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

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 (CuInS 2 ) 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 Pb 2+ S 2− ↔ Cu + In 3+ S 2− 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% YPO 4 , 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.

Geology and Mineralogy of Rare Earth Elements Deposits and Occurrences in Finland

“…As it is expected, and also demonstrated by many other studies [63,64], LREE are generally enriched in monazite, allanite and carbonate minerals, whereas xenotime concentrates HREE regardless the conditions of crystallization. The highest average LREE concentrations at around 60 wt % occur in monazite, bastnäsite and synchysite, but the range of these values is highly variable.…”

“…Previous studies concluded that enrichments of REE is due to a combination of magmatic and hydrothermal processes [60,[63][64][65]. The 1.64-1.63 Ga Kymi Granite Complex can be divided into three varieties of rapakivi granite: (1) granite with ovoid alkali feldspar phenocrysts larger than 3 cm in diameter, mantled by sodic plagioclase.…”

“…Previous studies concluded that enrichments of REE is due to a combination of magmatic and hydrothermal processes [60,[63][64][65]. …”

Critical Metals Mineralization in the Late-Stage Intrusions of Salmi Batholith, Ladoga Karelia, Russia

“…This formation pattern of roquesite is similar to the mechanism of removal of invisible gold from arsenopyrite's crystalline lattice, which provoked the formation of independent aggregates upon a rise in temperature [44]. Experimental data [45] and the study of In-bearing base-metal mineralization in Kumi greisens from Vyborg rapakivi batholiths in southern Finland [46] suggest that In could have penetrated isomorphically into galena. Upon further alteration of galena, In may have been released from it, binding in its own mineralroquesite.…”