Experimental study of phase relations in a lithium-bearing fluorine-rich haplogranite and nepheline syenite system

Phase relations were investigated in the model water-saturated system Si-Al-Na-Li-F-O at high fluorine contents, a temperature of 800°C, and a pressure of 1 kbar. The obtained aluminosilicate melts are widely variable from quartz- to nepheline-normative compositions with agpaitic indexes both higher and lower than one. Various fluoride, aluminofluoride, and oxide phases were observed in the equilibrium assemblage depending on the melt composition: quartz and cryolite associate with the silica richest aluminosilicate melts, topaz and corundum coexist with peraluminous melts, and villiaumite was observed in highly peralkaline melts. Extensive immiscibility between aluminosilicate and aluminofluoride melts was observed in the system. Aluminofluoride melt coexists with quartz- and nepheline-normative aluminosilicate melts with agpaitic indexes (K _a) of 0.7–1.4. The composition of aluminosilicate melt in equilibrium with aluminofluoride melt ranges from 33 to 70 wt % SiO₂, from 12 to 24 wt % Al₂O₃, and from 5 to 16 wt % alkalis. The aluminofluoride melt is variable in composition, its Al/Na ratio ranges from 20/80 to 40/60 depending on the composition of the equilibrium aluminosilicate melt. The experimental aluminosilicate melts equilibrated with cryolite, topaz, and aluminofluoride melt coincide in major component proportions with the bulk compositions of cryolite- and topaz-bearing granites and melt inclusions in minerals.     

The effect of CO<Subscript>2</Subscript> on the solubility of aqueous chloride fluid in dacite,phonolite, and rhyolite melts

The solubility of H₂O–CO₂–Cl-containing fluids of various concentrations (0, 3, 10, and 23 wt % of HCl and from 0 to ~8–15 wt % of CO₂) in dacite, phonolite, and rhyolite melts at 1000°C and 200 MPa was studied in experiments. It was shown that the Cl concentration in the melt increased substantially from rhyolite to phonolite and dacite (up to 0.25, 0.85, and 1.2 wt %, respectively). The introduction of CO₂ into the system resulted in an increase in the Cl content in the melt composition by 20–25%. One may suppose that Cl reactivity in a fluid increases in the presence of CO₂ to cause growth of the Cl content in the melt. The introduction of CO₂ into the system considerably affects the content of H₂O in aluminosilicate melts as well. Thus, the addition of CO₂ decreases the H₂O content in the melt by ~0.5–1.0 wt %. The decrease in the H₂O content in an aluminosilicate melt is probably caused by fluid dilution with CO₂ resulting in a decrease in the H₂O mole fraction and fugacity in the fluid.     

The effect of fluorine on liquidus phase relationships in the system Qz-Ab-Or with excess water at 1 kb

Liquidus phase relationships have been determined experimentally for the system Qz-Ab-Or with excess water and 1, 2, and 4 wt.% added fluorine at 1 kb pressure. With increasing fluorine content the position of the quartz-alkali feldspar field boundary moves away from the quartz apex. The position of the minimum melting composition and the minimum liquidus temperature change progressively from Qz₃₇Ab₃₄Or₂₉ and 730° C for the fluorine free system (Tuttle and Bowen 1958) to Qz₁₅Ab₅₈Or₂₇ and 630° C for the system with 4 wt.% added fluorine. Exploratory experiments have been carried out below the liquidus, and have indicated that for certain bulk compositions an assemblage consisting of two alkali feldspars, quartz, melt and vapour can exist at temperatures as low as 550° C at 1 kb.The experimental results suggest that there may be an interaction between fluorine and aluminosilicate complexes present within the melt, to produce aluminofluoride (AlF ₆ ^3– ) complex anions (Manning et al. 1980). The observed changes in liquidus phase relationships with increasing fluorine content indicate that the compositions of certain fluorine-rich granitic rocks are consistent with an origin by crystallisation of residual melts enriched in fluorine by magmatic differentiation. Such residual melts may exist at relatively low temperatures, and may form part of a continuum between granite magmatism and associated hydrothermal activity. Because of the observed preference of fluorine for aluminosilicate phases at the magmatic stage, the presence of fluorine alone is not considered to play a direct part in the generation of residual mineralising hydrothermal fluids.     

