Tourmaline-rich Rocks Research Articles

Geochemistry of tourmaline-rich rocks in the Gavião Block, Northern São Francisco craton: implications for its formation

Geochemistry of tourmaline-rich rocks in the Gavião Block, Northern São Francisco craton: implications for its formation

<sup>40</sup>Ar/<sup>39</sup>Ar DATING OF MARUYAMAITE (K-DOMINANT TOURMALINE) FROM DIAMOND-BEARING METAMORPHIC ROCKS OF THE KOKCHETAV MASSIF

40Ar/39Ar analyses were made on maruyamaite (potassium-dominant tourmaline) from tourmaline-quartz-feldspar rocks discovered within the Kumdy-Kol microdiamond deposit (Kokchetav massif, North Kazakhstan). Turmaline yielded well-defined 40Ar/39Ar plateau age spectra whose values coincide within the error - ages of 502.3±8.0, 502.2±8.0, 506.0±8.0 Ma. These ages are much younger than the age of 530±2 Ma determined for high-pressure metamorphism by different methods. Thus, the formation of tourmaline-rich rocks of the Kumdy-Kol deposit cannot be associated with the high-pressure metamorphic events, and, therefore, testifies in favor of the low-pressure nature of maruyamaite. Based on the coincidence of age data for tourmaline crystals with different potassium contents, it can be concluded that the K/Ar system in tourmaline can be used for dating metasomatic and metamorphic processes.

Tourmaline occurrence and gold mineralization at a granitoid-metasediment contact in the Upper Lom Basin, east Cameroon

Gold mineralization of the Upper Lom Basin in the eastern Cameroon goldfield is spatially associated with tourmaline. The goldfield belongs to the Adamawa-Yadé Domain of the Central African Fold Belt and is characterized by eluvial, alluvial and lode gold deposits of Pan-African age. This paper examines the chemistry and morphology of saprock gold and its link with tourmaline in the host rock. Tourmaline chemistry was obtained using the electron microprobe analysis (EMPA) technique while gold grains were examined for morphology and microchemistry using the EMPA equipped with scanning electron microscopy and energy dispersive spectrum instruments. The tourmaline compositional data plot in the dravite field, and belongs to the alkali group showing major element variations that are typical of tourmaline associated with granite-related orogenic gold deposits common along shear zones. Gold alloy composition (Ag, Cu) suggests multiple gold precipitation events due to episodic fluid influx, with a range of 572–1000 gold fineness. We infer that tourmaline development and gold deposition were coeval within the aureole around the pluton and involved reduced hydrothermal fluids with low salinity. The high δ11B in the tourmaline also points to fluid derivation from the Pan African granitic basement widely recognized to be fertile in gold in eastern Cameroon. Tourmaline textural-chemical features such as low Na content and gold microchemistry point to granite-related hydrothermal style of primary gold mineralization, and has implications for exploration as tourmaline-rich rocks in the vicinity of the felsic plutons in the Lom Basin would be optimal target areas for future exploration.

Magmatic and metamorphic evolution of tourmaline-rich rocks of the Sopron area, Eastern Alps

Magmatic and metamorphic evolution of tourmaline-rich rocks of the Sopron area, Eastern Alps

Genesis of the boron potential of the Taukha metallogenic zone, Sikhote Alin, and boron sources during formation of the Dal’negorsk boron deposit

It is shown that the formation of borosilicate skarn in the Taukha metallogenic zone completes a series of successive stages of the formation and transformation of the folded sequences of the Taukha accretionary wedge. The Early Cretaceous sedimentary stage, including accumulation of detrital tourmaline-rich sedimentary rocks, was implemented in the marginal sea of the Paleopacific Sino-Korean segment. In the Turonian–Campanian, the boron-bearing folded sequences of the accretionary wedge were involved in anatexis to generate siliceous S-type boron-bearing melts. The thus-formed magmatic chambers were emptied during catastrophic volcanic eruptions. At the final Middle Campanian volcanic stage, fluid-magmatic differentiation of the melt in the residual chambers generated fluid flow. The infiltration interaction of the fluids, which inherited enrichment in boron, with limestones of the olistostrome sequence resulted in the formation of a giant zone of grossular–wollastonite skarns and danburite lodes. The boron potential of the Taukha boron–lead–zinc metallogenic zone may be considered as a reproduction of the Precambrian boron metallogeny of the eastern Eurasian margin, which was implemented in the Late Mesozoic during recycling of the continental crust.

