Tourmaline Grains Research Articles

Provenance of the Oligocene Barail Sandstones Exposed In and Around Longsa Village, Wokha District, Nagaland: Reflections from Petrography and Heavy Minerals

Petrographic and heavy mineral compositions of the Oligocene Barail sandstones exposed in and around Longsa village of Wokha District, Nagaland, have been utilized for provenance interpretation. Grey and yellow coloured sandstones of the study area are comprised of angular to sub-rounded monocrystalline, non-undulatory quartz and the various lithic fragments. The overall composition of sandstones (Q-90.10%, F-1.70 %, RF-8.18%) matches with those of sublith-arenites category. The heavy mineral assemblage of the studied Barail sandstones is represented by euhedral/rounded to sub-rounded grains of zircon, tourmaline, rutile, kyanite, sillimanite, andalusite, titanite, staurolite and iron oxide. The studied sandstone composition suggests that the Barail sediments were derived from mixed provenance. Heavy mineral analysis also corroborates the above. Keywords: Petrography, Heavy Minerals, Provenance, Barail Group, Nagaland

Stable hydrogen isotope compositions of tourmalines from the Sopron metamorphic complex: Metamorphic closure temperature and fluid composition

AbstractStable hydrogen isotope compositions of metamorphic rocks and minerals can provide information on the origin of metamorphic fluids, which is especially important in systems that had experienced multiple metamorphic events. The Sopron orthogneiss-micaschist complex is a good target as it records signs of Variscan and Alpine metamorphic events as well as Variscan granitic magmatism. In this study tourmaline-bearing rocks (pegmatitic orthogneisses and kyanite-chlorite-muscovite schists) of the Sopron metamorphic complex were sampled and their tourmaline grains were analyzed for stable hydrogen isotope compositions (δ2H). The δ2H values (−23 ± 1‰, relative to V-SMOW) are in accordance with a fluid flux from devolatilization of subducted, seawater-containing rocks. Tourmaline-chlorite hydrogen isotope fractionations correspond to about 550 °C, indicating that δ2H values formed close to peak metamorphic temperatures are preserved without retrograde isotope exchange during cooling.

Chemistry and boron isotope composition of tourmaline as a robust tool to characterize the origin of porphyry molybdenum systems: The case of Donggebi deposit in East Tianshan, China

Tourmaline is a chemically and mechanically stable mineral that commonly occurs in hydrothermal mineral systems, and has been used to trace the mineralization processes of many genetic types of ore deposits. Here we apply tourmaline chemistry and isotopic composition to trace the genesis of a porphyry Mo system for the first time. Tourmaline is widespread in the Donggebi Mo deposit in East Tianshan, NW China, where it occurs in five distinct associations: type PG disseminated in the porphyritic granite, TS as hydrothermal tourmalite in the country rocks (strata), V1 in quartz-molybdenite vein, V2 in quartz-polymetallic sulfide vein, and V3 quartz-calcite-fluorite vein. All the tourmaline types belong to the alkali group, with the V3 being schorl and the other types being dravite and schorl. The PG-type tourmalines partly inherit the features of the porphyritic granite, and are rich in F, Na, Al and Sc, and poor in Fe, Mg, Ca, Sr, Ba, Pb, V and Mo compared to the other types. They have low Fe3+ concentrations and low Fe3+/(Fe3++Fe2+) ratios, suggesting a reducing fluid system. They also display weaker Eu-depletion (with an average (Eu/Eu*)N value of 0.96) than the porphyritic granite, and are considered to be the products of interaction of the reducing fluids and the granites. The average (Eu/Eu*)N values of tourmalines decrease from 6.66 of type V1, 5.65 of type V2, to 4.80 of type V3, indicating that the fluids became progressively less reducing along with sulfide precipitation. The δ11B values of the tourmalines range between −15.9 ‰ and −12.5 ‰, resembling those of S-type granites or clastic sediment-sourced hydrothermal fluids. The tourmaline grains in our study do not display any remarkable δ11B variation from the cores to rims, indicating that the δ11B values of the fluids and their source were relatively constant.

