Evolution of the Ladolam Geothermal System on Lihir Island, Papua New Guinea

Abstract

An active geothermal system and a 44 million ounce epithermal gold deposit are superimposed on Lihir (or Niolam) Island. Recent petrology studies and geophysical surveys indicate that the Ladolam geothermal system has a very complex history. Many previous authors have recognised that a major sector collapse has affected this system, but there is now evidence for two such collapse events. Those two sector collapses have had a significant effect on the geothermal system and on gold deposition.
The first collapse exposed a deep level hydrothermal system with potassic alteration and incipient porphyry copper-gold mineralisation. Subsequently a shallow geothermal system with epithermal gold mineralisation developed. Recent geophysical surveys and exploration drilling indicate that a second sector collapse then removed much (and in places all) of the clay cap from this epithermal system, exposing at the surface minerals like illite-smectite that typically form at a depth of 100 m or more.
Both sector collapse events probably contributed to gold deposition. The geothermal system has survived both of those events, and remains active and continues to deposit gold today. However, the hydrology of the system has been affected by the collapse events, with more rapid cooling where the clay cap has been removed.