Lithofacies types of
Paleoproterozoic meta-kimberlites from the Kimozero area and their
geochemical characteristics (Karelia, Russia)
Nosova A.A.*, Ruch`ov A.M.**
* - The Institute of
Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry,
Russian Academy of Sciences (IGEM RAS), Moscow, Russia
** - The Institute of
Geology KarRC RAS, Petrozavodsk, Russia
Kimberlites of Kimozero area are unique occurrence of
ancient Paleoproterozoic kimberlitic magmatism. Kimozero localized in
the central part of the Onega depression among shungite shales, basalts
and gabbro-dolerites of Zaonezhskiy complex of Paleoproterozoic age
(~2000 Ma). The age of formation of kimberlites of Kimozero is 1986 ±4
Ma (Samsonov et al., 2009). The first discoveries of kimberlite of
Kimozero were in 1992 and since this time are known just several
publications (Ushkov, 2001; Ustinov et al, 2009).
The main difficulty in the study of the kimberlites
of Kimozero is metamorphism in greenschist facies conditions and
intensive shear deformation with post-kimberlitic age (~1700 Ma). The
presence of these shear deformation have questioned the safekeeping of
primary morphological characteristics of the diatreme zone. Modern
contours of the kimberlitic structure may not correspond to the source
structure; to the same kimberlite output in different parts of the
kimberlitic structure have different level of erosion. However, despite
the above limits, kimberlites of the one tectonic blocks preserved the
original textural and structural characteristics which together with the
nature of the secondary alteration and petro- and geochemical
characteristics we can distinguish two types: carbonate and magnesium.
The kimberlites of these types form different lithofacies types. The
presence of several types of kimberlite (Carb- and Mg-kimberlites) as is
typical for other kimberlite provinces of the world, for example,
Daldyn-Alakit area, Yakutia (Ilupin and 1981).
Lithofacies varieties of kimberlites of Kimozero can
be grouped into two zones – diatreme and crater. In addition, there were
the thin dikes of Carb- and Mg-kimberlites.
consist of autolithic kimberlitic breccia (AKB), kimberlitic breccia
(KB) and kimberlitic xeno- tuff breccia (KTB). The kimberlitic breccia
consists of the xenoliths, megacrysts and groundmass. Among the
xenoliths dominated by fragments of dolerite, and found to be
subordinate carbonaceous shales. The size of xenoliths in the KB is
about a first cm, with their contents up to 15-20 vol. %; the size of
the xenoliths in the KTB can reach several decimeters, with their
contents up to 50-60 vol. %. Megacrysts represented by olivine, and
mica. Olivine is completely replaced by minerals of the serpentine
group, which in turn develops chlorite, and carbonates. Mica often forms
the deformed leaves, the size of them is 5-6 mm. The micas are
chloritized. The groundmass consists of serpentine-chlorite aggregate
with a mixture of carbonate material from the first to up to 30-40 vol.
%, small isometric grains of completely altered olivine of the second
generation, and fine disseminated of ore mineral up to 10-15%. The
relics of primary kimberlitic groundmass minerals are not stored. In AKB
notes the presence of autoliths up to 20 vol. %. Autoliths have a more
fine-grained structure with megacrysts of olivine which are completely
replaced by minerals of the serpentine group, and chloritized mica.
These minerals immersed in a carbonate-serpentine groundmass with fine
ore minerals up to 20 vol. %. Often autoliths have nuclear structure -
nuclei are xenoliths of country rocks or large xenocrysts of altered
olivine. Among the accessory minerals are garnet, pyroxene, spinel,
ilmenite and diamonds.
Crater zone consists
of kimberlitic tuffs and tuffites of pyroclastic and epiclastic origin.
