Mining
Lihir Island gold deposit consists of two orebodies, Minifie and Lienetz, located in the Luise Caldera on the east coast of Lihir Island. They will be mined by conventional open pit methods. Because of its lower stripping requirement and higher gold content, Minifie orebody will be mined first. The ultimate pit, eventually to remove both orebodies, will extend to within 100 m of the coast and its base will be 200 m below sea level. The overall planned pit area is approximately 2 km2.
Mine planning schedules the treatment of higher grade ore initially, with treatment of sequentially lower grades of ore over time. To accomplish this, lower grade ore will be stored separate stockpiles as it is mined. When mining and processing of the higher grade ore has been completed, those stockpiles will be recovered and the material processed.
Ore body modelling, grade control, blast design, mine planning, mine design, ground water dewatering and mining operations are carried out by LMC. Blasting is carried out by Orica.
Disposal of mine waste is by direct truck haulage and by a combined truck/barge operation. Hard waste materials are dumped directly in Luise Harbour, both by barges and by trucks, to form a platform for the offshore low grade ore stockpile, with some of the more durable portions of this material used to buttress the seaward face of the platform for additional stability. Soft mine waste, together with any remaining hard waste materials, are trucked to a barge loading dock, from where self-propelled, bottom-dump barges haul the waste 1.5 km offshore for discharge.
The mining operation is supported by all the necessary facilities and services, including haul roads, dewatering and geothermal control, mine maintenance and service buildings, a peripheral drainage system for the pit, waste disposal facilities, a bulk explosives manufacturing plant, and refuelling facilities. The mine facilities are located in the Ladolam Creek area close to the mining operations.
Between 1987 and 1992, geotechnical investigations for the Lihir project were completed in eight main areas: seismicity, pit slope design, mining operations, mine waste disposal, low grade ore stockpiles, process plant siting, infrastructure, and construction material.
Lihir Island is located in a region of moderate seismic activity some 200 km north of a zone of intense seismic activity. A seismic hazard study was undertaken to provide site-specific earthquake design criteria. Those criteria were used for structural design, liquefaction assessment, dynamic slope stability evaluation, and assessment of the tsunami threat.
Geotechnical field work included the comprehensive logging and point load testing of over 200 cored holes. Oriented core, obtained from 15 angled drill holes, provided approximately 2700 measurements. Borehole fracture and strength information was integrated into a computer data base. Laboratory testing established soil, marginal, and rock material and determined classification, strength, density and durability properties.
Structural analyses assisted rock pit slope design and circular failure analyses were used for soil and marginal material design. Overall wall stability analyses were completed employing representative rock mass strength properties. Groundwater slope depressurisation will be achieved by pumped wells and horizontal drains. Slope erosion control measures will be employed. Ultimate pit inter-ramp slope designs range from 22° to 34° in the upper soil and marginal rock zones, and from 35° to 47° in the lower rock area.
Geotechnical studies relating to mining operations assessed excavation characteristics, trafficability, and material handling characteristics. Because of limited land space, the majority of the waste is deposited offshore. An offshore drilling investigation determined sea bed material properties in Luise Harbour and established that a hard rock platform can be constructed in Luise Harbour for low grade ore stockpiling.
Geotechnical investigations were also completed for use in the design of the on-shore low-grade ore stockpiles, the Putput Point plant site, the key mine infrastructure elements (e.g. ship berth and barge loading dock in Luise Harbour, Lakunbut Dam, and airstrip at Kunaie), and the definition of construction material sources.
Since November 1996 all geotechnical work is conducted on site by LMC staff. Previous work is continuously being reviewed and updated.
Surface geothermal activity occurs within a 3 km2 area of the central caldera, a relic of the volcanic activity that formed the caldera. Gas seeps, fumaroles and thermal springs occur, and logging in drillholes showed that the temperature can exceed 200°C at a depth as little as 200 m.
Boiling water can be encountered at the water table, which is generally just above sea level. Almost half the proposed pit is within the geothermal system. The western portion of the pit will be excavated to a depth where the rock temperature is170°C, while in the east the pit will be in rocks which are significantly cooler. It has been estimated that about 25% of the mined material could be at a temperature in excess of 100°C at the time of mining. The pit, adjacent to Luise Harbour, will reach more that 200 m below sea level.
Hydrogeological investigations demonstrate that the orebody is within very permeable rocks bounded on three sides by very low permeability material. On the remaining side, the orebody is connected to Luise Harbour by high permeability material. Permeability factors in the orebody are sufficiently high to indicate that dewatering must precede excavation.
Dewatering will be achieved outside the pit largely though the use of pump wells to remove groundwater and to intercept sea water inflow. Some additional pit wells (which will pump water at 100°C-160°C) may also be used for localised pit dewatering.
Dewatering will reduce the pressure of the geothermal system, including the liberating steam and gas, thus creating the potential for their discharge to the pit. Though not required for dewatering, naturally flowing geothermal discharge wells will be installed to mitigate steam pressure accumulation.
Detailed computer simulations (using single and multi-phase three-dimensional models) assisted development of the dewatering geothermal management plan. A combination of pump wells (the first of which are now being drilled), geothermal discharge wells, and pressure relief wells are planned to ensure a 'dry' pit and prevent inflows of hot geothermal fluids and sea water through the sea wall. It is estimated that 39 pump wells, 70 geothermal wells, and 172 monitoring wells will be installed throughout the mine life. Pumping typically will total about 1250 L/s and geothermal discharge will average almost 200 kg/s. The pits will be dewatered progressively in accordance with the mine plan, with the water table lowered eventually to about 200 m below sea level.
Geological investigations, detailed down-hole temperature logging and hydraulic testing, indicate that there are no low-permeability layers which could form a cap and allow the development of high pressure steam zones in the mining area. However, monitoring wells, pressure relief wells and geothermal wells, are to be installed as a safety measure.
Sequential shells outlining pits of decreasing value were selected from floating cone evaluation in order to arrive at an 8-phase pit design, each phase representing 2 to 3 years of ore production. Cone miner phases were adjusted in the final design to meet practical mining constraints. A plan view phase design at Level 974 is attached which includes a cross sectional view along line B-B. Subsequent planning has reduced the number of phases from eight to six.
Annual production schedules were developed by an optimising variable (declining) cut-off grade strategy which maximises the net present value of the cash flows. Higher grade ore has been scheduled for initial processing, followed by successively lower grade ores. Ore below the declining cut-off grade, and above the break-even cut-off grade, will be stored in stockpiles over a mining period of 10 years. The stockpiles will be reclaimed and processed over a period of 13 years after the end of mining in the pit.
A fence of closely spaced drill holes indicated highly variable gold grades. Conditional simulation studies identified the need for selective mining to reduce dilution at high cut-off grades. Dilution factors from the simulation study were incorporated in the mineable ore reserve gold grade estimates.
Ore is mined in 6 m benches and waste is mined in 12 m benches with 450 t hydraulic excavators. Ore and waste is hauled from the pit in 140 t trucks.
Trucks transfer soft waste rock to 975 m3 bottom dump barges which transport it offshore for disposal below the 120 m depth, this depth being the deepest possible level of the ocean 'mixing zone'. Hard waste (barged and trucked) is used to construct a near-shore platform to support a low grade ore stockpile. Low grade ore is also stockpiled at one land sites and one marine platform.
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