The Lihir Project is 100% owned by Newcrest’s wholly-owned subsidiary, Lihir Gold Limited (Lihir Gold).
On November 6, 2023, Newmont Corporation completed the acquisition of Newcrest Mining Limited to create the world’s leading gold company with robust copper production.
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Summary:
The Lihir deposit is considered to be an example of an epithermal gold deposit. Lihir Island is part of a 155 mile (250-kilometers) long, northwest-trending, alkalic volcanic island chain that sits within an area where several micro-plates (Solomon Sea Plate, South Bismarck Plate and North Bismarck Plate) developed between the converging Australian and South Pacific plates. Lihir Island comprises two Plio–Pleistocene volcanic blocks, Londolovit Block and Wurtol Wedge and three Pleistocene volcanic edifices, Huniho, Kinami, and Luise.
Features of the Lihir deposit that classifies it as an alkalic epithermal gold deposit include:
- Island arc association;
- Hosted in a sector-collapse amphitheatre developed in oxidised alkaline igneous rocks;
- Mineralisation hosted in vein stockworks, disseminated zones and breccias;
- Gold association with pyrite; often refractory;
- Low temperature/low salinity fluids.
Intense alteration was intimately associated with ore-forming events. Early-stage potassic alteration occurred as porphyry-style alteration associated with the emplacement of alkalic stocks within the volcanic edifice, with peripheral and broadly contemporaneous propylitic alteration. Sudden collapse of the volcanic edifice is interpreted to have resulted in the rapid depressurising of the system and subsequent telescoping of epithermal alteration and associated gold mineralisation upon the porphyry environment. Argillic and advanced argillic alteration assemblages developed through continued geothermal activity, driven by post mineralisation magmatism. Geothermal activity continues to this day.
Three alteration styles are recognised:
- Clay zone: equates to argillic ± advanced argillic alteration, about 250 m thick, and subparallel to basal topography of amphitheatre; represents the modern geothermal system;
- Sulphide–adularia zone: equates to epithermal-style low sulphidation alteration; sub-parallel to basal topography of amphitheatre with crenulated local downward projecting base, defined by pyrite-cemented breccias, abundant adularia alteration and disseminated pyrite in altered wall rocks, the lower parts of the sulphide– adularia zone transitions gradationally into the biotite- and K-feldspar-altered rocks of the anhydrite zone. The upper parts are typically more adularia ± illite-altered;
- Anhydrite zone: equates to porphyry-style potassic alteration; vertically and horizontally extensive basal alteration unit; lateral and lower limits not demarcated, defined by the presence of > 1% anhydrite ± calcite ± quartz occurring as veins, breccia cement and/or intergranular disseminations within wall rocks, it is atypical of calc–alkalic porphyries in terms of lacking well-developed quartz stockwork veining.
The three alteration zones overprint each other at their basal contacts, and reflect distinct stages in the evolution of the magmatic–hydrothermal system.
The intense alteration from the early porphyry-style, late epithermal, and modern high-temperature geothermal system has obscured many of the primary rock types, and extends vertically and laterally beyond the mineralised zones, with poorly-constrained limits.
The Lihir deposit has dimensions of about 1,500 x 3,000 m and has about 500 m in depth extent. A long section through the Lihir deposit is included. The deposit remains open at depth, along strike, and to the east, where it is currently limited by the Pacific Ocean.
Gold is the only metal of economic significance present within the Luise Caldera. Mineralisation consists of a number of styles, ranging from early porphyry to late-stage epithermal mineralisation. Two of these gold mineralisation styles represent economically significant phases.
The most important mineralisation style is refractory K feldspar–sulphide mineralisation. In this association, gold occurs as solid solution gold in the crystal structure of sulphide grains. Overall sulphide content is relatively high, with the average sulphide grade of the Mineral Reserves being above 6%. The main sulphide mineral is pyrite, with accessory marcasite and rare arsenopyrite and chalcopyrite. Gold also occurs as small (less than 100 µm) blebs within fine pyrite crystals. The sulphides are characterised by their finegrained nature, and were deposited through wholesale flooding and deposition within all host rocks, imparting a sooty, dark-grey colouring to the host rocks.
Mineralisation is locally associated with strong leaching of the original lithologies, creating pinhole to open, vuggy textures. Cavities as large as 10 m in extent were encountered. This secondary porosity is thought to be the result of dissolution of host rock by hot alkaline fluids, or alternatively as the result of boiling. Gold locally occurs as electrum, gold tellurides, and native gold associated with quartz, calcite and bladed anhydrite.
The second significant style of gold mineralisation occurs as a quartz–chlorite–bladed anhydrite association which is more typical of porphyry-style mineralisation. This mineralisation likely resulted from mixing of magmatic fluids with oxidising near-surface water. Native gold several millimetres in size has been observed, although it is rare.