Tuffites formed by the recycling of pyroclastic material and the
kimberlitic material of diatreme zones. The rocks of crater zone consist
of disintegrated kimberlitic material (as similar to kimberlites
breccia) with the addition of fragments of country rocks represented by
dolerites or carbonaceous shales. Sometimes the contents of xenoliths
material, for example the carbonaceous shales, may dominate the
composition of the rocks. Textures of kimberlites of crater zone vary
from fine-grained to medium-grained; in the rocks are vary the
proportions of the contents of olivine's fragments, mica and the amount
of ore mineral (magnetite). Often there are remaining the relict layered
and gradational structures. They are indicating of the recycling of
kimberlitic material. For example, there are areas with alternating
layers of rich ore mineral (mainly magnetite, up to 80% vol.) and layers
of rich kimberlitic material. We may suppose in different parts of
kimberlite structures the existence of different sections of the crater
area and different level of erosion, because there are variations in
size and degree of sorting of mineral aggregates and the distribution of
packs which composed of one or another kimberlitic materials.
All the kimberlites were metamorphosed in the
greenschist facies. In result of this metamorphic process the primary
mineral assemblages have experienced a strong conversion. They were
replaced by carbonate-chlorite-actinolite association. Among the tuffs
and tuffites, especially near contacts with country rocks,
processes amphibolization and carbonation.
Carb-kimberlites are enriched in Al2O3,
CaO, P2O5, Li, Sr, Y, Nb, REE, Th, and U and
depleted in SiO2, MgO, Cr и Ni. The Fig. displays the
primitive mantle-normalized incompatible element diagram for the
Kimozero kimberlites. It shows that Carb-kimberlites do not have
positive anomalies of Zr-Hf and have negative anomalies of Ti as opposed
to Mg-kimberlites. In addition, Carb-kimberlites have a higher level of
fractionation of REE than Mg-kimberlites: relations (La/Yb)n
and (Gd/Yb)n are ranging in 5.7-7.0 and 71-115 respectively
for the Carb-kimberlites; and 20-48, and 3.1-5.2 for the Mg-kimberlites
respectively. It also shows that the distribution of trace elements for
the majority of representative samples tuffites comparable with the
distribution of trace elements for Mg-kimberlites. This evidences their
pyroclastic and epiclastic origin.
Fig. The primitive mantle-normalized incompatible
element diagram for the Kimozero kimberlites. Primitive mantle values
are (McDonough, Sun, 1995).
The resulting petro- and geochemical differences
suggest that the Carb-kimberlite had the more geochemically enriched
source than the Mg-kimberlites. High content of magnesium component in
melt of Mg-kimberlites associated with a higher degree of assimilation
of lithospheric mantle peridotite while the generation of Mg-kimberlites
melts (Sparks et al., 2008) in contrast to the Carb-kimberlites.
So, the formation of kimberlitic melts occurred in
two phases: 1) formation of Mg-kimberlites, their tuffs and products of
recycling; 2) Carb-kimberlites and the tuffs.
This study was financially
supported by grant of the
President of the Russian Federation for state support of young Russian
Ilupin I.P., Vaganov
V.I., Osipov, Yu.A., Podchasov V.M. Two groups of kimberlites in the
Daldyn field of Siberian kimberlite province // Dokl. USSR Academy of
Sciences, 1981. V. 261. №. 4. P. 957-960. [in
McDonough W.F., Sun S.S. The composition of the Earth
// Chem. Geol. 1995. V. 120. P. 223-253.
Samsonov A.V., Larionova
Yu.O., Salnikov, E.B. et al.
Isotope geochemistry and geochronology of Paleoproterozoic
meta-kimberlites of Kimozero
occurrence (Central Karelia) // Proceedings of the IV Russian
Conference on Isotope Geochronology «Isotopic Systems and the time of
geological processes». St. Petersburg, 2011. V. 2. P. 158-161. [in
Sparks R.S.J., Brooker R.A., Brown R.J. et al. The
nature of kimberlite melts, rock and magmas // IX Kimberlite Conf.,
Extended abstract, 2008. №9IKC-A00260. P. 1-3.
Ustinov V.N., Zagainyi
A.K., Smith C.B., Ushkov V.V., Lazko E.E., Lukyanova L.I., Lobkova L.P.
Early Proterozoic diamondiferous kimberlites of Karelia and the
peculiarities of their formation // Geology and Geophysics. 2009. V. 50.
№ 9. P. 963-977. [in Russian].
Kimozerskoe occurrence of diamondiferous kimberlites in the Onega
structure // Geology and Mineral Resources of Karelia.
2001. №. 3. P. 94-98.