Chemical Composition Of Tourmaline Research Articles

Dravite of Terny Meteoritic Crater (Kryvyi Rih District)

During mineralogical investigations of breccia and volcanic rocks of the Terny structure (Veseli Terny, Kryvyi Rih), so-called Terny meteoritic crater, the mineral tourmaline, previously unknown here, was found. Tourmaline crystals, with a length from 0.05 to 1.0 mm and more and up to 0.1—0.2 mm wide, are associated with quartz, potassium feldspar, graphite and apatite. The morphology and chemical composition of tourmaline were examined using optical microscope and EPMA in transparent thin sections. The tourmaline crystals are unusually parted along two transverse systems of microcracks oriented obliquely to the prism faces. EPMA reveals that tourmaline is dravite. The specific deformations of dravite and observed mineral associations probably indicate the rock formation under conditions of high pressures and temperatures of impact metamorphism. It is also confirmed by the presence of planar fractures in quartz.

Chemical composition of tourmalines from the Manjaka pegmatite and its exocontact, Sahatany Valley, Madagascar

Chemical composition of tourmalines from the Manjaka pegmatite and its exocontact, Sahatany Valley, Madagascar

Geochemistry and Genesis of Magnesium Tourmalines in Jian Forsterite Jade Deposits from Ji’an County, Jilin Province, Northeast China

The Jian forsterite jade, so named because of its enrichment in end-member forsterite, is a new type of jade found in Ji’an County (Jilin Province, Northeast China). Tourmaline is discovered in Jian forsterite jade deposits and is characterized by magnesium enrichment. In this study, three types of magnesium tourmaline were identified from the pegmatite veins (type 1), the contact zone (type 2), and the tourmaline veins in jade (type 3). The results are shown by the main test methods, such as EPMA, Micro-XRF, and LA-ICP-MS. The substitutions of Fe2+−1Mg2+−1, (□Al3+)−1 (Na+Mg2+)−1, (□Al3+2)−1 (Ca2+R2+2)−1, etc. are inferred by the variations in the major element compositions. From type 1 to type 2 tourmaline, the content of Mg, Sr, and Sn gradually increases, the content of Fe, Zn, K, Mn, Sc, Ga, and Co gradually decreases, the content of Ca initially decreases and then increases, and the content of Na initially increases and then decreases. Type 3 tourmaline has significantly higher Si and Al than the first two types, and the content of the remaining elements lies between the above two types. We propose that tourmalines in Jian forsterite jade deposits are typically of hydrothermal origins and are mainly constrained by magnesium, which is related to the contact metasomatic metamorphism of pegmatite-related hydrothermal fluid with the Jian forsterite jade, and the chemical composition of tourmaline indicates the fluid characteristics of gradual serpentinization of Jian forsterite jade.

Magmatic–hydrothermal evolution of the Koktokay No. 3 pegmatite, Altai, northwestern China: Constraints from in situ boron isotope and chemical compositions of tourmaline

Lithium–Cs–Ta-enriched (LCT-type) pegmatites are characterized by zoning texture and extreme enrichment of rare-metals. The role of fluids in zonation development and rare-metal mineralization remains a topic of debate. The Koktokay No. 3 pegmatite in northwestern China represents a typical LCT-type pegmatite characterized by economically significant Li–Be–Nb–Ta–Cs mineralization. This pegmatite exhibits a complete evolutionary sequence from outer to inner zones, with diverse rare-metal mineralization. Tourmaline is ubiquitously present in all textural zones of LCT-type pegmatites. In the Koktokay No. 3 pegmatite, the tourmalines exhibit a range of compositions from dravite and schorl to elbaite and rossmanite. The tourmalines in this pegmatite are characterized by significantly enriched light B isotope compositions (δ11B = −14.8‰ to −8.1‰), similar to those typically observed in most pegmatites derived from continental crust sources. The tourmalines in the outer zones have less variable δ11B values (−13.8‰ to −11.5‰) and are interpreted to have crystallized in a viscous magma. The tourmalines in the inner zones exhibit larger variations in δ11B values (−14.8‰ to −8.1‰) than those from the outer zones. The decreasing δ11B values of tourmalines from the outer to inner zones are attributed to the separation of an immiscible fluid phase that preferentially incorporated heavy B isotopes. The occurrence of hydrothermal tourmaline as veinlets indicates the exsolution of fluids in inner zones, with high δ11B values ranging from −10.2‰ to −9.3‰. Therefore, the hydrothermal fluid can be saturated when the magma has been strongly evolved. In addition to fractional crystallization, exsolved fluids play a significant role in the formation of internal zones and rare-metal mineralization in LCT-type pegmatites. The rare-metals are gradually enriched through fractional crystallization of magma, and the fluid contains mobile elements such as Cs that facilitate precipitation of rare-metal minerals.