Genesis of the Katugin rare-metal ore deposit: Magmatism versus metasomatism

Arguments in favor of magmatic or metasomatic genesis of the Katugin rare-metal ore deposit are discussed. The geological and mineralogical features of the deposit confirm its magmatic origin: (1) the shape of the ore-bearing massif and location of various types of granites (biotite, biotite–amphibole, amphibole, and amphibole–aegirine); (2) the geochemical properties of the massif rocks corresponding to A type granite (high alkali content (up to 12.3% Na₂O + K₂O), extremely high FeO/MgO ratio (f = 0.96–1.00), very high content of the most incoherent elements (Rb, Li, Y, Zr, Hf, Ta, Nb, Th, U, Zn, Ga, and REE) and F, and low concentrations of Ca, Mg, Al, P, Ba, and Sr); (3) Fe–F-rich rock-forming minerals; (4) no previously proposed metasomatic zoning and regular replacement of rock-forming minerals corresponding to infiltration fronts of metasomatism. The similar ages of the barren (2066 ± 6 Ma) and ore-bearing (2055 ± 7 Ma) granites along with the features of the ore mineralization speak in favor of the origin of the ore at the magmatic stage of the massif’s evolution. The nature of the ore occurrence and the relationships between the ore minerals support their crystallization from F-rich aluminosilicate melt and also under melt liquation into aluminosilicate and fluoride (and/or aluminofluoride) fractions.     

Phase relations in fluorine-bearing granite and nepheline syenite systems at 800°C and 1 kbar

The data obtained on the sodic part of the SiO₂-Al₂O₃-Na₂O-K₂O system with F at 800°C and 1 kbar provide the basis for constructing a phase diagram showing the region of an aluminosilicate melt. In this system, oxide and fluoride phases are identified that control the stability field of the melt and the solubility of F. Liquid immiscibility was detected in aluminous nepheline-and quartz-normative Li-bearing compositions (the latter compositions are characterized by a wider immiscibility field). Solubility of F was determined in an aluminosilicate melt saturated with respect to F, i.e., coexisting with phases rich in this element. The F concentrations in the glasses range from 2 to 20 wt %. The quartz-normative glasses are poorer in F (no more than 5 wt % F) than the nepheline-normative glasses (which contain mostly 5–10 wt % F). The maximum F concentrations (> 10 wt %) in the phase diagram lie on both sides of the albite composition point in the region of ultragpaitic nepheline-normative melts and in the region of normal syenite melts. Changes in the phase relations when Na is substituted for K were determined in the quartz-normative silicate melt.     

Leucocratic magmatic melts with the maximum fluorine concentrations: Experiment and relations in nature

Experimental data indicate that high F concentrations in leucocratic aluminosilicate melts (of granite and nepheline syenite composition) bring about the crystallization of F-rich minerals (topaz, villiaumite, and cryolite) on the liquidus. The crystallization of the minerals is controlled by the silicity, agpaitic coefficient, and proportions of alkalis in the system SiO₂-Al₂O₃-Na₂O-K₂O-F-H₂O. Our earlier experimental data on this system are compared with petrographic and petrochemical data on granites and nepheline syenites containing accessory topaz, cryolite, and villiaumite. The composition of topaz- and cryolite-bearing rocks is proved to correspond to the experimentally established equilibrium fields of F-rich aluminosilicate melt with these minerals. It is proved that the high-F minerals can crystallize from melt. The partial substitution of K and Na for Li modifies phase relations in the system, first of all, significantly expands the equilibrium field of aluminosilicate melt and alkaline aluminofluoride melts. The two melts are proved to be immiscible within broad compositional ranges in the SiO₂-Al₂O₃-Na₂O-Li₂O-F-H₂O system at 800–650°C and 1 kbar. Experimental data indicate that fluoride brine can coexist with aluminosilicate melts in nature. This finds support data on melt inclusions in granites and alkaline rocks whose contents of major components, water and fluorine are close to those in the experimental glasses. Our data lend support to the hypothesis that large cryolite bodies at the Ivigtut, Pitinga, Ulog-Tanzek, and other deposits were formed by fluoride salt melts that separated from F-rich aluminosilicate magmas late in the course of their differentiation. It is experimentally established that fluoride salt melts are able to concentrate valuable trace elements, such as Li, W, Nb, Hf, Sc, U, Th, and REE, which suggests that such melts can play an important role in the origin of rare-metal deposits genetically related to rocks that crystallize from magmas rich in F.     

