Tourmaline Nodules Research Articles

B-rich melt immiscibility in Late Cretaceous Nattaung granite, Myanmar: Implication by composition and B isotope in tourmaline

There is a strong controversy about the petrogenesis of tourmaline nodules and their relationship with coeval tourmaline-bearing granites, which are commonly associated with magmatic-hydrothermal rare metal deposits (e.g., Sn, Nb, and Ta). In this study, tourmaline is extensively distributed in tourmaline-bearing nodules (NT-type), pegmatite (PT-type), and fine-grained granitic dyke (GT-type), which are hosted by the Late Cretaceous Nattaung biotite granite (Myanmar). Zircon UPb dating results (~71 Ma) indicate that PT- and GT-types tourmaline-bearing magmatic rocks formed synchronously with biotite granite. All tourmalines have alkali group and schorl composition with Mg/(Mg + Fe) ratios of 0.03–0.45 and Na/(Na + Ca) ratios of 0.73 to 1.00. NT-type tourmaline shows high Nb/Ta ratios and V and Sr contents, indicating that they were likely crystallized from B-rich melts early immiscible with less evolved granitic magmas. PT- and GT-type tourmalines probably formed in aggregated B-rich melts that continuously separated from more evolved granitic magma, supported by their low Nb/Ta ratios and V and Sr contents. In addition, the studied tourmaline displays a slight variation of B isotopic compositions (δ11B values ranging from −12.2 ± 0.7‰ to −14.9 ± 0.4‰), likely resulted by Rayleigh fractionation of tourmaline. Their light B isotopic values are consistent with those of average continental crust, also revealing that the Nattaung granite was mainly derived from partial melting of the ancient Sibumasu crust. Importantly, late-stage PT- and GT-type tourmalines have higher Sn, Zn, Nb, and Ta contents than early-stage NT-type tourmaline, suggesting that the evolved B-rich magma could be one precursor to produce Sn-Nb-Ta-Zn deposits associated with extensive tourmaline in the world.

Elemental and B-O-H isotopic compositions of tourmaline and associated minerals in biotite-muscovite granite of Mashhad, NE Iran: Constraints on tourmaline genesis and element partitioning

Abundant tourmaline occurs in Late Triassic biotite-muscovite granites of Mashhad (NE Iran), with several distinct morphologies including tourmaline nodules in granite, pegmatitic tourmalines, aplitic tourmalines, and vein-related tourmalines (quartz-tourmaline vein, tourmaline-rich vein and radial tourmalines). Pegmatitic and aplitic tourmalines are compositionally Fe-rich schorl and the nodule, radial and vein tourmalines are Mg-rich schorl. The tourmalines have generally low abundances for most trace elements except for Sr, Ga, Li, Be, Sn, Pb and some transition elements. Tourmalines have low REE abundances and are enriched in LREE and depleted in HREE (mostly below detection limits) with positive Eu anomalies. The concentration of several trace elements (e.g., Pb, Be, Sn, Ga, Sr, Pb, and REE) correlates well with the major element composition of the tourmalines (e.g. Mg# (Mg/Mg + Fe), vacancy/)X-site vacancy + Na(and Na/)Na + Ca(). Trace element data of coexisting minerals with tourmaline indicate that the oligoclase and albite show the most preference in Sr, Pb and Eu and LREEs, biotite readily incorporates Rb, Ba, Zn, Li, Cs and Nb; K-feldspar has high contents of Rb, Ba, Sr and Pb; and garnet incorporates HREEs and Y during magma differentiation. The B-O-H isotopic compositions for all tourmalines are consistent with an S-type granite source, that was derived from partial melting of the continental crust. The pegmatitic and aplitic tourmalines with Fe-rich schorl composition indicate magmatic conditions and the radial, vein, and nodule tourmalines with Mg-rich schorl composition formed during the transition from magmatic to hydrothermal conditions due to the internal evolution of the granitic magma.

