Ridgeway Gold and Copper Mine Cadia Valley, New South Wales, Australia

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 is in the process of transitioning from the sub-level cave to a block cave beneath the existing mine. In 2007 the Newcrest board approved development of the Ridgeway Deeps block cave. It will be the first natural caving operation to be developed by Newcrest and will be the deepest block cave in Australia. The company says it is spending A$525m to develop it.

The mineral resource at Ridgeway Deeps was re-estimated following a geological reinterpretation which yielded an increased resource of 20 million tons containing 0.5 million ounces of gold, and 0.05 million tons of copper.

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. Reserves are 11.8Mt at 1.8g/t Au and, 0.68% Cu for 0.7Moz Au and 80kt Cu. Resources are 15Mt at 2.0g/t Au, 0.66% Cu for 1.0Moz Au and 100kt Cu.

RIDGEWAY MINING

Ridgeway is a sub-level caving operation, transitioning to a block cave. Crushed ore from the underground is delivered by conveyor to a surface stockpile, adjacent to the Ridgeway concentrator.

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. Concentrate from Ridgeway is combined with that from Cadia Hill before being pumped to the filtration plant in Blayney where it is dewatered prior to being transported by rail to Port Kembla for export to smelters throughout East Asia.

ORE PROCESSING

Ore production took place from five levels in, within the sub-level cave. The lowest level, at 830m below the surface, is the final production level before mining progresses below the crusher transfer horizon. Gold and copper head grades have decreased with depth. Trucking of ore and waste from deeper levels to the crusher has started.

PRODUCTION AND COSTS

Production for Ridgeway 2006–07 was down to 314,028oz gold, compared with 366,520 the previous year. Copper production was also lower at 37,939t compared with just under 40,000t for the previous year. "The effect of expected lower head grades was partially offset by increased mill throughout", Newcrest said in its 2007 annual report.

At achieved prices, the cash cost of production was negative A$228/oz (2005–06: negative A$52) and total cost was negative A$112/oz (2005–06: A$114). At spot prices, the cash cost of production was negative A$544/oz (2005–06: negative A$428) and total cost was negative A$428/oz (2005–06 negative A$263).

Block caving involves a mine design and process to ensure the rock 'naturally' caves progressively upwards under the influence of gravity due to the undercutting and removal of broken ore at the base of the block being mined. Block caving has a lower operating cost than sub-level caving as it relies on this natural caving process, requiring less drilling and blasting. The Ridgeway Deeps mining cost will be significantly lower than the existing Ridgeway mine due to the combined impact of lower block cave mining costs and the implementation of semi-automated ore loaders.

Capital cost for the project of A$545m comprises construction of the block cave, extension of the existing underground ore-handling system (including two new primary underground crushers), development of bulk underground mining technologies, including the application of automated remote loaders and modifications to the processing plant, adding additional secondary crushing and regrind facilities. The majority of the capital expenditure will be incurred over the 2008 and 2009 financial years with project commissioning occurring progressively over 2009.

FUTURE DEVELOPMENTS

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.