Primary platinum-bearing copper from the Lesnaya Varaka ultramafic alkaline complex, Kola Peninsula, northwestern Russia

Summary The Lesnaya Varaka ultramafic alkaline complex in the northeastern Fennoscandian Shield (Kola Peninsula, NW Russia) is a concentrically-zoned intrusion with a dominantly dunitic core (Fo_85-92) surrounded by clinopyroxenites. The complex resembles an Alaskan-type intrusion, but differs in its strong alkaline affinity. Native copper occurs as small (5 to 15 m), subhedral to euhedral crystals, isolated within titanomagnetite, in a dunite containing abundant titanomagnetite-perovskite mineralization (up to 30 modal %). Nickel-rich (4.1–4.5 wt.% Ni) tetraferroplatinum is also present as minute (up to 5 gmm) subhedral crystals, enclosed by titanomagnetite. They are typically partially rimmed by rhodian pentlandite ( 6 wt.% Rh). The copper crystals contain 0.6 to 10.1 wt.% Pt, 2.1 to 3.0 wt.% Ni, and essential Fe (approximately 2 to 3 wt.%). There is a wide variation in the Pt content between individual crystals, but its distribution within single crystals is fairly constant. Compared with Cu-Pt alloys from other localities, solid solution of Cu with Pt in the Lesnaya Varaka native copper is low. Unlike most occurrences in ultramafic rocks, the crystalline copper at Lesnaya Varaka appears to be a primary phase, which formed under moderately oxidizing conditions and at very low sulphur activities.

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Primäres Platin führendes Kupfer aus dem ultramafisch-alkalischen Lesnaya Varaka Komplex, Halbinsel Kola, nordwestliches Rußland

Zusammenfassung Der ultramafisch-alkalische Lesnaya Varaka Komplex im nordöstlichen Fennoskandischen Schild (Halbinsel Kola, NW Rufland) ist eine konzentrisch zonierte Intrusion mit einem Dunitkern (Fo_85-92) umgeben von Klinopyroxeniten. Der Komplex ähnelt einer Alaskan-Typ Intrusion, unterscheidet sich aber durch seine stark alkalische Affinität. Gedigen Kupfer tritt in einem Dunit, der verbreitet eine TitanomagnetitPerovskit Mineralisation führt, in Form kleiner (5 bis 15 m), sub- bis euhedraler Kristalle isoliert im Titanomagnetit auf. Nickel-reiches (4.l-4.5 Gew. % Ni Tetraferroplatin kommt ebenfalls in Form winziger (bis zu ca. 5 m subhedraler Kristalle im Titanomagnetit vor. Typischerweise sind diese Kristalle zum Teil von Rhodium-führendem Pentlandit (ca. 6 Gew.% Rh) umgeben. Die Kupferkristalle führen 0.6 bis 10.1 Gew.% Pt, 2.1 bis 3.0 Gew.% Ni und beträchtliche Gehalte an Fe (ca. 2–3 Gew.%). Die Pt-Gehalte zwischen verschiedenen Kristallen streuen stark, während sie innerhalb einzelner Kristalle ziemlich konstant sind. Im Vergleich mit Cu-Pt Legierungen von anderen Lokalitäten ist die Mischbarkeit von Cu und Pt im Kupfer von Lesnaya Varaka gering. Im Unterschied zu den meisten anderen ultramafischen Gesteinen, scheint das kristalline Kupfer von Lesnaya Varaka eine primäre Phase zu sein, die sich unter mäßig oxidierenden Bedingungen und bei sehr niedrigen Schwefelaktivitäten gebildet hat.

     

The formation conditions of labuntsovite-group minerals in the Kovdor massif,Kola Peninsula

In the Kovdor massif, labuntsovite-group minerals occur in dolomite carbonatite veins (labuntsovite-Mg), in a natrolite-calcite vein (lemmleinite-Ba and labuntsovite-Fe), and in calcite pockets and veinlets cutting fenites (late labuntsovite-Mg). They are closely intergrown with paragenetic carbonates, and this makes it possible to estimate their crystallization temperature from the fluid inclusions entrapped in dolomite (≥265°C) and calcite (175–225°C). The earlier labuntsovite-Mg was formed under relatively acidic conditions, whereas later labuntsovite-calcite mineralization was derived from alkaline solutions.     

