Dmitry Zozulya

Chevkinite-group minerals from Russia and Mongolia: new compositional data from metasomatites and ore deposits

Abstract Electron-microprobe analyses of Russian and Mongolian chevkinite-group minerals from little-known host lithologies, including various metasomatic rocks, quartzolites and an apatite deposit, are presented. The mineral species analysed include chevkinite-(Ce), perrierite-(Ce), polyakovite-(Ce) and Sr- and Zr-rich perrierite-(Ce). Compositional variation in the Sr-rich members of the group is broadly represented by the exchange vector (Fe + Mn + Al + REE) ↔ (Ca + Sr + Ti + Zr). Despite the varied parageneses, the chevkinite-(Ce) compositions are similar to previously published data. Many crystals have strong internal compositional variations, partly produced during primary crystallization and partly during low-temperature hydrothermal alteration.

Hydrothermal alteration of chevkinite-group minerals. Part 2. Metasomatite from the Keivy massif, Kola Peninsula, Russia

Abstract Chevkinite-(Ce) in a mineralized quartz-epidote metasomatite from the Keivy massif, Kola Peninsula, Russia, underwent at least two stages of low-temperature alteration. In the first, it interacted with hydrothermal fluids, with loss of Ca, Fe, LREE and Si and strong enrichment in Ti. The altered chevkinite was then rimmed and partially replaced by a zone of ferriallanite-(Ce) and davidite-(La), in turn rimmed by a zone of allanite-(Ce) richer in the epidote component. The allanite zone was in turn partially replaced by rutile-titanite-quartz assemblages, the formation of titanite postdating that of rutile. Aeschynite-(Y), aeschynite-(Ce) and REE-carbonates are accessory phases in all zones. The hydrothermal fluids were alkaline, with significant proportions of CO2 and F. At various alteration stages, the Ca, Si ± Al activities in the fluid were high. Formation of the aeschynite is discussed in relation to its stability in broadly similar parageneses; it was a primary phase in the unaltered chevkinite zone whereas in other zones it formed from Nb, Ti, REE and Th released from the major phases.

Ore geochemistry, zircon mineralogy, and genesis of the Sakharjok Y-Zr deposit, Kola Peninsula, Russia

The Sakharjok Y-Zr deposit in Kola Peninsula is related to the fissure alkaline intrusion of the same name. The intrusion ∼7 km in extent and 4–5 km2 in area of its exposed part is composed of Neoarchean (2.68–2.61 Ma) alkali and nepheline syenites, which cut through the Archean alkali granite and gneissic granodiorite. Mineralization is localized in the nepheline syenite body as linear zones 200–1350 m in extent and 3–30 m in thickness, which strike conformably to primary magmatic banding and trachytoid texture of nepheline syenite. The ore is similar to the host rocks in petrography and chemistry and only differs from them in enrichment in zircon, britholite-(Y), and pyrochlore. Judging from geochemical attributes (high HSFE and some incompatible element contents (1000–5000 ppm Zr, 200–600 ppm Nb, 100–500 ppm Y, 0.1–0.3 wt % REE, 400–900 ppm Rb), REE pattern, Th/U, Y/Nb, and Yb/Ta ratios), nepheline syenite was derived from an enriched mantle source similar to that of contemporary OIB and was formed as an evolved product of long-term fractional crystallization of primary alkali basaltic melt. The ore concentrations are caused by unique composition of nepheline syenite magma (high Zr, Y, REE, Nb contents), which underwent subsequent intrachamber fractionation. Mineralogical features of zircon-the main ore mineral—demonstrate its long multistage crystallization. The inner zones of prismatic crystals with high ZrO2/HfO2 ratio (90, on average) grew during early magmatic stage at a temperature of 900–850°C. The inner zones of dipyramidal crystals with average ZrO2/HfO2 = 63 formed during late magmatic stage at a temperature of ∼500°C. The zircon pertaining to the postmagmatic hydrothermal stage is distinguished by the lowest ZrO2/HfO2 ratio (29, on average), porous fabric, abundant inclusions, and crystallization temperature below 500°C. The progressive decrease in ZrO2/HfO2 ratio was caused by evolution of melt and postmagmatic solution. The metamorphic zircon rims relics of earlier crystals and occurs as individual rhythmically zoned grains with an averaged ZrO2/HfO2 ratio (45, on average) similar to that of the bulk ore composition. The metamorphic zircon is depleted in uranium in comparison with magmatic zircon, owing to selective removal of U by aqueous metamorphic solutions. Zircon from the Sakharjok deposit is characterized by low concentrations of detrimental impurities, in particular, contains only 10–90 ppm U and 10–80 ppm Th, and thus can be used in various fields of application.