Boron isotopes in tourmaline from drill core of the Jiajika granitic pegmatite type lithium deposit: Insights for granitic magma evolution and lithium enrichment

The Jiajika pegmatite-type Li deposit located in the southeastern Songpan-Ganze terrane of the eastern Tibetan Plateau is one of the largest hard-rock Li deposit in China, and its ore-genetic mechanism is still controversial. To understand the Li concentration of potentially economic levels in the Jiajika pegmatite-type Li deposit, the association between Li enrichment and granitic magma evolution is investigated using B isotope in tourmaline grains along the entire 3211-m-depth of the JSD-1 drill core in this work. Tourmalines hosted in granite, pegmatite, barrovian-type metamorphic rock and calc-magnesium-silicate metamorphic rock are being plotted in the fields of Li-poor granitoids and associated pegmatites and aplites on the Al-Fe-Mg and Ca-Fe-Mg ternary diagrams. These characteristics, along with their high Fe/Mg ratios and high Al cation numbers in Y position, are indicative of a magmatic origin. The B isotope (δ11B) of tourmalines in granite and pegmatite in Jiajika granitic pegmatite type Li deposits vary from −9.49 to −7.06‰, in line with the range observed in 90 % of worldwide granites and pegmatites. In comparison of the measured data of δ11B vs. Li to the modeled curves indicates that the differentiation process of granitic magma follows an equilibrium crystallization model better than a fractional crystallization model. The compiled evidence proves that magma differentiation accompanied by fluid exsolution facilitates the enrichment of Li during granitic magmatic evolution. The formation of the domal structure of granitic sheets due to the decompressing effect of magma ascent favors massive fluid exsolution at the late-stage granitic magma, which contributes to the deposition of spodumene dominated ore body at the shallower depth of 5–150 m.

Boron isotope compositions of coexisting kornerupine and tourmaline in high-grade metabasic rocks: an example from Akarui Point, Lützow-Holm Complex, East Antarctica

Boron isotope compositions were measured in kornerupine and tourmaline from lenses consisting primarily of kornerupine, plagioclase and corundum. The lenses occur within hornblende-gneiss or along the boundary between this gneiss and an amphibolite lens at Akarui Point in the Lützow-Holm Complex, Prince Olav Coast, East Antarctica. The peak metamorphic conditions have been estimated to be ~ 800-900 oC and ~ 8-11 kbar. The δ11B compositions of kornerupine, which is interpreted to have been a stable phase at the metamorphic peak, are -11.6 ± 0.4 to -7.8 ± 0.5 ‰ and -9.8 ± 0.3 to -6.1 ± 0.2 ‰ in two different samples. Grains of prograde tourmaline included in kornerupine and corundum yielded δ11B = -2.1 ± 0.3 to +0.6 ± 0.3 ‰, and the secondary tourmaline replacing kornerupine yielded δ11B = -4.6 ± 0.2 to -3.7 ± 0.2 ‰. Therefore, the isotopic fractionation between kornerupine and tourmaline, △11BTur–Krn (= δ11BTur - δ11BKrn), of the average prograde tourmaline and average host kornerupine is +6.7 ± 1.5 ‰, which is interpreted to indicate isotopic equilibrium at the metamorphic peak on the basis of previous studies of isotope fractionation between tourmaline and minerals of the kornerupine-prismatine series. The δ11B values obtained on prograde tourmaline are between whole rock δ11B of MORB and mantle rocks and of some sedimentary rocks, and are similar to the δ11B of blackwall tourmalines that crystallized during the decompression stage following high-pressure metamorphism. We infer that the syn-metamorphic B-bearing fluid present in the kornerupine-plagioclase-corundum lens is likely sourced from a mixture of sedimentary, mafic and ultramafic lithologies in a subduction setting. The metabasic and meta-ultramafic lenses found in Akarui Point could be interpreted as the remnant of mixing zone of Ediacaran to Cambrian subduction channel.

