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.

First Page Preview

First page PDF preview
You do not currently have access to this article.