Mg-rich Tourmaline Research Articles

Mg-rich mica and tourmaline in pink-colored “parelhas quartzite” intercalations in the equador formation, Borborema pegmatite province, NE-Brazil: record of evaporitic protolith?

Light gray, green to pink-colored banded quartzite intercalations in the NeoproterozoicCEquador Formation (Seridó Group), Seridó Fold Belt, Northeast Brazil, were exploited during several decades as dimension stones under the fashion name “Parelhas Quartzite” (PQ). They can be easily sliced along intensively foliated mm-thick laminae with relatively high content of light green or pink-colored mica, in alternation with more massive, a few cm-thick bands with lower mica contents. The variable ratios between pink and light green micas in the partition laminae, define the color of the produced rock plates, used as outdoor floor or wall coating. Massive, quartzites, meta-arcoses and meta-conglomerates of the Equador Formation (EF) are the exclusive host rock of a few Li-bearing rare element class granitic pegmatites of the Borborema Pegmatite Province, mined for the prized gemstone known as “Paraíba Tourmaline”. The EF hosts also nearly 10% of the Be-Li-Ta-bearing pegmatites of the same province. Pink-colored tourmaline in the form of millimeter- to decimeter-long crystals occur as prominent accessory mineral in the pink-colored PQ and are empirically described by miners and field geologists as “rubellite” and the pink-colored mica as “lepidolite”. The PQ could therefore be considered as a potential source of Lithium. The tourmaline crystals occur isolated or as radial arrangements, always flattened and oriented in the foliation planes. Light green micas are dominant in the greenish-gray PQ and are always referred in the literature as common muscovite. Tourmaline with the same textural relations as the pink-colored counterpart may also occur in the gray PQ plates but in this case is always black. Electron microprobe analyses of the pink mica and tourmaline allowed establish that they are respectively Al-rich phlogopite and magnesiofoitite (this one preferentially occurring in the crystal cores) with transition to dravite and uvite (rims). Both, mica and tourmaline present insignificant lithium contents. The light green micas were identified as the Fe-Mg-rich variety of muscovite known as “phengite”). The PQ can therefore be discarded as possible source of Li for economic exploration. They can also be excluded as source for Li-enrichment in pegmatites by “in situ” anatexis of quartzite or metasomatic assimilation of ascending pegmatite forming melts. There is also no support for the hypothesis of the genesis of the pink-colored mica and tourmaline by pegmatite-related exomorphic alteration. The most probable origin of the B- and Mg-enrichment in the PQ is related to syn-sedimentary evaporitic or volcano-exhalative incursions during the deposition of the original sediments.

Iron, boron, and sulfur isotope constraints on the ore-forming process of subseafloor replacement-style volcanogenic massive sulfide systems

The well-preserved Ashele subseafloor replacement-style volcanogenic massive sulfide (VMS) deposit in the Central Asian Orogenic Belt comprises two stages of Cu mineralization, i.e., early massive sulfides dominated by colloform and euhedral pyrite intergrown with chalcopyrite and sphalerite, which were replaced by late vein-dominated chlorite-chalcopyrite assemblages. In this study, a combined systematic Fe, B, and S isotope investigation was first applied to investigate the sulfide precipitation processes and the relative proportion of fluid sources in different alteration and mineralization stages of the Ashele deposit. Boron isotopes of Mg-rich tourmaline (δ11B from −5.57‰ to −2.73‰, average (avg.) −4.23‰) indicate significant seawater (∼19%) participated during the formation of massive sulfides. A two-component mixing model is used to estimate the contribution of seawater and igneous sulfur to the total sulfur budget, and the results show the increasing contribution of magmatic sulfur from the early (35%) to late (76%) stages. In addition, δ56Fe values of pyrite gradually increase from the massive ore (−0.46‰ to −0.02‰, avg. −0.24‰), quartz-pyrite (−0.09‰ to 0.07‰, avg. −0.01‰), chlorite-chalcopyrite-quartz-pyrite (0‰ to 0.21‰, avg. 0.08‰) to the quartz-sericite zone (−0.02‰ to 0.29‰, avg. 0.14‰), which is likely related to the different extent of isotopic exchange and formation temperature, and could be used in the exploration of VMS systems. The new two-stage ore-forming model shows that in the early stage, rapid mixing of hydrothermal fluid from underlying magma chamber with abundant cold seawater led to rapid deposition of pyrite and associated Cu mineralization under relatively oxidized condition, and long-term hydrothermal activities in relatively closed systems would promote the formation of upper massive ores, which resulted in an equilibrium system between pyrite, chalcopyrite, and associated fluid with wide ranges of δ56Fe in pyrite (−0.46‰ to −0.02‰) and chalcopyrite (−1.56‰ to −0.49‰). The late hydrothermal activities in relatively open system would contribute to stringer sulfides or stockworks underlying the massive ore in relatively reduced conditions with heavier Fe isotope compositions in pyrite (−0.09‰ to 0.29‰) and chalcopyrite (−0.60‰ to −0.04‰). Overall, our study demonstrates that the coupling of B, Fe, and S isotopes could be a useful tool to indicate long-term subseafloor infilling and replacement processes for subseafloor replacement-type VMS deposits, which are the prerequisite to form large-tonnage VMS deposits.

