Dravitic Tourmaline Research Articles

Accretionary and collisional processes of the Brasiliano Orogeny recorded by the Tonian La Tuna ophiolite, Dom Feliciano Belt, Uruguay

Accretionary to collisional orogenies such as the Brasiliano Orogeny of eastern South America present the opportunity to distinguish between oceanic-continental (accretionary) and continental (collisional) processes. However, the multitude of geotectonic processes involved and the reduced degree of preservation of the accretionary stage restrain the interpretation of evidence. This study focuses on a metasomatite associated with the La Tuna Ophiolite in the southern Dom Feliciano Belt of the Uruguayan Shield, as these can provide crucial insights into the accretionary stage of the Brasiliano Orogeny. The metasomatite occurs as a meter-sized massive tourmalinite body surrounded by a meter-wide chloritite lens, both enclosed by the La Tuna serpentinite. Our SEM-EDS and EPMA data revealed that the serpentines from the La Tuna ophiolite are antigorite and lizardite, while chlorite was identified as clinochlore. Abundant inclusions of sheridanite chlorite in dravite tourmaline suggests that tourmaline overgrew chlorite. Dravite crystals revealed the presence of three diffuse zones with minor compositional variations, mainly enrichments in Ca and Fe towards the rims. The U-Pb LA-ICP-MS dating of metasomatic zircon yielded ages of 743.2 ± 3.4 Ma (n = 22) for the chloritite and 743.1 ± 2.4 Ma (n = 31) for the tourmalinite. High δ11B values (+12.1‰ to + 18.1‰) of tourmaline (LA-ICP-MS) suggest that the metasomatic event was caused by seawater-derived fluids in a fluid-dominated environment. The single metasomatic U-Pb age combined with the high δ11B values suggest that the metasomatites underwent more intense metasomatic events than those associated with the Bossoroca (−8.5‰ to +1.8‰) and the Ibaré ophiolites (+3.2‰ to +5.2‰) further north in the São Gabriel Terrane of the Dom Feliciano Belt. Nonetheless, a few spots in tourmaline located within fracture zones and in contact with chlorite displayed negative (c. −5‰) and low positive δ11B values (c. 2‰). These are interpreted as overprinting by crustal fluids, either during the tectonic emplacement of the La Tuna Ophiolite or during exhumation. The calculated TDM Hf model ages for zircon in tourmalinite and chloritite range from 0.74 to 1.09 Ga and their ɛHf(T) values range from +4.9 to +11.2, indicating derivation from depleted mantle sources. The data corroborate earlier results indicating that the associated La Tuna amphibolites have composition similar to N-MORB. Furthermore, the trace element composition of zircon in the metasomatites is consistent with an oceanic environment (U/Yb < 0.1). Geotectonically, the La Tuna metasomatites formed at 743 ± 2 Ma in an oceanic setting, most likely in a juvenile Adamastor Ocean basin. During the collisional stage of the Brasiliano Orogeny, the La Tuna Ophiolite was tectonically emplaced within the <666 Ma La Micaela Schist of the Paso del Drágon Complex. As such, this study on the La Tuna Ophiolite contributes direct evidence on the oceanic evolution leading up to the accretionary stage of the Brasiliano Orogeny and provides new constraints on the transition towards the collisional stage.

Occurrence of dravitic tourmaline in a diamond-bearing breccia: a possible lamproite deposit in the Alto Paranaíba Igneous Province

The mineral chemistry of diamond indicator minerals is used to trace mineralizations in primary rocks such as lamproites/kimberlites and in secondary deposits. Subcalcic garnets and magnesian ilmenites are both minerals associated with the diamond potential described in many deposits in the Alto Paranaíba Igneous Province where several kimberlitic pipes and alluvial deposits occur. Our study was conducted on the Romaria diamond deposit of (Brazil), where garnet, ilmenite and tourmaline grains were recovered from a diamon-bearing breccia were analyzed under electron microprobe. The garnet composition is compatible with G10, G9 and G5 types. Ilmenite grains are Mg- to Mn-rich. Nearly 80% of the tourmaline grains are dravite-type. This association of dravite tourmaline with diamond in rocks of lamproitic affinity may be common, as evidenced in the Argyle, Ellendale, Prairie Creek, Sask, Presidente Olegário and Ymi-1 pipes. We therefore suggest a possible lamproite origin for the studied diamond deposit.

