Fe sulfide minerals are forming in the shallow-water hydrothermal system in Luise Harbor, Lihir Island, Papua New Guinea, due to the interaction of hydrothermal gas+ or -liquid, seawater, and Fe-rich sediments. Upon contact with oxygenated seawater, hydrothermal H 2 S is oxidized and forms H 2 SO 4 leading to the simultaneous dissolution of primary, Fe-rich sediment grains and the neoformation of mainly marcasite and pyrite. They are present as alternating colloform layers, as framboidal aggregates in vugs, or they replace organic fragments, primary olivine and primary magnetite. Deposition of either marcasite or pyrite appears to reflect variations in acidity due to varying admixture of air-saturated seawater. The gas phase is composed of mainly CO 2 , N 2 , and H 2 S. Assuming equilibration of the gases dissolved in a liquid phase and preservation of equilibrium CH 4 /CO 2 , the CH 4 -CO 2 equilibrium temperature is approximately 310 degrees + or -20 degrees C.