Topaz-bearing Granites Research Articles

Nature of the mineralizing fluids in the Balda and Motiya W-prospects, western India: Constraints from chemical and B-isotope composition of tourmaline

In the Proterozoic tungsten belts of Balda and Motiya in western India, tungsten mineralization is hosted in tourmaline-bearing quartz veins intrusive into pelitic schists and granites. Tourmaline is a ubiquitous phase in all rock types, and in this study, we use its major, trace element and B-isotope composition to constrain the nature of the tungsten (W)-bearing hydrothermal fluid and the processes involved in the precipitation of wolframite. We have also reconstructed the compositions of the W-precipitating fluids from mineral-fluid trace element partition coefficients that were extrapolated using the Lattice Strain Model. The tourmalines from both belts are of schorl composition and have high alkali, low Ca content, and moderate X-site vacancies. High Li, Mn, Zn, and Sn in the fluid in Motiya is suggestive of relatively saline fluid possibly derived from a granitic source. The high V/Sc ratios of tourmalines in the mineralized veins and wall-rock tourmalinites indicate an important role of biotite dissolution and fluid-rock interaction that contributed Fe–Mn for the precipitation of tourmaline and wolframite. The tourmalines in the mineralized veins at Balda (δ11Btur = −10.9 ± 0.7‰, 2σ; n = 10) and those in the associated granites (δ11Btur = −11.5 ± 0.7‰, 2σ; n = 6) have similar B-isotope composition, while those of the associated topaz granites and pegmatites (−13.9 ± 0.7‰, 2σ; n = 19) are isotopically lighter than the granites. The B-isotopic variation in the granite-pegmatite-vein system can be explained by fluid exsolution with the mineralized vein forming from exsolved fluid and the topaz-bearing granites and the pegmatites crystallizing from the residual melts. The δ11B of the mineralizing fluid is estimated to be ca. −7.4‰ at Balda and ca. −7.6‰ at Motiya, and were possibly derived from granites, consistent with extensive tourmalinization and muscovitization of the adjacent wall rocks, and high concentration of elements such as F, Li, B, Sn, Mn in the tourmalines and the reconstructed fluid. The chemistry and B-isotope composition of tourmalines in both the belts support the hypothesis that W-bearing hydrothermal fluid was primarily derived from a fractionated granitic source, and that precipitation of wolframite and tourmaline involved interaction of the granitic fluid with surrounding pelitic rocks.

Two Types of Highly Differentiated Topaz-Bearing Granites of the Salmi Batholith, Southern Karelia

Differences between two types of highly differentiated granites of the Salmi batholith: namely Li-siderophyllite topaz-bearing and Li–F topaz-zinnwaldite granites, were determined based on the study of their structural-morphological, mineralogical and geochemical features are determined. The high degree of the differentiation of these granites is confirmed by the presence of primary topaz, Li-micas of magmatic origin and by the presence of significative tetrad effect of M type. These granites are distinguished due to the types of their micas, the content of topaz (up to 1 and 15%, respectively), the morphology of granite intrusions and the geochemistry of rare elements. The Li–F zinnwaldite granites are depleted in REE and enriched in Ta, Hf, Ba, Sr compared to the Li-siderophyllite granites. The Li-siderophyllite granites clearly fit to the general evolution trend of the Salmi batholith granites, while the genesis of Li–F zinnwaldite granites seems to be associated not only with the processes of crystallization differentiation of the melt but also with the active addition of some components, primarily F and Li, probably from a deeper source, including the mantle.

Geochemical Specifics of Highly Differentiated Topaz-Bearing Granite of the Salmi Batholith

Two varieties of highly differentiated topaz-bearing granite of the Salmi Batholith are distinguished: Li-siderophyllite topaz-bearing granite (Li-Sdph) (Lupikko dome) and Li–F-topaz-zinnwaldite granite (Li–F-Znwl) (Uksinskii dome). Their geochemical patterns and their place in the evolution of the compositions of A-type granite magma derivatives are shown using indicators of crystallization differentiation and the mineragenic potential; the likely regime of the volatile components (water activity in the melt and F accumulation) is discussed. The physicochemical conditions of the formation of Li–F granite were studied. The solidus temperature was determined experimentally by the method of rock powder melting (570–640°C under water saturated and dry conditions). Based on the temperatures of rock formation obtained and the data on the composition of micas, the F content in the fluid in equilibrium with micas (~0.24–0.34 mol/dm3 HF) was estimated. This similarity of the evolutionary trends for granite magmatism of the studied rocks and commercial Ta–Nb objects of Eastern Transbaikalia (S-type granite) is shown.