Guidelines for assessing the provenance of Mesozoic and Cenozoic clastic successions sourced by pre-Jurassic basement complexes in southernmost North America

ABSTRACT Mexico is an attractive place for provenance studies focused on reconstructing the tectonic evolution of North America. This is because Mexico hosts a well-preserved clastic record associated with some of the major Mesozoic and Cenozoic tectonic processes that shaped the face of this continent. However, the available information on Mexican pre-Mesozoic source terranes is presently insufficient for provenance analysis. With the aim of drawing the guidelines for provenance determination, we present here detrital modes, geochemical data, and zircon U-Pb ages for detritus derived from pre-Jurassic basement complexes of Mexico. Our data show that the various basement complexes produce distinctive detrital modes and supply diagnostic and compositionally different detrital heavy minerals that represent powerful provenance tracers. The Oaxacan Complex, Ayú Complex, and East Mexico Arc are the main sources of quartzo-feldspathic and feldspatho-quartzose detritus. Quartz with rutile needles, mesoperthitic K-feldspar, orthopyroxene, augitic to diopsidic clinopyroxene, and Mg- to Ca-rich almandine (Alm71–52Grs7–3Prp43–23Sps3–1Alm74–56Grs21–19Prp23–2Sps5–2) are common minerals in detritus from the Oaxacan Complex. The Ayú Complex supplies detritus marked by the occurrence of sagenitic biotite and white mica, as well as Mn-rich almandine (Alm69–66Grs4–3Prp18–11Sps19–10). Detritus from the East Mexico Arc contains any of these mineral phases ubiquitous in the Oaxacan and Ayú complexes. The Acatlán Complex is the main source of detritus dominated by metamorphic lithic grains and quartz, with minor amounts of feldspar. Lithic grains are rank 2–4 metabasitic, metapelitic, and metapsammitic–metafelsitic fragments. Diagnostic mineral phases are schorl–dravitic tourmaline, Na-amphibole, and helycitic garnet varying from a Ca- to Mn-rich almandine (Alm74–55Grs34–15Prp16–3Sps12–1-Alm70–46Grs20–15Prp3–1Sps32–12). Zircon U-Pb geochronology proves to have some virtues but also major limitations because: 1) the zircon U-Pb age signature of many different sources in Mexico is similar and 2) zircon documents a limited number of sources because of variations in zircon fertility.

Origin of Monte Rosa whiteschist from in-situ tourmaline and quartz oxygen isotope analysis by SIMS using new tourmaline reference materials

Abstract A series of tourmaline reference materials are developed for in situ oxygen isotope analysis by secondary ion mass spectrometry (SIMS), which allow study of the tourmaline compositions found in most igneous and metamorphic rocks. The new reference material was applied to measure oxygen isotope composition of tourmaline from metagranite, meta-leucogranite, and whiteschist from the Monte Rosa nappe (Western Alps). The protolith and genesis of whiteschist are highly debated in the literature. Whiteschists occur as 10 to 50 m tube-like bodies within the Permian Monte Rosa granite. They consist of chloritoid, talc, phengite, and quartz, with local kyanite, garnet, tourmaline, and carbonates. Whiteschist tourmaline is characterized by an igneous core and a dravitic overgrowth (XMg &gt; 0.9). The core reveals similar chemical composition and zonation as meta-leucogranitic tourmaline (XMg = 0.25, δ18O = 11.3–11.5‰), proving their common origin. Dravitic overgrowths in whiteschists have lower oxygen isotope compositions (8.9–9.5‰). Tourmaline in metagranite is an intermediate schorl-dravite with XMg of 0.50. Oxygen isotope data reveal homogeneous composition for metagranite and meta-leucogranite tourmalines of 10.4–11.3‰ and 11.0–11.9‰, respectively. Quartz inclusions in both meta-igneous rocks show the same oxygen isotopic composition as the quartz in the matrix (13.6–13.9‰). In whiteschist the oxygen isotope composition of quartz included in tourmaline cores lost their igneous signature, having the same values as quartz in the matrix (11.4–11.7‰). A network of small fractures filled with dravitic tourmaline can be observed in the igneous core and suggested to serve as a connection between included quartz and matrix, and lead to recrystallization of the inclusion. In contrast, the igneous core of the whiteschist tourmaline fully retained its magmatic oxygen isotope signature, indicating oxygen diffusion is extremely slow in tourmaline. Tourmaline included in high-pressure chloritoid shows the characteristic dravitic overgrowth, demonstrating that chloritoid grew after the metasomatism responsible for the whiteschist formation, but continued to grow during the Alpine metamorphism. Our data on tourmaline and quartz show that tourmaline-bearing white-schists originated from the related meta-leucogranites, which were locally altered by late magmatic hydrothermal fluids prior to Alpine high-pressure metamorphism.

Myrmekitic intergrowth of tourmaline and quartz in eclogite-hosting gneisses of the Tso Morari ultrahigh-pressure metamorphic terrane (Eastern Ladakh, India): a possible record of high-pressure conditions

