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Mineralogy-geochemical specificity kimberlitic off Yakutia with the
different productivity
Rotman
A. Ya.
*, Ivanov А.S.
**, Pomazansky B.S.*
*
Geo-Scientific Research Enterprise, ALROSA, Mirny, Russia
**
Botuobinsky geological prospecting expedition ALROSA, Mirny, Russia
RotmanAYa@alrosa.ru,
arotman@mail.ru
The kimberlite appearances (diatrems, more rare veins, dykes and sills)
are the main source of diamonds and represent a characteristic magmatic
formation of ancient platforms. In Yakutian kimberlitic province (a
northeast part of the Siberian platform) they are connected mainly with
intense manifestations of
middle paleozoic (D3–C1
367-345
Мa)
and mesozoic (T 245-215 Ma, J3 160-149 Ma), taking various
spatial positions. The first include industrially diamondiferous
kimberlites. The basic volume kimberlitic rocks is presented volcanic
brecсias
(kimberlite brecсia
– KB, autholithic kimberlite brecсia
- AKB) and, in a smaller measure, massive porphyritic kimberlite (hypabyssal
kimberlite - HK) with a varying is qualitative-quantitative spectrum
diamond indicator minerals - Cr-pyropes, chromites and picroilmenite.
The chemical composition of kimberlites is characterised high magnesium
more often at almost equal quantities SiO2 and MgO (SiO2/MgO=1,1-1,2,
sometimes 1,0 and less – (pipes Internationalnaya, Aihal,
Udachnaya-east), high maintenance of Cr and Ni, high values of relations
Mg/Fe and Ni/Co. The kimberlytes of the region and, first of all,
industrially diamondiferous bodies on the parity of oxides of K and Ti,
Al and Ca, and also on the parity
of Zr and Nb, Nb and La, Ce and Sr correspond to kimberlites of the
group I of South Africa. Thus in middle paleozoic kimberlite fields with
industrial diamond’s higher values MgO/FeOtotal (3,7-5,8),
than in those mesozoic age (1,6-2,8) are fixed. Stand apart kimberlites
of the Nakynsky field in which, as well as in separate pipes
Daldyn-Alakitsky diamond area, it is marked rather low - less than 0,5
мас.
% - maintenance TiO2 at the lowered
values of La/Yb. Behaviour
К2О
is ambiguous with appreciable variations of concentration both in
middlle paleozoic, and in mesozoic kimberlites at frequent raised
potassium in productive bodies (Mir, Dachnaya, Internationalnaya,
Botuobinskaya, Njurbinskaya, Aihal, Udachnaya). Sometimes positive
correlation of diamond and potassium is found out, that was marked by
Vasilenko V.B. with colleagues [Vasilenko, etc., 1997]. As concentrator
K2O serves
phlogopite
which quantity considerably differs both between pipes, and in tubes
between crystallisation products melting different
intrusive phases.
The main feature of the chemical composition of kimberlites is the
antagonism of oxides Ca and Mg, their carriers reflecting share a
carbonate (mainly calcite) and olivine/serpentine.
Geochemical specificity is expressed in higher maintenances Ni, Cr, Li
and PGE in industrially diamondiferous kimberlites, and Co, Mn, V, Ti,
Sc, Be, Ga, Ba, Sr, Zr, Nb, Ta, REE, Zn and Sn - in poorly
diamondiferous and not diamondiferous displays at approximately equal
quantities Rb, Pb, Mo, Th and U.
Results of studying of correlation dependences of chemical compounds of
minerals-accompanying
of diamond (MAD), the maintenance and properties (morphology
varieties, colours of a photoluminescence
and other) diamonds representative selections from 18 kimberlitic
bodies of a southern part Yakutian kimberlitic province with using
clustaring (R and G methods) and factorial analyses show to a high and
significant factors of correlation between average frequencies of
occurrence of noted characteristics in communities of pipes which are
narrowly localised within this or that kimberlitic fields. With the help
cluster (k-means) the analysis chemical compounds pyropes selection
tests (more than 10 000 grains) are divided on 24 cluster groups (CG),
composition of picroilmenite (more than 10 000 grains) - into 16 CG by a
technique described earlier [Ivanov, 2004]. Frequency of occurrence of
groups accessory minerals of diamond, and also the basic forms of
crystals and colour of a photoluminescence of diamond along with
conditional level of efficiency kimberlitic rocks is defined.
For pyrope the ultrabasic paragenesis Daldynsky kimberlitic fields
raised maintenance Ca that it is possible to explain is characteristic,
being based on existing high positive correlation Са with Ni in this
area, higher PT-conditions of crystallisation pyrope concerning those
Mirninsky and Alakit-Marhinsky fields. The important difference of
average composition of CG pyrope eclogite paragenesis from kimberlitic
pipes of the Alakit-Marhinsky field under maintenance Ni and Ti-presence
of six groups of composition in the field of joints with diamonds. They
have higher level Na-content, than other groups. Little a bit more low
average maintenance Na is fixed in pyropes Mirninsky and Daldynsky
kimberlites.