Geochemistry of Sc in the magmatic process: Experimental evidence

Data on Sc distribution between aluminosilicate melt and fluoride phases (cryolite in the Na-K parts of the system and fluoride melt in its Li-bearing parts) were experimentally obtained for the Si-Al-Na-K-O-H-F system at 800, 750, and 700°C and $ P_{H_2 O} Data on Sc distribution between aluminosilicate melt and fluoride phases (cryolite in the Na-K parts of the system and fluoride melt in its Li-bearing parts) were experimentally obtained for the Si-Al-Na-K-O-H-F system at 800, 750, and 700°C and = 1000 bar, at the saturation of the aluminosilicate melt with respect to H₂O and F. The Sc partition coefficients between aluminosilicate melt and fluoride phases vary from 0.005 to 1.6 depending on phase relations in the system, which are, in turn, controlled by its bulk composition and experimental conditions. At 800°C, the Sc partition coefficients between aluminosilicate melt and fluoride phases are lower than 1 in all of the examined parts of the system; i.e., Sc is preferably distributed into the fluoride phase regardless of whether it is crystals of (Na, K-Na, or K)-cryolite or alkali-aluminofluoride melt. This is the fundamental difference of Sc behavior from those of REE, and most other elements, under the same conditions. A temperature decrease to 700°C and the transition from quartz-normative to nepheline-normative melts, as well as an increase in their agpaitic coefficient, leads to an increase in the Sc partition coefficients to values greater than 1. This tendency is related to an increase in the degree of depolymerization of the aluminosilicate melt. Sc is demonstrated to be characterized by strong affinity to F and can substitute up to half of Al atoms in the cryolite structure, and Sc is even more significantly incorporated into Li-bearing aluminofluoride melts. The high chemical affinity of Sc to F and the ability of the former to produce fluoride complexes in residual magmatic melts is one of the main reasons for Sc enrichment in pegmatites and high-temperature postmagmatic metasomatic rocks: greisens, skarns, and albitized granites. The data obtained on Sc behavior in the sequence of zones of reaction column developing during the interaction of dolerite with granitic melt do not validate the idea that Sc is selectively extracted during the migmatization and assimilation of host rocks. Original Russian Text ? T.I. Shchekina, E.N. Gramenitskii, 2008, published in Geokhimiya, 2008, No. 4, pp. 387–402.     

Formation conditions of the ore-bearing lithium-fluoride granites of the Shumilov tungsten deposit,central Transbaikalia

Silicate melt inclusions were studied in the quartz phenocrysts from the Li-F granite porphyry of the Shumilov Massif, which are considered to be the sources of ore-bearing fluids of the Shumilov tungsten deposit. The estimated water content in the granite melt was 2.1–7.6 wt %, at a water pressure in the magmatic chamber of 3.1–5.2 kbar. It was concluded that the fluid-magmatic systems of the Shumilov and Spokoinoe tungsten deposits were formed at similar physicochemical conditions.     