Abstract Alkaline schorlitic tourmaline with domains of myrmekitic quartz and tourmaline intergrowths is reported for the first time from quartzo-feldspathic gneisses of the Tso Morari Crystalline Complex (TMCC), eastern Ladakh Himalaya. Except for schorlitic tourmaline, the brown-green dravitic tourmaline occurs in melanocratic layers of the gneiss. The schorlitic tourmaline contains REE-rich apatite, which is a typical mineral formed under high-pressure (HP) conditions. The observed myrmekite, marked by vermicular quartz and tourmaline intergrowths, was probably formed during decompression as a consequence of excess silica released from recrystallized tourmaline. The recalculated composition of the tourmaline with quartz myrmekite suggests that Si also occupied the tourmaline octahedral Z site during the HP regime. During decompression excess Si from this tourmaline was replaced by Mg and Fe 3+ . At an early stage of exhumation needle-shaped schorlitic tourmaline II and mosaic zoning were formed. The excess of silica and the structural disorder suggest that the Si-oversaturated tourmaline was stable at high-pressure–ultrahigh-pressure (HP–UHP) conditions. The greater stability of dravitic tourmaline compared to schorlitic tourmaline at HP conditions is evidently recorded at the TMCC. The tourmaline-bearing gneisses of the TMCC most probably shared the same metamorphic conditions during Tertiary collision of the Indian and Eurasian plates, similar to that observed for the associated UHP eclogites.

Petrography and geochemistry of tourmaline breccia in the Longtoushan Au deposit, South China: genesis and its exploration significance

The Longtoushan Au deposit, located in the Qin-Hang Metallogenic Belt, South China, is a breccia-hosted epithermal Au deposit. It is characterized by the distinctive occurrence of magmatic-hydrothermal tourmaline-cemented breccia. Tourmaline-group minerals have an extensive range of chemical composition that could reflect their host environments, including some of economic interest. To investigate the potential of tourmaline as a geochemical indicator for ore exploration, chemical and boron isotope compositions of tourmaline from the Longtoushan Au deposit have been determined. Tourmaline mainly occurs as orbicular aggregates, clusters and veinlets in rhyolite porphyries, granite porphyries and in spatially associated hydrothermal breccias (Au-mineralized and Au-barren). Three types of tourmaline (I, II and III) are recognized at Longtoushan, most of which being alkali-deficient schorl-dravites. The pre-ore Tourmaline I in the rhyolite porphyry is Fe-rich schorl, contrasting with the dravitic ore-stage Tourmaline II in the mineralized breccia and Tourmaline III in the granite porphyry. Locally, Tourmaline II shows concentric zoning which correlates with core-rim chemical variations (higher Fe, Al in the rim than in the core). Tourmalines from Longtoushan are characterized by a dominant MgFe -1 exchange vector and have relatively low Na contents (avg. 0.66 apfu; formula normalized on 15 total cations), indicating they possibly crystallized in reduced fluids of low-salinity. The δ 11 B values of tourmalines from this study fall in a narrow range of −14.5 to −10.2‰, indicating dominant magmatic-hydrothermal fluids of continent crust origin. The results suggest that tourmaline from Longtoushan may originate from boron-rich vapor fluids unmixed from felsic magmas. Tourmaline breccia was likely formed by hydraulic fracturing, with open spaces infilled by boron-rich boiling fluids released from the magma. The fluid boiling process during brecciation would be critical for Au-bearing pyrite precipitation, which could compete with coexisting tourmaline for Fe, leaving tourmaline Fe-depleted. We propose that the Fe-poor dravitic tourmaline in the hydrothermal breccia could be used as prospecting guide at the Longtoushan Au deposit. Supplementary material: major element and boron isotope composition analyses of tourmaline are available at https://doi.org/10.6084/m9.figshare.c.4468808

High-pressure study of dravite tourmaline: Insights into the accommodating nature of the tourmaline structure

Author(s): O'Bannon, E; Beavers, CM; Kunz, M; Williams, Q | Abstract: The high-pressure behavior of dravite tourmaline [Na(Mg3)Al6(Si6O18)(BO3)3(OH)3(OH)] has been studied using luminescence spectroscopy and synchrotron-based single-crystal diffraction up to ~65 and ~24 GPa, respectively. Two emission bands associated with Cr3+/V2+ substitution are constant in energy up to ~9.0 GPa, and they shift to longer wavelength at higher pressures, suggesting that a change in compressional mechanism could occur at this pressure. Single-crystal diffraction data show subtle changes in ring ditrigonality occur near 9.0 GPa, which could cause the observed change in luminescence. Near 15 GPa, a splitting of one of the emission bands is observed, suggesting that a phase transition occurs at this pressure and that two unique octahedral sites are present in the high-pressure phase. Hysteresis is not observed on decompression, which indicates that this is a second-order transition, and the high-pressure structure appears to be metastable up to ~65 GPa. Single-crystal diffraction measurements show that a phase transition from rhombohedral R3m to rhombohedral R3 occurs at pressures near 15.4 GPa. The high-pressure phase is characterized by a distorted Si6O18 ring (e.g., the Si-Si-Si angles deviate from 120°), and the Si, Al, O6, O7, and O8 sites of the low-pressure phase split, implying that the high-pressure phase of tourmaline is a higher entropy phase. The large X-site exerts the primary control on compressibility, and the substitution of larger cations into this site will likely lower the pressure at which this transition occurs. Dravite tourmaline shows anisotropic compression with the c-axis being more compressible than the a-axis. The pressure and volume data up to ~15.4 GPa were fit with second- and third-order Birch-Murnaghan equations of state. We obtain a bulk modulus, K0 = 109.6(3.2) GPa, and a pressure derivative, K0′ c K'0 = 4.6(8) GPa, and with the pressure derivative set to 4, a bulk modulus of 112.0(1.0) GPa is derived. Moreover, our high-pressure results show that massive overbonding of the X and Y sites can be accommodated by the tourmaline structure. This unexpected result may explain the extraordinary structural tolerance with respect to chemical substitution on the X, Y, and Z sites.

