OZ Minerals’ scoping study on an expansion at the Carrapateena copper-gold project, in South Australia, has shown that converting the lower portion of the sublevel cave to a block cave from 2026 could yield up to 60,000 t/y more copper output at the same time as reducing operating costs.
The Carrapateena sublevel cave is still in the development phase and is expected to hit first production in the December quarter of this year. This project is expected to produce an average of 65,000 t/y of copper and 67,000 oz/y of gold over a 20-year mine life.
The study outlined a more than doubling of mine throughput from 4.25 Mt/y to 10-12 Mt/y from 2026 through the development of the block cave and expanded surface infrastructure. This was expected to cost A$1-$1.3 billion ($704-$916 million) in upfront capital and lead to all-in sustaining costs going from $1.05/Ib ($2,315/t) in the sublevel cave operation to $0.90-$0.95/Ib during block cave operation.
The plan would see OZ Minerals access the higher-grade bornite mineralisation first, via the top-down sublevel cave, followed by a bottom-up block cave, OZ said.
Mine expansion and transition to a block cave would require adjustment to the location of future underground infrastructure below crusher station two, including a change in orientation of the decline, conveyor and ventilation, OZ said. In the current operation plan, this is not due for installation until after 2021.
The materials handling system and crushing infrastructure would require additional drive motors and a faster conveyor system to hit the new 10-12 Mt/y capacity, while there would need to be upgrades to the primary and secondary ventilation systems; electricity and communications infrastructure; and water supply, dewatering and underground facilities.
In terms of the process plant, there would need to be either a new parallel processing plant installed or an upgrade of the current sublevel cave processing plant.
OZ Minerals CEO, Andrew Cole, said: “The Carrapateena Block Cave Expansion work showed the conversion to a block cave to be the most value accretive next step for the Carrapateena resource and conceptually for the entire province, as it potentially enables a series of future add-on block caves, which themselves will now be the subject of a Carrapateena Life of Province Plan scoping study.”
He said the sublevel cave construction project remained on schedule for first production later this year, with ramp up to full production of 4.25 Mt/y taking place over the following 18 months.
The company will now move onto a prefeasibility study for the block cave expansion plan, which is expected to be completed by mid-2020.
Factoring in the scoping study results increases life of mine tonnes from 84 Mt at 1.8% Cu and 0.7 g/t Au, to around 145 Mt at 1.2% Cu and 0.5 g/t Au over a 20-year period.