DIFFERENTIATING TOURMALINE SPECIES VIA CHEMISTRY AND REFLECTANCE SPECTROSCOPY AT THE GIANT COPPER PORPHYRY DEPOSIT AND ASSOCIATED TOURMALINE BRECCIA PIPES: TESTING TOURMALINE AS A MINERAL VECTOR

Abstract The A.M. breccia is part of the Giant Copper porphyry deposit in southern British Columbia. It is the only well-defined zoned tourmaline breccia pipe in the Canadian Cordillera. Tourmaline is a common alteration mineral within the A.M. breccia and is spatially associated with Cu mineralization. Observed changes in tourmaline chemistry range from alkali (schorlitic-dravitic) to calcic (feruvitic-uvitic). Tourmaline subspecies vary based on their spatial location within the A.M. breccia. Tourmaline outside of the pipe contains higher concentrations of Mg, whereas tourmaline preferentially incorporates Fe within the pipe. These chemical variations are indistinguishable in hand specimens. Spectral reflectance data were collected from 587 tourmaline grains to determine if discerning chemical changes in tourmaline can be made field-based and thus more cost-effective. Spectral reflectance differentiates tourmaline associated with mineralization and breccia textures from tourmaline occurring distal to the pipe contact or within barren tourmaline breccia pipes. Fe-rich tourmaline within the A.M. breccia shows spectral characteristics of end-member schorl (Fe-rich) spectra. Tourmaline distal to the A.M. breccia and within barren pipes demonstrates spectra of end-member dravite (Mg-rich). This grouping suggests that tourmaline subspecies can be inferred by spectral reflectance, enhancing the efficiency of tourmaline as a mineral vector. Tourmaline was also identified via airborne spectral surveys. However, the airborne spectral survey did not identify the end-member spectral properties identified by in situ analysis. Airborne spectral surveys can rapidly identify tourmaline breccia pipe exposures and expedite early stages of exploration in ore districts where tourmaline is a known gangue mineral.

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.

Chemical and boron isotopic variations of tourmaline deciphering magmatic-hydrothermal evolution at the Gejiu Sn-polymetallic district, South China

The world-class Gejiu Sn-polymetallic district (southwest China), with ca. 3.27 Mt Sn metal, is a magmatic-hydrothermal deposit associated with the Late Cretaceous biotite granites; however, the magmatic and hydrothermal processes involved in its formation remain poorly understood. Tourmaline is a ubiquitous phase in the Gejiu district, and here we employ in-situ major, trace element, and boron isotope compositions of tourmaline to investigate the source and evolution of the mineralizing fluid, and late-magmatic and hydrothermal processes related to ore formation. Based on petrographic observations, four types of tourmaline are identified: i) late-magmatic tourmaline (Tur I) disseminated in the biotite granites, ii) hydrothermal tourmaline (Tur II) from quartz veins with fracture-filling structure in the biotite granites, iii) hydrothermal tourmaline (Tur IIIa-1, IIIa-2 and Tur IIIb-1, IIIb-2) from fluorite-quartz veins with typical replacement texture against the greisenized granites, among which Tur IIIb-2 is directly related to cassiterite formation, and iv) hydrothermal tourmaline (Tur IV) from sulfide-calcite veins hosted by proximal skarn. Tourmaline (Tur I to Tur III) from the granites belongs mostly to the alkali group and schorl-dravite solid-solution series, with Fe/(Fe + Mg) ranging from 0.83–1.00 for Tur I, 0.68–1.00 for Tur II, and 0.19–1.00 for Tur III, respectively. By contrast, Tur IV belongs mostly to the vacancy group and has foitite–Mg-foitite composition, with Fe/(Fe + Mg) of 0.10–1.00. Large variations of Fe/(Fe + Mg) as well as Na/(Na + Ca) in hydrothermal tourmaline (Tur II to Tur IV), are related to different degrees of interaction between B-rich fluids with host rocks (solidified granite or carbonate). Most trace elements in tourmaline do not correlate with Fe/(Fe + Mg) and Na/(Na + Ca) ratios, implying the trace element compositions are predominantly controlled by melt/fluid compositions and local fluid-rock reactions. Relative to late-magmatic Tur I, hydrothermal tourmaline (Tur II to Tur IV) is enriched in Sr and depleted in Nb, Ta, and (REE + Y), whereas their Li, Be, Sc, V, and Sn concentrations are largely overlapped. The δ11B values in different types of tourmaline fall in a narrow range from −17.8 to −13.7‰, in favor of B-rich fluids episodically exsolved from the granitic melt.Tin enrichment in both late-magmatic and hydrothermal tourmaline grains, especially in those (Tur IIIb) from Sn-mineralized veins, is related to magmatic differentiation and late-stage fluid exsolution. Among all types of tourmaline, Tur IIIb-2 coexisting with cassiterite has elevated Fe3+/(Fe3+ + Fe2+) ratios (mean 0.23), indicating that cassiterite precipitated under relatively oxidized conditions. The distinct chemical (i.e. high Mg, Ti, V, Sc, Sr, and Sn contents) and B-isotope compositions (slightly lower δ11B values of −17.8 to −15.0‰) in syn-ore Tur IIIb-2, combined with the co-occurrence of liquid- and vapor-rich inclusions triggered by fluid boiling in Sn-mineralized veins, suggest that fluid boiling and acid-consuming reaction could be major processes triggering cassiterite precipitation in the quartz vein and greisen. Overall, the chemical and B-isotope signatures of tourmaline are ideal tracers to unravel the magmatic-hydrothermal evolution in the Gejiu Sn-polymetallic district, which could be further employed as a potential prospecting guide for SnW deposits.

