Saprolite

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A represents soil; B represents laterite, a regolith; C represents saprolite, a less-weathered regolith; beneath C is bedrock. Estructura-suelo.jpg
A represents soil; B represents laterite, a regolith; C represents saprolite, a less-weathered regolith; beneath C is bedrock.

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.

Contents

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 aquifers are capable of producing groundwater, often suitable for livestock. 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. [1]

Definition, description and locations

Saprolite is not as weathered as laterite; there is a continuum from the upper layer of saprolite to laterite. Laterite-saprolite cross section.PNG
Saprolite is not as weathered as laterite; there is a continuum from the upper layer of saprolite to laterite.

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 laterite.

Saprolites form in the lower zones of soil horizons [1] and represent deep weathering of the bedrock surface. [2] In lateritic regoliths 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. [1] Weathering formed thin kaolinitic [Al2Si2O5(OH)4] saprolites 1,000 to 500 million years ago; thick kaolinitic saprolites 200 to 66 million years ago; and medium-thick immature saprolites 5 million years ago in Sweden. [2] The general structure of kaolinite has silicate [Si2O5] sheets bonded to aluminium hydroxide [Al2(OH)4] layers.

Saprolite at Arranmore (Ireland). Transition from tectonized quartzite through saprolite to laterite. The weathered section is covered by glacial drift with scattered erratics, Holocene sandy soil and thin bog. Saprolite on Aranmore, Sept 2019.jpg
Saprolite at Arranmore (Ireland). Transition from tectonized quartzite through saprolite to laterite. The weathered section is covered by glacial drift with scattered erratics, Holocene sandy soil and thin bog.

Iron compounds are the primary coloring agents in saprolites. [3] At most outcrops the color comes from ferric compounds; the color relates to the mineralogy and particle size. [3] Submicron-sized goethite [FeO(OH)] is yellow; coarse goethite is brown. [3] Sub-micron-sized hematite [Fe2O3] is red; coarse hematite is gray to black. [3]

Regoliths vary from a few meters to over 150 m (490 ft) thick, depending on the age of the land surface, tectonic activity, climate, climate history and the composition of the bedrock. [1] Although these deeply weathered terrains 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. [1] In parts of Africa, India, South America, Australia and southeast Asia, regolith has been forming continuously for over 100 million years. [1] Deeply weathered regoliths are widespread in the inter-tropical belt, particularly on the continental landmasses between latitudes 35°N and 35°S. [1] 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. [1] In some localities it is possible to relatively date saprolite by considering that the saprolite must be younger than the parent material and older than any thick cover unit such a lava or sedimentary rock. This principle is useful in some contexts but in others, like certain parts of Sweden where grus is formed from Precambrian rocks and overlain by Quaternary deposits, it is of little value. [4]

Formation

The regolith of a region is the product of its long weathering history; leaching and dispersion are dominant during the initial phase of weathering under humid conditions. [1] Saprolites form in high rainfall regions which result in chemical weathering and are characterised by distinct decomposition of the parent rock's mineralogy. [5] Conditions for the formation of deeply weathered regolith include a topographically moderate relief flat enough to allow leaching of the products of chemical weathering. [1] A second condition is long periods of tectonic stability; tectonic activity and climate change partially erode the regolith. [1] Weathering rates of 20 m (66 ft) per million years suggest that deep regoliths require several million years to develop. [1] The third condition is humid tropical to temperate climate; higher temperatures enable reactions to occur more rapidly. [1] Deep weathering can occur in cooler climates, but over longer periods of time. [1]

Sulfides 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. [1] Carbonates are highly soluble, especially in acidic environments; the elements hosted by them calcium, magnesium, manganese and strontium are strongly leached. [1] Serpentinite oxidized and hydrolized low-silicon, iron- and magnesium-rich oxide igneous rocks are progressively weathered through this zone. [1] 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. [1] They are leached from the upper horizons and reprecipitate with secondary iron-manganese oxides in the mid- to lower saprolite. [1]

Uses

Aquifers in Western Australia are of saprolite grit. [6] Poorly weathered saprolite grit aquifers are capable of producing groundwater, often suitable for livestock. [6] Yields depend on the texture of the materials and their depth from which the aquifer is derived. [6]

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 ft). [1] The gold-carbonate association is also apparent in the Gawler Craton, South Australia. [1] Supergene enrichment occurs near the surface and involves water circulation with its resulting oxidation and chemical weathering. [1] 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. [1]

See also

Related Research Articles

Weathering Breaking down of rocks or other materials through exposure to the elements

Weathering is the breaking down of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs in situ, and should not be confused with erosion, which involves the transport of rocks and minerals by agents such as water, ice, snow, wind, waves and gravity.

