Distinguishing barren and auriferous veins in the Sigma Mine, Val-d'Or, Quebec

The Sigma Mine is an Archean lode gold deposit located in the Abitibi greenstone belt, Quebec. Barren quartz veins occur subparallel to auriferous veins within the mine and also occur north of the mine. The quartz and auriferous veins exhibit a similar mineralogy and alteration and are indistinguishable except for the presence or absence of gold. This study determines differences between (i) a vein that may be auriferous but lacks gold at that location and (ii) a barren vein that formed from fluids that never had the capability of carrying gold. Microthermometry distinguishes two distinct salinities within aqueous inclusions in the barren veins, compared to high salinities only reported in the auriferous veins. The CO2-H2O fluid inclusions within barren veins are remarkably similar to their counterparts in auriferous veins in composition, homogenization temperature, and CO2 effervescence. The most significant difference between the two vein types, and a potential exploration tool, is observed in the CO2-rich inclusions. Tight clustering of first melting temperatures between -57.4 and -56.4-degrees-C and Raman spectroscopy indicate the presence of only CO2 in the barren veins. These results contrast sharply with the -62.5 to -56.0-degrees-C range of first melting temperatures and the presence of H2S indicated by Raman spectroscopy in auriferous veins. Lack of CH4 and H2S gases in the barren CO2-rich inclusions reflects the oxidation state of the fluids, which was too high to permit transport of gold as a reduced complex.