Koděra, Peter

Murphy, P. J.

Rankin, A. H.

English

Koděra Peter

Rankin A. H.

Murphy P. J.

University of Szeged

Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003 IDENTIFICATION OF SOME FE-BEARING DAUGHTER MINERALS IN FLUID INCLUSIONS USING RAMAN SPECTROSCOPY KODERA. P.1. RANKIN, A. H.2, MURPHYJ 5 . J.2 1 Geological Survey of the Slovak Republic, Mlynska dolina 1, Bratislava, 817 04, Slovakia. 2 School of Earth Sciences and Geography, Kingston University, Kingston-upon-Thames, Surrey, KT1 2EE, UK. E-mail: kodera@gsssr.sk Introduction Hydrothermal, high temperature fluids related to acid to intermediate intrusives are often hypersaline (up to 80 wt % total dissolved salts) and particularly rich in Fe (up to 45 wt % FeCl2). Associated fluid inclusions usually contain Fe-bearing minerals but these are only rarely identified, except for simple Fe oxides, such as magnetite and hematite. Other Fe-bearing phases remain unidentified but are generally thought to be hydrated Fe chlorides or mixed Fe-K chlorides, based on their optical properties and semiquantitative SEM data from opened inclusions. Laser Raman Microspectroscopy, though unsuitable for some simple chlorides (such as NaCl), is particularly useful for the in situ identification of more complex daughter minerals, providing an important insight into Fe-contents, total salinity and redox state of the trapped fluid. Raman spectra of hydrated chlorides & OH-bearing minerals The essential features of Raman spectra of hydrated chlorides and OH-bearing minerals are the water bending and stretching bands and molecular OH bands. The number of OH peaks, their position and relative intensities vary from mineral to mineral and are controlled by the types and number of crystallographically equivalent sites of OH groups and the types of cation occupancies around OH sites, as well as the probabilities of these occupancies. Concerning salt hydrates, published Raman spectra exist for hydrohalite (NaC1.2H20), antarcticite (CaCl2 .6H20), MgCl2 .6H20, MgCl2 .12H20, FeCl3 .6H20, KCl.MgCl2 .6H20 and LiC1.5H20 (Dubessy et al„ 1982,1992), with major OH bands within the range 3300-3550 cm"' (mainly <3450 cm'1). The band widths are significantly narrower at low temperatures than at high temperatures, allowing better resolution of the closely spaced peaks. The OH stretches within hydroxyl groups in a crystal lattice are narrow even at room temperature. OH peaks within simple hydroxides occur in a very narrow range 3566-3578 cm'1 and within phyllosilicates in a relatively broader range -3550-3700 cm"1 (Lutz et al., 1994; Wang et al., 2002). Raman spectra of Fe-bearing hydrates As the Raman spectra of most Fe-bearing hydrated chlorides have never been reported, we attempted to record these spectra using synthetic compounds, both at room temperature and at low temperature (-180°C) and over the frequency range 100 to 4000 cm"1 and a time interval of 30-300 seconds (table 1). Fe+3C13.6H20 is yellow. It is stable in saturated water solutions at room temperature, but forms a series of Fe+3C13 compounds with varying degrees of hydration (Kroschwitz & Howe-Grant, 1995) on drying. The Raman spectra showed several very strong sharp peaks at low wavenumbers, probably corresponding to lattice vibration, in contrast to weak peaks in the O-H spectral region. Compared to the spectra published by Dubessy et al. (1982) there is a difference in the location of one peak (3271 vs. 3321 cm"'), probably due to orientation effects. Erythrosiderite (K2Fe+3Cl5 .H20) was prepared from a saturated solution with dissolved Fe+3C13.6H20 and KC1 that has been subsequently evaporated and the precipitated red crystals have been identified using quantitative SEM analyses. The Raman spectra showed again strong peaks at low wavenumbers in contrast to weak peaks in the O-H spectral region. Fe+2C12.4H20 is pale to bluish green, stable in saturated water solutions at room temperature. Below 12.3°C hexahydrate is stable, while dihydrate and monohydrate occur at >75°C and >150-160°C, respectively (Kroschwitz & Howe-Grant, 1995). Its Raman spectra showed several weak peaks at low wavenumbers, in contrast to very strong peaks in the O-H stretching spectral region. Table 1: Raman frequencies (in cm ') for Fc+1 and Fe*2 hydrates recorded at -I80°C determined in this study. Strong peaks are highlighted in bold. FeCl3.6H;Q K2FeClj.H;Q FeCl2.4H;Q FeCl2 .2H :0 174 178 194 217 231 243 259 305 304 402 340 377 424 618 598 1588 1621 1635 1634 1658 2192 2205 3004 3216 3175 3221 3271 3337 3381 3377 3415 3392 3421 3431 3428 3534 Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003 Synthesis of Fe+2C12.2H20 was attempted in two ways: by slow heating of Fe+2C12 .4H20 crystals (recrystallisation occurred > - 5 0 °C) and by evaporation of saturated solution at 100°C resulting in precipitation of transparent to reddish, needle-shaped crystals. In both cases the same type of Raman spectrum have been recorded. Attempts were made to synthesise mixed Fe+2-K hydrated chlorides from a solution with dissolved Fe+2C12 .4H20 and KCl. Evaporation produced sylvite, erythrosiderite, Fe+2C12.4H20 and a yellow to orange, needle-shaped hydrate, possibly douglasite (K2Fe+2Cl4.2H20). On evaporation to dryness the hydrate subsequently unmixed into sylvite and a brown-coloured