Fe sulfide formation due to seawater-gas-sediment interaction in a shallow-water hydrothermal system at Lihir Island, Papua New Guinea

Fe sulfide minerals are forming in the shallow-water hydrothermal system in Luise Harbor, Lihir Island, Papua New Guinea, due to the interaction of hy drothermal gas +/- liquid, seawater, and Fe-rich sediments. Upon contact wi th oxygenated seawater, hydrothermal H2S is oxidized and forms H2SO4 leadin g to the simultaneous dissolution of primary, Fe-rich sediment grains and t he neoformation of mainly marcasite and pyrite. They are present as alterna ting colloform layers, as framboidal aggregates in vugs, or they replace or ganic fragments, primary olivine and primary magnetite. Deposition of eithe r marcasite or pyrite appears to reflect variations in acidity due to varyi ng admixture of air-saturated seawater. The gas phase is composed of mainly CO2, N-2, and H2S. Assuming equilibrati on of the gases dissolved in a liquid phase and preservation of equilibrium CH4/CO2, the CH4-CO2 equilibrium temperature is approximately 310 degrees +/- 20 degrees C.