GRADE DISTRIBUTION OF THE GIANT OK TEDI Cu-Au DEPOSIT, PAPUA NEW GUINEA

Abstract

The Ok Tedi porphyry Cu-Au deposit in Papua New Guinea is a world-class mineral deposit. In its pre-mining stage, it contained 5.5 Mt Cu and 18.1 Moz Au; skarn accounts for ~80% of the current 1.9 Mt Cu and 7.7 Moz Au in total resources. Two composite felsic stocks intruded a tectonostratigraphic sequence of clastic and carbonate rocks at ~1.16 ± 0.10 Ma, in a postcollisional tectonic setting. Ensuing magmatic-hydrothermal mineralization occurred within a geologically short time span of ~200 k.y. On the mine scale, heterogeneous metal grades (0–2.5% Cu) reflect contrasting styles of mineralization, conforming to classic skarn- and porphyry-type mineralization. A three-dimensional geologic model, alteration distribution, mineralization styles, and grade distribution indicate that fluids, exsolved from a deeper magma chamber, used shallow stocks as their preferred permeable pathway upward. Where limestone was present at depth, high-grade skarn formed, with the permeable stock having very low (<0.1% Cu) grade. In contrast, where the intrusions were encapsulated solely by impermeable silicified siltstone, economic grade (avg 0.6% Cu) developed within the intrusion, showing concentric patterns interpreted to represent individual magmatic pulses within the stock. The rare opportunity to visualize this in three dimensions and interact with the model is provided in the interactive electronic supplementary material.