In-situ boron isotope and chemical composition of tourmaline in the Gyirong pegmatite, southern Tibet: Implications for petrogenesis and magma source

Leucogranitic rocks, mainly including leucogranite-pegmatite systems, have been found to be widely distributed in the South Tibetan Himalaya, and they have received considerable interest because of their significance in crustal evolution and associated rare-metal mineralization. Although the nature and geodynamic setting of the Himalayan leucogranites have been well documented by numerous studies, the pegmatites spatially associated with these leucogranites are still poorly understood. Tourmaline is a ubiquitous phase from the leucogranite to the pegmatite. We have therefore conducted in situ major and trace element and boron isotope investigations of tourmaline from the Gyirong pegmatite, synthesizing published data on the Gyirong leucogranite, to document the origin of tourmaline and its genetic implications. Two types of tourmaline (Tur-Ⅰ & Tur-Ⅱ) have been identified in this contribution and they are enriched in Fe, Si and Al but depleted in Mg and Ca, with Mg/(Mg+Fe) ratios ranging from 0.22 to 0.45. Accordingly, the tourmalines belong to the alkali group and have schorl composition. Trace elements, such as Zn, Ga, V, Sc, Li, Sn, Sr, and Co in the tourmalines are relatively enriched, whereas, other trace elements record low concentrations less than 10 ppm. The trace element concentrations of tourmaline are mainly controlled by melt composition. Morphological and geochemical characteristics reflect that the tourmalines from the Gyirong pegmatite are magmatic in origin. The Gyirong pegmatitic tourmalines have S-type granitoids and pegmatites boron isotopic signatures with a tight range of δ11B values between −11.8 and −9.7‰, which is consistent with the magmatic tourmalines (Mg-poor) of the Gyirong leucogranite. This study suggests that the Gyirong pegmatite was the product of crustal anatexis and that the crustal metapelitic rocks within the Greater Himalayan Crystalline Complex were the most likely source components.

Chemical composition of tourmaline in orogenic gold deposits

Tourmaline from 18 orogenic gold deposits and districts, hosted in varied country rocks and metamorphic facies, was investigated by EPMA (electron probe micro-analyzer) and LA-ICP-MS (laser ablation-inductively coupled plasma-mass spectrometry) to establish discriminant geochemical features to constrain indicator mineral surveys for gold exploration. Such tourmaline most commonly belongs to the alkali group, with a dravitic composition. LA-ICP-MS results were investigated with binary plots and PLS-DA (partial least square-discriminant analysis). PLS-DA suggests that the major element composition of tourmaline from orogenic gold deposits is buffered by the hydrothermal fluid, whereas trace element composition is strongly controlled by the composition and the metamorphic facies of the country rocks. Contents of Sn, Ga, Ti, rare earth elements (REE), Zr, Hf, Nb, Ta, Th, and U vary with the metamorphic facies of the country rocks. Tourmaline from orogenic gold deposits has high contents of Sr, V, and Ni and low Li, Be, Ga, Sn, Nb, Ta, U, and Th compared to tourmaline from other deposit types and geological environments. Binary plots such as Sr/Li vs. V/Sn, Sr/Sn vs. V/Nb, Sr/Sn vs. Ni/Nb, and Sr/Sn vs. V/Be, as well as PLS-DA, discriminate tourmaline from orogenic gold deposits from that of other settings. Binary plots highlight a transitional variation in the trace element composition of tourmaline from metamorphic, to magmatic-hydrothermal, to magmatic environments.