Geochemistry and formation of tourmaline nodules in Mashhad leucogranite, Iran

Nodular tourmalines composed of dark cores and white haloes occur in the Mashhad leucogranite. The cores are made up of tourmaline and quartz, and the haloes of quartz, muscovite, microcline and orthoclase. The host granite consists of quartz, plagioclase, microcline, muscovite and biotite. Geochemical analyses show that the studied tourmalines are schorl. They are characterized by oscillatory zoning, increasing concentrations of Ca and Mg from core to rim, moderate Fe/Fe + Mg ratios (56–63), and REE distribution patterns similar to those of the host granite. The tourmalines exhibit high Ti content due to breakdown of biotite. Petrographic and geochemical results show that the formation of tourmaline can be attributed to two stages. In the first stage, bubbles containing vapor-rich liquid, Fe, and B were generated by magma differentiation. During rise and cooling of the magma, core tourmaline was produced with Fe rich schorl composition. In the second stage, the tourmaline composition changed to that of Mg rich schorl because of the decomposition of biotite (biotite is considered as a source of Mg), and the mixing with vapor-rich liquid in bubbles

Chemical and boron isotopic compositions of tourmaline from the Nyalam leucogranites, South Tibetan Himalaya: Implication for their formation from B-rich melt to hydrothermal fluids

The Miocene Nyalam leucogranites from the South Tibetan Himalaya consist of two-mica leucogranite and tourmaline leucogranite, in which tourmaline is an important constituent in nodular tourmaline-quartz segregations. Within the two-mica leucogranite these nodules display two basic morphologies: rounded (2mg-ro type) and radial or dendritic (2mg-ra type). The nodules in the tourmaline leucogranite share similar textural features with the 2mg-ro type tourmaline nodules, but without the leucocratic halo. Tourmaline crystals in the tourmaline leucogranite display overgrowth textures — most commonly in the form of homogeneous cores (Tg-I type) surrounded by oscillatory-zoned rims (Tg-II type). Based on the petrography, chemical and boron isotopic compositions of the various types of tourmaline, it is hypothesized that the 2mg-ro type tourmaline nodules crystallized during the transition from magmatic to hydrothermal evolution of the granite, while the 2mg-ra type tourmaline nodules likely formed during syn-magmatic crystallization from boron-rich melts. In this hypothesis, the Tg-I type tourmalines also formed during the transitional stage from late magmatic to early hydrothermal process. Tg-II type tourmalines were obviously formed after the Tg-I type tourmaline, and can be attributed to the mixing of juvenile fluids and volatiles coming from the wall rocks to the nodule in a post-magmatic hydrothermal environment where Tg-I type tourmaline has already crystallized. Thus, the origin of the tourmaline nodules is related to the different stages of syn-magmatic crystallization from a B-rich melt, or the subsequent transition from magmatic to hydrothermal environment, or crystallized in the post-magmatic hydrothermal fluids.The average δ11B values of the 2mg-ra type and 2mg-ro type tourmaline are −15.1‰ and −14.4‰, respectively. The tourmalines in tourmaline leucogranite have slightly higher δ11B values, with the Tg-I type tourmalines averaging −12.5‰ and the Tg-II type tourmalines averaging −12.1‰. The boron isotopic composition of the tourmalines has implications for the origin of the leucogranites. We suggest that the distinction of the boron isotopic compositions between the source rocks of the two types of leucogranite could account for this boron isotope distinction in tourmalines.

Compositional variations in tourmalines from peraluminous rocks of the Dipilto Granitic Batholith, Eastern Chortis Terrane, Nicaragua: tracers of magmatic to hydrothermal evolution

The granitoids of the Dipilto Batholith in northern Nicaragua range in composition from amphibole–biotite tonalite to cordierite–biotite and biotite granite. The petrographic and geochemical features indicate hybrid origin for the metaluminous granodiorite and tonalite (El Paraiso Suite). Dominant peraluminous granites (leucogranite dykes, Yumpali and La Piedra suites) represent a typical product of fractional crystallization of crust-derived melts. Tourmaline occurs as rare mineral in peraluminous granites of the Yumpali Suite (randomly distributed grains and quartz– tourmaline nodules), but is common in the younger leucogranite, pegmatite dykes and hydrothermal veins. Chemical variations (mainly X Fe and F) in tourmaline as well as in biotite are consistent with magmatic differentiation processes in granites and pegmatites. The presence of quartz–tourmaline hydrothermal veins indicates involvement of B-rich post-magmatic hydrothermal fluids at the final stage of the Dipilto Batholith evolution. Tourmaline chemistry evolved from Mg-rich schorl in peraluminous granites, to F-rich schorl–foitite in the hydrothermal veins. This evolutionary trend is typical of F-poor granite–pegmatite systems.