Mineralogical mosaics from the Carpathian-Pannonian region 5

Further mosaic-like data were recorded on the mineral occurrences of the Carpathian–Pannonian region in this fifth member of the series arranged by countries and localities. Each “mosaic” contains a concise mineral description, mainly based on XRPD, SEM-EDX and EPMA measurements and a concise description of the mineral paragenesis. Some minerals are first-time descriptions from the entire discussed region, but all are newly documented occurrences for at least the described locality. From Hungary humboldtine and weddellite are described from the coalbed of Csordakút (Bicske), and data from Sr-rich and other Aluminium Phosphate Sulphate (APS) minerals from the ore mineralization of the Lahóca Hill at Recsk are also reported. Chemical data of Mg-rich tourmaline from the Harghita volcanic ridge are given, and also kyrgyzstanite is described from the ore mineralization of Baia Sprie, Romania. From Slovakia Na-containing sulphates (kröhnkite, ferrinatrite, tamarugite) from the ore mineralization of Farbište (Poniky) and an arsenate (bariopharmacosiderite) from the ore mineralization of Rožňava are also introduced.

Fluid sources in basement-hosted unconformity–uranium ore systems: tourmaline chemistry and boron isotopes from the Patterson Lake corridor deposits, Canada

The Patterson Lake corridor is a new uranium district located on the southwestern margin of the Athabasca Basin. Known resources extend almost 1 km below the unconformity in graphite- and sulfide-bearing shear zones within highly altered metamorphic rocks. Despite different host rocks and greater depths below the unconformity, alteration assemblages (chlorite, illite, kaolinite, tourmaline and hematite), ore grades and textures are typical of unconformity-related deposits. This alteration includes at least three generations of Mg-rich tourmaline (magnesio-foitite). The boron isotopic composition of magnesio-foitite varies with generation: the earliest generation, which is only observed in shallow samples from the Triple R deposit (Tur 1), contain the heaviest isotopic signature ( δ 11 B ≈ 19–26‰), whereas subsequent generations (Tur 2 and Tur 3) yield lighter and more homogeneous isotopic signatures ( δ 11 B ≈ 17.5–19.9‰). These results are consistent with precipitation from low-temperature, NaCl- and CaCl 2 -rich brine(s) derived from an isotopically heavy boron source (e.g. evaporated seawater) that interacted with tourmaline and silicates in the basement rocks and/or fluids derived from depth (with low δ 11 B values). The lower δ 11 B values in paragenetically later magnesio-foitite reflect greater contributions of basement-derived boron over time, whereas minor compositional variations reflect local metal sources (e.g. Cr, V, Ti) and evolving fluid chemistry (decreasing Na and Ca, increasing Mg) over time. The δ 11 B and chemical variation in magnesio-foitite over time reinforce the strong interactions with basement rocks in these systems while supporting incursion of basinal brines well below the unconformity contact. Supplementary material: Complete analytical dataset including reference materials are available at https://doi.org/10.6084/m9.figshare.c.5727555 Thematic collection: This article is part of the Uranium Fluid Pathways collection available at: https://www.lyellcollection.org/cc/uranium-fluid-pathways

Geochemistry of tourmaline of elbaite-dravite series from sapphire bearing pegmatites, proterozoic higher Himalayan Crystalline Complex Jammu and Kashmir, India: Implication for evolution of pegmatite melt