Tourmaline as a Recorder of Ore-Forming Processes in the Xuebaoding W-Sn-Be Deposit, Sichuan Province, China: Evidence from the Chemical Composition of Tourmaline

The Xuebaoding W-Sn-Be deposit located in the Songpan-Ganze Orogenic Belt (Sichuan Province, China) is a hydrothermal deposit with less developed pegmatite stage. The deposit is famous for the coarse-grained crystals of beryl, scheelite, cassiterite, apatite, fluorite, muscovite, and others. The orebody is spatially associated with the Pankou and Pukouling granites hosted in Triassic marbles and schists. The highly fractionated granites are peraluminous, Li-Rb-Cs-rich, and related to W-Sn-Be mineralization. The mineralization can chiefly be classified based on the wallrock and mineral assemblages as muscovite and beryl in granite (Zone I), then beryl, cassiterite and muscovite at the transition from granite to triassic strata (Zone II), and the main mineralized veins composed of an assemblage of beryl, cassiterite, scheelite, fluorite, and apatite hosted in metasedimentary rock units of marble and schist (Zone III). Due to the stability of tourmaline over a wide range of temperature and pressure conditions, its compositional variability can reflect the evolution of the ore-forming fluids. Tourmaline is an important gangue mineral in the Xuebaoding deposit and occurs in the late-magmatic to early-hydrothermal stage, and can thus be used as a proxy for the fluid evolution. Three types of tourmalines can be distinguished: tourmaline disseminations within the granite (type I), tourmaline clusters at the margin of the granite (type II), and tourmalines occurring in the mineralized veins (type III). Based on their chemical composition, both type I and II tourmalines belong to the alkali group and to the dravite-schorl solid solution. Type III tourmaline which is higher in X-site vacancy corresponds to foitite and schorl. It is proposed that the weakly zoned type I tourmalines result from an immiscible boron-rich aqueous fluid in the latest stage of granite crystallization, that the type II tourmalines showing skeletal texture directly formed from the undercooled melts, and that type III tourmalines occurring in the mineralized veins formed directly from the magmatic hydrothermal fluids. Both type I and type II tourmalines show similar compositional variations reflecting the highly fractionated Pankou and Pukouling granites. The higher Ca, Mg, and Fe contents of type III tourmaline are buffered by the composition of the metasedimentary host rocks. The decreasing Na content (<0.8 atoms per formula unit (apfu)) and increasing Fe3+/Fe2+ ratios of all tourmaline samples suggest that they precipitated from oxidized, low-salinity fluids. The decreasing trend of Al content from type I (5.60–6.36 apfu) and type II (6.01–6.43 apfu) to type III (5.58–5.87 apfu) tourmalines, and associated decrease in Na, may be caused by the crystallization of albite and muscovite. The combined petrographic, mineralogical, and chemical characteristics of the three types of tourmalines thus reflect the late-magmatic to early-hydrothermal evolution of the ore-forming fluids, and could be used as a geochemical fingerprint for prospecting W-Sn-Be mineralization in the Xuebaoding district.

Chemical composition of tourmaline in the biotite granite, the Dachang district: Insights into magmatic-hydrothermal evolution

Chemical composition of tourmaline in the biotite granite, the Dachang district: Insights into magmatic-hydrothermal evolution

Syn-metamorphic B-bearing fluid infiltrations deduced from tourmaline in the Main Central Thrust zone, Eastern Nepal Himalayas

Mode of occurrence and chemical composition of tourmaline in pelitic schists from the Main Central Thrust (MCT) zone of the Lesser Himalayan Sequences (LHS) are described in detail with the aim of deducing the chemical characteristics of tourmaline formed through B-bearing fluid infiltration and of estimating the composition of the syn-metamorphic fluids. Metasomatic tourmalines from the tourmalinized wall rocks show significant increases in XCa [=Ca/(Ca + Na)] at almost constant XMg [=Mg/(Mg + Fe2+)] from the cores or mantles to the rims. Tourmaline in tourmaline-rich (> 1.0 vol%) pelitic schists from the biotite zone to the kyanite zone also show marked increase in XCa at almost constant XMg, and are interpreted as a product of B-bearing fluid infiltration. Abundant margarite and anorthite formed in the pelitic schists intercalated with the metadolostone layers suggesting that the B-rich fluid became Ca enriched as it interacted with metadolostone layers, and metasomatically introduced Ca into the pelitic schists. Infiltration of such B- and Ca-rich fluids into pelitic schists likely resulted in production of abundant tourmaline with the compositional trend of increasing XCa at almost constant XMg. Most of the tourmaline in tourmaline-rich pelitic schists are in equilibrium with plagioclase, suggesting that the fluid composition was buffered by the pelitic schists. Composition of tourmaline in the pelitic schists with <1 vol% tourmaline mostly show increase in XCa and wider range of XMg values, reflecting its growth during prograde metamorphism. The composition of fluids that coexisted with tourmaline is estimated by applying experimentally-determined fluid/tourmaline chemical relationships to the composition of tourmaline from veins associated with tourmalinization, pelitic schists with <1 vol% tourmaline and tourmaline-rich pelitic schists. Assuming coexisting anion to be Cl, the salinity estimated for these rock types was ∼0.44–0.59 mol/l NaCl + CaCl2 that is similar to or slightly lower than the present-day seawater. Veins associated with tourmalinization and tourmaline-rich pelitic schists are sporadically distributed in the MCT zone of the LHS, suggesting that the syn-metamorphic, B-bearing saline fluid infiltrations took place widely in the MCT zone, and the fluid pathways were localized and channeled. Our observation supports the scenario whereby infiltration of fluid into the High Himalayan Crystallines (HHC) caused vapor-saturated partial melting of the HHC to give tourmaline leucogranite melts contemporaneous with inverted metamorphism in the MCT zone.