Origin and Formation of Tourmaline-rich Cordierite-bearing Metapelitic Rocks from Alpe Sponda, Central Alps (Switzerland)

Origin and Formation of Tourmaline-rich Cordierite-bearing Metapelitic Rocks from Alpe Sponda, Central Alps (Switzerland)

Multivariate statistical analysis of the REE-mineralization of the Maw Zone, Athabasca Basin, Canada

To evaluate the relationship between the enrichment of rare earth elements (REEs) and U, we carried out a principal component analysis (PCA) of sandstones in the REE-rich Maw Zone in the Athabasca Basin, Canada. The Maw Zone, a breccia pipe with surface exposure of 300×200m, consists of highly silicified, hematitized, tourmaline-rich rocks with high REEs (up to 8.1 wt.% as total REE oxides). It is ~4km southwest from the south end of the Phoenix U deposits, however, rocks in the Maw Zone do not show high U (<7.8ppm in most rocks). We used RQ-mode PCA to compute variable and object loadings simultaneously, which allows the displays of observations and the variables at the same scale. PCA biplots of the Maw Zone data show that U is strongly correlated with V, Cr, Fe, Ni, Cu, Na, Li and Ba, but very weakly correlated with heavy rare earth elements (HREEs)+Y, and inversely with light rare earth elements (LREEs) and P. Relative enrichment of HREEs, Y, and P suggests xenotime as the predominant host of the HREEs. The grouping of LREEs+Sr+Th+P suggests the probable occurrence of monazite and/or aluminum phosphate-sulfate (APS) minerals. A mineralogical study confirmed the common occurrence of xenotime and APS minerals. The positive association between U and Fe in the PCA plot in the Maw Zone suggests that U was transported by oxidized fluids. The absence of U mineralization in the Maw Zone is explained by low U in the oxidizing fluids, or a lack reducing fluids to precipitate U. The PC1 and PC2 may reflect the major element assemblages related to the REE mineralization. The comparison of the maps of PC scores and the concentrations of HREEs+Y and P shows that the scores of PCs are better indicating the hydrothermal REE mineralization. For prospecting HREEs, PC2 is more reliable than PC1 because PC2 has the largest variance of HREEs and the second largest variance of LREEs.

Chemical and boron isotopic compositions of tourmaline from the Paleoproterozoic Houxianyu borate deposit, NE China: Implications for the origin of borate deposit

