Generations Of Tourmaline Research Articles

Tourmaline as an indicator of ore-forming processes: Evidence from the Laodou gold deposit, Northwest China

A detailed textual and geochemical study of tourmaline is presented to trace fluid evolution and ore genesis of the Laodou gold deposit in West Qinling Orogen, Northwest China. Based on petrographic observations and relationships to gold mineralization, three generations and five types of tourmaline were identified. The pre-ore tourmaline generation (Tur A) occurs as phenocryst replacement within quartz diorite porphyry (Tur A1) and metasomatic filling in hydrothermal breccia (Tur A2). It shows intense oscillatory zoning and variable Mg/(Mg + Fe) ratios (0.30–0.67), implying rapid compositional changes controlled by host-rock composition and fluid-rock interaction. The syn-ore tourmaline generation (Tur B) forms as open-space filling (Tur B1) and peripheral overgrowth (Tur B2) of Tur B1. It occurs in quartz-tourmaline and quartz-stibnite-sphalerite-tourmaline veins. Narrow Mg/(Mg + Fe) range (0.48–0.76), poor chemical zoning and more homogeneous compositions of Tur B reflect a higher fluid flux. The post-ore tourmaline generation (Tur C) occurs as monopolar overgrowths developed at the − c pole of previous tourmaline within granitoids and hydrothermal veins. This has a composition between dravite and foitite with Mg/(Mg + Fe) from 0.65 to 0.95 and no zoning, which reflect formation under low-temperature conditions. Relatively high Sn, Nb and Ta concentrations, LREE enrichment accompanied by strong positive Eu anomalies of Tur A mostly support a magmatic-hydrothermal origin. Slight LREE enrichment and low abundances of ΣREE of Tur B are caused by progressive input of meteoric waters. Tur C shows relative enrichment in Ba, depletions in V and Ni, and a flat REE pattern with negative Eu anomalies, suggesting a metamorphic source. Boron isotope compositions suggest that boron of Tur A (δ11B average − 6.49 ‰) is sourced from an I-type melt involved with the Laodou quartz diorite porphyry, while the lighter δ11B values in paragenetically younger tourmaline (Tur C with δ11B average − 9.10 ‰) support cooling and a metamorphic input sourced from argillaceous slate of the Lower Permian Daguanshan Group. The variations in textural characteristics, chemical and boron isotope compositions from Tur A to Tur C support a fluid mixing between a magmatic-hydrothermal fluid and meteoric waters leaching metasedimentary rocks. Typical morphology, core-to-rim decrease in Mg/(Mg + Fe), Fe3+Al−1, (□XAl)(NaR2+)−1 and (AlO)(R2+OH)−1 vectors, decreasing Na, Ca concentrations and Eu anomalies of tourmaline may record cooling, oxidation and dilution during fluid evolution. This process is possibly caused by fluid mixing and results in gold precipitation. Boron sources, as well as fluid provenance and evolution interpreted by tourmaline provide insight into the Laodou deposit as an intrusion-related gold deposit.

Tourmalines in the Namacotche Li-Cs-Ta granitic pegmatite group, Mozambique: crystal chemistry and origin

The field work, backscattered electron images and detailed microanalyses of three generations of tourmaline from the Namacotche LCT pegmatites allows de distinction between the compositional magmatic and hydrothermal tourmalines. The generation 1 occurs in the outer intermediate zone of the pegmatite. It consists of zoned crystals with an oscillatory inner core of foitite and schorl, an outer core of schorl and an Fe-rich fluor-elbaite rim. Unzoned Fe-rich fluor-elbaite crystals occur in the inner intermediate zone of the pegmatite. All the crystals are derived by fractionation of a (Al, Li, B)-rich pegmatite melt. However, the rim of zoned crystals and some compositions of unzoned crystals show evidence of hydrothermal fluids, as they plot outside the fractionation trends. The zoned fluor-elbaite crystals of the generation 2 are from the inner intermediate zone of the pegmatite. They have a pink core and a green rim. The rim has higher YFe2+, Na, F contents YFe2+/(YFe2++Licalc.) value and lower Si, YAl, Licalc. and X-site vacancy contents, X-vacancy/(Na+X-vacancy) value than the core. Both zones are hydrothermal. The rim is an overgrowth. The fluor-elbaite gemmy crystals of the generation 3 occur in sheared breccia blasts and clasts with a cookeite matrix. They depend mainly on the fluid-rich hydrothermal environment of low temperatures (280-150ºC). Some from the cycle a may result from the dissolution of magmatic tourmaline crystals of the generation 1 from the sheared outer and inner intermediate zones of the pegmatite due to reaction with late fluids in chemical disequilibrium, followed by growth of tourmaline with low temperature hydrothermal fluids. The evolution from the cycle a to the cycle b and to the cycle c of the generation 3 implies that the hydrothermal reacting fluids were undergoing fractionation and becoming richer in Li and poorer in Fe2+ during the late hydrothermal crystallization of the pegmatites.