General characteristics of the geochemical evolution of silicic melts during the development of massive sulfide magmatic ore systems: Evidence from the investigation of melt inclusions

The investigation of melt inclusions in the minerals of volcanic rocks from the massive sulfide deposits of Siberia and the Urals revealed some specific features in the development of their magmatic ore systems. It was shown that the petrochemical and rare earth element compositions of melt inclusions reflect the geodynamic conditions of their formation: island arc conditions for the massive sulfide deposits of Rudny Altai, eastern Tuva, and the Salair Range and a back arc basin environment for the Yaman-Kasy deposit. The silicic melts of inclusions from the volcanic rocks of massive sulfide deposits show some specific features with respect to the contents of volatile components. In all of the ore deposits studied, fluorine content was always low (0.03–0.08 wt %), whereas chlorine content (0.13–0.28 wt %) was higher than the average value for silicic melts (0.17 wt %). There is a strong differentiation of water content in melt inclusions, both between deposits and between various volcanics from a single deposit. Ore-bearing melts show the highest water contents of 3.34–4.07 wt %. High Cu contents in the silicic melts of the Yubileinoe and Kyzyl-Tashtyg deposits (up to 7118 and 3228 ppm, respectively) may indicate the affinity of some ore components to particular silicic magmas. This is supported by the elevated contents of Cu in the porphyry Cu deposits of Romania (Valea Morii), Mongolia (Bayan Ula), and Bolivia. On the other hand, the silicic melts of inclusions from the molybdenum-uranium deposit of the Strel’tsovka ore field show high contents of another group of ore components (U and F).     

Magmatic evolution of Li–F, rare-metal granites: a case study of melt inclusions in the Khangilay complex, Eastern Transbaikalia (Russia)

We report compositions of homogenized quartz-hosted melt inclusions from a layered sequence of Li-, F-rich granites in the Khangilay complex that document the range of melt evolution from barren biotite granites to Ta-rich, lepidolite–amazonite–albite granites. The melt inclusions are crystalline at room temperature and were homogenized in a rapid-quench hydrothermal apparatus at 200 MPa before analysis. Homogenization runs determined solidus temperatures near 550 °C and full homogenization between 650 and 750 °C. The compositions of inclusions, determined by electron microprobe and Raman spectroscopy (for H₂O), show regular overall trends of increasing differentiation from the least-evolved Khangilay units to apical units in the Orlovka intrusion. Total volatile contents in the most-evolved melts reach over 11 wt.% (H₂O: 8.6 wt.%, F: 1.6 wt.%, B₂O₃: 1.5 wt.%). Concentrations of Rb range from about 1000 to 3600 ppm but other trace elements could not be measured reliably by electron microprobe. The resulting trends of melt evolution are similar to those described by the whole-rock samples, despite petrographic evidence for albite- and mica-rich segregations previously taken as evidence for post-magmatic metasomatism.

Melt variation trends in most samples are consistent with fractional crystallization as the main process of magma evolution and residual melt compositions plot at the granite minimum in the normative Qz–Ab–Or system. However, melts trapped in the highly evolved pegmatitic samples from Orlovka deviate from the minimum melt composition and show compositional variations in Al, Na and K that requires a different explanation. We suggest that unmixing of the late-stage residual melt into an aluminosilicate melt and a salt-rich dense aqueous fluid (hydrosaline melt) occurred. Experimental data show the effectiveness of this process to separate K (aluminosilicate melt) from Na (hydrosaline melt) and high mobility of the latter due to its low viscosity and relatively low density may explain local zones of albitization in the upper parts of the granite.     

The effect of water on accessory phase solubility in subaluminous and peralkaline granitic melts

The solubilities of columbite, tantalite, wolframite, rutile, zircon and hafnon were determined as a function of the water contents in peralkaline and subaluminous granite melts. All experiments were conducted at 1035 °C and 2 kbar and the water contents of the melts ranged from nominally dry to approximately 6 wt.% H₂O. Accessory phase solubilities are not affected by the water content of the peralkaline melt. By contrast, solubilities are affected by the water content of the subaluminous melt, where the solubilities of all the accessory phases examined increase with the water content of the melt, up to 2 wt.% H₂O. At higher water contents, solubilities are nearly constant. It can be concluded that water is not an important control of accessory phase solubility, although the water content will affect diffusivities of components in the melt, thus whether or not accessory phases will be present as restite material. The solubility behaviour in the subaluminous and peralkaline melts supports previous spectroscopic studies, which have observed differences in the coordination of high field strength elements in dry vs. wet subaluminous granitic glasses, but not for peralkaline granitic glasses. Lastly, the fact that wolframite solubility increases with increasing water content in the subaluminous melt suggests that tungsten dissolved as a hexavalent species.     