Non-invasive assessment of the formation of tourmaline nodules by X-ray microtomography and computer modeling

Tourmaline nodules occurring in the Capo Bianco (Elba Island, Italy) aplitic rocks are here investigated by X-ray microtomography 3D imaging. This non-invasive technique provides 3D images of the tourmaline nodules, revealing an irregular morphology consisting of branches that extend radially from the cores. The nodules present scale-invariant features that can be described by a box-counting fractal dimension. The value of the fractal dimension is proportional to the size of the nodules and tends asymptotically to a value of 2.5, in agreement with the results obtained from the simulation of virtual nodules, by means of a diffusion-limited aggregation model based on a Monte Carlo Metropolis algorithm, in which the growth probability at the tips of the nodule is an inverse function of the diffusion coefficient. The results support the hypothesis that tourmaline formed by a disequilibrium magmatic process, in which diffusion represents the rate-limiting step, inducing the formation of nodules with irregular shapes. This study shows the potential of X-ray microtomography, in combination with numerical modeling, as a probe for accessing the 3D microstructural information of complex mineral morphologies with a non-invasive approach. The combination of numerical and experimental, non-invasive, 3D techniques represents a fundamental step forward in bridging the gap between the observation of microstructures and the interpretation of the associated processes.

THE CORUNDUM + TOURMALINE NODULES RELATED TO HYDROTHERMAL ALTERATION OF SPODUMENE IN THE NANPING No. 31 PEGMATITE DYKE, FUJIAN PROVINCE, SOUTHEASTERN CHINA

The Nanping No. 31 pegmatite is a spodumene-subtype LCT granitic pegmatite in southeastern China. Spodumene mainly occurs as rock-forming mineral in the quartz – coarse albite – spodumene zone (zone III) and the quartz – spodumene – amblygonite zone (zone IV). Various alteration-induced phenomena were observed in spodumene. Spodumene in zone III is usually altered along its {110} cleavages to networks of albite + muscovite, resulting from a replacement of spodumene by an alkaline fluid, which likely begins by Na metasomatism. Extensive alteration of spodumene in zone IV to quartz and muscovite veinlets is related to replacement by K+- and H+-rich fluid. However, alteration by strongly acid fluids also affected spodumene, leading to formation of fine-grained white mica in zones III and IV. In this paper, we focus on the occurrence of nodules of corundum + tourmaline, which are typically restricted to the aggregates of white mica in zone IV. Fibrous corundum and radial tourmaline signify a hydrothermal origin. Their textural relationship with secondary muscovite suggest a close genetic relation with the latter. The corundum + tourmaline nodules arise by alteration of the secondary muscovite by late B-, Mg- and Fe-rich fluids.

Contrasting tourmaline types from peraluminous granites: a case study from Moslavačka Gora (Croatia)

Two texturally and chemically distinct types of tourmaline are found inside peraluminous granites of the Moslavacka Gora, Croatia: nodular tourmaline in the two-mica granite and disseminated tourmaline in the cross-cutting leucogranite dykes. Both tourmaline types belong to alkali tourmaline group, nodular tourmaline being dravite to schorl and disseminated tourmaline corresponding to schorl. Comparison of characteristic parameters of nodular (Nt) and disseminated tourmaline (Dt) shows significant differences in #Fe (0.40–0.65 for Nt vs. 0.74–0.85 in Dt) along with variations in the calculated X-site vacancy (0.22–0.37 pfu in Nt and 0.33–0.44 in Dt) and □/(□+Na) ratio (0.23–0.40 in Nt and 0.34–0.45 in Dt). Disseminated tourmaline from the MG leucogranites is regarded as an early crystallized magmatic phase, while the interstitial tourmaline from the cores of tourmaline nodules originated from more complex mineralogical and chemical interactions inside the two-mica granite melt. Major element gain (Mg) and loss (Fe, Ca, Na, K) for the “idealized nodule” (34 vol. % core + 66 vol. % halo) when compared to the host granite shows that the nodule’s volume is not a completely independent and closed system. Based on the observed characteristics, nodule’s halo can be considered as a “transitional zone” between the tourmaline-bearing core and the host granite, texturally and mineralogically related to the host two-mica granite, chemically being an integral part of the nodule’s volume at the same time.

Tourmaline nodules: products of devolatilization within the final evolutionary stage of granitic melt?

