Abstract

The porphyry copper prospect at Plesyumi, New Britain, occurs near the island's center in a complex of dated upper Oligocene intrusions and volcanic rocks. The large copper-bearing sulfide system reveals several styles of mineralization and alteration, reflecting both veinlet and wall-rock control. Veins consist of high-total-sulfide, quartz-sericite assemblages in rocks of the volcanic suite and of low-total-sulfide quartz-biotite-magnetite-feldspar assemblages in the equigranular intrusion which composes the wall rock. The mineralization occurs within, and is genetically related to, a group of unusual high-silica, high-soda, low-potash porphyritic rocks.The volcanic rocks which are interpreted to have formed at the surface now lie in fault contact with the equigranular intrusive rocks. The preferred interpretation is that the volcanic column has been dropped to its present position, an interpretation based upon the disparity of the two environments of formation, upon the nearly identical radiometric ages of the two rock types, and upon the dissimilarity of gross alteration types in the two rock groups.A first stage of secondary sulfide enrichment evolved prior to deposition of Pliocene (?) sediments and volcaniclastic rocks. Geologically recently, however, and perhaps within the past 250,000 years, renewed uplift has exposed the system to further oxidation and leaching, and a new supergene sulfide blanket is forming. Results of water analyses from samples within the oxidizing column suggest that deposition of chalcocite is taking place under conditions of progressively reducing pH.

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