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MICROMINERAL COMPOSITION
AND PHYSICOCHEMICAL CONDITIONS OF
RARE-METAL ROCKS FORMING IN
ALTAI
Smirnov S.Z.*,
Annikova I.Yu.*, Vladimirov A.G.*, Tomas V.G.*, Sokolova E.N.**
*Institute
of Geology and Mineralogy SB RAS, Novosibirsk, Russia,
ira99@uiggm.nsc.ru
**Novosibirsk State
University, Novosibirsk, Russia, ssmr@uiggm.nsc.ru
The new data on mineralogy and
conditions of ongonite and elvan generation in Kalguta ore-magmatic
system, Southern Altai, are reported. The results of former
geological, petrological and thermobarogeochemical investigations
are reported in (Vladimirov et. al., 1998; Titov et. al., 2001;
Annikova et. al., 2006).
Ongonite-elvan
magmatism of Suothern Altai appears within the Eastern-Kalguta dike
belt both spatially and genetically associated with Kalguta
rare-metal granite massif and of the same name Mo-W ore deposit. The
belt has 10 km extension and from 1,5 to 3 km spread. Within belt
elvans composed about 90 of a 120 mapped dikes. In addition, the
dyke complex consists of apatite- and fluorite-bearing granite
porphyries, ongonites and ultra rare-metal ongonites. The latter
show an unusual mineral composition (apatite instead of topaz in
phenocrysts and matrix) and anomalous rare-element composition
(contents of Cs2O
reach 1800 ppm, P2O5
– to 0,96 %).
Mineralogy
of ongonites and elvans.
Porphyric rocks from Easrtern-Kalguta dike belt were studing using
petrographic polarizing microscope and scanning electronic
microscope (the SEM method). It allowed to ascertain that
phenocrysts in most cases are quartz and feldspar (K feldspar and
plagioclase) with small amount of muscovite and biotite. Feldspar
phenocrysts have different alterations. Replacement of plagioclase
by K feldspar are observed. Matrix in studied rocks consist of
quartz, albite, muscovite. Main accessory minerals in ongonites and
elvans are apatite, Ti-magnetite and pyrite. Provisional data shows
that most
important typomorphic mineral – apatite – correspond to
F-apatite and enriched in Mn and sometimes Ce. Pyrite presented as
idiomorphic phenocrysts and contain occasionally silicate minerals.
Ti-magnetite occur as idiomorphic crystals and crystal joints
contains silicates, apatite, zircon.
Micophenocrysts
and crystal inclusions in ongonite and elvan minerals.
Detailed study by SEM method allow to find the new earlier not fixed
in Eartern Kalguta dike belt mineral phases. Among microphenocrysts
the most common are zircon, monazite and xenotime included in
apatite, Ti-magnetite, quartz and feldspars. Most
interesting are the finds of Ti-niobates, REE- and Th- phosphates
and silicates in apatite and biotite phenocrysts. Ti-niobates
present a mineral range polycraze-euxenite, and REE-phosphates –
monazite and xenotim. X-Ray spectrum from SEM allows to expect the
Ce-britholite presence. Important feature of listed minerals is
their Th-specificity: it was determined Th prevalence above U.
Fluid
and melt inclusions. The
most informative mineral for microthermometrical and microprobe
study is a quartz contains both melt and fluid inclusions. Inclusion
dimensions are different. Large inclusiuons (50 micron and more)
usually founded in phenocryats cores whereas small inclusions
founded in rims. In large phenocrysts melt inclusions associate with
fluid ones. Fluid inclusions dimensions are not exceeding 6 micron.
There were determined two types of initial fluid inclusions: the
essentially hydrous inclusions with water solution and gas bubble
and the water-carbon dioxide ones contain water solution and carbon
dioxide phase. Microthermometrical study show that carbon dioxide
phase contain pure carbon dioxide with near critical density. The
total melting of crystal phases in inclusions included both under
fluid-saturated crystallization and fluid-unsaturated one has been
observed at 635-650°C. In some elvan samples in fluid-saturated
crystallization phenocrysts at this temperature the most amount of
inclusions became homogeneous. In phenocrysts cristallized under
fluid-unsaturated conditions the last crystal phases ans gas bubble
presence at 650°C.
Conclusion.
Thus crystallization of early-generation phenocrysts happened in
fluid-satirated and phase-heterogeneous system contain silicate melt
and carbon dioxide – water fluid. Seemingly the phenocrysts
crystallization proceeded under temperature not lower than
635-650°C. Melts forming porphyric rocks was enriched in
volatile and fluxing components. Muscovite presence not only in
phenocrysts but also in matrix evident the high H2O
partial pressure. The high F chemical potential reduce to
replacement from plagioclase to fluorite on late stage of dikes
generation. The high phosphorus concentration in melts reduce to
binding REE and Th as phosphates and remove them from the melt. Most
likely the apatite as main phosphatic phasewas an early mineral. At
final crystallization stages apatite became unstable and was
replaced by K silicates (K feldspar and muscovite).
Investigations
were carried out with Financial Support of Presidium SB RAS
(Integration project ¹¹ 7.10.2, 99 and 6.11) and Russian Fund of
Basic Researches
(project ¹ 06-05-65137).
References
Annikova
I.Yu., Vladimirov A.G., Vystavnoy S.A. et. al. U-Pb, 39Ar/40Ar
dating and Sm-Nd, Pb-Pb isotope research of Kalguta Mo-W
ore-magmatic system (Southern Altai, Russia) // Petrology, 2006. V.
14. ¹ 1. P.90-108.
Vladimirov A.G., Vystavnoy S.A., Titov A.V. Petrology
of Early Mesozoic raremetal granites in Southern Altai: problem of
generation anomalous enriched rare alkali and phospohorus melts //
Russian Geology and Geophysics, 1998. V. 39. ¹. 7. P.901-916
Titov A.V. Vladimirov A.G. Vystavnoy S.A., Pospelova L.N. Unusual high-temperature felsite
porphyres in postgranite dike belt of Kalguta raremetal-granite
massif (Altai Mountains) // Geoñhemistry.
2001. ¹ 6. P.677-682.
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