DISTINGUISHING BARREN AND AURIFEROUS VEINS IN THE SIGMA MINE, VAL-DOR, QUEBEC

The Sigma Mine is an Archean lode gold deposit located in the Abitibi greenstone belt, Quebec. Barren quartz veins occur subparallel to auri ferous veins within the mine and also occur north of the mine. The qua rtz and auriferous veins exhibit a similar mineralogy and alteration a nd are indistinguishable except for the presence or absence of gold. T his study determines differences between (i) a vein that may be aurife rous but lacks gold at that location and (ii) a barren vein that forme d from fluids that never had the capability of carrying gold. Microthe rmometry distinguishes two distinct salinities within aqueous inclusio ns in the barren veins, compared to high salinities only reported in t he auriferous veins. The CO2-H2O fluid inclusions within barren veins are remarkably similar to their counterparts in auriferous veins in co mposition, homogenization temperature, and CO2 effervescence. The most significant difference between the two vein types, and a potential ex ploration tool, is observed in the CO2-rich inclusions. Tight clusteri ng of first melting temperatures between -57.4 and -56.4-degrees-C and Raman spectroscopy indicate the presence of only CO2 in the barren ve ins. 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 i n the barren CO2-rich inclusions reflects the oxidation state of the f luids, which was too high to permit transport of gold as a reduced com plex.