Effect of Heat Treatment on Madagascar Dravite Tourmaline: UV-Visible and Diffuse Reflectance Infrared Spectroscopic Characterization

The color change and chemical behavior of Madagascar dravite when subjected to heating in air atmosphere were studied by energy-dispersive X-ray fluorescence spectrometry, UV-visible (UV-vis) spectroscopy and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. Heat treatment was performed with temperatures of 500, 600, and 700°C for 2 h. Colors and color differences were measured and evaluated using CIELAB color measurement. The origin of color and its change after treatment are assessed via UV-vis spectra. The diffuse reflectance infrared spectra of the dravites revealed major absorption bands of OH, BO3, Si6O18, and R–OH (where R = Fe, Mg). The variation of infrared spectra with increasing temperature indicates the deformation of R–OH bonds in an octahedron and, further, the deformation of BO3 and Si6O18. The results of color change may be utilized to enhance the color or clarity in dravite tourmaline and for generating satisfactory color.

Oxy-dravite from Wołowa Góra Mountain, Karkonosze massif, SW Poland: Crystallochemical and structural studies

ABSTRACTYellowish dravitic tourmaline (dominated by the oxy-dravite component) associated with secondary fluor-dravite/fluor-schorl and dravite/schorl tourmalines was found in a quartz vein cropping out in the eastern part of the Karkonosze Mountains range, SW Poland. The crystal structure of this tourmaline was refined to an R1 value of 1.85% based on single-crystal data, and the chemical composition was determined by electron-microprobe analysis. The tourmaline, a representative of the alkali-tourmaline group, has the structural formula: (Na0.75Ca0.12□0.13)Σ1(Mg1.93Al0.95Ti0.06${\rm Fe}_{{\rm 0}{\rm. 04}}^{{\rm 2 +}} $ V0.01)Σ3(Al5.38Mg0.62)Σ6B3Si6O27(OH)3(O0.46OH0.33F0.21)Σ1, and is characterized by an extremely high Mg/(Mg + Fe) ratio of 0.97–0.99, the WO2– content that reaches 0.59 apfu resulting in a local predominance of the oxy-dravitic component and Mg–Al disorder on the octahedral Y and Z sites of the order of 0.64 apfu. This disordering results in an increasing &lt;Z–O&gt; distance with ~1.925 Å, and unit-cell parameters a = 15.916(1) Å and c = 7.180(1) Å. The tourmaline formed during Variscan prograde metamorphism under the influence of a released (H2O,B,F)-bearing fluid. The fluid mobilized the most soluble components of partly altered silicic volcaniclastic material of the Late Cambrian to Early Ordovician bimodal volcanism to become the protolith for adjacent quartzo-feldspathic schists and amphibolites, and propagated them into the surrounding granitic gneisses of the Kowary unit in the eastern metamorphic cover of the Karkonosze granite.

An experimental study on K and Na incorporation in dravitic tourmaline and insight into the origin of diamondiferous tourmaline from the Kokchetav Massif, Kazakhstan

Tourmaline was synthesized in the system MgO–Al2O3–B2O3–SiO2–KCl–NaCl–H2O from an oxide mixture and excess fluid at 500–700 °C and 0.2–4.0 GPa to investigate the effect of pressure, temperature, and fluid composition on the relative incorporation of Na and K in dravitic tourmaline. Incorporation of K at the X-site increases with pressure, temperature, and KCl concentration; a maximum of 0.71 K pfu (leaving 0.29 X-vacant sites pfu) was incorporated into K-dravite synthesized at 4.0 GPa, 700 °C from a 4.78 m KCl, Na-free fluid. In contrast, Na incorporation depends predominately on fluid composition, rather than pressure or temperature; dravite with the highest Na content of 1.00 Na pfu was synthesized at 0.4 GPa and 700 °C from a 3.87 m NaCl and 1.08 m KCl fluid. All synthesized crystals are zoned, and the dominant solid solution in the Na- and K-bearing system is between magnesio-foitite [□(Mg2Al)Al6Si6O18(BO3)3(OH)3OH] and dravite [NaMg3Al6Si6O18(BO3)3(OH)3(OH)], with the dravitic component increasing with the concentration of Na in the fluid. In the K-bearing, Na-free system, the dominant solid solution is between magnesio-foitite and K-dravite [KMg3Al6Si6O18(BO3)3(OH)3(OH)], with the K-dravitic component increasing with pressure, temperature, and the concentration of K in the fluid. The unit-cell volume of tourmaline increases with K incorporation from 1555.1(3) to 1588.1(2) A3, reflecting the incorporation of the relatively large K+ ion. Comparison of our results to the compositional data for maruyamaite (K-dominant tourmaline) from the ultrahigh-pressure rocks of the Kokchetav Massif in Kazakhstan suggests that the latter was formed in a K-rich, Na-poor environment at ultrahigh-pressure conditions near the diamond-stability field.

