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Lagerstätten

Magnesit- und Talklagerstätten



Charakteristik der Magnesit- und Talk-Verwitterungslagerstätten

Accompanying minerals include magnesite, dolomite, chlorite, serpentine, calcite, chlorite, and in some cases sulfides, quartz, tremolite or vermiculite.

In naturally occurring crystals, the magnesium in talc can be replaced by iron or nickel and the silicium by aluminium or chromium.

Talc has a high capacity for absorbing organic substances. The opposite applies to water: talc is hydrophobic and insoluble. It is also acid-resistant, chemically inert and non-toxic. Talc has neither aroma nor taste. Formation and deposits of talc

Talc can be formed in different geological environments or processes. Several types of talc deposits may be distinguished according to the present composition and parent rock from which they are derived. There are four types of talc deposits; the two main ones contributing to the world's talc production are described below.

Dolomite-hosted talc and talc-chlorite rocks Formation of talc rock from dolomite

The talc is formed by the alteration of sedimentary magnesium carbonate rocks (dolomite and magnesite) at elevated temperature and pressure below the earth's surface. While the magnesium is fixed in situ, the silica is transported by silica-containing hot fluids to react with the Magnesium- bearing carbonates to form talc. These altered rocks are then talc-rich dolomites or magnesites. This type of deposits usually deliver the massive talcs accompanied by carbonates, chlorite and some quartz. The mineral composition is generally 30 - 100% talc, 0 - 70% chlorite/carbonates and 0.1 - 0.5% quartz.

Ultramafic-hosted talc magnesite rocks These talc deposits result from hydrothermal alteration of magnesium-rich magmatic parent rocks. They are called ultramafic rocks while they consist mostly of mafic (dark Magnesium-rich) minerals. Alteration process is two-fold: first hydration of these mafic minerals such as olivine or pyroxene by H2O influx into serpentine which is a hydrated Magnesium-silicate. Second step is alteration of serpentine into talc and magnesite by CO2- addition.

These rocks consist of talc, magnesite, chlorite, sulfides and other minerals with no or very low quartz content.

Since talc in these deposits is not massive but occurs as talc-magnesite rock, the crude ore must be crushed and grinded prior to refining by flotation to increase the talc content and whiteness before this talc can be used as an industrial mineral.

This kind of talc deposits occur in Finland, Norway, Sweden, Canada and Russia.



Magnesitlagerstätte Eugui in Navarra

Eugui liegt als Lager im Namur und wird seitlich von Dolomiten und Kalken vertreten. Die Genese wird verschieden gedeutet. Einerseits wird sie als synsedimentär im Oberkarbon entstanden gesehen; ndere Autoren (CLAR, LOTZE, DESTOMBES, D. RICHTER, ADLER, PILGER) erklären sie als metasomatisch, wobei Dolomite verdrängt wurden. Nach Ansicht der drei zuletzt genannten Autoren entstanden die Magnesitvorkommen in den Westpyrenäen zusammen mit der austrischen Faltung vor der Oberkreide.

The Eugui-Asturreta magnesite deposit (Western Pyrenees, Spain) forms a discoidal body with a maximum thickness of 130 m, located within a folded Namurian carbonate sequence. The magnesite rocks are composed of lens-shaped crystals (max 8 cm) arranged in black and white bands (zebra structure). The morphology and textural characteristics of magnesite and the structural relationships between magnesite and dolostone host rocks indicate that magnesite replaced the host dolostone.

The magnesite crystals replaced the host dolostones, growing perpendicularly from stylolites and meeting between two adjacent stylolite sets. The portions of the crystals close to the stylolites are black colored because they incorporated carbonaceous matter and clay laminations of the replaced dolostone (incorporative growth), whereas the crystal terminations are white because impurities have been displaced and lie at an intercrystalline position and into the meeting zone (displacive growth). These texture differentiations could be interpreted by changes in the ratios of dolomite dissolution versus magnesite precipitation due to increasing amounts of Ca released during the replacement process. Late dolomite is commonly pseudomorphous after magnesite crystals.

Dolostone host rocks and magnesite show similar Mn, Al, and K contents. The host dolostones have a lower average FeO content (0.24 wt %) than magnesite (1.82 wt %) and than later dolomite after magnesite (1.04 wt %). The black magnesite bands with a low percentage of impurities show REE contents and patterns similar to those of the relatively pure dolostone host rocks, suggesting an origin by metasomatic replacement. The white magnesite bands display REE contents similar, but slightly lower in LREE, to those of the host dolostones, as would be expected by magnesite replacing dolomite. REE patterns of dolomite after magnesite are very similar to those of magnesite, suggesting that REE behaved conservatively during the replacment of magnesite by dolomite and that the process has been induced by fluids impoverished in REE and equilibrated with the carbonate sequence.

The petrography and geochemistry of the Eugui magnesite indicate that the deposit originated by metasomatic replacement of a dolostone precursor promoted by fluids moving through stylolite and bedding planes. Sedimentary, structural, textural, and geochemical relics of the replaced dolostones are preserved in the magnesite rocks. The peculiar textural characteristics of the magnesite represent replacement features and cannot be interpreted by diagenetic or metamorphic recrystallization of an original marine microcrystalline magnesite deposit.

In the area of the magnesite deposits of Eugui (Navarra, Spain) studies on illite crystallinity, the degree of graphitization of carbonaceous material, measurements of vitrinite reflectivity, and fluid inclusions have been carried out on dolomites, magnesites, schists, and carbonaceous matter. These rocks have suffered metamorphism of very low to low grade. The magnesite appears generally concordant with the Namurian dolomitic rocks showing a typically banded structure. The genetic model proposed involves an early Mg concentration during sedimentation (syndiagenetic dolomitization), lateral circulation of saline solutions, and formation of diagenetically crystallized rhythmites (DCR), and final compaction. The formation of magnesite took place under the conditions of low pressure and temperatures close to 150°C, very similar to all strata-bound ore deposits. Deformation and regional metamorphism only caused minor removal, recrystallization, and transformation of the clay minerals and carbonaceous matter.

Dolomit-Lagerstätte
Dolomit-Lagerstätte

Dolomit-Magnesit-Steinbruch Azcarrate, Eugui, Navarra, Spanien;
Foto: Gobierno de Navarra, Geografico Nacional de Espana

Geografico Nacional de Espana
Gebändertes Magnesiterz
Gebändertes Magnesiterz

Eugui, Navarra, Spanien;
Bildautor: Frederic Varells

Fabre Minerals


Literatur

  • Lugli, S., Ruiz, J.T., Garuti, G., Olmedo, F., 2000; Petrography and Geochemistry of the Eugui Magnesite Deposit (Western Pyrenees, Spain): Evidence for the Development of a Peculiar Zebra Banding by Dolomite Replacement; Econ. Geol., V 95, 1775-1791
  • Pilger,A., 1963; Die Magnesitlagerstätten in den westlichen Pyrenäen; Ztschr. Deutsche Geolog. Ges.: 115, p. 913 - 925
  • Rayner, J.H.,Brown,G., The crystal structure of talc: Clays and Clay Minerals: 21: 103-114.
  • Velasco, F.,Pesquera, A., Arce, R., Olmedo, F., 1987; A contribution to the ore genesis of the magnesite deposit of Eugui, Navarra (Spain); Mineralium Deposita, Vol.: 22, Number 1 (1987), 33-41, DOI: 10.1007/BF00204241




Durchblättern

Geologisches Portrait/Lagerstätten [ Vorherige: Literatur | Nächste: Klassifizierung von Lagerstätten ]