Water solubility in aluminosilicate melts

We have compiled water solubility data for a wide range of natural and synthetic aluminosilicate melts in a search for correlations between melt composition and solubility. The published data reveal some interesting systematics. For example, molar water solubility increases with decreasing silica content in binary and pseudobinary silicates, and much higher solubilities are associated with alkali systems compared to alkaline earth silicate melts. Water solubility increases regularly with decreasing silica content along the silica-nepheline join. From the limited data available for potassium and calcium aluminosilicate melts, these systems appear to behave differently to sodium aluminosilicates. The compiled data are not nearly extensive enough to begin to understand the effects of melt composition on solubility. We suggest that many more systematic studies for a wide range of aluminosilicate melts will be necessary before we can systematize and understand the compositional dependence of water solubility. We have also examined results of experiments designed to probe the details of the water dissolution mechanism, and discuss the present state of interpretation of these data. We conclude that although considerable progress has been made, the water dissolution process is still not well understood at the molecular level, and remains an important research problem.     

The Composition of Melt Inclusions in Minerals from Tephra of the Soil–Pyroclastic Cover of Simushir Island (Central Kuril Islands)

The compositions of approximately 70 naturally quenched melt inclusions in olivine, clinopyroxene, orthopyroxene, and plagioclase phenocrysts from tephra of the soil–pyroclastic cover of Simushir Island (Central Kuril Islands) were studied. The concentrations of the major rock-forming components, H₂O, S, and Cl were analyzed in inclusions. The reconstructed melts contain 48.6–78.4 wt % SiO₂, 0.3–8.26 wt % MgO, and 0.12–1.72 wt % K₂O. The concentration of S and Cl in the melts changes regularly with increasing SiO₂ content: from 0.14 to ~0.02 wt % S and from ~0.05 to ~0.28 wt % Cl. The content of H2O in parental melts is 4.2–4.5 wt %.     

Formation conditions of allivalites, olivine-anorthite crystal enclaves, in the volcanics of the Kuril-Kamchatka arc

In this paper we address allivalites, coarse-and giant-textured olivine-anorthite rocks occurring as separate blocks in the eruption products of many volcanoes from the frontal part of the Kuril-Kamchatka arc. New data are reported on the petrography, mineralogy, and composition of melt inclusions in minerals from ten allivalite samples from Ksudach, Il’inskii, Zavaritskii, Kudryavyi, and Golovnin volcanoes. The crystallization temperatures of allivalite minerals were estimated as 970–1080°C at a melt water content of 3.0–3.5 wt % and oxygen fugacity NNO = 1–2. A genetic link was established between the compositions of melt inclusions and interstitial glasses in allivalites and volcano rocks. The cumulate nature of allivalites was demonstrated. Using mass balance calculations, the degree of fractionation of primary melts during the formation of cumulate layers was estimated for various volcanoes as 22–46%.     

The petrology of the Upper Albian tuffites from the Bakhchysarai district,southwestern Crimea

We present the first data on the composition of plagioclase and quartz-hosted melt inclusions, plagioclase primary magmatic zoning and mineral compositions for the Upper Albian tuffites from the Bakhchysarai district. The estimated composition of the magmatic melts corresponds to the high potassium rhyolites. The measured water content in the melt inclusions reaches 4 wt %. The phenocrysts crystallized at the temperature within the range 820–860°C and under the pressure not less than 0.1 GPa. The Upper Albian tuffites from the Bakhchysarai district were formed in the active continental margin environment.     