Significant boron isotopic fractionation in the magmatic evolution of Himalayan leucogranite recorded in multiple generations of tourmaline

The late-stage evolution of leucogranite magmas and related magmatic-hydrothermal processes which could lead to rare-metal mineralization are still poorly studied in the Himalayas. Since tourmaline is among the minerals that are present in evolved granites of the Himalayas, the geochemistry of this mineral, and in particular the boron isotopic composition, is a powerful tool to trace evolution processes and sources of these magmas. This study focuses on the analysis of tourmalines in the Gurla Mandhata tourmaline leucogranites (GMTL) from the northwest of the Himalayan orogen. Two generations of tourmaline show clear differences in texture, major element, and B isotopic compositions. Early-stage tourmalines have high-Mg# [Mg/(Mg + Fe) 0.39–0.45] and occur as inclusions in late-stage tourmalines [Mg# <0.34] and other minerals. The δ11B values are in the range of −7 ~ −8‰ for the early-stage tourmaline and in the range of −12 ~ −15‰ for the late-stage tourmaline. The occurrence of tourmalines with contrasting B isotopic compositions from the same leucogranite body (even in a same specimen) can be explained either by mixing of magmas from different sources with different B isotopic compositions or by remarkable B isotopic fractionation during magma differentiation. As indicated by mineral textures and compositions, the mixing of different magma batches can be ruled out as a major cause in the case of GMTL. Alternatively, the bimodal distribution of tourmaline δ11B most likely suggests the occurrence of multiple-stage B isotopic fractionation during the magma evolution. Our modeling based on Rayleigh fractionation shows that this fractionation may have been induced mainly by extraction of fluid at a late magmatic stage, but crystallization of tourmaline may have also played a minor role. Interestingly, a compilation of δ11B values (>260 data points) in magmatic tourmaline from eight Himalayan leucogranite bodies systematically shows a similar bimodal distribution (i.e., with peaks at −7‰ and − 13‰), implying that this significant B isotopic fractionation may be a common scenario during magma evolution of the Himalayan leucogranites. However, multiple magma sources with contrasting B isotopic compositions cannot be fully ruled out for explaining the bimodal distribution of B isotopic composition. Combining our results with SrNd isotopic and other geochemical characteristics of the Himalayan leucogranites, we propose that the granitic magmas with high δ11B originated either from fluid-present melting involving boron-rich fluids, or more probably from dehydration melting of the Greater Himalayan Sequence that had been metasomatized by boron-rich fluids. The geological context implies that these fluids were probably derived from dehydration of mica-rich rocks from the Lesser Himalayan Sequence underlying the Greater Himalayan Sequence.

Multiple episodes of hydrothermal alteration and uranium mineralization in the Singhbhum Shear Zone, eastern India: constraints from chemical and boron isotope composition of tourmaline

In this study, the chemical and B-isotope composition of tourmalines that coprecipitated with uraninite in schistose rocks of the Mohuldih and Bagjata uranium deposits along the Singhbhum Shear Zone are used to constrain the sources and the nature of the mineralizing fluids, along with the mechanism of primary uraninite precipitation. Three generations of tourmaline are identified, which can be correlated with three generations of uraninite in the host rocks. These correspond to three major episodes of hydrothermal alteration of the shear zone rocks at 1.8-1.9 Ga, 1.66-1.56 Ga, and ca. 1.0 Ga, synchronous with metamorphism/granitoid activity. The texturally earliest tourmaline with δ11B of 8.8 ± 0.3 ‰ precipitated together with primary uraninite from externally-sourced high saline fluids derived from subducting slab during the 1.8–1.9 Ga hydrothermal alteration event. The precipitation of uraninite can be linked to the decreasing salinity of the hydrothermal fluid as a consequence of fluid-rock interaction. Dissolution of chlorite/biotite and the precipitation of more Mg-rich tourmalines resulted in decrease of fluid salinity, which destabilized Fe-chloride and U-chloride complexes leading to the precipitation of Fe-rich tourmaline together with primary uraninite. The texturally later second and third generation tourmalines (δ11B = 2.5 to 6.3 ‰) and uraninites formed from mixed fluids derived from the slab and dehydration of pelitic assemblages during renewed episodes of hydrothermal alteration synchronous with metamorphism at 1.66-1.56 Ga and ca. 1.0 Ga. The δ11B of the tourmalines from the schists of the Singhbhum Shear Zone correspond to the range of values (–10.4 ‰ to +6.0 ‰) reported for IOCG-type deposits associated with metamorphic slab-derived fluid sources.

