Abstract The exceptionally large gold resource at Ladolam (>1,300 metric tons (Mt) Au), Lihir Island, resulted from the transition of an early-stage, low-grade porphyry gold system to a low-sulfidation epithermal gold event. The original porphyry stage is indicated by remnant hydrothermal breccia clasts of strongly biotitemagnetite altered monzodiorite with disseminated pyrite ± chalcopyrite and poorly developed pyrite ± quartz stockwork veins. The breccia matrix is strongly mineralized with disseminated auriferous pyrite. The breccias are cut by late-stage epithermal quartz-chalcedony-illite-adularia-pyrite veins that locally contain bonanza gold grades of up to 120 g/t. Isotope data suggest a magmatic source of sulfur in the gold-bearing fluids at Ladolam. Ladolam is hosted by alkaline rocks that range from porphyritic trachybasalts, trachyandesites, and latites to rare phonolites and olivine-clinopyroxene cumulates that are cut locally by monzodiorite stocks. Oxygen barometry on olivine-spinel-clinopyroxene phases in these rocks indicates very high oxygen fugacities ( f O2 ), 1.4 to 4.8 log units above that of the fayalite-magnetite-quartz (FMQ) buffer. Gold-copper mineralization is commonly associated with alkaline rocks with high f O2 values and abundant primary magnetite contents; this study provides direct determination of the f O2 of such rocks. High f O2 of parental melts delays the early crystallization of magmatic sulfides, in which metals, such as gold and copper, preferentially partition and are thus depleted in the melt during fractionation. The geochemical signature of the host rocks at Ladolam is typical of primitive to relatively evolved compositions. Their high K 2 O content (up to 4.7 wt %), high average K 2 O/Na 2 O ratios (0.8), and high average Ce/Yb ratios (14) are typical of high K igneous rocks transitional to shoshonites. The rocks formed by decompression melting along extensional structures either related to back-arc rifting in the Manus basin or to a flexure in the subducting Solomon plate. However, their high large-ion lithophile element (LILE) and very low high-field strength element (HFSE) contents are typical of potassic igneous rocks from oceanic (island)-arc settings. This unusual composition has probably been derived by partial melting of subduction-modified lithospheric mantle, as developed in a stalled subduction zone. Mica phenocrysts in the rocks have unusually high halogen concentrations. Igneous phlogopites contain high fluorine (up to 5.6 wt %) and elevated chlorine (<0.08 wt %) contents. Hydrothermal biotites from rocks that display potassic alteration have low fluorine (<0.08 wt %) but very high chlorine concentrations (up to 0.15 wt %). This data makes the Ladolam rocks comparable to those from the Grasberg copper-gold deposit, Indonesia.