Gold is the only metal of economic significance present within the Luise Caldera. A number of mineralization styles have been recognized, ranging from early porphyry to late stage epithermal mineralization. Two of these represent economically significant phases of gold mineralization at Lihir.

• The most important is refractory potassium feldspar-sulphide mineralization. In this association, gold occurs primarily as sub micron size particles in sulphide minerals. Overall sulphide content is relatively high, with the average sulphur grade of the reserves being above 6%. The main sulphide mineral is pyrite, with the marcasite form present as an accessory mineral and rare arsenopyrite.

Gold also occurs as small (less than 100 micron) blebs within fine pyrite crystals. The sulphides are characterized by their fine grained nature, and have been deposited through wholesale flooding and deposition within all host rocks, imparting a sooty, dark grey coloring to the host rocks. Within the Lienetz and Kapit orebodies (and in some localized portions of Minifie) this style of mineralization has been associated with strong leaching of the original lithologies creating pinhole to open, vughy textures.

• The second significant style of gold mineralization occurs as a low sulphidation quartz-chlorite-bladed anhydrite association, more typical of epithermal style mineralization, deposited through the mixing of magmatic fluids with oxidizing near-surface water. Occasional free gold, up to several millimetres in size, has been observed in association with this mineralization style.

Mineralization occurs as discrete fracture filled veins through all levels of the deposits, and is inferred to be an overprinting style of epithermal mineralization associated with the cooling of the active geothermal system within the Luise Caldera. It is also characterized by relatively high, though erratic, gold grades when compared with the high sulphidation style of mineralization.

Production mining operations at Lihir are conducted by Newcrest using their own equipment fleet and workforce. A separate mining contractor operation using a smaller pioneering fleet is developing new areas on the steeply dipping caldera slopes. Production mining uses a fleet of 500t class hydraulic face shovels loading into 135t rear dump haul trucks. Ore and waste is drilled and blasted on 12m benches and mined in a single pass, except where excessive heave has required a second lift for safety reasons. The ground is frequently too hot for conventional explosives, requiring high temperature blasting products and specialized blasting procedures for mining in hot ground.

Ore is crushed in two primary crushing circuits. The first circuit consists of a 42 65” gyratory crusher and Abon toothed rolls crusher. Competent ore is crushed in the gyratory crusher and softer ore types in the Abon crusher. Both crushers discharge on to an overland conveyor up to a radial stacker for stockpiling ahead of the grinding circuits, providing mainly high grade ore to the third milling circuit (known as HGO2 circuit) installed as part of the major plant expansion.

The second primary crushing, installed for the plant expansion, consists of two jaw crushers operating in parallel. This circuit typically receives more competent lower grade ores and supplies the FGO and HGO mills via a separate overland conveyor and radial stacker.

Ground flotation grade ore from the FGO circuit Mill is subjected to simple bulk rougher flotation in a single roughing stage consisting of a bank of five 150m3 flotation tank cells. Ground flotation ore from the HGO circuit Mill is processed by five 300m3 flotation tank cells.

Thickened ore slurry is pumped to the three parallel autoclave circuits, and a fourth larger autoclave installed as part of the last major plant expansion. The fourth autoclave is 2.2 times the capacity of the existing three autoclaves. Conventional gold processing Sherritt autoclave technology at a temperature of 205° C and a pressure of 2,650 kPa is utilized. Feed slurry is first preheated in heat recovery vessels before being pumped under pressure to each of the six-compartment agitated autoclave vessels.

Oxidized slurry passes through two trains of two stage counter-current decantation (CCD) circuit, where it is washed with process water and seawater as required minimizing slurry acidity. Note that a second train of CCD washing, neutralization, CIL and tailings disposal system was installed as part of the major plant expansion to handle the additional feed from the fourth autoclave.

The washed slurry is then neutralized with lime slurry prepared from slaking imported quicklime, and then gold is recovered from the neutralized slurry by cyanide leaching using conventional CIL technology and a series of agitated tanks. The slurry is conditioned with lime in the first tank and cyanide is added to the second tank. The slurry is then agitated with granulated carbon in the remaining tanks and passes through the tanks while the carbon is retained by screens.

Loaded carbon from the CIL circuit is stripped of gold in an elution system. The gold is eluted from carbon using hot caustic/cyanide solution and the carbon is then rinsed with water. The resulting gold solution is circulated through electro-winning cells where gold is recovered through electroplating to form a gold sludge. The sludge is dried and then smelted to produce doré bars which are shipped to a refinery. Barren carbon is regenerated in a rotary kiln.

The CIL leach residue tailings are detoxified by formation of strong metal complexes such as ferrocyanide, and through dilution with seawater (cooling water return) in the junction box upstream of the de-aeration tank.

Under these conditions weak acid dissociable cyanide (CNWAD) converts to stable ferrocyanide. The tailings gravitate to a common disposal system which also collects the flotation tailings; remaining CCD wash water as well as oxygen plant and power plant cooling water return streams.