1. Overview of Mining and its Impacts1 Proposed mining projects vary according to the these metal ores are the subject of this Guidebook. type of metals or materials to be extracted from the The Guidebook does not discuss the mining of earth. The majority of proposed mining projects ores that are extracted using strip mining methods, involve the extraction of ore deposits such as including aluminum (bauxite), phosphate, and copper, nickel, cobalt, gold, silver, lead, zinc, uranium. The Guidebook also does not discuss molybdenum, and platinum. The environmental mining involving extraction of coal or aggregates, impacts of large-scale mining projects involving such as sand, gravel, and limestone. 1.1 PHASES OF A MINING PROJECT There are different phases of a mining project, EIA for the exploratory phase of a mining project beginning with mineral ore exploration and because the impacts of this phase can be ending with the post-closure period. What profound and because further phases of mining follows are the typical phases of a proposed may not ensue if exploration fails to find sufficient mining project. Each phase of mining is quantities of high-grade mineral ore deposits. associated with different sets of environmental impacts. 1.1.2 Development 1.1.1 Exploration If the mineral ore exploration phase proves that there is a large enough mineral ore deposit, of A mining project can only commence with sufficient grade, then the project proponent may knowledge of the extent and value of the mineral begin to plan for the development of the mine. ore deposit. Information about the location and This phase of the mining project has several value of the mineral ore deposit is obtained during distinct components. the exploration phase. This phase includes surveys, field studies, and drilling test boreholes 1.1.2.1 Construction of access roads and other exploratory excavations. The construction of access roads, either to The exploratory phase may involve clearing of provide heavy equipment and supplies to the wide areas of vegetation (typically in lines), to mine site or to ship out processed metals and allow the entry of heavy vehicles mounted with ores, can have substantial environmental impacts, drilling rigs. Many countries require a separate especially if access roads cut through ecologically Chapter 1 3 sensitive areas or are near previously isolated In almost every case, metallic ores are buried communities. If a proposed mining project under a layer of ordinary soil or rock (called involves the construction of any access roads, then ‘overburden’ or ‘waste rock’) that must be moved the environmental impact assessment (EIA) for the or excavated to allow access to the ore deposit. project must include a comprehensive assessment The first way in which proposed mining projects of the environmental and social impacts of these differ is the proposed method of moving or roads. excavating the overburden. What follows are brief descriptions of the most common methods. 1.1.3.1 Open-pit mining Open-pit mining is a type of strip mining in which the ore deposit extends very deep in the ground, necessitating the removal of layer upon layer of overburden and ore. In many cases, logging of trees and clear-cutting or burning of vegetation above the ore deposit may precede removal of the overburden. The use of heavy machinery, usually bulldozers and dump trucks, is the most common means of Erosion near a mining road, Pelambres mine, Chile removing overburden. Open-pit mining often PHOTO: Rocio Avila Fernandez involves the removal of natively vegetated areas, 1.1.2.2 Site preparation and clearing and is therefore among the most environmentally- destructive types of mining, especially within If a mine site is located in a remote, undeveloped tropical forests. area, the project proponent may need to begin by clearing land for the construction of staging areas that would house project personnel and equipment. Even before any land is mined, activities associated with site preparation and clearing can have significant environmental impacts, especially if they are within or adjacent to ecologically sensitive areas. The EIA must assess, separately, the impacts associated with site preparation and clearing. 