Chromium mineralization in the Khibiny alkaline massif (Kola Peninsula,Russia)

This paper presents new data on chromium mineralization in a fenitized xenolith in Mt. Kaskasnyunchorr in the Khibiny alkaline massif (Kola Peninsula, Russia) and summarizes data on Cr mineralogy in the Khibiny Mountains. Protolith silicates that contained Cr³⁺ admixture are believed to be the source of this element in the fenite. Cr-bearing (maximum Cr₂O₃ concentrations, wt %, are in parentheses) aegirine (5.8), crichtonite-group minerals (2.1), muscovite (1.3), zirconolite (1.1), titanite (1.0), fluorine-magnesioarfvedsonite (0.8), biotite (0.8), ilmenite (0.6), and aenigmatite (0.6) occur in the fenite. The late-stage spinellides of the FeTi-chromite-CrTi-magnetite series, which are very poor in Mg and Al and which formed after Crrich aegirine and ilmenite, are the richest in Cr (up to 42% Ct₂O₃). Cr concentrations grew with time during the fenitization process. Unlike minerals in the Khibiny ultramafic rocks where Cr is associated with Mg, Al (it is isomorphic with Cr), and with Ca, chromium in the fenites is associated with Fe, Ti, and V (with which Cr³⁺ is isomorphic) and with Na in silicate minerals. Cr³⁺ Mobility of Cr³⁺ and the unique character of chromium mineralization in the examined fenites were caused by high alkalinity of the fluid.     

PGE and Au minerals from the low-sulfide ore of the Pana Tundra Pluton,Kola Peninsula

The PGE minerals (PGM) in the low-sulfide ore of the Pana Tundra intrusion have been studied in the Northern and Southern reefs, Eastern Chuarvy and Churozero sites. Forty mineral species and five mineral phases of PGE and Au have been identified. The early magmatic sulfide-telluride-sulfarsenide assemblage and the late redeposited arsenide-telluride assemplage are distinguished on the basis of relationships between minerals and morphology of PGM. Toward the upper part of the intrusion and from the west eastward, the contribution of Pt- and Pd-sulfides to ore regularly decreases along with an increase in the amounts of Pt- and Pd-arsenides depending on the sequence of crystallization of host rocks and degree of their postmagmatic alteration. Microprobe analyses and variation in chemical compositions of PGM pertaining to the braggite-vysotskite, eoncheite-merenskyite, and kotulskite-sobolevskite isomorphic series are given.     

REE mineralogy and geochemistry of the Western Keivy peralkaline granite massif,Kola Peninsula,Russia

The authors have studied the geology, geochemistry, petrology and mineralogy of the rare earth elements (REE) occurring in the Western Keivy peralkaline granite massif (Kola Peninsula, NW Russia) aged 2674 ± 6 Ma. The massif hosts Zr- and REE-rich areas with economic potential (e.g. the Yumperuaiv and Large Pedestal Zr-REE deposits), where 25% of ΣREE are represented by heavy REE (HREE). The main REE minerals are: chevkinite-(Ce), britholite-(Y) and products of their metamict decay, bastnäsite-(Ce), allanite-(Ce), fergusonite-(Y), monazite-(Ce), and others. The areas contain also significant quantities of zircon reaching potentially economic levels. We have discovered that behavior of REE and Zr is controlled by alkalinity of melt/solution, which, in turn, is controlled by crystallization of alkaline pyroxenes (predominantly aegirine) and amphiboles (predominantly arfvedsonite) at a late magmatic stage. Crystallization of mafic minerals leads to a sharp increase of K₂O content and decrease of SiO₂ content that cause a decrease of melt viscosity and REE and Zr solubility in the liquid. Therefore, REE and zirconium immediately precipitate as zircon and REE-minerals. There are numerous pod- and lens-like granitic pegmatites within the massif. Pegmatites in the REE-rich areas are also enriched in REE, but HREE prevails over light REE (LREE), about 88% of REE sum.     

Trace elements in various genetic types of zircon from syenite of the Sakharjok massif,Kola Peninsula

