Peculiar features of
accessory mineralization of vein series of carbonatites
from Chetlassky complex
Kozyreva I.V. *, Shvetsova I.V. *, Nedosekova I.L. **, Kapitanova V.A. *
*Institute of Geology
Komi SC UB RAS, Syktyvkar, Russia
**IGG UB RAS,
Rare earth-thorium-rare metal mineralization is known
in phenites as well as in carbonatites and in vein rocks related to
Chetlassky magmatites developed within Middle Timan (Ivensen, 1954,
Cherny, 1972, Stepanenko, 1975, 1979, 1984, Kostyakhin, Stepanenko,
The Chetlassky complex of dyke ultrabasic rocks is
close to the early and medium stages of autonomous picrite-lamprophyric
series associated with ultrabasic alkaline complexes with its
specificity and lack of feldspatholites and presence of kimpicrites and
aylikites (Nedosekova et al., 2011).
A number of dyke aggregates and related fields are
determined in the basins of Kosyua and Bobrovaya rivers. This is SN
trending Kosyuskoe→Bobrovskoe→Oktyabrskoe, where smaller areas are
determined. The depth of drilling decreas in the same direction. Within
the fields in different areas picrites and carbonatites, different
phenites (melanocratic and leucocratic) and also vein structures are
determined. Hydrothermal-metasomatic rocks are confined, as magmatites,
to faults with NE inclination. The areas of their localization represent
metasomatically transformed areas of enclosing rocks – phenites. The
zones have a complex configuration due to inhomogeneous structure and
transformation of sedimentary metamorphic rocks related to the
The phenites compose zones with symmetrically zonal
pattern, where axial (rear) part is composed of carbonatites, and
external parts – by phenites changing to aegirinized and amphibolized
varieties of sedimentary metamorphic rocks. With the absence of
carbonatites in the rear zones, the phenites look like vein-like bodies
in sedimentary metamorphic rocks, which they are connected with by
The development of structures of vein series,
goethite-feldspar and quartz-goethite-hematite, is observed within all
the fields. Rare earth-rare metal minerals are concentrated in these
rocks and form large groups.
The collection of mineral monofractions from vein
rocks (crushed samples by V.I. Stepanenko, I.V.Shvetsova, B.Ya.
Yatskevich) was studied by scanning electron microscope (JSM-6400) with
energy-dispersive spectrometer (ISIS Link) and wave spectrometer (Microspec)
in Institute of Geology, Komi SC UB RAS. New data on crystal morphology
and their chemical composition were received.
The Kosyuskoe field (Kosyu site), late carbonatite
veinlets 2545 (dolomite-phlogopite-chalcopyrite vein), 2556
(quartz-calcite-khlorite-pyrite vein). The known minerals of
carbonatites (Ivensen, 1964, Kostykhin, Stepanenko, 1984, Kovalchuk et
al., 2011, Kovalchuk, 2011): zircon, monazite, kodazite, ilmenorutile,
columbite, pyrochlore. The known minerals of phenites and vein series,
described earlier (Ivensen, 1964): zircon, monazite, ilmenorutile,
columbite, allanite, pherritorite.
Bobrovskoe field (Novobobrovsky site) № 460 –
phenite, № 8, 10-A-1 – quartz-goethite vein. The known minerals of
phenites and vein series, described earlier (Ivensen, 1964):
Mn-columbite, ilmenorutile, pyrochlore, monazite, auerlite, codazzite,
xenotime, tengerite, hydrothorite, chiblite, galenite, zircon, apatite,
allanite and pherrithorite.
Oktyabrskoe field (Oktyabrsky site) №751, 753, 754,
756 – phenite (albitites). The known minerals of phenites and vein
series, described earlier (Ivensen, 1964): apatite, xenotime,
ilmenorutile, monazite, hematite.
The studied crushed samples, apart from rock-forming
minerals of plagioclase group (albite) and K feldspar (orthoclase and
microcline), quartz, micas, rare earth-thorium-rare metal minerals are
represented by monazite, xenotime, columbite, ilmenorutile and unusual
varieties of thorium phases in rare earth carbonates up to clear thorium
silicate. The observed associations often have mixed contents, divided
into different groups and suggesting that we observe isomorphous range
of ferrous phosphate ↔ thorium silicate. Strontium apatite is observed
(SrO content to 8 wt%).