Hydrothermal alteration of chevkinite-group minerals: products and mechanisms. Part 1. Hydration of chevkinite-(Ce)

Abstract Samples from Russia and Scotland are used to examine the interaction of the REE-Ti silicate chevkinite-(Ce) with hydrothermal fluids. Altered zones in crystals are distinguished by using areas of low intensity on backscattered-electron images, low analytical totals, increasingly large departures from stoichiometry and, in some cases, the presence of micropores. Initial alteration of the chevkinite results in strong Ca enrichment. With increasing degrees of alteration, Ca abundances drop sharply, as do those of the REE, Fe and Si. In contrast, Ti levels increase strongly, usually accompanied by higher Nb ± Th levels. The most altered zones contain up to 36 wt.% TiO2 and the formula cannot be expressed in the standard chevkinite formula. In detail, samples follow different alteration trends, presumably reflecting different P, T, fO₂ and fluid composition. The Ti enrichment may have been related to a reaction front of dissolution-reprecipitation passing through the outer zones of the original chevkinite, leaving behind a reprecipitated Ti-enriched phase which may or may not be chevkinite.

Multiple crystallization of zircon in the Sakharjok rare-earth element-zirconium deposit, Kola Peninsula

120 Zircon is characterized by striking typomorphic features (morphology, anatomy, chemical composi� tion), which allow the reconstruction of conditions and stages of its crystallization in different geological processes (magmatic, postmagmatic, and metamor� phic). Therefore, the study of these features could be very informative in deciphering the sequence and duration of geological and, especially, oregenerating processes. The Sakharjok deposit is presently one of Russias most promising complex sources for zirconium, rare� earth elements (REEs), and yttrium (1). The Sakhar� jok alkaline massif is located in the western part of the Keivy terrane (Kola Peninsula). The latter is made up mainly of Late Archean basic to felsic metavolcanics and metasediments lying on the tonalite-trondhjemite-gra� nodiorite basement of the Central Kola block, as well as intraformational intrusions of alkali granites. The Sakharjok Massif is restricted to the southern part of the Western Keivy alkaline granite massif. This is a fis� suretype intrusion consisting of alkali gabbroids (essexites), nepheline syenites, and alkali syenites (2). The alkaline magma intruded as extended (>7 km long) steep dikelike bodies along vertical faults between alkaline granites and gneiss diorites. The massif has maximal width (1.5-2 km) in its northern part. Its western and southwestern parts are occupied by alka� line syenites, while the eastern part is made up of nepheline syenites with large (up to 80 × 200 m in size) blocks of alkaline gabbroids. Petrographically, they are represented by very abundant trachytoid mesoand leucocratic (most abundant) lepidomelane-egirine syenites, porphyritic ferrohastingsite syenites devel� oped in the marginal parts of the massif, and pegma� toid lepidomelane-ferrohastingsite syenites that occur as lenslike bodies in the trachytoid syenites.

Hydrothermal alteration of a chevkinite-group mineral to a bastnäsite-(Ce)-ilmenite- columbite-(Fe) assemblage: interaction with a F-, CO2-rich fluid

The results are presented of a textural and mineral chemical study of a previously undescribed type of hydrothermal alteration of chevkinite-(Ce) which occurs in a syenitic pegmatite from the Vishnevye Mountains, Urals Region, Russia. The progressive alteration of the chevkinite to a bastnäsite-(Ce)-ilmenite-columbite-(Fe) assemblage through a series of texturally complex intermediate stages is described and electron microprobe analyses are given of all the major phases. Unusual Nb ± Th-rich phases formed late in the alteration sequence provide evidence of local Nb mobility. The main compositional fluxes are traced, especially of the REE, HFSE, Th and U. It appears that almost all elements, with the exception of La, released from the chevkinite-(Ce) were reincorporated into later phases, such that they did not leave the alteration crust in significant amounts. The hydrothermal fluids are inferred to have been F- and CO2-rich, with variable levels of Ca activity, and with fO2 mainly between the nickel-nickel oxide and magnetite-hematite buffers. This occurrence represents a new paragenesis for a columbite-group mineral.

