Fischer-Tropsch synthesis of hydrocarbons during sub-solidus alteration of the Strange Lake peralkaline granite, Quebec/Labrador, Canada

The composition of the carbonic phase(s) of fluid inclusions in pegmatite quartz from the Strange Lake peralkaline complex has been analysed by gas chromatography using online extraction of inclusion contents and a PoraPLOT(R) Q capillary column. The measured gas species are, in order of abundance, CH4, H-2, C2H6, CO2, N-2, C3H8, n-C4H10, n-C5H12, C2H2, i-C4H10, and C2H4. Minor amounts of i-C5H12, n-C6H14, i-C6H14, and neo-C6H14 were also detected (but not quantified) in some samples. A suite of quartz samples from Ca-metasomatised pegmatites contains fluid inclusions with a similar distribution of hydrocarbons but much higher proportions of CO2. The carbonic fluid coexisted immiscibly with a brine (Salvi and Williams-Jones, 1992), which on the basis of field and petrographic evidence, was interpreted to have originated from the magma. However, thermodynamic calculations indicate that the above gas species, specifically the hydrocarbons, could not have coexisted at equilibrium in the proportions measured, at any geologically reasonable conditions either prior to or post entrapment. We propose, instead, that the gas compositions measured in the Strange Lake inclusions, and in inclusions from other alkalic complexes, resulted from the production of H-2 during the alteration of arfvedsonite to aegirine, and the subsequent reaction of this H-2 With orthomagmatic CO2 and CO to form hydrocarbons in a magnetite-catalysed Fischer-Tropsch synthesis. Locally, influx of an oxidised calcic brine, derived externally from the pluton, altered the original composition of the fluid by converting hydrocarbons to CO2. Copyright (C) 1997 Elsevier Science Ltd