The company said it could construct a second smaller decline in parallel to the existing Tjati decline.

The additional decline would initially provide ventilation and emergency egress, and later in the mine life, a separate pathway for the ore conveyer from the mobile fleet.

OZ said the extra decline was cost neutral, as it would see a number of previously included vent rises no longer required.

However, the company said the change provided a “significantly lower risk profile” and increased future operational flexibility.

More details will be provided when OZ releases the Carrapateena feasibility study on April 28.

In November, OZ released the prefeasibility study for Carrapateena.

Pre-production capital costs for a 4 million tonne per annum sub-level cave operation are $830 million, while the company plans to build a $150 million concentrate treatment plant (CTP) in Whyalla.

Average annual production is set to be 61,000 tonnes of copper and 63,000 ounces of gold at C1 operating costs of US82c per pound, including by-products.

The mine life is at least 20 years, with operations to generate revenue over that time of $A10.6 billion.

Using a 9.5% discount rate, the project has a post-tax net present value of $820 million, excluding a one-off deferred acquisition payment of $US50 million, and an internal rate of return of 20%.

OZ had $A656 million in cash at the end of December, but managing director Andrew Cole said last month that a funding decision wouldn’t be made until later in the year.

“We’re looking at strategic options and strategic partnerships,” he said.

“Based on everything we’ve seen to date, the project is getting stronger, not weaker.”

Project development work continues, as does the permitting process, with an Environmental Protection and Biodiversity Conservation Act referral lodged.

Shares in OZ dropped by 2.2% to $8.03, the lowest price this year.