Coulsonite FeV 2 O 4 —A Rare Vanadium Spinel Group Mineral in Metamorphosed Massive Sulﬁde Ores of the Kola Region, Russia

: This work presents new data on a rare vanadium spinel group mineral, i.e., coulsonite FeV 2 O 4 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 µ m, and has a uniformly high content of Cr 2 O 3 (20–30 wt.%), ZnO (up to 4.5 wt.%), and MnO (2.8 wt.%), high microhardness (743 kg / mm 2 ) and coe ﬃ cient 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 TiO 2 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 µ m, 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 / mm 2 , the reﬂection coe ﬃ cient changes in relation to iron, vanadium, and chromium content.

Vanadium mineralization in the Kola region was earlier partially described [22]. This work provides data on a more detailed study of coulsonite, i.e., the main vanadium mineral in the Kola region massive sulfide ores. Data on coulsonite from the Bragino and Pyrrhotite Ravine occurrences after [8] were obtained by the author of this work.

Geological Setting
A detailed description of the geological setting was given in [23], therefore in this work only brief overview is presented.
The Bragino massive sulfide ore occurrence is located in basic volcanics of the Mennel formation, which belongs to the central part of the South Pechenga structural zone ( Figure 1). These rocks are depleted in K, Sr, Rb, Ta, Zr, Hf, Ti, light and heavy rare-earth elements, and enriched in Ba, Th, and Nb. Host rock for the Bragino occurrence belong to low-alkali and Fe-Mg-enriched rocks 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 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 amphibolite facies [28,29].  [22,24,26,28,32]).
According to a previous study [32] and the author's field observations, an ore body of the Bragino occurrence is a lens up to 7 m in thickness, ca. 100 m in length, and 70-75° dip. The most part  [22,24,26,28,32]).
According to a previous study [32] and the author's field observations, an ore body of the Bragino occurrence is a lens up to 7 m in thickness, ca. 100 m in length, and 70-75 • dip. The most part of 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.
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 V 2 O 5 in these ores does not exceed 0.06%.

Results
Coulsonite in massive sulfide ores of the Bragino occurrence is one of the most common vanadium minerals. Three varieties of coulsonite were defined based on the chemical composition, some physical properties and mineral association: coulsonite-I, coulsonite-II, and coulsonite-III. These varieties are briefly described in (Table 1).

Coulsonite-I
Coulsonite-I was found in massive pyrrhotite ores of type I. It consists of a hexagonal and monocline pyrrhotite mixture (80% of ores) (1 in Figure 2a-c) and microveins of hexagonal pyrrhotite (2 in Figure 2a,b). In the oxidation zone, pyrrhotite is replaced by a pyrite-marcasite intergrowth with "bird's eye" structure (up to 18 % of this ore) (3 in Figure 2a-c). The mineral composition of this ore type is rather poor, 2 % is accessory minerals: chalcopyrite, galena, molybdenite, coulsonite, V-bearing phlogopite and clinochlore intergrowth, Sc-V-bearing senaite, and W-V-bearing rutile [23].
Coulsonite-I occurs as octahedral crystals and crystal clusters up to 500 µm (Figure 2d-f). It is saturated by a sulfide inclusion (Figure 2f) or V-W-bearing rutile. In reflected light, coulsonite-I is grey, the internal reflection is not observed (Figure 2f). The chemical composition of coulsonite-I is given in ( Table 2). It is characterized by uniformly high content of

