Progressive 18O depletion during CO2 separation from a carbon dioxide-rich hydrothermal fluid: evidence from the Grey River tungsten deposit, Newfoundland

Abstract

Oxygen isotope data for the Grey River tungsten prospect, Newfoundland, Canada, indicate a progressive depletion in δ¹⁸O_fluid during mineralization. Early veins with pegmatitic affinities were deposited at 470 °C and pressures greater than 1 kbar (100 MPa), from a fluid with a δ¹⁸O composition of 7.4‰, presumed to be of magmatic origin. Successive vein deposition, at progressively lower temperatures and pressures, culminated in the precipitation of wolframite-bearing veins at a temperature of 300 °C and pressures of 150–320 bar (15–32 MPa), from a low salinity fluid with a δ¹⁸O composition in the range 3.2–1.6‰.Low values of δ¹⁸O (and δD_fluid) are recorded in many vein tungsten deposits and are normally interpreted as reflecting mixing of isotopically light meteoric fluids or formation brines with magmatic fluids. However, fluid-inclusion evidence for the Grey River mineralization indicates that a 40 mol% CO₂ loss occurred by immiscibility and retrograde boiling of the hydrothermal fluid between 420 and 300 °C. Such a chemical change would have significantly altered the oxygen isotopic character of the hydrothermal fluid since CO₂ fractionates ¹⁸O relative to coexisting water by ~10‰ at 400 °C and ~14‰ at 300 °C. Calculations using available CO₂–H₂O fractionation factors reveal that up to a 7‰ depletion in δ¹⁸O of the residual aqueous fluids may occur as a result of the 40 mol% CO₂ loss from the hydrothermal fluid.