Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003

FLUID INCLUSIONS IN THE IANAPERA EMERALD, SOUTH MADAGASCAR

VAPNIK. Ye. ' . SABOT, B.2, M O R O Z , I.3

' Department of Geological and Environmental Sciences, Ben-Gurion University of the Negev, P .O.Box 653, 84105 Beer-Sheva, Israel. 2 B P 34, 42450 Sury le Comtal , France. 3 Division of applied Physics, School of Applied Science, The Hebrew University of Jerusalem, 91904, Jerusalem, Israel. E - m a i l : vapnik@bgumail .bgu.ac.i l

The Ianapera emerald deposit is located at 200 km North East of Tulear, Vohibory region, South Madagascar . The Ianapera deposit belongs to Vohibory formations attributed to Precambrian age (Nicollet, 1985). T h e Vohibory format ions are constituted by intermediate-pressure granulite facies to high-pressure granulite facies rocks. They are consti tuted by gneisses (orthogneiss and migmatitic gneiss), leptynites, marbles, serpentinites, amphiboli tes ( some of them can bear co rundum) and pyroxenites.

The emerald mineralisation is concentrated at the interaction between migmatitic gneisses (garnet-biot i te-feldspar-quartz-sillimanite-disthene paragenesis) and serpentinites. Both lithologies are subject to a metasomatic alteration. Serpentinite shows a wide range of transition between non metasomatised composit ion characterized by an hourglass texture and those of a tremolite-carbonate-rich talcschist. Some veinlets of chrysolite are observed within the serpentinite. Metasomat ised gneiss is characterised by a garnet-biotite-sillimanite-quartz-tourmaline paragenesis. The total width of metasomatic co lumn is varying f rom 50 cm to 2 m.

The Ianapera deposit is located within the Ampanihy major shear zone described as rooted to the mantle and which direction (N10°) is controlled by a transpressional regime leading to pure shear in shear zone due to convergence of D e w h a r craton westward and Tanzania and Zambian craton eastward during pan-Afr ican orogeny (Martelat et al., 1997).

Fluid inclusions were investigated in several chips of emerald. The microthermomertry , R a m a n spectroscopy and S E M were used for the study. Fluid inclusions show mainly spacious distribution. In a single case inclusions were found in trail elongated along emerald growth face. Inclusions are evidently of primary origin. Several types of inclusions are usually present within the single inclusion population (Fig. 1). (a) Mult iphase inclusions composed f rom several birefr ingent and isotropic solid phases and aqueous solution with or without the gas bubble. Solid phases somet imes occupy up to 50-60 vol. % of f luid-inclusion volume, (b) Single phase C0 2 - r i ch inclusions often with one or several birefr ingent and isotropic solid phases. C 0 2

gaseous phase is appeared only during cooling runs, (c) Solid inclusions composed f rom multi-grained aggregates. Aqueous multiphase inclusions show the first melting of ice mainly between - 1 5 and -3°C whereas final melt ing was

usually observed between - 5 and - 1 ° C . Most of the solid phases are insoluble during the heating, the homogeniza t ion of the gas bubble was observed mainly between 240 and 315°C. Solid phases are R a m a n active and consider ing the obta ined spectrums are represented by dolomite, aragonite and calcite (Burke, 1994). In a few inclusions nahcolite was identif ied. Considering microthermometric data, f inding of nahcolite and constant presence of carbonates within the inclusions the aqueous part is likely composed of HC0 3 "-NaCl solutions. The salinity of solution is usually about 5 wt .% NaCl equiv.

C0 2 - r i ch inclusions show the melting temperature around -57 .5°C and homogenizat ion between - 1 7 and 8°C, most inclusions homogenize at - 10°C . Considering Raman data the solid phases in C 0 2 - r i c h inclusions are also represented by carbonates.

Multi-grained solid inclusions are not Raman active and/or give the similar spectrum of emerald-host . Several inclusions were exposed by polishing on the surface and analyzed by SEM. S E M data on several inclusions show very similar results: analyses are carbon-rich with high silica, magnesium and aluminum contents. Consider ing e lement atomic ratios it seems that multi-grained inclusions are formed by beryl-emerald and magnesite aggregate.

Although all types of inclusions are usually present in the single inclusion populat ion only a few aqueous-r ich inclusions with C 0 2 bubble occupying 5-10 vol. % of inclusion volume were observed. Joint occurrence of aqueous and C 0 2 inclusions in the same primary populations of fluid inclusions is a strong argument on contemporaneous trapping of both fluid inclusion systems although generally low-salinity of the aqueous inclusions makes problematic the appearance of two fluid inclusions types due to fluid heterogenization.

The microthermometric data on the contemporaneous aqueous and C 0 2 inclusions were used to evaluate the PT-condi t ions of inclusion trapping. Typical isochors and their intersection were used (Fig. 2). The field of isochors intersection is within the sillimanite stability field, at the PT-condit ions of migmati te formation but has to be restricted by beryl stability field. T h e likely condition of inclusion trapping during the emerald growth are as follows: T=620-710°C and P=5-6 kbars. These except ional ly high PT-conditions of emerald growth are in accordance with the suggestion on metasomatic emerald format ion in relation to high-grade metamorphic processes.

The Ianapera emerald is quite unique considering the emerald geological setting and emerald growth in highest PT-conditions that up to now were recorded for emerald formation (Groat et al., 2002).

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Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003

aqueous solution

O LCO,

carbonates nàhcolite

GCO-LCO:-¿)

LCO;

LCO, crystalline agregates

\

carbonates nahcolite 6 0 (xm

Likely conditions of inclusions trapping and emerald formation %

w M Hj £1 M

0) H

in m ai M PK

400 600 800 Temperature (°C)

F i g u r e 1. F igu re 2.

F i g . l . Ske t ch of p r i m a r y f lu id - inc lus ion popu la t ion p ro jec ted to the plain. F ig . 2. P T - c o n d i t i o n s of t h e I anape ra e m e r a l d fo rma t ion d e t e r m i n e d by the in tersec t ion of typica l i socho re s of C 0 2

( S c h m u l o v i c h et al . , 1982) and a q u e o u s inc lus ions (Bodnar , Vi tyk , 1994). Kyan i t e -S i l l iman i t e -Anda lus i t e s tabi l i ty f ie lds (Ker r i ck , 1990), beryl s tabi l i ty field (Bar ton , 1986) and wate r - sa tura ted grani te so l idus ( Johannes , Hol tz , 1996) a re s h o w n in re la t ion to the emera ld g r o w t h .

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