Comparison between magmatic activity and gold mineralization at Conical Seamount and Lihir Island, Papua New Guinea

Grab samples from the submarine Conical Seamount, located about 10 km south of the giant Ladolam gold deposit, Lihir Island, reveal the highest gold concentrations yet reported from the modern seafloor. Lavas from Conical Seamount are characterized by high K2O contents, high K2O/Na2O ratios, and high Ce/Yb ratios, which are typical of high-K igneous rocks from oceanic (island) arc-settings. The primitive character of the rocks from Conical Seamount implies a magmatic evolution related to a single eruptive phase, which contrasts with the more evolved rocks forming Lihir Island. Geochemical as well as mineral chemical data suggest that the melts from both Conical Seamount and Lihir Island originate from the same magma source. In common with the samples from Lihir Island, elevated oxygen fugacities of 0.7-2.5 log units above the FMQ buffer are recorded from the Conical Seamount lavas. There are distinct differences between the mineralization styles at Conical Seamount and at the Ladolam gold deposit, Lihir Island. While early-stage pyritic stockwork mineralization at Conical Seamount is hosted by clay-silica altered basaltic rocks with local alunite+/-kaolinite alteration, main-stage Au-mineralization occurs in sericite-alkali feldspar altered polymetallic sulfide-bearing siliceous veins. By contrast, early-stage pyritic stockwork mineralization at Ladolam is restricted to biotite-magnetite +/- silica-altered monzodiorite, while the main-stage bulk-tonnage mineralization occurs as auriferous pyrite-bearing hydrothermal breccias which, in places, are cut by quartz-chalcedony-illite-adularia-pyrite+/-marcasite veins containing isolated bonanza gold grades.