Multiple generations of tourmaline from Yushishanxi leucogranite in South Qilian of western China record a complex formation history from B-rich melt to hydrothermal fluid

Abstract One tourmaline-bearing leucogranite dike occurs in the Yushishanxi Nb-Ta mining area in the Yushishan district of the South Qilian orogenic belt in western China. Abundant tourmalines have been identified in the leucogranite, including disseminated, crosscutting quartz-tourmaline veins and tourmaline veinlets. Detailed petrological, geochemical, and boron isotopic studies indicate that these tourmalines have distinctive core-rim zoning signatures and significant chemical variations, which can be divided into four paragenetic generations. Generation-I tourmalines in the leucogranite have a magmatic origin and were followed by hydrothermal Generation-II, Generation-III, and Generation-IV. Tourmalines from all four generations belong to the alkali group and the schorl-dravite solid-solution series. The most notable features are the variations in Mg, Fe, and Ca contents. The variable Mg/(Mg+Fe) ratios in generations I to IV may be attributed to the multiple influx and/or interaction of initially magma-derived hydrothermal fluid with surrounding rocks, magma-derived Fe-rich hydrothermal fluid, and external metamorphic fluid. In situ B-isotope analyses of tourmaline yield a total range of δ11B values from –11.7 to –6.0‰. The earliest Generation-I tourmalines have δ11B values of –11.1 to –9.6‰, whereas Generation-II and Generation-III tourmalines record a higher δ11B value of –9.5 to –6.0‰ and –9.3 to –6.9‰, respectively. Such an increase is mainly controlled by boron-isotope fractionation between melt-fluid and tourmaline-fluid, Rayleigh fractionation, and also, to some extent, by a hydrothermal recharge from the same magma source. Generation-IV tourmalines have the lightest B-isotope values (down to –11.7‰). This shift back to lighter B-isotopes in the Generation-IV cannot be explained by closed-system crystallization and isotope fractionation of known phases. Alternatively, a lighter source of boron via fluid circulation from the surrounding metamorphic rocks can explain this light boron isotope composition. Therefore, we propose that an isotopically light fluid may have derived from B release during mica breakdown within the surrounding metamorphic rocks.

Tourmaline as a Tracer of Late-Magmatic to Hydrothermal Fluid Evolution: The World-Class San Rafael Tin (-Copper) Deposit, Peru