1.1.3 Active mining Open-pit mine in Cerro de Pasco, Peru Once a mining company has constructed PHOTO: Centro de Cultura Popular LABOR, Peru access roads and prepared staging areas that Because open-pit mining is employed for ore would house project personnel and equipment, deposits at a substantial depth underground, it mining may commence. All types of active usually involves the creation of a pit that extends mining share a common aspect: the extraction below the groundwater table. In this case, and concentration (or beneficiation) of a metal groundwater must be pumped out of the pit to from the earth. Proposed mining projects allow mining to take place. A pit lake usually differ considerably in the proposed method for forms at some point in time after mining stops and extracting and concentrating the metallic ore. the groundwater pumps are turned off. 4 Guidebook for Evaluating Mining Project EIAs 1.1.3.2 Placer mining off-site. Mining projects that only involve the reworking of abandoned mine waste piles avoid Placer mining is used when the metal of interest the environmental impacts of open-pit mining is associated with sediment in a stream bed or and placer mining, but still entail environmental floodplain. Bulldozers, dredges, or hydraulic jets impacts associated with purification (beneficiation) of water (a process called ‘hydraulic mining’) of metals from the waste piles. are used to extract the ore. Placer mining is usually aimed at removing gold from stream 1.1.4 Disposal of overburden and sediments and floodplains. Because placer waste rock mining often occurs within a streambed, it is an environmentally-destructive type of mining, In almost every project, metallic ores are buried releasing large quantities of sediment that can under a layer of ordinary soil or rock (called impact surface water for several miles downstream ‘overburden’ or ‘waste rock’) that must be moved of the placer mine. or excavated to allow access to the metallic ore deposit. For most mining projects, the quantity 1.1.3.3 Underground mining of overburden generated by mining is enormous. The ratio of the quantity of overburden to the In underground mining, a minimal amount of quantity of mineral ore (called the ‘strip ratio’) overburden is removed to gain access to the ore is usually greater than one, and can be much deposit. Access to this ore deposit is gained by higher. For example, if a proposed mining project tunnels or shafts. Tunnels or shafts lead to a more involves the extraction of 100 million metric tons horizontal network of underground tunnels that of mineral ore, then the proposed mining project directly access the ore. In an underground mining could generate more than one billion metric tons method called ‘stoping’ or ‘block caving,’ sections of overburden and waste rock. or blocks of rock are removed in vertical strips that leave a connected underground cavity that is These high-volume wastes, sometimes containing usually filled with cemented aggregate and waste significant levels of toxic substances, are usually rock. deposited on-site, either in piles on the surface or as backfill in open pits, or within underground Although underground mining is a less mines. Therefore, the EIA for a proposed mining environmentally-destructive means of gaining project must carefully assess the management access to an ore deposit, it is often more costly options and associated impacts of overburden and entails greater safety risks than strip mining, disposal. including open-pit mining. While most large- scale mining projects involve open-pit mining, 1.1.5 Ore extraction many large underground mines are in operation around the world. After a mining company has removed overburden, extraction of the mineral ore begins using 1.1.3.4 Reworking of inactive or specialized heavy equipment and machinery, abandoned mines and tailings such as loaders, haulers, and dump trucks, which transport the ore to processing facilities using Some mining projects involve the reworking haul roads. This activity creates a unique set of waste piles (often tailings) from inactive or of environmental impacts, such as emissions of abandoned mines, or older waste piles at active fugitive dust from haul roads, which an EIA for a mines. Typically, this is proposed when more proposed mining project should assess separately. efficient methods of metal beneficiation have made it economical to re-extract metals from old mining waste. The material from the piles may be sent to processing facilities on-site or Chapter 1 5 1.1.6 Beneficiation Although metallic ores contain elevated levels of metals, they generate large quantities of waste. For example, the copper content of a good grade copper ore may be only one quarter of one percent. The gold content of a good grade gold ore may be only a few one-hundredths of a percent. Therefore, the next step in mining is grinding (or milling) the ore and separating the relatively small quantities of metal from the non- metallic material of the ore in a process called ‘beneficiation.’ Heap leach, Bighorn gold mine, CA PHOTO: Bender Environmental Consulting Milling is one of the most costly parts of 1.1.7 Tailings disposal beneficiation, and results in very fine particles that allow better extraction of the metal. However, As previously discussed, even high-grade mineral milling also allows a more complete release ores consist almost entirely of non-metallic of contaminants when these particles become materials and often contain undesired toxic tailings. Tailings are what remains following metals (such as cadmium, lead, and arsenic). milling of the ore to fine particles and extraction of The beneficiation process generates high-volume the valuable metal(s).