Chemical composition,volatile components,and trace elements in melts of the Karymskii volcanic center,Kamchatka, and Golovnina volcano,Kunashir Island: Evidence from inclusions in minerals

Melt inclusions were examined in phenocrysts in basalt, andesite, dacite, and rhyodacite from the Karymskii volcanic center in Kamchatka and dacite form Golovnina volcano in Kunashir Island, Kuriles. The inclusions were examined by homogenization and by analyzing glasses in more than 80 inclusions on an electron microscope and ion microprobe. The SiO₂ concentrations in the melt inclusions in plagioclase phenocrysts from basalts from the Karymskii volcanic center vary from 47.4 to 57.1 wt %, these values for inclusions in plagioclase phenocrysts from andesites are 55.7–67.1 wt %, in plagioclase phenocrysts from the dacites and rhyodacites are 65.9–73.1 wt %, and those in quartz in the rhyodacites are 72.2–75.7 wt %. The SiO₂ concentrations in melt inclusions in quartz from dacites from Golovnina volcano range from 70.2 to 77.0 wt %. The basaltic melts are characterized by usual concentrations of major components (wt %): TiO₂ = 0.7–1.3, FeO = 6.8–11.4, MgO = 2.3–6.1, CaO = 6.7–10.8, and K₂O = 0.4–1.7; but these rocks are notably enriched in Na₂O (2.9–7.4 wt % at an average of 5.1 wt %, with the highest Na₂O concentration detected in the most basic melts: SiO₂ = 47.4–52.0 wt %. The concentrations of volatiles in the basic melts are 1.6 wt % for H₂O, 0.14 wt % for S, 0.09 wt % for Cl, and 50 ppm for F. The andesite melts are characterized by high concentrations (wt %) of FeO (6.5 on average), CaO (5.2), and Cl (0.26) at usual concentrations of Na₂O (4.5), K₂O (2.1), and S (0.07). High water concentrations were determined in the dacite and rhyodacite melts: from 0.9 to 7.3 wt % (average of 15 analyses equals 4.5 wt %). The Cl concentration in these melts is 0.15 wt %, and those of F and S are 0.06 and 0.01 wt %, respectively. Melt inclusions in quartz from the dacites of Golovnina volcano are also rich in water: they contain from 5.0 to 6.7 wt % (average 5.6 wt %). The comparison of melt compositions from the Karymskii volcanic center and previously studied melts from Bezymyannyi and Shiveluch volcanoes revealed their significant differences. The former are more basic, are enriched in Ti, Fe, Mg, Ca, Na, and P but significantly depleted in K. The melts of the Karymskii volcanic center are most probably less differentiated than the melts of Bezymyannyi and Shiveluch volcanoes. The concentrations of water and 20 trace elements were measured in the glasses of 22 melt inclusions in plagioclase and quartz from our samples. Unusually high values were obtained for Li concentrations (along with high Na concentrations) in the basaltic melts from the Karymskii volcanic center: from 118 to 1750 ppm, whereas the dacite and rhyolite melts contain 25 ppm Li on average. The rhyolite melts of Golovnina volcano are much poorer in Li: 1.4 ppm on average. The melts of the Karymskii volcanic center are characterized by relative minima at Nb and Ti and maxima at B and K, as is typical of arc magmas.     

Experimental constraints on pre-eruption conditions of pantelleritic magmas: Evidence from the Eburru complex, Kenya Rift

The phase relationships and compositions of a pantellerite from the Eburru complex in the Kenya Rift Valley have been determined at 150 MPa and under reducing conditions, 2 log units below the Ni–NiO solid buffer. The effects of temperature and melt water content on phase relationships have been explored. Alkali feldspar and quartz crystallise alone at temperatures above 700 °C, irrespective of melt water content. Below 700 °C, sodic amphibole and clinopyroxene also crystallise; the amphibole being the liquidus phase under water-rich conditions. The coexistence of amphibole phenocrysts with alkali feldspar and quartz in a crystal-poor pantellerite implies temperatures below 700 °C and melt water contents higher than 4 wt.%, possibly up to 5–6 wt.%. Pantellerites have lower liquidus temperatures than associated comendites, which supports a parent–daughter relationship between the two magma types. The melts produced in the experiments extend the compositional trend displayed by the natural rock series, and reproduce some extreme compositions occasionally observed in alkaline volcanic series, with FeO contents above 12 wt.% and Na₂O contents approaching 10 wt.%. Pantellerites are therefore the true near-minimum melt compositions of alkaline oversaturated magma series.     

