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Abstract

The Gayna River deposit is located in the Mackenzie Mountains of NW Canada and is hosted by the platform carbonates of the Neoproter ozoic Little Dal Group. The mineralization occurs as a series of sphalerite cemented breccias and veins which have a clear structural control. The deposits at Gayna have the following main paragenetic components: pre-ore (pyrite, limestone, dolostone, calcite cement and early sphalerite), ore-related (dolos par, quartz, sphalerite, and galena) and post-ore (quartz, pyrite, calcite, and pyrobitumen). In total the δ 34 S composition of sulphides at Gayna River range from -4.4 to 25.7‰ (CDT). The pre-ore phase pyrite and sphalerite samples have the lower δ 34 S values (-4.4 to 2.4‰ and 1.5 to 5.2‰ respectively). Carbon and oxygen isotope results for the host dolomite were δ 13 C = 1.8 to 4.6‰ (PDB) and δ 18 O = 22.1 to 25.2‰ (SMOW). The ore-phase sphalerite has δ 34 S values from 12.2 to 25.7‰ and the galena ranges from δ 34 S = 14.9 to 23.2‰. The dolospar phase has δ 13 C values of 0.7 to 2.5‰ (PDB) and δ 18 O values of 20.0 to 26.6‰ (SMOW). Primary fluid inclusion data in sphalerite suggest the main phase of mineralization was formed by fluids that ranged in homogenization temperature (Th) from 107 to 227°C (mean 204°C) and 23.0 to 34.4 wt.% NaCl equiv. Pyrites from the post-ore phase have δ 34 S values of 11.5 to 21.2‰. The late calcite phase had δ 13 C values of -3.0 to 3.0‰ (PDB) and δ 18 O values of 16.6 to 20.3‰ (SMOW). Inclusions in late quartz and calcite have Th values that range from 99 to 233°C and 59 to 159°C respectively and corresponding salinities of 13.7 to 32.9 wt % NaCl equiv. for quartz, and 13.6 to 23.7 % NaCl equiv for calcite. The data suggests that the Gayna River deposit likely was formed by mixing of at least two fluids and one of those fluids originated as seaw ater. The sulphur for the deposit was derived from the evaporite sequences found in the Little Dal Group possibly by thermochemical sulphate reduction. Post-mineralization fluids are cooler and less saline, possibly due to the mixing of the mineralizing brine with a more dilute fluid.