The Lihir deposit is considered to be an example of an epithermal gold deposit.

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 hightemperature 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.

Production mining is conducted by Newcrest using Owner-operated equipment fleet and an Owner workforce. A separate mining contractor operation using a smaller pioneering fleet is used to develop new working areas on the steep caldera slopes.

Production mining is by conventional open pit method, using a fleet of 500 t class (operating weight) hydraulic face shovels loading into 135 t capacity rear-dump haul trucks, with a recently demonstrated mining rate of 33 Mt/a ex-pit. Ore and waste are drilled and blasted on 12 m benches and mined in a single pass. Where practicable, walls are drilled with a pre-split to assure stable wall rock conditions. The ground is frequently too hot for conventional explosives, requiring high-temperature blasting products and specialised blasting procedures for mining in hot ground.

A majority of ex-pit ore is allocated by gold and sulphur grade into a blend plan agreed with process plant staff along with existing stockpiled ore. Mill feed is based on the blend plan and can be comprised of reclaimed ore from the ROM stockpiles, direct ex-pit ore and existing stockpile ore.

Waste rock from the mine is either placed into 1,500 t capacity barges for off-shore submarine disposal or stockpiled for use as road base, bench sheeting, stemming or construction fill. Submarine waste disposal is carefully planned and controlled to achieve a continuous rill along the steeply-sloping sea floor and minimise the potential for uncontrolled slumping.

Ore is crushed in two primary crushing circuits. The first circuit consists of a 42–65” gyratory crusher and second an MMD toothed rolls crusher. Competent ore is crushed in the gyratory crusher and softer ore types in the MMD crusher. Both crushers discharge on to an overland conveyor and then to a radial stacker for stockpiling ahead of the grinding circuits.

The second primary crushing circuit, installed during the 2010–2012 MOPU plant expansion, consists of two jaw crushers operating in parallel. Separate overland conveyors are used.

There are three grinding circuits. One circuit (HGO2) generally treats high grade ore that is fed direct to the downstream oxidising autoclaves. The second and third circuits (FGO circuit and HGO circuit) are generally directed to the flotation plants. However, all three circuits can be directed to flotation as necessary, and all three circuits can go “direct” to the autoclaves as necessary.

All three grinding circuits have a primary semi-autogenous grind (SAG) mill followed by a secondary ball mill in closed circuit with classifying hydro-cyclones. Pebbles from the HGO and HGO2 circuits are combined and directed to two cone pebble crushers. Crushed pebbles are directed back to the HGO mills.

The current capacity of the HGO, FGO and HGO2 mills is approximately 6.5, 5.0 and 6.5 Mt/a respectively. Ground ore is thickened and washed in a one or two stage grinding thickener counter-current decant (CCD) washing circuit with raw water to minimise chloride concentration in the autoclave feed.

As the gold mineralisation is refractory, the plant consists of crushing and grinding followed by partial flotation, pressure oxidation, and then recovery of gold from washed oxidised slurry using conventional cyanidation. The plant currently has a nameplate capacity of 15 Mt/a.

The Lihir Operations have changed from a “full oxidation” treatment plant to a partial oxidation plant. The current operating strategy, termed the “Lihir operating strategy” or LOS, was developed to exploit the benefits of partial oxidation to maximise gold production rates.

The LOS is a self-correcting system. If a feed is presented to the autoclave that is too low in sulphide sulphur, then the autoclaves will slow down to maintain front-end temperatures; hence, forcing more ore to flotation, which then increases sulphur grade, allowing increased throughput reaching a new operating equilibrium.

The LOS maximises and optimises the gold production rate at all times irrespective o ........