The relationship between CH_4 and CO_2 inclusions and Fe-O-S mineralization in intrusions of the Kola alkaline province

摘要

Studies on the origin and relationship of methane and higher hydrocarbons in alkaline complexes of Khibina and Lovozero in the Kola Peninsula, along with a comparative study of fluid inclusions in the carbonatites and phoscorites of Kovdor and Sokli, have revealed a close relationship between reduced carbonic fluids and late-stage hydrothermally altered assemblages. The assemblages have become increasingly oxidised with the production of secondary minerals rich in ferric ions such as aegerine, biotite and magnetite replacing earlier mafic minerals. An abiogenic origin for the hydrocarbons is favoured involving Fischer-Tropsch reactions between primary magmatic CO_2-rich fluids and H_2 produced during late-stage hydrothermal reactions at temperatures of 350 deg and pressures of 0.5-1.2kbar. The presence of magnetite and hydrated oxidised silicates catalyse the Fischer-Tropsch reaction. The sulphide mineralization at Sokli and Khibina is associated with H_2O-rich fluids, not with the reduced carbonic fluids as might have been predicted.