Abstract The origin of tourmaline nodules, and of their peculiar textures found in peripheral parts of the Moslavačka Gora (Croatia) Cretaceous peraluminous granite are connected with the separation of a late-stage boron-rich volatile fluid phase that exsolved from the crystallizing magma. Based on field, mineralogical and textural observations, tourmaline nodules were formed during the final stage of granite evolution when undersaturated granite magma intruded to shallow crustal horizons, become saturated and exsolved a fluid phase from residual melt as buoyant bubbles, or pockets. Calculated P–T conditions at emplacement level are c. 720 °C, 70–270 MPa, and water content in the melt up to 4.2 wt%. Two distinct occurrence types of tourmalines have been distinguished: disseminated and nodular tourmalines. Disseminated tourmaline, crystallized during magmatic stage, is typical schorl while nodular tourmaline composition is shifted toward dravite. The increase of dravite in nodular tourmaline is attributed to mixing of the fluid phase from the residual melt with fluid from the wall rocks. The pressure decrease and related cooling at shallow crustal levels can be considered as a major factor controlling fluid behaviour, formation of a volatile phase, and the crystallization path in the Moslavačka Gora granite body.

Tourmaline nodules from Capo Bianco aplite (Elba Island, Italy): an example of diffusion limited aggregation growth in a magmatic system

The morphology of tourmaline nodules occurring in the Capo Bianco aplite (Elba Island, Italy) is studied. Outcrop features indicate that tourmaline nodules are the product of magmatic crystallization, as they are aligned along flow fields developed within the magmatic hosting mass. Mesoscopic observations indicate that nodule morphologies are very variable, from rounded to dendritic. Morphometric analyses show that tourmaline nodules are fractals and that fractal dimension quantifies their degree of irregularity. Numerical simulations of nodule growth are performed by using a Diffusion-Limited Aggregation process. The presence in natural samples of nodules with different morphologies is explained by considering a chaotic magmatic system characterized by a complex interplay between growth rate in different dynamical regions, latent heat of crystallization, and local convection dynamics. It is suggested that higher growth rates correspond to growth of tourmaline nodules in dynamical regions where the transfer of nutrients is very efficient. In such conditions, the latent heat released by the growing nodule is high, inducing strong local convection dynamics, destabilizing the nodule interface, and promoting the formation of dendritic morphologies. On the contrary, the growth of nodules in dynamical regions characterized by weak transfer of nutrients is inhibited leading to weak local convection dynamics and, consequently, to the formation of rounded morphologies.

Origin and petrogenetic implications of tourmaline-rich rocks in the Sierra Nevada (Betic Cordillera, southeastern Spain)

Tourmaline-rich rocks (up to 60% tourmaline) associated with low–medium grade metamorphic assemblages occur in the Sierra Nevada area (Betic Cordillera, southeastern Spain). Tourmaline appears in a variety of forms: (1) stratiform tourmalinites; (2) quartz–tourmaline nodules; (3) porphyroclasts in felsic orthogneisses; and (4) disseminations in psammopelitic metasediments and gneisses. Tourmaline within these lithologic groups exhibits textural and chemical variations that reflect complex premetamorphic growth under open-system conditions, and subsequent changes due to Alpine regional metamorphism. Microprobe analyses of the tourmalines reveal a wide compositional variation between schorl and dravite end members with variable contents of X-site vacancies (av. 0.084–0.225 apfu), Ca (av. 0.095–0.269 apfu), and excess of Al (up to 6.588 apfu) compared with the theoretical value of 6 in ideal schorl and dravite. The amount of Ca may be significant in porphyroclasts from the gneisses. Fe/(Fe+Mg) ratios for tourmalines in tourmalinites, metasediments, and gneisses range from 0.34 to 0.95, 0.16 to 0.92, and 0.28 to 0.97, respectively. Na/(Na+Ca) ratios are also variable, mostly ranging from 0.5 to 0.9. Many of the tourmalines have complex chemical and colour zoning patterns, including significant fluctuations in Al, Fe, Mg, Na, Ca, Ti, and F. Based on petrographic and chemical data, three generations of tourmaline have been established. The first generation corresponds to magmatic–postmagmatic tourmaline that is represented by tourmaline porphyroclasts within the orthogneisses. The second generation of tourmaline formed during tourmalinization of psammopelitic rocks giving rise to tourmalinites. The third generation of tourmaline is represented by cellular textures, pale reaction rims and overgrowths developed during the Alpine regional metamorphic overprint.