Synthesis of K-dominant tourmaline

K-dominant tourmaline was synthesized in the system MgO-Al2O3-B2O3-SiO2-KCl-H2O at 700 °C and 4.0 GPa. The crystals were zoned and characterized by less-potassic cores (1.46 wt% K2O) and more-potassic rims (up to 3.44 wt% K2O). The K-dominant tourmaline rims are represented by the average structural formula (K0.60(3)□0.36(3))(Mg2.60(7)Al0.40(7))(Al5.98(3)Si0.02(3))Si6O18(BO3)3(OH)3.92(7)O0.08(8), which is analogous to the structural formula of dravite and is referred to here as “K-dravite”; the maximum analyzed K content (3.44 wt% K2O) represents occupancy of the X site by 0.71 K pfu. The addition of Na to the system in approximately equal molar proportions to K results in the crystallization of K-bearing, Na-rich dravitic tourmaline, dramatically reducing the K content to an average value of 0.47 wt% K2O, corresponding to 0.10 K pfu. This suggests that a K-dominated bulk composition is necessary for K-dominant tourmaline crystallization. Compositional zoning shows that solid solution exists between end-member compositions of “K-dravite” [KMg3Al6Si6O18(BO3)3(OH)3(OH)] and dravite via the isovalent exchange XK(XNa)−1, magnesio-foitite via the coupled substitution XKYMg(X□YAl)−1, and “K-olenite” via the coupled substitution YMgOH(YAlO)−1. Structural refinement of the powder X-ray diffraction data provides a unit-cell volume for the synthesized “K-dravite” of 1580.1(5) A3, which is greater than that determined for K-bearing dravitic tourmaline synthesized at the same conditions [1574.9(4) A3]. We interpret this to reflect expansion of the crystal structure due to incorporation of the relatively large K+ ion.

Cathodoluminescence, fluid inclusion and stable C–O isotope study of tectonic breccias from thrusting plane of a thin-skinned calcareous nappe

Basal hydraulic breccias of alpine thin-skinned Muraň nappe were investigated by means of cathodoluminescence petrography, stable isotope geochemistry and fluid inclusions analysis. Our study reveals an unusual dynamic fluid regime along basal thrust plane during final episode of the nappe emplacement over its metamorphic substratum. Basal thrusting fluids enriched in 18O, silica, alumina, alkalies and phosphates were generated in the underlying metamorphosed basement at epizonal conditions corresponding to the temperatures of 400–450°C. The fluids fluxed the tectonized nappe base, leached evaporite-bearing formations in hangingwall, whereby becoming oversaturated with sulphates and chlorides. The fluids further modified their composition by dedolomitization and isotopic exchange with the host carbonatic cataclasites. Newly formed mineral assemblage of quartz, phlogopite, albite, potassium feldspar, apatite, dravite tourmaline and anhydrite precipitated from these fluids on cooling down to 180–200°C. Finally, the cataclastic mush was cemented by calcite at ambient anchizonal conditions. Recurrent fluid injections as described above probably enhanced the final motion of the Muraň nappe.

Geochemical and geological controls on the genesis of gem-quality "Paraiba Tourmaline" in granitic pegmatites from northeastern Brazil

The electric blue Paraiba Tourmaline (PT) occurs in northeastern Brazil only in four deposits of zoned and highly fractionated granitic pegmatites of the Borborema Pegmatitic Province (BPP). These PT-bearing pegmatites are rich in spodumene or lepidolite, and are emplaced in iron-poor quartzites or metaconglomerates of the Neoproterozoic Equador Formation. Black comb-textured dravitic tourmaline in the border zone indicates crystallization from a boron-bearing granitic magma. The PT occurs at the transition between the albite-rich inner intermediate zone (IIZ) and the quartz core of the pegmatites, in primary wedge-shaped crystals of near-end-member color-zoned elbaite, arranged in fan-like, radial or comb-textured groups of crystals. The PT most commonly forms the intermediate growth-zone of color-zoned elbaite crystals having a pink to red core and a green to dark green rind. The PT-bearing pegmatites are distinguished by intensive late replacement of primary K-feldspar, spodumene, elbaite and Li-phosphates by lepidolite. The lepidolite replacement of zoned elbaite is highly selective; it begins in the core, then affects the light blue intermediate zones, and preferentially leaves intact the green rim. PT crystals or crystal zones preserved from lepidolite replacement are rare, and occur enveloped in quartz of the pegmatite core or in vug fillings. According to electron-microprobe data on color-zoned elbaite crystals, PT crystal-zones are distinguished not only by high Cu contents, but also by higher values of Li and F (mostly fluor-elbaite), Fe concentrations near zero, and lower concentrations of Mn than the contiguous zones with other colors. Increasing Fe + Mg concentrations are responsible for the green color in the rims enveloping PT, even if Cu values remain elevated. Values of Al, X □ and Mn are slightly elevated in the pink zones in comparison with PT in the same crystal. Textural relations indicate that PT formed by direct crystallization from a pegmatite-forming melt rich in B, Li and F, at the early stage of formation of the quartz core, and prior to the appearance of secondary lepidolite and other late hydrothermal minerals. The difference between PT-bearing and other elbaite-rich pegmatites may not necessarily be the failure to produce primary PT, but also the failure to preserve it from late conversion to lepidolite. The formation of PT is linked with low content of mafic minerals in the wallrock and in the pegmatite-forming melt itself.