The Houxianyu borate deposit in northeastern China is one of the largest boron sources of China, hosted mainly in the Paleoproterozoic meta-volcanic and sedimentary rocks (known as the Liaohe Group) that are characterized by high boron concentrations. The borate ore-body has intimate spatial relationship with the Mg-rich carbonates/silicates of the Group, with fine-grained gneisses (meta-felsic volcanic rocks) as main country rocks. The presence of abundant tourmalinites and tourmaline-rich quartz veins in the borate orebody provides an opportunity to study the origin of boron, the nature of ore-forming fluids, and possible mineralization mechanism. We report the chemical and boron isotopic compositions of tourmalines from the tourmaline-rich rocks in the borate deposit and from the tourmaline-bearing fine-grained gneisses.Tourmalines from the fine-grained gneisses are chemically homogeneous, showing relatively high Fe and Na and low Mg, with δ11B values in a narrow range from +1.22‰ to +2.63‰. Tourmalines from the tourmaline-rich rocks, however, commonly show compositional zoning, with an irregular detrital core and a euhedral overgrowth, and have significantly higher Mg, REE (and more pronounced positive Eu anomalies), V (229–1852ppm) and Sr (208–1191ppm) than those from the fine-grained gneisses. They show varied B isotope values ranging from +4.51‰ to +12.43‰, which plot intermediate between those of the terrigenous sediments and arc rocks with low boron isotope values (as represented by the δ11B=+1.22‰ to +2.63‰ of the fine-grained gneisses of this study) and those of marine carbonates and evaporates with high boron isotope values. In addition, the rim of the zoned tourmaline shows notably higher Mg, Ti, V, Sn, and Pb, and REE (particularly LREEs), but lower Fe, Co, Cr, Ni, Zn, Mn, and lower δ11B values than the core. These data suggest that (1) the sources of boron of the borate ore-body are mainly the Paleoproterozoic meta-volcanic and sedimentary rocks, and (2) the ore-forming fluids should be the high temperature metamorphic fluids related to the amphibolite-facies metamorphism of the Paleoproterozoic foldbelt, which leach boron from the boron-rich meta-volcanic and sedimentary rocks of the Liaohe Group, and the boron-rich metamorphic fluids subsequently interacted with the marine Mg-rich carbonates and evaporates, forming borate deposit, the tourmaline overgrowth in the rim and the tourmaline-rich rocks.

Stability of chloritoid + biotite-bearing assemblages in some metapelites from the Palaeoproterozoic Singhbhum Shear Zone, eastern India and their implications

Abstract The arcuate Singhbhum Shear Zone (SSZ) of the East Indian Shield area, fringing the Archaean Singhbhum Craton, exposes a mélange-like ensemble of polymetamorphosed and highly tectonized siliciclastic sediments, mafic–ultramafic extrusive, tourmaline-rich rocks, magnetite–apatite rocks, granites and Cu–Fe–U sulphide ores of Meso- to Palaeoproterozoic age (1.5–1.77 Ga). Metapelites from two localities in the SSZ developed the enigmatic assemblage chloritoid–biotite–garnet–chlorite, which are the first reported from India and the eighth in world occurrence. Textural studies and algebraic analyses of the phase compositions in the KFMASH system indicate the operation of the reaction, chloritoid+biotite→garnet+chlorite+H 2 O and this reaction has a negative slope in pressure–temperature ( P – T ) space. Quantitative geothermobarometry and pseudosections in the NCMnKFMASHO system indicate that this reaction occurred during prograde metamorphism that culminated at 6.3±1 kbar and 490±40 °C. The stability of the chloritoid+biotite is also sensitive to the MnO content of the bulk rock, and to the chemical potential of H 2 O and oxygen ( μ H 2 O and μ O 2 , respectively), of the ambient fluid phase. Thrusting of continental crust in a collisional setting is invoked to explain the peak metamorphic temperatures and the clockwise P – T trajectory is construed from the petrological study of the chloritoid–biotite schist.

Tourmaline 40Ar/39Ar chronology of tourmaline-rich rocks from Central Iberia dates the main Variscan deformation phases

