Overburden and waste rock are materials that must be moved aside to gain access to ore, which is mined and processed to recover metals for commercial purposes. Many metals occur in nature as sulfide minerals. When ore is mined and overburden and waste rock containing sulfides is left exposed to the elements, the action of water, oxygen and naturally occurring bacteria has the potential to create sulfuric acid. If not properly managed, this acidic water can dissolve metals contained in overburden and waste rock and cause adverse environmental impacts to water resources. This process is known as acid rock drainage (ARD).

At our mine sites, we use interdisciplinary engineering and environmental groups and risk assessments to develop technical and environmental approaches to managing overburden and waste rock that meet site-specific topographical and hydrologic needs and environmental conditions. At all our operations, we manage overburden and waste rock in compliance with applicable local regulations, permits and environmental impact studies. Excavated overburden and waste rock are generally segregated into separate stockpiles to facilitate future handling and reclamation. Surface covers, compaction and revegetation to decrease infiltration, water diversion channels, downstream storm water collection ponds, and monitoring are all techniques that can be utilized during or after operations to minimize and mitigate ARD and assure environmental protection.

At many of Freeport-McMoRan Copper & Gold Inc.’s mine sites located in arid climates in the southwest United States, Peru and Chile, the potential for ARD is significantly reduced because of the lack of rainfall to mobilize metals in overburden and waste rock. PT Freeport Indonesia handles overburden under a comprehensive overburden management plan approved by the Government of Indonesia. Independent audits of PT Freeport Indonesia’s environmental management system concluded that our overburden management programs are well integrated and consistent with international practice.

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