How Macraes gold mine cut assay turnaround time in half

The Challenge

Macraes is a low-grade ore body with a cutoff of  0.2–0.3 grams per tonne. At low grade, the mine can’t afford to wait.

Under fire assay, the target turnaround for grade control samples was 48 hours. It rarely landed there, and any repeat assay pushed it further out. Meanwhile the diggers sat.

“We can’t have diggers parked up waiting for assays to get back. If the diggers aren’t working, we aren’t making money,” said Ben Baxter, Superintendent, Geology.

Precision was the second problem. Fire assay reads a 30–50 gram sub-sample, reduced by hand through multiple steps, with each step adding uncertainty.

The Solution

Macraes didn’t switch on a promise. OceanaGold ran comparisons against fire assay, down to the mine’s lowest cutoff grades, where the margin for error is thinnest. Only then did they agree to take a PhotonAssay™ unit on site.

The unit came online on 15 March 2025. Every sample on site runs through it: open pit grade control, underground grab samples, metallurgical samples and exploration. Grade control turnaround has halved. Exploration results that once took months now come back in days.

With the comparisons behind them, Macraes decommissioned the fire assay entirely.

“We had a big program of comparisons with fire assay through our current lab, and when we were happy with that, we just cut the ties and decommissioned the fire assay, and we’ve never looked back,” said Baxter.

Shutting down the fire assay shut down the furnaces and the LPG supply that ran them.  “Down at the lab we used a lot of LPG for the fire assay furnaces. We no longer require that,” he added.

No furnaces. No gas. One less hazard to manage on site.

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