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Coulsonite Fev2o4—A Rare Vanadium Spinel Group Mineral in Metamorphosed Massive Sulfide Ores of the Kola Region, Russia

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Coulsonite Fev2o4—A Rare Vanadium Spinel Group Mineral in Metamorphosed Massive Sulfide Ores of the Kola Region, Russia minerals Article Coulsonite FeV2O4—A Rare Vanadium Spinel Group Mineral in Metamorphosed Massive Sulfide Ores of the Kola Region, Russia Alena A. Kompanchenko Geological Institute of the Federal Research Centre “Kola Science Centre of the Russian Academy of Sciences”, 14 Fersman Street, 184209 Apatity, Russia; [email protected]; Tel.: +7-921-048-8782 Received: 24 August 2020; Accepted: 21 September 2020; Published: 24 September 2020 Abstract: This work presents new data on a rare vanadium spinel group mineral, i.e., coulsonite FeV2O4 established in massive sulfide ores of the Bragino occurrence in the Kola region, Russia. Coulsonite in massive sulfide ores of the Bragino occurrence is one of the most common vanadium minerals. Three varieties of coulsonite were established based on its chemical composition, some physical properties, and mineral association: coulsonite-I, coulsonite-II, and coulsonite-III. Coulsonite-I forms octahedral crystal clusters of up to 500 µm, and has a uniformly high content of 2 Cr2O3 (20–30 wt.%), ZnO (up to 4.5 wt.%), and MnO (2.8 wt.%), high microhardness (743 kg/mm ) and coefficient of reflection. Coulsonite-II was found in relics of quartz–albite veins in association with other vanadium minerals. Its features are a thin tabular shape and enrichment in TiO2 of up to 18 wt.%. Coulsonite-III is the most common variety in massive sulfide ores of the Bragino occurrence. Coulsonite-III forms octahedral crystals of up to 150 µm, crystal clusters, and intergrowths with V-bearing ilmenite, W-V-bearing rutile, Sc-V-bearing senaite, etc. Chemical composition of coulsonite-III is characterized by wide variation of the major compounds—Fe, V, Cr. In some crystals of coulsonite-III, relics of chromite are observed. The microhardness of coulsonite-III is 577 kg/mm2, the reflection coefficient changes in relation to iron, vanadium, and chromium content. Keywords: spinel group mineral; coulsonite; vanadium; massive sulfide ore; metamorphism; Paleoproterozoic; Kola region 1. Introduction Coulsonite is a naturally occurring iron-vanadium spinel group mineral with formula FeV2O4. According to www.midat.org, the type locality of coulsonite is the Buena Vista Hills east of Lovelock, Nevada (USA) where coulsonite was identified in magnetite-bearing rocks. The mineral occurs as an ilmenite-like exsolution in magnetite and disseminated subhedral crystals in silicate gangue separated from magnetite [1]. Many findings of rare vanadium minerals, including vanadium spinel, are connected with high-grade metamorphic complexes where two main conditions are required: primary enrichment of host rock in vanadium and high degree of metamorphism (up to amphibolite facies). For example, the only tanzanite deposit in the world is in Tanzania [2,3]. A lot of vanadium minerals were found in metamorphic rock of the Sludyanka complex in the southern Baikal area (Russia), such as new mineral species, including magnesiocoulsonite MgV2O4 [4–7]. Yet, the most significant findings of vanadium spinel group minerals are associated with metamorphosed massive sulfide ores described only in supracrustal units of several Paleoproterozoic rift-related structures of ancient cratons worldwide. Coulsonite, along with other vanadium minerals, was identified in massive sulfide ores in the Kola region (Russia), the Vihanti (Finland) and the Rampura Agucha (India) deposits [8–11]. The third Minerals 2020, 10, 843; doi:10.3390/min10100843 www.mdpi.com/journal/minerals Minerals 2020, 10, x FOR PEER REVIEW 2 of 14 Minerals 2020, 10, 843 2 of 14 Coulsonite, along with other vanadium minerals, was identified in massive sulfide ores in the Kola region (Russia), the Vihanti (Finland) and the Rampura Agucha (India) deposits [8–11]. The vanadium spinel, vourelainenite MnV2O4, was discovered in massive sulfide ores in the Sätra deposit third vanadium spinel, vourelainenite MnV2O4, was discovered in massive sulfide ores in the Sätra (Sweden) [12] and also found in the Outokumpu deposit (Finland) [13]. All these massive sulfide deposit (Sweden) [12] and also found in the Outokumpu deposit (Finland) [13]. All these massive ore deposits and occurrences have similar main characteristics: geological position (Paleoproterozoic sulfide ore deposits and occurrences have similar main characteristics: geological position rift-related(Paleoproterozoic structure), rift- timerelated of formationstructure), (ca.time 1.9 of formation Ga), high degree(ca. 1.9 Ga), of metamorphism high degree of (upmetamorphism to amphibolite or(up even to amphibolite granulite facies), or even and granulite mineral facie compositions), and mineral (the composition main mineral (the is main pyrrhotite) mineral [14 is– pyrrhotite)21]. [14–Vanadium21]. mineralization in the Kola region was earlier partially described [22]. This work providesVanadium data on mineralization a more detailed in the study Kola of coulsonite,region