Petrogenetic implications of the mineral-chemical characteristics of scheelite and associated phases from Miniki Gol (Chitral), NW Pakistan

The stratabound tungsten mineralization in Chitral, northern Pakistan, lies within the Hindu Kush terrane and is located to the north-west of the Main Karakoram Thrust (MKT) that represents the suture zone between the Kohistan complex and Asian plate. The area has undergone amphibolite facies metamorphism followed by retrogression and at least two episodes of deformation. Scheelite occurs mainly in the calc-silicate quartzite and subordinate tourmalinite associated with metapelite of the Jurassic Arkari Formation at Miniki Gol. The scheelite host rocks are intruded by leucogranite, which is exposed ∼400 m east of the studied tungsten mineralization. The leucogranite was emplaced after the culmination of the amphibolite facies metamorphism. The scheelite-bearing calc-silicate rock consists of clinozoisite, quartz, calcic-amphibole, plagioclase, chlorite, biotite, calcite, sphene and garnet. This mineral assemblage correlates with a skarn-type environment. Scheelite is intergrown with clinozoisite in the calc-silicate rock and also associated with tourmaline and spessartine-rich garnet in the tourmalinite. The extensive tourmalinisation leading to the formation of abundant (i.e., up to 80 modal%) tourmaline in the scheelite-bearing tourmalinite appears to have been caused by a post-magmatic hydrothermal activity. The mentioned leucogranite in the area might have acted as a source of fluids for this borondominated metasomatic process. The abundant development of epidote, making up to 60% by volume of the calc-silicate rock, signifies its formation through a hydrothermal phenomenon. The chemical compositions of tourmaline, clinozoisite, sphene, garnet and scheelite from the calc-silicate rock also support their hydrothermal origin. Similarly, the compositional characteristics of both tourmaline and spessartine-rich garnet from the tourmalinite indicate the precipitation of these phases from evolved acidic magmaderived fluids. More importantly, the investigated scheelite contains significant amounts of Zr (up to 0.46 wt% ZrO2) and Ta (up to 0.35 wt% Ta2O5), which are believed to replace W in scheelite from post-magmatic environments. As all the mentioned minerals are associated and some of them even intergrown with scheelite, it appears that the tungsten mineralization also is genetically linked to the solidification and emplacement of leucogranite in the area. Although itself generally unmineralized, the Miniki Gol leucogranite’s average W content (5.3 ppm) is higher than that of low-Ca granites.

Pegmatite dykes and quartz veins with tourmaline: an example of partial melting in the contact aureole of the Chandman Massif intrusion, SW Mongolia

*Corresponding author Pegmatite dykes with tourmaline and quartz–tourmaline veins in the Chandman Khayrkhan Crystalline Complex (the Eastern Mongolian Altay range, SW Mongolia) are spatially related to the contact aureole of the Variscan granitoids of the Chandman Massif. Tourmalines are also present in pegmatites that intruded into the I-type granitoids of the Chandman Massif near the contact with surrounding migmatites. The fluid-present melting of metapelites in wall rocks of the I-type granodiorites and tonalities can be considered as the main process inducing the boron-rich granitic melt formation. Variations in chemical compositions of tourmaline from several pegmatite dykes, mainly in Na, Ca, Al, Fe and Mg, are interpreted as a result of fractionation of the parental melt. The quartz–tourmaline veins represent a product of crystallization of aqueous fluid exsolved from the pegmatite melt. Chemical compositions of tourmaline with elevated Ca and Mg are interpreted to reflect mixing of the boron-rich postmagmatic fluids with Mg–Ca enriched external component represented by fluids from the biotite-rich metaluminous igneous host rocks.

