Abstract

Fluid-inclusion and stable isotope evidence are presented for the transition from magmatic-related mesothermal to meteoric water-dominated epithermal activity at the giant Porgera gold deposit, Papua New Guinea. Reflecting this transition, Au-Ag mineralization occurs in two main stages: disseminated auriferous pyrite in phyllic alteration zones (stage I); and fault-related, quartz-roscoelite-cemented hydrothermal breccias and veins carrying locally abundant free gold and Au-Ag-tellurides (stage II). The deposit is spatially and temporally associated with a late Miocene (6 Ma) epizonal intrusive complex emplaced in continental crust immediately prior to an early Pliocene continent- island-arc collision. Stage I ore formation was associated in part with fluids of magmatic origin (∼200 to >500 °C, 7 to 12 wt% and 31 to 58 wt% NaCl equivalent [eq.], δ18O = 8.1‰ to 9.4‰, δD = -50‰ to -32‰) and appears to represent a new type of Au-rich, Cu-poor, porphyritic, intrusion- related mineralization associated with volatile-rich, mafic, alkalic magmatism in a continent island-arc collision environment. Stage II vein-type mineralization crosscuts this earlier disseminated orebody; it formed at depths of 2 to 3 km from lower temperature, lower salinity, isotopically exchanged ground waters (∼180 °C, 3 to 10 eq. wt% NaCl, δ18O = 3.1‰ to 6.4‰, δD = -62‰ to -34‰), similar to fluids from other Au-Ag-Te epithermal deposits. Gold in these late veins was probably remobilized from deeper stage I-type mineralization, and stable isotope and textural evidence suggest that Au was deposited following tectonically induced fluid phase separation.

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