GRASBERG MINE LOCATED IN PAPUA, INDONESIA
The Grasberg minerals district includes open-pit and underground mines.
Did you know?
PT Freeport Indonesia (PT-FI) commenced mining operations in 1972 and in 1988 discovered the Grasberg mine. Today, after significant production, the Grasberg mining district contains one of the world’s largest recoverable copper reserve and the largest gold reserve.
The remote highlands of the Sudirman Mountain Range in the province of Papua, Indonesia, which is on the western half of the island of New Guinea.
Processes and facilities
The Grasberg minerals district includes the following underground mines that are being operated; the Grasberg Block Cave, the Deep Mill Level Zone (DMLZ), Big Gossan and the Deep Ore Zone (DOZ). In September 2015, PT-FI initiated pre-commercial production, which represents ore extracted during the development phase for the purpose of obtaining access to the ore body, at the Deep Mill Level Zone (DMLZ) underground mine. PT-FI also has several projects in progress in the Grasberg minerals district related to the development of the large-scale, long-lived, high-grade underground ore bodies located beneath and nearby the Grasberg open pit. In aggregate, these underground ore bodies are expected to produce large-scale quantities of copper and gold.
Grasberg open pit. PT-FI began open-pit mining of the Grasberg ore body in 1990. The final phase of the Grasberg open pit was mined during 2019, including the removal of the open pit ramps. In aggregate, the Grasberg open pit produced over 27 billion pounds of copper and 46 million ounces of gold in the 30-year period from 1990 through 2019.
Grasberg Block Cave. The Grasberg Block Cave ore body is the same ore body historically mined from the surface in the Grasberg open pit. Undercutting, drawbell construction and ore extraction activities in the Grasberg Block Cave underground mine continue to track expectations. Monitoring data on cave propagation in the Grasberg Block Cave underground mine is providing confidence in growing production rates over time. As existing drawpoints mature and additional drawpoints are added, cave expansion is expected to accelerate production rates to an average of over 98,000 metric tons of ore per day in 2022. Production at the Grasberg Block Cave underground mine is expected to continue through 2041.
DOZ underground mine. The DOZ ore body lies vertically below the now depleted Intermediate Ore Zone. PT-FI began production from the DOZ ore body in 1989 using open-stope mining methods, but suspended production in 1991 in favor of production from the Grasberg open pit. Production resumed in September 2000 using the block-cave method and is at the 3,110-meter elevation level.
TThe DOZ is a mature block-cave mine that previously operated at 80,000 metric tons of ore per day. Current operating rates from the DOZ underground mine are driven by the value of the incremental DOZ ore grade compared to the ore from the DMLZ and Grasberg Block Cave underground mines. Ore milled from the DOZ underground mine averaged 20,900 metric tons of ore per day in 2020, and production at the DOZ underground mine is expected to continue through 2022.
DMLZ underground mine. The DMLZ ore body lies below the DOZ underground mine at the 2,590-meter elevation and represents the downward continuation of mineralization in the Ertsberg East Skarn system and neighboring Ertsberg porphyry. During third-quarter 2018, PT-FI commenced hydraulic fracturing activities to manage rock stresses and pre-condition the DMLZ underground mine for large-scale production following mining induced seismic activity experienced in 2017 and 2018. Results to date have been effective in managing rock stresses and pre-conditioning the cave. Ongoing hydraulic fracturing operations combined with continued undercutting and drawbell openings in the two currently active production blocks are expected to expand the cave, supporting higher production rates that are expected to average approximately 74,000 metric tons of ore per day in 2022. Estimates of timing of future production continue to be reviewed and may be modified as additional information becomes available. Production at the DMLZ underground mine is expected to continue through 2041.
To mitigate the impact of these events, PT-FI implemented a revised mine sequence; upgraded support systems, blasting and reentry protocols; and improved mine monitoring and analysis processes. Development activities and mining are taking place in unaffected areas while impacted areas are being assessed, rehabilitated and prepared to be placed back into use.
Big Gossan underground mine. The Big Gossan underground mine was on care-and-maintenance status during most of 2017 and production restarted in fourth-quarter 2017. The Big Gossan mine lies underground and adjacent to the current mill site. It is a tabular, near vertical ore body. The mine utilizes a blasthole stoping method with delayed paste backfill. Stopes of varying sizes are mined and the ore dropped down passes to a truck haulage level. Trucks are chute loaded and transport the ore to a jaw crusher. The crushed ore is then hoisted vertically via a two-skip production shaft to a level where it is loaded onto a conveyor belt. The belt carries the ore to one of the main underground conveyors where the ore is transferred and conveyed to the surface stockpiles for processing. Ore milled from the Big Gossan underground mine averaged 7,000 metric tons per day in 2020. Production at the Big Gossan underground mine is expected to continue through 2041.
Description of Ore Bodies
Our Indonesia ore bodies are located within and around two main igneous intrusions, the Grasberg monzodiorite and the Ertsberg diorite. The host rocks of these ore bodies include both carbonate and clastic rocks that form the ridge crests and upper flanks of the Sudirman Range, and the igneous rocks of monzonitic to dioritic composition that intrude them. The igneous-hosted ore bodies (the Grasberg open pit and block cave, and portions of the DOZ block cave) occur as vein stockworks and disseminations of copper sulfides, dominated by chalcopyrite and, to a lesser extent, bornite. The sedimentary-rock hosted ore bodies (portions of the DOZ and all of the Big Gossan) occur as “magnetite-rich, calcium/magnesian skarn” replacements, whose location and orientation are strongly influenced by major faults and by the chemistry of the carbonate rocks along the margins of the intrusions.
The copper mineralization in these skarn deposits is dominated by chalcopyrite, but higher bornite concentrations are common. Moreover, gold occurs in significant concentrations in all of the district’s ore bodies, though rarely visible to the naked eye. These gold concentrations usually occur as inclusions within the copper sulfide minerals, though, in some deposits, these concentrations can also be strongly associated with pyrite.
48.8% of PT-FI owned by Freeport-McMoRan Inc.; 51.2% of PT-FI owned by PT Indonesia Asahan Aluminum (Persero) (PT Inalum), a state-owned enterprise that is wholly owned by the Indonesian government.
Note: FCX's economic interest in PT-FI is expected to approximate 81.28% through 2022.