An oxygen isotope study of two contrasting orogenic vein gold systems in the Meguma Terrane, Nova Scotia, Canada, with implications for fluid sources and genetic models

Abstract

Sampling of quartz vein material from two gold deposits of similar geological setting but different ages (The Ovens, 408 Ma; Dufferin, 380 Ma) in the Meguma Terrane of Nova Scotia has been done to compare and contrast their δ18Oquartz signatures. Despite different ages of formation, quartz from all vein types in each of the deposits (i.e. saddle-reef, bedding-concordant leg reefs, en echelon, discordant) shows limited intra-deposit variation with similar average δ18O values of +15.2 ± 0.9‰ (n = 16) for The Ovens and +15.7 ± 0.6‰ (n = 12) for Dufferin. Using an average δ18O value of +15.4‰ for the two deposits, the corresponding δ18OH2O values, calculated for 400°C and 350°C based on constraints from mineral assemblages and fluid inclusion studies, indicate averages of 11.4 ± 0.2‰ and +10.2 ± 0.2‰, respectively. The isotopic data indicate that: (1) the vein-forming fluids have a metamorphic signature, (2) all vein types in the two deposits represent formation from a single, isotopically homogeneous fluid, and (3) a fluid of similar isotopic signature was generated by two contrasting tectono-thermal events in the Meguma Terrane that were separated by 30 Ma. Integration of these results with previously published data for 14 Meguma gold deposits indicate a general stratigaphic dependence in δ18OH2O values for deposits when arranged in their relative stratigraphic position such that δ18OH2O values increase upwards in the stratigraphy. This apparent trend cannot be explained by models involving either fluid mixing or cooling of the vein-forming fluids, but instead is modelled using fluid/rock interaction taking into account a change in the modal mineralogy of the metasedimentary rocks upwards in the stratigraphy.