Tourmaline of rare elbaite-dravite series has been reported for the first time from the sapphire bearing pegmatite from the Paddar area, Kishtwar District (Jammu and Kashmir), India. The sapphire-bearing pegmatites are hosted by ultramafic rocks of the Higher Himalayan Crystalline metamorphic complex. These pegmatites are dominantly composed of sodic-plagioclase, perthite, tourmaline, and sapphire, whereas microcline, biotite, and muscovite make up the minor phases. The tourmaline from these pegmatites contains high Mg, Ca, Na, and F content as well as low K, Fe, and Cr content. They are classified as alkali tourmaline, characterized by higher Mg/Mg + Fe and Na/Na + Ca ratios. They display complex substitution mechanisms during the crystallization (Li + Al↔Mg, Fe↔Mg, Na↔Ca, B + Al↔Si) and post crystallization alteration by the hydrothermal fluids. The geochemistry of tourmaline suggests that the tourmaline is crystallized from peraluminous, silicious pegmatitic melt (enriched in Al, Li, B, and H2O). This melt was most likely formed through partial melting of the metasediments of Higher Himalaya Crystalline during the M2 phase of regional metamorphism. The pegmatitic melt was injected in the ultramafics, resulting in bimtasomatism reaction evidenced by the development of systematic alteration zones next to the contact. Bimetasomatism results in the diffusion of the Ca and Mg from host rock to pegmatitic melt, and removal of silica from the pegmatitic melt, which leads to the increase of Al in pegmatitic melt. Finally, a saturation of the Al in the melt with a high Mg influx from host rock results in the favourable environment for crystallization of Mg-rich tourmaline and sapphire.

Characterizing fluids associated with the McArthur River U deposit, Canada, based on tourmaline trace element and stable (B, H) isotope compositions

The origin and nature of fluids responsible for U deposits of the Athabasca Basin have been in debate. This study presents the trace element abundances and stable isotope ratios (D/H and 11B/10B) of well characterized tourmaline samples determined by in situ analytical techniques, in order to evaluate the nature of fluids during the mineralization of the McArthur River U deposit, the world's largest high-grade U deposit. Magnesio-foitite, an alkali-deficient Mg-rich tourmaline, is abundant along the 13-km long P2 fault, a reverse structure that controls the location of the McArthur River deposit. Magnesio-foitite contains variable concentrations of rare earth elements (REEs, up to 46ppm in total) and Y (up to 35ppm). Chondrite normalized patterns indicate low light REEs (LREEs) relative to heavy REEs (HREEs) (CeN<YN, Y as a proxy for HREEs) and slight negative Eu anomalies, similar to those of uraninite from the deposit. The data combined with textural evidence suggest that magnesio-foitite and uraninite co-crystallized with LREE-rich aluminum phosphate sulfate minerals. Relatively high REE contents of magnesio-foitite in the ore zone indicates the incursion of a REE-rich basement fluid. Low values of δD (−98 to −41‰) for magnesio-foitite suggests that the mineralizing fluid originated from groundwater. Variably high values of δ11B (+13.1 to +23.2‰) are explained by the dissolution of B from carbonate or evaporitic rocks and preferential removal of 10B by the crystallization of illite and kaolinite. The results of this study support the ascent of a REE-rich basement fluid during mineralization and extensive modification of basinal fluids through the crystallization of clay minerals.

Geology, mineral chemistry and tourmaline B isotopes of the Córrego Bom Sucesso area, southern Serra do Espinhaço, Minas Gerais, Brazil: Implications for Au–Pd–Pt exploration in quartzitic terrain

The Palaeo-Mesoproterozoic metasiliciclastic rocks of the southern Serra do Espinhaço, Minas Gerais, Brazil, are host to historically important alluvial deposits of diamonds and gold. Detrital gold grains often comprise Au–Pd–Pt intermetallic compounds, with low Ag contents, which contain inclusions of tourmaline and titaniferous hematite (up to ~ 6 wt.% TiO 2). The latter minerals connect the alluvial mineralisation to the rutile–hematite–quartz veins and tourmalinisation observed in the quartzitic country rocks of the alluvial gravel. The quartzite (Sopa-Brumadinho Formation of lacustrine to fan-deltaic origin) is affected by pervasive B metasomatism with F-bearing tourmaline replacing the recrystallised quartz fabric. The tourmaline belongs to the alkali group, with Mg/(Mg + Fe) and X/(X + Na) ratios in the ranges from 0.5 to 0.7 and 0.18 to 0.29, respectively, where X represents vacancies in the X site. Boron-isotopic values of tourmaline vary from ~ 1 to − 10.4‰ δ 11B. The B-isotope range, in conjunction with the Na–Mg-rich tourmaline composition, and the widespread occurrence of tourmalinite in the Sopa-Brumadinho Formation suggest a derivation from non-marine evaporitic brines. Brines are capable of transporting otherwise immobile Ti and explain, under oxidising conditions, the fractionation of Ag from Pd to precipitate palladiferous gold with extremely high Pd/Ag ratios. Zirconium-in-rutile and Ti-in-quartz temperatures for a variety of hematite-rich veins suggest episodic vein emplacement over a temperature range from around 500 °C to ~ 350 °C. Cross-cutting relationships and episodic vein emplacement indicate a late-Brasiliano age.

