Unconventional hydrocarbons in abiogenic Archean crystalline rocks : insight into ancient fluid chemistry in the Lupin Gold Deposit, Nunavut, Canada

Abstract

Archean gold-bearing quartz veins at the Lupin mine occur in amphibolitic iron-formation in a deformed metaturbidite sequence. Stable isotope, fluid inclusion microthermometry, gas chromatography, laser ablation IC-PMS, and laser Raman microscopy techniques were used to study quartz-hosted fluid inclusions, the associated altered host rocks, and the relative timing and role fluids played in late stage gold precipitation. Bulk GC crush analysis of vein quartz showed that fluid inclusion volatiles are dominated by CH4, but also contain aliphatic species up to at least C6, and aromatic species that are suspected to be of a biogenic origin but could have been re-speciated by abiogenic processes (e.g., reductive coupling oligomerization). The vein quartz, at 20˚C, contains two petrographically distinct single-phase non-aqueous fluid inclusion types, classified as type-I and type-II. Type-II fluid inclusions (T[subscript hCarbonic(avg)] = -22.4˚C) contain primarily CO[subscript 2]-(CH[subscript 4]-N[subscript 2]-H[subscript 2]S), and are early secondary in origin, forming as grain boundaries sealed up after most of the quartz had already grown. Type-I fluid inclusions (T[subscript hCarbonic(avg)] = -89.9˚C) contain primarily CH[subscript 4]-(N[subscript 2]-H[subscript 2]S), and are late secondary, crosscutting the healed grain boundaries that host type-II fluids. The type-I fluids were trapped heterogeneously with two-phase aqueous fluids between 320 - 490˚C and 1.2 – 2.2 kbar, as deduced by the method of intersecting isochores. Sulfur found in vein and host sulfides has [Sigma superscript 34]S[subscript VCDT] values of +1.8 to +3.4‰, which is in the endogenic (processes that originate within the earth) or magmatic source range. Nitrogen isotope analysis of fluid inclusions in vein quartz found a [Sigma superscript 15]N[subscript AIR] value of +7.4‰, which indicates that organics and micas from the BIF and associated metasedimentary rocks are source possibilities. Carbon isotope analysis of fluid inclusions in vein quartz found [Sigma superscript 13]C[subscript VPDB] values of -29.9 to -22‰, in accordance with a thermogenic origin of hydrocarbons, or originating from Fischer-Tropsch-type reactions. Three possibilities for secondary gold precipitation are suggested based on ore assemblages and petrographic/compositional characteristics of fluid inclusion assemblages: (i) an increase in pH, which is suggested by the occurrence of magnetite with gold, decreasing the Au-complexing sulfur and Au concentrations in solution; (ii) minor changes in ƒO[subscript 2] in a highly oxidized system causing Au to drop out; or (iii) the partitioning of H[subscript 2]S (and Au-complexing HS[superscript -) from aqueous fluid to carbonic fluid as they mingled immiscibly, derived from different sources initially.