Boron isotope composition of detrital tourmaline: A new tool in provenance analysis

Tourmaline is a borosilicate mineral commonly found in sedimentary rocks worldwide. It is chemically and mechanically stable and, as a host of boron in crustal rocks, it could be a useful indicator of rock sources. Despite this, its potential as a source indicator using the boron (B) isotope has remained largely under-explored. In this study, we propose a new integrated approach to fingerprint the origin of detrital tourmaline grains and test whether the approach is useful for identifying source rocks. Tourmaline grains in modern river sands from Southern Tibet were analyzed for optical properties, major elements, and B isotope composition. The geochemistry of detrital tourmaline in sand from the Nianchu and Pumchu rivers draining the Himalayan ranges is similar, and the B isotope values indicated they were mainly from Himalayan leucogranites. Tourmalines in the Lhasa River sand were derived from two major sources. Grains with high Al, low Mg, and strongly negative B isotope values (−12‰ to −16‰) were interpreted from the Gangdese arc batholith, whereas those with low Al, high Fe, and B isotope values of −2‰ to 2‰ were proposed most likely from Nyaingentanglha orthogneisses. We examined the relationships between tourmaline properties and found that B isotope values are useful as a source indicator. We also established a database of B isotope values for 2724 tourmaline crystals in different types of exposed rock spanning 37 countries, to promote the use of B isotope values in tourmaline in provenance studies.