Geochemical Features of High Differentiated Topaz-Bearing Granites of Salmi Batholith

Geochemical Features of High Differentiated Topaz-Bearing Granites of Salmi Batholith

Relation between cathodoluminescence and trace-element distribution of magmatic topaz from the Ary-Bulak massif, Russia

AbstractIn order to define the cathodoluminescence (CL) properties of magmatic topaz and its relation with trace-element composition, we studied topaz phenocrysts from the Ary-Bulak ongonite massif, Russia using a wide array of analytical techniques. Scanning electron microscopy CL panchromatic images reveal strong variations, which define micrometre-scale euhedral growth textures. Several truncations of these growth textures occur in single grains implying multiple growth and resorption events. The CL-spectra of both CLbright and -dark domains have a major peak in the near-ultraviolet centred at 393 nm. Cathodoluminescence images taken after several minutes of electron bombardment show decreasing emission intensity. Electron microprobe analyses indicate high F concentrations (average OH/(OH + F) = 0.04 calculated by difference, 100 wt.% – total from electron probe microanalyses), consistent with what has been found previously in topaz-bearing granites, and the OH stretching vibration (∼3653 cm–1) was detected in Raman spectra. Laser ablation inductively-coupled plasma mass spectrometry traverses performed across the CL textures detected trace elements at ppm to thousands of ppm levels, including: Fe, Mn, Li, Be, B, P, Nb, Ta, W, Ti, Ga, light rare-earth elements, Th and U. Lithium, W, Nb and Ta appear to be correlated with CL intensity, suggesting a role for some of these elements in the activation of CL in topaz. In contrast, no clear correlation was found between CL intensity and F contents, despite the fact that the replacement of OH for F is known to affect the cell parameters of topaz.

Tracking magmatic and hydrothermal Nb–Ta–W–Sn fractionation using mineral textures and composition: A case study from the late Cretaceous Jiepailing ore district in the Nanling Range in South China

The Jiepailing mining district in the Nanling range in South China is well-known for its granite-related Sn–Be–F-mineralization. Recently, drill holes have exposed an Nb–Ta–W–Sn mineralized granitic porphyry and topaz-bearing granite–greisen at depth, which we have studied here, using mineral (columbite, rutile, wolframite, cassiterite, zircon, and mica) major- and trace-element compositional data, mineral textures, and zircon and columbite U–Pb geochronology. Our age data shows that the porphyry and the granite and their mineralization formed at ~91–89±1Ma in the late-Cretaceous, and thus subsequent to the main ore-forming events of the region. Continuous mineral compositional trends indicate that the studied granitoids are related by progressive fractionation. We propose that: (1) subhedral–euhedral, low-Ta columbite crystallized from melt; (2) euhedral–subhedral rutile and wolframite and subhedral and subhedral cassiterite up to ~30μm in size formed at the magmatic–hydrothermal transition of the system; and (3) high-Ta columbite and subhedral cassiterite up to ~10μm in size formed from subsolidus hydrothermal fluids. In combination with the Nb, Ta, W, and Sn compositions of zircon and mica, their textures and compositional variation allow us to track the magmatic to hydrothermal rare-metal fractionation (concentration, mobilization, and deposition) of the system in detail, despite our limited access to it through only two exploration drill cores. Using the Nb, Ta, W, and Sn concentrations in zircon (refractory, early-crystallized) and in micas (late equilibrated), respectively, was particularly useful for tracing the partial loss of Sn and W ore components from the intrusion, and to constrain the information which is crucial for any rigorous ore exploration.

