Orthomagmatic deposit
An orthomagmatic or intramagmatic deposit ( English magmatic segregation deposit ) is an ore deposit that was deposited from a still glowing magma by fractional crystallization or by liquid mixing.
Enrichment through fractional crystallization
Certain minerals with a particularly high melting point crystallize out of the rock melt and accumulate at the bottom of the magma chamber. The resulting accumulates often show a distinct lithological alternation in which certain mineral sequences are repeated several times. The causes of this stratification are controversial. Some workers assume mechanical sorting by gravity or by magma flows; others prefer a rhythmic separation model, or magma mixing .
Well-known examples are the chromite - seams in the Bushveld Complex , a layered intrusion ( lopolith ) in South Africa and in the Great Dyke in Zimbabwe who are already in the Precambrian to the stable continental crust ( Kraton penetrated). In the Bushveld complex, because of its vanadium content, the magnetite enriched in the upper zones is now also mined. The platinum- bearing “ Merensky Reef ” lies between the chromite bands and the magnetite band . The genesis of these oxide-rich ligaments is still poorly understood.
In addition to such stratiform ore enrichments of the Bushveld type, there are also many small podiform chromite deposits (alpine type), e.g. B. in the southern Urals and in the Philippines . Since they are located in ophiolite complexes, they are believed to be former intrusions into oceanic crust in back-arc basins , not near mid-ocean ridges . Their age is Phanerozoic , and thus significantly younger than that of the Precambrian occurrences.
While chromium deposits are almost exclusively bound to ultra-basic rocks and only rarely to gabbroid or noritic intrusions, there are also important orthomagmatic titanium deposits, such as B. at Tellness in Norway or Allard Lake in Québec , Canada, which, however, have a close relationship with anorthosites and anorthosite gabbros. The differences in mineralization are obviously due to the differences between the parent magmas. The most important ore mineral here is ilmenite .
Enrichment by mixing liquids
Certain immiscible components of the melt can separate from one another even in the molten state. Sulphides in particular in a sulphide - silicate melt separate out, combine to form droplets and sink to the bottom. In contrast to fractional crystallization, liquid mixing takes place not only in magma chambers, but also at the base of basic and ultra-basic lava flows and sills (e.g. komatiite within archaic greenstone belts ). Chalcophilic elements such as iron , copper , nickel and the metals of the platinum group accumulate in the sulfide droplets . Here deposits with a high ratio of copper to nickel are bound to gabbroide rocks; those with a low ratio of ultrabasite.
The most important orthomagmatic Sulfiderzanreicherungen the world are the nickel deposits in the Sudbury Basin ( Ontario ). It is located in a Norit - Intrusion who has suffered a catastrophic release of energy, probably the impact of an asteroid . Further examples can be found in Petschenga ( CIS ), Jinchuan ( China ) and in the Yilgarn Block ( Australia ). The nickel deposit of Noril'sk-Talnakh (GUS) is a special case. It is not only tied to the sub-volcanic production crevices of basaltic basalts , but is also unusually young ( Triassic ). The bulk of the orthomagmatic sulphide deposits, however, come from the Archean and early Proterozoic .
See also
literature
- Anthony M. Evans: Mineral Deposition Studies . Ferdinand Enke, Stuttgart 1992, ISBN 3-432-99801-5 .