Technological investigations of alluvial garnet sands from the Kola Peninsula

The paper presents the investigation results on two technological samples of alluvial garnet sands from Yavr area located in the southwest of the Kola Peninsula. The first sample TP-11/2 (mass 250 kg) was used to study the granulometric, mineralogical and chemical composition of garnet sands. The second sample TP-12 (mass about 6.5 ton) was used to manufacture some garnet concentrates for the waterjet cutting testing. It has been established that the main material of TP-11/2 sample is concentrated in the size of -0.63 + 0.10 mm. The larger particles make 5.0%, the smaller particles make 7.0%. The size distribution in the TP-12 sample is similar. The main minerals of the sands are quartz, feldspar, garnet, amphiboles, pyroxenes, mica, rutile and ilmenite. The percentage of secondary minerals represented by sillimanite, kyanite, apatite, tourmaline, graphite, magnetite, and single zircon grains is less than 2%. Garnet in Javr area sands is 9–-97% in the free phase. The mass concentration of garnet in the sands is 15–19%, the mass concentration of rutile is 1.0%. Garnet corresponds to almandine in composition. A small part of garnets (about 15–20%) contains a pyrope component. The prevailing shape of garnet grains is angular; the profile of the grains is predominantly isometric and elongated. The garnet sands were dressed at the pilot concentration plant of the Mining Institute, in the gravitymagnetic-electric circuit using the following equipment: a fine screen, a spiral sluice, a concentration table, a weak field magnetic separator, a high-intensity magnetic separator, an electrostatic separator for scavenging of rough garnet concentrates. According to the test results, all garnet concentrates with the size of –0.63 + 0.315 mm, –0.315 + 0.18 mm, –0.18 + 0.10 mm were assumed suitable for the waterjet cutting.

Tourmalines as a Tool in Provenance Studies of Terrigenous Material in Extra-Carpathian Albian (Uppermost Lower Cretaceous) Sands of Miechów Synclinorium, Southern Poland

The mature transgressive Albian quartz sands in the Miechów Synclinorium contain a poor (<1%) heavy mineral suite consisting of tourmaline, rutile, garnet, staurolite, ilmenite, zircon, monazite, kyanite, and gahnite. The other minerals, especially those containing Fe and Ti (e.g., biotite), are subordinate. Over 512 tourmaline grains from seven outcrops in the Miechów Segment were analysed using electron probe microanalysis (EPMA). The majority of grains belong to the alkali tourmaline group in which the X-site is dominated by Na (0.4 to 0.9 apfu). Detrital tourmalines are mainly dravite with a prevalent schorl end-member with average XMg values over 0.6. Apart from Mg and Fe, the other Y-site cations rarely exceed 0.1 apfu. Fluorine content is usually below the detection limit. Their chemical composition suggests that most tourmaline grains were sourced from Al-rich and Al-poor metapelites/metapsammites. The main source rocks for the Albian sands were rocks from low- to medium-grade metamorphism, probably from Al-rich quartz-muscovite schist and/or muscovite rich gneisses. Additional minor source rocks were granites and pegmatites coexisting with them. A comparison of the examined tourmaline to tourmaline from possible source areas indicates that these areas were located in the eastern part of the Bohemian Massif and/or eastern Sudetes.

Tourmaline microboudinage: An indicator of its host rheology

Columnar tourmaline in Cenozoic granitic mylonites from the Chongshan metamorphic zone (CMZ), Yunnan, China has deformed, at 450–300 °C, by tensile fractures normal to the foliation and lineation. Sequential fracturing and separation of the fragments during progressive cooling and exhumation upward the brittle-ductile transition produced a series of microboudins with decreasing length/width ratios. Here we present a method to apply the microboudinage paleopiezometry, combined with strain analyses, microstructural observations and thermochronological data to provide constraints on strain rates (ε˙), differential stresses (σ) and their correlations with temperature for the polycrystalline quartz matrix in which tourmaline grains have been fragmented into microboudins. The results yield a flow law of ε˙ = Aσnexp(-Q/RT) with n = 4, Q = 184 kJ/mol and A = 7.180 × 10−7 MPa-ns−1 for naturally deformed quartzite. The differential stresses calibrated by this flow law are very close to the geometrical mean of the lower and upper bounds given by Luan and Paterson (1992) and Gleason and Tullis (1995), respectively. Utilization of this flow law to the present-day crust of the CMZ provides a reliable interpretation for the distribution of focal depths and electrical resistivity structure. The microboudinage piezometer also permits to estimate the mechanical strengths of rocks deformed in the elastic-brittle regime.