Boron-isotopic constraints on the petrogenesis of hematitic phyllite in the southern Serra do Espinhaço, Minas Gerais, Brazil

Metasiliciclastic rocks predominate in the lower units of the Palaeo-Mesoproterozoic Espinhaço Supergroup, in the southern Serra do Espinhaço, Minas Gerais, Brazil. The lower units also comprise rocks with locally preserved igneous fabrics, but which have very unusual chemistries. These rocks, collectively known as hematitic phyllite, are characterised by abundant fine-grained muscovite, i.e. sericite, and variable amounts of titaniferous hematite, rutile and tourmaline. Currently, the hematitic phyllite has been interpreted as a metamorphosed palaeosol after basaltic rocks and, as such, has been used as a palaeoclimatic indicator. However, the lateritic nature of the hematitic phyllite cannot unambiguously be determined because of the K metasomatism, hematitisation and tourmalinisation recorded in the hematitic phyllite and in the arenaceous country rocks. Here we report the B-isotopic and chemical compositions of tourmaline from the hematitic phyllite. Our δ11B data are in the range between −15‰ and 4‰. The tourmaline compositions fall along the povondraite–"oxy-dravite" join, which defines a meta-evaporitic tourmaline trend. A meta-evaporitic B source is constrained by the B-isotopic data as non-marine. Our model for the hematitic phyllite suggests that B- and K-rich brines were derived from the metamorphic dewatering of non-marine evaporites. Such brines extensively altered volcanic rocks of basaltic and rhyolitic compositions, leading to tourmaline-bearing, hematite–sericite assemblages of the hematitic phyllite.

Fluid sources and metallogenesis in the Blackbird Co-Cu-Au-Bi-Y-REE district, Idaho, U.S.A.: insights from major-element and boron isotopic compositions of tourmaline

Tourmaline is a widespread mineral in the Mesoproterozoic Blackbird Co–Cu–Au–Bi–Y–REE district, Idaho, where it occurs in both mineralized zones and wallrocks. We report here major-element and B-isotope compositions of tourmaline from stratabound sulfide deposits and their metasedimentary wallrocks, from mineralized and barren pipes of tourmaline breccia, from late barren quartz veins, and from Mesoproterozoic granite. The tourmalines are aluminous, intermediate in the schorl–dravite series, with Fe/(Fe + Mg) values of 0.30 to 0.85, and 10 to 50% X -site vacancies. Compositional zoning is prominent only in tourmaline from breccias and quartz veins; crystal rims are enriched in Mg, Ca and Ti, and depleted in Fe and Al relative to cores. The chemical composition of tourmaline does not correlate with the presence or absence of mineralization. The δ 11 B values fall into two groups. Isotopically light tourmaline (−21.7 to −7.6‰) occurs in unmineralized samples from wallrocks, late quartz veins and Mesoproterozoic granite, whereas heavy tourmaline (−6.9 to +3.2‰) is spatially associated with mineralization (stratabound and breccia-hosted), and is also found in barren breccia. At an inferred temperature of 300°C, boron in the hydrothermal fluid associated with mineralization had δ 11 B values of −3 to +7‰. The high end of this range indicates a marine source of the boron. A likely scenario involves leaching of boron principally from marine carbonate beds or B-bearing evaporites in Mesoproterozoic strata of the region. The δ 11 B values of the isotopically light tourmaline in the sulfide deposits are attributed to recrystallization during Cretaceous metamorphism, superimposed on a light boron component derived from footwall siliciclastic sediments ( e.g. , marine clays) during Mesoproterozoic mineralization, and possibly a minor component of light boron from a magmatic–hydrothermal fluid. The metal association of Bi–Be–Y–REE in the Blackbird ores suggests some magmatic input, but involvement of granite-derived fluids cannot be conclusively established from the present database.

