Genetic features of tourmaline from the magnesian skarns of Kuhilal deposit, Southwestern Pamir
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Background. Although the Kuhilal deposit of Mg skarns has been thoroughly studied, tourmaline in their composition has never been considered [5, 6, 16]. Uvite, a tourmaline variety, was mentioned in the list of rare minerals only with its crystallochemical formula [17]. L.N. Shabynin, a great connoisseur of skarns, noted the rarity of tourmaline in Mg skarns of various deposits. Tourmaline of this type is characterized by a strong variation in ferruginosity and a significant content of Ca. At the same time, Ca:Na varies from 2:3 to 20:1 [15]. According to N.N. Pertsev, another major specialist in skarns, tourmaline is a rare mineral of Mg skarn deposits [11]. The boron-iron ore deposit of Tayozhnoe, Aldan shield, is an exception. This deposit belongs to skarns, where a widespread development of tourmaline was noted [12]. Tourmaline (dravite) of this deposit was found only in silicate metamorphic rocks containing marbles and Mg skarns. Aim. To determine the paragenesis, crystal morphology, chemical composition, and genetic characteristics of tourmaline from Kuhilal Mg skarns. Materials and methods. Large aggregates (more than 10 cm across) and crystals of apple-green tourmaline from the Mg skarns of the Kuhilal deposit were investigated. Samples of Mg skarns containing tourmaline were collected and documented by the authors during fieldwork research at the Kuhilal deposit in 2017. The samples were examined by mineralogical and petrographic methods, X-ray diffraction analysis using a DRONE-3M (analyst A.V. Fedorov, Sergo Ordzhonikidze Russian State University for Geological Prospecting), microprobe analysis using a Cameca SX 100 in 15 kV 30 nA shooting mode (analyst V.I. Taskaev, IGEM RAS), and X-ray fluorescence analysis using an AXIOS advanced spectrometer with an X-ray tube equipped with a 3 kW Rh anode and Philips PW-2400 with a sensitivity of 10-4% (analyst A.I. Yakushev, IGEM RAS). The majority of studies were conducted at the Department of Mineralogy and Gemology of the Sergo Ordzhonikidze Russian State University for Geological Prospecting. Results. Tourmaline from the Mg skarns of the Kuhilal deposit has been studied for the first time. This mineral is rare for skarn rocks. Two tourmaline mineral associations were identified: with spinel and forsterite, and with chlorite and serpentine. In terms of chemical composition, tourmaline isattributed to fluorine-containing uvite with a close to zero ferruginosity. Uvite is characterized by a lenticular shape with a rarely observed, underdeveloped prism. Its mineralogical properties, chemical composition, and formation conditions were analyzed. Tourmaline can be used to make inexpensive jewelry inserts. Conclusion. Tourmaline crystallization occurred under the conditions of granulite facies of regional metamorphism. For boron formation, an apo-sedimentary, evaporite source is assumed.Keywords:
Tourmaline
Pleochroism
The Varėna Suite comprises a set of ultramafic rocks: olivinic, pyroxenic, magnetitic, dolomitic, and apatite bearing rocks, which form complex bodies of few sq. km in the Precambrian crystalline basement near the Varėna town in Southern Lithuania. Occurrences of few mineral commodities are related to the Varėna Suite. Magnetitic rocks contain essential resources of high grade iron ores. Phlogopite, apatite, REE and Th mineralization is related to the Varėna Suite. The assessment of the potential for these commodities is primarily dependent on the origin of Varėna Suite, which is still disputed. The models of metasomatic (skarn), and igneous (layered intrusion) origin are proposed earlier. The article presents an overview and reinterpretation of the recent data on the Varėna Suite, its petrographic and geochemical characteristic, as well as the arguments for igneous origin of the Varėna Suite, as the polyphase intrusion with subsequent metasomatic alteration, with alkaline trend.
Metasomatism
Ultramafic rock
Layered intrusion
Tourmaline
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Numanoite, the Cu analogue of borcarite, is found in an irregular patch in crystalline limestone near gehlenite–spurrite skarns at the Fuka mine, Okayama Prefecture, Japan. Numanoite (up to 1 mm across) is observed as a core or zones in borcarite crystals up to 5 mm long. The mineral is also found as veinlets up to 0.4 mm wide in aggregates of borcarite crystals. The associated minerals are nifontovite, bultfonteinite, calcite and an unidentified magnesium silicate mineral. In hand specimen, the mineral is blue-green to colorless and transparent with a vitreous luster. The streak is white to pale blue-green. Numanoite is monoclinic, space group C 2/ m , a 17.794(2), b 8.381(1), c 4.4494(7) A, β 102.42(2)° and Z = 2. The strongest seven lines in the X-ray powder-diffraction pattern [ d in A( I )( hkl )] are 7.57(100)(110), 2.671(84)(421), 2.727(68)(221), 1.887(52)(041,440), 2.272(48)(331), 2.899(44)(600) and 1.698(34)(640). Electron-microprobe and thermogravimetric analyses gave B 2 O 3 24.09, CaO 38.11, CuO 10.32, MgO 1.02, ZnO 0.51, CO 2 15.80, H 2 O 9.75, sum 99.60 wt.%. The empirical formula, calculated on the basis of O = 18, is Ca 3.898 (Cu 0.744 Mg 0.145 Zn 0.036 ) ∑0.925 B 3.969 O 5.615 (OH) 6.208 (CO 3 ) 2.059 , ideally Ca 4 CuB 4 O 6 (OH) 6 (CO 3 ) 2 . The mineral is optically biaxial negative, α 1.618(2), β 1.658(2), γ 1.672(2), and 2 V calc = 60°. The mineral has perfect cleavages in two directions. The density is 2.96(2) g/cm 3 (meas.) and 2.93 g/cm 3 (calc.). The Vickers microhardness is 376 (290–464) kg/mm 2 (25 g load), and the Mohs hardness number is 4½. The differential thermal analysis curve shows two endothermic peaks at 489° and 692°C. It is likely that numanoite from the Fuka mine formed by precipitation from late Cu- and Mg-bearing hydrothermal solutions.