Average compositions similar clusters groups picroilmenite also have the
prominent features in everyone kimberlitic a field. Among picroilmenite
from kimberlite the Mirninsky field the greatest quantity (6) CG with
raised maintenances Cr2O3 concerning other fields
is noted.
Kimberlitic bodies of Daldyn-Alakitsky area find out appreciable
differences on morphology, colour, photoluminescence and other
properties of diamonds. The general quality sign of diamonds of
kimberlitic bodies of these fields is predominance of blue, yellow-green
photoluminescence above frequency of appearance of crystals with of
pink-lilac colour luminescence.
On a ratio gabitus forms of crystals of diamonds 3 groups of bodies are
allocated. In the first group comparable maintenances of diamonds
octahedral, transitive between octahedral and rombododecahedral gabitus,
laminar rombododecahedral are observed at the least quantity
dodecahedral. All get to this group practically known industrial and
semicommercial deposits Daldynsky and Alakit-Marhinsky kimberlitiс
fields. In the second group comparable quantities of octahedrons,
transitive forms, laminar rombododecahedral at domination dodecaedroids.
For the third group the comparable quantity of transitive forms, laminar
and roundish rombododecahedrons is characteristic at domination
octahedron crystals. Groups 2 and 3 are presented unproductive
kimberlitic pipes. For the investigated bodies of the Alakit-Marhinsky
field it is noticed, that high positive correlation with diamond
concentrations find out CG of ferrimagnetic picroilmenite
Р3,
Р4,
and
Р6.
Occurrence such picroilmenite in highly productive bodies and
correlations with frequencies of occurrence of diamonds defined gabitus
or photoluminescences can be connected with possibility of growth of
diamonds in conditions concerning raised fugitive oxygen [Palianov,
etc., 2000]. Only in highly productive kimberlite there are groups
pyrope, allocated on known classifications [Dawson J.B., Stephens, 1975,
etc.] G17, G18 and G20 (G17 concerns to lherzolitic, G18 - to diamond it
is low - and mildly-chromic dunite-harzburgites, and G20 - to
lherzolitic high chromic paragenesis).
For the Daldynsky field high and significant factors of correlation
between groups of pyrope composition G5, G12 and G21, G16 and occurrence
of diamonds in the form of laminar rombododecahedrons, crystals with an
orange photoluminescence are defined (G5 concerns to eclogitic to
paragenesis, G12 - to lherzolitic it is low - chromic low titanic
paragenesis, G21 - to diamond high chromium dunite-harzburgites
paragenesis, and G16 represents verlitic paragenesis. Significant
correlation of frequency of occurrence clusters groups ferrimagnetic
picroilmenite
Р6
and
Р7
which meet from 1 to 6 grains on one hundred in casual samples, and
level of efficiency of bodies on diamonds is appreciable.
Among kimberlite bodies of the Mirninsky field stand apart high diamond
grade pipe Mir and Internationalnaya, XXIII congress and Dachnaya. High
maintenances octahedron diamonds, transitive between oktahedrical and
rombododecahedritical gabitus are peculiar to them at the minimum
maintenance laminar and roundish rombododecahedron, pink and pink-lilac
colours of photoluminescence, and also the raised maintenances eclogite
pyropes G7 and G10 and granates of all groups of composition of chromic
ultrabasic paragenesis G20-G23, switching on diamond. In samples of
diamonds of these bodies, along with the high maintenance octahedron
crystals and prevalence of a pink-lilac photoluminescence, rather raised
maintenance of diamonds with zone distribution of colour of a
photoluminescence, and also with uncertain yellow colour of a
luminescence is noted. Kimberlite Mir pipe contains the greatest
quantity of pyropes. Among them CG eclogite paragenesis G5, G6 and G8
with lowered value Na2O from joints with diamonds and groups
G12 and G11 from megacrystals with raised values TiO2 prevail.
In a Mir pipe rather raised maintenance of diamonds with a violet
photoluminescence, prevalence ferrimagnetic picroilmenite cluster
groups P4 is noted.
In kimberlite underproductive bodies of the Mirninsky field (vein An-21,
pipe Amakinskaya, Tayognaya) which high maintenances of diamonds in the
form of laminar and roundish rombododecahedrons with blue, whitish,
green, yellow and violent colours of a photoluminescence are
characteristic, prevalence paramagnetic not chromic groups picroilmenite
CG
Р8
and
Р11,
pyrope G13 high titanic paragenesis, and also G14 and G17 the ultramafic
and G18 diamond poorly chromic dunite-harzburgites paragenesis is noted.
The high (+0.96) coefficient of correlation for frequencies of
occurrence picroilmenite cluster group P2 and group G16 granates verlite
paragenesis is indicative.
Kimberlite the studied displays find out is qualitative-quantitative
variations of mineralogy-geochemical characteristics. Frequency of
occurrence of some CG of minerals-accompanying
of diamond is in direct or return significant correlation communication
with defined crystal morphology and physical properties of diamonds only
in communities of bodies defined kimberlitic fields. It is characterizes
as a territorial unit with specific features of processes deep mineral
formation. Presence of minerals-accompanying of diamonds with raised
maintenance Ti and Fe in process diamond formation possibly promotes
formation of crystals dodecahedron gabitus, is more rare laminar
rombododecahedron.
References
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