solution (oxidation of Fe+2 to Fe+3). The resulting Raman spectra of this hydrate were much affected by this decomposition. Spectra of daughter minerals in fluid inclusions The experimentally derived spectra, together with other published reference spectra, were used to identify the Fe-daughters present in inclusions in samples from four districts, each related to different types of hydrothermal mineralization. Fluid inclusion studies on the Hodrusa granodiorite related to Fe-skarn (Slovakia), showed Raman spectra typical of hydrates but the exact position of peaks did not match with any of the experimentally determined spectrum of hydrates. However, SEM-EDX results and the absence of sylvite in these inclusions indicate the presence of both Fe and K in their structures, possibly present in variable ratios. These daughters were yellow-green, anisotropic with high relief and showed two modes of dissolution temperatures ( -112 and ~130°C). Raman spectroscopy determined here also a common presence of OH bearing daughter minerals, typically with sharp OH peaks both at room and low temperatures. Based on published spectra they have been identified as biotite and ferropyrosmalite [Fe8Si6OI5(OH,Cl)10], respectively, and an unknown, isotropic, green-yellow mineral with high relief, dissolving in the range from 324 to 465°C (sharp peak at 3449 cm'1 in addition to two smaller peaks at 385 and 199 cm"1) - perhaps hibbingite [Fe2(OH)3Cl]. Raman spectroscopic study of inclusions in quartz from the Chorloque Sn-porphyry (Bolivia) detected two types of hydrates. Compared to the experimentally determined spectra, the first type can be identified as FeCl2 .2H20. The second, with a greenish colour, is probably a Fe-K hydrate with an unknown K-Fe ratio. Furthermore, the spectrum of the unknown OH-bearing daughter mineral, apparent in the Slovakian sample (hibbingite?), was also recorded in the Chorloque inclusions Raman spectroscopic study of inclusions in topaz from the Mole Granite related to W-Mo-Sn in silexite (Australia) detected two hydrates, showing distinctive spectra. Limited data does not allow a positive identification. However, their optical properties (green colour, anisotropy) indicate the presence of Fe-K hydrates. A preliminary Raman spectroscopic study of inclusions in quartz from the Dartmoor Granite related Sn-W-Fe veins (UK) failed to detect any hydrates. However, the spectra of the same unknown OH-bearing daughter mineral, as detected in the Slovakian and Bolivian samples (hibbingite?), were also recorded here. Conclusions Hydrated Fe+2 and Fe+3 chlorides and erythrosiderite show characteristic, but previously unpublished, Raman bands, which can be used to identify certain Fe-Cl-daughter minerals. Of particular note is the fact that ferrous and ferric chloride hydrate species differ substantially in their Raman spectra. The presence of simple ferrous chloride hydrates as daughter minerals in fluid inclusions appears to be more rare than previously expected, while no evidence has been found for the presence of ferric hydrates. This is not surprising, as data obtained from experiments suggest that ferrous iron is the dominant species in natural hydrothermal solutions, whereas ferric species generally occur in abnormal oxidised solutions (e.g. Kwak et al, 1986). Mixed Fe+2-K chloride hydrates are probably more common, while the Fe/K ration can probably largely differ, depending on the fluid composition and temperature of FeCl2 saturation. The present study also demonstrates the common presence of an OH- and Fe-bearing daughter mineral, whose identity, at present, remains speculative (hibbingite?). Acknowledgement The research has been performed thanks to financial support provided by NATO and the Royal Society of London during the tenure of a NATO-Royal Society Fellowship (PK) in the School of Earth Sciences and Geography, Kingston University. References DUBESSY, J., AUDEOUD, D, WILKINS, R., KOSZTOLÁNYI, C. (1982): The use of Raman microprobe MOLE in the determination of the electrolytes dissolved in the aqueous phase of fluid inclusions. Chemical Geology, 37, 137-150. DUBESSY, J . , BOIRON, M . C . , MOISSETTE, A . , MONNIN, C . , SRETENSKAYA, N . ( 1 9 9 2 ) : D e t e r m i n a t i o n s o f w a t e r , h y d r a t e s a n d p H in fluid inclusions by micro-Raman spectrometry. European Journal of Mineralogy, 4, 885-894. KWAK, T. A. P., BROWN, W. M., ABEYSINGHE, P. B., TAN, T. H. (1986): Fe solubilities in very saline hydrothermal fluids: their relations to zoning in some ore deposits. Economic Geology, 81, 447-465. KROSCHWITZ, I., HOWE-GRANT, M. ED. (1995): Kirk-Othmer encyclopedia of chemical technology. John Willey &Sons, N.Y. LUTZ, H. D., BECKENKAMP, K., MÖLLER H. (1994): Weak Hydrogen Bonds in Solid Hydroxides and Hydrates. J. Mol. Struct. 322,263-266. d WANG A., FREEMAN, J., KUEBLER, K. E. (2002): Raman spectroscopic characterization of phyllosilicates. Proc. of the 33r Lunar and Planetary Science conference, abstract 1370. 102 

Identification of some Fe-bearing daughter minerals in fluid inclusions using raman spectroscopy

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Identification of some Fe-bearing daughter minerals in fluid inclusions using raman spectroscopy

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