Aluminous and alkali-deficient tourmaline from the Singhbhum Shear Zone, East Indian shield: Insight for polyphase boron infiltration during regional metamorphism

In the western part of the Singhbhum Shear Zone (SSZ), East Indian Shield, borosilicate-bearing veins of variable thickness (tens of micrometers to 1 m thick) are hosted in kyanite-quartzite and kyanite-mica schist. The veins have been classified into three types, which are, from oldest to youngest, generation I (tourmaline), II (dumortierite + tourmaline), and III (tourmaline) veins. Alkali- and Mg-rich tourmaline [ X Mg = Mg/(Mg + Fe) = 0.68 ± 0.09; X = Na, Ca, K, □ (vacancy) = 0.40 ± 0.12] is the sole borosilicate in generation I veins, which have been folded in response to regional deformation. Generation II veins were emplaced along shear bands (1 mm to 1 m thick) developed parallel to the axial planes of these folds. Long axes of fibrous dumortierite and prismatic tourmaline of generation II veins are oriented along the shear bands and have been bent around lenticular remnants of host kyanite-quartzite. Generation III veins have a dendritic pattern, crosscut generation II veins and show aggregates of fibrous to acicular tourmaline. Prismatic tourmaline in generation II veins is optically zoned with a green tourmaline core that is variably replaced and rimmed by blue tourmaline. Fibrous to acicular tourmaline in generation III veins is comprised up of blue tourmaline with compositions similar to the rim composition of prismatic tourmaline in generation II veins. Green and blue tourmaline is aluminous (Al total >7 apfu) and alkali-deficient (X = 0.71 ± 0.08). High Y Al content, high X, low X Mg (0.19 ± 0.10), and excess cation charge indicate tourmaline in generation II veins is rich in an “oxy-foitite” component. Foitite-rich tourmaline in generation III veins has tetrahedral Al and a slightly lower Mg-content and X than those of generation II veins. Optical zoning in prismatic tourmaline corresponds to an abrupt compositional change with paragenetically older green tourmaline having higher Al and X Mg , but lower alkali content in the X-site than the blue tourmaline rim. The compositional variation in green and blue tourmaline can be explained by a combination of coupled substitutions represented by AlO[R(OH)] −1 and Al(NaR) −1 , where R = (Fe 2+ + Mg). Pseudosections in the system Na 2 O-K 2 O-Al 2 O 3 -SiO 2 -H 2 O constructed from bulk chemical compositions of the studied rocks and the P-T slopes of two isochors computed from brine-rich inclusions trapped in quartz grains indicate that borosilicate formation in generation II and III veins occurred within 4.1 ± 0.5 kbar and 377 ± 21 °C. The mineral assemblages and textures suggest that the borosilicate-bearing veins formed from infiltration-driven alteration of host kyanite-quartzite and kyanite-mica schist along structurally controlled conduits by more than one batch of chemically distinct boron-rich aqueous fluids.

Crystal structure analyses of four tourmaline specimens from the Cleopatra's Mines (Egypt) and Jabal Zalm (Saudi Arabia), and the role of Al in the tourmaline group

Fe-rich “oxydravite” and dravite from the Late Proterozoic ophiolitic mélange of the Arabo-Nubian Shield, located in Egypt and Saudi Arabia, were structurally and chemically characterized by using crystal structure refinement based on single-crystal X-ray diffraction data, electron microprobe analysis, and Mössbauer spectroscopy. Structural formulae obtained by optimization procedures indicate disordering of Al, Mg, and Fe 2+ over the Y and Z sites, and an ordering of Fe 3+ at Y. The disordering can be explained by the substitution mechanisms 2 Y Mg+ Z Al+ W OH = 2 Y Al+ Z Mg+ W O 2 - and 2 Y Fe 2+ + Z Fe 3+ + W OH = 2 Y Fe 3+ + Z Fe 2+ +WO 2- , which are consistent with reducing the mismatch in dimensions between YO 6 and ZO 6 octahedra. To explain the Mg-Al disordering process, as well as the occurrence of B at the T site in tourmaline, analogies have been drawn between the crystal structure of tourmaline and that of lizardite. A critical constraint in both structures is the geometrical fit of the six-membered tetrahedral ring with the attached group of three YO 6 octahedra. In tourmaline, the disordering of Mg and Al over Y and Z relieves the strain due to the misfit in dimensions of the larger triads of edge-sharing MgO 6 octahedra and the smaller Si 6 O 18 tetrahedral rings. In Al-rich tourmaline, where the octahedral cluster is smaller, the strain can be relieved by incorporating B in the tetrahedra. An opposite effect is observed by substitution of Al for Si at the tetrahedral site in Mg-rich tourmaline. Because the Al radius is intermediate between those of Mg and Si, Al plays an important structural role in accommodating the potential misfit between YO 6 , ZO 6 , and TO 4 polyhedra. The amount of Al and its distribution in the structure strongly affects the values of the unit-cell parameters of tourmaline and yields volume variations according to a quadratic model. This results from the effect of Z Al combined with the occurrence of B at T in Al-rich tourmaline. Z Al has a greater effect than Y Al as long as Al does not fully occupy the Z site.