Abstract The world-class San Rafael tin (-copper) deposit (central Andean tin belt, southeast Peru) is an exceptionally large and rich (&amp;gt;1 million metric tons Sn; grades typically &amp;gt;2% Sn) cassiterite-bearing hydrothermal vein system hosted by a late Oligocene (ca. 24 Ma) peraluminous K-feldspar-megacrystic granitic complex and surrounding Ordovician shales affected by deformation and low-grade metamorphism. The mineralization consists of NW-trending, quartz-cassiterite-sulfide veins and fault-controlled breccia bodies (&amp;gt;1.4 km in vertical and horizontal extension). They show volumetrically important tourmaline alteration that principally formed prior to the main ore stage, similar to other granite-related Sn deposits worldwide. We present here a detailed textural and geochemical study of tourmaline, aiming to trace fluid evolution of the San Rafael magmatic-hydrothermal system that led to the deposition of tin mineralization. Based on previous works and new petrographic observations, three main generations of tourmaline of both magmatic and hydrothermal origin were distinguished and were analyzed in situ for their major, minor, and trace element composition by electron microprobe analyzer and laser ablation-inductively coupled plasma-mass spectrometry, as well as for their bulk Sr, Nd, and Pb isotope compositions by multicollector-inductively coupled plasma-mass spectrometry. A first late-magmatic tourmaline generation (Tur 1) occurs in peraluminous granitic rocks as nodules and disseminations, which do not show evidence of alteration. This early Tur 1 is texturally and compositionally homogeneous; it has a dravitic composition, with Fe/(Fe + Mg) = 0.36 to 0.52, close to the schorl-dravite limit, and relatively high contents (10s to 100s ppm) of Li, K, Mn, light rare earth elements, and Zn. The second generation (Tur 2)—the most important volumetrically—is pre-ore, high-temperature (&amp;gt;500°C), hydrothermal tourmaline occurring as phenocryst replacement (Tur 2a) and open-space fillings in veins and breccias (Tur 2b) and microbreccias (Tur 2c) emplaced in the host granites and shales. Pre-ore Tur 2 typically shows oscillatory zoning, possibly reflecting rapid changes in the hydrothermal system, and has a large compositional range that spans the schorl to dravite fields, with Fe/(Fe + Mg) = 0.02 to 0.83. Trace element contents of Tur 2 are similar to those of Tur 1. Compositional variations within Tur 2 may be explained by the different degree of interaction of the magmatic-hydrothermal fluid with the host rocks (granites and shales), in part because of the effect of replacement versus open-space filling. The third generation is syn-ore hydrothermal tourmaline (Tur 3). It forms microscopic veinlets and overgrowths, partly cutting previous tourmaline generations, and is locally intergrown with cassiterite, chlorite, quartz, and minor pyrrhotite and arsenopyrite from the main ore assemblage. Syn-ore Tur 3 has schorl-foititic compositions, with Fe/(Fe + Mg) = 0.48 to 0.94, that partly differ from those of late-magmatic Tur 1 and pre-ore hydrothermal Tur 2. Relative to Tur 1 and Tur 2, syn-ore Tur 3 has higher contents of Sr and heavy rare earth elements (10s to 100s ppm) and unusually high contents of Sn (up to &amp;gt;1,000 ppm). Existence of these three main tourmaline generations, each having specific textural and compositional characteristics, reflects a boron-rich protracted magmatic-hydrothermal system with repeated episodes of hydrofracturing and fluid-assisted reopening, generating veins and breccias. Most trace elements in the San Rafael tourmaline do not correlate with Fe/(Fe + Mg) ratios, suggesting that their incorporation was likely controlled by the melt/fluid composition and local fluid-rock interactions. The initial radiogenic Sr and Nd isotope compositions of the three aforementioned tourmaline generations (0.7160–0.7276 for 87Sr/86Sr(i) and 0.5119–0.5124 for 143Nd/144Nd(i)) mostly overlap those of the San Rafael granites (87Sr/86Sr(i) = 0.7131–0.7202 and 143Nd/144Nd(i) = 0.5121–0.5122) and support a dominantly magmatic origin of the hydrothermal fluids. These compositions also overlap the initial Nd isotope values of Bolivian tin porphyries. The initial Pb isotope compositions of tourmaline show larger variations, with 206Pb/204Pb(i), 207Pb/204Pb(i), and 208Pb/204Pb(i) ratios mostly falling in the range of 18.6 to 19.3, 15.6 to 16.0, and 38.6 to 39.7, respectively. These compositions partly overlap the initial Pb isotope values of the San Rafael granites (206Pb/204Pb(i) = 18.6–18.8, 207Pb/204Pb(i) = 15.6–15.7, and 208Pb/204Pb(i) = 38.9–39.0) and are also similar to those of other Oligocene to Miocene Sn-W ± Cu-Zn-Pb-Ag deposits in southeast Peru. Rare earth element patterns of tourmaline are characterized, from Tur 1 to Tur 3, by decreasing (Eu/Eu*)N ratios (from 20 to 2) that correlate with increasing Sn contents (from 10s to &amp;gt;1,000 ppm). These variations are interpreted to reflect evolution of the hydrothermal system from reducing toward relatively more oxidizing conditions, still in a low-sulfidation environment, as indicated by the pyrrhotite-arsenopyrite assemblage. The changing textural and compositional features of Tur 1 to Tur 3 reflect the evolution of the San Rafael magmatic-hydrothermal system and support the model of fluid mixing between reduced, Sn-rich magmatic fluids and cooler, oxidizing meteoric waters as the main process that caused cassiterite precipitation.

The nature and sources of ore-forming fluids in the Bhukia gold deposit, Western India: constraints from chemical and boron isotopic composition of tourmaline