Argillization of topaz-bearing granites in the Hub stock, Horni Slavkov-Krasno Sn-W ore district (Bohemian Massif, Czech Republic)

Argillization of topaz-bearing granites in the Hub stock, Horni Slavkov-Krasno Sn-W ore district (Bohemian Massif, Czech Republic)

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 Filfila (NE Algeria) topaz-bearing granites and their rare metal minerals: Petrologic and metallogenic implications

The Langhian Filfila granite complex (Internal Maghrebides, Lesser Kabylia) comprises a Fe-biotite and cordierite-granite together with a protolithionite–tourmaline-granite and late zinnwaldite–topaz-granite and aplites. The topaz-bearing granite occurs as small stocks with stockscheider borders. Several rare metal minerals were found in the Filfila topaz-bearing rocks: rare cassiterite (mostly included into albite and zinnwaldite) and ferberite (included in K-feldspar); and very abundant Ta-rutile included in zinnwaldite or K-feldspar and itself including rare small columbo-tantalite crystals. Filfila ferberites are slightly enriched in Nb, and contain traces of Ta, Ti and sometimes Sc. Columbo-tantalites (Ta/Ta + Nb between 0.08 and 0.13, Fe/Fe + Mn between 0.65 and 0.96) have compositions shifted towards the wodginite end-member. Ta-rutiles are members of the rutile-wodginite solid solutions, close to the rutile end-member. They are iron-rich (Fe/Fe + Mn between 0.96 and 1.0), variably enriched in Ta (Ta/Ta + Nb between 0.03 and 0.32), and always contain Sn and W. Under SEM examination, Ta-rutiles exhibit complex zoning patterns that may be interpreted as recording both fractionation and magma mixing processes. The geological, geochemical and mineralogical evidence points to the mixing of a meta-aluminous melt with several peraluminous melts, including a rare metal melt. Although this mixing inhibited the Ta potential of the Filfila topaz-granites, our study raises the possibility of Ta-bearing RMG being present at depth in the Internal Zone of the Algerian Maghrebides.

Origin and evolution of topaz-bearing granites from the Nanling Range, South China: a geochemical and Sr–Nd–Hf isotopic study

The F-rich Hongshan pluton in the eastern Nanling Range, southern China, is a topaz-bearing albite leucogranite. It is distinctive from other topaz-bearing felsic rocks in South China with respect to age, size, geochemical evolution and topaz mode and morphology. The Hongshan granites are highly peraluminous and characterized by high K2O/Na2O, Si, Rb, Cs, Nb, Ta and F, and low Ca, Ba, Sr, Zr, Hf, P, K/Rb, Zr/Hf and Eu/Eu*. The granites show significant trace-element variations with magma evolution, with increasing Rb, Cs, Nb, Ta, Sn, W and decreasing Sr, Ba, Zr, Hf, Y, REE, Pb, Th, K/Rb, Zr/Hf, Th/U and Eu/Eu*. These changes dominantly reflect fractional crystallization of plagioclase, biotite and accessory minerals such as zircon and monazite. The granites also exhibit a decrease in ɛNd(t = 225 Ma) from −7.9 to −11.7 with magma evolution. Modeling shows that the Nd isotopic variation could result from assimilation of the Taoxi Group wall rocks during fractional crystallization.

The specific case of the Mid-Proterozoic rapakivi granites and associated suite within the context of the Columbia supercontinent

Rapakivi granites and the associated suite (anorthosite, mangerite and charnockite) that developed during the Mid-Proterozoic escape the global rules that control usual granitic magma formation. About 40 points that characterise the Proterozoic magmatism confirm the originality of the magmatism. Rapakivi granites developed without a global orogenic context within the supercontinent Columbia. The intrusions extend from the East European shield to the western US, through Fennoscandia, Greenland and Labrador. Other occurrences in the Amazonian shield, Australia or South China are less well documented. They show no definite trend in age or chemistry that would explain large-scale (mantle plume) effects. A mantle upwelling of material is contradictory with the re-assembly of a supercontinent because it occurred before this magmatic episode (1.9–1.8 Ga). At ∼1.7 Ga, the supercontinent Columbia amalgamated with the collision of the Yapavai and Mazatzal Provinces. A model is suggested that takes into account the supercontinent re-assembly, defining a downwelling flow in the mantle that anchors the continent above it. In contrast to a material flow, heat is still delivered to the base of the continental lithosphere, and is focused toward the juvenile suture zone. The base of the crust must reach 1200–1300 °C before producing anorthosite magmas. Under such a high temperature gradient lasting over a long time, magmas are transferred toward the upper crust, giving the thin (5 km) and square-shape (100 km) that the intrusions presently have. Heat delivery is essentially conductive, leading to long time-spans for intrusions. The presence of the supercontinent, immobile over a descending cell (poloidal mode of convection) developed a tangential force (toroidal mode of convection) that partly split the continent through strike–slip deformation due to plate rotation. It developed progressively between 1.57 and 1.3 Ga, starting from Fennoscandia, and then passing to Amazonia, western US and Labrador in a clockwise sense. The associated rotation induced sinistral shear manifested by small-scale shear zones and the orientation of late magmatic facies (topaz-bearing granites) in each province. The Proterozoic magmatism appears to be unique because it requires a supercontinent with a zone of juvenile crust surrounded by older cratons. The present Moho still shows remnants of this process, having bumpy undulations that may reach 22 km in amplitude over a distance of 200 km.

