The Mount Fubilan Intrusion is part of a geologically young hypabyssal stock in the Star Mountains of Papua New Guinea. This stock was mapped as the Ok Tedi Intrusive Complex and divided into four separate bodies: the Mount Fubilan, Sydney Intrusion, Kalgoorlie, and Ningi Intrusions. Hydrothermal fluids caused alteration of the Mount Fubilan, and parts of the other intrusions, to potassic and propylitic mineral assemblages and deposited gold and copper. This investigation documents similarities and differences between the least-altered intrusive rocks of the complex and those that have undergone potassic metasomatism. The study involved detailed petrographic examination of more than two hundred thin-sections, major-oxide and trace element chemistry, and microprobe analyses of individual minerals. The magmas that crystallized to form the stock are shown to be intermediate in composition between andesite and latite. They were quartz-saturated, metaluminous, weakly iron-rich, and crystallized under oxidizing conditions. The principal mineral phases in the least-altered intrusive rocks are andesine, pyroxene, orthoclase, and quartz. The accessory mineral suite in least-altered rocks includes biotite, sphene, apatite, magnetite, and zircon. Hornblende is present in a few samples Magmatic pyroxene is diopsidic in composition; hornblendes is cdenitic; and biotite is annitic. Potassic alteration has converted andesine to orthoclase, or mixtures of albite and orthoclase, ferro magnesian minerals to hydrothermal biotite, sphene to rutile, and magnetite to pyrite and chalcopyrite. Hydrothermal biotite is phlogopitic in composition. Gold and copper were concentrated in the zone of potassic alteration. The mineralogical transformation of the intrusive rocks of the Mount Fubilan and associated intrusions was caused by the infiltration of hydrothermal fluids that deposited potassium, gold, and copper and that leached and removed virtually all other rock constituents. Leached components were transported away from the zone of potassic alteration and deposited in peripheral parts of the intrusive complex to form propylites, endoskarn, and massive replacement bodies or removed from the system entirely. The Mount Fubilan intrusion was closely similar in chemistry and mineralogy to the other intrusions of the complex prior to alteration. Petrochemical differences between the Mount Fubilan Intrusion and the other intrusions were produced entirely by hydrothermal alteration. / Graduation date: 2000