Rare minerals Nb, Ta, V in vein rocks are represented
by minerals: columbite, ilmenorutile. Vanadium doesn’t form minerals and
is included in ilmenorutile (up to 6-13 wt%).
Columbite is observed as grains in phenites and
carbonatite veins, it forms large crystals and crystal joints in vein
rocks. According to our data columbite from carbonatite veins contains
(wt%) Nb2O5 72-79, MnO not above 2, FeO at level
of 20 and related to pherrocolumbites. TiO2 content is not
above 1.5 wt % , and maximal content Та is 2
wt %. Columbite from veins contains (wt %) Nb2O5
76, MnO up to 14, FeO 7, and also small quantity of V2O5
(0.1), TiO2 (1) and related to Mn-columbites. All
columbites possess a characteristic typomorphic feature – the presence
of ilmenorutile inclusions.
Thus, process trend: vein carbonatites (Fe-Col) →
phenites (Col (Mn=Fe)) → quartz-goethite veins (Mn-Col or Col (Mn³Fe))
is marked by wave-like input of Mn and Fe and change of pherrocolumpbite
of vein carbonatites to phenite columbite and further to mangancolumbite
of quartz-goethite veins.
In ilmenorutiles from phenites, as inclusions in
columbites, Nb2O5 content makes 20 wt.% and more,
and V2O5 content increases to 6, and ilmenorutiles
contain up to 13 wt.% of inclusions in columbites from veins.
The rare earth minerals are represented by monazite,
xenotime, rare earth carbonates. Monazite forms crystals and I observed
as small inclusions in various minerals.
Monazite in veins and associated with Mn-columbite
contains wt.%: La2O3 at level 8, Ce2O3
- 26-28, Nd2O3 – 16-18, ThO2 at level
5. Thus, cerium dominates in monazite; the high concentration of
neodymium is observed; and thorium is constantly present; however
strontium content varies even within one grain.
According to N.S.Kovalchuk et al. monazite from
carbonatites at microlevel has various structures like small grains
(several mcm in size), forms different aggregates and is observed in
different mineral associations (Kovalchuk et al., 2011). Chemically they
are subdivided into three groups by predominating cation: type 1 (La2O3≥Ce2O3),
type 2 (La2O3=Ce2O3), type 3
Xenotime is observed as crystals and grains and also
as inclusions in other minerals. Microprobe studies reveal xenotime as
poikolite inclusions in complex, hard-to-diagnose, but stable phases. We
define the phases as isomorphous Fe[PO4]-Th[SiO4]
minerals. However taking into account that earlier works mentioned the
presence of hattonite Th[SiO4] (structural analog of
monazite), we seem to deal with P-hattonite CeThSi[PO8]) or
Th, Si-rabdophanite (CeThP[SiO8]×3H2O), and then
the stable ferric admixture can be resulted from goethite or
hydrogoethite admixture in the mineral. Further structural
investigations will find answers to these questions.
Rare earth carbonates. In such minerals as
bastnaesite – mineral from carbonate group – iron is always present.
Images show the presence of rare earth phases in matrix of siderite or
ankerite. Thorium-rich phases are well defined. However thinner
intergrowths are preset that is like graphic, where alternation of Ca
and Fe rich carbonates is observed; more Fe-rich ones show higher
content of REE and Sr. Generally La2O3 и
Ce2O3 content makes 15-30 wt.%
with predominance of cerium.
The studied features of mineral chemical composition
of vein carbonatites from Chetlassky complex can support sorting source
areas for gold-bearing and diamondiferous placers in Middle Timan.
Minerals of rare earth, radioactive and rare metals
occur in vein carbonatites developed within the Chetlassky Kamen in
Middle Timan. The formation of mineralization is connected with the
effect of evolutioning mantle fluid on sedimentary metamorphic rocks.
The investigations are carried out within the
framework of RAS Program 12-P-5-1015 (block 4).
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