Gravitation is a natural Phenomenon by which objects with Mass attract one another Geophysics, a major discipline of Earth sciences, is the study of the Earth by quantitative physical methods especially by seismic, electromagnetic Gravity surveys aim to detect significant accumulations of lead and zinc due to their greater density relative to their surrounding host rocks.

Finally, the 'pointy end' of an exploation programme is to drill each and every one of the gravity targets in sequence, with no favour or prejudice given to the strength or amplitude of any anomaly. It is well known that unsubtle and unsophisticated methods of pattern drilling have found MVT deposits missed by more selective explorers, for instance the Lennard Shelf Deposits in Western Australia were found on the second last hole of an extensive drilling programme.

Similar deposit styles

Similar deposit styles may be encountered in sheared and deformed carbonate belts where zinc-lead sulfides are hosted at the sheared contact of carbonates with siliciclastic sequences. Examples include the Dharwar Basin zinc-lead deposits, India where sulfides are hosted in shears within dolomite sequences.

Examples

See also

References

SULFIDE DEPOSITS USGS

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