Mineralogical and Petrographic Characteristics of Indium and REE-Bearing Accessory Phases in the Kymi Granite Stock , Southern Finland

The Wiborg rapakivi batholith (1.64 Ga) in southeastern Finland with documented occurrences of REE, indium and Zn-Cu-Pb sulphide mineralization was studied. Hydrothermal greisen and quartz vein type Fe-Sn and Zn-Cu-Pb are found in the Kymi granite stock as intrusions. They are enriched with indium and rare earth elements, with roquesite (CuInS2) being a major indiumcarrier, whereas monazite (Ce), allanite (Ce), bastnäesite (Ce), xenotime-(Y) and thorite are the main REE carriers. Combination of optical and field emission scanning electron microscopy (FE-SEM) and electron probe microanalysis (EPMA) were used to study the indium and REE-bearing mineral assemblages. EPMA of roquesite found in galena had a composition of 26.16% S, 0.02% Fe, 25.06% Cu, 0.03% Zn, 1.06% As, 0.31% Sb and 47.14% In. Substitution reaction PbS ↔ CuInS is the cause of the incorporation of indium in the galena structure. The majority of the LREE are carried by monazite, bastnäesite and allanite, and the HREE by xenotime and zircon. There is a partial solid solution between monazite and xenotime with minor or trace amounts of LREE in xenotime grains (6.0 wt%). LREE (>95 mol% LREE) and less than 5 mol% HREE + Y reflects the enrichment of chondrite-normalized REE of the monazite grains of the Kymi granite stock. The xenotime grains (small and irregular) main composition contains 71 76 mol% YPO4, 16 27 mol% HREE, and 6 8 mol% LREE. It is believed that indium and REE-mineralization presence is due to the combination of magmatic and postmagmatic processes, particularly at later stages by fluid fractionation.


Introduction
The Wiborg rapakivi batholith (1.64 Ga) in southeastern Finland (Figure 1) is one of a few regions in the Fennoscandian Shield with several documented occurrences of REE, indium and Zn-Cu-Pb sulphide mineralization.Petrological, field observations and many studies on the rapakivi granites in southern Finland, suggest that REE enrichment in the studied rapakivi samples is due to a combination of magmatic and postmagmatic processes (alteration and mineralization), which play an important role in the genesis of REE mineralization [1] [2] [3] [4].In the Wiborg rapakivi batholith of southeastern Finland, several promising exploration targets have been investigated previously with respect to indium (In) and Zn-Cu-Pb sulphide.The first observation of a mineral with essential indium (roquesite) in the Fennoscandian Shield was that of Burke & Kieft [5], and not least due to increasing global interest in this rare metal, new discoveries have recently been made, particularly in southeastern Finland [6] [7] [8].These occurrences include In-bearing magnetite-sphalerite ore (Pahasaari and Getmossmalmen), Zn-Cu-Pb-Ag-In-bearing greisen veins (Jungfrubergen) and indium-bearing polymetallic veins in Loviisa.According to these authors, only the sphalerite has shown significant indium contents from the multiple greisen veins of the wiborgite rapakivi granites, which indicates that these veins related to brittle tectonic evolution and associated hydrothermal activity.Indium typically occurs in trace concentrations in copper and zinc sulfide minerals, of which sphalerite is the most important for global indium production, even though chalcopyrite exhibits the highest observed contents [9].In the Toyoha mine, Japan, the world's largest indium producer in the 1990s -early 2000s, the main carriers of indium besides indium-bearing sphalerite and stannite-kësterite were indeed roquesite together with sakuraiite, chalcopyrite, and a poorly defined Zn-In mineral [10] [11].An elevated indium contents that were identified at the sulfide-polymetallic and tin-sulfide deposits of Siberia and Far East, Russian; therefore, these deposits are of economic interest.Sphalerite and chalcopyrite and chalcopyrite, bornite, and sphalerite are the major indium carriers in the base-metal massive sulfide and tin-sulfide ores, respectively [12] [13].
The data presented in this study are the outcome of unique set of boreholes drilling during 2014 as part of a critical minerals project carried out by the Geological Survey of Finland (GTK).The study aimed to assess the potential for indium and REE minerals hosted by rapakivi granite and associated rocks in southern Finland.Here, we provide a detailed description of the accessory phases and mineral chemistry of indium and REE-bearing minerals within late-stage intrusions, which is important not only for the Wiborgite rapakivi granites, but also for other granitic complexes in the Palaeoproterozoic bedrock.

Analytical Methods
Thin sections were prepared from 14 samples of which 12 were selected from topaz-greisen mineralization veins, stockscheider pegmatite and even aplitic DOI: 10.4236/nr.2018.92003

Petrography and Mineral Chemistry
The Kymi granite complex consists mainly of three modes of rocks; these include ovoidal alkali feldspar phenocrysts mantled by sodic plagioclase, quartz-feldspar porphyrtic texture with angular shape phenocrysts and porphyritic hornblende rapakivi texture composed of plagioclase feldspar, K-feldspar, quartz, biotite and hornblende [4] [14].and FeO (total content > 2 wt%).Furthermore, there is no important indium enrichment in the selected cassiterite grains (In 2 O 3 between 0.0114 to 0.0047 wt%) (Table 2).3. The chemical determinations confirmed that Zn, Fe, Ag, Sb and Bi are the most important minor elements and have an uneven distribution in the studied galena.

