Recent Exploration Drilling at Lihir Geothermal Field, PNG: Effects of Catastrophic Sector Collapse on a Magmatic-Hydrothermal System

Abstract

Detailed logging and petrological analysis of cores from three exploratory wells (GW47, GW52, GW54) at Luise Harbour, Lihir Island, reveal an intrusive complex underlying lavas and volcaniclastic breccias of the Ladolam breccia complex. The intrusive rocks consist of porphyritic to equigranular, cumulate clinopyroxene diorite, which were emplaced as sub-horizontal bodies, cut by sub-vertical porphyritic diorite and rare aplite dikes. The similar mineralogy of plutonic rocks and crosscutting dikes implies a common magmatic source, however the change in their emplacement mechanisms, along with textural differences, suggests a change in the orientation of least principal stress that may have been caused by rapid unloading during volcanic sector collapse of the Luise crater. High temperature (>300°C) hydrothermal veining (garnet, clinopyroxene, magnetite) in the intrusive complex predates the porphyritic dikes, and hence sector collapse. Recent alteration assemblages that overprint the high temperature assemblage are in thermal equilibrium with the current hydrothermal system. A moderate temperature (>240°C) propylitic assemblage (epidote, chlorite, adularia, quartz, illite) occurs beneath a shallow zone of phyllic alteration (interlayered illite-smectite, quartz, pyrite) where cooler (<200°C) fluids have infiltrated the upper, faulted, regions of the intrusive complex.