Exploration of driving mechanisms of equilibrium boron isotope fractionation in tourmaline group minerals and fluid: A density functional theory study

The equilibrium boron isotope fractionations (α3–4, Δ11B(Tur-fluid)) between tourmaline mineral groups and fluids at 0.5 GPa, 600–1000 K are investigated using density functional theory calculations. The first solvent shell controls boron isotope fractionation in solution, where the β values of both H3BO3 and B(OH)4− decrease with increasing numbers of hydrogen bonds. In supercritical fluids, the weakening of hydrogen bonds and the diversity in configurations of hydrated boron species both contribute to variations in the vibrational frequencies. The 1000lnαTur-fluid value increases with increasing Mg/(Fe+Mg) ratios in the dravite-schorl solid solution series. This effect, together with the crystallization sequence predicted from the Gibbs free energies of tourmaline formation, accounts well for the association of boron isotope and chemical zonation observed in zoned tourmaline grains. The dependence of boron isotope fractionation on the BO bond length in tourmaline group minerals reflects the changes in relevant molar volumes caused by differences in the mass and charge of the atoms occupying the X, Y, and Z sites of tourmalines. This study suggests that the chemical composition of the tourmaline plays an important role in controlling the boron isotope composition in tourmalines crystallized from hydrothermal systems.

Boron Measurement in Tourmaline from Pegmatite Veins, Simdega Area, Chhotanagpur Gneissic Complex, Eastern India using Electron Probe Microanalysis

Tourmaline group of minerals, the primary source of boron, are cyclosilicates which are widespread in the earth’s crust. Earlier studies involving the nomenclature and classification of tourmaline were based on the measurement of its common elements (Al, Mn, Fe, Mg, etc.). In all such studies, boron was assumed to be fixed in the composition and restricted only to the triangular structural site. However, recent discovery of the presence of boron in the tetrahedral structural site as well, necessitates the measurement of boron content. Much of the earlier attempts to measure boron were based on solution methods, and electron microprobe analysis (EPMA) was the least used due to low levels of detection of its analytical crystals. In the present study, we quantify boron – particularly along with fluorine and other major elements – in tourmaline grains using high-sensitivity PC3 analytical crystal. We found that the measured boron content slightly exceeds that of the stoichiometrically calculated boron. Also, the studied tourmalines come under the alkali group in general and belong to the schorl– dravite solid solution series in particular.

Badanie wpływu zawartości minerałów ciężkich na naturalną promieniotwórczość skał miocenu zapadliska przedkarpackiego

In the Miocene rocks of the Carpathian Foredeep, the occurrence of overestimating the measurements of natural radioactivity logs for mudstones was noticed. This phenomenon causes problems with recognition on the basis of natural radioactivity profiling of these rocks from claystones, characterized by inferior petrophysical parameters. In this study, another possible source of increased radioactivity of mudstone rocks, i.e. the content of heavy minerals that tend to accumulate radioactive elements - uranium and thorium - was investigated. Heavy minerals, belonging to the accessory minerals, such as zircon, apatite, monazite, rutile, garnet and titanite, are characterized by increased radioactivity due to substitutions of U and Th in the crystal lattice. Natural radioactivity of the test samples was measured using the gamma spectrometry method with the RT-50 device from the Georadis company. The heavy minerals were then removed from the rock samples using a heavy liquid, previously separating the samples into grain fractions to facilitate the separation of the heavy fraction. The individual grain fractions deprived of heavy minerals were put back together and the uranium, thorium (and potassium) content was re-measured. Microscopic observations were also made to qualitatively analyze the mineralogical structure of the separated heavy fraction. In the tested samples, among others: zircon, tourmaline, garnet, rutile, apatite, chlorite or titanite grains were identified. The results of the measurements of natural radioactivity, before and after the separation of heavy minerals, do not give a clear answer to the problem of excessive indications of the gamma logs. It is necessary to increase the number of analyzed samples. The trend of decreasing amounts of U and Th in the samples from which the heavy fraction was isolated was observed. However, changes in radioactivity are so small that they do not allow to draw final conclusions. Additional analyzes are needed to assess the impact of other sources of uranium and thorium in analyzed rocks, such as the content of organic matter or Th content in clays.

