Geology, alteration, and mineralisation of the Golpu Porphyry and Wafi Epithermal deposit, Morobe Province, Papua New Guinea

摘要

The Golpu porphyry and Wafi epithermal deposit of Papua New Guinea contains contrasting stylesudof epithermal veins and alteration that overprinted early-formed porphyry-style mineralisation. Golpu is theudeasternmost and oldest porphyry deposit in a previously unrecognised belt that includes most of the porphyryuddeposits in the New Guinea Orogen. This metallogenic belt was formed in an orogenic regime that was driven byudwest-directed, low-angle subduction of an antiformal slab of oceanic lithosphere (the Solomon Sea Plate).udThe regional stratigraphy at Golpu comprises Mesozoic metasedimentary rocks of lower to middle greenschistudmetamorphic facies. These rocks were intruded in the study area by Golpu and Nambonga diorites, whichudhave calc-alkaline to high-K calc-alkaline arc geochemical characteristics. Early porphyry veins and alterationudfollowed emplacement of the Golpu diorites. Porphyry activity culminated in the main stage of Cu-Au mineralisationudaround 8.67 ± 0.02 Ma, resulting in an unusually high-grade (1 Gt @ 0.90% Cu and 0.63 ppm Au) andudvertically elongated (> 1.8 km) orebody. Golpu sulphides define a zonation pattern from chalcopyrite with traceudbornite, to chalcopyrite > pyrite, to distal pyrite > chalcopyrite. They also define zonation in sulfur isotopic compositionsud(e.g., O34s sulfides from proximal -0.9 to distal +2.0 ‰).udA maar-diatreme breccia complex was emplaced after the Golpu porphyry mineralisation, followed byudthe main stage of Wafi epithermal mineralisation and alteration. Zones of pervasive high sulfidation alterationudcontaining quartz, alunite, pyrophyllite, kaolinite, dickite, and diaspore occur above and overprint the Golpuudporphyry. These zones contain vein and disseminated pyrite - covellite - chalcocite - tennantite ± enargite ±udbornite atop the porphyry, to distal disseminated pyrite - sphalerite ± tennantite. O34S alunite values vary fromud+6.3 ‰, directly above Golpu, to +19.4 ‰, 600 metres west of Golpu.udAn arcuate zone of intermediate sulfidation epithermal carbonate - sulfi de ± quartz ± adularia veinsudand montmorillonite - chlorite ± muscovite ± illite alteration occurs along the margins of the high sulfidationuddomain. Most of the Wafi epithermal gold resource (136 Mt @ 1.70 ppm Au) occurs in the intermediate sulfidation zone nearest the Wa! high sulfidation alteration. The intermediate sulfidation epithermal mineralisationudtransitions down the north-dipping Compass fault into the Golpu porphyry environment, where sparse veins ofudtennantite - sphalerite - galena - chalcopyrite are interpreted as the basal component of intermediate sulfidation epithermal veins.udThe temporal and spatial configuration of veins and alteration assemblages at Golpu and Wafi indicateudthat the Golpu porphyry was the causative intrusive complex for both porphyry and epithermal mineralisation.udUplift and exhumation during the life of the porphyry system was driven by low-angle subduction, and resultedudin a shift from porphyry to epithermal magmatic-hydrothermal activity over a period of 0.25 to 0.40 m.y.