Gold mineralization in the newly discovered Bhukia deposit in northwestern India is hosted in Proterozoic metamorphosed volcano-sedimentary rocks of the Aravalli-Delhi Belt. Three generations of tourmaline occurring in different textural settings are recognized in the host rocks of the deposit. Fine-grained texturally early tourmalines (Tur-I) precipitated during the gold-sulfide mineralization stage. They are dravitic in composition and occur parallel to the S1 tectonic foliation as fine-grained crystals in tourmaline-albite layers within quartz-albite rock and as scattered grains in calc-silicate rocks. Coarse-grained texturally later schorls (Tur-II) are also restricted to calc-silicate and quartz-albite rocks and characterized by sector zoning. Late-stage type III tourmaline (Tur-III), also of schorl composition, replaces Tur-II along fractures and margins. They are interpreted to have formed during a phase of ore remobilization. All tourmalines, especially the schorls, are strongly enriched in Li, Ga, Mn and Zn with Ga concentrations being the highest ever reported in natural tourmalines (up to 1380 ppm). The boron isotope composition is similar in all three tourmaline types with the consistently light δ11B (−10.4‰ to −7.2‰) indicative of a continental source for B. The chemical and B-isotopic composition of tourmaline is suggestive of the involvement of two fluids, a granitic-derived hydrothermal fluid and metapelite-derived metamorphic fluid. The δ11B variations in the tourmalines can be explained by mixing between 11B-poor granitic-derived hydrothermal fluid and 11B-rich metamorphic-hydrothermal fluid. While high V (2110–4247, avg. 3339 ppm) in the early dravitic tourmalines indicate mixing of granitic-derived hydrothermal fluid with pelite-derived metamorphic hydrothermal fluids during gold-sulfide mineralization, the enrichment of Li, Mn, Zn and Ta, and depletion of V in the later schorlitic tourmalines suggest increasing influence of granitic-derived hydrothermal fluids during ore remobilization.

Mineralogy of Ti-bearing, Al-deficient tourmaline assemblages associated with lamprophyre dikes near the O&amp;rsquo;Grady Batholith, Northwest Territories, Canada

Calc-alkaline lamprophyre dikes are hosted by tourmalinized metasedimentary rocks in the Northwest Territories, Canada. Some of these lamprophyre dikes are cross-cut by aplite and pegmatite dikes, as well as tourmaline-bearing quartz veins that were all derived from the nearby granitic O'Grady Batholith. The lamprophyre dikes are composed of actinolite to magnesio-hornblende, plagioclase, K-feldspar and quartz with minor phlogopite (up to 4.13 wt. % TiO2), titanite, apatite, pyrite, allanite-(Ce), and zircon. A zone near the margin of one of the dikes has been altered to tourmaline associated with actinolite to magnesio-hornblende, clinochlore, titanite and quartz, with minor clinopyroxene and apatite. Two generations of tourmaline are recognized: Tur I occurs in quartz at the margin of the dike and Tur II forms a massive aggregate with common inclusions of other minerals in an altered lamprophyre zone near the margin of the dike. The vast majority of the analyzed tourmaline is Al-deficient, with less than 6 apfu Al at the Z site (on average 5.691 apfu in Tur I and 5.601 apfu in Tur II). Tur I is mostly dravite with uvite, plus minor feruvite and fluor-uvite, while Tur II contains a greater proportion uvite, feruvite, and fluor-uvite. The most evolved tourmaline compositions observed are feruvite with up to 2.17 wt. % TiO2, and fluor-uvite with up to 0.84 wt. % F. The tourmaline composition reflects the unique geochemical environment in which it crystallized; from Tur I to Tur II, tourmaline becomes richer in Ca-, Fe-, and Ti, presumably due to the reaction of B-bearing fluids with the Al-poor, Ca-, Mg-Fe-, and Ti-bearing minerals in the lamprophyre dike. The high F contents of some tourmaline species suggest that it crystallized from fluids derived from the aplite and pegmatite dikes.

Bosiite, NaFe3+ 3(Al4Mg2)(Si6O18)(BO3)3(OH)3O, a new ferric member of the tourmaline supergroup from the Darasun gold deposit, Transbaikalia, Russia

