First find of cerianite in zircons from metasomatites of the Terskii greenstone belt, Baltic shield

Abstract

) is one of the most scarce rareearth minerals. Because of difficulties in discovery andidentification of this mineral, E.I. Semenov proposedconsidering cerianite as an object of nano (not micro)mineralogy [1]. Since its discovery in the 1950s, nomore than 20 reliable finds of this mineral (mainly inthe form of microscopic grains and microinclusions inother minerals) have been documented. Cerianite wasdiscovered for the first time in Sudbury (Canada) inthe carbonate dikes cutting across nepheline syenites[2]. In addition to Ce, cerianite contains isomorphicadmixtures of other LREE (La, Pr, Nd). Significantamounts of Th and other elements (Xe, Cs, Pb, Rh)were found in cerianite from pegmatites of East Antarctica [3]. Supergene cerianite occurs as a weatheringproduct of bastnaesite [1] and fluocerite [4], as well asin the weathering crust of different compositions [1,5–7]. In the oceanic Fe–Mn ores, cerianite isobserved as submicron films on apatite [8]. The formation of cerianite in granites is considered to berelated to hydrothermal alteration, accompanied byfluid leaching of Ce from monazite [9]. Cerianite associates with minerals and ores of noble metals: Au, Pt(Sukhoi Log deposit [10]); Au, Pd, and REE (oremanifestations of the Polar Urals [11] and others]).Overgrowths of cerianite of a few microns thick ondiamond phase were found in the carbonado fromBrasilia [12].Cerianite is a sensitive indicator of conditions ofmineral formation, because Ce is the only compatibleREE, which in natural conditions can occur not onlyin trivalent but also in tetravalent form. Cerianite isformed only in strongly oxidizing conditions, mainlyin alkaline solutions (fluids) [13].Submicron inclusions of cerianite were found by usin the zircon from twomica garnet–chlorite–quartz–plagioclase metasomatite (sample 2508) in the Terskiigreenstone belt in the southern framing of the Imandra–Varzuga structure (southeastern part of the KolaPeninsula). Complex isotopic–geochemical study ofzircons from metasomatites of the Terskii greenstonebelt allowed us to distinguish two stages of metamorphism (2680 and 2025 Ma) and two stages of metasomatism (2600 and 1800 Ma) [14]. Practically all zircons of different ages were completely or partiallymetasomatized, which caused their enrichment inREE (especially LREE), Th, U, Sr, and Ba, and formation of flat REEs with a reduced Ce anomaly.The structural features, majorelement composition of the zircons, and the composition of foreignmineral inclusions, if present, were studied in theregime of compositional contrast on a JEOL JSM6460LV scanning electron microscope equipped withan Oxford INCA detector at the St. Petersburg MiningInstitute. U–Pb dating of zircons from metasomatiteswas conducted on a SHRIMPII ion microprobe atthe Center for Isotopic Research, Karpinskii AllRussia Research Institute of Geology. The REE and traceelement composition of zircons was analyzed at thesame grains and points as U–Pb dating, on a CamecaIMS4f ion microprobe at the Yaroslavl Branch of thePhysicoTechnical Institute of the Russian Academyof Sciences.Zircon containing cerianite differs in the heterogeneous structure. Its weakly altered right upper part(point 2.1 in Fig. 1) retains traces of faceting and primary growth zoning. At the final stage of the Svecofennian metasomatism, most zircon was transformed with the formation of “patches” of irregularreniform shape, which form a band up to 100 µm insize with a characteristic dark tint in backscatteredelectrons (BSE) (point 2.2 and inset