Geochemistry and mineralogy of gold in the PACMANUS and Susu knolls hydrothermal systems, eastern Manus basin, Papua New Guinea

grantor: University of TorontoThe eastern Manus back-arc basin is located off the east coast of New Guinea, in the west Pacific. Active vent fields here, are some of the most gold-rich on the modern seafloor. Unaltered volcanic rocks from eastern Manus Basin have an average gold content of 6 ppb, higher than that of mid-ocean ridge basalts and northern Mariana and Izu Ogasawara arc volcanic rocks, but similar to other back-arc volcanic rocks. This suggests that back-arc volcanic rocks may constitute an enriched source of gold. Mass balance modeling of the PACMANUS hydrothermal system indicates that it is unlikely that the gold found in the precipitates resulted from a single episode of leaching. Movement of the high temperature reaction zone during evolution of the hydrothermal system may, however, expose the fluid to sufficient rock to account for the PACMANUS gold. Helium isotope ratios of fluids from inclusions in hydrothermal precipitates range from 1.7 Ra to 7.17 Ra, indicating a mantle source. The fluids are also enriched in CO2 and N2 over mid-ocean ridge basalt volatiles, compatible with their derivation from subducted sediments. A CO2 > CH4 > N2 enrichment trend of volatiles in inclusions from the Suzette vent site indicates that the vent fluid may have undergone phase separation. Hydrothermal precipitates contain up to 56.8 ppm Au. Gold occurs as silver-poor inclusions in three associations: massive chalcopyrite associated with bornite and covellite; tennantite associated with chalcopyrite and sphalerite; and tennantite and chalcopyrite. Gold is carried in solution predominantly as the Au(HS)° complex consistent with the indicated high activity of sulfur. Reaction path modeling confirms that Au(HS)° is the dominant gold complex. A conductive cooling simulation predicts the formation of a chalcopyrite-bornite-covellite assemblage similar to that found in chalcopyrite-rich portions of chimneys. Mixing with seawater results in precipitation of a low temperature sphalerite assemblage that includes gold-rich electrum, and bears similarities to sphalerite+barite-rich samples. The gold-rich nature of the hydrothermal precipitates is best explained by a dual source of gold, from both leaching of gold-enriched sub-seafloor rocks, and a magmatic fluid, coupled with efficient transport and deposition of gold by the hydrothermal fluid.Ph.D