Smirnov, S. Z.

English

Smirnov S. Z.

Peretyazhko I. S.

Zagorsky V. Ye.

Mikhailo M. Yu.

University of Szeged

Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003 THE EVIDENCE OF UNUSUAL MELTS FROM MELT INCLUSIONS IN POCKET QUARTZ OF THE OKTYABRSKAYA PEGMATITE MINE M A L K H A N RIDGE, CENTRAL TRANSBAIKALIA. S M I R N O V . S. Z . ' . P E R E T Y A Z H K O , I. S.2, Z A G O R S K Y , V. Ye.2, M I K H A I L O , M. Yu.1 1 IMP SB RAS, Koptyga, 3, Novosibirsk 90, Russia. 2 I G C SB RAS, Favorskogo, la , Irkutsk, Russia. E-mail: ssmr@uiggm.nsc.ru Magmatic nature of pegmati tes has long been established and traced up to the early stages of pocket format ion (Kosukhin et al., 1984). Nevertheless, the information about composit ions of the latest pegmati t ic melts remains scarce. This is especial ly true for the pegmatites containing relatively large crystal-bearing cavities (pockets) with rare-metal mineralization. These pegmatites provide good examples of magmatic-hydrothermal transition taking place within a relatively closed system. This paper presents the data on composit ions and P-T conditions of the latest pegmatitic melts, entrapped in the roots of pocket quartz crystals of the single pocket cavity of the Oktyabrskaya pegmatite body. Oktyabrskaya is one of the largest pegmatites, productive for gem tourmaline within Malkhan pegmati te field. It is a 250-m long gently dipping lenticular pegmati te body, up to 17 m thick. The central part of the body contains numerous pocket cavi t ies ranging in volume f rom tens of cubic centimeters to first cubic meters. Typically, the pocket walls are lined by quar tz -K-feldspar-albite mineral complex with subordinate lepidolite and color tourmaline protruding into the cavity as wel l - formed crystals. The pocket infilling contains various color tourmaline, as well as subordinate pink beryl, hambergi te , danburi te , mica and clay materials. The material discussed in study was obtained f rom a small (about 15 cm across) pocket , l ined by clevelandite albite-quartz drusy complex with minor lepidolite and accessory cassiterite and apatite. The pocket is surrounded by medium- to coarse-grained oligoclase pegmatite with minor schorl tourmaline and almandine-spessar t ine garnet. The quartz grains of oligoclase pegmatite were found to contain very small (1-10 p.m) primary melt inclusions (MI), which associate with rare fluid inclusions (up to 15 pm). Larger (up to 100+ | im) crystallized melt inclusions appear in coarse-grained quartz nearby the pocket wall. These inclusions are omnipresent in the roots of the pocket quartz crystals and typical ly associate with abundant multiphase fluid inclusions (FI). The Mis typically consist of aggregate of silicate minerals and a fluid isolation. The latter contains water solution, a gas bubble and a daughter sassolite crystal. Silicate/fluid ratio varies significantly even within a single group of Mis , indicat ing heterogeneous entrapment of coexist ing fluid and melt. Silicate phases dominate the M i s in the crystals roots. R a m a n spectra of Mis without visible fluid isolations, nevertheless, display a prominent line at 880 cm"1. Therefore , daughter sassolite and water solution exist in these inclusions and are distributed within interstices of crystall ine aggregate. M i s with well-separated fluid isolations become dominant toward the middle parts of pocket crystals. T h e M i s have not been observed in the latest zones of studied crystals. Raman spectroscopy, SEM E D S and E M P A demonstrate that the aggregate of daughter silicate minerals is composed mainly of mica. Some mica crystals appear to have unusually high (up to 20-24 wt. %) C s 2 0 content . Taking into account ratios of K 2 0 , A1203 and C s 2 0 the micas should be considered a mixture of muscovite, nanpingite and probably lepidolite. T h e minor minerals are topaz, K-feldspar