Zircon is an accessory mineral in alkali and nepheline syenites of the Neoarchean Sakharjok intrusion. Zircon in association with britholite and pyrochlore forms orebodies in nepheline syenite of this massif. Zircon crystals reveal an inhomogeneous zonal, occasionally mosaic structure comprising fragments and zones related to magmatic, hydrothermal, and metamorphic stages of mineral formation. Magmatic zircon differs by a high REE concentration (1769 ppm, on average), distinct Ce maximum (Ce/Ce* = 105, on average), and Eu minimum (Eu/Eu* = 0.19) as compared with other genetic types. No correlation between these parameters has been established. Hydrothermal zircon is characterized by a low Ce/Ce* ratio (0.7–3.9 and 2.0, on average), elevated LREE contents, and lowered ratios of MREE and HREE to La. Metamorphic zircon differs from magmatic by a sharply lower REE concentration (385 ppm, on average), lowered Th/U (0.32) and Ce/Ce* (31.9, on average) ratios. In the Ce/Ce* versus MREE/La plot, the lowest values of these ratios are typical of hydrothermal zircon, while the intermediate and maximum values are inherent to metamorphic and magmatic zircons, respectively. These variations make it possible to delineate reliable fields of their compositions. The distribution of data points in the above-mentioned plots shows that REE chemical activity depends on the redox conditions of zircon crystallization.     

Geochemistry and age of the complex of alkaline metasomatic rocks and carbonatites of the Gremyakha-Vyrmes massif, Kola Peninsula

This paper presents new geochemical data on the complex of alkaline metasomatic rocks and carbonatites, which hosts the rare-metal mineralization of the Gremyakha-Vyrmes massif. The contents of major and trace, including rare-earth elements were determined in the albitites, aegirinites, and carbonatites. Two types of the rare-metal ores are distinguished: niobium albitite and zirconium aegirinite ores. It was shown that the albitites and aegirinites have similar trace element distribution patterns, being most geochemically close to the foidolites. The carbonatites, albitites, and aegirinites were dated by Rb-Sr and Sm-Nd methods at 1887 ± 58 Ma, which corresponds to the formation age of the Gremyakha-Vyrmes massif. The ultrabasic rocks, foidolites, alkaline metasomatic rocks, and carbonatites were formed successively within a relatively narrow range. The geological observations and geochemical data led us to conclude that the emplacement of the fluid-saturated carbonatite solutions-melts at the final stages of the massif formation against a background of fault tectonics caused a pervasive metasomatism of the ultrabasic and alkaline rock complexes and, as a result, the formation of the alkaline albitites and aegirinites. The carbonatites could be sources of rare-metals, while foidolites served as a geochemical barrier, and their metasomatic alteration led to the formation of Zr-Nb mineralization in the albitites and aegirinites.     

Age of the Moncha Tundra fault, Kola Peninsula: Evidence from the Sm-Nd and Rb-Sr isotopic systematics of metamorphic assemblages

The first Sm-Nd and Rb-Sr dates were obtained for the dynamometamorphic processes associated with the origin and evolution of the Moncha Tundra fault, Kola Peninsula, which separates two large Early Paleoproterozoic layered intrusions: the Monchegorsk Ni-bearing mafic-ultramafic intrusion and the Main Range massif of predominantly mafic composition. The fault belongs to the regional Central Kola fault system, whose age was unknown. The material for the dating included metamorphic minerals from blastomylonitic rocks recovered by structural borehole M-1. Mineralogical thermobarometry suggests that the metamorphism occurred at 6.9–7.6 kbar and 620–640°C, which correspond to the amphibolite facies. The Sr and Nd isotopic systems were re-equilibrated, and their study allowed us to date the dynamometamorphic processes using mineral isochrons. It was established that the Moncha Tundra fault, and, respectively, the whole Central Kola fault system appeared in the middle of Paleoproterozoic ～2.0–1.9 Ga, simultaneously with the Svecofennian orogen in the central part of the region and the Lapland-Kola orogen in its northeastern part. Another episode of dynamometamorphism that occurred at 1.60–1.65 Ga is envisaged.     

The Early Paleoproterozoic Monchegorsk layered mafite-ultramafite massif in the Kola Peninsula: Geology, petrology, and ore potential

