Abstract

Rutile, ruthenian rutile and titanian ruthenium dioxide inclusions were found in isoferroplatinum nuggets obtained as by-products of gold sluicing in the historical gold-rush area of Finnish Lapland. Analytical data suggest that an extensive solid-solution exists between the end-member compositions TiO2 and RuO2. Ruthenian phases occur almost exclusively with laurite inclusions in the nuggets, in some cases as inclusions and intergrowths in laurite, suggesting the primary nature of the ruthenian phases. The TiO2 phase is considered to be rutile rather than anatase or brookite, because there seems to be an extensive solid-solution between the TiO2 and RuO2 phases, and RuO2 is known to be isostructural with rutile. The abundant mafic and ultramafic metamorphosed intrusive and extrusive rocks occurring southwest of sluicing sites could represent source rocks of the isoferroplatinum nuggets. After the Cenozoic weathering event, the nuggets drifted in the Holocene ice sheets to their present position in till overlying the migmatitic to enderbitic bedrock of the Lapland granulite belt.

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