During crustal thickening, metapelites taken to depth release boron-bearing hydrothermal fluids because of progressive heating and dehydration. These fluids swiftly percolate upwards, especially if the crust is being actively deformed, to form tourmaline where the PT conditions and the chemical composition of the host-rock are favorable. The age of the so-formed tourmaline would record the age of the upward admittance of B-bearing fluids and, presumably, the age of the deformation. This process has been documented in the Martinamor Antiform of Central Iberia, a region where tourmaline-bearing rocks are particularly abundant. Metasomatic tourmaline from the Late Cambrian San Pelayo orthogneisses (zircon U-Pb age of 496 ± 5 Ma) yielded 40Ar/39Ar plateau ages at 370 ± 5 Ma and 342 ± 5 Ma. The first value represents the crystallization age of the tourmaline and is so far the most precise estimation of the age of crustal thickening in Central Iberia (D1). The second value reflects a partial loss of Ar caused by the second deformation phase (D2). Tourmaline from mylonitized and folded tourmalinites developed above D2 shear zones yield perturbed spectra with mean “plateau” ages of 347 ± 9 Ma and 342 ± 9 Ma which may represent either the resetting of older tourmaline or the formation of new tourmaline by focused boron metasomatism. After the metamorphic peak and simultaneously with the emplacement of the main granitoids of the Avila Batholith (310-315 Ma), another episode of boron metasomatism precipitated a new generation of tourmaline, which appears either concentrated in fine-layered tourmalinites (318 ± 2 Ma) or disseminated within Ediacaran-Cambrian metasediments (316 ± 2 Ma). The source of boron was the breakdown of previously formed tourmaline during melting reactions. Lastly, tourmaline from a leucogranitic body yielded a saddle-shaped age spectrum with a minimum age of ca. 296 Ma, roughly coeval with the youngest leucogranites. Although further work is required, our results suggest that tourmaline can be a useful chronological marker for dating deformation and magmatism.

Petrogenesis of tourmaline rocks associated with Fe-carbonate–graphite metapelite, metabasite and strata-bound polymetallic sulphide mineralisation, Peloritani Mountains, Sicily, Southern Italy

Tourmalinite and tourmaline-rich rocks associated with Fe-carbonate–graphite phyllite, strata-bound polymetallic sulphide deposits, metabasite and marble were studied, for information on the mechanism of tourmaline formation in the pre-Hercynian low-grade metamorphic sequence of the Mandanici Unit in the Peloritani Mountains of Sicily, southern Italy. The major and trace element compositions of the tourmaline rocks suggest the existence of a sedimentary protolith with pre-metamorphic black shale and bedded chert. Boron was interpreted to be accumulated in a restricted sedimentary basin, between platform carbonate formations, with abundant organic matter and Fe–Al–Ti-rich laterite–bauxite soil-derived clastic supply, under a continental volcano-tectonic extensional regime accompanied by a local convective hydrothermal system along faults. Petrographic, crystal–chemical and δ 11B isotopic data are compatible with a model of marine sediment dewatering at temperatures below 200 °C, which caused the removal of boron from clay. Metamorphism led to the development of tourmaline in an Al–Ti-rich environment, in equilibrium with other minerals such as ilmenite, albite and muscovite. The upper temperature of metamorphism (almost 375 °C), estimated on the basis of δ 11B, fits geothermometric results from Δ 13C carbonate–graphite on associated rocks. The estimated value of δ 11B in the tourmalinite protolith, − 7.5‰ , is also compatible with continental-derived Al-rich sediments.

Rochas ricas em turmalina da formação Morro da Pedra Preta: relações com os turmalinitos portadores de ouro do Grupo Serra do Itaberaba, SP

Tourmaline is a common mineral in the Serra do Itaberaba Group (Guarulhos-Brazil), especially in its basal, volcanic-sedimentary sequence, the Morro da Pedra Preta Formation. Tourmalinites, basic metatuffs, intermediate to acid metavolcaniclastic rocks, metapelites and quartz veins are among the tourmaline-rich rocks. The presence of gold disseminated in a tourmalinite of the Tapera Grande prospect led to the investigation of the relation between the environments of tourmalinite genesis and mineralizations in the Serra do Itaberaba Group. The investigation was extended to tourmaline-rich basic metatuffs, intermediate to acid metavolcaniclastic rocks, and metapelites that also show some silicification, carbonatization, potassification, chloritization and sulfidation. The tourmaline chemistry showed that even in the regions proximal to the exhalative center, the fluid/rock ratio is low because the tourmaline preserves most of the host rock's characteristics rather than those of the boron-rich fluids themselves. Such hydrothermal-metamorphic fluids promoted the redistribution of synsedimentary or syngenetic gold. The composition (aqueous-carbonic, low-salinity) of these fluids, however, did not favor the formation of massive sulfide deposits.