Provenance and diagenesis of Guamá Sandstone, northeastern Pará, Brazil: A Silurian link between the Amazonas and Parnaíba basins

The Guamá Sandstone, exposed in small isolated outcrops in the northeastern region of the State of Pará (Brazil), which are surrounded by Cenozoic sediments, represents a Silurian relic of the Amazonas and Parnaiba basins connected during the Paleozoic and separated only since the Triassic period through the development of the Marajó Basin. Previous outcrop-based studies showed that the Guamá Sandstone is a fine to medium-grained quartzarenite, with high compositional and textural maturity, which was deposited in a coastal marine setting (foreshore/shoreface), correlatable with the environment of the Silurian Nhamundá Formation of the Amazonas Basin. The principal primary sources of the sandstone are associated with rocks formed during the Transamazonico and Brasiliano Cycles according to isotopic dating of zircon. In order to better understand provenance and diagenesis of the Guamá Sandstone core samples of a 50 m deep bore hole, located 6 km north of the town of São Miguel do Guamá, have been studied. The sandstone is usually bioturbated and massive, but planar stratification (foreshore) and cross bedding (shoreface), the latter in the lower part of the profile, are also present indicating a transgressive to regressive coastline. In addition to quartz, the Guamá Sandstone is composed of the following detrital accessory minerals: zircon, tourmaline, rutile, epidote, mica, ilmenite and chromite while kamacite occurred only sporadically indicating, at least for the latter, origin from Fe-Ni-meteorites. Provenance data obtained from the composition of zircon and tourmaline grains, as well as from internal textures of zircon, point to primary igneous and metamorphic source rocks of the Guamá Sandstone. While zircon is principally of magmatic origin, tourmaline grains are derived dominantly from metapelitic, metapsammitic and granitoid rocks. However, pre-existing sediments probably represent, to a lesser extent, the ultimate direct source as suggested by the well-rounded grains of the quartz arenite and worn tourmaline overgrowths. The diagenetic phases of the sandstone include syntaxial quartz overgrowths, microquartz, kaolinite booklets, illite and anatase. The quartz overgrowths are pseudo-zoned and engulf parts of the kaolinite booklets. These may be surrounded by microquartz which usually replaced quartz overgrowths. Illite, which is rare, is probably the product of replacement of kaolinite, and anatase may be related to partial dissolution of ilmenite. The main diagenetic processes including precipitation of quartz and kaolinite began during eodiagenesis, in shallow burial conditions, and proceeded through mesodiagenesis as suggested by multi-episodic quartz zoning and formation of illite. The early precipitation of kaolinite is related to alteration of feldspar and mica caused through flushing of meteoric water during forced marine regression and uplift, probably still in Silurian time. The sources for quartz cementation may include kaolinitization of feldspar and pressure dissolution, the latter associated to concavo-convex and sutured grain contacts, although these types of contact are relatively rare. Therefore, external silica-rich solutions likely occurred.

Provenance of Cretaceous sandstones in the Banda Arc and their tectonic significance

The provenance of Cretaceous sandstones in the Banda Arc islands differs from west to east. Sandstones in Sumba and West Timor contain significant amounts of feldspar (K-feldspar and plagioclase) and lithic fragments, suggesting a recycled to magmatic arc origin. In comparison, East Timor and Tanimbar sandstones are quartz rich, and suggest a recycled origin and/or continental affinity. Heavy mineral assemblages in Sumba and West Timor indicate metamorphic and minor acidic igneous sources and include a mixture of rounded and angular zircon and tourmaline grains. In East Timor, Babar and Tanimbar, an ultimate origin from a mainly acid igneous and minor metamorphic source is interpreted, containing a mixture of rounded and angular zircon and tourmaline grains.Detrital zircon ages in all sandstones range from Archean to Mesozoic, but variations in age populations indicate local differences in source areas. Sumba and West Timor are characterised by zircon age peaks at 80–100 Ma, 200–240 Ma, 550 Ma, 1.2 Ga, 1.5 Ga and 1.8 Ma. East Timor and Tanimbar contain 80–100 Ma, 160–200 Ma, 240–280 Ma, 550 Ma and 1.5 Ga zircon peaks. Most populations are also common in Triassic and Jurassic formations along the Outer Banda Arc and in many other areas of SE Asia. However, the abundance of Jurassic and Cretaceous populations was unexpected. We interpret Cretaceous sandstones from Sumba, Timor and Tanimbar to have been deposited in SE Sundaland. Syn-sedimentary Cretaceous (68–140 Ma) sources are suggested to include the Schwaner Mountains in SW Borneo and Sumba. Material derived mainly from older recycled sediments that had their main sources in the Bird's Head, Western and Central Australia, and local sources close to Timor.

