Overburden is the rock that must be moved aside to access ore to be mined for further processing to produce metals for commercial purposes. PT Freeport Indonesia handles overburden under a comprehensive Overburden Management Plan approved by the Government of Indonesia. Most metals occur in nature as minerals called sulphides. When ore is mined and overburden containing sulphides is left exposed to the elements, the action of water, oxygen and natural bacteria has the potential to create sulphuric acid. This acidic water can dissolve metals contained in overburden rock which has the potential to be transported in water drainage systems and, if not properly managed, to cause adverse environmental impacts. This process is called acid rock drainage.

PT Freeport Indonesia manages and monitors acid rock drainage from its operations. Independent audits of PT Freeport Indonesia’s environmental management system concluded that our overburden management programs are “…consistent with international practice.” Under the approved overburden management plan, PT Freeport Indonesia places overburden in areas around the open pit. One of these areas, the Wanagon basin, is a relatively small area of approximately six hectares (less than 15 acres) and is gradually being filled with overburden as part of the approved plan.
 

PT Freeport Indonesia’s acid rock drainage mitigation plans provide for the short-term capture and treatment of the existing acid rock drainage, in conjunction with limestone blending and limestone capping of existing overburden placement areas, which is expected to significantly reduce future acid rock drainage generation. Throughout 2002, acid rock drainage was captured in piping systems and directed to the mill area for lime neutralization.

Beginning in mid-2003, overburden material will be placed in the Lower Wanagon Valley, just below the Wanagon basin, using the stacker system previously used to place material in the Wanagon basin. The remainder of the material will be placed in the Wanagon basin using conventional truck placement. Based on a series of trials over the past seven years, an appropriate blend of sulfide-bearing overburden, and acid-consuming limestone mined from the outer limits of the Grasberg mine, will be placed in the Lower Wanagon Valley to ensure that there is minimal production of acid rock drainage in this area.