Timing and thermochemical constraints on multi-element mineralisation at the Nori/RA Cu–Mo–U prospect, Great Bear magmatic zone, Northwest Territories, Canada

The timing of Cu–Mo–U mineralisation at the Nori/RA prospect in the Paleoproterozoic Great Bear magmatic zone has been investigated using Re–Os molybdenite and 40Ar–39Ar biotite geochronology. The Re–Os molybdenite ages presented are the first robust sulphide mineralisation ages derived from the Great Bear magmatic zone. Cu–Mo–U mineralisation is hosted in early to syn-deformational hydrothermal veins consisting of quartz and K-feldspar or more commonly tourmaline-biotite-quartz-K-feldspar, with associated wall-rock alteration assemblages being predominantly biotite. Sulphide and oxide minerals consist of chalcopyrite, molybdenite and uraninite with lesser pyrite and magnetite. Elevated light rare earth elements and tungsten concentrations associated with the Cu–Mo–U mineralisation have also been reported at the prospect by previous workers. Molybdenite and uraninite occur intimately in dravitic tourmaline growth zones and at grain margins, attesting to their syngenetic nature (with respect to hydrothermal veining). Two molybdenite separates yield Re–Os model ages of 1,874.4 ± 8.7 (2σ) and 1,872.4 ± 8.8 Ma (2σ) with a weighted average model age of 1,873.4 ± 6.1 Ma (2σ). Laser step heating of biotite from the marginal alteration of the wall-rock adjacent to the veins yields a 40Ar–39Ar maximum cooling age of 1,875 ± 8 Ma (MSWD = 3.8; 2σ), indistinguishable from the Re–Os molybdenite model age and a previously dated ‘syn-tectonic’ aplitic dyke in the region. Dravitic tourmaline hosts abundant primary liquid–vapour–solid-bearing fluid inclusions. Analytical results indicate liquid–vapour homogenisation at >260°C constraining the minimum temperature of mineralisation. The solids, which are possibly trapped, did not homogenise with the liquid–vapour by 400°C. Salinities in the inclusions are variable. Raman spectra identify that at least some of the solids are calcite and anhydrite. Raman spectra also confirm the vapour phases contain some CO2; whereas clathrates or CH4 was not observed or detected. Quartz grains only host secondary fluid inclusions, which fluoresce under ultraviolet light, indicating trapped hydrocarbons. We speculate that these resulted from Phanerozoic fluid circulation through the Proterozoic basement. The collective interpretation of the age, hydrothermal character and associated metals, high temperature and variable salinity suggests that the Nori/RA Cu–Mo–U mineralisation can be linked with the earliest stages of plutonism in the Great Bear magmatic zone. From a regional perspective, the mineralisation may pre-date the extensive multi-element mineralisation now recognised as part of the iron oxide copper–gold (IOCG) spectrum of deposits. As IOCG provinces generally contain a variety of mineralisation styles, we interpret this as the earliest phase of the extensive mineralising system.

Boron-isotope fractionation between tourmaline and fluid: an experimental re-investigation

The fractionation of boron isotopes between synthetic dravitic tourmaline and fluid was determined by hydrothermal experiments between 400 and 700°C at 200 MPa and at 500°C, 500 MPa. Tourmaline was crystallized from an oxide mix in presence of water that contained boron in excess. In one series of experiments, [B]fluid/[B]tour was 9 after the run; in another series it was 0.1. All experiments produced tourmaline as the sole boron-bearing solid, along with traces of quartz and talc. Powder XRD and Rietveld refinements revealed no significant amounts of tetrahedrally coordinated boron in tourmaline. 11B always preferentially fractionated into the fluid. For experiments where [B]fluid/[B]tour was 9, a consistent temperature-dependent boron isotope fractionation curve resulted, approximated by Δ11B(tour–fluid) = −4.20 · [1,000/T (K)] + 3.52; R2 = 0.77, and valid from 400 to 700°C. No pressure dependence was observed. The fractionation (−2.7 ± 0.5‰ at 400°C; and −0.8 ± 0.5‰ at 700°C) is much lower than that previously presented by Palmer et al. (1992). Experiments where [B]fluid/[B]tour was 0.1 showed a significant larger apparent fractionation of up to −4.7‰. In one of these runs, the isotopic composition of handpicked tourmaline crystals of different size varied by 1.3‰. This is interpreted as resulting from fractional crystallization of boron isotopes during tourmaline growth due to the small boron reservoir of the fluid relative to tourmaline, thus indicating larger fractionation than observed at equilibrium. The effect is eliminated or minimized in experiments with very high boron excess in the fluid. We therefore suggest that values given by the above relation represent the true equilibrium fractionations.