Li-bearing, disordered Mg-rich tourmaline from a pegmatite-marble contact in the Austroalpine basement units (Styria, Austria)

Pale-blue to pale-green tourmalines from the contact zone of Permian pegmatites to mica schists and marbles from different localities of the Austroalpine basement units (Rappold Complex) in Styria, Austria, are characterized. All these Mg-rich tourmalines have small but significant Li contents, up to 0.29 wt% Li2O, and can be characterized as dravite, with FeO contents of ~ 0.9–2.7 wt%. Their chemical composition varies from X (Na0.67Ca0.19 K0.02☐0.12) Y (Mg1.26Al0.97Fe2+ 0.36Li0.19Ti4+ 0.06Zn0.01☐0.15) Z (Al5.31 Mg0.69) (BO3)3 Si6O18 V (OH)3 W [F0.66(OH)0.34], with a = 15.9220(3), c = 7.1732(2) A to X (Na0.67Ca0.24 K0.02☐0.07) Y (Mg1.83Al0.88Fe2+ 0.20Li0.08Zn0.01Ti4+ 0.01☐0.09) Z (Al5.25 Mg0.75) (BO3)3 Si6O18 V (OH)3 W [F0.87(OH)0.13], with a = 15.9354(4), c = 7.1934(4) A, and they show a significant Al-Mg disorder between the Y and the Z sites (R1 = 0.013–0.015). There is a positive correlation between the Ca content and distance for all investigated tourmalines (r ≈ 1.00), which may reflect short-range order configurations including Ca and Fe2+, Mg, and Li. The tourmalines have XMg (XMg = Mg/Mg + Fetotal) values in the range 0.84–0.95. The REE patterns show more or less pronounced negative Eu and positive Yb anomalies. In comparison to tourmalines from highly-evolved pegmatites, the tourmaline samples from the border zone of the pegmatites of the Rappold Complex contain relatively low amounts of total REE (~8–36 ppm) and Th (0.1–1.8 ppm) and have low LaN/YbN ratios. There is a positive correlation (r ≈ 0.91) between MgO of the tourmalines and the MgO contents of the surrounding mica schists. We conclude that the pegmatites formed by anatectic melting of mica schists and paragneisses in Permian time. The tourmalines crystallized from the pegmatitic melt, influenced by the metacarbonate and metapelitic host rocks.

Metamorphic Na- and OH-rich disordered dravite with tetrahedral boron, associated with omphacite, from Syros, Greece: chemistry and structure

Metamorphic Fe-bearing dravite from a glaucophane schist from Syros, Greece, associated with omphacite, has been characterized by chemical analyses (EMPA, SIMS, Mossbauer study) and by crystal structure determination. The optimized formula, calculated using chemical and structural data (including from Mossbauer spectroscopy) is x (Na 0.96 Ca 0.02 □ 0.02 ) Y (Mg 1.29 Al 0.99 Fe 2+ 0.44 Fe 3+ 0.19 Ti 0.05□ 0.04 ) Z (Al 4.90 Mg 1.10 ) T (Si 5.83 B 0.17 ) B 3 O 27 [(OH) 3.93 F 0.07 ], with a = 15.9443(3), c = 7.2094(3) A, R = 0.017. The OH- content is nearly 4 apfu , thus there is no significant O 2− at the V and W site. The X site is nearly completely filled with Na, contrary to most natural tourmalines. We conclude that tourmaline samples from the dravite-schorl series where the O1 site ( W site) is mainly occupied by OH- and/or F, but not by O 2− , favour an X -site occupation with Na. Mg and Al are strongly disordered in this tourmaline sample. This Mg-rich tourmaline is an unusual example of Al-Mg disorder which is not driven by the short-range requirements of O 2− at the O 1 site. Surprisingly, we found small amounts of [4] B in this Mg-rich dravite. This is confirmed by the chemical analysis (including light elements), by the refinement (∼ 0.26 [4] B apfu ), as well as by the relatively small T -O> distance of 1.6179 A. This is the first known example of a Mg-rich tourmaline which contains significant amounts of [4] B. The formation of this dravite (at PT conditions of ∼ 6 to 7 kbar/∼ 400°C) took place in a subduction-exhumation environment at undersaturated SiO 2 -conditions.

