diamond prospecting and the study of volcanic pipes on the ... diamond concentration in two pipes, Bolvantsy ..... Zander, V. N. Geologicheskoye stroyeniye.
EARTH SCIENCE SECTIONS ~ Volume
,
222/May-June
\-
\ \
published Scripta
Publishing
in cooperation
American
by
with
Geological
Co the
Institute
1975
i "
l
DOKLADY
180
AKAD.
NAUK
OF DIAMONDS
NORTHERN
PART
IN
REFERENCES 1. Bazarova, T. Yu. and E. A. Dmitriyev. Dokl. Akad. Nauk, 177, No. 1, 185, 1967. 2. Naumov, V. B. and A. R Polyakov. Geokhimiya, No. 4, 379, 1971. 3. Sobolev, V. S. Chteniya im. V.1. Vernadskogo (V. 1. Vernadskiy Lectures), ~, p. 34, Nauka Press, 1973. 4. Sobolev, V. S., T. Yu. Bazarova and 1. T. Bakumenko. Bull. Volcanol., 35, No. 2, 479, 1972.
ALKAlIC
OF THE
BASALTOIDS
RUSSIAN
IN
THE
1
PLATFORM
S. v. Piotrovskiy,
E. G. Sochneva and
by Academician A. V. Sidorenko, July 8, 1974)
Between 1970 and 1973, we were engaged in diamond prospecting and the study of volcanic pipes on the Onega Peninsula of the White Sea. The work was done at the suggestion of Academician A. V. Sidorenko because of the discovery of volcanic pipes, composed of alkali c basaltoid breccia, near the village of Nenonsa. In all, eight volcanic pipes and nine untested tubular magnetic anomalies were discovered and studied here. In stream alluvium and beach sand of the Onega Peninsula we found about 140 pyrope grains, including varieties with as much as 5. 6 percent Cr203• We also observed evidence of diamond concentration in two pipes, Bolvantsy and Karakhta. The volcanic pipes are confined to a fault on the northeast side of the buried Onega graben of Riphean age [1]. The same fault, running near the town of Kandalaksha, Kola Peninsula, also contains pipes of similar composition [2]. On the Onega Peninsula the pipes are vertical or
"Tr-anslated from: Priznaki almazonosnosti shchelochnylfu bazal'toidov severa Russkoy platformy. Doklady Akademii Nauk SSSR, 1975, Vol. 222, No. 4, pp. 939-941.
'!e,-
222
Received April 10, 1974
F. V. Kaminskiy, Yu. A. Klyuyev, A. A. Konstantinovskiy, V. M. Yuzhakov (Presented
Vol.
high as 1300° to 1350°, and, above all, 3) the high homogenization temperatures of inclusions in pyroxene (1340° to 1380, .
absence of inclusions homogenizing over this temperature range. Between 1120· and 1040· the crystallization was intensive despite the sharp decrease in temperature. Leucite crystallized extremely intensively while the temperature decreased sharply from 1250° to 1150·. The rock probably began to harden at about 1040°, judging by the absence of pseudosecondary inclusions in biotite. The above crystallization temperatures of leucite are not inconsistent with its ability to crystallize at higher temperatures in other rocks. For example, leucite formed at higher temperatures is known in fergusite porphyry of the eastern Pamirs and in Holocene lavas of Vesuvius. Even though the inclusions in this leucite could not be homogenized, the following facts indicate its earlier crystallization at higher temperatures: 1) the absence of crystalline inclusions in pyr ox ene, 2) the fact that complete melting of the contents of the inclusions required temperatures as
EVIDENCE
SSSR,
steeply dipping bodies with a round or isometric shape and a diameter of 100 to 475 m. Their average cross -section area is 4.2 hectares. The geology of these pipes has already been described by Stankovskiy et al. [3]. The pipe -forming rock is an alkalic basaltoid breccia of the auto lithic type. Round and oval fragments of earlier generated rocks (autoliths) , as much as 10 to 15 cm in diameter, are cemented by material that is similar in composition and accounts for 50 to 70 percent of the rock by volume. Xenoliths account for 1 to 5 percent of the rock by volume. They consist predominantly of the surrounding Wendian mudstone of the Redkino stratigraphic unit. Archean gneiss xenoliths are less common. Both autoliths and the cement are brecciated. Chondrules ranging in diameter from O. 5 to 7 mm are cemented by material that is similar in composition and accounts for 10 to 90 percent of the rock by volume. They have a generally highly altered glassy mesostasis, which, together with the microlites embedded in it, is their main component. Clinopyroxene phenocrysts generally occur in the middle of chondrules and appear to form their "core". Chondrules are probably relics of the first stages of magma crystallization within the pipe at very shallow depths.
