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Characteristics of fluid
components in the alkaline magmatic systems, participating in the
formation of the Konder platinum-bearing ultrabasic massif (Siberian
Platform)
Simonov V.A.*,
Prikhod’ko V.S.**, Agafonov L.V.*, Kotlyarov A.V.*, Kovyazin S.V.*
* Institute of Geology
and Mineralogy SB RAS, Novosibirsk, Russi
** Institute of
Tectonics and Geophysics FEB RAS, Khabarovsk, Russia
vladimir@itig.as.khb.ru
To examine ultrabasic
platinum-bearing massifs, problems of hyperbasite genesis are of great
importance. Up to now, there are different viewpoints on participation
of magmatic systems in the genetic processes. Special attention is paid
to the formation of ore components. Most authors who studied original
platinum mineralization in ultrabasic massifs suggest a significant and
leading role of fluids in the transfer of precious metals and formation
of their ore congregations.
The investigations carried
out enabled us to detect melted inclusions in chromospinellide from the
dunites of the Konder and Inaglinsky platinum-bearing massifs
(southeastern Siberian Platform) that is a direct evidence of magmatic
systems participation in crystallization of the ultrabasic rocks. The
analysis of the compositions of heated and hard-tempered melted
inclusions shows that the studied ultramafites originated from picritic
magmas. High magnesian inclusions in their basic components are
equivalent to data on biotite-pyroxene alkaline picrites, pointing to an
active participation of ultrabasic alkaline magmatic systems in the
formation of the Konder and Inaglinsky massifs (Simonov et al., 2010,
2011).
To ascertain the
peculiarities of fluid components in the alkaline magmatic systems,
taking part in the formation of the Konder platinum-bearing ultrabasic
massif and to solve the problems of the volatile components effect on
the distribution and accumulation of precise metals, the chromatographic
analysis was used. The samples (selected in 2011 in the inner parts of
the Konder massif) were tested with gas chromatography by using earlier
published methods (Osorgin, 1990; Simonov, 1993) at the Institute of
Geology and Mineralogy SB RAS, Novosibirsk. Volatile components were
extracted from monomineral fractions (chromites and olivines) with the
help of step-type heating in the helium atmosphere. Low-temperature
gases (isolated up to 400°C) were not considered. High-temperature gases
(isolated at 1000°C) which are most probably similar to endogenous
syngenetic fluids were followed in further constructions.
Compositions of fluid
components in chromospinellide (and in associated olivines) from
different-type chromite manifestations in the dunites of the Konder
massif, such as accessories, relatively rare impregnation, abundant
impregnation, small schlieres, large veins, and a chromite lens with
apparent platinoids, were examined with the use of high-temperature gas
chromatography.
Investigations of
monofractions from the studied chromite manifestations by scanning
microscope (IGM SB RAS, Novosibirsk) revealed that chromospinellide from
schlieres, lenses and veins are the most platinoid-enriched minerals.
Vein chromites contain isoferroplatinum, Os sulfides, laurite,
ferronickelplatinum sulfides. The chromite lenses and schlieres include
isoferroplatinum, native Os and Pt in association with monazite and
zircon. The first paragenetic mineral association is typical for most
high-temperature systems, whereas the other one is common for
low-temperature systems.
The analysis fulfilled by
gas chromatography displayed the lower water content (0.2-0.29 wgt%) and
carbon dioxide (240-360 ppm) in platinoid-enriched chromospinellide from
schlieres and veins, as compared with the accessory and impregnated
chromites which are depleted in precious metals – H2O (about
0.3 wgt%) and CO2 (about 680 ppm) respectively. In general,
there is established a distinct direct correlation between water and
carbon dioxide contents in chromites and olivines from the dunites of
the Konder massif.
A substantially greater
role of H2O relative to CO2 has been elucidated in
platinoid-enriched chromite-olivine associations (H2O/CO2
= 8.1-10.6), and also obviously increased contents of reduced volatiles
(H2+CH4+CO + 280-760 cm3/kg), as
compared with depleted ones (maximum H2O/CO2 to
7.6 and 33-177 cm3/kg). Accordingly, the first are
characterized by low oxidation of fluids (dominantly about 40%) in
relation to the second, practically fully oxidized volatiles – 70-90%.
The results of the
chromatographic investigations on the chromites and olivines of the
Konder massif are substantiated by our earlier obtained data on the
Kempirsaisky hyperbasite massif in Kazakhstan (Simonov, 1993), which
indicate positive correlation of Pt with H2O/CO2
and a direct relation between Pd and Os with high values of the reduced
volatile components. Thus, accumulation of platinoids in the chromite
schlieres, lenses and veins of the Konder massif was favoured by an
increased role of water, as compared with carbon dioxide (high H2O/CO2
ratio), and also considerable contents of the reduced gases and
accordingly low fluid oxidation, participating in the formation of
noticeable chromospinellide congregations in the dunites.
Comparing the obtained
compositions of volatile components in the chromospinellide of the
Konder massif with the data on the chromatographic analysis of chromites
from other ultrabasic complexes of Asia (Siberia, Far East and
Kazakhstan) one can note that these compositions are most similar to the
fluids from the chromites of Koryakia (ultrabasitic massifs of the
Krasnaya and Chirynay Mountains).
This study was supported by
the Russian Foundation for Basic Research (project №12-05-00959) and
Project DES RAS №2.
References
Osorgin N.Yu.
Chromatographic analysis of a gaseous phase in minerals (methods,
apparatus, metrology). Novosibirsk, 1990. 32 p. (Preprint/ IG&G SB Acad.
Sciences of the USSR, No 11).
Simonov V.A. Prtogenesis of
ophiolites (thermobarogeochemical investigations). Novosibirsk: OIGGM SB
RAS, 1993. 247 p.
Simonov V.A., Prikhod’ko
V.S., Kovyazin S.V. Formation conditions of the platinum-bearing
ultrabasic massifs of southeastern Siberian platform // Petrology. 2011.
V. 19. No 6. P. 579-598.
Simonov V.A., Prikhod’ko
V.S., Kovyazin S.V., Tarnavsky A.V. Formation conditions of the Konder
platinum-bearing ultrabasic massif (Aldan Shield) // Dokl. RAS. 2010. V.
434. No 1. P. 108-111. |