© 2015 T. Al-Ani, published by De Gruyter Open. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License. The article is published with open access at www.degruyter.com. Open Geosci. 2015; 7:646–662 Research Article Open Access Thair Al-Ani* REE-bearing minerals in the Rapakavi Grante, Southern Finland DOI 10.1515/geo-2015-0057 Received Januar 26, 2015; accepted August 16, 2015 Abstract: The studied rock samples belong to the classic 1615–1645 Ma Wiborg Rapakivi granite terrane of south- eastern Finland. Geochemical studies show that the ra- pakivi granites and associated rocks form metaluminous to peraluminous A-type granites and plot in the "within plate granites (WPG)" field on the tectonic discriminate di- agrams from the Onkamaa, Suomenniemi and Luumäki. The rapakivi granite displays enrichment of light over heavy REE (LREE/HREE = 2-34) and usually negative Eu anomalies (Eu/Eu * = 0.01 - 1.4). Enrichment in REE in some studied samples is confined to highly fractionated por- tions of the Rapakivi granite. Fractional crystallization of the evolving fluorite-rich peraluminous granitic magma was accompanied, particularly at later stages by fluid frac- tionation, which plays an important role in the genesis of the REE-mineralization. The studied rapakivi granites host REE-minerals includ- ing monazite-(Ce), allanite (Ce), bastnäsite (Ce), xenotime, thorite and REE-bearing mineral apatite. Monazite and al- lanite are the most important REE carriers in the studied granites and these minerals are strongly enriched in the LREE. Monazites are hosted in apatite, quartz, plagioclase, K-feldspar, and biotite. Grain size of monazite is variable ranging from 50 to >100 μm. Monazite contains 48-68 wt% REE 2 O 3 , 24.3-29.3 wt% P 2 O 5 and low Th<1.5 wt%ThO 2 . The Y, REE, U, Th-bearing minerals are not commonly asso- ciated with the primary minerals except for Th-bearing minerals, which occur as silicates (e.g. thorite, ThSiO 4 ); and/or replace other elements in the structure of some ac- cessory mineral, especially xenotime, brabantite, zircon, and apatite. Electron probe microanalysis (EPMA) provides an indi- cation of solid solution series between thorite-xenotime- zircon, which are related to hydrothermal solutions en- riched in REE, Y, P, U, F, and Zr. Keywords: Rapakivi; Finland; REE; Monazite; Allanite; Bastnäsite; Xenotime; Thorite *Corresponding Author: Thair Al-Ani: Geological Survey of Fin- land, FI-02151 Espoo, Finland, Email: thair.alani@gtk.fi 1 Introduction The Rapakivi granites of southern Finland form four large batholiths (Wiborg, Aland, Vehmaa, Laitila), and several smaller satellitic batholiths and stocks (Suomenniemi, Ahvenisto, Onas, Bodom, Obbnäs, Kökar, Fjälskär, Eura- joki, Kokemaki) [1]. The studied area is located within the Wiborg Rapakivi granite batholith of 1.64–1.63 Ga in age (Figure 1). Wiborg Rapakivi granites are associated with a variety of types of mineralization (e.g. occurrence of topaz, Li-Fe mica and albite; high F, Li, Rb, Ga, In, Sn and Nb; low Mg, Ti, Zr, Ba, Sr and Eu), which can be used to distinguish them from barren granites [2]. Many stud- ies since 1970’s have considered the Rapakivi granite com- plexes in Finland to be magmatic in origin, whose miner- alogy have been modified by the postmagmatic fluids [e.g. [3–8]], which altered their metallogeny [9, 10]. The typi- cal rapakivi texture is coarse-grained, with alkali feldspar ovoids either surrounded by sodic plagioclase (Wiborgite) or with majority of alkali feldspar megacrysts not mantled by plagioclase (Pyterlite) [1, 4–6]. Many of these granites show the mineralogical and geochemical characteristics of tin granites and in several cases, greisens-type [9–13] and rare skarn-type [14] Sn–Be–W–Zn-In mineralizations asso- ciated with them. This study presents detailed petrological, mineralogi- cal and geochemical characterization of the Rapakivi gran- ite and associated rocks. We provide a detailed description of REE behavior and mineral chemistry of REE-bearing minerals (monazite-(Ce), xenotime-(Y)) and REE-bearing minerals (apatite, fluorite, and zircon) within mineralized lamprophyre dyke, and discussed the genesis of these min- erals in the context of the Rapakivi granites. 2 Methods A total of 28 samples from the Onkamaa Hamina, Suomen- niemi, Luumäki and Kymi in southern Finland were col- lected for geochemical and mineralogical analyses. Ma- jor and minor elements were determined by XRF (method 175X), rare earth elements and trace elements by ICP-MS (method 308 M) and Co, Sc, V, Y and Zr by ICP-AES (method 308P) in the Espoo laboratory of Labtium. Unauthenticated Download Date | 7/26/18 9:33 AM