Volcanogenic-related origin of sulfide rich quartz veins: evidence from O and S isotopes at the Giant Dormant gold mine, Abitibi belt, Canada

The Geant Dormant gold mine is a sulfide-rich quartz vein gold deposit host ed by a volcano-sedimentary sequence and an associated felsic endogenous do me and dikes. The auriferous quartz-sulfide veins were preceded by two synv olcanic gold-bearing mineralizing events: early sulfidic seafloor-related a nd later disseminated pyrite in the felsic dome. This deposit differs from classical Archean auriferous quartz vein deposits by the low carbonate and high sulfide contents of the veins and by their formation prior to ductile penetrative deformation. The delta(18)O values of quartz associated with se afloor-related auriferous sulfides average 11.9 +/- 0.6 parts per thousand, (n = 3). The seafloor hydrothermal fluids had a delta(18)O value of 3.2 pa rts per thousand, calculated at 250 degrees C. The oxygen isotope compositi on of quartz and chlorite from veins average 12.5 +/- 0.3 parts per thousan d (n = 20) and 5.9 +/- 1.1 parts per thousand, (n = 4) respectively. Assumi ng oxygen isotope equilibrium between quartz and chlorite, the veins formed at a temperature of similar to 275 degrees C, which is consistent with the calculated temperature of 269 +/- 10 degrees C from chlorite chemistry. Th e gold-bearing fluids had a delta(18)O value of 4.7 parts per thousand calc ulated at 275 degrees C. The delta(34)S values of sulfides from the three g old events range from 0.6 to 2.8 parts per thousand (n = 32) and are close to magmatic values. Sulfur isotope geothermometry constrains the sulfide pr ecipitation in the gold-bearing veins at a temperature of similar to 350 de grees C. The similarity of the isotope data, the calculated delta(18)O of t he mineralizing fluids and the likely seawater fluid source suggest that th e three mineralizing events are genetically related to a volcanogenic hydro thermal system. The high value of the auriferous fluids (delta(18)O = 4.7 p arts per thousand) is attributed to a significant magmatic fluid contributi on to the evolved seawater-dominated convective hydrothermal system. The tw o-stage filling of veins at increasing temperature from quartz-chlorite (27 5 degrees C) to sulfides (350 degrees C) may reflect the progressive matura tion of volcanogenic hydrothermal systems. These results, together with fie ld and geochemical data, suggest that formation of gold-rich volcanogenic s ystems require specific conditions that comprise a magmatic fluid contribut ion and gold from are-related felsic rocks, coeval with the mineralizing ev ents. This study shows that some auriferous quartz-vein orebodies in Archea n terranes are formed in volcanogenic rather than mesothermal systems.