научная статья по теме HYDROSILICATE LIQUIDS IN THE SYSTEM NA2O SIO2 H2O WITH NAF, NACL AND TA: EVALUATION OF THEIR ROLE IN ORE AND MINERAL FORMATION AT HIGH T AND P Геология

Текст научной статьи на тему «HYDROSILICATE LIQUIDS IN THE SYSTEM NA2O SIO2 H2O WITH NAF, NACL AND TA: EVALUATION OF THEIR ROLE IN ORE AND MINERAL FORMATION AT HIGH T AND P»

Hydrosilicate Liquids in the System Na2O-SiO2-H2O with NaF, NaCl and Ta: Evaluation of Their Role in Ore and Mineral Formation at High T and P

© 2012 S. Z. Smirnov"b, V. G. Thomas", V. S. Kamenetskyc, O. A. Kozmenko", R. R. Largec

aInstitute of Geology and Mineralogy SB RAS, pr. Ak. Koptyuga 3, Novosibirsk, 630090 Russia; e-mail: ssmr@igm.nsc.ru bNovosibirsk State University, Pirogova st. 2, Novosibirsk 630090, Russia cARC Centre of Excellence in Ore Deposits, University of Tasmania, Hobart, Tasmania 7001, Australia;

e-mail: Dima.Kamenetsky@utas.edu.an Received February 28, 2011; in final from, September 15, 2011

Consideration of the existence of hydrosilicate liquids (HSL) in nature can help in understanding the accumulation and transport of some mineral- and ore-forming components at the transition from magmas to hydrothermal fluids. W studied the experimental formation of HSL using a base system Na2O—SiO2—H2O with addition of NaF, NaCl and metallic Ta. The interaction between quartz and aqueous solution, performed at 1.5 kbar and 600°C and followed either by cooling or by quench, showed that the formation of HSL occurred when initial Na2O exceeded 2 wt%. Neither NaF nor NaCl have a significant effect on the formation of HSL. The HSL concentrates I; whereas Cl partitions into the aqueous fluid. With addition of Ta to the system, the HSL becomes metal-enriched. Natural analogs of experimental HSL can be found among "melt/fluid" inclusions entrapped in quartz and other minerals of miaroles in granite pegmatites and rare-metal granites.The HSL is a novel medium enabling extreme concentrations of lithophile ore metals at the magmatic-hydrothermal transition.

INTRODUCTION

For granite intrusions, the processes and compositions at the magmatic-hydrothermal transition, including formation of granite-related mineralization, have been highlighted in recent research, e.g., (Halter, Webster, 2004), but the exact physico-chemical path from a given magma to a given mineral assemblage or ore remains controversial. The existing models imply magma evolution with exsolution of either supercritical aqueous fluid (Barnes, 1979; Burnham, 1979) or of hydrosaline melt/brine (e.g., Lowenstern, 1994; Roedder, 1992; Shinohara, 1994; Solovova et al., 1991; Veksler et al., 2002; Webster, 1997). Aqueous fluids ofvariable salinity, and brines, are considered capable of carrying metals from magmas to deposition sites (Audetat et al., 2000; Duc-Tin et al., 2007; Kamenetsky et al., 2004; Rankin et al., 1992; Yardley, 2005).

Both supercritical aqueous fluid and hydrosaline melt/brine are characteristically Si-poor1, and thus significant amount of quartz or other silicate minerals, associated with economic mineralization, currently presents a paradox. Thus experimental modeling of systems that can concentrate and transport significant quantities ofboth silica and ore elements is very important to this problem. Previous experiments showed that in the systems silicate— H2O—AX (A—alkali ion and X—hydroxyl, carbonate, borate, sulphate or fluoride ions), at T= 250—800°C and P= 1—3 kbar silicate minerals and hydrous fluid coexist with liquids, which are composed mostly ofSiO2 and H2O in al-

1 The only example of Si-rich hydrosaline melt is shown in (Rickers et al., 2006; Thomas et al., 2006).

most equal molar proportions (Balitsky et al., 2000; Butu-zov, Bryatov, 1957; Ganeev, Rumyantsev, 1971; Kotel'nik-ova, Kotel'nikov, 2002, 2004, 2008, 2009a; 2009b, 2010a; 2010b; Kravchuk, Vlyashko, 1979; Morey, Fenner, 1917; Morey, Fleischer, 1940; Mustart, 1972; Peretyazhko et al., 2010; Rumyantsev, 1999; Smirnov et al., 2005; Tuttle, Friedman, 1946). Hereafter we use the term hydrosilicate liquids (HSL) after (Vksler etal., 2002). Importantly, the HSL are stable in presence of aqueous fluids within the P-T range, which is usually considered for hydrothermal processes only. Previously, Wilkinson et al. (Wilkinson et al., 1996) suggested that fluids similar to HSL may be a medium for the mass transport in the lithosphere, and Peretyazhko et al. (Peretyazhko et al., 2004a) discussed their important role at the magmatic-hydrothermal transition in granitic pegmatite systems. These results suggest that development of HSL could be an alternative evolutionary path at the end ofwater-saturated magma solidification, i.e. silicate melt —► magmaticsilicate rock + hydrosilicate liquid + aqueous fluid. Our new experimental studies provide insights into mobilization and transport of elements and metals, which are important in syn- and post-magmatic ore formation, by hydrosilicate liquids and their mineral- and ore-forming potential in felsic magma derived hydrothermal systems.