Coulsonite-I
Coulsonite-I was found in massive pyrrhotite ores of type I. It consists of a hexagonal and monocline pyrrhotite mixture (80% of ores) (1 in Figure 2a-c) and microveins of hexagonal pyrrhotite (2 in Figure 2a,b). In the oxidation zone, pyrrhotite is replaced by a pyrite-marcasite intergrowth with "bird's eye" structure (up to 18 % of this ore) (3 in Figure 2a-c). The mineral composition of this ore type is rather poor, 2 % is accessory minerals: chalcopyrite, galena, molybdenite, coulsonite, Vbearing phlogopite and clinochlore intergrowth, Sc-V-bearing senaite, and W-V-bearing rutile [23].
Coulsonite-I occurs as octahedral crystals and crystal clusters up to 500 µm (Figure 2d-f). It is saturated by a sulfide inclusion (Figure 2f) or V-W-bearing rutile. In reflected light, coulsonite-I is grey, the internal reflection is not observed (Figure 2f). The chemical composition of coulsonite-I is given in (Table 2) [33], apart from those introduced by the author for rare minerals. SE-secondary electron.  [33], apart from those introduced by the author for rare minerals. SE-secondary electron. Microhardness of coulsonite-I is 743 kg/mm 2 (average of 15 measurements), which exceeds microhardness of coulsonite in the literature (338 kg/mm 2 ) [8,34]. A coefficient of coulsonite-I reflection (Table 3) significantly differs from coulsonite from the Pyrrhotite Ravine, which is close to standard coulsonite [8].  Microhardness of coulsonite-I is 743 kg/mm 2 (average of 15 measurements), which exceeds microhardness of coulsonite in the literature (338 kg/mm 2 ) [8,34]. A coefficient of coulsonite-I reflection ( Table 3) significantly differs from coulsonite from the Pyrrhotite Ravine, which is close to standard coulsonite [8].

Coulsonite-II
Coulsonite-II was found in relics of quartz-albite veins from massive pyrrhotite ores of type II (Figure 4a,b) and the massive pyrite ore. The relics have a zonal and banded structure where their axial band is composed of albite and two marginal bands built up by quartz. Only the albite axial band hosts a Cr-Sc-V-bearing mineralization (Figure 4c,d).

Coulsonite-II
Coulsonite-II was found in relics of quartz-albite veins from massive pyrrhotite ores of type II (Figure 4a,b) and the massive pyrite ore. The relics have a zonal and banded structure where their axial band is composed of albite and two marginal bands built up by quartz. Only the albite axial band hosts a Cr-Sc-V-bearing mineralization (Figure 4c,d).  Coulsonite-II forms thin tabular (needle-like in a section) crystals and crystal clusters (Figure 4e,f), dimensions of which vary from 2 × 10 to 10 × 30 µm. Crystals of coulsonite-II are usually randomly located in albite, yet, in some cases, some orientation is observed. In association with coulsonite-II roscoelite, V-bearing muscovite, thortveitite, Sc-V-bearing crichtonite group minerals, rutile, byrudite and tivanite were defined. In reflected light, coulsonite-II is light grey, and the enrichment of crystals in TiO 2 has much higher coefficient of reflection and cream tint (Figure 4f).
