With the 1:5.3 incline-decline now complete at Newcrest's Ridgeway gold mine at Cadia near Orange in NSW (see October issue, page 8, contractor Eroc Pty Ltd is establishing the basic underground infrastructure, due for completion in early 2002.
The emphasis now is on effective establishment of a sub-level caving operation for what is essentially a low-grade underground operation, at an average reserve grade of 2.7g/t gold.
The Ridgeway ore body is an upright bulbous body of stockwork and sheeted quartz veins accompanied by copper sulphides developed around a small, sub-vertical monzonite that intrudes a sequence of flat-lying volcanics and underlying sediments. The deposit is blind, the top being 500m below surface, with maximum dimensions of some 450m east-west and 250m north-south.
Until the conveyor to be fitted to the incline-decline is operational in 2002, waste and development ore will be hauled up the single decline. Over the last 12 months, 1.68mt of waste and ore has been trucked up at the same time as a multitude of contractors have gone underground.
But the emphasis is now on progressing the sub-level caving, to ramp monthly production of ore from 100,000t to 330,000t or higher for a 12-year mine life following completion of construction.
Three production levels, at 25m intervals, will be active at any one time. The first level at 5330RL (450m from surface ) is already complete, with 5305mRL in production, and 5280, 5255 and 5230 levels under development below this.
A perimeter drive is first made at each level, with 16-24 cross cuts about 200m long from each at 14m centres. These will then be drilled out, fired and ore retrieved in retreat to the perimeter drive. Cable bolting and shotcrete is installed at the turnouts to secure the ground.
The drives on the higher levels are 5m wide by 4m high but on the lowest levels will go to 6m wide by 4m high to improve draw from the cave.
International benchmarking with other sub level caving operations and mines like LKAB in Sweden and Henderson in the USA has assisted Newcrest to develop a suitable cave management strategy.
At present, the "loosening zone" is propagating at around 4m a month, which means it should reach surface in about five years time.
However, a risk management regime has seen $1m spent on a monitoring system. Holes drilled from surface allow borehole cameras and a microseismic system to assist in monitoring the behaviour of the cave.
The ore body is open at depth below the present mine plan to 4855mRL, although it is tending to become narrower. Newcrest intends to drill it out so see whether a further portion of the ore body could be developed.
Ore at present is being put through the Cadia mill. This, and a lower head grade, has seen the milling recovery at the Cadia mill reduced to 71%.
However assembly of the 6.5kW AG and ball mills adjacent to the Cadia mill to handle Ridgeway ore is advanced, as is installation of major mechanical equipment.
The mill has been designed as a conventional grinding and flotation process to produce copper gold concentrate with a gravity circuit to recover free gold as bullion. Average recoveries of gold and copper over the life of mine are estimated at 91% and 93.7% respectively.
Other infrastructure such as tailings storage and water supply are projected to be complete this month (November).
Meanwhile the nearby Cadia open cut mine continues to expand rapidly, with the final north and east interim walls now established. The west wall is currently being established although there is still some distance to go to the south.
The pit currently measures almost circular at 1,100m by 1,200m, with a fleet of 14 Caterpillar 793C 230t trucks servicing two Demag Komatsu H6555 shovels.