Sn-polymetallic greisen-type deposits associated with late-stage rapakivi granites, Brazil: fluid inclusion and stable isotope characteristics

Tin-polymetallic greisen-type deposits in the Itu Rapakivi Province and Rondônia Tin Province, Brazil are associated with late-stage rapakivi fluorine-rich peraluminous alkali-feldspar granites. These granites contain topaz and/or muscovite or zinnwaldite and have geochemical characteristics comparable to the low-P sub-type topaz-bearing granites. Stockworks and veins are common in Oriente Novo (Rondônia Tin Province) and Correas (Itu Rapakivi Province) deposits, but in the Santa Bárbara deposit (Rondônia Tin Province) a preserved cupola with associated bed-like greisen is predominant. The contrasting mineralization styles reflect different depths of formation, spatial relationship to tin granites, and different wall rock/fluid proportions. The deposits contain a similar rare-metal suite that includes Sn (±W, ±Ta, ±Nb), and base-metal suite (Zn–Cu–Pb) is present only in Correas deposit. The early fluid inclusions of the Correas and Oriente Novo deposits are (1) low to moderate-salinity (0–19 wt.% NaCl eq.) CO 2-bearing aqueous fluids homogenizing at 245–450 °C, and (2) aqueous solutions with low CO 2, low to moderate salinity (0–14 wt.% NaCl eq.), which homogenize between 100 and 340 °C. In the Santa Bárbara deposit, the early inclusions are represented by (1) low-salinity (5–12 wt.% NaCl eq.) aqueous fluids with variable CO 2 contents, homogenizing at 340 to 390 °C, and (2) low-salinity (0–3 wt.% NaCl eq.) aqueous fluid inclusions, which homogenize at 320–380 °C. Cassiterite, wolframite, columbite–tantalite, scheelite, and sulfide assemblages accompany these fluids. The late fluid in the Oriente Novo and Correas deposit was a low-salinity (0–6 wt.% NaCl eq.) CO 2-free aqueous solution, which homogenizes at (100–260 °C) and characterizes the sulfide–fluorite–sericite association in the Correas deposit. The late fluid in the Santa Bárbara deposit has lower salinity (0–3 wt.% NaCl eq.) and characterizes the late-barren-quartz, muscovite and kaolinite veins. Oxygen isotope thermometry coupled with fluid inclusion data suggest hydrothermal activity at 240–450 °C, and 1.0–2.6 kbar fluid pressure at Correas and Oriente Novo. The hydrogen isotope composition of breccia-greisen, stockwork, and vein fluids ( δ 18O quartz from 9.9‰ to 10.9‰, δD H 2O from 4.13‰ to 6.95‰) is consistent with a fluid that was in equilibrium with granite at temperatures from 450 to 240 °C. In the Santa Bárbara deposit, the inferred temperatures for quartz-pods and bed-like greisens are much higher (570 and 500 °C, respectively), and that for the cassiterite-quartz-veins is 415 °C. The oxygen and hydrogen isotope composition of greisen and quartz-pods fluids ( δ 18O qtz-H 2O =5.5–6.1‰) indicate that the fluid equilibrated with the albite granite, consistent with a magmatic origin. The values for mica ( δ 18O mica-H 2O =3.3–9.8‰) suggest mixing with meteoric water. Late muscovite veins ( δ 18O qtz-H 2O =−6.4‰) and late quartz ( δ 18O mica-H 2O =−3.8‰) indicate involvement of a meteoric fluid. Overall, the stable isotope and fluid inclusion data imply three fluid types: (1) an early orthomagmatic fluid, which equilibrated with granite; (2) a mixed orthomagmatic-meteoric fluid; and (3) a late hydrothermal meteoric fluid. The first two were responsible for cassiterite, wolframite, and minor columbite–tantalite precipitation. Change in the redox conditions related to mixing of magmatic and meteoric fluids favored important sulfide mineralization in the Correas deposit.

