PT-FI makes headway at Grasberg Block Cave underground copper-gold mine

Freeport McMoRan has confirmed that, during the second quarter of 2019, PT Freeport Indonesia (PT-FI) commenced extraction of ore from the Grasberg Block Cave underground mine, in Papua, Indonesia.

This is the same orebody historically mined from the surface in the open pit, Freeport noted.

Since its discovery in 1988, production from the Grasberg open pit has totalled approximately 27,000 MIb (12.25 Mt) of copper and 46 Moz of gold. Over its life, PT-FI expects to produce an additional 17,000 MIb of copper and 14 Moz of gold from the Grasberg Block Cave underground mine, making Grasberg one of the world’s largest copper and gold deposits.

In 2023, PT-FI expects to produce an average of 130,000 t/d of ore from five production blocks spanning 335,000 sq.m in the large-scale Grasberg Block Cave underground mine. At average reserve grades of 0.96% Cu and 0.72 g/t Au, this is expected to equate to production of 850 MIb of copper and 700,000 oz of gold per year.

During the June quarter, Freeport noted:

  • Undercutting in the Grasberg Block Cave underground mine exceeded 20,000 sq.m, over 20% above forecast. Inception to date, undercutting in the Grasberg Block Cave underground mine totals 48,000 sq.m;
  • Initiated drawbelling in two additional production blocks, bringing the active production blocks to three. Eighteen drawbells were opened, exceeding forecast. Open drawbells in inventory in the three active production blocks currently total 29;
  • Ore extraction ramped up from an average of 5,000 t/d in March quarter 2019 to an average of around 9,000 t/d in June 2019 and is expected to reach 15,000 t/d by the end of 2019, and;
  • The fully-autonomous, state-of-the-art underground rail system is supporting efficient transport of ore to the oreflow system for delivery to the mill processing facility.

Freeport said: “Monitoring data on cave propagation in the Grasberg Block Cave underground mine is providing increased confidence in growing production rates over time. As existing drawpoints mature and additional drawpoints are added, cave expansion is expected to accelerate production from an average of 30,000 t/d of ore per day in 2020 to 130,000 t/d in 2023.

The Deep Mill Level Zone (DMLZ) underground mine, located east of the Grasberg orebody and below the Deep Ore Zone (DOZ) underground mine, is expected to produce 8,000 MIb of copper and 8 Moz of gold over its life.

In 2022, PT-FI expects to produce from three production blocks in the DMLZ underground mine spanning 200,000 sq.m. At average reserve grades of 0.92% Cu and 0.76 g/t Au, 80,000 t/d of ore is expected to equate to production of 500 MIb/y of copper and 560,000 oz/y of gold.

During the quarter hydraulic fracturing operations at the DLMZ were ongoing and continue to be successful in conditioning the rock for large-scale mining, according to Freeport. Undercutting approached 4,000 sq.m, in-line with forecast (inception to date, undercutting totals 58,000 sq.m).

In June 2019, undercutting commenced on the second production block to establish two large production blocks in the DMLZ underground mine for ore extraction, while a total of four drawbells were opened, bringing total inventory of drawbells to 74.

Lastly at DMLZ, production ramped up from an average of 6,800 t/d of ore in the March quarter to an average of some 9,000 t/d in June. The DMLZ underground mine is expected to reach 11,000 t/d of ore by the end of 2019.

Ongoing hydraulic fracturing operations combined with continued undercutting and drawbell openings in the two production blocks are expected to expand the cave, supporting higher rates of production with an average of 28,000 t/d of ore estimated in 2020 and 80,000 t/d of ore in 2022.

Meanwhile, PT-FI continues to mine the final stages of the Grasberg open pit. During the most recent quarter, PT-FI opened an additional mining area to extend pit life options into the September quarter and potentially longer (previous estimates were based on mining through June 2019).

“The mine sequencing changes in the open pit delayed access to the high-grade material previously forecast to be produced during second-quarter (June quarter) 2019, resulting in lower copper and gold production from the open pit than the April 2019 estimates,” the company said.

Revised 2019 mine plans for the Grasberg open pit are expected to meet and provide an opportunity to exceed the April 2019 estimates for copper and gold production for the year 2019, Freeport noted. PT-FI will continue to monitor geotechnical conditions to determine the extent of mining in the open pit, with material not mined from the open pit available to be mined from the Grasberg Block Cave.