119
12. As a result of various speed and time of the elevation kimberlite magma
to the surface of cratons and the vector’s extent of the uplift pathway from
initial mantle sources of diamond in the limit of kimberlite provinces the
zones with deposits of various degree of diamondiferous was formed. These
zones are differed by average of weight, morphology and grade of value of
the diamond crystals. Thus, in kimberlite provinces
of Siberian and Africa the
central zones are marked by diamond subfacies, then the intermediate zones
– of diamond-pyrope subfacies and peripheral zones only by pyrope
subfacies (Table 8, Figure 5,6,7) (Krutoyarskiy and Milashev, 1964;
Milashev, 1974)
13. The high quality crystals of diamond are kept in diamondiferous
kimberlites I and II types, in which flat facet of rather fine technical crystals
prevail, with a significant quota of large jewel stones. There are worse quality
of diamonds in lamproites (kimberlite III and IV types), where many fine
technical crystals (up to 95 %) prevail and a few jewel stones. The high
quality of diamonds is mainly dated to low degree diamondiferous
kimberlites of diamond-pyrope subfacies, where large curve facet crystals
prevail and a lot of jewel (70-80 %). The latter ones
are genetically connected
with the ancient late Proterozoic kimberlites, which usually settle on the
periphery of cratons.
14. The next major condition of safety of the diamondiferous initial sources
is the degree of the subsequent metamorphic and metasomatic changes of
these rocks. As a vivid example can serve the graphitization of the initial
octahedron crystals of diamond in Beni Bousera peridotite massif located in
NW Morocco, that was obducted in Cretaceous period. That massif has
undergone metamorphic transformation and graphitization in Miocene (21.5
Ма), therefore diamonds were completely destroyed (Figure 11.), (Pearson et
al., 1993).
15. It was made the calculation of the recovered diamonds from kimberlite
and lamproite deposits at the time of their formation, as well as the reserves
of the diamonds, that were leftover in the bowels of the Earth. As a result it
shows that out of 5 established diamond-bearing kimberlite epochs the most
favorable were: late Proterozoic (16%), late Paleozoic (18 %), early
Mezozoic (22), late Mesozoic (30%) and Cenozoic (14%) (Table 5, Figure
2). The resourses of diamonds in lamproites prevail above kimberlites only
among late Proterozoic and middle Riphean deposits. In our opinion, the
general increasing resources of diamonds in more younger epochs of
kimberlite magmatism, is caused by the increasing the depth of occurrence of
initial mantle sources of diamond in connection with growth of sickness of
the depleted pyrolite upper mantle, consisted from pyroxene-olivine rocks
(Krutoyarskiy at al., 2000).
120
16. Especially, it is necessary to emphasize the interrelation between
favorable epochs of formation lithophilic terra-rare elements and diamonds,
which settle within the limits of cratons, accordingly with the ages 2500,
1200, 600, 400, 220 and 100 Ма (Table 5, Figure 3.) (Krutoyarskiy et al.,
2000). The lithophilic rare elements and alkaline was extracted from non-
depleted hypolite layer of the upper mantle composed by garnet peridotite,
pyroxenite and eclogite and its transformation in depleted
pyrolite layer of the
upper mantle, consisted from pyroxene-olivine rocks. They are carried out by
abyssal intratelluric fluid which appearance is determined by the degassing of
protonic hydrogen from hydridic core during the expansion of the Earth. At
the same time in the non-depleted upper mantle at the depths of 240-170 km
some " hot points " started up above the jets of superheat protonic hydrogen
and partial melting of garnet peridotites layers began. The result of this was
generation of alkaline-ultramafic magmas and than the displays of
diamondiferous kimberlites and lamproites under the platforms.
.17. In our opinion, each diamondiferous kimberlite epoch within the limits
of platforms is finished by the formation of large ring impact structures,
caused by brisant exhalation of superheated hydrogen gases of huge capacity
from the hydridic earth’s core in places of crossing deep faults. If in such
places the graphite gneisses or layers of carbonic schists are available, under
influence of high temperatures (2000
o
C) and extreme impact pressure
graphite passes in hexagonal diamond – lonsdaleite. For example, into the
Popigay ring impact structure (diameter about 100 km) on the North Siberian
platform was detected specific fine diamonds. They represent by
polycrystalline aggregates of cubic and hexagonal modification (lonsdaleite)
of carbon (Masaitis et al., 1998 ). The conditions of genesis and
location of
impact diamonds sharply differ from all known magmatic and metamorphic
deposits of diamonds. The resources of diamonds, located in the impact rocks
of the Popigay ring structure, as a whole, exceed those in all known in the
World diamondiferous kimberlite provinces. However, the quality of impact
diamonds is very low, because of small size of crystals and friable texture of
joints, that’s why they are suitable only for the abrasive industry.
18. If take into account the time of the start intrusion of the diamondiferous
kimberlites in the Yakutian province (376-353 Ма) and the age of the
finishing non-diamond kimberlites and picrites of the pipe Obnagennaya (100
Ма), the duration of kimberlite magmatism will be approximately 276-253
Ма. However, if the time of formation Popigay impact diamonds (36 Ма) is
taken into account to, the total duration of Yakutian diamond-bearing
province will be 340-317 Ма or about 330 Ма. In our opinion, the general
cause of the diamondiferous on this territory was the start and the end of
exhalation the superheated protonic hydrogen gas from the hydridic core of
the Earth, which has caused the global activity of
tectono-magmatic processes
into East-Siberian craton.