Characteristic features of alkaline rocks hosting the Ladolam gold deposit, Lihir Island, Papua New Guinea

摘要

The Ladolam gold deposit on Lihir Island, Papua New Guinea, contains about >1300 metric tons of Au, hence represents one of the largest epithermal gold deposits discovered to date. The Au mineralization resulted from the transition of an early-stage, low-grade porphyry Au system to a low-sulfidation epithermal Au event. Ladolam is hosted by alkaline rocks that range from trachybasalts through trachyandesites to latites that are cut locally by monzodioritic stocks. The geochemical signature of the host rocks at Ladolam is typical of high K igneous rocks transitional to shoshonites. Mica phenocrysts in the rocks have unusually high halogen concentrations of up to 5.6 wt% F and up to 0.15 wt% Cl. Oxygen barometry on olivine-spinel-clinopyroxene in the rocks indicates very high oxygen fugacities of 1.4 to 4.8 log units above that of the fayalite-magnetite-quartz (FMQ) buffer. Elevated precious-metal (Au-Pt-Pd) concentrations of fresh and unaltered alkaline rocks from Lihir are interpreted as primary magmatic features suggesting a Au pre-enrichment of the mantle sources beneath the island. LA-ICPMS analyses of melt inclusions are in progress to investigate magma evolution before, during and after porphyry- and epithermal-style mineralization.