Environments of ore deposition at the porgera gold deposit, papua new guinea

摘要

The Porgera gold mine is associated with 6 Ma-old mafic alkalic intrusions emplaced at shallow levels into unconsolidated Jurassic and Cretaceous sediments. The mineralization consists of an earlier high-temperature (300 deg- 350 deg) base-metal-sulfide-rich stage with sericite-carbonate alteration (stage I), and a later lower temperature epithermal-type phase represented by quartz-roscoelite (-Au) veins (stage II). New styles of mineralization and alteration have been recognized at depth in association with stage I, consisting of the assemblages magnetite-chalcopyrite-pyrrhotite and biotite-actinolite-anhydrite. Rare vapor-rich fluid inclusions coexisting with liquid-rich inclusions in stage II quartz provide evidence for ore deposition by phase separation. Stage II fluids are rich in CO_2 relative to typical epithermal fluids as evidenced by fluid inclusions in which liquid CO_2 is preserved, and by widespread presence of clathrate during melting. Recent recognition of adjlaria in some stage II veins confirms the similarity of this style of mineralization with other adularia-sericite or low-sulfidation epithermal deposits, but formation at greater depths (approx 2 km).