The Esis porphyry copper deposit is located in the head-waters of the Esis River in East New Britain. The deposit has a typical island-arc setting. It occurs at the margin of a composite pluton of upper Oligocene age intruding Eocene, basic volcanics forming the basement of the New Britain arc. Hydrothermal alteration and sulfide mineralization extend into the country rocks from a complex, elongate mass of mineralized, intrusive breccias and plagioclase-quartz porphyry dikes that occupy a portion of the intrusive volcanic contact zone. Chlorite-, sericite-, and biotite-dominated alteration assemblages are present, although the zones do not have a regular distribution. The biotite zone has not been recognized in the volcanics and is of limited extent in the granitic intrusives. Changes in bulk chemistry of altered rocks can be qualitatively related to progressive mineralogical changes accompanying alteration. Chloritized intrusives, for instance, are particularly low in K, Rb, and Ba. There is a high proportion of intrusive breccias within the mineralized area. They range from those in which fragments are angular and rotation is minimal, to those in which fragments are well rounded. The breccias represent the magmatic and hydrothermal activity that took place between the emplacement of the premineralization granitic intrusives and the intrusion of those porphyry dikes that are not mineralizedWe consider that the association between the mineralization and a specific suite of intrusive rocks supports an orthomagmatic model of deposit genesis and has important implications for porphyry copper search in island arcs.

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