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Features of Exploration
for Rare-Earth Orebody by Geophysical Methods in Proterozoic Syenite
Massif of the Azov Ring Intrusion
Nikolaev
I.Yu., Fedotov S.M.
Ukrainian
State Research and Design Institute of Mining Geology, Rock Mechanics
and Mine Surveying (UkrNIMI) of
the National Academy of Sciences of Ukraine (NANU), Donetsk, Ukraine
EvgSheremet@yandex.ru
The Azov
deposit of zirconium-rare-earth ores is located within similarly-named
stock that is layered syenite intrusions amid rocks of the Volodarsky
massif of the South Kalchik complex of Low Proterozoic within the
Eastern Priazovie of Ukrainian Shield (US) Priazovsky megablock.
Plan view of the Azov
structure is a zonal-ring ellipse of the area of 3.8 km2 of
north-south strike. As its geology and structure are concerned, a
deposit of the same name is located in the south and north parts of the
Azov structure. Cross-section of syenites is intrusion body (stock)
confined to intersection node of faults of north-south, north-west and
east-west strike (Strekozov, 1998). Syenite stock is drilled over down
to the depth of more than 350 m; according to geophysical data its depth
can achieve several kilometers.
In 1988 ‑ 1992 within the
boundaries of the Azov structure exploration for zirconium ores were
conducted with large scope of drilling: 230 bore holes have been drilled
along 21 profiles. It is determined that complex of taxites of
south-east part of flat contact can be considered as productive zone
with impregnated zircon-britholite-orthite orebody.
Methods of well gamma-ray
logging and audiomagnetotelluric sounding (AMTS) were used in
exploration for rare-earth orebody.
The Azov rare-earth deposit
of US Priazovsky megablock is a perfect example of a deposit containing
radioactive minerals. Well gamma-ray logging was conducted according to
standard technique with the purpose of identification of different types
of rock units, evaluation of gamma activity of rocks, testing anomalous
zones for content of main radio nuclides, and also for detection of ore
zones.
Qualitative well gamma-ray
layering is quite accurate. Ore zones due to uranium and thorium
isomorphically embedded in ore minerals (orthite, britholite, and
others) have increased natural radioactivity (50‑500 µR/hour).
Background values of natural gamma activity in enclosing rocks and
barren interbedded layers are 10‑17 µR/hour (Sheremet, 2007).
Detailed geoelectrical
survey along geological survey lines of the Azov deposit was conducted
based on AMTS (Sheremet, 2007b).
Analysis of resistivity
sections along the profiles of geological survey lines shows that
geochemical anomalies coincide rather accurately with the zones of
maximum low electrical resistance (10‑30 Ohm·m) everywhere where there
are manifestations of sulphide impregnation. If there is no sulphide
impregnation, these anomalies are not detected. That's exactly why
anomalies can serve as additional exploration criterion for rare-earth
orebody.
From the outcomes of
geoelectric survey of the Azov structure we conclude that such areas can
be near the main depth granosyenite intrusion (that breaks through
gabbro-syenites of the Volodarsky massif) located to the west and also
to the south of it at the outcrops of syenites.
Geoelectric survey of the
Volodarsky massif and geological interpretation of the obtained data
show that rocks have varied thickness (from several thousand meters to
seven kilometers). Most of granosyenite and gabbro-syenite masses of the
Azov structure propagate down to the depth not exceeding 1,500 m. They
are branches of depth (down to 10,000 m) intrusion having endocontact
facies. Regional geoelectric survey of the Azov and Volodarsky massifs
reveal that formations of the Azov structure are not differentiates of
the Volodarsky gabbro-syenite massif but branches of granite-granosyenite
intrusion cutting the Volodarsky massif, which contains one more
intrusion similar to that which branch is the Azov deposit.
References
Strekozov, S. N.,
Vasilchenko V. V., Gursky D. S. et al.
Geologic structure and mineralization pattern of the Azov deposit //
Mineral Resources of Ukraine.
1998.
No. 3.
P.
6-9.
Sheremet
E. M.,
Strekozov S. N., Omelchenko A. A.
Ecological problems in development of the Azov deposit of
rare-metal‑rare-earth ores of increased radio activity of the Eastern
Priazovie (Ukrainian Shield) // Ecological Problems. Donetsk: DonNTU.
No. 1-2.
2007.
P. 125-129.
Sheremet E. M.,
Strekozov S. N., Krivdik S. G. et al.
Prediction of Orebodies of Rare Elements of Ukrainian Shield /
under the editorship of
S. G. Krivdik.
Donetsk, Veber.
2007b.
220 p. |