Indium and REE-Bearing Minerals
The indium and REE-bearing mineral assemblages were investigated in polished thin sections of separated heavy mineral and petrographic thin sections by using a combination of a combination of optical and electronics microscopies (SEM& EPMA).Roquesite is a major indium carrier, while monazite, bastnäesite, alla-  1.90 wt%) and inversely correlated with ThO 2 (Table 5).The higher grade metamorphic and related igneous rocks are the best source areas for monazites with a high thorium content [16].Monazites with low Th are common in rocks that have experienced significant fluid involvement, including authigenic precipitation (e.g., [17]) or as hydrothermal precipitates (e.g., [18]).

The Fractionation of the REE-Bearing Minerals
Kymi granite stock in southeastern Finland and especially the related greisens are enriched in REE, In, F and Sn and contain various sulphide mineralization such as sphalerite, galena, chalcopyrite and arsenopyrite.An examination of the distribution of REE in the studied rapakivi granites revealed that a large proportion of the LREE were hosted in monazite and bastnäesite, whereas the HREE were commonly hosted in xenotime, thorite and zircon (Figure 4).Enrichment of REE in some of the selected samples of the studied Kymi granite stock is confined to highly fractionated portion at the rapakivi granites.Therefore, the coexisting volatile-rich fluid at the later stage, which play an important role in the genesis of REE-mineralization in the wiborgite and Kymi granite stock.Figures    6).
The solid solution between monazite and xenotime is limited between 6 -8 mol% LREE in xenotime and between 4 -5 mol% HREE + Y in monazite (Figure 5).The LREE content of melts buffered by xenotime and HREE content of melts buffered by monazite have a strong temperature dependence, which can be used as a geothermometer [20].The mole fraction of HREE + Y in monazite ranges between 4 -5 mol%, corresponding to low temperatures of 500˚C -600˚C (Mogilevsky, 2007 in figure 9).The solid solutions of monazite and xenotime in the studied Kymi granite complex were equilibrated hydrothermally at peak temperatures ranging from 500˚C -600˚C.The essential point is that the concentrations of HREE + Y in monazite and LREE (La to Sm) in xenotime are strongly dependent upon the growth temperature and degree of metamorphism [21] [22].

Mineralization of Indium-Bearing Minerals
Mineralized greisen veins and deposits generally occur in association with highly fractionated granitic intrusions [9] [23].The highly evolved Kymi granite complex contains several greisen veins within the zonal structure, and they occur in  the release of Zn from the crystal lattice of sphalerite present in galena is also possible.

Conclusions
1) The occurrence of greisen intrusions within the Kymi granite stock and as well in the surrounding wiborgite rapakivi granites, such as albite-topaz-REE microgranite dikes and mineralization greisen veins of F-Sn-Zn-Pb-Cu, is due to primary magmatic fluids and postmagmatic processes, which strongly enriched in fluorine and tin, respectively.These intrusions host In-REE minerals, with roquesite (CuInS 2 ) being a major indium carrier, whereas monazite (Ce), allanite (Ce), Bastnäesite (Ce), xenotime-(Y) and thorite are the main REE carriers.
2) Mineralogical investigations show the dominating LREE-bearing minerals of monazite, allanite and bastnäesite, while the HREE-bearing minerals include thorite, xenotime and zircon.F-rich granitic melt has been intruded and cross-cutting magmatic system at later stages, which plays an important role in the genesis of REE mineralization.
3) The studied Kymi granite stock may include variable amounts of sulphides, such as galena, sphalerite, pyrite and chalcopyrite.These sulphide assemblages occur in variable amounts and may be precipitated from hydrothermal solutions during the late stage of crystallization as filling of fractures, of open vugs and of the spaces within the studied rapakivi granites.Indium association with sulphide, mainly galena, can be distinguished on the basis of a microanalytical study of the sulphide assemblages.The results demonstrate that indium prefers galena over sphalerite and chalcopyrite.This could be due to possible substitution of copper and indium with lead in galena.A probable substitution is Pb 2+ S 2− ↔ Cu + In 3+ S 2− , which gives a similar ionic radius and the same charges.Furthermore, the strong correlation between copper, iron and zinc in galena could also play an important role.However, another likely suggestion is that zinc presented in galena is replaced by indium, although the coupled substitution 2(Pb 2+ Zn + ) = Cu + In 3+ is also present.

Figure 1 .
Figure 1.Geological map of the Kymi topaz granite stock in the Wiborg rapakivi batholith, southeastern Finland.The selected borehole locations are marked with gray dots.Modified from Haapala & Lukkari [26].
Apatite (AP) is present in most of the studied samples as disseminations biotite, quartz and feldspar or in contact with zircon, sulphides, chlorite and REE-bearing minerals.A BSI image of some apatite crystals shows an approximately hexagonal basal section, 70 µm in diameter (Figure 2(a)).It was also commonly observed as inclusions within zircon (Figure 2(b)).Apatite grains (fluorapatite mainly) are characterized by higher Ca (53.

