Carbonate hosted lead zinc ore deposits

Carbonate-hosted lead-zinc ore deposits are important and highly valuable concentrations of lead

Lead sulfide

Lead sulfide is an ionic compound of lead and sulfur, having two possible proportions:*Lead sulfide, the ionic compound containing lead in the +2 oxidation state*Lead sulfide, the ionic compound containing lead in the +4 oxidation state...

and zinc sulfide

Zinc sulfide

Zinc sulfide is a inorganic compound with the formula ZnS. ZnS is the main form of zinc in nature, where it mainly occurs as the mineral sphalerite...

ore

Ore

An ore is a type of rock that contains minerals with important elements including metals. The ores are extracted through mining; these are then refined to extract the valuable element....

s hosted within carbonate

Carbonate rock

Carbonate rocks are a class of sedimentary rocks composed primarily of carbonate minerals. The two major types are limestone, which is composed of calcite or aragonite and dolostone, which is composed of the mineral dolomite .Calcite can be either dissolved by groundwater or precipitated by...

(limestone

Limestone

Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

, marl

Marl

Marl or marlstone is a calcium carbonate or lime-rich mud or mudstone which contains variable amounts of clays and aragonite. Marl was originally an old term loosely applied to a variety of materials, most of which occur as loose, earthy deposits consisting chiefly of an intimate mixture of clay...

, dolomite

Dolomite

Dolomite is a carbonate mineral composed of calcium magnesium carbonate CaMg2. The term is also used to describe the sedimentary carbonate rock dolostone....

) formations and which share a common genetic

Ore genesis

The various theories of ore genesis explain how the various types of mineral deposits form within the Earth's crust. Ore genesis theories are very dependent on the mineral or commodity....

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

Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...

and zinc

Zinc

Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...

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

Mining

Mining is the extraction of valuable minerals or other geological materials from the earth, from an ore body, vein or seam. The term also includes the removal of soil. Materials recovered by mining include base metals, precious metals, iron, uranium, coal, diamonds, limestone, oil shale, rock...

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

Mississippi River

The Mississippi River is the largest river system in North America. Flowing entirely in the United States, this river rises in western Minnesota and meanders slowly southwards for to the Mississippi River Delta at the Gulf of Mexico. With its many tributaries, the Mississippi's watershed drains...

in the United States, where such ores were first recognised; these include the famed Southeast Missouri Lead District of southeastern Missouri

Missouri

Missouri is a US state located in the Midwestern United States, bordered by Iowa, Illinois, Kentucky, Tennessee, Arkansas, Oklahoma, Kansas and Nebraska. With a 2010 population of 5,988,927, Missouri is the 18th most populous state in the nation and the fifth most populous in the Midwest. It...

, and deposits in southeast Iowa

Iowa

Iowa is a state located in the Midwestern United States, an area often referred to as the "American Heartland". It derives its name from the Ioway people, one of the many American Indian tribes that occupied the state at the time of European exploration. Iowa was a part of the French colony of New...

, southwest Wisconsin

Wisconsin

Wisconsin is a U.S. state located in the north-central United States and is part of the Midwest. It is bordered by Minnesota to the west, Iowa to the southwest, Illinois to the south, Lake Michigan to the east, Michigan to the northeast, and Lake Superior to the north. Wisconsin's capital is...

, and northwest Illinois

Illinois

Illinois is the fifth-most populous state of the United States of America, and is often noted for being a microcosm of the entire country. With Chicago in the northeast, small industrial cities and great agricultural productivity in central and northern Illinois, and natural resources like coal,...

.

Similarly Irish-type carbonate lead-zinc ores, exemplified by Lisheen Mine

Lisheen Mine

The Lisheen Mine is a lead and zinc mine located between the villages of Moyne and Templetuohy in North Tipperary, Ireland. In the Rathdowney Trend Lisheen is an underground mine where the Lisheen deposit lies at an average depth of 170 metres below surface....

in County Tipperary

County Tipperary

County Tipperary is a county of Ireland. It is located in the province of Munster and is named after the town of Tipperary. The area of the county does not have a single local authority; local government is split between two authorities. In North Tipperary, part of the Mid-West Region, local...

, are formed in similar ways.

