L. M. Lyalina

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.

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.

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.

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.

Fluorbritholite(Y) and Yttrialite(Y) from Silexites of the Keivy Alkali Granites, Kola Peninsula

Investigation of the morphology, anatomy, and chemical composition of fluorbritholite-(Y) and yttrialite-(Y) from silexites of the Keivy alkali granites in Kola Peninsula has shown that these minerals are the main REE concentrators in this area and that their content reaches 10–15 vol %. Britholite and yttrialite are associated with zircon, aeschynite-(Y), chevkinite-(Ce), fergusonite-(Y), thorite, monazite-(Ce), xenotime-(Y) and bastnaesite-(Ce). Three morphological types of fluorbritholite-(Y) have been identified: (I) subhedral crystals and grains, (II) anhedral grains intergrown with yttrialite-(Y), and (III) poikilitic crystals and skeletal aggregates. These morphological types of fluorbritholite-(Y) are characterized by successive (I to III type) decreases in P content down to the pure silicate fluorbritholite-(Y). Crystals of the first type are heterogenous: the P content decreases and the HREE content increases from core to rim. The total REE content increases insignificantly from types I to II and drastically decreases in fluorbritholite-(Y) of type III. The successive prevalence of HREE over LREE indicates the hydrothermal conditions of mineral crystallization. The chemical composition of yttrialite-(Y) is distinguished by the relatively high Th content and depletion in Al. The compositional trend (from core to rim) in heterogeneous grains of yttrialite-(Y) testifies that their heterogeneity was caused by metasomatic alteration of the mineral. The interrelation of fluorbritholite-(Y) and yttrialite-(Y) indicate that fluorbritholite-(Y) of types II and III were formed later than yttrialite-(Y). Evidence for fluorbritholite-(Y) and yttrialite-(Y) formation suggests the significant role of hydrothermal processes in the genesis of silexites.

Behoite and Mimetite from the Saharjok Alkaline Intrusion, Kola Peninsula

The detailed study of the mineral composition of the nepheline syenite pegmatite from the Saharjok Intrusion has resulted in the finding of behoite and mimetite, a mineral species identified in the Kola region for the first time. The pegmatite body at the contact between nepheline syenite and essexite is unusual in textural and structural features and combination of mineral assemblages including unique beryllium mineralization. Behoite Be(OH)2 is an extremely rare beryllium mineral. It occurs as powderlike aggregates in the leaching cavities between euhedral pyroxene crystals. Behoite was identified by comparison of X-ray powder diffraction data of the studied mineral phase and behoite from the Be-bearing tuff in the type locality of this mineral (Utah, United States). Mimetite was found in the same pegmatite of the Saharjok intrusion. It forms unusual parallel-fibrous aggregates with individual fibers as long as ∼1 mm and only ∼1 μm across. X-ray powder diffraction data and the chemical composition characterize the mineral as hexagonal phase Pb5[AsO4]3Cl. Both behoite and mimetite are the products of late hydrothermal alteration of primary minerals (meliphanite, galena, arsenopyrite, and loellingite). The secondary phases freely crystallized in the cavities remaining after the leached nepheline.

REE distribution in zircon from reference rocks of the Arctic region: Evidence from study by the LA-ICP-MS method

The results of the LA-ICP-MS analysis of the concentrations of REEs, U, Th, and Hf in zircon from Paleo- and Neoarchean reference rocks of the Kola region (garnet–amphibole gneiss, basic and acid granulites, and granite pegmatite) are reported. A new methodology of the study of accessory zircons has been validated and modified. The accuracy of the results is confirmed by analysis of standard zircons Temora 1 and 91 500 and by comparison with the data obtained in other laboratories.

Autonomous anorthosites of the Anabar Shield: Age, geochemistry, and formation mechanism

The new high-accuracy data on U–Pb zircon geochronology, Sm–Nd systematics, and geochemistry of anorthosites of the Anabar Shield are discussed. It is established that anorthosite massifs are composed of gabbro–anorthosites (1.96 Ga old) and oligoclasites (1.93 Ga old) in association with monzodiorites (1.84–1.90 Ga old) and porphyroblastic granites. These rocks were generated in the Archean (3.2–2.7 Ga ago) in the lower crust from quartz–diorite melts under the plume tectonics regime in line with the filterpressing mechanism. The rocks were successively exhumed to upper levels of the crust owing to the Paleoproterozoic impact-triggered process to form a tectonically juxtaposed complementary magmatic complex.

Time of formation and genesis of yttrium-zirconium mineralization in the Sakharjok massif, Kola Peninsula

The Kola geotectonic province in the northeastern Fennoscandian Shield accommodates a significant number of alkaline rock massifs differing in age. They are of mantle and mantle-crustal origin (alkali and nepheline syenites, carbonatites) and related to crustal sources (Neoarchean alkali granites). Among them, the Neoarchean Sakharjok nepheline syenite massif is related to the oldest intrusions of this kind bearing yttrium-zirconium mineralization. The crystallization of alkali syenite pertaining to the first intrusive phase of the intrusive Sakharjok massif is dated to 2645 ± 7 Ma, and this implies that this syenite postdated alkali granites (2.66–2.67 Ga). To date the yttrium-zirconium ore, we applied the local U-Pb method to zircon crystals occurring in the mineralized block hosted in nepheline syenite. The earliest fragments of zircon crystallized 1832 ± 7 Ma ago; the age of metamorphism is estimated at 1784 ± 13 Ma. These dates indicate the Paleoproterozoic age of the yttrium-zirconium mineralization, which was formed as a product of fluid reworking of the Neoarchean nepheline syenite of the Sakharjok massif.