Jasperoid
Encyclopedia
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
, which is a form of metamorphosed chemical sedimentary rock, and from jasper
which is a chemical sediment.
Sulfidic jasperoids are typical examples of silica-sulfide
metasomatism
of dolostone
s, and are found in Nevada
and Iran
. They are hard, dense purple-black rocks with considerable content of pyrite
. The bodies in Nevada are quite thin (seldom greater than 8 m) and stratabound.
alteration, and are known only from the Proterozoic
rocks of the Glengarry Basin in Australia
. These jasperoids are hard, purple to dark purple rocks composed primarily of amethyst
quartz and fine disseminated hematite
and some magnetite
.
While contentious, these jasperoids are thought to form by extreme alteration of wall rocks within a shear zone
, and may occur in sediments, andesite
s, trachyte
s and basalt
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
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
or spilite
. These are subordinate in volume to the shear-hosted forms and are usually quite thin (less than 3 m).
of the wall rocks to a shear zone. The presence of carbonate alteration, talc-carbonated high-magnesian basalts and ultramafic 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
, nickeline and gersdorffite
are encountered in the footwall of the Miitel nickel mine.
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.
Metasomatism
Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids.Metasomatism can occur via the action of hydrothermal fluids from an igneous or metamorphic source. In the igneous environment, metasomatism creates skarns, greisen, and may affect hornfels in the contact...
alteration and occurs in two main forms; sulfidic jasperoids and hematitic jasperoids. True jasperoids are different from jaspillite
Jaspillite
Jaspillite, or jaspilite, is a chemical rock formed similar to chert, but is generally quite iron rich. It is also known as taconite. Jaspillite is also a mixture of hematite and quartz common in the banded iron formation rocks of Proterozoic and Archaean age in the Canadian shield.Jaspillite is...
, which is a form of metamorphosed chemical sedimentary rock, and from jasper
Jasper
Jasper, a form of chalcedony, is an opaque, impure variety of silica, usually red, yellow, brown or green in color; and rarely blue. This mineral breaks with a smooth surface, and is used for ornamentation or as a gemstone. It can be highly polished and is used for vases, seals, and at one time for...
which is a chemical sediment.
Sulfidic jasperoids are typical examples of silica-sulfide
Sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...
metasomatism
Metasomatism
Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids.Metasomatism can occur via the action of hydrothermal fluids from an igneous or metamorphic source. In the igneous environment, metasomatism creates skarns, greisen, and may affect hornfels in the contact...
of dolostone
Dolostone
Dolostone or dolomite rock is a sedimentary carbonate rock that contains a high percentage of the mineral dolomite. In old U.S.G.S. publications it was referred to as magnesian limestone. Most dolostone formed as a magnesium replacement of limestone or lime mud prior to lithification. It is...
s, and are found in Nevada
Nevada
Nevada is a state in the western, mountain west, and southwestern regions of the United States. With an area of and a population of about 2.7 million, it is the 7th-largest and 35th-most populous state. Over two-thirds of Nevada's people live in the Las Vegas metropolitan area, which contains its...
and Iran
Iran
Iran , officially the Islamic Republic of Iran , is a country in Southern and Western Asia. The name "Iran" has been in use natively since the Sassanian era and came into use internationally in 1935, before which the country was known to the Western world as Persia...
. They are hard, dense purple-black rocks with considerable content of 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...
. The bodies in Nevada are quite thin (seldom greater than 8 m) and stratabound.
Hematitic jasperoids
Hematitic jasperoids are examples of advanced silica-hematiteHematite
Hematite, also spelled as haematite, is the mineral form of iron oxide , one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum...
alteration, and are known only from the Proterozoic
Proterozoic
The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life"...
rocks of the Glengarry Basin in Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
. These jasperoids are hard, purple to dark purple rocks composed primarily of amethyst
Amethyst
Amethyst is a violet variety of quartz often used in jewelry. The name comes from the Ancient Greek ἀ a- and μέθυστος methustos , a reference to the belief that the stone protected its owner from drunkenness; the ancient Greeks and Romans wore amethyst and made drinking vessels of it in the belief...
quartz and fine disseminated hematite
Hematite
Hematite, also spelled as haematite, is the mineral form of iron oxide , one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum...
and some magnetite
Magnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...
.
While contentious, these jasperoids are thought to form by extreme alteration of wall rocks within a shear zone
Shear zone
A shear zone is a very important structural discontinuity surface in the Earth's crust and upper mantle. It forms as a response to inhomogeneous deformation partitioning strain into planar or curviplanar high-strain zones. Intervening blocks stay relatively unaffected by the deformation...
, and may occur in sediments, andesite
Andesite
Andesite is an extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. In a general sense, it is the intermediate type between basalt and dacite. The mineral assemblage is typically dominated by plagioclase plus pyroxene and/or hornblende. Magnetite,...
s, trachyte
Trachyte
Trachyte is an igneous volcanic rock with an aphanitic to porphyritic texture. The mineral assemblage consists of essential alkali feldspar; relatively minor plagioclase and quartz or a feldspathoid such as nepheline may also be present....
s and basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
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
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
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....
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
Volcanogenic massive sulfide ore deposit
Volcanogenic massive sulfide ore deposits are a type of metal sulfide ore deposit, mainly Cu-Zn-Pb which are associated with and created by volcanic-associated hydrothermal events in submarine environments....
chert
Chert
Chert is a fine-grained silica-rich microcrystalline, cryptocrystalline or microfibrous sedimentary rock that may contain small fossils. It varies greatly in color , but most often manifests as gray, brown, grayish brown and light green to rusty red; its color is an expression of trace elements...
or spilite
Spilite
Spilite is a fine-grained igneous rock, resulting particularly from alteration of oceanic basalt.The term was introduced into the geological literature by Alexandre Brongniart in 1827...
. 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 metasomatismMetasomatism
Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids.Metasomatism can occur via the action of hydrothermal fluids from an igneous or metamorphic source. In the igneous environment, metasomatism creates skarns, greisen, and may affect hornfels in the contact...
of the wall rocks to a shear zone. The presence of carbonate alteration, talc-carbonated high-magnesian basalts and ultramafic 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
Yilgarn craton
The Yilgarn Craton is a large craton which constitutes the bulk of the Western Australian land mass. It is bounded by a mixture of sedimentary basins and Proterozoic fold and thrust belts...
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
Arsenopyrite is an iron arsenic sulfide . It is a hard metallic, opaque, steel grey to silver white mineral with a relatively high specific gravity of 6.1. When dissolved in nitric acid, it releases elemental sulfur. When arsenopyrite is heated, it becomes magnetic and gives off toxic fumes...
, nickeline and gersdorffite
Gersdorffite
Gersdorffite is a nickel arsenic sulfide mineral with formula NiAsS. It crystallizes in the isometric system showing diploidal symmetry. It occurs as euhedral to massive opaque, metallic grey-black to silver white forms. Gersdorffite belongs to a solid solution series with cobaltite, CoAsS....
are encountered in the footwall of the Miitel nickel mine.
Geographical distribution
True jasperoid is found in dolomitic formations in Nevada, USA, and within dolomitic limestone sequences in Iran, Iraq and Kurdistan.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.