EXPERIMENTAL DESIGN AND ANALYTICAL METHODS

Experimental Setup and Strategy

The system Na2O—SiO2—H2O is considered to be a simplified example of hydrous silicate systems, where

silicate liquids are stable down to low temperatures and pressures and thus it can be a general model of acidic and intermediate magmas, coexisting with alkaline aqueous fluids. The system, which is hereafter called base system, was studied in details by Mustart (Mustart, 1972) in the area of peralkaline aliminosili-cate compositions and by Kravchuk (Kravchuk, 1979) in area of silica-rich compositions. Therefore this study was not focused to determination of specific phase equilibria, but to the study of properties of the silicate liquids, which appear under experimental conditions. The aims of experiments were to determine effect of Cl and F, which are important to element transport in nature, on the process of HSL formation, and the ability of HSL to concentrate and transport rare metals.

The experiments were performed in the hermetically sealed 8.5 ml gold ampoules in the stainless-steel autoclave. Synthetic quartz, reagent grade NaF, NaCl, NaOH, were used to obtain HSL under experimental conditions. Pure Ta plates were used for the study of dissolution and transport of metals in HSL. Starting compositions of experimental runs are given in Table 1. The pressure was not controlled independently. A half of remaining free volume of ampoule was filled with double distilled water in order to reach 1.5 kbar pressure at 600°C following 0.5 g/cm3 isochore of the pure water. The autoclaves were heated up to 600°C for 8 hours, and then this temperature was maintained for several days. Temperature was controlled by two chromel-alumel thermocouples placed above and below the working space of the autoclave. The temperature gradient was < 3°C in the ampoule.

After the isothermal exposition the autoclaves were cooled to room temperature. Two cooling strategies were employed to record 1) a snapshot of compositions at high temperature (quenching at 100°/min for the experiments in small 20 ml autoclaves after isothermal heating for 2, 4 and 9 days) and 2) possible changes of experimental products with decreasing temperature and pressure (slow cooling, ~10 hrs for the experiments in large 250 ml autoclaves after 18 days heating period).

After cooling the ampoules were weighted for control of the leak tightness. Aqueous solutions were extracted first. The amount of liquid products was calculated as a difference between the loaded and empty ampoule weights without the weight of dry solid products.

Analytical Methods

Solid products of the experiments are compact mixtures of X-ray amorphous (vitreous) and crystalline phases. Major elements (Si, Na, Cl, F) in both vitreous and crystalline phases were analysed with a Cameca SX-100 electron microprobe (University of Tasmania) using 15 kV accelerating voltage. The instrument was calibrated using natural minerals: Mg-olivine (Si), tugtupite (Cl), apatite (F), and synthetic

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glass UTAS-3 (Na). The problem of "Na-loss" during the microprobe analysis of the vitreous phases (Line-veawer, 1962; Nielsen, Sigurdsson, 1981; Morgan, London, 1996; Morgan, London, 2005) was avoided by use of low 10 nA electron beam current and defo-cused to 40 ^m beam. Measuring the intensity of NaXa for 30 s showed that Na decrease was around 1% rel. This allowed estimation of H2O contents in the vitreous phases as "volatiles by difference".

Concentrations of Ta in the vitreous products were measured both by EMPA and LA ICP-MS. LA ICP-MS analyses were performed using a New Wave Research UP-213 Nd-YAG (213 nm) laser coupled to an HP 4500 mass-spectrometer (University of Tasmania, Australia) in a He atmosphere by ablating 110 ^m diameter spots (80 ^m where crystal phases were present). Instrument background was measured for 30 s. Analytical signal was collected for 40—50 s. Data reduction was undertaken according to standard methods using Na (analysed by EMPA) as the internal standard. NIST-612 and USGS BCR-2g glasses were used as primary and secondary standards, respectively.

The accuracy of data obtained by in-situ methods were tested using wet chemistry analysis of the vitreous phase devoid of crystals. The sample (0.5 g) was fused with 1.25 g of lithium metaborate and dissolved in 2.5% HNO3. Si was determined by spectrophotometry of heteropoly molybdenum-silica blue complex (Fon-er, Gal, 1981), and F — of Th-arsenazo (III) complex (Jeffrey, 1970). Na was determined by AAS on Perkin-Elmer 400 and Ta by ICP-AES on Thermo Jarrell IRIS spectrometer.

Determination of Si, Na, and Ta in residual aqueous liquids was performed immediately after experiments by using wet chemistry methods (see above). The aliquots of liquids were diluted in order to analyse elements with high concentrations and acidified to prevent rapid polymerisation of silica. The crystalline phases were identified by the electron microprobe WDS and EDS methods.

EXPERIMENTAL RESULTS

After experiments, the ampoules contained aqueous solution and a compact mixture of vitreous and crystalline products (Tables 1 and 2).

Vitreous Products

Quench experiments. The quench experiments produced columns composed mostly of the vitreous phase. The c

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