During the second quarter of 2019, PT Freeport Indonesia (PT-FI) commenced extraction at its newly commissioned underground mining operations in the Grasberg minerals district in Papua, Indonesia. The Grasberg Block Cave underground mine is the same ore body historically mined from the surface in the open pit. Since its discovery in 1988, production from the Grasberg open pit has totaled approximately 27 billion lb of copper and 46 million ounces (oz) of gold. Over its life, PT-FI expects to produce an additional 17 billion lb of copper and 14 million oz of gold from the Grasberg Block Cave mine, sustaining Grasberg as one of the world’s largest copper and gold deposits, the company said.
In 2023, PT-FI expects to produce an average of 130,000 metric tons per day (mt/d) of ore from five production blocks spanning 335,000 m2 (covering over 80 acres) in the large-scale Grasberg Block Cave mine. At average reserve grades of 0.96% copper and 0.72 grams/mt of gold, 130,000 mt/d of ore is expected to equate to production of 850 million lb of copper and 700,000 oz of gold per year. Access to higher grades is expected in the early years of production.
PT-FI reported that undercutting in the Grasberg Block Cave mine exceeded 20,000 m2, more than 20% above forecast. Inception to date, undercutting in the Grasberg Block Cave mine totals 48,000 m2. Draw-belling initiated in two additional production blocks brought the active production blocks to three. A total of new 18 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 mt/d in first-quarter 2019 to an average of approximately 9,000 mt/d in June 2019 and is expected to reach 15,000 mt/d by the end of 2019. 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.
Monitoring data on cave propagation in the Grasberg Block Cave mine is providing increased confidence in growing production rates over time. As existing draw points mature and additional draw points are added, cave expansion is expected to accelerate production from an average of 30,000 mt/d of ore in 2020 to 130,000 mt/d of ore in 2023.
The Deep Mill Level Zone (DMLZ) mine, located east of the Grasberg ore body and below the Deep Ore Zone (DOZ) mine, is expected to produce 8 billion lb of copper and 8 million oz of gold over its life.
In 2022, PT-FI expects to produce from three production blocks in the DMLZ mine spanning 200,000 m2 (a footprint approaching 50 acres). At average reserve grades of 0.92% copper and 0.76 g/mt of gold, 80,000 mt/d of ore is expected to equate to production of 500 million lb of copper and 560,000 oz of gold per year.
During the second quarter, hydraulic fracturing operations at DMLZ were ongoing and continue to be successful in conditioning the rock for large-scale mining. Undercutting approached 4,000 m2, in-line with forecast. Inception to date, undercutting totals 58,000 m2. In June 2019, undercutting commenced on the second production block to establish two large production blocks. A total of four drawbells were opened, bringing total inventory of drawbells to 74. Production ramped up from an average of 6,800 mt/d of ore in first-quarter 2019 to an average of approximately 9,000 mt/d of ore in June. The DMLZ mine is expected to reach 11,000 mt/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 mt/d of ore estimated in 2020 and 80,000 mt/d of ore in 2022.
PT-FI continues to mine the final stages of the Grasberg open pit. During second-quarter 2019, PT-FI opened an additional mining area to extend pit life options into third-quarter 2019 and potentially longer (previous estimates were based on mining through June 2019). .
Material not mined from the open pit would be available to be mined from the Grasberg Block Cave.