Source

The source of sulfur and metals within carbonate-hosted lead-zinc ore deposits is the host carbonate sequence where diagenetic

Diagenesis

In geology and oceanography, diagenesis is any chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration and metamorphism. These changes happen at relatively low temperatures and pressures...

recrystallisation of low-temperature aragonite

Aragonite

Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...

into a higher-temperature calcite

Calcite

Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...

form prompts adsorbed zinc to be metamorphically ejected from the crystal lattice of the carbonate minerals. The zinc then enters the formational solutions and typically forms a chelated

Chelation

Chelation is the formation or presence of two or more separate coordinate bonds between apolydentate ligand and a single central atom....

bond with hydrocarbons which typically contain sulfur

Sulfur

Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

Transport

The low-temperature hydrothermal solution formed by diagenetic recrystallization of the carbonates then migrates into stratigraphic highs, such as folds

Fold (geology)

The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in...

, and typically also concentrates in fault zones at the margins of basement graben

Graben

In geology, a graben is a depressed block of land bordered by parallel faults. Graben is German for ditch. Graben is used for both the singular and plural....

s.

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

Sedimentary exhalative deposits

Sedimentary exhalative deposits are ore deposits which are interpreted to have been formed by release of ore-bearing hydrothermal fluids into a water reservoir , resulting in the precipitation of stratiform ore....

lead-zinc deposits.

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

Pyrolysis

Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures without the participation of oxygen. It involves the simultaneous change of chemical composition and physical phase, and is irreversible...

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

Galena

Galena is the natural mineral form of lead sulfide. It is the most important lead ore mineral.Galena is one of the most abundant and widely distributed sulfide minerals. It crystallizes in the cubic crystal system often showing octahedral forms...

, sphalerite

Sphalerite

Sphalerite is a mineral that is the chief ore of zinc. It consists largely of zinc sulfide in crystalline form but almost always contains variable iron. When iron content is high it is an opaque black variety, marmatite. It is usually found in association with galena, pyrite, and other sulfides...

, marcasite

Marcasite

The mineral marcasite, sometimes called white iron pyrite, is iron sulfide with orthorhombic crystal structure. It is physically and crystallographically distinct from pyrite, which is iron sulfide with cubic crystal structure. Both structures do have in common that they contain the disulfide...

and pyrite

Pyrite

The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...

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

Galena

, and zinc sulfide sphalerite

Sphalerite

. Weathered equivalents form anglesite

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 celestine. It contains 74% of lead by mass and...

, cerussite

Cerussite

Cerussite is a mineral consisting of lead carbonate , and an important ore of lead. The name is from the Latin cerussa, white lead. Cerussa nativa was mentioned by Conrad Gessner in 1565, and in 1832 F. S. Beudant applied the name cruse to the mineral, whilst the present form, cerussite, is due to...

, hydrozincite

Hydrozincite

Hydrozincite, also known as zinc bloom, is a white carbonate mineral consisting of Zn526. It is usually found in massive rather than crystalline form.It occurs as an oxidation product of zinc ores and as post mine incrustations...

and secondary galena and sphalerite within the supergene

Supergene (geology)

In ore deposit geology, supergene processes or enrichment occur relatively near the surface. Supergene processes include the predominance of meteoric water circulation with concomitant oxidation and chemical weathering. The descending meteoric waters oxidize the primary sulfide ore minerals and...

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

Pyrite

-marcasite

Marcasite

contents typically below 30% of the mass of sulfides. This makes MVT lead-zinc deposits particularly easy to treat from a metallurgical

Metallurgy

Metallurgy is a domain of materials science that studies the physical and chemical behavior of metallic elements, their intermetallic compounds, and their mixtures, which are called alloys. It is also the technology of metals: the way in which science is applied to their practical use...

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

Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

veining

Vein (geology)

In geology, a vein is a distinct sheetlike body of crystallized minerals within a rock. Veins form when mineral constituents carried by an aqueous solution within the rock mass are deposited through precipitation...

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

Coral reef

Coral reefs are underwater structures made from calcium carbonate secreted by corals. Coral reefs are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. The polyps...

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 survey is often carried out, which is the only geophysical technique

Geophysics

Geophysics is the physics of the Earth and its environment in space; also the study of the Earth using quantitative physical methods. The term geophysics sometimes refers only to the geological applications: Earth's shape; its gravitational and magnetic fields; its internal structure and...

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
Pine Point Mine
The Pine Point Mine is located west of Fort Resolution, Northwest Territories and east of Hay River, Northwest Territories, on the south shore of Great Slave Lake, Northwest Territories. Pine Point was a Mississippi Valley type deposit. It produced lead and zinc ores from 1964 to 1988...

, 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.