Syros Metasomatic Tourmaline: Evidence for Very High- 11B Fluids in Subduction Zones

High-pressure (HP) metamorphic blocks enclosed in a mafic to ultramafic matrix from a melange on the island of Syros are rimmed by tourmaline-bearing reaction zones (blackwalls). The B isotopic composition of dravitic tourmaline within these blackwalls was investigated in situ by secondary ion mass spectrometry. Boron in these tourmalines is unusually heavy, with d 11 B values exceeding þ18‰ in all investigated samples and reaching an extreme value of þ28.4‰ in one sample. Blackwalls formed during exhumation of the HP melange at a depth of 20-25 km at temperatures of 400-430 � C, by influx of external hydrous fluids. The compositions of the fluids are estimated to be in the range of 100-300 mg/g B with d 11 B values of þ18 to þ28‰. The high d 11 B values cannot be explained by tourmaline formation from unmodified slab-derived fluids. However, such fluids could interact with the material in the exhumation channel on their way from the dehydrating slab to the site of tourmaline formation in the blackwalls. This could produce exceptionally high d 11 B values in the fluids, a case that is modelled in this study. The model demonstrates that subduction fluids may be effectively modified in both trace element and isotopic composition during their migration through the material over- lying the subducting slab. Blackwall tourmaline from Syros has a large grain size (several centimetres), high abundance, and an exceptionally high d 11 B value. The formation of tourmaline at the contact between mafic or felsic HP blocks and their ultramafic matrix involved fluids released during dehydration reactions in the subducting slab. It forms a heavy-boron reservoir in hybrid rocks overlying the subducting slab, and may, thus, have a significant impact on the geochemical cycle of B and its isotopes in subduction zones.

Kornerupine parageneses in whiteschists and other magnesian rocks: is kornerupine + talc a high-pressure assemblage equivalent to tourmaline + orthoamphibole?

Kornerupine, (□,Fe,Mg)(Mg,Fe,Al)9(Si,Al,B)5 (O,OH,F)22, has been reported with talc in rocks from six localities worldwide, but only at Chilapila Hill in the Lufilian Arc, Zambia do textural relationships imply that kornerupine (Krn) equilibrated with talc (Tlc) during a prograde metamorphic event at T≈ 640 °C, P≈ 13 kbar; a prograde Krn + Tlc assemblage has also been reported from Mautia Hill, Tanzania (P ≤ 13 kbar). In order to estimate possible constraints on the stability range for the kornerupine + talc paragenesis in nature, we constructed a P-T diagram in the model system MgO-Al2O3-SiO2-H2O (MASH) for seven phases quartz (Qtz), B-free kornerupine sensu stricto, anthophyllite (Ath), chlorite (Chl), cordierite (Crd), kyanite (Ky), and talc. The minimum pressure for Krn + Tlc + Ky stability in MASH is close to that for Ky + Tlc stability, i.e., 6–8 kbar, at T≤ 780 °C. However, in the natural system, B2O3 and Na2O are major constituents in Krn and orthoamphibole (Oam), respectively, and dravitic tourmaline (Tur) is widespread. The critical assemblage alternative to Krn + Tlc in nature is Tur + Oam. The upper pressure limit of Tur + Ath is determined by the upper pressure for anthophyllite: 7.7–10.5 kbar at 682–794 °C in the MgO-SiO2-H2O system (Chernosky et al. 1985, Am Mineral 70:223–236), and is undoubtedly higher in the presence of Na2O, CaO, and Al2O3. At three of the six localities, talc is a retrograde phase; nonetheless, it possibly equilibrated with kornerupine on the retrograde path or during a later metamorphic event at P-T conditions appropriate for Ky + Tlc. At the sixth locality (Mulvoj, southwestern Pamir Mountains, Tajikistan), Krn is found in the same thin section as talc and kyanite and all three minerals formed during a prograde metamorphic event at T≥ 650 °C, P near 7 kbar. However, Krn is restricted to a lens 4 to 6 mm thick of phlogopite + anthophyllite + Tur and it does not touch either talc or kyanite. A reaction relating the Mulvoj and Chilapila Hill (Krn + Tlc + Ky + Qtz + Tur) parageneses is calculated from compositions in the Mulvoj rock to be 0.40Tur + 2.55Ath + 1.33H2O + 0.27F = Krn + 2.16Tlc + 0.36B2O3 + 0.02Rutile + 0.19Na2O + 0.17CaO. Given the difference in metamorphic pressures estimated for Mulvoj and Chilapila Hill, Krn + Tlc is inferred to be favored by increasing pressure as well as by low Na2O and CaO contents. Some FeO, F, Fe2O3, and BeO are present in measurable amounts in at least one of the phases in the Mulvoj and Chilapila Hill whiteschists (e.g., Krn contains 0.24–0.67 wt% BeO), but the effect of these constituents is subordinate to that of Na2O, CaO and B2O3. The Krn + Tlc could be a more important assemblage in B-bearing whiteschists than has been reported to date, particularly at pressures where orthoamphibole is no longer stable.