,
;
" 181
PETROGRAPHY Table 1 Chemical composition of alkalic basaltoid breccia from the Onega Peninsula and its minerals (in percent)
No. 6
42.0 0.30 21.1 6.3
41.0 0.01 15.3 10.6
No. 2
No. 3
48.49 0.57 7.46 0.04 5.42 2.41
39.64 0.16 20.48 0.01 8.16 18.25
22.3
7.8
-
-
-
Ign.loss
46.88 0.78 10.57 0.018 5.72 3.31 0.017 0.27 13.55 7.20 3.51 1.24 0.42 0.02 0.09 6.36
Total
99.95
SiO, TiO, AIzO. Cr,O. Fe,O. FeO NiO MnO MgO CaO l'\a,O
K,O P,O,
\',0,
sa,
Si AJI" Ti AI"I Cr FeH FeH Mn
l\fg Ca ]\a K
-
0.14 13.70 20.84 1.04 0.11 0.05
-
0.09 0.43
0.67 6.23 6.00 0.27 0.13 0.01
-
No. 4
No. 5
No. 1
Oxide
52.8 0.22 11.7 1.05
-
-
0.41 9.6 2.0
-
0.18 17.7 4.6
-
-
-
-
-
6.8 -
0.3 19.2 6.3
-
-
No. 7
No. 8
41.7
2.00 46.00 I.72 0.10 0.60 41.90 0.01 0.54 5.67 1.46
-
17.2 8.8 -
6.8
0.28 23.4 1.4
-
-
-
-
-
--
0,04
-
-
-
-
100.79
100.01
reo.os
99.98
99.51
99.58
100.04
1.78 0.22 0.02 0.11
3.08
4.08
3.11
3.02
3.00
-
0.01 1.91
-
0.88
-
-
0.07 0.15
0.75 0.82 0.07 0.01
-
1.21 0.49 0.09 0.73 0.51 0.04 0.01
-
-
0.02 1.38 0.08 1.86
0.03 1.42 0.21
-
-
-
-
-
-
0.02 1.87 0.38 0.49
1.33 0.61 0.42
1.46 0.50 0.41
0.01 1.86 0.37
0.02 2.10 0.50
0.02 2.50 0.11
-
-
-
-
-
-
-
0.88 0.01 0.01 0.22
-
-
Note. No. 1) brecc ia (mean of 108 analyses); No. 2) pyroxenes, Karakhta, Kurtyayevo, and Lyvozero pipes (mean of 4 analyses); No. 3) pink garnet, Bolvantsy pipe; No. 4) violet garnet, Kar akhta pipe; No. 5) greenish-violet garnet, Bolvantsy pipe; No. 6, 7) garnets, Kyanda River alluvium; No. 8) ilmenite, Bolvantsy pipe. Analysts: N9s. 2, 3, 8) G. I. Kireyeva, Central Prospecting Research Institute (chemical method); Nos. 4, 5) N.A. Arsen'yeva, Central Prospecting Research Institute (microspectral method); No.6, 7) data of T. T. Nikolayeva, LOPI (MS-46 Cameca microprobe.