Sulphides:
Most widespread sulphide minerals found in the studied wiborgite rapakivi granites are sphalerite (Sp), galena (Gn) and chalcopyrite (Cp) are the dominants sulphide phases.Galena and sphalerite are intimately intergrowth and develop a rim replacement texture of sphalerite-galena (Figure 2(a), Figure 2(e)).Chalcopyrite occurs as an individual crystal and commonly associated with acanthite (Ag 2 S) and galena (PbS) (Figure 2(f)).The number of galena crystals with an octahedral shape and other sulphide grains was derived from heavy mineral assemblages of the two samples, L433 R1/18.95 and L433 R1/21.10.The FE-SEM-EDS examination of multiple galena grains established overgrowth development of large early octahedral-cubic crystals by numerous smaller cubic crystals with triangular symmetry and a size below 500 µm (Figure 2(g), Figure 2(h)).Inclusions of an indium-bearing mineral (roquesite) sometimes occur in DOI: 10.4236/nr.2018.9200328 Natural Resources T. Al-Ani et al. galena, mostly as later sulphide phases, in selected rapakivi granite samples (Figure 3(a), Figure 3(b)).The chemical composition of the sulphide minerals obtained by EPMA is given in Table nite and xenotime are the dominant light rare earth element (LREE) minerals studied Kymi granite stock, southern Finland.Roquesite (Roq) is a major indium carrier (ideal formula CuInS2) from the Kymi granite complex.It mainly occurs as 10 -30-µm subhedral to anhedral, often found as angular crystals in galena (Figure2(g), Figure2(h)).Overall, the mineral appears to be relatively widespread, yet not abundant; it has been observed within a two-metre section of the drill core sample from the studied section L434R1/20.75-21.10 m, and some of roquesite appear as several individual It becomes clear, that the low Th content in the monazite from the mineralization greisen and quartz vein in The Kymi granite stock is indicative of hydrothermal origin of alteration.Bastnäesite (Bsn) in the studied rapakivi granite is mostly present as acicular or needle-shaped crystals forming either radial accumulations or intricate crosscutting grids within a variety of minerals such as monazite, fluorite and biotite (Figure 3(c)).It forms needle-like aggregates of crystals in small cavities or occurs as fillings of thin fractures in fluorite and quartz (Figure 3(g)).In some cases, it is found as a replacement of allanite.It appears that a bright bastnäesite ring (Figure 3(h)) replaced the outer rims of the former allanite.The presence of fluorite, and REE bearing minerals (allanite, bastnäesite) in most fluorite-rich samples is indicative to fractionated of lanthanides (rare-earth elements, REE) in DOI: 10.4236/nr.2018.9200330 Natural Resources

Table 1 .
Representative electron microprobe analysis of apatite and fluorite from the Kymi granite stocks, wt%.

Table 2 .
Representative microprobe analysis of cassiterite from the Kymi granite stocks, wt%.
crystals about 100 µm in length, and in contact with biotite (Figure2(d)).The crystals of cassiterite from the studied samples are homogeneous in composition and consist of nearly pure SnO 2 (>98 wt%) with small amounts of Nb 2 O 5 , TiO 2

Table 3 .
Representative microprobe analysis of sulphide minerals from the Kymi granite stocks, wt%.

Table 4 .
Representative electron microprobe analysis of roquesite from drill core L434R1 with the drilling interval depth between 20.75 -21.10 m in Kymi granite stocks, wt%.
3(2.5 to 3.2 wt%) and F (6.7 to 9.3 wt%).Bastnäesite grains also revealed the presence of HREE (0.5 to 1.5 wt%), ThO 2 (0.3 to 1.3 wt%) and Y 2 O 3 (1.7 to 3.4 wt%) (Table5).The overwhelming presence of fluorite, REE bearing minerals, the alkaline nature of the igneous phases, carbonate flooding, sodic and potassic alteration in the district has led some authors to call upon a hidden carbonatite as the REE metal source and hydrothermal heat source (Schreiner, 1993 and McLemore, 2010).Allanite (Aln) analysed in the present study (Figure 3(h)) has an almost rectangular shape (400 µm), with irregular subhedral contacts with biotite (Bt) and, more externally, quartz (Qtz).Allanite is enriched in REE, particularly the light REE (Ce, La, Pr and Nd), and shows a relatively uniform composition with 31granite stock also occur in the monazite-xenotime solid solution, which indicates the presence of hydrothermal solutions highly enriched in REE, Y, P, F and Sn.Hydrothermal and solid solutions can be distinguished by a high content of DOI: 10.4236/nr.2018.9200332 Natural Resources T. Al-Ani et al.

Table 5 .
Chemical composition of LREE-bearing minerals from the Kymi granite stocks, wt%.

Table 6 .
Chemical composition of xenotime and thorite from the Kymi granite stocks, wt%.