Bosiite, NaFe3+3(Al4Mg2)(Si6O18)(BO3)3(OH)3O, is a new mineral species of the tourmaline supergroup from the Darasun gold deposit (Darasun mine), Vershino-Darasunskiy, Transbaikal Krai, Eastern-Siberian Region, Russia (52°20′24″N, 115°29′23″E). Bosiite formed as a hydrothermal phase in a gold-bearing quartz-vein spatially related to the Amudzhikan–Sretensky subvolcanic K-rich granodiorite-porphyry intrusion. Ores of this deposit are enriched in sulfides (up to 60%). Bosiite is intimately associated with other tourmalines. The first tourmaline generation is bosiite, which is followed by a second generation of oxy-dravite and a third generation of dravite. Bosiite also coexists with quartz and pyrite; further associated minerals in the vein are gangue minerals (quartz, calcite, and dolomite), sulfides (pyrite, arsenopyrite, chalcopyrite, pyrrhotite, tetrahedrite, sphalerite, and galena) and native gold. Crystals of bosiite are dark brown to black with a pale-brown streak. Bosiite is brittle and has a Mohs hardness of 7; it is non-fluorescent, has no observable parting and cleavage. It has a measured density of 3.23(3) g/cm3 (by pycnometry) and a calculated density of 3.26(1) g/cm3. In plane-polarized light, it is pleochroic, O = yellow-brown, E = red-brown. Bosiite is uniaxial negative, ω = 1.760(5), ϵ = 1.687(5). The mineral is trigonal, space group R 3 m , a = 16.101(3), c = 7.327(2) A, V = 1645.0(6) A3. The eight strongest X-ray diffraction lines in the (calculated) powder pattern [ d in A( I ) hkl ] are: 2.606(100)(50-1), 8.051(58)(100), 3.008(58)(3-1-2), 4.025(57)(4-20), 3.543(50)(10-2), 4.279(46) (3-11), 2.068(45)(6-1-2), 4.648(28)(300). Analysis by a combination of electron microprobe (EMPA), inductively coupled plasma mass spectrometry (ICP-MS), Mossbauer spectroscopic data and crystal-structure refinement results in the empirical structural formula: ![Formula][1] According to the IMA-CNMNC guidelines, the dominant valence at the Y site is 3+ and the dominant cation is Fe3+. To accommodate the disorder and allocating cations to the Z and Y sites, the recommended procedure leads to the optimized empirical formula (based on 31 O): X (Na0.73Ca0.23□0.04) Y (Fe3+2.40Fe2+0.59Ti4+0.01) Z (Al3.36Mg1.69Fe3+0.95) T (Si5.92Al0.08O18) (BO3)3 V (OH)3 W [O0.85(OH)0.15]. Bosiite, ideally NaFe3+3(Al4Mg2)(Si6O18)(BO3)3(OH)3O, is related to end-member povondraite, ideally NaFe3+3(Fe3+4Mg2)(Si6O18)(BO3)3(OH)3O, by the substitution Z Al4 → Z Fe3+4. Further, bosiite is related to oxy-dravite, ideally Na(Al2Mg)(Al5Mg)(Si6O18)(BO3)3(OH)3O, by the substitutions [6]Fe3+3 → [6]Al3. Bosiite is named after Dr. Ferdinando Bosi, researcher at the University of Rome La Sapienza, Italy, and an expert on the crystallography and mineralogy of the tourmaline-supergroup minerals and the spinels. [1]: /embed/mml-math-1.gif

Episodic tourmaline growth and re-equilibration in mica pegmatite from the Bihar Mica Belt, India: major- and trace-element variations under pegmatitic and hydrothermal conditions

AbstractMica pegmatites from the Bihar Mica Belt contain three distinct generations of tourmaline. The major-element composition, substitution vectors and trajectories within each group are different, which indicates that the three types of tourmalines are not a part of one evolutionary series. Rather, the differences in their chemistries as well their mutual microtextural relations, can be best explained by growth of tourmaline from pegmatitic melts followed by episodic re-equilibration during discrete geological events. The euhedral, coarse-grained brown type I tourmaline cores have relatively high Ca, Mg (XMgc. 0.37) and Al with correlated variation in Sr, Sc, Ti, Zr, Y, Cr, Pb and Rare Earth elements (REEs). They are inferred to have crystallized from pegmatitic melts. Monazites included within these tourmalines give chemical ages of 1290−1242 Ma interpreted to date the crystallization of the pegmatitic tourmaline. The bluish type II and greyish type III tourmalines with low Ca and Mg contents (XMg = 0.16−0.27) and high Zn, Sn, Nb, Ta and Na, formed by pseudomorphic partial replacement of the pegmatitic tourmaline via fluid-mediated coupled dissolution–reprecipitation, are ascribed to a hydrothermal origin. The ages obtained from monazites included in these tourmalines indicate two alteration events at c. 1100 Ma and c. 950 Ma. The correlated variation of Ca, Mg and Fe and the trace elements Sr, Sn, Sc, Zn and REE within the tourmalines indicates that the trace-element concentrations of tourmaline are controlled not only by the fluid chemistry but also by coupled substitutions with major-element ions.

Tourmaline from quartz lenses of the Urtui granite pluton, Strel’tsovka orefield, Ttansbaikal krai

Quartz-tourmaline lenses, around which host granite is impregnated by uraninite, have been found among porphyritic granite with large phenocrysts of the Urtui pluton in the Ttansbaikal krai framing the Strel’tsovka volcano-tectonic structure. Two generations of tourmaline are distinguished. Most individual crystals belong to the first generation attributed to “fluor-schorl”; tourmaline-II attributed to schorl occurs as thin rims overgrowing tourmaline-I. The major type of cation isomorphic substitution in both tourmalines is Fe2+ → Mg. The Fe3+/Fetot value and Li content in the average sample are 2% and 80 ppm, respectively. The high F content, comparatively high Li, low Fe3+/Fetot value, and character of cation isomorphic substitution indicate that the tourmaline relates to greisens. The combination of these features allows one to distinguish greisen-type tourmaline-bearing rocks. The impregnated uranium mineralization in granite of the Urtui pluton, one of the probable sources of uranium in economic U ore of the Strel’tsovka deposit, is suggested to be caused by greisenization and the formation of quartz-tourmaline lenses.