and several undetermined minerals. The small melt inclusions in pegmatite quartz were homogenized at atmospheric pressure. The first melt ing occurs < 550-580°C, while total homogenizat ion was observed at 600-650°C. The large M i s in coarse-grained pegmati te and pocket quartz have been re-melted under external water pressure of 2.0-2.5 kbar in an autoclave at 500, 550, 600 and 650±10°C during 14-24 hours. The melting of crystalline aggregate occurs within 550-610°C. After the final quench at 650±10°C the samples contain melt inclusions with glass, unmelted muscovite and topaz, and fluid isolation. T h e fract ion of fluid within re-melted M I varies significantly even within single group of inclusions. Total homogenizat ion was not observed in large inclusions under the experimental conditions applied. The concentrations of water, major and some trace elements in quenched glasses were measured using E M P A and S I M S microanalysis. The glasses are enriched in Cs, F, B and H 2 0 , whereas Fe, Mg, Mn, P, and CI appear to be below the detect ion limits of the EMPA. K and Cs are dominant alkaline metals, while Li, Na and R b are minor. S I M S demonstra ted that quenched glass contains elevated amounts of Li, Be and extremely high concentrations of Ta and Nb, while Sn and W appear to be be low the detection limits. However , even after combination the E M P A and SIMS data, analytical totals remain significantly less than 100%. This could be explained by loss of water during SIMS analysis. Assuming that, water concentrat ion could be estimated up to 12-15 wt. %. The composition of fluid inclusions and fluid isolations of MI was assessed using the micro- thermometr ic data. FIs associated with Mis show very wide range of eutectic temperatures (-28 +• -6.5°C) and ice melt ing temperatures f rom -6.5 to -2.9°C. Several measurements of fluid isolations in M i s display relatively high eutectic (-9.5°C) and ice melt ing temperatures (-3.7 +• -7.6). H3BO3 concentrations in FIs and fluid isolations of Mis are estimated at 12-20 and 15-17 wt. % respectively. Salt concentration in NaCl-eq ranges f rom 5 to 9 wt % in both. Thus, composi t ions of f luids trapped in FIs and M i s are quite similar. The composit ions of fluids in re-melted at 650°C and 2.5 kbar M i s appear to be similar to that of the fluids in the 188 Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003 unhea ted M i s . M e a n w h i l e , gas -wa te r so lu t ion h o m o g e n i z a t i o n t empera tu re s of re -mel ted M i s are s o m e 100° h ighe r than t h o s e of the unhea ted M i s . T h i s indica tes d i s so lu t ion of f lu id c o m p o n e n t s in the inc lus ion mel t . T h e h o m o g e n i z a t i o n t e m p e r a t u r e d i f f e r e n c e d e m o n s t r a t e s that in the c o u r s e of c rys ta l l iza t ion of M I the p ressure shou ld increase f r o m 2.2 to 3.7 kbar w h e n t empe ra tu r e dec reases f r o m 6 1 5 to 5 5 0 ° C . A c c o r d i n g ( K o s u k h i n et al. , 1984; K o v a l e n k o et al., 1996) the late mel t s of the topaz-bery l pegmat i t e s o r ig ina te f r o m d e e p d i f f e ren t i a t ion of initial g ran i t e pegma t i t e m a g m a s . H o w e v e r , recent e x p e r i m e n t s b y (Veks l e r et al., 2 0 0 2 ) c lear ly ind ica te that at appropr i a t e P - T - X cond i t i ons the me l t s wi th c o m p o s i t i o n s s imi lar to those d e t e r m i n e d in this work coexis t wi th wa te r f lu ids and the so-ca l led h y d r o s a l i n e mel t s ( that in f ac t a re h y d r o u s a lumino-boros i l i ca t e mel ts ) . C o - g e n e t i c F Is and M i s in the coa r se -gra ined p e g m a t i t e and p o c k e t quar tz indica te c lear ly c o e x i s t e n c e of immisc ib l e h y d r o u s s i l ica te mel t and bor ic a c i d - w a t e r f lu id at the f ina l s tage of m a g m a t i c pegma t