DISORDERED Mg-BEARING OLENITE FROM A GRANITIC PEGMATITE AT GOSLARN, AUSTRIA: A CHEMICAL, STRUCTURAL, AND INFRARED SPECTROSCOPIC STUDY

The core and rim of a short prismatic dark brown tourmaline crystal (10 mm in diameter) from a granitic pegmatite near Goslarn, Lower Austria, were structurally and chemically characterized, including H2O determination and Mossbauer analysis. The optimized formula, calculated using chemical and structural data for the core, is X (Na0.64Ca0.11K0020.23) Y (Al1.58Mg0.91Fe 2+ 0.41Ti0.10) Z (Al4.90Mg1.10) T (Si6.00) B3O27 [(OH)3.07O0.90F0.03], with a 15.9149(3), c 7.1757(2) A, R = 0.014. The optimized formula for the rim is X (Na0.71Ca0.10K0010.18) Y (Al1.42Mg1.10Fe 2+ 0.39Ti0.09) Z (Al4.94Mg1.06) T (Si5.95Al0.05) B3O27 [(OH)3.05O0.92F0.03], with a 15.9238(4), c 7.1822(2) A, R = 0.014. Mg and Al are highly disordered in this sample of tourmaline, and the O1 site is occupied by ~[O0.9(OH)0.1]. Such samples of magnesian olenite provide a very good example of disorder driven by the short-range requirements of O 2– at the O1 site that is associated with the ~2Al + Mg configuration at the coordinating Y sites. To date, these tourmaline samples have the highest Al content at the Y site (up to ~1.6 apfu) for Mg-rich tourmaline. Three quadrupole-splitting distributions were used to represent the contributions of Y-site Fe 2+ in the Mossbauer spectrum. The absence of Fe 3+ reflects the low fugacity of oxygen in this pegmatite. The infrared (IR) spectrum shows four bands. These bands, at ~4528, 4457, 4358 and 4241 cm –1 , are combinations of OH stretching + MOH bending motions. In the fundamental OH region, only three bands at ~3737, 3630 (sh) and 3552 cm –1 are clearly observed. The relatively sharp bands and the very low R-values of the crystal-structure refinements indicate a high degree of crystallinity. The strong band at ~3552 cm –1 can be assigned to O–H3 stretching, which is in agreement with an occupancy of ~(OH)3 at the V site. The band at ~3630 cm –1 , which can be assigned to O–H1 stretching, is relatively small. This is in agreement with the occupancy of ~[O0.9(OH)0.1] at the W site. We assess the assignment of Mg-rich tourmaline to dravite or olenite, and different methods to approximate the Al–Mg order, where there is no possibility for a single-crystal determination of the structure.

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.

Estimation and testing of standard molar thermodynamic properties of tourmaline end-members using data of natural samples

Different estimation methods have been combined with calorimetric data for a natural Mg-rich tourmaline and dumortierite, a structurally similar borosilicate, to estimate thermodynamic properties of five tourmaline end-members (dravite, schorl, alkali-free dravite, aluminofoitite, and ferrifoitite). The results were tested by predicting the solubility of Fe-rich tourmaline in natural, low-variance hydrothermal mineral assemblages, for which the B concentration was independently known from Laser Ablation-ICP-MS analysis of coexisting fluid inclusions. Agreement within the uncertainty of experimental and thermodynamic data supports the validity of the estimation methods that used the fictive properties of the IIIB2O3 polyhedron for calculating standard enthalpy and entropy, and reactions involving dumortierite to estimate the heat-capacity functions. This study indicates more generally that the development of new microanalytical techniques for measuring the composition of natural fluid inclusions in well-constrained mineral assemblages represents a promising approach to estimating or testing thermodynamic data for complex silicates.