Chemical and growth zoning in trapiche tourmaline from Zambia — a re-evaluation

Trapiche tourmalines from Zambia show the hexagonal prism a {112 - 0}, the positive pyramids r {101 - 1} and o {022 - 1} and the negative pyramid -r {011 - 1 } as dominant external crystal forms. The trapiche pattern is formed by the positioning of liquid and solid inclusions which are trapped at growth boundaries between pyramidal and prismatic growth sectors. Growth-induced elongate voids are also formed perpendicular to the dominant growth faces, which are inclined at 27°, at 46° or at 90° to the pedion {0001} of the tourmaline hosts. All the trapiche tourmalines are chemically zoned with isomorphic substitutions of Ca by Na and Mg by Al, which is characteristic for tourmalines of the uvite-dravite solid solution series. Content of Na increases and of Ca decreases from core to rim of all tourmalines. However, at the boundaries between different growth sectors (forming the geometric trapiche pattern of the samples), tourmaline compositions with higher Na and lower Ca contents than measured in adjacent pyramidal and prismatic growth sectors were found. The formation of this complex structural and chemical pattern is consistent with the formation of the tourmaline crystals in a two step process, with the skeletal growth of a sodium-rich dravite in a fi rst step and the subsequent, layer-by-layer growth of a second tourmaline generation. The growth of the second generation started with calcium-rich fl uor-uvite and was followed step-by-step by a more sodium-rich dravite. The green coloration of the samples is caused by minor contents of vanadium and chromium.

Tourmaline in the Vetka porphyry copper-molybdenum deposit of the Chukchi Peninsula of Russia

Three generations of tourmaline have been identified in propylite in the Vetka porphyry copper-molybdenum deposit of the Chukchi Peninsula of Russia. Tourmaline-I is characterized by its Fetot/(Fetot + Mg) value, which ranges from 0.33 to 0.49. Tourmaline-II, which crystallizes at a lower temperature, overgrowing tourmaline-I or occurring as isolated crystals, is distinguished by a higher Fetot/(Fetot + Mg), which varies from 0.46 to 0.72. The Fetot/(Fetot + Mg) ratio in tourmaline-III, which overgrows tourmaline-II is lower (0.35–0.49), and is identical to that of the first tourmaline generation. This is probably caused by the beginning of sulfide deposition. Tourmalines in the deposit characterized by complex isomorphic substitutions can be attributed to the intermediate members of the dravite—“hydroxy-uvite”-“oxy-uvite” and schorl-“hydroxy-feruvite”-“oxy-feruvite” series. Tourmaline starts to crystallize at temperatures above 340°C. The fluid responsible for the tourmaline deposition was magmatic, with a significant admixture of meteoric water (δ18OH2O = −0.85 to −0.75‰). The high Fe3+/Fetot ratio (0.50) indicates high oxygen activity when the tourmaline precipitated. It has been established that the isomorphic substitution Fetot → Al is typomorphic of tourmalines from porphyry copper deposits worldwide.

GEOLOGY AND CHEMICAL VARIATIONS IN TOURMALINE FROM THE QUARTZ-TOURMALINE BRECCIAS WITHIN THE KERKENEZ GRANITE-MONZONITE MASSIF, CENTRAL ANATOLIAN CRYSTALLINE COMPLEX, TURKEY

A hydrothermal breccia zone 6 km long and 2 km wide, crossed by quartz–tourmaline veins, occurs to the south of the Late Cretaceous Kerkenez granite–monzonite massif in the northern part of the Central Anatolian Crystalline Complex, Yozgat, Turkey. This breccia zone is characterized by numerous veins, a few millimeters to 1 meter in width, and display typical micro-tectonic features of multiple brecciation. Trace amounts of albite, K-feldspar, epidote, muscovite and rutile are associated with quartz and tourmaline. Petrographic features indicate three successive stages of tourmaline generation. The first generation is represented by feruvite, and the second and third generations consist of schorl. These distinct generations of tourmaline show successive enrichment in Al and Na, and depletion in Ti and Ca. The Na/(Na + Ca) values vary in the range 0.41–0.49 in the first generation, and 0.53–0.86 and 0.91–0.99 in the second and third generations. The development of the breccia zone and formation of three successive generations of tourmaline are most probably related to fluid-assisted brecciation and infiltration of low- to medium-pH hydrothermal fluids under subsolidus conditions. Successive enrichment in Na and Al in the younger generations is ascribed to late-stage fractionation in the B-rich granitic source.

