Grasberg copper mill at 67%: recovery, cave ramp‑up and risk notes for mine engineers
Reviewed by Joe Ashwell
First reported on MINING.com
30 Second Briefing
Freeport-McMoRan’s Grasberg copper-gold complex in Central Papua has restored milling to about 140,000 tonnes of ore per day, roughly 67% of normal rates, a year after an 800,000-tonne mudslide from the former open pit killed seven workers and forced a shutdown. The Grasberg Block Cave supplied about 70,000 tonnes per day of mill feed in Q3, with underground material-handling upgrades due by early 2027 and Production Block 1S targeted to restart by mid-2027, lifting output towards 80% of capacity. PT Freeport Indonesia’s East Java smelter is ramping up alongside the PT Smelting facility, together able to produce up to 800,000 tonnes of copper cathode annually, while Freeport guides to Q3 copper production of 830 million lb and expects realised prices above $6.50/lb.
Technical Brief
- Company investigation identified accelerated ore removal from a narrow, clay‑rich production block as creating a high‑velocity flow channel linking surface mud to underground voids.
- Existing monitoring systems and drawpoint sampling reportedly provided no precursory warning, indicating limitations in current geotechnical and flow‑path surveillance for cave‑surface interactions.
- For other block and panel caves, the event points to the need for explicit design checks on potential surface‑to‑underground flow paths, especially where fine, clay‑rich materials overlie active draw zones.
Our Take
With Grasberg’s removed copper representing 1.2% of global mine output, any prolonged milling constraint in Central Papua would tighten the same copper market that has underpinned Freeport-McMoRan’s recent $100‑billion‑plus valuation milestone noted in our April 23, 2026 coverage.
The move from a July unit net cash cost estimate of $2.00/lb to $2.10/lb at Grasberg slightly erodes margins but remains well covered if realised prices do exceed $6.50/lb, consistent with the elevated realised copper prices that helped Freeport beat second‑quarter profit forecasts in the July 23, 2026 article.
Freeport-McMoRan’s role in Morgan Stanley’s “Space 60” list for copper and other critical minerals means operational disruptions at the Grasberg copper‑gold complex are now material not just for traditional metals markets but also for downstream aerospace and electronics supply chains highlighted in that April 21, 2026 piece.
Prepared by collating external sources, AI-assisted tools, and Geomechanics.io’s proprietary mining database, then reviewed for technical accuracy & edited by our geotechnical team.
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