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Geology, alteration, and mineralisation of the Golpu Porphyry and Wafi Epithermal deposit, Morobe Province, Papua New Guinea
thesisposted on 2023-05-27, 12:37 authored by Rinne, ML
The Golpu porphyry and Wafi epithermal deposit of Papua New Guinea contains contrasting styles of epithermal veins and alteration that overprinted early-formed porphyry-style mineralisation. Golpu is the easternmost and oldest porphyry deposit in a previously unrecognised belt that includes most of the porphyry deposits in the New Guinea Orogen. This metallogenic belt was formed in an orogenic regime that was driven by west-directed, low-angle subduction of an antiformal slab of oceanic lithosphere (the Solomon Sea Plate). The regional stratigraphy at Golpu comprises Mesozoic metasedimentary rocks of lower to middle greenschist metamorphic facies. These rocks were intruded in the study area by Golpu and Nambonga diorites, which have calc-alkaline to high-K calc-alkaline arc geochemical characteristics. Early porphyry veins and alteration followed emplacement of the Golpu diorites. Porphyry activity culminated in the main stage of Cu-Au mineralisation around 8.67 ¬¨¬± 0.02 Ma, resulting in an unusually high-grade (1 Gt @ 0.90% Cu and 0.63 ppm Au) and vertically elongated (> 1.8 km) orebody. Golpu sulphides define a zonation pattern from chalcopyrite with trace bornite, to chalcopyrite > pyrite, to distal pyrite > chalcopyrite. They also define zonation in sulfur isotopic compositions (e.g., O34s sulfides from proximal -0.9 to distal +2.0 ‚ÄövÑ‚àû). A maar-diatreme breccia complex was emplaced after the Golpu porphyry mineralisation, followed by the main stage of Wafi epithermal mineralisation and alteration. Zones of pervasive high sulfidation alteration containing quartz, alunite, pyrophyllite, kaolinite, dickite, and diaspore occur above and overprint the Golpu porphyry. These zones contain vein and disseminated pyrite - covellite - chalcocite - tennantite ¬¨¬± enargite ¬¨¬± bornite atop the porphyry, to distal disseminated pyrite - sphalerite ¬¨¬± tennantite. O34S alunite values vary from +6.3 ‚ÄövÑ‚àû, directly above Golpu, to +19.4 ‚ÄövÑ‚àû, 600 metres west of Golpu. An arcuate zone of intermediate sulfidation epithermal carbonate - sulfi de ¬¨¬± quartz ¬¨¬± adularia veins and montmorillonite - chlorite ¬¨¬± muscovite ¬¨¬± illite alteration occurs along the margins of the high sulfidation domain. 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 mineralisation transitions down the north-dipping Compass fault into the Golpu porphyry environment, where sparse veins of tennantite - sphalerite - galena - chalcopyrite are interpreted as the basal component of intermediate sulfidation epithermal veins. The temporal and spatial configuration of veins and alteration assemblages at Golpu and Wafi indicate that the Golpu porphyry was the causative intrusive complex for both porphyry and epithermal mineralisation. Uplift and exhumation during the life of the porphyry system was driven by low-angle subduction, and resulted in a shift from porphyry to epithermal magmatic-hydrothermal activity over a period of 0.25 to 0.40 m.y.
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