The Ridgeway underground mine cave is located in the mineral-rich Cadia Valley province in central New South Wales. A gold and copper mine, it is 100% owned by Australia’s Newcrest Mining, and is one of three operations owned by the company in the province. The others are Cadia East and Cadia Hill, the largest of the three.Ridgeway was successfully transitioned from contract mining to owner operated mining in April 2005.
The Newcrest board approved development of the Ridgeway Deeps block cave in June 2007. The Ridgeway Deeps block cave project involved development of resources below the Ridgeway Underground mine at a depth of 300m to about 1,100m. It is the first natural caving operation and the deepest block cave in Australia.
Ridgeway Deeps was developed at a cost of A$525m by Ausenco. The project involved construction of a block cave, extension of the existing underground ore-handling system (including two new primary underground crushers), development of bulk underground mining technologies such as the application of automated remote loaders and modifications to the processing plant. The project also involved construction of an additional secondary crushing and regrind circuit.
The mine commenced full production from the block cave beneath the existing mine,
Ridgeway Deeps will be producing about 1.6 million ounces (Moz) of gold and 0.21 million tons (Mt) of copper over the eight-year mine life.
Geology and reserves
The large porphyry system at Cadia Valley Operations contains a number of deposit styles and structural settings including one of the world’s highest grade gold-rich porphyry’s at Ridgeway.
The Ridgeway ore body, which has no surface presence, lies around half a kilometre below the surface and resembles an inverted teardrop measuring around 450m by 250m and extending over 850m in depth.
As of June 2010, the total reserves including those in the Ridgeway Deeps, were 101mt at 1.8g/t Au and, 0.38% Cu for 2.6Moz Au and 380kt Cu and the total resources were estimated at 155Mt graded at 0.73g/t Au, 0.38% Cu for 3.6Moz Au and 580kt Cu.
Ridgeway Deeps mining
The transformation of Ridgeway Underground – a sub-level caving operation – into a block cave involved a mine design and a process to ensure the rock ‘naturally’ caves progressively upwards by a gravitational force of the undercutting and removal of broken ore at the base of the block being mined.
Crushed ore from the mine is delivered by conveyor to a surface stockpile, adjacent to the Ridgeway Underground concentrator.
Ore extracted from Ridgeway Deeps is processed using the floatation and gravity of Ridgeway Underground circuit which has a nominal capacity of 5.6Mtpa.
A two-stage grinding process forms a part of the comminution circuit. The grinding circuit comprises a 6.5MW SAG mill and 6.5MW ball mill. Using similar processes to the adjacent Cadia Hill concentrator, gold and copper are recovered in a conventional floatation circuit to produce a copper concentrate containing elevated gold levels.
An additional secondary crushing circuit and two Verti mills were installed at the plant to further process the hard ore from Ridgeway Deeps. The secondary crushing circuit maintains high grades. The first Verti mill, which was installed in the ball mill circuit, has allowed for high throughput. The second mill is installed within the flotation circuit for regrinding works.
The concentrate is combined with that from Cadia Hill before it is pumped to the filtration plant in Blayney. It is dewatered at the filtration plant prior to transport by rail to Port Kembla for export to smelters throughout East Asia.
Recent exploration drilling below the area designated for the Ridgeway Deeps block cave has indicated a continuation of the ore body. Preliminary assessments are being undertaken into the potential development of a second block cave which could further extend the mine life of the Ridgeway and Ridgeway Deeps mines.