Noble gas isotopic signatures of sulfides in carbonatites of the Vuoriyarvi alkaline-ultrabasic complex (Kola Region, NW Russia)

Isotope signatures of noble gases incorporated into sulfides from alkaline-ultrabasic carbonatite complexes of the Devonian Kola Alkaline Province (Kola region, NW Russia) have not been studied previously. We investigated the isotope composition of helium and argon in pyrite and pyrrhotite from several carbonatite varieties of the Vuoriyarvi massif. The obtained data showed that the noble gas isotope systems are well preserved in sulfides from carbonatites. We found that sulfides from the Vuoriyarvi trapped both crustal (meteoric) waters and the modified mantle fluids. Fluids with a pronounced mantle He signature captured by pyrrhotite are isotopically homogeneous. In contrast, isotope signatures of He and Ar in pyrite evidence that this mineral was formed at a dynamic change of fluid sources. Evidence of fractionation of He and Ar due to degassing was also revealed. These results suggest that the isotopic signatures of noble gases in sulfides from carbonatites are promising as a tool for identifying the fluid phase associated with sulfide mineralization and paragenetic (i.e., REE-rich) minerals.