Gas chromatographic investigation of the light hydrocarbon compositions of fluids associated with various geological environments and regimes: A first comparison

English Abstract:

The light, saturated hydrocarbon signatures (C1-C4 saturated species) of volatile fluids hosted in a variety of distinct geological environments (trapped in fluid inclusions and/or as occluded gas) have been analyzed. Fluid composition data was acquired directly by in-line gas chromatographic (GC) analysis, and from a number of published literature sources. The geological environments investigated include a variety of sedimentary, metamorphic and igneous terranes, both ore-bearing and non-ore-bearing. Although studies using bulk GC analysis to investigate the hydrocarbon signatures of trapped fluids from certain environments have been performed previously, a study comparing the hydrocarbon signatures from a diversity of geological environments has yet to be compiled and may offer insight into the influence of non-aqueous fluid constituents in these systems. Hydrocarbon-bearing fluids of known biogenic/thermogenic origin (i.e., from oil and gas fields and gas hydrates) are markedly richer in C3-C4 hydrocarbons and possess elevated C4/C3 and C3/C2 hydrocarbon ratios relative to hydrocarbon-bearing fluids of suspected abiogenic origin. Fluids from various igneous environments (e.g., mantle, porphyry-epithermal, alkali-agpaitic, mafic-ultramafic-associated Ni-Cu-PGE sulfide deposits, and rare metal pegmatitic environments) share very similar hydrocarbon signatures and consistently display a log linear decrease in hydrocarbon abundance with increasing carbon number (Shultz-Flory distribution). This suggests that light hydrocarbons hosted in these igneous terranes may share similar origins and genetic controls, likely abiogenic in nature, due to their magmatic, organic-poor setting of entrapment. These processes may include, but are not limited to, catalytic Fischer-Tropsch synthesis and reductive coupling reactions, which generate higher order hydrocarbon via step-wise homologation reactions. Understanding the non-aqueous fluid chemistry associated with various geological environments will help elucidate the complex physicochemical controls responsible for hydrocarbon generation under geologically relevant conditions (T, P, fO₂, pH, etc.) and the potential for ore metal remobilization by these non-aqueous compounds.