The chemical composition of coulsonite-II (Table 2) is rather similar to coulsonite-III. This is characterized by a higher content of FeO and a lower content of V 2 O 3 and Cr 2 O 3 . A content of ZnO does not exceed 3 wt.%, while MnO is under 0.5 wt.%. For some crystals of coulsonite-II, an extremely high TiO 2 content was established (up to 18 wt.%). These crystals are clearly visible in BSE images (Figure 4e) and reflected polarized light photos (Figure 4f). After [35], a maximum content of TiO 2 (3 wt.%) in coulsonite was described for that from Kalgoorli. Due to its unusual morphology and chemical composition, coulsonite-II is similar to nolanite described in [36]. It is impossible to measure microhardness and precise value of the reflection coefficient due to a small size of coulsonite-II crystals. For the same reason, we have only an EDS analysis of coulsonite-II. For coulsonite-II, the EBSD confirmation method was used ( Figure 5). It was based on comparison of data obtained for coulsonite-II and taken from AMCSD [37] CIF data for coulsonite (0015774), nolanite (0000910) in an Oxford Instruments AZtecHKL analysis software where coulsonite was confirmed on 12 bands. Coulsonite-II forms thin tabular (needle-like in a section) crystals and crystal clusters ( Figure  4e,f), dimensions of which vary from 2 × 10 to 10 × 30 µm. Crystals of coulsonite-II are usually randomly located in albite, yet, in some cases, some orientation is observed. In association with coulsonite-II roscoelite, V-bearing muscovite, thortveitite, Sc-V-bearing crichtonite group minerals, rutile, byrudite and tivanite were defined. In reflected light, coulsonite-II is light grey, and the enrichment of crystals in TiO2 has much higher coefficient of reflection and cream tint (Figure 4f).
The chemical composition of coulsonite-II (Table 2) is rather similar to coulsonite-III. This is characterized by a higher content of FeO and a lower content of V2O3 and Cr2O3. A content of ZnO does not exceed 3 wt.%, while MnO is under 0.5 wt.%. For some crystals of coulsonite-II, an extremely high TiO2 content was established (up to 18 wt.%). These crystals are clearly visible in BSE images ( Figure 4e) and reflected polarized light photos (Figure 4f). After [35], a maximum content of TiO2 (3 wt.%) in coulsonite was described for that from Kalgoorli. Due to its unusual morphology and chemical composition, coulsonite-II is similar to nolanite described in [36]. It is impossible to measure microhardness and precise value of the reflection coefficient due to a small size of coulsonite-II crystals. For the same reason, we have only an EDS analysis of coulsonite-II. For coulsonite-II, the EBSD confirmation method was used ( Figure 5). It was based on comparison of data obtained for coulsonite-II and taken from AMCSD [37] CIF data for coulsonite (0015774), nolanite (0000910) in an Oxford Instruments AZtecHKL analysis software where coulsonite was confirmed on 12 bands.

