Abstract
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 indium- carrier, 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 Pb2+S2-Cu+In3+S2- 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.
Highlights
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, which play an important role in the genesis of REE mineralization [1] [2] [3] [4]
The present study demonstrated that indium was initially concentrated in galena, and we have discovered new occurrences of the copper-indium sulphide mineral roquesite as minute grains included in galena
Summary
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. 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, in southeastern Finland [6] [7] [8]. These occurrences include In-bearing magnetite-sphalerite ore (Pahasaari and Getmossmalmen), Zn-CuPb-Ag-In-bearing greisen veins (Jungfrubergen) and indium-bearing polymetallic veins in Loviisa. 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]
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