Application of bulk fluid volatile chemistry to exploration for metasedimentary rock-hosted orogenic gold deposits: An example from the Meguma terrane, Nova Scotia, Canada

Abstract

Disseminated gold (Au) deposits hosted in metasedimentary rocks present unique exploration challenges because the Au is generally very fine-grained or refractory, and not vein-hosted. Exploration programs in such settings rely on extensive lithogeochemical surveys, lesser structural analysis, and little emphasis on ore-fluid geochemistry. The Moose River anticline (MRA) in the Lower Paleozoic Meguma terrane (Nova Scotia, Canada) hosts several high-tonnage (~10–20 Mt), low-grade (≤1–4 g/t Au) Au deposits comprised of disseminated ± quartz vein-hosted mineralization in metamudstones. Here, the Touquoy deposit hosted in the MRA is used to evaluate a new approach to geochemical exploration by examining if the volatile chemistry of host rocks preserves a signature related to Au mineralization, using a gas chromatographic (GC) technique that analyzes bulk volatiles released from crushed rock. Since GC is a bulk analytical technique, (i.e., multiple generations of fluid inclusion volatiles yielding a mixed analytical volume), the volatile composition of fluid inclusions were also investigated in-situ using laser Raman microspectroscopy to reconcile bulk analyses with the composition and abundance of different inclusion types. Inclusions in quartz pressure shadows associated with porphyroblasts host N2 ± CH4-dominant vapor phases and represent wall rock-equilibrated fluids. In comparison, fluid inclusion vapor phases in quartz veinlets crosscutting the metamudstones are more complex in composition and show a continuum between CO2-dominant and N2 ± CH4-dominant compositions; the former is interpreted as the fluid responsible for Au mineralization in the Meguma terrane (e.g., H2O-CO2 ± CH4 ± N2 fluids). GC shows statistically significant differences in the compostion of the bulk volatiles released from crushed metamudstones from the Touquoy deposit (n = 21) and barren (n = 24) settings. Whereas the former commonly contain detectable CO2 (50% of samples; LoQ(CO2) ~ 10−10 mol/g), only ~4% of metamudstones from barren settings along the MRA contain CO2, yielding a t-test p-value of 0.0103 between the two sample populations. Whereas the more frequent detection of CO2 in mineralized samples is related to increasing carbonate alteration proximal to Au mineralization, no correlation is observed between bulk CO2 abundance and Au grade. Additionally, anomalous differences in abundances of released C3 (t-test p-value = 0.043) and C4 (t-test p-value = 0.03) hydrocarbons (HC) are recognized between the two populations (LoQ(HC) ca. 10−13 to 10−12 mol/g), with Touquoy metamudstones having higher Σ(C3)/CH4 and Σ(C4)/CH4 ratios compared to barren equivalents. The compositional differences suggest that more aqueous-carbonic fluids infiltrated metamudstone units where locally favorable physical and/or chemical conditions were present, resulting in increased Au endowment. The relative increase in fluid flux, and thus Au grade, resulted in more trapped CO2 (i.e., resulting from more trapped inclusions and/or greater CO2 fluid component) and modification to hydrocarbon signatures through homologation and/or degradation reactions. Rigorous sample preparation ensures these compositional differences are related to paleofluids and not contamination.Tentatively, the results suggest that the bulk analysis of volatiles released from fluid inclusions in metamudstones may be a useful vectoring tool for disseminated Au mineralization in Meguma-type gold systems and similar environments (e.g., Carlin-type). This technique may be appropriately scaled to complement exploration drilling programs and may be more sensitive in determining ore proximity than often ambiguous pathfinder elements.