Effects of F,B2O3 and P2O5 on the solubility of water in haplogranite melts compared to natural silicate melts

The effects of F, B₂O₃ and P₂O₅ on the H₂O solubility in a haplogranite liquid (36 wt. % SiO₂, 39 wt. % NaAlSi₃O₈, 25 wt. % KAlSi₃O₈) have been determined at 0.5, 1, 2, and 3 kb and 800, 850, and 900°C. The H₂O solubility increases with increasing F and B content of the melt. The H₂O solubility increase in more important at high pressure (2 and 3 kb) than at low pressure (0.5 kb). At 2 kb and 800°C, the H₂O solubility increases from 5.94 to 8.22 wt. % H₂O with increasing F content in the melt from 0 to 4.55 wt. %, corresponding to a linear H₂O solubility increase of 0.53 mol H₂O/mol F. With addition of 4.35 wt. % B₂O₃, the H₂O solubility increases up to 6.86 wt. % H₂O at 2 kb and 800°C, corresponding to a linear increase of 1.05 mol H₂O/mol B₂O₃. The results allow to define the individual effects of fluorine and boron on H₂O solubility in haplogranitic melts with compositions close to that of H₂O-saturated thermal minima (at 0.5–3 kb). Although P has a dramatic effect on the phase relations in the haplogranite system, its effect on the H₂O solubility was found to be negligible in natural melt compositions. The concominant increase in H₂O solubility and F can not be interpreted on the basis of the available spectroscopic data (existence of hydrated aluminofluoride complexes or not). In contrast, hydrated borates or more probably boroxol complexes have been demonstrated in B-bearing hydrous melts.     

Genesis and mechanisms of formation of rare-metal peralkaline granites of the Khaldzan Buregtey massif,Mongolia: Evidence from melt inclusions

Melt inclusions were studied by various methods, including electron and ion microprobe analysis, to determine the compositions of melts and mechanisms of formation of rare-metal peralkaline granites of the Khaldzan Buregtey massif in Mongolia. Primary crystalline and coexisting melt inclusions were found in quartz from the rare-metal granites of intrusive phase V. Among the crystalline inclusions, we identified potassium feldspar, albite, tuhualite, titanite, fluorite, and diverse rare-metal phases, including minerals of zirconium (zircon and gittinsite), niobium (pyrochlore), and rare earth elements (parisite). The observed crystalline inclusions reproduce almost the whole suite of major and accessory minerals of the rare-metal granites, which supports the possibility of their crystallization from a magmatic melt. Melt inclusions in quartz from these rocks are completely crystallized. Their daughter mineral assemblage includes quartz, microcline, aegirine, arfvedsonite, polylithionite, a zirconosilicate, pyrochlore, and a rare-earth fluorocarbonate. The melt inclusions were homogenized in an internally heated gas vessel at a temperature of 850°C and a pressure of 3 kbar. After the experiments, many inclusions were homogeneous and consisted of silicate glass. In addition to silicate glass, some inclusions contained tiny quench zircon crystals confined to the boundary of inclusions, which indicates that the melts were saturated in zircon. In a few inclusions, glass coexisted with a CO₂ phase. This allowed us to estimate the content of CO₂ in the inclusion as 1.5 wt %. The composition of glasses from the homogeneous melt inclusions is similar to the composition of the rare-metal granites, in particular, with respect to SiO₂ (68–74 wt %), TiO₂ (0.5–0.9 wt %), FeO (2.2–4.6 wt %), MgO (0.02 wt %), and Na₂O + K₂O (up to 8.5 wt %). On the other hand, the glasses of melt inclusions appeared to be strongly depleted compared with the rocks in CaO (0.22 and 4 wt %, respectively) and Al₂O₃ (5.5–7.0 and 9.6 wt %, respectively). The agpaitic index is 1.1–1.7. The melts contain up to 3 wt % H₂O and 2–4 wt % F. The trace element analysis of glasses from homogenized melt inclusions in quartz showed that the rare-metal granites were formed from extensively evolved rare-metal alkaline melts with high contents of Zr, Nb, Th, U, Ta, Hf, Rb, Pb, Y, and REE, which reflects the metallogenic signature of the Khaldzan Buregtey deposit. The development of unique rare metal Zr–Nb–REE mineralization in these rocks is related to the prolonged crystallization differentiation of melts and assimilation of enclosing carbonate rocks.     

