Grasberg mine has the single largest known gold reserve and the second largest copper reserves in the world. It is located 96km north of Timika, at Tembagapura in Irian Jaya – the most easterly of Indonesia’s provinces – on the western half of the island of New Guinea.
Grasberg minerals district includes open-pit and underground mines. It has produced 528 billion ounces of copper and 53 million ounces (Moz) of gold, including more than 432 billion ounces of copper and 46Moz of gold from the Grasberg open pit during 1990 to 2019.
Extraction of ore from the Grasberg Block Cave underground mine commenced in the second quarter of 2019, which is the same ore body mined from the surface in the Grasberg open pit. The mining of the final phase of the Grasberg open pit was completed in Q4 2019 and the mine transitioned from open-pit mining to large-scale underground mining.
Grasberg Block Cave is estimated to contain proven and probable reserves of 275.2 billion ounces of copper and 14.2Moz of gold as of December 2019.
Freeport McMoran holds a 48.8% interest in the mine while the remainder (51.2%) is held by Inalum.
Freeport McMoran and Rio Tinto sold a majority interest in Grasberg to Indonesia’s state-owned aluminium mining company, Indonesia Asahan Aluminium (Inalum). Prior to the $3.85bn transaction, Freeport held a 90.64% stake in the mine operation while Rio Tinto had a 40% participating interest. Under the deal, the government agreed to provide a special licence to Freeport to enable the company to continue the extraction of ore until 2041. To support the mine and its workforce, PT Freeport has built an airport, a port at Amamapare, 119km of access road, a tramway, hospital, housing, schools and other facilities.
The mine stands at the collision of the Indo-Australian and the Pacific tectonic plates. Two distinct phases of intrusion have led to the production of nested coaxial porphyry ore bodies and sulphide-rich skarn at the margins, while sedimentary strata include Eocene clastic carbonate limestone with siltstones and sandstones near the base.
The Dalam Diatreme (DD) forms the first intrusive stage, being highly fragmental and characterised by clasts and a matrix of dioritic composition.
Mineralisation is largely disseminated and chalcopyrite dominant, having average grades of 1.2% copper and 0.5g/t gold.
The second intrusive stage, the Main Grasberg Stock (MG), is composed of non-fragmental, porphyritic monzodiorites, forming a quartz-magnetite dilational stockwork with veinlet-controlled copper-gold mineralisation. This is a high-grade resource, with averages of 1.5% copper and 2g/t gold.
There is also a third intrusive stage, associated with the South Kali Dykes, which was the final intrusion and the most weakly mineralised.
Mining at Grasberg Open Pit
The workings comprise an open-pit mine, an underground mine and four concentrators. The open-pit mine, which forms a mile-wide crater at the surface, is a high-volume low-cost operation, producing more than 67 million tonnes of ore and providing more than 75% of the mill feed in 2006.
Designed to be fully mechanised, using 6.2m³ Caterpillar R1700 load-haul-dump vehicles (LHDs) at the extraction level with a truck haulage level to the gyratory crusher, the Deep Ore Zone (DOZ) block cave mine is one of the largest underground operations in the world.
After 2004, when the DOZ mine averaged 43,600tpd, a second underground crusher and additional ventilation were installed to increase daily capacity to 50,000 tonnes. Ramp-up production has begun at the Deep Mill Level Zone (DMLZ) underground mine, which lies below the DOZ underground mine and to the east of the Grasberg ore body.
Ore from both operations is transported by conveyor to centralized mine facilities, feeding a combined daily average total of some 225,000t of ore to the mill and 135,000t to the stockpiles.
Production equipment includes 30m³–42m³ buckets, a 170-strong fleet of 70t–330t haul trucks, together with 65 dozers and graders, with radar, GPS and robotics used in the mine’s state-of-the-art slope-monitoring system.
Processing at Grasberg Open Pit
The ore undergoes primary crushing at the mine, before being delivered by ore passes to the mill complex for further crushing, grinding and flotation. Grasberg’s milling and concentrating complex is the largest in the world, with four crushers and two giant semi-autogenous grinding (SAG) units processing a daily average of 240,000t of ore.
A flotation reagent is used to separate concentrate from the ore. Slurry containing 60-40 copper concentrate is drawn along three pipelines to the seaport of Amamapare, more than 70 miles away, where it is dewatered. Once filtered and dried, the concentrate – containing copper, gold and silver – is shipped to smelters around the world.
The facilities at the port also include the PT Puncak Jaya coal-fired power station, which supplies the Grasberg operations.
Details of Grasberg Block Cave
Ore extraction from the Grasberg Block Cave underground mine began in H1 2019 and it is expected to produce an additional 272,000Moz of copper and 14Moz of gold over the mine life.
Total undercutting in the underground mine as of July 2019 stands at 48,000m², while the number of active production blocks increased to three.
Ore extraction increased to an average of 17,000t a day in December 2019.
Production is expected to subsequently increase to an average of 30,000t of ore per day in 2020, 60,000t in 2021 and 130,000t in 2023 from a total of five production blocks.
The ore is transported to the processing facility by a fully autonomous underground railway system.