i t e c rys ta l l i za t ion wi th in at least 6 5 0 - 5 0 0 ° C and u p to 4 kbar . T h e d e t e r m i n e d c h e m i c a l c o m p o s i t i o n s and the d o m i n a t i o n of micas a m o n g daugh te r c rys ta l s p rov ide e v i d e n c e fo r unusua l c o m p o s i t i o n of la te -s tage mel t s , par t ic ipa t ing in c rys ta l l i za t ion of pegma t i t e and p o c k e t quar tz . Our da ta ind ica te tha t t hese unusua l me l t s can a c c u m u l a t e H 2 0 , B , F in add i t ion to ra re g ran i tophy le e l e m e n t s l ike Cs , Li , Be , T a and N b . T h e concen t r a t i ons of these e l e m e n t s exceed , by one o r two o rde r s of m a g n i t u d e , the bu lk c o m p o s i t i o n of M a l k h a n p e g m a t i t e s d e t e r m i n e d b y ( Z a g o r s k y and P e r e t y a z h k o , 19921 - 2) . C o n c e n t r a t i o n s of mos t m a j o r e l e m e n t s in s tudied M i s a re c lo se to the c o m p o s i t i o n s o f M i s in q u a r t z f r o m p e g m a t i t e s of E h r e n f r i e d e r s d o r f S n - W depos i t , G e r m a n y ( T h o m a s et al., 2 0 0 3 ) . H o w e v e r , the c o m p o s i t i o n s of M i s f r o m the s tudied p o c k e t d i f f e r s s ign i f i can t ly f r o m the latter p e g m a t i t e s in h igher Cs , Li c o n t e n t a l o n g wi th the a b s e n c e of CI, P and Sn. Th i s ind ica tes that c o m p o s i t i o n s of the latest pegmat i t i c me l t s m a y vary s ign i f ican t ly . T h e mos t unusua l f e a tu r e of s tudied mel t s is lack of cons i s t ency b e t w e e n the M i ' s d a u g h t e r minera l a s s e m b l a g e s and the p o c k e t mine ra l a s s e m b l a g e . O n e exp lana t ion of this p h e n o m e n o n is preferent ia l c rys ta l l iza t ion of quar tz f r o m f lu id that coex i s t ed wi th mel t . T h e mine ra l s that c rys ta l l ized f r o m the mel t s in this ca se mus t b e s ca r ce and could b e f o u n d as c rys ta l l ine inc lus ions wi th in coeva l g r o w t h zones of p e g m a t i t e and p o c k e t minera l s . H o w e v e r , in s o m e cases these unusua l me l t s c o u l d b e a c c u m u l a t e d in app rop r i a t e p laces of the p e g m a t i t e c h a m b e r and par t ic ipa te in f o r m a t i o n of a lb i t e - tourmal ine - l ep ido l i t e near -pocke t c o m p l e x e s wi th m i n o r pol luci te , beryl and severa l T a - N b minera ls . Such c o m p l e x e s are typical of a n u m b e r of pegma t i t e s in the M a l k h a n f ie ld . T h e f i n d i n g of the M i s r ep resen t ing the latest po r t ions of the p e g m a t i t e m a g m a c a p a b l e of concen t ra t ion of r a re me ta l s is ve ry impor tan t fo r inves t iga t ion of t he or ig in o f the o r e - f o r m i n g so lu t ions , der ived f r o m grani t ic m a g m a . T h e s tudy is suppor t ed by R F B R gran ts 0 1 - 0 5 - 6 4 6 7 7 a n d 0 3 - 0 5 - 6 4 4 3 6 . References KOSUKHIN, O. , BAKUMENKO, I., CHUPIN, V . (1984) : M a g m a t i c s tage of grani t ic p e g m a t i t e fo rma t ion . N a u k a , N o v o s i b i r s k , 1 3 6 pp . KOVALENKO, V. I., TSARYEVA, G. M „ NAUMOV, V. B. , HERVIG, R . L „ NEWMAN, S. (1996) : M a g m a of p e g m a t i t e s f r o m Vo lhyn ia : c o m p o s i t i o n and crys ta l l iza t ion p a r a m e t e r s de t e rmined by m a g m a t i c inc lus ion s tudies : Pe t ro logy , 4, 3, 2 6 5 - 2 7 6 . ZAGORSKY, V. YE., PERETYAZHKO, I. S. 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The evidence of unusual melts from melt inclusions in pocket quartz of the Oktyabrskaya pegmatite Mine Malkhan Ridge, Central Transbaikalia

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The evidence of unusual melts from melt inclusions in pocket quartz of the Oktyabrskaya pegmatite Mine Malkhan Ridge, Central Transbaikalia

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