OXYGEN EXCHANGE IN THE THE HIDDEN HYDROGEOSPHERE: TOWARDS A NEW EQUILIBRIUM? (Invited Presentation)

Fracture networks bearing Ca-Na-Cl fluids with extended (Ma-Ga) residence times are common to crystalline Precambrian rocks globally. These fluids characteristically plot to the left of the Global Meteoric Water Line (GMWL) in δ¹⁸O- δ²H space. This is typically attributed to site-specific processes affecting both δ¹⁸O and δ²H values (e.g. silicate hydration coupled with formation of secondary minerals, radiolytic H₂ production, isotopic exchange with a H₂-rich gas, or isotope exchange with O and/or H-bearing minerals).

Using a global dataset compiled from published and unpublished data from S. Africa, Fennoscandia and the Canadian Shield, late-stage meteoric-non-meteoric fluid mixing can be deconvoluted and constraints placed on the isotopic composition of the most saline end-members. From here, the proposed mechanisms which produce these isotopic signatures can be revisited and re-evaluated. Importantly, these fluids are found to differ principally in O isotope space but have a relatively uniform range of H isotope values. Common processes must be at play in Precambrian crystalline rock settings around the world. This study explores the role of high-temperature hydrothermal/metamorphic activity defining primary fluids with δ¹⁸O- δ²H fluid values initially plotting to the right of the GMWL, as proposed by previous, site-specific models. Following this, low temperature isotopic exchange at low water-to-rock ratios over characteristically long (Ma) geologic timescales result in progressive ¹⁸O depletion in the fluids as the system approaches low-temperature equilibrium. The low H content of the rocks result in minimal changes to the δ²H. Lastly, late stage addition of (paleo)-meteoric water and mixing results in decreased salinities and isotopic regression back towards the GMWL.

This global framework is successfully applied to fluid samples collected from Kidd Creek Mine, Canada, where fluids from two levels of the mine had been sampled with significantly different residence times. Fluids at both levels approach low-temperature ¹⁸O/¹⁶O equilibrium with the fracture mineralogy, but have comparable δ²H values. These findings are consistent with the proposed model of decoupled ¹⁸O/¹⁶O isotopic exchange at low temperatures and water-to-rock ratios with no major ongoing processes affecting ²H.