Petrographic, Chemical and B-Isotopic Insights into the Origin of Tourmaline-Rich Rocks and Boron Recycling in the Martinamor Antiform (Central Iberian Zone, Salamanca, Spain)

Tourmaline in the Martinamor antiform occurs in tourmalinites (rocks with >15–20% tourmaline by volume), clastic metasedimentary rocks of the Upper Proterozoic Monterrubio formation, quartz veins, pre-Variscan orthogneisses and Variscan granitic rocks. Petrographic observations, back-scattered electron (BSE) images, and microprobe data document a multistaged development of tourmaline. Overall, variations in the Mg/(Mg þ Fe) ratios decrease from tourmalinites (0 36–0 75), through veins (0 38–0 66) to granitic rocks (0 23–0 46), whereas Al increases in the same order from 5 84–6 65 to 6 22–6 88 apfu. The incorporation of Al into tourmaline is consistent with combinations of cAl(NaR)–1 and AlO(R(OH))–1 exchange vectors, where c represents X-site vacancy and R is (Mg þ Fe2þ þ Mn). Variations in c/( c þ Na) ratios are similar in all the types of tourmaline occurrences, from 0 10 to 0 53, with low Ca-contents (mostly <0 10 apfu). Based on field and textural criteria, two groups of tourmaline-rich rocks are distinguished: (1) pre-Variscan tourmalinites (probably Cadomian), affected by both deformation and regional metamorphism during the Variscan orogeny; (2) tourmalinites related to the synkinematic granitic complex of Martinamor. Textural and geochemical data are consistent with a psammopelitic parentage for the protolith of the tourmalinites. Boron isotope analyses of tourmaline have a total range of dB values from 15 6 to 6 8%; the lowest corresponding to granitic tourmalines ( 15 6 to 11 7%) and the highest to veins (1 9 to 6 8%). Tourmalines from tourmalinites have intermediate dB values of 8 0 to þ2 0%. The observed variations in dB support an important crustal recycling of boron in the Martinamor area, in which pre-Variscan tourmalinites were remobilized by a combination of mechanical and chemical processes during Variscan deformation, metamorphism and anatexis, leading to the formation of multiple tourmaline-bearing veins and a new stage of boron metasomatism.

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.

Origin of tourmaline in the metamorphosed Sikait pelitic belt, south Eastern Desert, Egypt

