Chapter 27: Geology of the Porgera Gold Deposit, Papua New Guinea

Abstract Porgera is a ~974-metric ton (t) Au, low-sulfidation, alkalic, epithermal gold deposit located in Papua New Guinea. The deposit is spatially associated with 6 Ma stocks of the mafic alkalic Porgera Intrusive Complex, which were emplaced within Cretaceous carbonaceous mudstones in a transpressional orogenic setting linked to continent-island arc collision. As with many other major magmatic-hydrothermal ore deposits in New Guinea, deep-seated, arc-normal transfer structures have been suggested as controls on intrusion emplacement through the creation of a localized extensional environment favorable for magma ascent. Gold mineralization occurred in two distinct phases, both within ≤0.2 m.y. of emplacement of the Porgera Intrusive Complex. Stage 1 mineralization of intrusion-related carbonate-base metal association consists of extensional vein swarms dominated by coarse intergrown pyrite ± galena and sphalerite, generally hosted within or proximal to the intrusive bodies of the Porgera Intrusive Complex. These veins represent the lowest grade and economically least significant mineralization phase. Overprinted high-grade epithermal Stage 2 mineralization consists of roscoelite, pyrite, and quartz veins and breccia veins ± subordinate amounts of barite, marcasite, sphalerite, tetrahedrite, galena, hematite, and tellurides. Gold mineralization is commonly associated with the roscoelite-rich coatings on vein walls or breccia clasts. Stage 2 mineralization is controlled by a deposit-scale extensional fault-fracture mesh and displays a variety of textural styles including: (1) <5-mm veinlets dominated by roscoelite, pyrite, and gold; (2) thicker veins up to 10 cm wide with roscoelite, pyrite, and gold on the margins with central bands of alternating crustiform quartz and thin layers of roscoelite-pyrite-gold; (3) hydrothermal breccias with roscoelite, pyrite, and gold coating breccia margins and internal clasts, with crustiform quartz forming the matrix. The giant endowment of the Porgera gold system is attributed to its favorable tectonic location and local extensional setting, its vertical extent, the oxidized nature of the mineralizing fluids, and highly efficient gold precipitation.