Chemical composition of tourmaline from the Asarcık Pb–Zn–Cu ± U deposit, Şebinkarahisar, Turkey

Abundant tourmaline, commonly as rosettes up to 1.5 cm in diameter, is present in the Upper Cretaceous Asarcik granitoid (Şebinkarahisar), Northeastern Turkey. The tourmalines also occur in quartz–tourmaline veins up to 4 m thick that cut the Asarcik granitoid. This granitoid was emplaced in the eastern Pontide Metallogenic Belt, where accompanying tourmalines are found in close association with a vein-type, uranium-bearing Pb–Zn–Cu deposit. Tourmaline crystals are generally optically zoned from core to rim with a bluish green core. Electron-microprobe studies show that tourmalines from the Asarcik granitoid and quartz–tourmaline veins display mainly dravite–schorl solid solutions with a tendency to schorl compositions. The tourmaline from the Asarcik granitoid has Fe/(Fe + Mg) ratios from 0.28 to 0.69 (mean = 0.52) and Na/(Na + Ca) ratios from 0.57 to 0.93 (mean = 0.76). In contrast, tourmaline in the quartz–tourmaline veins has Fe/(Fe + Mg) ratio from 0.14 to 0.92 (mean = 0.59) and Na/(Na + Ca) ratio from 0.78 to 1.00 (mean = 0.96). Tourmaline in the veins has a more alkali-deficient trend than that in the granitoid. Substitution mechanisms for the chemical evolution of tourmalines at Asarcik are the MgFe−1, (Fe2+Fe3+)(MgAl)−1, □Al(NaR)−1, AlOR2+−1(OH)−1, □Al2ONa−1R2+−2(OH)−1, Ca0.5□0.5Na−1, CaMg2□−1Al−2, CaMgO□−1Al−1(OH)−1, CaMg3OH□−1Al−3O−1, CaMgNa−1Al−1, CaONa(OH)−1, CaMg2OHNa−1Al−2O−1, CaMg2□−1Al−2, and CaMg3OH□−1Al−3O−1 exchange vectors. Chemical zoning in the tourmalines is consistent with these substitutions. Stoichiometric calculations of electron-microprobe data in the Asarcik granitoid suggest a relatively oxidizing environment during tourmaline crystallization.

Hydrothermal gold mineralization at the Hira Buddini gold mine, India: constraints on fluid evolution and fluid sources from boron isotopic compositions of tourmaline

We determined the boron isotope and chemical compositions of tourmaline from the Hira Buddini gold deposit within the Archean Hutti-Maski greenstone belt in southern India to investigate the evolution of the hydrothermal system and to constrain its fluid sources. Tourmaline is a minor but widespread constituent in the inner and distal alteration zones of metabasaltic and metadacite host rocks associated with the hydrothermal gold mineralization. The Hira Buddini tourmaline belongs to the dravite–schorl series with variations in Al, Fe/(Fe+Mg), Ca, Ti, and Cr contents that can be related to their host lithology. The total range of δ11B values determined is extreme, from −13.3‰ to +9.0‰, but 95% of the values are between −4 and +9‰. The boron isotope compositions of metabasalt-hosted tourmaline show a bimodal distribution with peak δ11B values at about −2‰ and +6‰. The wide range and bimodal distribution of boron isotope ratios in tourmaline require an origin from at least two isotopically distinct fluid sources, which entered the hydrothermal system separately and were subsequently mixed. The estimated δ11B values of the hydrothermal fluids, based on the peak tourmaline compositions and a mineralization temperature of 550°C, are around +1 and +10‰. The isotopically lighter of the two fluids is consistent with boron released by metamorphic devolatilization reactions from the greenstone lithologies, whereas the 11B-rich fluid is attributed to degassing of I-type granitic magmas that intruded the greenstone sequence, providing heat and fluids to the hydrothermal system.

