Jasperoid is a rare, peculiar type of metasomatic alteration and occurs in two main forms; sulfidic jasperoids and hematitic jasperoids. True jasperoids are different from

jaspillite Jaspillite, or ''jaspilite'' also ''itabirite'', is a chemical rock formed similar to chert, but is generally quite iron rich. It is also known as ''jasper taconite''. Jaspillite is typically a banded mixture of hematite and quartz common in the b ...

, which is a form of metamorphosed chemical sedimentary rock, and from

jasper Jasper, an aggregate of microgranular quartz and/or cryptocrystalline chalcedony and other mineral phases,Kostov, R. I. 2010. Review on the mineralogical systematics of jasper and related rocks. – Archaeometry Workshop, 7, 3, 209-213PDF/ref> ...

which is a chemical sediment. Sulfidic jasperoids are typical examples of

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is o ...

sulfide Sulfide (British English also sulphide) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to chemical compounds l ...

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 c ...

of dolomites, and are found in

Nevada Nevada ( ; ) is a state in the Western region of the United States. It is bordered by Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. Nevada is the 7th-most extensive, ...

, Australia and

Iran Iran, officially the Islamic Republic of Iran, and also called Persia, is a country located in Western Asia. It is bordered by Iraq and Turkey to the west, by Azerbaijan and Armenia to the northwest, by the Caspian Sea and Turkm ...

. They are hard, dense purple-black rocks with considerable content of

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 ...

. The bodies in Nevada are quite thin (seldom greater than 8 m) and stratabound.

Hematitic jasperoids

Hematitic jasperoids are examples of advanced silica-

hematite Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of ...

alteration, and are known only from the

Proterozoic The Proterozoic () is a geological eon spanning the time interval from 2500 to 538.8million years ago. It is the most recent part of the Precambrian "supereon". It is also the longest eon of the Earth's geologic time scale, and it is subdivided ...

rocks of the Glengarry Basin in Australia. These jasperoids are hard, purple to dark purple rocks composed primarily of

amethyst Amethyst is a violet variety of quartz. The name comes from the Koine Greek αμέθυστος ''amethystos'' from α- ''a-'', "not" and μεθύσκω (Ancient Greek) / μεθώ (Modern Greek), "intoxicate", a reference to the belief that ...

quartz and fine disseminated

and some

magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With ...

. While contentious, these jasperoids are thought to form by extreme alteration of wall rocks within a

shear zone In geology, a shear zone is a thin zone within the Earth's crust or upper mantle that has been strongly deformed, due to the walls of rock on either side of the zone slipping past each other. In the upper crust, where rock is brittle, the shea ...

, and may occur in sediments,

andesite Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomina ...

trachyte Trachyte () is an extrusive igneous rock composed mostly of alkali feldspar. It is usually light-colored and aphanitic (fine-grained), with minor amounts of mafic minerals, and is formed by the rapid cooling of lava enriched with silica and ...

s and

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of a ...

s. These bodies are often discordant to stratigraphy and are quite podiform in nature. The bodies in the Glengarry Basin are up to 120 m thick and over 3 km in length. These jasperoids are an important source of

gold Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile ...

ore within the region. Some hematitic jasperoids may be sourced from metamorphosed and altered jaspillite, and are located above areas identified as submarine basalt vents. These, therefore, may represent a type of exhalite

chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a ...

or spilite. These are subordinate in volume to the shear-hosted forms and are usually quite thin (less than 3 m).

Conditions of metasomatism

The formation of hematitic jasperoids is considered to be the product of highly oxidised

of the wall rocks to a shear zone. The presence of carbonate alteration, talc-carbonated high-magnesian basalts and

ultramafic Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are compos ...

rocks and the remnant mineralogy being restricted to hematite, silica, and sulfides indicates oxidising fluid chemistry. Relict volcaniclastic textures in some jasperoids indicate that aluminosilicates have been replaced pervasively by silica + hematite. Transitional forms and poorly developed analogs are present in some gold camps within the Yilgarn craton where deep, reduced methane and carbonate bearing alteration fluids mix with shallower oxidised fluids, resulting in purple-pinkish carbonate flooding alteration within basalts and ultramafic rocks. One example of this is the Widgiemooltha Dome, where carbonated hematite-bearing siliceous basalts containing native gold,

arsenopyrite Arsenopyrite (International Mineralogical Association, IMA List of mineral symbols, symbol: Apy) is an iron arsenic sulfide (FeAsS). It is a hard (Mohs scale of mineral hardness, Mohs 5.5-6) metallic, opaque, steel grey to silver white mineral wi ...

, nickeline and gersdorffite are encountered in the footwall of the Miitel nickel mine.

Geographical distribution

True jasperoid is found in

dolomitic Dolomite () is an anhydrous carbonate mineral composed of calcium magnesium carbonate, ideally The term is also used for a sedimentary carbonate rock composed mostly of the mineral dolomite. An alternative name sometimes used for the dol ...

strata In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as e ...

in Nevada, USA, and within dolomitic limestone sequences in Azerbaijan, Iran, Iraq and Turkey. Hematitic jasperoid is only found within the Glengarry basin, Western Australia, although it probably represents an end-member state of a variety of alteration and metasomatism styles found elsewhere.

References

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External links

Nevada Jasperoid photos; Stanford
Sedimentary rocks Chert Metamorphic rocks Economic geology