Comparison of Proterozoic and Phanerozoic rift-related basaltic-granitic magmatism

This paper compares the 1.67–1.47 Ga rapakivi granites of Finland and vicinity to the 1.70–1.68 Ga rapakivi granites of the Beijing area in China, the anorogenic ∼130 Ma granites of western Namibia, and the 20–15 Ma granites of the Colorado River extensional corridor in the Basin and Range Province of southern Nevada. In Finland and China, the tectonic setting was incipient, aborted rifting of Paleoproterozoic or Archean continental crust, in Namibia it was continental rifting and mantle plume activity that led to the opening of southern Atlantic at ∼130 Ma. The 20–15 Ma granites of southern Nevada were related to rifting that followed the Triassic–Paleogene subduction of the Farallon plate beneath the southwestern United States. In all cases, extension-related magmatism was bimodal and accompanied by swarms of diabase and rhyolite–quartz latite dikes. Rapakivi texture with plagioclase-mantled alkali feldspar megacrysts occurs in varying amounts in the granites, and the latest intrusive phases are commonly topaz-bearing granites or rhyolites that may host tin, tungsten, and beryllium mineralization. The granites are typically ferroan alkali-calcic metaluminous to slightly peraluminous rocks with A-type and within-plate geochemical and mineralogical characteristics. Isotope studies (Nd, Sr) suggest dominant crustal sources for the granites. The preferred genetic model is magmatic underplating involving dehydration melting of intermediate-felsic deep crust. Juvenile mafic magma was incorporated either via magma mingling and mixing, or by remelting of newly hybridized lower crust. In Namibia, partial melting of subcontinental lithospheric mantle was caused by the Tristan mantle plume, in the other cases the origin of the mantle magmatism is controversial. For the Fennoscandian suites, extensive long-time mantle upwelling associated with periodic, migrating melting of the subcontinental lithospheric mantle, governed by heat flow and deep crustal structures, is suggested.

The amblygonite (LiAlPO4F)-montebrasite (LiAlPO4OH) solid solution: A combined powder and single-crystal neutron diffraction and solid-state6Li MAS, CP MAS, and REDOR NMR study

The amblygonite-montebrasite series of minerals, common constituents of granitic pegmatites and topaz-bearing granites, show complete solid solution with ideal composition LiAlPO{sub 4}(F, OH). These compounds are ideal for studying F {leftrightarrow} OH solid solution in minerals because natural members of the series generally show little deviation from the ideal composition. In this study, we used powder and single-crystal neutron diffraction and solid-state {sup 6}Li MAS, CP MAS, and REDOR NMR techniques to study the effect of F {leftrightarrow} OH substitution on the series. Lattice parameters refined from single-crystal neutron diffraction data show increasing b and decreasing a, c, and V with increasing F/(F + OH). The volume is highest for the OH end-member because of the presence of an additional atom (H). The a and c parameters decrease with increasing F/(F + OH) because the O-H vector is close to the a-c plane and the Al-OH/F vectors are approximately parallel to c. Lattice parameters refined from neutron powder diffraction patterns collected at lower T show that thermal contraction increases with F/(F + OH), presumably because the F anion takes up less space than the OH molecule. The results show that the OH/F position is always fully occupied. The H displacementmore » ellipsoid shows little change with occupancy, which obviously corresponds negatively with increasing F/(F + OH). However, the Li displacement ellipsoid becomes extremely large and anisotropic with increasing F fraction. Most of the distortion is associated with the U{sub 3} eigenvalue, which lies between the c and c* directions. U{sub eq} values corresponding to the Li atom show a greater reduction with decreasing temperature than the other atoms. The temperature dependence of Li is the same regardless of F content. Even when extrapolated to absolute zero the Li displacement ellipsoid is very large, which implies a large static disorder.« less

Trace elements and cathodoluminescence of igneous quartz in topaz granites from the Hub Stock (Slavkovsk� Les Mts., Czech Republic)