Stable Isotope Constraints on Ore Formation at the San Rafael Tin-Copper Deposit, Southeast Peru

The San Rafael tin-copper deposit in the Eastern Cordillera of the Peruvian Central Andes is the world’s largest hydrothermal tin lode, with a total resource of about 1 million metric tons metal, at an average grade of 4.7 wt percent Sn. The mineralization is of the cassiterite-sulfide type and occurs in a vertically extensive veinbreccia system centered on a shallow-level, late Oligocene granitoid stock. The tin ores form cassiterite-quartzchlorite‐bearing veins and breccias hosted by several large fault-jogs at depth in the lode. By contrast, the copper ores, which contain disseminated acicular cassiterite, are localized in the upper part of the system. Both ore types are associated with a very distinctive, strong chloritic alteration, which was preceded by intense sericitization, tourmalinization, and tourmaline veining. The δ34S values of the sulfides range between 2 and 6 per mil, and vary very little with location in the deposit. This indicates that the hydrothermal system was large, with a relatively homogeneous source of sulfur, likely of magmatic origin. This is confirmed by stability relationships of ore minerals, which indicate that the ore fluids were initially reducing. Microthermometric studies of fluid inclusions in cassiterite, quartz, tourmaline, and fluorite show that the fluids responsible for the early, barren stage were hot, hypersaline brines (380°‐540°C, 34‐62% NaCl equiv), whereas the ore-stage fluids had moderate to low salinity (0‐21 wt % NaCl equiv), and were of moderate temperature (290°‐380°C). In addition to the marked dilution of the ore fluids with evolution of the hydrothermal system, they became progressively more oxidizing, as inferred by the local association of minor hematite with cassiterite and the ubiquitous replacement of pyrrhotite by pyrite and marcasite. The δ18O values of the fluid decreased systematically with time, as indicated by the δ18O values of different generations of tourmaline, cassiterite, and quartz. This evolution was paralleled by an increase in the δD values of the fluid, inferred from the δD values of tourmaline and chlorite. This trend is consistent with mixing of the ore fluids with a cooler fluid that had substantially lower δ18O, and cannot be explained by fluid boiling. Based on structural evidence for an opening of the vein system and a transition from lithostatic to hydrostatic conditions at the onset of mineralization, we infer that ore deposition was caused by an influx of hot groundwater of meteoric origin which mixed repeatedly with tin-bearing magmatic brines. The oxidation, dilution, cooling, and acid neutralization of the ore fluids destabilized chloride complexes of tin and triggered the large-scale precipitation of cassiterite.

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.

A bone fragment of a probable aquatic vertebrate in the Serra Alta Formation (upper permian of Paraná Basin)

much darker than the cores, variations due to varying Fe and Mg contents. A second generation of light green tourmalines also occurs in quartz veins of Quartzite. These are alkalideficient, Crand V-bearing tourmalines with higher Mg# than those of the schorl-dravite series. The occupancy of the X-site, according to X-ray fluorescence data, is δ0,51Ca0.33Na0.15, thus corresponding to foitite, considered as an alkali-deficient schorl. Raman studies also discriminated two groups of tourmalines, one belonging to the buerguerite-schorl series and the other to the dravite-buerguerite-uvite series. Stable isotope data allowed to define sediment and hydrothermal waters as fluid sources, ruling out the association of the tourmalines with e.g. the Brasiliano granitoid bodies found in the area. δ18O compositions for tourmalines (+12 per mil) and host metachert and quartz veins (+13 per mil) are very similar, showing fluid equilibration during (re)crystallization of quartz and tourmaline. The presence of at least two distinct groups of tourmalines indicates distinct environments and timing for tourmaline generation. In Tapera Grande, tourmalines were formed in a submarine exhalative-sedimentary environment. Their composition was not strongly affected by medium-grade metamorphism. In Quartzite, tourmaline compositions reflect that of the country rock, once fluid percolation along Sertaozinho fault and associated fractures caused leaching of Cr (and V) and the crystallization of alkali-deficient, Cr(V-)bearing tourmalines in veins, together with quartz. The heat source for mineralizing fluids must have been a granitoid body (Pau Pedra) south of Tapera Grande and intermediate to acid pipes in the Quartzito area. These fluids were also responsible for distinct types of mineralization, characterized in Tapera Grande by the assemblage gold-pyrite-pyrrhotite-chalcopyrite, and in Quartzite by electrum-pyrite-chalcopyrite-sphaleritegalena-scheelite-molibdenite. — ( December 14, 2001 ) .