Ferrocarbonatite

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Ferrocarbonatites are igneous rocks that belong to the group of carbonatites . They consist of more than 50 percent by volume of iron-rich carbonates .

definition

Classification diagram of the carbonatites according to Woolley (1989), the ferrocarbonatite field in red

Ferrocarbonatites are defined in two ways:

  • via the modal mineral inventory as carbonatite, which consists of more than 50 percent by volume of iron-rich carbonate.
  • on the chemical composition as a carbonatite variety which, with an SiO 2 content of less than 20 percent by weight, the weight percentages
    • CaO / (CaO + MgO + FeO + Fe 2 O 3 + MnO) <0.8
    • MgO <(FeO + Fe 2 O 3 + MnO)

Fulfills.

mineralogy

The main minerals of ferrocarbonatites are the carbonates anchorite , iron-rich dolomite , siderite and also calcite .

These are accompanied by magnetite , hematite , ilmenite , goethite , iron-rich phlogopite , chlorite , apatite , allanite , pyrite , chalcopyrite , quartz , albite , microcline , sodium amphibole and aegirine .

Monazite , pyrochlore , bastnasite , melilite , cancrinite , analcime and lamprophyllite act as accessories . Track manner can barite , Cerit , fluorite , halite , sylvite , sphalerite , molybdenite and galena be present. The carbonates ankylite , parisite and synchisite as well as aeschynite , monazite, thorite and uraninite are very rarely found .

Chemical composition

The following table should clarify the chemical composition of ferrocarbonatites:

Weight% Average ferrocarbonatite Ferrocarbonatite veins
Swartbooisdrif		 Ferrocarbonatite
(Le Bas, 1981):		 Ferrocarbonatite
Kangankunde		 Ferrocarbonatite
Mount Homa
SiO 2 4.70 1.43 1.49 3.24 0.83
TiO 2 0.42 0.25 0.22 0.00 0.07
Al 2 O 3 1.46 0.42 1.11 0.20 0.65
Fe 2 O 3 12.72 21.54 6.97 11.50 11.00
MnO 1.65 2.43 1.58 5.18 5.53
MgO 6.05 7.98 1.62 10.74 0.36
CaO 32.77 27.06 46.29 25.85 43.60
Na 2 O 0.39 0.25 0.34 0.00 0.05
K 2 O 0.39 0.00 0.22 0.00 0.06
P 2 O 5 1.97 0.01 1.63 1.27 0.42
CO 2 30.74 35.38 33.97 32.62 30.42
BaO 3.25 0.02 1.93 2.48 4.00
SrO 0.88 0.52 0.12 0.73 0.07
REEO 1.04 2.82 1.50

Ferrokarbonatite end members are the carbonatitic Differentiationsreihe Sövit - Alvikit - Beforsit -Ferrokarbonatit or Calcitkarbonatit - Dolomitkarbonatit (Magnesiokarbonatit) -Ferrokarbonatit, which is reflected in the sequence of the predominant carbonate minerals (calcite-dolomite Ankerit-siderite). Compared to calcite carbonatites, they are characterized by an enrichment of the following components:

  • SiO 2 : from 2.72 percent by weight for calcite carbonatites to 4.70 percent by weight for ferrocarbonatites
  • TiO 2 : from 0.15 to 0.42 percent by weight
  • Iron oxides: from 3.26 to 12.72 percent by weight
  • MnO: from 0.52 to 1.65 percent by weight
  • BaO: from 0.34 to 3.25 percent by weight
  • F: Increase in the F / Cl ratio from 3.6 to 22.5
  • Th: from 52 to 280 ppm , with a simultaneous increase in the Th / U ratio from 6 to 40
  • Rare earths: generally very strong enrichment (from 3730.7 ppm to 10372.3 ppm), especially the LREE:
    • La: from 608 ppm to 2670 ppm
    • Ce: from 1687 to 5130 ppm

Occurrence

The carbonatite volcano Mount Homa in Kenya

Individual evidence

  1. ↑ Le Bas, MJ: Carbonatite-Nephelinite Volcanism . Wiley, London 1977, pp. 347 .
  2. ↑ Pirajno, F. and González-Álvarez, I .: The ironstone veins of the Gifford Creek ferrocarbonatite complex, Gascoyne Province . Record 2013/12. Geological Survey of Western Australia, 2013.
  3. ↑ Woolley, AR and Kempe, DRC: Carbonatites: Nomenclature, average chemical compositions and element distribution . Ed .: Bell, K., Carbonatites: Genesis and Evolution. Unwin Hyman, London 1989, pp. 1- 14 .
  4. ↑ Le Bas, MJ: Carbonatite Magmas . In: Mineralogical Magazine . tape 44 , 1981, pp. 133-140 .