Saprolite
Encyclopedia
Saprolite is a chemically weathered
rock. Saprolites form in the lower zones of soil profiles and represent deep weathering of the bedrock
surface. In most outcrops its color comes from ferric
compounds. Deeply weathered profiles are widespread on the continental landmasses between latitudes 35°N and 35°S.
Conditions for the formation of deeply weathered regolith
include a topographically moderate relief flat enough to prevent erosion
and to allow leaching
of the products of chemical weathering. A second condition is long periods of tectonic stability; tectonic activity and climate change can cause erosion. The third condition is humid tropical
to temperate climate.
Poorly weathered saprolite grit aquifer
s are capable of producing groundwater
, often suitable for livestock. Deep weathering causes the formation of many secondary and supergene
ore
s – bauxite
, iron ores, saprolitic gold
, supergene copper
, uranium
and heavy minerals in residual accumulations.
.
Saprolites form in the lower zones of soil horizons and represent deep weathering of the bedrock surface. In lateritic regolith
s – regoliths are the loose layer of rocks that rest on the bedrock – saprolite may be overlain by upper horizons of residual laterite; most of the original profile is preserved by residual soils or transported overburden. Weathering formed thin kaolinitic
[Al2Si2O5(OH)4] saprolites 1,000 to 500 million years ago; thick kaolinitic saprolites 200 to 65 million years ago; and medium-thick immature saprolites 5 million years ago. The general structure of kaolinite has silicate [Si2O5] sheets bonded to aluminium
hydroxide [Al2(OH)4] layers.
Iron
compounds are the primary coloring agents in saprolites. At most outcrops the color comes from ferric compounds; the color relates to the mineralogy and particle size. Submicron-sized goethite
[FeO(OH)] is yellow; coarse goethite is brown. Submicron-sized hematite [Fe2O3] is red; coarse hematite is gray to black.
Regoliths vary from a few meters to over 150 m (492.1 ft) thick, depending on the age of the land surface, tectonic activity, climate
, climate history and the composition of the bedrock. Although these deeply weathered terrain
s now occur in a wide variety of climates ranging from warm humid to arid, tropical to temperate, they were formed under similar conditions in the past. In parts of Africa, India, South America, Australia and southeast Asia, regolith has been forming continuously for over 100 million years. Deeply weathered regoliths are widespread in the inter-tropical belt, particularly on the continental landmasses between latitudes 35°N and 35°S. Similar weathered regoliths exist at much higher latitudes – 35–42°S in southeast Australia (Victoria and Tasmania),
40–45°N in the United States (Oregon and Wisconsin) and 55°N in Europe (Northern Ireland, Germany) – although these are not regionally extensive.
and dispersion
are dominant during the initial phase of weathering under humid conditions. Saprolites form in high rainfall regions which result in chemical weathering and are characterised by distinct decomposition of the parent rock's mineralogy. Conditions for the formation of deeply weathered regolith include a topographically moderate relief flat enough to allow leaching of the products of chemical weathering. A second condition is long periods of tectonic stability; tectonic activity and climate change partially erode the regolith. Weathering rates of 20 m (65.6 ft) per million years suggest that deep regoliths require several million years to develop. The third condition is humid tropical to temperate climate; higher temperatures enable reactions to occur more rapidly. Deep weathering can occur in cooler climates, but over longer periods of time.
Sulfide
s are some of the most unstable minerals in humid, oxidizing environments; many cadmium
, cobalt
, copper
, molybdenum
, nickel
and zinc
sulfides are easily leached to deep in the profile. Carbonates are highly soluble, especially in acidic environments; the elements hosted by them – calcium, magnesium, manganese and strontium – are strongly leached. Serpentinite
– oxidized and hydrolized low-silicon, iron- and magnesium-rich oxide igneous rock
s – are progressively weathered through this zone. Ferromagnesian minerals are the principal hosts for nickel, cobalt, copper and zinc in sulfide-poor mafic and ultramafic rocks, and are retained higher in the profile than sulfide-hosted metals. They are leached from the upper horizons and reprecipitate
with secondary iron-manganese oxides in the mid- to lower saprolite.
s in Western Australia
are of saprolite grit. Poorly weathered saprolite grit aquifers are capable of producing groundwater, often suitable for livestock. Yields depend on the texture of the materials and their depth from which the aquifer is derived.
The distributions of gold
and calcium carbonate
or calcium magnesium carbonates are closely correlated and documented in the southern Yilgarn Craton
, Western Australia, in the top 1 to 2 m (3.3 to 6.6 ft) of the soil profile and locally as deep as 5 m (16.4 ft). The gold-carbonate association is also apparent in the Gawler Craton
, South Australia
. Supergene enrichment occurs near the surface and involves water circulation with its resulting oxidation and chemical weathering. Deep weathering causes the formation of many secondary and supergene ores – bauxite, iron ores, saprolitic gold, supergene copper, uranium and heavy minerals in residual accumulations.
Weathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...
rock. Saprolites form in the lower zones of soil profiles and represent deep weathering of the bedrock
Bedrock
In stratigraphy, bedrock is the native consolidated rock underlying the surface of a terrestrial planet, usually the Earth. Above the bedrock is usually an area of broken and weathered unconsolidated rock in the basal subsoil...
surface. In most outcrops its color comes from ferric
Ferric
Ferric refers to iron-containing materials or compounds. In chemistry the term is reserved for iron with an oxidation number of +3, also denoted iron or Fe3+. On the other hand, ferrous refers to iron with oxidation number of +2, denoted iron or Fe2+...
compounds. Deeply weathered profiles are widespread on the continental landmasses between latitudes 35°N and 35°S.
Conditions for the formation of deeply weathered regolith
Regolith
Regolith is a layer of loose, heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids, and other terrestrial planets and moons.-Etymology:...
include a topographically moderate relief flat enough to prevent erosion
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
and to allow leaching
Leaching (pedology)
In pedology, leaching is the loss of mineral and organic solutes due to percolation. It is a mechanism of soil formation. It is distinct from the soil forming process of eluviation, which is the loss of mineral and organic colloids. Leached and elluviated materials tend to be lost from topsoil and...
of the products of chemical weathering. A second condition is long periods of tectonic stability; tectonic activity and climate change can cause erosion. The third condition is humid tropical
Tropical climate
A tropical climate is a climate of the tropics. In the Köppen climate classification it is a non-arid climate in which all twelve months have mean temperatures above...
to temperate climate.
Poorly weathered saprolite grit aquifer
Aquifer
An aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...
s are capable of producing groundwater
Groundwater
Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...
, often suitable for livestock. Deep weathering causes the formation of many secondary and 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...
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 – bauxite
Bauxite
Bauxite is an aluminium ore and is the main source of aluminium. This form of rock consists mostly of the minerals gibbsite Al3, boehmite γ-AlO, and diaspore α-AlO, in a mixture with the two iron oxides goethite and hematite, the clay mineral kaolinite, and small amounts of anatase TiO2...
, iron ores, saprolitic 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...
, supergene copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
, uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
and heavy minerals in residual accumulations.
Definition, description and locations
Saprolite (from Greek σαπρος = putrid + λιθος = rock) is a chemically weathered rock (literally, it means "rotten rock"). More intense weathering results in a continuous transition from saprolite to lateriteLaterite
Laterites are soil types rich in iron and aluminium, formed in hot and wet tropical areas. Nearly all laterites are rusty-red because of iron oxides. They develop by intensive and long-lasting weathering of the underlying parent rock...
.
Saprolites form in the lower zones of soil horizons and represent deep weathering of the bedrock surface. In lateritic regolith
Regolith
Regolith is a layer of loose, heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids, and other terrestrial planets and moons.-Etymology:...
s – regoliths are the loose layer of rocks that rest on the bedrock – saprolite may be overlain by upper horizons of residual laterite; most of the original profile is preserved by residual soils or transported overburden. Weathering formed thin kaolinitic
Kaolinite
Kaolinite is a clay mineral, part of the group of industrial minerals, with the chemical composition Al2Si2O54. It is a layered silicate mineral, with one tetrahedral sheet linked through oxygen atoms to one octahedral sheet of alumina octahedra...
[Al2Si2O5(OH)4] saprolites 1,000 to 500 million years ago; thick kaolinitic saprolites 200 to 65 million years ago; and medium-thick immature saprolites 5 million years ago. The general structure of kaolinite has silicate [Si2O5] sheets bonded to aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
hydroxide [Al2(OH)4] layers.
Iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
compounds are the primary coloring agents in saprolites. At most outcrops the color comes from ferric compounds; the color relates to the mineralogy and particle size. Submicron-sized goethite
Goethite
Goethite , named after the German polymath Johann Wolfgang von Goethe, is an iron bearing oxide mineral found in soil and other low-temperature environments. Goethite has been well known since prehistoric times for its use as a pigment. Evidence has been found of its use in paint pigment samples...
[FeO(OH)] is yellow; coarse goethite is brown. Submicron-sized hematite [Fe2O3] is red; coarse hematite is gray to black.
Regoliths vary from a few meters to over 150 m (492.1 ft) thick, depending on the age of the land surface, tectonic activity, climate
Climate
Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elemental measurements in a given region over long periods...