Regolith A layer of loose, heterogeneous superficial deposits covering solid rock

Regolith is a blanket of unconsolidated, loose, heterogeneous superficial deposits covering solid rock. It includes dust, broken rocks, and other related materials and is present on Earth, the Moon, Mars, some asteroids, and other terrestrial planets and moons.

Spheroidal weathering

Spheroidal weathering is a form of chemical weathering that affects jointed bedrock and results in the formation of concentric or spherical layers of highly decayed rock within weathered bedrock that is known as saprolite. When saprolite is exposed by physical erosion, these concentric layers peel (spall) off as concentric shells much like the layers of a peeled onion. Within saprolite, spheroidal weathering often creates rounded boulders, known as corestones or woolsack, of relatively unweathered rock. Spheroidal weathering is also called onion skin weathering,concentric weathering,spherical weathering, or woolsack weathering.

Skarn Hard, coarse-grained, hydrothermally altered metamorphic rocks

Skarns or tactites are hard, coarse-grained metamorphic rocks that form by a process called metasomatism. Skarns tend to be rich in calcium-magnesium-iron-manganese-aluminium silicate minerals, which are also referred to as calc-silicate minerals. These minerals form as a result of alteration which occurs when hydrothermal fluids interact with a protolith of either igneous or sedimentary origin. In many cases, skarns are associated with the intrusion of a granitic pluton found in and around faults or shear zones that intrude into a carbonate layer composed of either dolomite or limestone. Skarns can form by regional, or contact metamorphism and therefore form in relatively high temperature environments. The hydrothermal fluids associated with the metasomatic processes can originate from either magmatic, metamorphic, meteoric, marine, or even a mix of these. The resulting skarn may consist of a variety of different minerals which are highly dependent on both the original composition of the hydrothermal fluid and the original composition of the protolith.

Bedrock Lithified rock under the regolith

Bedrock in geology is solid rock that lies under loose softer material (regolith) within the crust of Earth or another terrestrial planet.

Copper extraction Process of extracting copper from the ground

Copper extraction refers to the methods used to obtain copper from its ores. The conversion of copper consists of a series of physical and electrochemical processes. Methods have evolved and vary with country depending on the ore source, local environmental regulations, and other factors.

The pedosphere is the outermost layer of the Earth that is composed of soil and subject to soil formation processes. It exists at the interface of the lithosphere, atmosphere, hydrosphere and biosphere. The pedosphere is the skin of the Earth and only develops when there is a dynamic interaction between the atmosphere, biosphere, lithosphere and the hydrosphere. The pedosphere is the foundation of terrestrial life on Earth.

Ore genesis How the various types of mineral deposits form within the Earths crust

Various theories of ore genesis explain how the various types of mineral deposits form within the Earth's crust. Ore-genesis theories vary depending on the mineral or commodity examined.

In ore deposit geology, supergene processes or enrichment are those that occur relatively near the surface as opposed to deep hypogene processes. Supergene processes include the predominance of meteoric water circulation with concomitant oxidation and chemical weathering. The descending meteoric waters oxidize the primary (hypogene) sulfide ore minerals and redistribute the metallic ore elements. Supergene enrichment occurs at the base of the oxidized portion of an ore deposit. Metals that have been leached from the oxidized ore are carried downward by percolating groundwater, and react with hypogene sulfides at the supergene-hypogene boundary. The reaction produces secondary sulfides with metal contents higher than those of the primary ore. This is particularly noted in copper ore deposits where the copper sulfide minerals chalcocite Cu2S, covellite CuS, digenite Cu18S10, and djurleite Cu31S16 are deposited by the descending surface waters.

Gaspéite

Gaspéite, a very rare nickel carbonate mineral, with the formula (Ni,Fe,Mg)CO
3
, is named for the place it was first described, in the Gaspé Peninsula, Québec, Canada.

Kambaldaite

Kambaldaite, NaNi4(CO3)3(OH)3·3H2O, is an extremely rare hydrated sodium nickel carbonate mineral described from gossanous material associated with Kambalda type komatiitic nickel ore deposits at Kambalda, Western Australia, and Widgie Townsite nickel gossan, Widgiemooltha, Western Australia.

Violarite (Fe2+Ni23+S4) is a supergene sulfide mineral associated with the weathering and oxidation of primary pentlandite nickel sulfide ore minerals.

