The basis of the alternate process is scavenging unleached sulphide concentrates from the existing carbon-in-leach tailings and subjecting them to ultra-fine grinding and intense cyanidation to improve the overall leach recovery.

This is a similar appoach to what has been used successfully for years at Granny Smith mine and Kalgoorlie Consolidated Gold Mines’ Super Pit operations in Western Australia.

Millennium started the Sulphide Ore Expansion Study, aimed at unlocking the full economic potential of the Nullagine resource, last year, and the bulk of the work was done last quarter.

That study was initially based on using an integrated CIL and flotation circuit. That would have involved the integration of a flotation circuit capable of treating sulphide ore with the existing 2 million tonne per annum CIL plant.

However, it would also include the installation of a ball mill for additional grinding of fresh ore, a conventional flotation circuit and associated concentrate handling facilities.

The overall bill for that was likely to run to $40 million to $46 million, including contingency.

Processing costs for the free-milling fresh ore was estimated to be $19 per tonne while processing the sulphide ore was estimated to cost $27.50/t.

Millennium brought in Minescope Services and its principal process engineer Dale Harrison helped it identify the alternative processing option.

Preliminary financial estimates show the new processing option could reduce the capital cost of the original flotation circuit pathway of $40 million to $46 million to $12 million to $15 million.

Millennium chief executive Peter Cash said the company was pleased to be working with Ausenco and Minescope Services to pursue a processing configuration that could be a “game-changer” for the project.

“The Sulphide Ore Expansion Study, which underpins our corporate objective of becoming a 100,000 ounce a year producer by unlocking the existing large sulphide mineral resource inventory at Nullagine, is now in its final and most important stages,” Cash said.

“The identification of this new processing option could transform the economics of the expansion project, dramatically reducing both capital and operating costs and resulting in a major reassessment of our reserve inventory.

“We still have further work to do to confirm that this is the case, but the early indications are extremely positive.

“The imminent completion of the Expansion Study dovetails with our move underground at Nullagine, with development of our first underground mine at Bartons set to begin by the end of this quarter.

“It also comes at a time when our exploration team is continuing to enjoy success with deeper drilling identifying the potential to significantly grow the mineral resource base below and along strike of the existing open pits.”