Pb IN THE PACMANUS SEA-FLOOR HYDROTHERMAL SYSTEM, EASTERN MANUS BASIN: NUMERICAL MODELING OF A MAGMATIC VERSUS LEACHED ORIGIN

Abstract

Mass balance calculations of metal leaching and deposition in the PACMANUS submarine hydrothermal vent field in the eastern Manus basin in the Bismarck Sea off the east coast of Papua New Guinea indicate that simple leaching of the underlying volcanic pile by hydrothermal fluids cannot account for the Pb found in associated sulfide deposits and Pb-enriched altered volcanic rocks. With 100 percent leaching, transport, and precipitation efficiencies, 1.69 km3 of volcanic rock would need to be leached to account for Pb. Applying realistic leaching efficiencies of 50 to 60 percent and transport-precipitation efficiencies of 10 to 20 percent, 14 to 34 km3 of volcanic rock would need to be leached, assuming a static reaction zone. The estimated available volume of leachable rock is 0.144 km3, much less than required. Applying a dynamic reaction zone model, 98 to 235 volumes of fresh, leachable rock (1 volume = 0.144 km3) would need to be exposed to hydrothermal fluids. This translates into a 588 to 1,410 m migration of the reaction zone. A model incorporating such values would not be tenable. Previously published compositions of melt inclusions in volcanic rocks from PACMANUS indicate an aver-age of 199 ppm Cu, 68 ppm Zn and 5.2 ppm Pb in the melt and 1.3 to 1.7 wt percent of degassed H2O. These data suggest that 37.7 million tonnes (Mt) of magmatic fluid containing 7.2 wt percent Cu and 2.3 wt percent Zn could be released from a 1 km3 body of magma. Using the same metal transfer efficiency of 80 percent, the degassed fluid would have a maximum concentration of 0.17 wt percent Pb, which translates to 58,000 t of Pb from the melt. For the highest known fluid discharge rate of 92,000 g/s and over a 100,000-year period, a concentration of only 12 ppm Pb in the original magmatic component could account for all the Pb in the PACMANUS deposits, assuming that 1 percent of the mixed end-member fluid is of magmatic origin. A wide range of durations of hydrothermal activity and discharge rates, assuming a maximum 0.17 wt percent Pb concentration in the degassed magmatic fluid, can account for the total Pb in the deposit.