Tourmaline-bearing rocks in the Singhbhum shear zone, eastern India: Evidence of boron infiltration during regional metamorphism

Frozen-in reaction textures combined with mineral chemistry of tourmaline-bearing metamorphic assemblages provide valuable information about fluid-rock interaction during orogenesis. Tourmaline occurs in four distinct mineralogical associations in the Singhbhum Shear Zone (SSZ) of the Precambrian East Indian craton. The tourmaline-bearing rocks are associated intimately with pelitic to psammopelitic and quartzofeldspasthic rocks, including meta-granite. The rocks were affected by two sets of folding (F1 and F2) and ductile shearing associated with F1 during ca. 1.6–1.8 Ga tectonic activity in this belt. Quantitative geothermobarometry and the stability relations of the metamorphic assemblages developed in the pelitic rocks establish the presence of two episodes of metamorphism; a prograde M1 event that culminated at 480 ± 40 °C, 6.4 ± 0.4 kbar, and an M2 event that caused retrogression of the M1 assemblages. Field and petrographic observations in combination with data on chemical composition of tourmaline indicate three stages of tourmaline growth. The earliest stage is represented by pretectonic (pre−metamorphic) greenish-yellow tourmaline cores in the tourmalinite from the Surda area (Association A). The textural relations demonstrate that in the second stage, greenish-yellow cores of oscillatory zoned tourmaline in the metagranite (albite + quartz + biotite + chlorite ± muscovite ± apatite ± magnetite, Association C), very fine tourmaline grains in muscovite schist (muscovite ± quartz, Association B) and greenish yellow cores of tourmaline in biotite-muscovite schist (biotite + muscovite + magnetite + chlorite + apatite+ quartz, Association D) crystallized during the F1-M1 events. Post-tectonic, greenish-blue tourmaline, belonging to the third stage, replaces the earlier tourmaline grains along their margins and cracks. Compositionally, tourmalines of all four associations fall in the alkali group. Most of the data fall in the dravite field with a few analyses straddling the boundary between schorl and dravite. An abrupt compositional change was noted across the different color zones in the tourmaline grains. This change is explained by one or more of the coupled substitutions, □Al(NaR)−1, CaR(NaAl) −1, and Mg(Fe) −1, where R = Mg + Fe2+ + Mn. Integrating the field, petrographic, and phase-compositional data, it has been demonstrated that tourmaline growth in the second and third stages was induced by infiltration-driven, B metasomatism during prograde metamorphism of the rocks. Deep-seated magma was the source of the B-bearing hydrothermal fluid.

Tourmaline chemistry in the Miocene and Paleozoic granites, Central Nepal Himalaya

Tourmalines are found in almost all the formations, from the Lesser Himalaya to the Tibetan-Tethys Himalaya. They are equally found in the Miocene and Paleozoic granites in Central Nepal. Aplite-pegmatite dykes and Miocene granite from the Higher Himalaya, Tibetan-Tethys Himalaya also contain the tourmaline.&#x0D; The chemical composition of tourmalines from the Miocene granite, Paleozoic granite, and aplite pegmatites is presented in schorlite (Fe)-dravite (Mg)-elbaite (Al) diagram. The tourmalines from the granites are schorlitic (rich in Fe) in composition. No compositional variation between rim and core of the tourmaline in augen gneiss of the Formation III of the Higher Himalaya could correspond to the recrystallization of the tourmaline during the Himalayan metamorphism. The tourmalines from Manaslu and Chhokang granites can be evolved from Mg rich composition in higher temperature towards the Fe rich in lower temperature during the crystallization reflected by the XFe variation. Similarly, the composition of tourmalines is found to evolve from dravite (Mg rich) in the aplite-pegmatite dykes of the Higher Himalaya to schorlite (Fe rich) and elbaite (Al) in the aplite-pegmatite dykes of the Annapurna Formation of the Tibetan-Tethys Himalaya and Manaslu granite.&#x0D; The aplite-pegmatite dykes intruded into the pelitic rocks contain Mg rich tourmalines while Fe rich tourmalines are found in calcareous host rocks. So, the composition of the tourmalines of aplite-pegmatites can not be controlled by the composition of host rocks, but it is indirectly controlled by the composition of fluid phase of magma.