A Neoproterozoic metapelitic schist belt at the Sikait area of the south Eastern Desert is a favourable environment for localisation of tourmaline mineralisation in the Pan-African Belt in Egypt. Local concentrations of tourmaline in the Sikait area are closely associated with stratiform metapelitic schists. They are confined to the Nugrus shear zone, along which various leucocratic rocks (leucogranite, pegmatite, aplite) are syntectonically emplaced with various stages of hydrothermal quartz veins. Tourmaline occurs either as disseminated isolated clusters of crystals or as discontinuous tourmalinite bands within the metapelitic rocks, pegmatites and quartz veins. Four types of tourmaline-rich rocks were distinguished: (i) fine-grained tourmaline-rich rocks, which are associated with the biotite schist along contacts with gneissose granites (TR1); (ii) tourmaline-rich rocks, associated with the metapelitic and amphibolitic closely to Nugrus thrust zones (TR2); (iii) tourmaline-rich rocks, associated with the pelitic hornfels at the contact between metapelitic schist and leucogranite (TR3); and (iv) tourmaline-rich rocks and quartz veins, associated with chlorite-graphite schist in contact aureoles with leucogranite and pegmatite veins (TR4). Microprobe analyses revealed that tourmalines are Al saturated for the given Fe/Mg alkali-deficient group tourmaline with minor X-site vacancy and substitution of Ca for Na. Tourmalines belong to the schorl-dravite solid solution series and have a wide compositional range, from nearly end member dravite for TR3 tourmalines to schorl for TR4 tourmalines; TR1 and TR2 tourmalines have intermediate compositions. The Fe/(Fe + Mg) varies between 0.02 and 0.89. Variation in composition of Al-rich tourmalines is essentially caused by variations in Mg, Fe, Na, Ca and Ti. The whole rock chemical analyses of tourmaline-rich rocks closely resemble the trends observed for metapelitic schist and leucocratic rock and reflect mixing between phyllosilicate-rich and quartz-rich end members, which indicates that tourmaline-rich rocks do not contain a significant detrital component. Chondrite-normalised patterns of rare earth elements (REE) in tourmaline-rich rocks and quartz-rich tourmalines are similar to those of the surrounding unaltered metapelitic schists and leucocratic rocks, respectively. Minor depletions of LREE and local negative Ce anomalies characterise the chondrite-normalised REE pattern of TR1 tourmaline-rich rocks, suggesting its formation in the presence of seawater-derived fluids. However, TR4 types are characterised by low content ΣREE, such as that of leucocratic rocks. Thus, the geochemical data imply relative immobility of Al, Ti, Cr and HREE during hydrothermal alteration and later metamorphism. The different tourmaline varieties and their respective compositions are interpreted in terms of multistage evolution. Formation of the TR1 tourmaline-rich rocks probably was the net result of several processes, including direct precipitation from B-rich hydrothermal fluids or colloids, early diagenetic reactions of biotite-pelitic sediments with these fluids and subsequent recrystallisation during late regional deformation and metamorphism to give TR2 tourmaline-rich rocks. The TR3 tourmaline-rich rocks mainly developed by the thermal metamorphic recrystallisation of TR1. Tourmaline-rich rocks and veins adjacent to leucogranite and pegmatite veins (TR4) are the result of B-metasomatism; the primary B having been recycled from tourmalinites during regional metamorphism and magmatism.

Chemistry and genetic implications of tourmaline and Li-F-Cs micas from the Valdeflores area (Caceres, Spain)

Pervasive metasomatism that involved the formation of tourmaline-rich rocks and influx of Li. F. and Cs into Ordovician psammo-pelitic metasediments occurred in the Valdeflores area (Cáceres. Spain). Numerous Li- and Sn-bearing. mineralized, greisen-type veins also can be observed here, in the vicinity of geochemically specialized granites. Tourmaline- rich rocks appear as: (1) massive, fine-grained, dark green to black rocks: and (2) fine- scale tourmaline-rich laminae, which alternate with quartz-rich layers parallel to the bedding. Electron microprobe analyses indicate that the tourmaline lies mostly within the space defined by the exchange vectors from dravite: FeMg -1 (schorl), ⃞AlNa -1 Mg -1 (foitite), AlOMg -1 (OH) -1 (olenite), and CaMgNa -1 Al -1 (uvite). The Fe/(Fe+Mg) ratio ranges main­ly from 0.87 to 0.54 and increases with Al in the Y-site. Analytical results and substitu­tional relations show an insignificant elbaite component. Mica in the tounnalinized rocks is very fine-grained (mostly &lt;50 μm). White mica ranges from lithian muscovite-phengite to lepidolite/zinnwaldite, containing up to 8.40 wt% F. 6.0 wt% Li 2 O, and 10.73 wt% FeO. Dark mica shows a variable color and has compositions characterized by relatively high contents of Cs 2 O (1.14-2.78 wt%) and F (1.94-8.08 wt%), with a deficit in K 2 O (5.75- 9.04 wt%). Log (f H₂O /f HF ) of fluids in equilibrium with biotite in the tourmaline-rich rocks was 4.02-4.17 at T ≈ 400 °C. Log (f H₂O /f HF ) values of fluids in equilibrium with topaz (X F ≈ 0.8) in country rock adjacent to contacts with veins, and in equilibrium with am- blygonite-montebrasite (X amb = 0.2) in the veins were about 4.30-4.60 and 6.4-6.7, respectively. Tliese variations denote the existence of gradients in relative a HF more than differences of temperature during metasomatism. The lack of tourmaline in the veins is interpreted to reflect the alkalinity and low Fe-Mg contents in the fluids, which precluded the formation of tourmaline. Consequently, most of the boron was expelled into metase­diments where tourmaline was produced as a result.

