HOME

TheInfoList



OR:

Carbonate-hosted lead-zinc ore deposits are important and highly valuable concentrations of
lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...
and
zinc sulfide Zinc sulfide (or zinc sulphide) is an inorganic compound with the chemical formula of ZnS. This is the main form of zinc found in nature, where it mainly occurs as the mineral sphalerite. Although this mineral is usually black because of various ...
ore Ore is natural rock or sediment that contains one or more valuable minerals, typically containing metals, that can be mined, treated and sold at a profit.Encyclopædia Britannica. "Ore". Encyclopædia Britannica Online. Retrieved 7 Apr ...
s hosted within
carbonate A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate ...
(
limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
, marl,
dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...
) formations and which share a common genetic origin. 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 Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...
and
zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...
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
mine Mine, mines, miners or mining may refer to: Extraction or digging * Miner, a person engaged in mining or digging *Mining, extraction of mineral resources from the ground through a mine Grammar *Mine, a first-person English possessive pronoun ...
s. This classification of ore deposits is also known as Mississippi Valley Type or ''MVT'' ore deposits, after a number of such deposits along the
Mississippi River The Mississippi River is the second-longest river and chief river of the second-largest drainage system in North America, second only to the Hudson Bay drainage system. From its traditional source of Lake Itasca in northern Minnesota, it fl ...
in the United States, where such ores were first recognised; these include the famed
Southeast Missouri Lead District The Southeast Missouri Lead District, commonly called the Lead Belt, is a lead mining district in the southeastern part of Missouri. Counties in the Lead Belt include Saint Francois, Crawford, Dent, Iron, Madison, Reynolds, and Washingt ...
of southeastern
Missouri Missouri is a state in the Midwestern region of the United States. Ranking 21st in land area, it is bordered by eight states (tied for the most with Tennessee): Iowa to the north, Illinois, Kentucky and Tennessee to the east, Arkansas t ...
, and deposits in northeast
Iowa Iowa () is a state in the Midwestern region of the United States, bordered by the Mississippi River to the east and the Missouri River and Big Sioux River to the west. It is bordered by six states: Wisconsin to the northeast, Illinois to th ...
, southwest
Wisconsin Wisconsin () is a state in the upper Midwestern United States. Wisconsin is the 25th-largest state by total area and the 20th-most populous. It is bordered by Minnesota to the west, Iowa to the southwest, Illinois to the south, Lake M ...
, and northwest
Illinois Illinois ( ) is a state in the Midwestern United States. Its largest metropolitan areas include the Chicago metropolitan area, and the Metro East section, of Greater St. Louis. Other smaller metropolitan areas include, Peoria and Rockf ...
. Similarly Irish-type carbonate lead-zinc ores, exemplified by
Lisheen Mine Lisheen Mine is a former lead-zinc-silver mine located between the villages of Moyne and Templetuohy in County Tipperary, Ireland. In the Rathdowney Trend, Lisheen was an underground mine where the Lisheen deposit lies at an average depth of 170 ...
in
County Tipperary County Tipperary ( ga, Contae Thiobraid Árann) is a county in Ireland. It is in the province of Munster and the Southern Region. The county is named after the town of Tipperary, and was established in the early 13th century, shortly after t ...
, are formed in similar ways.


Sources

The ultimate source of the mineralizing fluid(s) in MVT deposits is unknown. The ore fluids of MVT deposits are typically low temperature (100 °C-150 °C) and have the composition of basinal brines (10-30 wt.% NaCl equivalent) with pH's of 4.5-5 (buffered by host carbonates). This hydrothermal fluid may or may not carry the required sulfur to form sulfide minerals. Mobile hydrocarbons may have played a role in delivering reduced sulfur to certain MVT systems, while methane and other organic matter can potentially reduce sulfate carried by an acidic fluid. The ore fluid is suspected to be derived from clastic red bed sequences (potential metal source) that contain evaporites (potential sulfur source).


Transport

Two potential transportation mechanisms for the metal-bearing ore fluid have been proposed. The first involves compaction of sediments in basins with rapid sedimentation. Mineralizing fluids within the basin become trapped within discrete, over-pressured aquifers and escape episodically and rapidly. The second fluid transportation mechanism is topographically-driven gravitational fluid flow. This occurs during uplift that is commonly associated with an orogenic event. One edge of a basin is uplifted during the formation of a foreland fold and thrust belt, and basinal fluids migrate laterally away from the deformation front as the basin is uplifted. Migration of the fluids through deep portions of the basin may result in the acquisition of metals and sulfur contained within the basin.