, climate history and the composition of the bedrock. Although these deeply weathered terrain
Terrain
Terrain, or land relief, is the vertical and horizontal dimension of land surface. When relief is described underwater, the term bathymetry is used...
s now occur in a wide variety of climates ranging from warm humid to arid, tropical to temperate, they were formed under similar conditions in the past. In parts of Africa, India, South America, Australia and southeast Asia, regolith has been forming continuously for over 100 million years. Deeply weathered regoliths are widespread in the inter-tropical belt, particularly on the continental landmasses between latitudes 35°N and 35°S. Similar weathered regoliths exist at much higher latitudes – 35–42°S in southeast Australia (Victoria and Tasmania),
40–45°N in the United States (Oregon and Wisconsin) and 55°N in Europe (Northern Ireland, Germany) – although these are not regionally extensive.
Formation
The regolith of a region is the product of its long weathering history; leachingLeaching (pedology)
In pedology, leaching is the loss of mineral and organic solutes due to percolation. It is a mechanism of soil formation. It is distinct from the soil forming process of eluviation, which is the loss of mineral and organic colloids. Leached and elluviated materials tend to be lost from topsoil and...
and dispersion
Dispersion (chemistry)
A dispersion is a system in which particles are dispersed in a continuous phase of a different composition . See also emulsion. A dispersion is classified in a number of different ways, including how large the particles are in relation to the particles of the continuous phase, whether or not...
are dominant during the initial phase of weathering under humid conditions. Saprolites form in high rainfall regions which result in chemical weathering and are characterised by distinct decomposition of the parent rock's mineralogy. Conditions for the formation of deeply weathered regolith include a topographically moderate relief flat enough to allow leaching of the products of chemical weathering. A second condition is long periods of tectonic stability; tectonic activity and climate change partially erode the regolith. Weathering rates of 20 m (65.6 ft) per million years suggest that deep regoliths require several million years to develop. The third condition is humid tropical to temperate climate; higher temperatures enable reactions to occur more rapidly. Deep weathering can occur in cooler climates, but over longer periods of time.
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 :...
s are some of the most unstable minerals in humid, oxidizing environments; many cadmium
Cadmium
Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, bluish-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Similar to zinc, it prefers oxidation state +2 in most of its compounds and similar to mercury it shows a low...
, cobalt
Cobalt
Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....
, copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
, molybdenum
Molybdenum
Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...
, nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
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...
sulfides are easily leached to deep in the profile. Carbonates are highly soluble, especially in acidic environments; the elements hosted by them – calcium, magnesium, manganese and strontium – are strongly leached. Serpentinite
Serpentinite
Serpentinite is a rock composed of one or more serpentine group minerals. Minerals in this group are formed by serpentinization, a hydration and metamorphic transformation of ultramafic rock from the Earth's mantle...
– oxidized and hydrolized low-silicon, iron- and magnesium-rich oxide igneous rock
Igneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
s – are progressively weathered through this zone. Ferromagnesian minerals are the principal hosts for nickel, cobalt, copper and zinc in sulfide-poor mafic and ultramafic rocks, and are retained higher in the profile than sulfide-hosted metals. They are leached from the upper horizons and reprecipitate
Precipitation (chemistry)
Precipitation is the formation of a solid in a solution or inside anothersolid during a chemical reaction or by diffusion in a solid. When the reaction occurs in a liquid, the solid formed is called the precipitate, or when compacted by a centrifuge, a pellet. The liquid remaining above the solid...
with secondary iron-manganese oxides in the mid- to lower saprolite.
Uses
AquiferAquifer
An aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...
s in Western Australia
Western Australia
Western Australia is a state of Australia, occupying the entire western third of the Australian continent. It is bounded by the Indian Ocean to the north and west, the Great Australian Bight and Indian Ocean to the south, the Northern Territory to the north-east and South Australia to the south-east...
are of saprolite grit. Poorly weathered saprolite grit aquifers are capable of producing groundwater, often suitable for livestock. Yields depend on the texture of the materials and their depth from which the aquifer is derived.
The distributions 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...
and calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
or calcium magnesium carbonates are closely correlated and documented in the southern 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...
, Western Australia, in the top 1 to 2 m (3.3 to 6.6 ft) of the soil profile and locally as deep as 5 m (16.4 ft). The gold-carbonate association is also apparent in the Gawler Craton
Gawler craton
The Gawler Craton covers approximately 440,000 square kilometres of central South Australia. Its Precambrian crystalline basement crustal block was cratonised ca. 1550-1450 Ma...
, South Australia
South Australia
South Australia is a state of Australia in the southern central part of the country. It covers some of the most arid parts of the continent; with a total land area of , it is the fourth largest of Australia's six states and two territories.South Australia shares borders with all of the mainland...
. Supergene enrichment occurs near the surface and involves water circulation with its resulting oxidation and chemical weathering. Deep weathering causes the formation of many secondary and supergene ores – bauxite, iron ores, saprolitic gold, supergene copper, uranium and heavy minerals in residual accumulations.