The Early Paleoproterozoic Monchegorsk Complex is exposed over an area of 550 km² and comprises two layered mafite-ultramafite intrusions of different age: the Monchegorsk pluton of ultramafic and mafic rocks and the predominantly gabbroid Main Range Massif (also referred to as the Moncha-Chuna-Volch??i Tundras Massif), which are separated by a fault. Both massifs consists of intercalating cumulates (first of all, Ol ± Crt, Ol + Opx ± Crt, Opx, Opx + Pl ± Cpx, and Pl), they were produced by similar melts of siliceous high-Mg series but differ in the stratigraphy of their cumulates: while the Monchegorsk pluton is dominated by ultramafites, the Main Range Massif consists mostly of gabbroids, first of all, of gabbronorites. The complex is accompanied by PGE-Cu-Ni ore mineralization, low-sulfide Pt-Pd mineralization, and chromite mineralization. Judging from geological data and isotopic dates, the Monchegorsk Complex is a long-lived magmatic center, which evolved over a time span of 50 Myr at 2.50?C2.46 Ga. The Main Range Massif is younger and likely truncates the western continuation of the Monchegorsk pluton. The complex is spatially restricted to the zone of the Middle Paleoproterozoic regional Central Kola Fault and is now tectonic collage whose rocks were variably affected by overprinted metamorphism in the course of deformations. These processes most significantly affected rocks along the peripheries of the Monchegorsk pluton in the south. These rocks were completely transformed under greenschist-facies conditions but often preserved their primary textures and structures. The processes overprinted both the marginal portions of the pluton itself and the rocks of its second phase, which are accompanied by economic low-sulfide PGE deposits. The PGE-Cu-Ni ore mineralization of the Monchegorsk Complex is genetically related to two distinct evolutionary episodes with a quiescence period in between:

1. The emplacement of large layered mafite-ultramafite intrusions at 2.5?C2.45 Ga. Economic deposits of sulfide Cu-Ni ores with subordinate PGE mineralization occur within the Monchegorsk pluton, and the moderate-grade low-sulfide PGE ores are related to its second evolutionary phase (in the foothills of Vuruchuaivench and in the Moroshkovoe Lake, and Southern Sopcha areas). The primary magmatic ore mineralization is predominantly Cu-Fe-Ni sulfide with PGE bismuthides-tellurides.
2. The Monchegorsk Complex was involved in the zone of the Central Kola Fault at 2.0?C1.9 Ga and was broken in a collage of tectonic blocks. The rocks were sheared along the boundaries of the blocks and were affected by overprinted metamorphism, which proceeded under greenschist-facies conditions in the structures surrounding the Monchegorsk pluton in the south. Thereby the primary PGE-Cu-Ni ore mineralization underwent metamorphic processes was recrystallized with the formation of Pt-Pd arsenides, stannides, antimonides, selenides, etc. This processes was associated with the partial redistribution of PGE with their local accumulation (up to economic concentrations), and the orebodies themselves acquired diffuse outlines. In other words, the second episode was marked by the transformation of the older primary magmatic ore mineralization.

     

Proterozoic Gremyakha-Vyrmes Polyphase Massif, Kola Peninsula: An example of mixing basic and alkaline mantle melts

The paper presents newly obtained data on the geological structure, age, and composition of the Gremyakha-Vyrmes Massif, which consists of rocks of the ultrabasic, granitoid, and foidolite series. According to the results of the Rb-Sr and Sm-Nd geochronologic research and the U-Pb dating of single zircon grains, the three rock series composing the massif were emplaced within a fairly narrow age interval of 1885 ± 20 Ma, a fact testifying to the spatiotemporal closeness of the normal ultrabasic and alkaline melts. The interaction of these magmas within the crust resulted in the complicated series of derivatives of the Gremyakha-Vyrmes Massif, whose rocks show evidence of the mixing of compositionally diverse mantle melts. Model simulations based on precise geochemical data indicate that the probable parental magmas of the ultrabasic series of this massif were ferropicritic melts, which were formed by endogenic activity in the Pechenga-Varzuga rift zone. According to the simulation data, the granitoids of the massif were produced by the fractional crystallization of melts genetically related to the gabbro-peridotites and by the accompanying assimilation of Archean crustal material with the addition of small portions of alkaline-ultrabasic melts. The isotopic geochemical characteristics of the foidolites notably differ from those of the other rocks of the massif: together with carbonatites, these rocks define a trend implying the predominance of a more depleted mantle source in their genesis. The similarities between the Sm-Nd isotopic characteristics of foidolites from the Gremyakha-Vyrmes Massif and the rocks of the Tiksheozero Massif suggest that the parental alkaline-ultrabasic melts of these rocks were derived from an autonomous mantle source and were only very weakly affected by the crust. The occurrence of ultrabasic foidolites and carbonatites in the Gremyakha-Vyrmes Massif indicates that domains of metasomatized mantle material were produced in the sublithospheric mantle beneath the northeastern part of the Fennoscandian Shield already at 1.88 Ga, and these domains were enriched in incompatible elements and able to produce alkaline and carbonatite melts. The involvement of these domains in plume-lithospheric processes at 0.4–0.36 Ga gave rise to the peralkaline melts that formed the Paleozoic Kola alkaline province.