Role of Halogen Elements on the Panguna Porphyry Copper Mineralization, Bougainville, Papua New Guinea

Chemical compositions were measured on the apatite and biotite of magmatic and hydrothermal origin from the Panguna porphyry copper deposit, Papua New Guinea. The magmatic apatite and biotite in the intrusives which predated the mineralization were characterized by a high Cl/OH ratio. The Cl/OH ratios of the earlier hydrothermal minerals (biotitization and early veining stages) were almost as high as in the magmatic stage. The fHCl/fH_2O ratios of the magmatic and early hydrothermal stages were rather high, compared with other mining areas in the island arcs. The apatites showed the effect of chemical exchange during the mineralization and/or later hydrothermal stage. There were two exchanging processes; Cl leaching in the surrounding potassic alteration zone (high Cu precipitation zone), and F additional process in the central chlorite-sericite-clay alteration zone (low Cu zone). The apatite and biotite in the Biuro Granodiorite which postdated mineralization were relatively poor in Cl and F. The chemical compositions and paragenesis of the hydrous minerals suggest the following process of mineralization. A Cl rich fluid emanating from Cl rich magma and carrying base metals precipitated Cl rich hydrous minerals. The meteoric water mixed with this magmatic fluid in the surrounding potassic zone, and precipitated ore minerals from magmatic fluid. The meteoric water which entered into the central zone lost the ability to precipitate ore minerals by mixing with the magmatic fluid in the surrounding potassic zone. The central chlorite-sericite-clay zone was enriched in F by the fluid from a more differentiated magma