Paragenesis and chemistry of multistage tourmaline formation in the Sullivan Pb-Zn-Ag deposit, British Columbia

Detailed petrographic study, scanning electron microscope imaging, and electron microprobe analyses of tourmalines from the Sullivan Pb-Zn-Ag massive sulfide deposit (British Columbia, Canada) document multiple paragenetic stages and large compositional variations. The tourmalines mainly belong to two common solid-solution series: dravite-schorl and dravite-uvite. Ca- and Fe-rich feruvite and alkali-deficient tourmalines are present locally. Products of tourmaline-forming stages include (from oldest to youngest): (1) rare Fe-rich dravite-schorl within black tourmalinite clasts in footwall fragmental rocks; (2) widespread Mg-rich, very fine grained, felted dravite in the footwall (the main type of tourmaline in the footwall tourmalinite pipe); (3) recrystallized, Fe-rich dravite-schorl (locally Ca-Fe feruvite) in the tourmalinite pipe, which preferentially occurs near postore gabbroic intrusions; (4) Mg-rich dravite or uvite associated with chlorite-pyrrhotite and chlorite-albite-pyrite-altered rocks in the shallow footwall and hanging wall; (5) discrete Mg-rich tourmaline grains associated with chlorite and discordant Mg-rich tourmaline rims which occur on disseminated Fe-rich schorl in the bedded Pb-Zn-Ag ores. The timing of rare Fe-rich schorl in the bedded ores is uncertain, but it most likely occurred during or between stages 2 and 3.The different paragenetic stages and their respective tourmaline compositions are interpreted in terms of a multistage evolution involving contributions from: (1) variable mixtures of synsedimentary, Fe-rich hydrothermal fluids and entrained seawater; (2) postore, Fe-rich, gabbro-related hydrothermal fluids; and (3) postore metamorphic reactions. Early synsedimentary, Fe-rich hydrothermal fluids which contained little or no entrained seawater formed Fe-rich black tourmalinite clasts locally in the footwall. The major type of tourmaline in the footwall tourmalinite pipe is Mg rich, recording seawater entrainment under high water/rock conditions, rather than control by the chemical composition of the original host sediments. Rare Fe-rich schorl within the bedded Pb-Zn-Ag ores is believed to have formed on the sea floor by reaction of an Fe-rich brine pool with detrital aluminous sediments. Postore emplacement of gabbro sills and local dikes in the footwall produced Fe-rich hydrothermal fluids, which were responsible for formation of minor Fe-rich dravite-schorl which overprinted earlier dravite. Postore, but synsedimentary, hydrothermal alteration involving entrained seawater was responsible for deposition of dravite and uvite in the hanging wall and for dravite in the brown tourmalinites of the shallow footwall. Mg-rich dravite-uvite associated with chlorite and in discordant rims on schorl in the bedded ores formed by sulfide-silicate reactions during greenschist facies regional metamorphism.

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.

Mineralogic reaction zones at a calc-silicate/metapelite interface: an example of trace element mobility in a metamorphic environment

AbstractMetasomatic interaction on a cm scale between calc-silicate pods and the enclosing sillimanite + biotite + tourmaline gneiss at Partridge Breast Lake, northern Manitoba, Canada, led to the development of an inner (by calc-silicate rock), hornblende-rich reaction zone and an outer, biotite-rich zone. The boundary between the reaction zones is interpreted as the original calc-silicate/metapelite interface. Compared with its metapelitic protolith, the biotite zone shows a two- to twenty-fold depletion in the concentrations of incompatible trace elements (notably the light rare earths, U, Th, Nb, Ta, Zr and Hf). In contrast, the relative concentrations of trace elements remained nearly constant during the mineralogical transformation of the calc-silicate rock to the hornblende zone. The depletion of trace elements in the biotite zone is attributed to the dissolution of accessory phases (e.g. monazite). Although stable at the metamorphic conditions (∼600–650°C at ∼ 4.5 kbar) prevalent during metasomatism, Mg-rich tourmaline is absent in the biotite zone, suggesting that either the pH or composition (e.g. the (Al + Si)/(Ca + Mg + Fe) ratio) of the aqueous fluid phase was inappropriate for the preservation of this mineral.

Metamorphism of magnesium-altered felsic volcanic rocks from Bergslagen, central Sweden. A transition from Mg-chlorite- to cordierite-rich rocks