Li, B-rich Rhaetian metabauxite, Tuscany, Italy: reworking of older bauxites and igneous rocks

Li, B-rich metabauxite layers occur in two outcrops of talus metabreccias intercalated between Norian and Liassic marbles in the Alpi Apuane, Tuscany, Italy. Field evidence, petrographical, geochemical, and paleogeographic, paleoclimatologic considerations suggest that bauxitic material was derived from a multistage reworking of Late Carboniferous–early Autunian primary deposits now completely removed. The identified lithotypes are: cookeite-rich metabauxite, cookeite-bearing metabreccia, chloritoid schist and chloritoid phyllite. The Mg content of calcite, and the Si content of muscovite and mineral assemblages indicate temperatures of 350°–380°C, pressures of <7 kbar and f O 2 -values of 10 −30 to 10 −24 for the Alpine metamorphism. In comparison with the average Mesozoic bauxite of Bronevoy et al. (1985)the metabauxite is: strongly enriched in Si, K, Ca, Mg, Li, B, Be, Ni and Ce; it shows comparable contents of Co, La, Eu, Tb, Ta and Th; it is depleted in Ga, Cr, V, Zn, Mn, Hf, U; and is strongly depleted in Sr, F, Cu. A metabauxite layer shows decreasing abundance of Fe and Ga from bottom to top with a pattern opposite to that of primary bauxite deposits (inverted profile). The strong enrichment of Li and B is not related to evaporitic conditions, since the MgO, Na 2O and Sr contents are too low, and indicator minerals such as dravitic tourmaline, albite and scapolite are missing. During the Permian, volcanic products similar to the coeval peralkaline–peraluminous rocks of Corsica–Sardinia were widespread in Tuscany. B, Li-rich minerals were produced by circulation of hot hydrothermal fluids within the Permian volcanic rocks during and soon after their emplacement. The erosion of such hydrothermal products supplied detrital material to the talus metabreccia.

The sulphur springs geothermal field, St. Lucia, lesser Antilles: Hydrothermal mineralogy of wells SL-1 and SL-2

Two wells have been drilled to depths of 1413 and 2213 meters in the geothermal field of Sulphur Springs, St. Lucia, and reveal a complex volcanic sequence characterized by collapse episodes followed by the emplacement of dacite domes. The geothermal reservoir consists of fractured volcanic rocks and produces superheated steam. Well-bottom temperatures are around 270–290°C. The hydrothermal alteration found in both the productive SL-2 well and the non-productive SL-1 is strongly reminiscent of that of porphyry copper deposits, with (1) an inner, high-temperature potassic zone characterized by the occurrence of dravitic tourmaline, quartz, and biotite, (2) an outer propylitic alteration zone that is partly superimposed on (3) a potassic alteration zone. The alteration mineral assemblages indicate that the hydrothermal system has cooled at the levels sampled.

Stream sediment tourmaline geochemistry in massive sulfide exploration: an example from Virginia, U.S.A.

Abstract Semi-quantitative analyses of tourmaline grains were made using a scanning electron microscope equipped with an energy dispersive X-ray analyzer. The tourmaline was collected from 60 stream sediment samples within the Cambrian (?) central Virginia volcanic-plutonic belt. Compositionally, most of the tourmaline is dravite or schorl, with a minor uvite component in some samples. A probability plot of FeO/(FeO + MgO) in tourmaline has a sharp positive inflection at 0.235. Nine of the 10 samples with tourmaline FeO/(FeO + MgO) ratios For comparison, concentrations of Zn and Fe were also determined in minus-80 mesh stream sediment samples. Of the 10 samples considered statistically anomalous, only six are downstream from sulfide prospects. The downstream distance of these samples from mineralization averages 1 km and none of the samples is more than 2 km downstream. Based on relationships in this study, the presence and distribution of strongly dravitic tourmaline in stream sediment may be a sensitive and reliable guide to the location of submarine exhalative sulfide deposits. Although the cost per sample is greater than that for analyses of minus-80 mesh sediment, the total cost of a survey could be similar if increased downstream detectibility results in fewer samples being required to adequately cover an area for exploration.