Britholite ores of the Sakharjok Zr–Y–REE deposit, Kola Peninsula: Geochemistry, mineralogy, and formation stages

Britholite ores in the complex Sakharjok Zr–Y–REE deposit (Kola Peninsula) form linear bodies in nepheline syenite and contain britholite-group minerals and zircon as main ore minerals. Geochemical data indicate that the formation of the britholite ores of the Sakharjok Massif was mainly controlled by magmatic differentiation and lateto post-magmatic reworking of the rocks by alkaline and F, CO2-bearing fluids. The elevated content of ore components in magma is caused by its derivation from enriched mantle source. It was established that crystallization of britholite occurred at the late and post-magmatic stages of the massif formation and was assisted by fluids with different physicochemical properties. The widest spread fluorbritholite-(Ce) typical of the trachytoid nepheline syenite crystallized mainly during albitization from highly alkaline, F-rich and CO2-bearing fluids/solutions. Britholite-(Ce) and fluorbritholite-(Y) found in the most recrystallized porphyritic nepheline syenite were formed at the later hydrothermal stage from F-bearing water-rich (metamorphic?) solutions. Fluorcalciobritholite crystallized from high-temperature pegmatite melt/solution at high CO2 activity. Postcrystallization alterations of the britholite-group minerals from the Sakharjok deposit resulted in the formation of altered zones within crystals and rims around them. The composition of overgrowth rims indicates the removal of F, Ce, and La from britholite.

Solid solution between potassic alkali amphiboles from the silica-rich Kvaløya lamproite, West Troms Basement Complex, northern Norway

Alkali amphibole of rare compositions occurs as a rock-forming mineral in a high-Si phlogopite lamproite from Kvaloya, northern Norway. The amphibole typically occurs as small grains forming irregular and rosette-shaped aggregates in a matrix dominated by Fe-rich K-feldspar and quartz. Amphibole shows compositions ranging between the three limiting compositions: A : A,B ( K 1.01 Na 1.99 ) C ( Na 0.26 Mg 1.58 Mn 0.03 Fe 2 + 0.91 Fe 3 + 1.6 Ti 0.47 □ 0.13 ) T Si 8 O 22 W [ F 0.97 O 1.03 ] B : A,B ( KNa 2 ) C ( Na 0.04 Mg 1.04 Mn 0.22 Fe 2 + 0.65 Fe 3 + 2.07 Ti 0.25 □ 0.7 ) T Si 8 O 22 W [ F 0.68 Cl 0.01 O 0.13 ( OH ) 1.18 ] C : A,B ( K 0.9 Na 2.1 ) C ( Na 0.04 Mg 3.54 Mn 0.02 Fe 2 + 0.28 Fe 3 + 1.04 Ti 0.03 □ 0.02 ) T Si 8 O 22 W [ F 1.34 O 0.06 ( OH ) 0.6 ] Composition C shows significant content of fluoro-potassic-magnesio-arfvedsonite, while composition A is a Fe 2+ , Fe 3+ and C Na rich variety of potassic-obertiite. Composition B is characterized by an exceptional high value of C □. It is emphasized that the presence of C □ and C Na in amphibole needs to be confirmed by other methods. The relationship between W O 2− , C □,Ti 4+ and Fe 3+ of amphibole can be expressed by the following exchange operators, choosing potassic-magnesio-arfvedsonite [KNa 2 (Mg 4 Fe 3+ )Si 8 O 22 (OH) 2 ] as the additive component: Ti 4 + Mg 2 + − 1 H + − 2 Fe 3 + Mg 2 + − 1 H + − 1 Ti 4 + □ Mg 2 + − 2 Fe 3 + 2 □ Mg 2 + − 3 The two first exchange operators result in deprotonation of OH, while the two others result in the formation of vacancies on the C sites. The presence of amphibole both in the lamproite and in the adjacent fenitized granite suggests that the mineral formed during reactions between rock and fluids derived from the volatile-rich lamproite magma. Possibly, amphibole core (composition A and B), formed in equilibrium with the fluid phase during crystallization of the melt, while amphibole rim (composition C) formed during subsequent mineral-fluid reactions. Presence of hematite in the lamproite matrix in addition to oxo-amphibole indicates that the rock formed during highly oxidizing conditions.