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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.
Tourmaline
Metasomatism
Breccia
Magmatic water
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ABSTRACT Study of the mineralogy of the Chester sandstones of southwestern Indiana shows an abundance of a very few minerals, and a trace of many more. The heavy minerals consist of 50 to 100 percent leucoxene, 25 to 50 percent zircon, and 10 to 25 percent tourmaline. Rutile, ilmenite and brookite compose from 1 to 10 percent of the heavy minerals. Magnetite, anatase, garnet, hornblende, kyanite, muscovite, hypersthene, pyrite, chlorite and barite are present sporadically and always in small quantity. The light minerals consist of quartz, feldspar, and calcite. The outstanding feature of the mineralogy of the Chester sandstones is the occurence and nature of the mineral leucoxene together with the other closely associated titanium-bearing minerals, brookite and rutile. It is recognized that the titanium oxide minerals, brookite and rutile, are developed authigenically from the decomposition of leucoxene.
Hornblende
Tourmaline
Ilmenite
Brookite
Muscovite
Rutile
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Tourmaline
Pegmatite
Cassiterite
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Molla Taleb pegmatites (northeast of Aligudarz) are located in the western part of the metamorphic-igneous Sanandaj-Sirjan Zone (SSZ). Slates and schists along with siliceous veins and veinlet and black Hornfels, as well as metamorphic sandstones are among the oldest deposits of this area. The most important geological event in this area is the development and intrusion of granitoid masses into schists of the Molla Taleb area during the Middle Jurassic. The rocks of the study area are in the range of gabbro, diorite, granodiorite, and granite. Granites are in the range of type-I granites. Most specimens are calc-alkaline and mainly contain peraluminous. Microprobe electron analysis of tourmalines present in pegmatites, tourmaline- aplite-pegmatite veins, nodular tourmalines, and quartz-tourmaline veins shows that all tourmalines are in the Schorl region and the range of alkaline tourmalines. These tourmalines with FeO / FeO + MgO ratios between 0.6 and 0.8 are in the range of magmatic-hydrothermal tourmalines and more than 0.8 in the magmatic range. Therefore, the studied tourmalines are dependent on granite environments and are formed by a hydrothermal fluid of magmatic origin.
Pegmatite
Tourmaline
Hornfels
Diorite
SLATES
Petrogenesis
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Background. Although the Kuhilal deposit of Mg skarns has been thoroughly studied, tourmaline in their composition has never been considered [5, 6, 16]. Uvite, a tourmaline variety, was mentioned in the list of rare minerals only with its crystallochemical formula [17]. L.N. Shabynin, a great connoisseur of skarns, noted the rarity of tourmaline in Mg skarns of various deposits. Tourmaline of this type is characterized by a strong variation in ferruginosity and a significant content of Ca. At the same time, Ca:Na varies from 2:3 to 20:1 [15]. According to N.N. Pertsev, another major specialist in skarns, tourmaline is a rare mineral of Mg skarn deposits [11]. The boron-iron ore deposit of Tayozhnoe, Aldan shield, is an exception. This deposit belongs to skarns, where a widespread development of tourmaline was noted [12]. Tourmaline (dravite) of this deposit was found only in silicate metamorphic rocks containing marbles and Mg skarns. Aim. To determine the paragenesis, crystal morphology, chemical composition, and genetic characteristics of tourmaline from Kuhilal Mg skarns. Materials and methods. Large aggregates (more than 10 cm across) and crystals of apple-green tourmaline from the Mg skarns of the Kuhilal deposit were investigated. Samples of Mg skarns containing tourmaline were collected and documented by the authors during fieldwork research at the Kuhilal deposit in 2017. The samples were examined by mineralogical and petrographic methods, X-ray diffraction analysis using a DRONE-3M (analyst A.V. Fedorov, Sergo Ordzhonikidze Russian State University for Geological Prospecting), microprobe analysis using a Cameca SX 100 in 15 kV 30 nA shooting mode (analyst V.I. Taskaev, IGEM RAS), and X-ray fluorescence analysis using an AXIOS advanced spectrometer with an X-ray tube equipped with a 3 kW Rh anode and Philips PW-2400 with a sensitivity of 10-4% (analyst A.I. Yakushev, IGEM RAS). The majority of studies were conducted at the Department of Mineralogy and Gemology of the Sergo Ordzhonikidze Russian State University for Geological Prospecting. Results. Tourmaline from the Mg skarns of the Kuhilal deposit has been studied for the first time. This mineral is rare for skarn rocks. Two tourmaline mineral associations were identified: with spinel and forsterite, and with chlorite and serpentine. In terms of chemical composition, tourmaline isattributed to fluorine-containing uvite with a close to zero ferruginosity. Uvite is characterized by a lenticular shape with a rarely observed, underdeveloped prism. Its mineralogical properties, chemical composition, and formation conditions were analyzed. Tourmaline can be used to make inexpensive jewelry inserts. Conclusion. Tourmaline crystallization occurred under the conditions of granulite facies of regional metamorphism. For boron formation, an apo-sedimentary, evaporite source is assumed.
Tourmaline
Pleochroism
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