Polydymite

Polydymite, Ni2+Ni23+S4, is a supergene thiospinel sulfide mineral associated with the weathering of primary pentlandite nickel sulfide.

Carbonate-hosted lead-zinc ore deposits

Carbonate-hosted lead-zinc ore deposits are important and highly valuable concentrations of lead and zinc sulfide ores hosted within carbonate formations and which share a common genetic origin.

Grus (geology) Accumulation of angular, coarse-grained fragments resulting from the granular disintegration of crystalline rocks

Grus is an accumulation of angular, coarse-grained fragments resulting from the granular disintegration by the processes of chemical and mechanical weathering of crystalline rocks generally in an arid or semiarid region. Grus sand, when cemented into a sandstone, will form an arkose.

Laterite Product of rock weathering in wet tropical climate rich in iron and aluminium

Laterite is both a soil and a rock type rich in iron and aluminium and is commonly considered to have formed in hot and wet tropical areas. Nearly all laterites are of rusty-red coloration, because of high iron oxide content. They develop by intensive and prolonged weathering of the underlying parent rock, usually when there are conditions of high temperatures and heavy rainfall with alternate wet and dry periods. Tropical weathering (laterization) is a prolonged process of chemical weathering which produces a wide variety in the thickness, grade, chemistry and ore mineralogy of the resulting soils. The majority of the land area containing laterites is between the tropics of Cancer and Capricorn.

In ore deposit geology, hypogene processes occur deep below the earth's surface, and tend to form deposits of primary minerals, as opposed to supergene processes that occur at or near the surface, and tend to form secondary minerals.

Residuum is often used to refer to the soil and subsoil that forms as the result of long weathering over carbonate rocks bedrock. It is defined primarily as “the unconsolidated weathered at least partly, mineral material that has accumulated as consolidated rocks disintegrated in place. It is a type of soil parent material which has formed in place of origin. This distinguishes residuum from most other types of parent material. Parent material is classified by mode of transport.

Primary mineral

A primary mineral is any mineral formed during the original crystallization of the host igneous primary rock and includes the essential mineral(s) used to classify the rock along with any accessory minerals. In ore deposit geology, hypogene processes occur deep below the earth's surface, and tend to form deposits of primary minerals, as opposed to supergene processes that occur at or near the surface, and tend to form secondary minerals.

Regolith-hosted rare earth element deposits

Regolith-hosted rare earth element deposits are rare-earth element (REE) ores in decomposed rocks that are formed by intense weathering of REE-rich parental rocks in subtropical areas. In these areas, rocks are intensely broken and decomposed. Then, REEs infiltrate downward with rain water and they are concentrated along a deeper weathered layer beneath the ground surface.

References

  1. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Butt, C.R.M.; Lintern, M.J.; Anand, R.R. (1997). "Evolution of Regoliths and Landscapes in Deeply Weathered Terrain Implications for Geochemical Exploration" (PDF) (40). Retrieved April 22, 2010.Cite journal requires |journal= (help)
  2. 1 2 Lidmar-Bergström, Karna; Olsson, Siv; Olvmo, Mats (1997). "Palaeosurfaces and associated saprolites in southern Sweden". Geological Society, London, Special Publications. 120 (1): 95. Bibcode:1997GSLSP.120...95L. doi:10.1144/GSL.SP.1997.120.01.07. S2CID   129229906 . Retrieved April 21, 2010.
  3. 1 2 3 4 Hurst, Vernon J. (February 1977). "Visual estimation of iron in saprolite". GSA Bulletin. Geological Society of America. 88 (2): 174. Bibcode:1977GSAB...88..174H. doi:10.1130/0016-7606(1977)88<174:VEOIIS>2.0.CO;2.
  4. Migoń, Piotr; Lidmar-Bergström, Karna (2002). "Deep weathering through time in central and northwestern Europe: problems of dating and interpretation of geological record". Catena. 49 (1–2): 25–40. doi:10.1016/S0341-8162(02)00015-2.
  5. Dippenaar, Mattys; Van Rooy, Louis; Croucamp, Leon (2006). The Use of Index laboratory Testing to Determine the Engineering Behaviour of Granitic Saprolite (PDF) (Report). IAEG. Retrieved May 3, 2010.
  6. 1 2 3 George, Richard J. (January 1992). "Hydraulic properties of groundwater systems in the saprolite and sediments of the wheatbelt, Western Australia". Journal of Hydrology. Elsevier B.V. 130 (1–4): 251. Bibcode:1992JHyd..130..251G. doi:10.1016/0022-1694(92)90113-A.