The cement of these chondrules also consists of a matrix and phenocrysts in the ratio of about 4: 1. Its glass generally is paler than in chondrules. Microlites consist of completely saussuritized plagioclase with locally preserved evidence of twinning. Phenocrysts account for 10 to 40 percent of the rock by volume and range in size from 1 to 5 mm. They consist of clinopyroxene, olivine , nepheline, garnet and, less commonly, mica and hornblende Pyroxenes form tabular crystals that range in color from dark green (y = 1. 73, er = 1. 701) to pale green (y = 1. 709, er = 1. 68). According to chemical and X -ray diffraction analyses, they belong to the diopside-augite series. The average chemical composition of clinopyroxenes (mixture) is shown in Table 1 (No. 2). Calculation gave the following end-member ratio (in percent): diopside 62. 2, hedenbergite 22. 8, clinoenstatite 7.8, jadeite 7.2. As compared with pyroxenes from kimberlite pipes of Siberia,
those from the pipes of the Onega Peninsula contain less magnesium but more calcium. On the Poldervaart-Hess diagram our pyroxenes form a compact group within the diopside-satite field. Olivine occurs in small quantities in nearly all the pipes. It generally is replaced entirely by anisotropic serpentine (with locally admixed carbonate) and can be identified only by the shape of pseudomorphs. From measurements of the refractive index (y = 1. 722 to 1. 754), the ferruginosity of this olivine ranges from 25 to 55 percent. Garnet occurs in all pipes; its concentration in the heavy fraction of crushed samples may reach 30 to 40 percent. In thin sections it forms well-faceted isometric crystals that range in size from 0.2 to 0.5 mm. Virtually all the garnets ( 98 percent) are pink and can be classified as alrnandine (n = 1. 78 to 1. 79, ao = 11.53 to 11.552 A) (Table 1, No. 3). Upon detailed examination, however, we were able to identify orange and violet grains with a specific gravity
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Table 2 X-ray diffraction analysis of samples Bolvantsy I
e
10.0 2.3 4.5 2.3 3.2
21°55' 37°55' 45°40' 59°30' 71°10'
Karakhta I
8
10.0 1.1 6.5 7.8 5.3
22°00' 37"10' 4,5°35' 59°00' 71°15'
dIn. A
2.070 1.254
urn
0.8947 0.8143
dIn of dramond [5], dIn, A
2.060 1.270 1.079 0.8994 0.8139
A 2.060 1.261 1.075 0.8916 0.8182
* Photographedby O. V. Gal.liu.lina(All-Union DIamondResearch Institute}. BSV-6tube with Cu anode, voltage25 kV, current 5 mA. gravity of 3.5 to 3.55, n ~ 1. 737 to 1. 745 and ~ = 11.523 ± 0.002 A. Microspectral analysis of one such grain from the Karakhta pipe showed that it contained 43.3 percent pyrope (Table 1, No. 4). Several grains from the Bolvantsy pipe proved to be still more interesting (Table 1, No. 5). For example, they contain as much as 63.3 percent and 6.3 percent CrZ03• Such garnets are characteristic of kimberlite pipes in Yakutia. One item of considerable importance for prospecting is the find of similar garnets in stream alluvium under which no volcanic pipes have so far been discovered (Table 1, Nos. 6 and 7). Nepheline occurs in all the pipes studied as completely substituted square, tabular and hexagonal crystals and is locally transformed into chlorite and cancrinite. In the Bolvantsy pipe we also found iltnenite and were able to classify it, because of its relatively high magnesium content, as picroilmenite (Table 1, No. 8). The average chemical composition of breccia samples from the Onega Peninsula (Table 1, No. 1) sbows that they contain more silica, but less magnesium than kimberlite, and have a sodium content one order of magnitude higher (because of the presence of nepheline). Thus, breccia of the Onega Peninsula is similar in chemical composition to monchiquite and "ankar atr-ite" of the Soviet Maritime region (4]. One important result of our work was the discovery of evidence of diamond concentration in two pipes, Bolvantsy and Karakhta, where magnesium and chromium garnets had previously been found. A fine size fraction of crushed br eccia ( -0.375 mm), after flotation, was sintered with alkalies and dissolved in weak acids.
The product was then X -rayed in a URS-50IM unit. In Table 2 we give the results of the survey and the identification of diamond lines with the standard (5]. Thus, diamonds have been discovered for the first time in alkalic basaltoids, a fact that confirms their polygene tic nature (6]. The Bolvantsy and the Karakhta pipes are the first bedrock sources of diamonds found on the Russian platform. Received July 4, 1974
REFERENCES 1. Zander, V. N. Geologicheskoye stroyeniye i per spektivy rudonosnosti fundamenta sklonov Baltiyskogo shchita ( Geologic Structure and Ore Potential of the Basement on the Flanks of the Baltic Shield) , Leningrad, 1972. 2. Kuryleva, N. A and V. V. Nosikov, Razv. i okhrana nedr, No. 3, 5, 1959. 3. Stankovskiy , A F., M. A. Danilov et al. Sov. geol., No. 8, 69, 1973. 4. Gapeyeva, G. M. Geologiya i geofizika, No. 10, 46, 1960. 5. Mikheyev, V.1. Rentgenometricheskiy opredelitel' mineralov (X-ray Identification of Minerals), Moscow, 1957. 6. Kaminskiy, F. V. In: Zakonomernosti razmeshcheniya i prognozirovaniya almaznykh mestorozhdeniy ( Distribution and Prediction of Diamond Deposits), p. 45, Leningrad, 1973.