Coulsonite-III
Coulsonite-III is the most common variety in sulfide ores of the Bragino occurrence. It was found in massive pyrrhotite ores of type II and massive pyrite ores. Massive pyrrhotite ores of type II differ from the type I by a strong cracking, presence of relics of quartz-albite veins, hexagonal pyrrhotite predominance (Figure 6a,b), and more diversified mineral composition [23]. The major minerals of this ore type are pyrrhotite, chalcopyrite, sphalerite, pyrite, and marcasite. Arsenopyrite, molybdenite, galena, cobaltite, altaite, hessite, volynskite, rucklidgeite, kotulskite, gold, etc. were defined as minor and rare minerals. The massive pyrite ore consists of coarse-grained pyrite ( Figure  6a) and pyrrhotite, chalcopyrite, sphalerite, quartz, albite and siderite in interstitial space. Pyrite contains a lot of inclusions of individual minerals (sulfides, oxides, quartz, etc.) and a mineral association observed only in these pyrite ores [23].

Coulsonite-III
Coulsonite-III is the most common variety in sulfide ores of the Bragino occurrence. It was found in massive pyrrhotite ores of type II and massive pyrite ores. Massive pyrrhotite ores of type II differ from the type I by a strong cracking, presence of relics of quartz-albite veins, hexagonal pyrrhotite predominance (Figure 6a,b), and more diversified mineral composition [23]. The major minerals of this ore type are pyrrhotite, chalcopyrite, sphalerite, pyrite, and marcasite. Arsenopyrite, molybdenite, galena, cobaltite, altaite, hessite, volynskite, rucklidgeite, kotulskite, gold, etc. were defined as minor and rare minerals. The massive pyrite ore consists of coarse-grained pyrite ( Figure 6a) and pyrrhotite, chalcopyrite, sphalerite, quartz, albite and siderite in interstitial space. Pyrite contains a lot of inclusions of individual minerals (sulfides, oxides, quartz, etc.) and a mineral association observed only in these pyrite ores [23]. The morphology of coulsonite-III is extremely diverse. It forms individual corroded octahedral crystals up to 150 µm, crystal clusters, and intergrowths with other minerals. For coulsonite-III, four separate mineral associations are typical: 1-coulsonite-III containing relics of chromite, with an intergrowth with V-bearing ilmenite (up to 3 wt.% V2O3) (Figure 6c-i) [9]. Relics of chromite in the central part of coulsonite-III crystals are perfectly distinguished in the reflected light image and element mapping (Figure 6e), however, are not observed in the BSE images because of the proximity of vanadium and chromium atomic mass. It has a lower coefficient of reflection (Table 3), the V2O3 and Cr2O3 contents are 22 wt.% and 32 wt.%, respectively. Ilmenite is saturated in a ferberite inclusion (Figure 6e,i). 2-coulsonite-III in an intergrowth with W-V-bearing rutile and ilmenite (and kyzylkumite only in the pyrite ores) (Figure 6j). 3-coulsonite-III surrounded by a V-bearing magnetite rim V2O3 up to 1.5 wt.% (Figure 6k). 4-coulsonite-III with nolanite in the marginal area or The morphology of coulsonite-III is extremely diverse. It forms individual corroded octahedral crystals up to 150 µm, crystal clusters, and intergrowths with other minerals. For coulsonite-III, four separate mineral associations are typical: 1-coulsonite-III containing relics of chromite, with an intergrowth with V-bearing ilmenite (up to 3 wt.% V 2 O 3 ) (Figure 6c-i) [9]. Relics of chromite in the central part of coulsonite-III crystals are perfectly distinguished in the reflected light image and element mapping (Figure 6e), however, are not observed in the BSE images because of the proximity of vanadium and chromium atomic mass. It has a lower coefficient of reflection (Table 3), the V 2 O 3 and Cr 2 O 3 contents are 22 wt.% and 32 wt.%, respectively. Ilmenite is saturated in a ferberite inclusion (Figure 6e,i). 2-coulsonite-III in an intergrowth with W-V-bearing rutile and ilmenite (and kyzylkumite only in the pyrite ores) (Figure 6j). 3-coulsonite-III surrounded by a V-bearing magnetite rim V 2 O 3 up to 1.5 wt.% (Figure 6k). 4-coulsonite-III with nolanite in the marginal area or tin inclusion only in the pyrite ores (Figure 7). The microhardness of coulsonite-III is 577 kg/mm 2 . The reflection coefficient of coulsonite-III is given in Table 3.

Genesis of Coulsonite of the Bragino Occurrence
The vanadium mineralization of the Bragino occurrence was found only in massive sulfide ores and was not established in the host rock. The vanadium mineralization genesis is partially discussed elsewhere [22]. Findings of vanadium oxides or silicates in massive sulfide ores are rather unusual. Formation of vanadium mineralization requires two pre-conditions: (1) primary enrichment of the host rock in vanadium and (2) high degree of metamorphism (up to amphibolite facies). These conditions are met both for the Bragino occurrence and for other objects in which vanadium mineralization was described [9][10][11][12][13]22]. In dispersed state, vanadium is located in sulfide in volcanichosted massive sulfide (VHMS) complexes [38]. The source of vanadium can be an organic suspension, which absorbs it from seawater and precipitates with sulfides captured by them [39]. After [40][41][42][43], the metamorphism and secondary hydrothermal stage of primary (where pyrite is the main mineral) massive sulfide ores led to the remobilization and redistribution of elements located in sulfides in the dispersed state and to their concentration in newly formed sulfide minerals or in their own mineral phases. The metamorphic recrystallization of sulfides can possibly form vanadium oxides containing coulsonite-I and coulsonite-III.
The findings of V-Sc-bearing minerals in relics of primary hydrothermal veins of the Bragino sulfide ores suggest that these veins were formed at a hydrothermal event that preceded the metamorphic recrystallization of the sulfide ores and that coulsonite-II is of a hydrothermal origin.

