Summary:
Deposit Type
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.
Deposit Geology
The Luise volcano consists of a 4 by 3.5 km wide amphitheater, elongated and breached to the northeast. This is inferred to be a remnant of the original approximately 1.1 km high volcanic cone that underwent sector collapse(s). The Lihir deposit is located in the footwall of the sector collapse detachment surface.
Gold mineralization at the Lihir deposit is a complex and refractory assemblage associated mainly with pyrite and marcasite veinlets, disseminations, replacements, and breccia fillings. The sector collapse event(s) superimposed late-stage, gold-rich, alkalic low-sulfidation epithermal mineralization upon early-stage, porphyry-style alteration.
Several structural trends appear important in localizing and confining individual breccia units as well as gold mineralization. The predominant regional orientation of dykes and faults on Aniolam Island are north-northeast-trending (-025°), which is interpreted to be associated with deepseated tensional faults, and which may have controlled the long axis of the Luise volcanic edifice.
Other strong structural trends occurring within the Luise Caldera include:
• East-northeast-trending structures dipping moderately (60°) to the north;
• Arcuate generally east-west-trending, north-dipping, listric-shaped structures believed to be associated with the collapse of the volcanic edifice;
• Sub-vertical northwest-trending structures;
• Steeply eastward-dipping north-south-trending structures. East-northeast- and northeast-trending structures are most common, and coincide with aligned offshore islands, aeromagnetic features and elongation of inferred volcanoes and intrusions. The most prominent faults on Aniolam Island are normal faults striking 040° to 050° and dipping 40° to 50° to the northwest. North-, northwest- and west-northwest-trending structures are defined by magnetic lineaments and truncations.
A broad, three-fold vertical alteration zonation is interpreted to represent this evolution. With increasing depth, the alteration zones consist of:
• 0.2-Ma, surficial, generally barren, steam-heated clay alteration zone that is a product of modern high-temperature geothermal activity;
• 0.6-0.2 Ma, high-grade (> 3 g/t Au), refractory sulfide and adularia alteration zone that represents the ancient epithermal environment;
• 0.9-0.3 Ma, comparatively low-grade (< 1 g/t Au) zone rich in anhydrite ± carbonate, coupled with biotite alteration, that represents the ancient porphyry-style environment.
Post sector collapse volcanism occurred during the modern geothermal stage, with the emplacement of several diatreme breccia bodies.
Texturally-destructive hydrothermal alteration and mineralization often obscures texture and composition in volcanic and volcaniclastic rocks where these units are cut by multiple diatremes and subvolcanic intrusions.
Mineralization
The Lihir deposit has dimensions of about 1,500 x 3,000 m and has about 500 m in depth extent. The deposit remains open at depth, along strike, and to the east, where it is currently limited by the Pacific Ocean.
Mineralization consists of a number of styles, ranging from early porphyry to late-stage epithermal mineralization. Two of these gold mineralization styles represent economically significant phases.
The most important mineralization style is refractory potassium feldspar-sulfide mineralization. In this association, gold occurs as solid solution gold in the crystal structure of sulfide grains. Overall sulfide content is relatively high, with the average sulfide grade of the mineral reserves being above 6%. The main sulfide 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 sulfides are characterized by their fine-grained nature and were deposited through wholesale flooding and deposition within all host rocks, imparting a sooty, dark-grey coloring to the host rocks. Mineralization 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 the 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 mineralization occurs as a quartz-chlorite-bladed anhydrite association, which is more typical of porphyry-style mineralization. This mineralization likely resulted from the mixing of magmatic fluids with oxidizing near-surface water. Native gold several millimeters in size has been observed, although it is rare.