Unique Accessory Ti-Ba-P Mineralization in the Kvalöya Ultrapotassic Dike, Northern Norway

Unusual ultrapotassic dikes were recently found on the Kvalöya Island in Northern Norway. The dikes crosscutting granites 1.8 Ga in age are 0.1–1.0 m thick and consist of phlogopite phenocrysts in a fine-grained groundmass of K-magnesioarfvedsonite, orthoclase, apatite, and secondary chlorite. According to the composition of the rock-forming minerals (4.5–6.0 wt % K2O and 0.7–3.5 wt % TiO2 in magnesioarfved-sonite, 1.6–3.6 wt % FeO in orthoclase, 9.2–10.7 wt % Al2O3 and 2.1–2.6 wt % TiO2 in phlogopite) and its bulk chemical composition (K/Na = 2.3–2.9, K/Al = 1.0–1.2, (Na + K)/Al = 1.4–1.7, Mg# V = 65–73, (La/Yb)n = 100–140, 3.2–4.0 wt % TiO2, 0.55–1.47 wt % BaO, 2.5–3.0 wt % P2O5, 2650–3000 ppm Zr, 900–1260 ppm REE total, 2300–2500 ppm Sr), the rock corresponds to lamproite of the transitional type. The unique chemical composition of the rock resulted in uncommon Ti-Ba-P accessory mineralization, including baotite Ba4(Ti,Nb)8Si4O28Cl (up to 5 vol %), Sr-apatite (5–7 vol %), and previously unknown Na-Mg-Ba phosphate. Baotite forms anhedral elongated and isometric grains 10–500 μm in size. It is characterized by low Nb (0.03–0.05 f.c.); admixtures of K (0.04–0.12 f.c.) and Sr (0.04–0.07) replacing Ba and Fe (0.01–0.03 f.c.); and Al (0.03–0.04 f.c.) substituting Ti. Euhedral elongated zonal apatite crystals are extremely enriched in SrO (8–12 wt %) and REE2O3 + Y2O3 (6–9 wt %) in the marginal zone. Na-Mg-Ba phosphate occurs as prismatic grains 10–100 μm in size. The atomic ratio of its major cations Na: Mg: Ba: P ∼ 2: 1: 1: 2 corresponds to the conventional formula Na2MgBa(PO4)2; the mineral contains Sr, Mn, Fe, Ca, Si, and Al admixtures.

Minerals of the gadolinite-(Y)-hingganite-(Y) series in the alkali granite pegmatites of the Kola Peninsula

Minerals of the gadolinite-(Y)-hingganite-(Y) series pertaining to the gadolinite-datolite group have been found in the alkali granite pegmatites of the Kola Peninsula. Gadolinite-(Y) is distinguished by its unique natural crystalline state. The unit-cell parameters of this mineral have elevated values as compared with those of gadolinite-(Y) from other deposits and occurrences: (i) a = 10.11 Å, b = 7.63 Å, c = 4.79, V = 369.30 Å3; (ii) a = 10.05 Å, b = 7.69 Å, c = 4.76, V = 367.99 Å3. The increase in unit-cell parameters is not correlated with variation in chemical composition. The variable chemical compositions of particular individuals, especially as concerns REE and Y contents, assume two gadolinite-(Y) generations being contained in the intragranite pegmatites. Gadolinite-I is characterized by a high LREE content (LREEN/HREEN = 1.6) with a prevalence of total REE over Y (REE/Y = 1.36). Gadolinite-II is significantly depleted in LREE (LREEN/HREEN = 0.3) with a prevalence of Y over REE (REE/Y = 0.29). Hingganite-(Y), which has also been found in the alkali granite pegmatites of the Kola Peninsula for the first time, is characterized by elevated unit-cell parameters as well: a = 10.05 Å, b = 7.72 Å, c = 4.76 Å, V = 369.12 Å3. The mineral is enriched in Ca (up to 5 wt % CaO); and, by contents of REE and Y, the hingganite-(Y) from inter-granite pegmatites keeps the marginal position between its Y-dominant and REE-dominant varieties. The chondrite-normalized REE patterns assume that hingganite-(Y) crystallizes between the first and the second generations of gadolinite-(Y) and that alkali intragranite pegmatites are formed at the late magmatic stage, whereas amazonite-bearing pegmatites are formed under postmagmatic hydrothermal conditions.