Chemical Composition of Coulsonite of the Bragino Occurrence
The chemical composition of coulsonite of the Bragino occurrence has several special features distinguishing it from the one of the Pyrrhotite Valley or Vihanti, which are the nearest similar objects. It is related to the genesis of ores enclosing vanadium mineralization and their metamorphic alteration.
Coulsonite-I and coulsonite-III have the similar genesis but they were established in different types of pyrrhotite ores of the Bragino occurrence, and clearly differ in their chemical composition (Table 2, Figure 3). The chemical composition of coulsonite is different because of the wide isomorphic substitution in both structural positions.
In tetrahedral A-position, the main cation is Fe 2+ (more than 0.8 apfu) in all coulsonite varieties, Zn and Mn admixtures are more substantial in coulsonite-I (Figure 3a). In octahedral B-position, the main cation is V 3+ mainly substituted by Cr 3+ and Fe 3+ (Figure 3b), Al, Ti, Si are present in small amounts. The vanadium content is approximately the same within coulsonite varieties (about 1 apfu). In B-position of coulsonite-I, V 3+ is mainly substituted by Cr 3+ , Fe 3+ is absent according to formulae calculation. In coulsonite-II and coulsonite-III the inverse dependence between Cr 3+ and Fe 3+ content The microhardness of coulsonite-III is 577 kg/mm 2 . The reflection coefficient of coulsonite-III is given in Table 3.

Genesis of Coulsonite of the Bragino Occurrence
The vanadium mineralization of the Bragino occurrence was found only in massive sulfide ores and was not established in the host rock. The vanadium mineralization genesis is partially discussed elsewhere [22]. Findings of vanadium oxides or silicates in massive sulfide ores are rather unusual. Formation of vanadium mineralization requires two pre-conditions: (1) primary enrichment of the host rock in vanadium and (2) high degree of metamorphism (up to amphibolite facies). These conditions are met both for the Bragino occurrence and for other objects in which vanadium mineralization was described [9][10][11][12][13]22]. In dispersed state, vanadium is located in sulfide in volcanic-hosted massive sulfide (VHMS) complexes [38]. The source of vanadium can be an organic suspension, which absorbs it from seawater and precipitates with sulfides captured by them [39]. After [40][41][42][43], the metamorphism and secondary hydrothermal stage of primary (where pyrite is the main mineral) massive sulfide ores led to the remobilization and redistribution of elements located in sulfides in the dispersed state and to their concentration in newly formed sulfide minerals or in their own mineral phases. The metamorphic recrystallization of sulfides can possibly form vanadium oxides containing coulsonite-I and coulsonite-III.
The findings of V-Sc-bearing minerals in relics of primary hydrothermal veins of the Bragino sulfide ores suggest that these veins were formed at a hydrothermal event that preceded the metamorphic recrystallization of the sulfide ores and that coulsonite-II is of a hydrothermal origin.