Tourmaline chemistry in the metamorphic rocks of the central Nepal Himalaya

Tourmaline occurs in various tectonic units of the Himalaya. It is found in almost all formations, from the Lesser Himalaya to the Tibetan-Tethys Sedimentary Series. They are also found in the Cambro-Ordovician granite of the Kathmandu nappe and Miocene leucogranites of the Higher Himalaya.&#x0D; The chemical composition of tourmalines from the central Nepal Himalaya is presented in schorlite (Fe)-dravite (Mg)­ elbaite (Al) diagram. The variation in chemistry of tourmaline in different types of rock is found to depend on the composition of the host rocks, the grade of metamorphism and the position in the structural edifice. The chemical results show that the Mg-rich tourmaline (dravite) is found to be concentrated in marble, calcic gneiss, whereas schorlite elbaite end members are found in the pelitic gneiss, mica schist, migmatitc, and quartzite, with little increase of elbaite content.&#x0D; The Lesser Himalayan tourmalines show a correlation between their chemistry and the grade of metamorphism of the host rocks. The Mg content of the tourmaline increases whereas Fe and Al contents decrease from chlorite to kyanite isograd. In the chlorite isograd rocks, the tourmalines are detritic in origin and are affected very little by the low-grade metamorphism. With increasing grade of metamorphism, the fluids rich in boron are mobilized in the schist and quartzite, and reacted with aluminium silicate associated elements resulting in the crystallization of the tourmalines. In the Higher Himalayan Crystallines, where the temperature remains constant and the pressures decreases upward in the hanging wall of the MCT, the composition of the tourmaline is found to be evolved with an increase of Fe content and decrease of Mg and Al contents in the higher structural level.

Chemical and boron isotopic composition of magmatic and hydrothermal tourmalines from the Sinceni granite–pegmatite system in Swaziland

The Archean Sinceni granite is a geologically and geochemically well-studied example of the transition from granite to pegmatite, with subsequent development of hydrothermal alteration in metatonalite wallrocks. Tourmaline occurs in all settings of this system, from the granite to the pegmatites and in the hydrothermal rocks. The chemical and B-isotopic composition of tourmalines was studied by electron- and ion microprobe to understand how tourmaline composition varies during the stages of magmatic to hydrothermal development. The chemical composition of tourmaline from each rock type (granite, pegmatite, hydrothermal vein in metatonalite) is distinctive. In terms of their Na/Na+Ca and Fe/Fe+Mg ratios, the tourmalines correlate with the whole-rock ratios of the corresponding host rock. Tourmalines from the pegmatites show significantly higher Al contents than those from the parental granite, and a correspondingly greater degree of alkali-site vacancies (ca. 30% in pegmatite, 15–20% in granite). Tourmalines in granite have a narrow range of isotopic composition ( δ 11 B , mean −15.7‰, σ=1.3), which is considered to approximate the composition of the magma from which they crystallized. Tourmalines from aplitic zones in pegmatite have the same range of boron isotopic composition as the granite (mean −15.2‰, σ=1.7). In contrast, tourmaline from a coarse, zoned pegmatite and hydrothermal tourmalines are isotopically lighter ( δ 11 B values of −16.6 to −23.0‰). These differences are consistent with a model of isotopic fractionation due to fluid exsolution. Tourmaline–fluid fractionation factors allow us to estimate that the exsolved fluid had a δ 11 B composition of about −13‰. The B-isotopic compositions of hydrothermal tourmalines are consistent with formation from this fluid at temperatures of 350 to 500°C, which are in line with temperatures from fluid inclusion microthermometry.

Chemical compositions of tourmaline in the Yindongzi-Tongmugou Pb-Zn deposits, Qinling, China: Implications for hydrothermal ore-forming processes

A microprobe study has been carried out on the chemical composition of tourmaline from the Yindongzi and Tongmugou stratabound Pb-Zn ore deposits, eastern Qinling, China. Tourmaline was analysed from a variety of rock types representative of its various occurrences associated with the ore bodies. All the tourmalines studied here belong to the schorl-dravite series. Most are of hydrothermal origin with Mg > Fe and Na > Ca. Some detrital cores of tourmaline have been recognized from their geometry and chemistry, with Fe > Mg. The chemical trends from core to rim in zoned grains suggest a multi-stage model for the growth of tourmaline and genesis of the ore bodies. The first stage was represented by a more Mg-rich hydrothermal fluid in the submarine hydrothermal system, producing Mg-rich tourmalines by selective replacement of clay-rich sediments close to the sediment-water interface. The second stage was dominated by Fe-rich hydrothermal fluid and resulted in overgrowth of Fe-rich tourmaline rims. This stage also led to the nucleation and growth of new tourmaline crystals and was responsible for the formation of the main massive sulphide orebodies. Finally, a further period of hydrothermal activity or a metamorphic event led to the formation of an additional rim of Mg-rich tourmaline.