The ca. 1.9 Ga old felsic volcanic rocks of the Bergslagen province have been affected by synvolcanic hydrothermal Mg-metasomatism. A spatial relationship exists between Mg-alteration zones and calc-silicate/carbonate banded-iron mineralizations and a genetic link is suggested. This is supported by the occurrence of the Mg-rich tourmaline dravite, which exists in the Mg-alteration zones as well as in the wall-rocks to iron-oxide mineralizations. Following later regional metamorphism and deformation, these rocks occur as large zones of Mg-rich schists parallel to sub-parallel with the regional foliation (S1). The dravite crystals have been affected by this regional deformation. A second deformational episode folded S1 along steeply dipping fold axes. Locally, this resulted in a crenulation cleavage and the development of a new generation of micas (S2) crosscutting S1. In the westernmost areas the Mg-schists contain a fine-grained network of Mg-chlorite and muscovite with little or no preferred orientation, indicating low pressures and low-grade metamorphic conditions. The grade of recrystallization and deformation increase towards the east and a boundary between low- and medium-grade metamorphic terrains is crossed. The metamorphic boundary is marked by the disappearance of chlorite and the appearance of cordierite in the Mg-schists. Porphyroblasts of Mg-chlorite or cordierite grew across S1 and cordierite porphyroblasts are transected by S2 micas. This indicates that the peak of regional metamorphism occurred between the two deformational episodes. Retrogression of cordierite to chlorite and muscovite is confined to the vicinity of crosscutting S2 micas.

Tourmalines from Appalachian-Caledonian massive sulfide deposits; textural, chemical, and isotopic relationships

Tourmaline is a common gangue mineral in several strata-bound massive sulfide deposits of the Appalachian-Caledonian orogen. Most of the tourmaline-rich deposits occur in dominantly metasedimentary terranes such as Bleikvassli, Norway; Black Hawk, Maine; Ore Knob, North Carolina; and Elizabeth, Vermont. Minor amounts of tourmaline are also known in association with volcanic-hosted deposits in the Appalachians and in the Trondheim district of Norway. The tourmaline forms isolated euhedral crystals or crystal aggregates in massive sulfide and adjacent wall rocks; in a few areas, it is found in conformable tourmaline-rich layers (tourmalinites). Associated minerals may include quartz, plagioclase, phlogopite, and dolomite, with pyrrhotite, chalcopyrite, galena, and sphalerite. Many of the tourmaline crystals are optically zoned, in which brown zones concentrically alternate with pale yellow or green zones. A few specimens of tourmaline intergrown with massive sulfide (e.g., Ore Knob) display fine, delicate growth lamellae similar to those commonly observed in hydrothermal (vein) sphalerites, skarn garnets, and sphalerites from the massive sulfide deposits of the Kuroko district, Japan.Detailed geochemical studies indicate that the tourmalines have characteristic major and trace element compositions. Electron microprobe analyses show that, with rare exception, the tourmalines are Mg-rich dravites (avg. FeO/(FeO + MgO) = 0.21). Abrupt transitions between color zones within some tourmaline are accompanied by relative variations in FeO/(FeO + MgO) of as much as 30 percent; yellow-brown zones are more Mg rich than blue or green zones. In contrast, the fine growth lamellae are not characterized by such variations in chemistry. Spectrographic analyses of pure mineral separates indicate that the tourmalines also contain high average contents of Cr (78 ppm), Cu (77 ppm), Pb (114 ppm), Sr (98 ppm), and V (195 ppm), while uniformly low in Li (7 ppm) and Mn (382 ppm). In all analyzed samples, fluorine is much more abundant than chlorine; the average F/Cl ratio is 33.Tourmalines from tourmalinites and massive sulfide deposits are typically characterized by heavier delta 18 O values (9.5-15.5ppm) and less variable, heavier delta D values (generally -45 to -60ppm) than tourmalines from unmetamorphosed igneous pegmatites. The latter typically have delta 18 O < 9.5 per mil and delta D < or = -60 per mil. Oxygen isotope fractionations between quartz and tourmaline (delta = 1.27-4.01) do not correlate directly with inferred grade of subsequent metamorphism, although in areas of high metamorphic grade the delta values approach those of igneous quartz-tourmaline pairs. The delta 18 O values of quartz and tourmaline are believed to reflect variation in the oxygen isotope compositions of the host lithologies. The similar, narrow range in hydrogen isotope composition for tourmaline in all studied deposits is compatible with a modified seawater origin for the tourmaline- and sulfide- forming fluids.The distinctive textural and chemical characteristics of the tourmalines reported here have both exploration and genetic significance. Though it may have undergone recrystallization and some accompanying chemical and isotopic exchange during regional metamorphism, tourmaline in the massive sulfide deposits is an early formed, perhaps diagenetic, mineral in which fine lameliar zoning may be a preserved premetamorphic feature. In any case, the tourmalines from the massive sulfide deposits are Mg-rich dravites with trace element and stable isotope compositions substantially different from those of the Fe-rich schorls typical of most granitic rocks. These geochemical and isotopic signatures are attributed to the original environment of formation of the associated sulfide deposits and suggest that Mg-rich tourmaline may be a valuable prospecting guide in the search for massive sulfide deposits.