¶Igneous quartz of the late-Variscan topaz-bearing granites from the Hub Stock (Slavkovský Les, Czech Republic) was investigated by cathodoluminescence (CL) and electron probe micro-analysis (EPMA) to demonstrate the intra-granular heterogeneity of growth patterns and trace element distribution in quartz. We show that EPMA is well suited for the in situ study of Al and Ti in zoned quartz, because of its high spatial resolution down to 5 µm in conjunction with the ability to combine spot analyses with CL imaging. In the quartz phenocrysts of the topaz granites high Ti is associated with blue luminescent growth zones. High Ti (>40 ppm) in quartz indicates a high crystallisation temperature and pressure. The groundmass quartz of the granites which is almost free of Ti, has higher Al than the phenocrysts which may reflect an increase of lithophile elements and water content in melt during the late magmatic stage. The occurrence of similar quartz phenocrysts in most of the late-Variscan granites and rhyolites of the Krusne Hory/Erzgebirge which intruded over a period of about 40 Ma points to a similar crystallisation environment and origin of the quartz phenocrysts in the lower to middle crust.

Petrography and geochemistry of the topaz-bearing granite stocks in Artjärvi and Sääskjärvi, western margin of the Wiborg rapakivi granite batholith

LUKKARI, SARI 2002. Petrography and geochemistry of the topaz-bearing granite stocks in Artjarvi and Saaskjarvi, western margin of the Wiborg rapakivi granite batholith. Bulletin of the Geological Society of Finland 74, Parts 1–2, 115– 132. The Artjarvi and Saaskjarvi granite stocks at the western margin of the Wiborg rapakivi batholith are multiphase rapakivi granite intrusions in which the most evolved phase is topaz-bearing granite. The Artjarvi stock is composed of porphyritic and even-grained biotite granite and even-grained topaz granite, and the Saaskjarvi stock comprises even-grained biotite granite and porphyritic topaz granite. The granites are metaluminous to peraluminous A-type granites, showing within-plate (WPG) geochemical characteristics. The topaz granites from the Artjarvi and Saaskjarvi stocks are petrographically and geochemically similar to other topaz-bearing rapakivi granites in Finland. The anomalous geochemistry of the topaz granite is essentially magmatic; postmagmatic reactions have only slightly modified its composition. Greisen veins, some of which are mineralized, are widely found associated with the Artjarvi and Saaskjarvi stocks. The most characteristic feature of the Artjarvi granite stock is a stockscheider at the roof contact of the topaz granite. The stockscheider is composed of schlieren layering and pegmatite layers parallel to the contact. The most probable mechanism for the formation of the schlieren layering is velocity-gradient sorting parallel to the flow, which led to accumulation of mafic minerals along the upper contact of the topaz granite. Cooling and contraction of the topaz granite formed fractures parallel to the roof contact and the residual pegmatite magmas were injected along the fractures forming pegmatite layers. Textures like graphic intergrowths, aplitic groundmass and fan-shaped alkali feldspar crystals associated with the Artjarvi porphyritic biotite granite and pegmatite indicate crystallization from undercooled magma. Miarolitic cavities found in the porphyritic biotite granite and topaz granite indicate at least local volatile saturation.

Amblygonite-montebrasite solid solutions as monitors of fluorine in evolved granitic and pegmatitic melts

The distribution of F between amblygonite (Amb, LiAlPO 4 F)-montebrasite (Mbr, LiAlPO 4 OH) solid solutions and metaluminous haplogranitic melt has been calibrated at 585 °C and 200 MPa H 2 O. The partition coefficient for F between the crystalline phase and melt, D F Mbr/melt , is linear between 0 to ~10 wt% F in amblygonite, which contains 13 wt% F at the end-member: C F Mbr = 3.65C F melt + 0.07, r 2 = 0.995, n = 6. Values of D F Amb/melt decrease sharply above 10 wt% F in amblygonite as the amblygonite reaches saturation in F at 200 MPa H 2 O. In natural occurrences, however, the vast majority of primary amblygonite- montebrasite solid solutions contain ~4-7 wt% F, well within the linear range of the calibrated exchange reaction, and the montebrasite-bearing assemblages are among the last to crystallize. If the F contents of the montebrasite are magmatic, then these most-fractionated residual melts of the LCT (Li-Cs-Ta, and mostly peraluminous S-types) rare-element class generally contained up to ~1.0-1.8 wt% F near the end of their crystallization. The modest F contents of pegmatites are consistent with the common association of Li aluminosilicates and with the general paucity of topaz in these occurrences. In topaz-bearing granites of Western Europe, however, high-F amblygonite (~10- 11 wt% F) reflects >3 wt% F in melt during crystallization of these magmatic phases.