Manganiferous ironstones in the early Ordovician Manx Group, Isle of Man: a protolith of coticule?

Abstract Spessartine-garnet quartzites (coticule) are now widely recognized as extremely useful marker horizons throughout the Appalachian-Caledonian Orogen. In many instances, they appear to be of early Ordovician age. They reflect a syn-sedimentary, volcanic exhalative origin and in many areas, e.g. the Leinster region in southeast Ireland, they are intimately associated with, inter alia , base metal mineralization and with tourmalinite. However, though the metamorphic coticule is common, the pre-metamorphic coticule precursor has proved difficult to recognize. Manganiferous ironstone rocks of coticule aspect are a distinctive feature in parts of the Manx Group exposed along the west coast of the Isle of Man. Though they lack any trace of the characteristic manganese garnet, perhaps reflecting a relatively low metamorphic grade, they do bear a strong morphological resemblance in outcrop to the typical coticule lithology of the Ribband Group in southeast Ireland. In both places, tourmaline-rich rocks occur nearby. On the Isle of Man, a major tourmalinite occurrence is linked to an important shear zone. In briefly reviewing the field occurrence, petrography and chemistry of the manganiferous ironstone beds, and by comparison with coticule elsewhere, the possibility that these ironstones are a coticule precursor or a coticule facies variant is introduced. That a lithology now termed coticule might be a significant component in the Manx Group (‘Slates’) was noted no less than 70 years ago. Any recognition of coticule rock would have an important bearing on the understanding of the stratigraphy, metamorphism and mineralization of the Manx Group.

Salt dome-controlled sulfide precipitation of Paleoproterozoic Fe-Cu sulfide deposits, eastern Liaoning, northeastern China

The lower portion of the Paleoproterozoic Northern Liaohe Group, Liaoning, northeast China, comprises metavolcanics (tuff-dominated) and metasediments (mostly arkoses and dolostones) that host stratiform-strata bound Fe(Cu) sulfides and Ca(Ba) sulfates. The ore-bearing sequence consists of, in ascending order, pyroclastic rocks, terrigenous clastics, and carbonates, all of which have been metamorphosed to greenschist amphibolite grade. Extensive albite-rich breccias are present at the base of the succession and as pipes crosscutting a variety of lithologic units in the Lieryu Formation. The geometry, geochemistry, and associated structures of these albite-rich rocks suggest that they are metamorphosed salt domes.The sulfide-sulfate Fe(Cu) ores occur in a transitional zone between tuffs and dolomitic carbonates, with those in the upper cycle being economically more important. The sulfides were precipitated at two sites: around venting centers as breccias, lenses, and veins; and as stratiform pyrite-chalcopyrite associated with anhydrite that is distal to the salt domes. Within each mining district the proximal ores are associated with brecciated, albite- and tourmaline-rich rocks. The sulfides in the proximal ores have delta 34 S values of 8.9 to 12.7 per mil, whereas the distal ores have delta 34 S values of 2.6 to 8.8 per mil for the sulfides and 7.9 to 19.0 per mil for the anhydrite; suggesting a sulfate-dominated sulfur source, with the pyrite delta 34 S variations arising mainly from decreasing temperatures away from the centers of the deposits.The origin of the deposits is best described by a model in which sulfate-carbonate rocks were deposited in peripheral sinks during salt diapirism. The salt domes then acted as focuses for subsequent hydrothermal venting and were the centers of sulfide mineralization. The sulfides precipitated during interaction of the reduced, metal-bearing hydrothermal fluids with the in situ sulfates that were the major source of the sulfur.