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 absorbed 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 The pyrolysis (or devolatilization) process is the thermal decomposition of materials at elevated temperatures, often in an inert atmosphere. It involves a change of chemical composition. The word is coined from the Greek-derived elements ''py ...
in place to form bitumens. 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 The mineral marcasite, sometimes called “white iron pyrite”, is iron sulfide (FeS2) with orthorhombic crystal structure. It is physically and crystallographically distinct from pyrite, which is iron sulfide with cubic crystal structure. Both ...
and
pyrite The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic luster and pale brass-yellow hue giv ...
thus form. 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. Weathered equivalents form
anglesite Anglesite is a lead sulfate mineral with the chemical formula PbSO4. It occurs as an oxidation product of primary lead sulfide ore, galena. Anglesite occurs as prismatic orthorhombic crystals and earthy masses, and is isomorphous with barite and ...
,
cerussite Cerussite (also known as lead carbonate or white lead ore) is a mineral consisting of lead carbonate (PbCO3), and is an important ore of lead. The name is from the Latin ''cerussa'', white lead. ''Cerussa nativa'' was mentioned by Conrad Gessner ...
,
smithsonite Smithsonite, also known as zinc spar, is the mineral form of zinc carbonate ( Zn CO3). Historically, smithsonite was identified with hemimorphite before it was realized that they were two different minerals. The two minerals are very similar in a ...
,
hydrozincite Hydrozincite, also known as zinc bloom or marionite, is a white carbonate mineral consisting of Zn5( CO3)2( OH)6. It is usually found in massive rather than crystalline form. It occurs as an oxidation product of zinc ores and as post mine incru ...
and secondary galena and sphalerite within the supergene zone. 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 The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic luster and pale brass-yellow hue giv ...
-
marcasite The mineral marcasite, sometimes called “white iron pyrite”, is iron sulfide (FeS2) with orthorhombic crystal structure. It is physically and crystallographically distinct from pyrite, which is iron sulfide with cubic crystal structure. Both ...
contents typically below 30% of the mass of sulfides. This makes MVT lead-zinc deposits particularly easy to treat from a
metallurgical Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are known as alloys. Metallurgy encompasses both the sc ...
view. 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 Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...
veining and colloform silica, however silicate gangue minerals are often rare.


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 relatively 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 A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups. C ...
accumulations. 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. Finally, once a basin model of the carbonate sequence is formulated, and the primary basin-margin faults are roughly identified, a
gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...
survey is often carried out, which is the only geophysical technique which can directly detect MVT deposits. 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 exploration 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

* Admiral Bay, Zn-Pb-Ag deposit, Northwest Shelf, Western Australia, theorised to be an MVT replacement type (undeveloped) *
Pine Point Mine The Pine Point Mine is located on the south shore of Great Slave Lake between Hay River to the west and Fort Resolution to the east, in the Northwest Territories of Canada. It produced lead and zinc ores from a Mississippi Valley Type deposit b ...
, Zn-Pb, deposit, Northwest Territories, Canada. (producer, 1964-1988) * Manbarrum-Sorby Hills zinc and lead deposits, Bonaparte Basin, Western Australia and Northern Territory (undeveloped)
Lennard Shelf
Lead-Zinc deposits, Lennard Shelf, Kimberleys, Western Australia. *
Tara Mine Tara Mines is a zinc and lead mine near Navan, County Meath, Ireland. Tara is an underground mine where the orebody lies between 50 and 900 metres below the surface in carbonate-hosted lead-zinc ore deposits. The deposit was discovered in 1970 ...
, Ireland * Topla and Mežica mines on Petzen, Austrian-Slovenian border.


See also

*
Lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...
,
zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...
*
Limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
*
Coral reef A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups. C ...
*
Ore genesis Various theories of ore genesis explain how the various types of mineral deposits form within Earth's crust. Ore-genesis theories vary depending on the mineral or commodity examined. Ore-genesis theories generally involve three components: sou ...
* Fault (geology) *
Metasomatism Metasomatism (from the Greek μετά ''metá'' "change" and σῶμα ''sôma'' "body") is the chemical alteration of a rock by hydrothermal and other fluids. It is the replacement of one rock by another of different mineralogical and chemical co ...


References

*Guilbert, John M. and Charles F. Park, 1986, ''The Geology of Ore Deposits'', W. H. Freeman, pp 889–907, *S. Paradis, K. Dewing and P. Hannigan, ''Mississippi Valley-type Lead-Zinc deposits (MVT)''
Geological Survey of Canada
*Trygve Hõy, B.C. Geological Survey
IRISH-TYPE CARBONATE-HOSTED Zn-Pb
*Nora K. Foley, ''ENVIRONMENTAL GEOCHEMISTRY OF PLATFORM CARBONATEHOSTED SULFIDE DEPOSITS'
USGS
* Dörling, S.L, D.I. Groves and Muhling P., 1998. ennard Shelf Mississippi Valley-type (MVT) Pb-Zn deposits, Western Australia. https://web.archive.org/web/20090205191034/http://www.ga.gov.au/image_cache/GA5372.pdf AGSO Journal of Australian geology and geophysiscs, 17(4), 115-120. * Hanilçi, N., and Öztürk, H., 2011. eochemical/isotopic evolution of Pb–Zn deposits in the Central and Eastern Taurides, Turkey. http://www.tandfonline.com/doi/full/10.1080/00206811003680008#.UhxhPNLwmSo International Geology Review, Volume 53, Issue 13, pages 1478-1507, 2011. {{Ores Economic geology # Ore deposits