Fluid and melt inclusion evidence for the origin of idiomorphic quartz crystals in topaz-bearing granite from the Salmi batholith, Karelia, Russia

Idiomorphic quartz crystals in topaz-bearing granite from the Salmi batholith contain primary inclusions of silicate melt and abundant mostly secondary aqueous fluid inclusions. Microthermometric measurements on melt inclusions give estimates for the granite solidus and liquidus of 640–680°C and 770–830°C, respectively. Using published solubility models for H 2O in granitic melts and the obtained solidus/liquidus temperatures from melt inclusions, the initial water concentration of the magma is deduced to have been approximately 3 wt.% and the minimum pressure about 2 kbar. At this initial stage, volatile-undersaturation conditions of magma were assumed. These results indicate that the idiomorphic quartz crystals are magmatic in origin and thus real phenocrysts. During subsolidus cooling and fracturing of the granite, several generations of aqueous fluid inclusions were trapped into the quartz phenocrysts. The H 2O inclusions have salinities and densities of 1–41 wt.% NaCl eq. and 0.53–1.18 g/cm 3, respectively.

Magmatic and Postmagmatic Processes in Tin-mineralized Granites: Topaz-bearing Leucogranite in the Eurajoki Rapakivi Granite Stock, Finland

The 1.57 Ga Eurajoki stock in southwestern Finland is composed of biotite–hornblende–fayalite granite, biotite granites and late-stage topaz-bearing granite, as well as related topaz-bearing rhyolite (ongonite) dykes. Miarolitic cavities indicate the presence of a separate fluid phase during late stages of crystallization of topaz-bearing granite, and greisen-type Sn–Be–W–Zn mineralization is closely associated with it. Subsolidus reactions have modified the petrography and geochemistry of the topaz-bearing granite. The presence of magmatic topaz is indicated by petrographic features and crystallized melt inclusions entrapped in topaz. The magmatic origin of accessory cassiterite in the topaz-bearing granite (average 80 ppm Sn) is indicated by mode of occurrence and chemical composition of cassiterite: the Nb2O5 + Ta2O5 content of cassiterite is 4.8–8.7 wt % in granite, 2.3–3% in pegmatite and 0.0–0.6% in greisen. Some of the rhyolite dykes contain topaz as phenocrysts and as small prismatic crystals in the groundmass showing locally fluidal texture. These features indicate that the late-stage magma was highly enriched in F and Sn. Strong exsolution of alkali feldspar has produced intergranular albite rims and grain rows. De-anorthitization of primary plagioclase has produced turbid albite with inclusions of topaz, fluorite and quartz. Metasomatic albitization of K-feldspar and K-feldspathization of plagioclase have only locally been important. In greisenized granite, metasomatic addition of topaz and quartz is common. The topaz-bearing granite had originally anomalous geochemistry and mineralogy, and its anomalous character was further increased during postmagmatic fluid–rock interaction. The petrographic and geochemical peculiarities of the topaz–bearing granite of Eurajoki are characteristic of many other late-stage intrusive phases of rapakivi complexes and for many tin granites of different ages. In interpreting the origin of such highly evolved granites, the role of Na-metasomatic albitization has often been overemphasized at the expense of exsolution and recrystallization.

Magmatic and Postmagmatic Processes in Tin-mineralized Granites: Topaz-bearing Leucogranite in the Eurajoki Rapakivi Granite Stock, Finland

The 1·57 Ga Eurajoki stock in southwestern Finland is composed been overemphasized at the expense of exsolution and reof biotite–hornblende–fayalite granite, biotite granites and late-stage crystallization. topaz-bearing granite, as well as related topaz-bearing rhyolite (ongonite) dykes. Miarolitic cavities indicate the presence of a separate fluid phase during late stages of crystallization of the topazbearing granite, and greisen-type Sn–Be–W–Zn mineralization is closely associated with it. Subsolidus reactions have modified the