TITLE:
Mineralogical and Petrographic Characteristics of Indium and REE-Bearing Accessory Phases in the Kymi Granite Stock, Southern Finland
AUTHORS:
Thair Al-Ani, Timo Ahtola, Janne Kuusela, Nadhir Al-Ansari
KEYWORDS:
Indium, Roquesite, REE-Minerals, Kymi Stock, Finland
JOURNAL NAME:
Natural Resources,
Vol.9 No.2,
February
14,
2018
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.