Compositions of magmatic melts and evolution of mineral-forming fluids in the banska Stiavnica epithermal Au-Ag-Pb-Zn deposit, Slovakia: A study of inclusions in minerals

Melt and fluid inclusions were investigated in minerals from igneous rocks and ore (Au-Ag-Pb-Zn) veins of the Stiavnica ore field in Central Slovakia. High H₂O (7.1–12.0 wt %) and Cl (0.32–0.46 wt %) contents were found in silicate melt inclusions (65–69 wt % SiO₂ and 5.2–5.6 wt % K₂O) in plagioclase phenocrysts (An 68–36) from biotite-homblende andesites of the eastern part of the caldera. Similar high water contents are characteristic of magmatic melts (71–76 wt % SiO₂ and 3.7–5.1 wt % K₂O) forming the sanidine rhyolites of the Vyhne extrusive dome in the northwestern part of the Stiavnica caldera (up to 7.1 wt %) and the rhyolites of the Klotilda dike in the eastern part of the ore field (up to 11.5 wt %). The examination of primary inclusions in quartz and sanidine from the Vyhne rhyolites revealed high concentrations of N₂ and CO₂ in magmatic fluid (8.6 g/kg H₂O and 59 g/kg H₂O, respectively). Fluid pressure was estimated as 5.0 kbar on the basis of primary CO₂ fluid inclusions in plagioclase phenocrysts from the Kalvari basanites. This value corresponds to a depth of 18 km and may be indicative of a deep CO₂ source. Quartz from the granodiorites of the central part of the Stiavnica-Hodrusa complex crystallized from a melt with 4.2–6.1 wt % H₂O and 0.24–0.80 wt % Cl. Magmatic fluid cogenetic with this silicate melt was represented by a chloride brine with a salinity of no less than 77–80 wt % NaCl equiv. Secondary inclusions in quartz of the igneous rocks recorded a continuous trend of temperature, pressure, and solution salinity, from the parameters of magmatic fluids to the conditions of formation of ore veins. The gold mineralization of the Svyatozar vein system was formed from boiling low-salinity fluids (0.3–8.0 wt % NaCl equv.) at temperatures of 365–160°C and pressures of 160–60 bar. The Terezia, Bieber, Viliam, Spitaler, and Rozalia epithermal gold-silver-base metal veins were also formed from heterogeneous low-salinity fluids (0.3–12.1 wt %) at temperatures of 380–58°C and pressures of 240–10 bar. It was found that the salt components of the solutions were dominated by chlorides (high content of fluorine, up to 0.45 mol/kg H₂O, was also detected), and sulfate solutions appeared in the upper levels. The dissolved gas of ore-forming solutions was dominated by CO₂ (0.1–8.4 mol %, averaging 1.3 wt %) and contained minor nitrogen (0.00–0.85 mol %, averaging 0.05 mol %) and negligible methane admixtures (0.00–0.05 mol %, averaging 0.004 mol %). These data allowed us to conclude that the magmatic melts could be sources of H₂O, Cl, CO₂, and N₂. The formation of the epithermal mineralization of the Stiavnica ore field was associated with the mixing of magmatic fluid with low-concentration meteoric waters, and the fluid was in a heterogeneous state.