was i.e.,earlier the partially main vanadium described mineral [22]. This in work the Kola regionprovides massive data on sulfide a more ores. detailed Data onstudy coulsonite of coulsonite, from thei.e., Bragino the main and vanadium Pyrrhotite mineral Ravine in occurrencesthe Kola afterregion [8] massive were obtained sulfide byores. the Data author on coulsonite of this work. from the Bragino and Pyrrhotite Ravine occurrences after [8] were obtained by the author of this work. 2. Geological Setting 2. GeologicalA detailed Setting description of the geological setting was given in [23], therefore in this work only brief overviewA detailed is presented. description of the geological setting was given in [23], therefore in this work only briefOccurrences overview is presented. and deposits of massive sulfide ores of the Kola region are located in the Pechenga–Imandra–VarzugaOccurrences and deposits Paleoproterozoic of massive sulfide riftogenic ores of the belt, Kola which region formed are located during in 2.5–1.7the Pechenga Ga [24––27]. ImandraThe Bragino–Varzuga massive Paleoproterozoic sulfide ore riftogeni occurrencec belt, is which located formed in basic during volcanics 2.5–1.7 of Ga the [24 Mennel–27]. formation, whichThe belongs Bragino to themassive central sulfi partde ore of occurrence the South Pechengais located in structural basic volcanics zone of (Figure the Mennel1). These formation, rocks are depletedwhich belongs in K, Sr,to the Rb, central Ta, Zr, part Hf, of Ti, the light South and Pechenga heavy rare-earth structural elements,zone (Figure and 1). enriched These rocks in Ba,are Th, anddepleted Nb. Hostin K, Sr, rock Rb, for Ta, the Zr, BraginoHf, Ti, light occurrence and heavy belong rare-earth to low-alkali elements, and enr Fe-Mg-enrichediched in Ba, Th, rocks and of theNb. normal-type Host rock for mid-ocean the Bragino ridge occurrence basalts (MORB) belong [to28 ,low29].-alkali The vanadium and Fe-Mg content-enriched in volcanicsrocks of the of the normal-type mid-ocean ridge basalts (MORB) [28,29]. The vanadium content in volcanics of the Mennel formation varies between 160 and 450 ppm [28], which is just slightly higher than that in the Mennel formation varies between 160 and 450 ppm [28], which is just slightly higher than that in the Earth’s crust (138 ppm) [30]. There is an imprecise Sm-Nd age of 1.89 0.04 [29], as well as Rb-Sr Earth’s crust (138 ppm) [30]. There is an imprecise Sm-Nd age of 1.89 ± 0.04± [29], as well as Rb-Sr isochron whole-rock ages of 1.87 0.05 [31]. The Bragino occurrence host rock was metamorphosed in isochron whole-rock ages of 1.87± ± 0.05 [31]. The Bragino occurrence host rock was metamorphosed amphibolite facies [28,29]. in amphibolite facies [28,29]. FigureFigure 1. 1.Geological Geological map map of theof the South South Pechenga Pechenga structural structural zone zone and positionand position of the Braginoof the Bragino occurrence (modifiedoccurrence after (modified [22,24, 26after,28 ,[3222]).,24,26,28,32]). AccordingAccording to to a a previous previous study study [32 [32]] and and the the author’s author’s field field observations, observations, an orean bodyore body of the of Braginothe occurrenceBragino occurrence is a lens up is a to lens 7 m up in to thickness, 7 m in thickness, ca. 100 m ca. in 100 length, m in andlength, 70–75 and◦ dip.70–75 The° dip. most The part most of part the ore body is covered with quaternary sediments. Thirty samples of massive sulfide ores with an average weight of 2 kg were cut from bedrock outcrops. Minerals 2020, 10, 843 3 of 14 Based on the texture of the ores, four of ores types were distinguished within the Bragino occurrence: massive, banded, brecciated, and disseminated. Massive ores are the most common. This ore type can be divided into three subtypes based on their mineralogical composition: massive pyrrhotite ores of type I (mPo-I), massive pyrrhotite ores of the type II (mPo-II), and massive pyrite ores [23]. These ore types will be considered below. The content of V2O5 in these ores does not exceed 0.06%. 3. Methods The microscopic study of ore minerals in reflected light was conducted using an optical microscope Axioplane. Chemical analyses of the minerals were carried out by the Cameca MS-46 electron probe microanalyzer (Geological Institute, Kola Science Centre of the RAS, Apatity, Russia), WDS (Wavelength Dispersive Spectrometer) mode, 22 kV, 20–30 nA, 5–20 µm beam diameter (Cameca, Gennevilliers, France). The following standards (and analytical lines) were used: diopside (SiKα, CaKα), anatase (TiKα), MnCO3 (MnKα), pyrope (AlKα, MgKα), magnetite (FeKα), chromite (CrKα), and metallic vanadium (VKα). Detection limits of electron microprobe analyses (wt.%): Al—0.05, Si—0.05, Mn—0.01, Fe—0.01, Zn—0.01, Ti—0.02, Cr—0.02, V—0.02. Element distribution, morphology, and intraphase heterogeneity were also determined using a LEO-1450 scanning electron microscope (SEM) (Carl Zeiss, Oberkochen, Germany) equipped with a Bruker XFlash-5010 Nano GmbH (Bruker, Bremen, Germany) energy dispersive spectrometer (EDS) and an SEM Hitachi S-3400N (Hitachi, Tokyo, Japan) equipped with a EDS Oxford X-Max 20 (Oxford Instruments, Abington, UK).
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