Chemical Composition of Coulsonite of the Bragino Occurrence
The chemical composition of coulsonite of the Bragino occurrence has several special features distinguishing it from the one of the Pyrrhotite Valley or Vihanti, which are the nearest similar objects. It is related to the genesis of ores enclosing vanadium mineralization and their metamorphic alteration.
Coulsonite-I and coulsonite-III have the similar genesis but they were established in different types of pyrrhotite ores of the Bragino occurrence, and clearly differ in their chemical composition (Table 2, Figure 3). The chemical composition of coulsonite is different because of the wide isomorphic substitution in both structural positions.
In tetrahedral A-position, the main cation is Fe 2+ (more than 0.8 apfu) in all coulsonite varieties, Zn and Mn admixtures are more substantial in coulsonite-I (Figure 3a). In octahedral B-position, the main cation is V 3+ mainly substituted by Cr 3+ and Fe 3+ (Figure 3b), Al, Ti, Si are present in small amounts. The vanadium content is approximately the same within coulsonite varieties (about 1 apfu). In B-position of coulsonite-I, V 3+ is mainly substituted by Cr 3+ , Fe 3+ is absent according to formulae calculation. In coulsonite-II and coulsonite-III the inverse dependence between Cr 3+ and Fe 3+ content is observed. The chemical composition of coulsonite-II is similar to coulsonite-III, yet some crystals of coulsonite-II have the high TiO 2 content (up to 18 wt.%). Ti probably substitutes Cr 3+ and Fe 3+ ( Table 2).
A massive chromium admixture could be associated with mafic-ultramafic volcanism. The central part of some crystals is enriched in chrome, and can indicate a presence of chromite in primary ores like in the Urals, for example [40] and its subsequent substitution by coulsonite.
After [43], in pyrrhotite ores of massive sulfide deposits embedded in rocks metamorphosed until the amphibolite facies level, there are not only metamorphic alteration of iron sulfides, but significant changes in mineral occurrence forms of zinc, barium, and other elements. These changes in zinc mineralogy were studied in pyrite deposits of the Appalachian and Scandinavian Caledonides, where a substitution of sphalerite by zinc-bearing spinels and silicates was observed [44]. Zinc started accumulating in coulsonite due to metamorphic transformations of sphalerite-bearing ores and redistribution of zinc. Each vanadium spinel has a chrome analogue: chromite FeCr 2 O4-coulsonite FeV 2 O 4 , magnesiochromite MgCr 2 O 4 -magnesiocoulsonite MgV 2 O 4 , manganochromite MnV 2 O 4 -vuorelainenite MnV 2 O 4 . Zincochromite ZnCr2O4 still does not have a vanadium analogue, yet, the ability of zinc to substitute iron in coulsonite can help to discover this mineral in massive sulfide ores rich in sphalerite.
It is noted that the Cr content in coulsonite has a direct impact on its physical properties. The high content of Cr 2 O 3 (up to 20 wt.% and higher) leads to microhardness being doubled, on average. Reflection coefficient increases up to 550 nm and decreases after (Table 3). In polarized light, coulsonite enriched by chromium looks darker (Figure 6c,d). A difference between the Cr content in the dark central part and the brown boundary part of the crystal in (Figure 6c,d) is ca. 10 wt.%.

Conclusions
Findings of vanadium spinels are typical of the Paleoproterozoic high metamorphic VHMS ores like the Vihanty, Outokumpu or Kola region deposits. Three varieties of coulsonite were defined and described in the Bragino massive sulfide ores in the Kola region, Russia based on their chemical composition, some physical properties, and mineral association: coulsonite-I, coulsonite-II, and coulsonite-III. Wide variations in the chemical composition of coulsonite are caused by the isomorphic substitution between V, Cr and Fe. Coulsonite-I forms octahedral crystal clusters of up to 500 µm, and has a uniformly high content of Cr 2 O 3 , ZnO, and MnO. Coulsonite-II was found in relics of quartz-albite veins in association with other vanadium minerals. It forms a thin tabular shape and is enriched in TiO 2 . Coulsonite-III is the most common variety in massive sulfide ores of the Bragino occurrence. Coulsonite-III forms octahedral crystals and crystal clusters, intergrowths with V-bearing ilmenite, W-V-bearing rutile, etc. In some crystals of coulsonite-III, relics of chromite are observed. The Cr admixture mainly has a direct influence on coulsonite physical properties, such as microhardness and reflection coefficient. Increasing the content of Cr 2 O 3 leads to microhardness being doubled, on average. Three varieties of coulsonite were formed due to metamorphic and hydrothermal alteration of the primary massive sulfide ore, which contains vanadium in a dispersed state in sulfide. In addition, it should be noted that conventional BSE images do not reveal the features of the distribution of Cr and V in Cr-V spinels, and elemental mapping should be used.