The term carbonatite is defined, and it is explained that carbonatites are
not of sedimentary origin even though they may be composed of more than 50%
carbonate minerals. The mineralogy and petrography of carbonatites are then
briefly described. Next, the relationship of carbonatites with other igneous rocks
such as kimberlite, ijolite, essexite, nepheline syenite and alkali syenite are discussed.
It is noted that carbonatites have chronological, spatial, chemical, and tectonic
relationships with kimberlite and alkaline rocks.
Fenetization is then described as the alkali metasomatism of the country
rocks.
The field occurrence and distribution of carbonatites are summarized. It is
explained that they commonly occur in continental shield and platform areas which
have experinced tensional stresses and differential up - and - down move -ments .
Four previously proposed theories are then given for the origin of carbonatites:
1 ) mobilization of limestones and marbles, 2 ) alkalic metasomatism 3) hyd ,
rothermal replacement of alkali igneous rocks and 4) magmatic origin. There is
more favourable evidence (including experimental results) for the theory of magmatic
origin.
The economic value of different elements and minerals m carbonatites
(apatite, niobium, barite rare earths) is briefly discussed.
The petrographical, mineralogical, and geological information useful for the
discovery of carbonatites (Deans, 1967 ) is briefly summarized.
So far, no definite occurrence of carbonatities has been reported in Iran, but
favourable areas and possible regions for their occurrence are mentioned.
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