Magmatic and hydrothermal history of the porphyry-style deposits of the Ardlethan tin field, New South Wales, Australia

The cassiterite-sulfide deposits of the Ardlethan tin field are classified as porphyry-style Sn deposits on the basis of the intimate spatial and temporal relationships of the breccia pipes and Sn deposits with the Ardlethan Granite, and in particular, the Mine porphyry; the continuum of salinity and temperature conditions from high-temperature and high-salinity fluids in the Ardlethan Granite to lower temperature and less saline fluids in the breccia pipes in the Mine granite; and the isotopic character of the ore fluids which is consistent with a magmatic fluid source characterized by primitive isotopic signatures for carbon and sulfur. The deposits occur in breccia pipes in the Siluro-Devonian Mine granite (417 + or - 2.5 Ma) close to its contacts with the Ardlethan Granite (410 + or - 2.5 Ma) and Ordovician metasediments. The Ardlethan Granite and related phases are moderately to strongly fractionated and enriched in Rb, Th, U, Nb, Y, Li, As, Ta, W, F, and Sn. Tourmaline nodules occur within the Ardlethan Granite and halite-bearing fluid inclusions found in quartz from the nodules homogenized between 250 degrees and 485 degrees C. One of the most fractionated phases of the Ardlethan Granite--the Mine porphyry-- was emplaced into the major breccia pipe of the Mine granite following the onset of brecciation. Brecciation and mineralization are linked to the release of magmatic volatiles and saline brines from the Ardlethan Granite. The vug-filling minerals in all the deposits show a paragenetic sequence from cassiterite, wolframite, arsenopyrite, milky quartz, and tourmaline to pyrite, chalcopyrite, sphalerite, galena, tourmaline, and clear quartz to late-stage toothy quartz, fluorite, and cookeite. Fluid inclusion data from quartz show that the deposition of milky quartz and cassiterite occurred between 300 degrees and 370 degrees C, sulfide and clear quartz between 210 degrees and 2800C, and toothy quartz, fluorite, and cookeite between 105 degrees and 245 degrees C. Fluid salinities ranged..

Tourmaline nodules: indicators of hydrothermal alteration and SnZn(W) mineralization in the Cape Granite Suite, South Africa

Tourmaline and quartz-tourmaline nodular aggregates are common in S-type granitoids of the Cape Granite Suite in the Tygerberg terrane of the Neoproterozoic Saldania belt, South Africa. Most of the aggregates occur within the 200 km long Yzerfontein-Helderberg linear zone, which hosts a diversity of exo- and endomagmatic base and precious metal deposits. The conspicuous dark spherical nodules, with diameters of up to 40 cm, are surrounded by a leucocratic halo and consist mainly of tourmaline (schorl) and quartz. Spatial and temporal relationships indicate that the nodules are features of post-magmatic replacement related to the hydrothermal alteration of crystallized granite. Their distribution is possibly controlled by fluid movement along micro-fractures and diffusion along grain boundaries. Nodule composition suggests that the hydrothermal fluids that formed them were oxidizing and chemically simple, mainly B-(F)-rich and consequently acidic with anomalously high concentrations of Zn, Sn and Ga. Where proximal to Sn-Zn-(W) quartz-vein deposits, spatial relations show that nodule formation is more widespread than and preceded the vein mineralization. This relationship, coupled with the similar metal association of the nodules and veins, suggests a common hydrothermal fluid source. It also allows the use of tourmaline nodules as regional indicators of hydrothermal alteration and of late-stage vein deposits of similar metal association.

The age, petrogenesis and emplacement of the Dalmatian Granite, H.U. Sverdrupfjella, Dronning Maud Land, Antarctica

The ∼470 Ma Dalmatian Granite forms sheet-like bodies intruded discordantly into orthogneisses, paragneisses and calcareous rocks belonging to the ∼1000 Ma Jutulrora, Sveabreen and Fuglefjellet formations respectively. The Dalmatian Granite is muscovite + biotite bearing. Two varieties are recognized, one that is magnetite-bearing and another that is characterized by tourmaline nodules. At some localities, development of the tourmaline-bearing variety is spatially associated with the presence of carbonates. Physical conditions of emplacement for the Dalmatian Granite are estimated to be approximately 700°C and 6kbar with pH2O = Pload. The emplacement of the granite is considered to have occurred syntectonically during D3 approximately 470 Ma ago. The granites are therefore similar in age to Pan African age granites in Mozambique as well as Ross Orogeny age granites in the Transantarctic Mountains.