Indicator minerals of the Meliadine orogenic gold deposits, Nunavut (Canada), and application to till surveys

The Meliadine Gold District is located about 25 km north of Rankin Inlet, Nunavut (Canada), in the Archean Rankin Inlet greenstone belt. The bedrock is composed of greenschist facies metamorphic rocks. The composition of tourmaline and scheelite is investigated by electron probe micro-analyser (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA ICP-MS). The chemical composition of these indicator minerals from the Meliadine deposits is compared with the composition of grains extracted from till samples along a 2 km transect parallel to the direction of ice flow at the Mustang showing. Tourmaline from the deposit is characterized by a flat rare earth element (REE) pattern having a positive europium anomaly also found in tourmaline grains in till samples down-ice of the Mustang showing. Scheelite from the Mustang showing shows a bell-shaped REE pattern with a negative europium anomaly, which is also found in scheelite from down-ice till samples. Scheelite and tourmaline recovered from till down-ice from the Mustang showing have a composition diagnostic of the gold mineralization. Determining the composition of tourmaline and scheelite in unconsolidated sediments can serve to identify gold mineralization and has the potential to be applied in mineral exploration in the Meliadine Gold District and elsewhere.

Application of the inverse Batschelet distribution to measuring the preferred orientation of tourmaline grains

We have used the inverse Batschelet distribution to perform a statistical analysis of mineral lineation defined by the preferred orientation of tourmaline grains on a foliation surface. Orientation data for 542, 617, 3202, and 3621 tourmaline grains from four metachert specimens were characterized by the parameters ξ, κ, ν, and λ, representing the location, concentration, skewness, and peakedness of the distribution, respectively. The inverse Batschelet distribution can be represented by either one full model or one of three sub-models, which comprise varying combinations of the adopted parameters. These are the full four-parameter family model and its skew-von Mises (λ = 0), symmetric (ν = 0), and von Mises (λ = 0 and ν = 0) sub-models. Statistical analysis reveals that the full four-parameter family, and symmetric models best represent our orientation data. We conclude that the inverse Batschelet distribution is a useful analytical technique, having three important advantages over alternative approaches: (1) the orientation of a mineral lineation is represented by the mode of the data, defined as half of ξ – 2ν; (2) the degree of symmetry of the orientation data is represented by ν; and (3) the strength of the preferred orientation can be quantified by κ and λ.

Statistical model selection between elastic and Newtonian viscous matrix models for the microboudin palaeopiezometer

We carried out statistical evaluations of two probability density functions for the microboudin palaeopiezometer using the Akaike information criterion (AIC) and the cross-validation (CV) technique. In terms of the relevant stress-transfer model, these functions are defined as the elastic matrix model and the Newtonian viscous matrix model, respectively. The AIC and CV techniques enable us to evaluate the relative quality of both models when applied to nine data sets collected from metachert samples containing tourmaline grains in a quartz matrix, collected from the East Pilbara Terrane, Western Australia. The results show that the elastic matrix model is the more appropriate probability density function for analysis of fracturing of tourmaline grains in a quartz matrix. This statistical evaluation shows the validity of the elastic matrix model for the microboudin palaeopiezometer when analysing such data sets.Graphical abstract.