TOURMAL: software package for tourmaline, tourmaline-rich rocks and related ore deposits

TOURMAL is a user-friendly software package developed for editing, storing, calculating and plotting microprobe tourmaline analyses from different geologic environments such as granites and their related aplites and pegmatites, metasomatic and vein type ore deposits, stratabound base metal deposits, gold deposits, and sedimentary and metamorphic rocks. Calculated results, saved in comma delimited ASCII file format, permit the user to search for other applications related to tourmaline compositions. The program prepares 28 binary and seven ternary diagrams. It is written in Quickbasic for IBM PC and compatibles, and runs under DOS and Windows operating systems with a VGA graphic card. This program permits the user to convert standard binary graphs and ternary diagrams into PCX file format for high quality printouts.

Mineralogy, geochemistry and geological significance of tourmaline-rich rocks from the Paleozoic Cinco Villas massif (western Pyrenees, Spain)

Tourmaline-rich rocks associated with clastic metasedimentary rocks of Carboniferous age occur in the Cinco Villas massif, western Pyrenees. Three types of tourmaline-rich rocks were distinguished: (1) Fine-grained stratiform tourmaline-rich rocks, which are associated with carbonaceous metapelites (TR1); (2) stratabound tourmaline-rich rocks, associated with metapelites in the contact aureole of the Aya granitoid pluton (TR2); (3) stratabound to massive tourmaline-rich rocks, associated with psammopelites in contact with granites and pegmatites (TR3). Tourmalines belong to the schorl–dravite solid solution series and have a wide compositional range, from nearly end-member dravite for TR1 tourmalines to schorl for TR3 tourmalines; TR2 tourmalines have intermediate compositions. The Fe/(Fe+Mg) typically varies between 0.02 and ≈0.55, increasing from TR1 to TR3. The TR1 tourmalines commonly display a discontinuous chemical zoning with Fe-rich green cores (8–8.5% FeO) and Mg-rich colorless rims (10–11% MgO). In contrast, crystals that exhibit fine growth lamellae appear to lack significant chemical zoning. Oxygen and hydrogen isotope compositions also reveal major differences between TR1 and TR3 tourmalines, the former displaying heavier δ18O values (17.7–19‰) and δD values (−35 to −42‰) than TR3 tourmalines 11 to 13‰ and −47 to −76‰, respectively. The TR2 tourmalines show intermediate values of 11.3 to 14.6‰ for δ18O and −40 to −55‰ for δD. Linear and continuous chemical variations obtained for major and trace elements of the whole rocks reflect mixing between clay-rich and quartz-rich end-members, indicative that some tourmaline-rich rocks contain a significant detrital component. Chondrite normalized REE (rare earth element) patterns of tourmaline-rich rocks are similar to those of surrounding unaltered clastic metasediments, except for some TR1 rocks which are characterized by low contents of ΣREE. Mass-balance calculations show that tourmaline-forming processes plus metamorphism led to mass and volume changes at mesoscopic scales (≈10% for the TR1 tourmalinites). Silicon, Fe, Mn, and REE elements were partially lost from sedimentary rocks, whereas Mg and particularly B were added to pelitic sediments. Available data, nevertheless, do not allow an assessment of the boron source. Formation of the TR1 tourmaline-rich rocks probably was the net result of several processes, including direct precipitation from B-rich hydrothermal fluids or colloids, early diagenetic reactions of carbonaceous pelitic sediments with these fluids, and subsequent recrystallization during regional metamorphism. The TR2 tourmaline-rich rocks mainly developed by metamorphic recrystallization of TR1. Tourmaline-rich rocks and veins adjacent to pegmatites and granitic rocks (TR3) are the result of boron metasomatism; the primary boron having been recycled from stratiform tourmalinites during regional metamorphism and magmatism.