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HARDROCK MINING ON FEDERAL LANDS APPENDIXES
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HARDROCK MINING ON FEDERAL LANDS This page in the original is blank.
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HARDROCK MINING ON FEDERAL LANDS Appendix A The Nature of Mining The purpose of this Appendix is to provide an introduction and basic understanding of the nature of ore deposits and various mining activities. Some environmental issues are mentioned, but a detailed discussion of potential environmental impacts of mining activities is presented in Appendix B. Appendix D includes a discussion of the directions that mining will likely take in the future, along with identification of research needs for the industry, particularly regarding environmental issues. DESCRIPTION OF ORE DEPOSITS Ore deposits form as variants of such geologic processes as volcanism, weathering, and sedimentation operating with an extraordinary intensity. Ore deposits typically are parts of large-scale (several miles across and perhaps just as deep) ore-forming systems in which many elements, not just those of economic interest, have been enriched. For example, arsenic, antimony, thallium, and mercury are commonly enriched in or near Carlin-type gold deposits. Explorationists continually seek to discern trace chemical haloes or geophysical patterns to combine with geological observations and concepts to recognize faint clues to the location of the ore deposit. Known ores constitute less than 1 part in 10,000 of the metal endowment of the upper 1 km of continental crust; thus, by far the largest portion of metals resides in ordinary rocks as a low-level background geochemical signature in amounts too meager for economic mining. Most ore deposits that cropped out in the lower 48 states have been discovered; new discoveries come from unrecognized extensions of known deposits or from “blind” ore bodies buried under alluvium or sedimentary or volcanic rocks. The Twin Creeks deposit near Getchell, Nevada is a good example of such an ore body. Many discoveries of ore deposits are made in or near old mining districts, in part because the same hydrothermal system that formed the high-grade, easy-to-find deposits that were previously mined commonly formed other deposits nearby (either
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HARDROCK MINING ON FEDERAL LANDS laterally or deeper). The Round Mountain deposit mentioned later is an excellent example. Many hardrock commodities are associated with magmatic and hydrothermal processes (Guilbert and Park, 1986). These processes, in turn, are associated with modern or ancient mountain belts. Mountainous or sparsely vegetated terrains, such as those in the western states, expose possibly productive rocks much more fully than do, for example, the mid-continent prairies. In addition, the West is blessed with geologic conditions, including abundant igneous rocks and associated hydrothermal systems, that have led to the formation of ore deposits. Thus, the prime prospecting ground is in land that many people regard as valuable for aesthetic reasons, which creates potential for conflict among uses of the land. Nonetheless, society requires both a healthy environment and sources of materials, many of which can be supplied only by mining. One of the common features of most ores and the reason underlying much environmental concern is the presence of large quantities of sulfur, usually as the mineral pyrite (FeS2), which creates the potential for environmental problems caused by acid runoff (see Sidebar 3–1 and Appendix B). A few mineral types (e.g., chromium or aluminum and placer deposits of gold, titanium, or tin) contain little sulfur and constitute minimal geochemical environmental problems. Many sulfide-bearing deposits contain abundant carbonate minerals, which buffer acid produced from the oxidation of sulfides, and many sulfide deposits that have been thoroughly oxidized by natural weathering no longer pose an acid mine drainage threat. Many hydrothermal ore deposits have haloes of low-grade mineralization surrounding the ore. With better technologies (or higher prices) low-grade waste rock can become ore. A good example is the Round Mountain mine in Nevada (Figure A-1), where early production (350,000 ounces of gold from 1906 to 1969) was from high-grade veins mostly mined underground. From 1977 through 1997 about 4 million additional ounces were produced from a large open pit exploiting relatively low-grade disseminated mineralization in the rock surrounding the high-grade veins mined long ago; the projected eventual output will be more than 11 million ounces of gold (Tingley and Bonham, 1998), worth close to $3 billion at the current price of approximately $250/ounce. Heap leaching has made possible the economic recovery of gold from low-grade ores. In some of today's major mining operations, grades can be lower than 0.01 ounce of gold per short ton of ore (0.3 gram of gold per metric ton of ore). Typical grades for open-pit gold mines in Nevada are about 0.07 ounce of gold per short ton of ore; underground mining is more costly and requires ores of about 0.3 ounce of gold per short ton to be profitable (Tingley and Bonham, 1998).
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HARDROCK MINING ON FEDERAL LANDS FIGURE A-1 Map view of the Round Mountain gold Mine. SOURCE: Tingley and Berger. 1985. Round Mountain Gold Corporation. 1994. Some minerals seldom occur in sufficient quantities to be mined just for themselves. Instead they are recovered as by-products or co-products with other commodities. Examples of by-products are cadmium, indium, gallium, germanium, and thallium with lead-zinc ores and rhenium with copper-molybdenum ores. Coproducts (e.g., gold and copper in many porphyry copper deposits, lead and zinc in many deposits hosted in limestones, and silver and gold in many epithermal vein deposits) augment the value of each other and make deposits economic that might be unmineable for only one of them.
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HARDROCK MINING ON FEDERAL LANDS Most metallic mineral deposits that formed by magmatic or hydrothermal processes have complex geometries that influence the selection of mining method; in contrast, most economically viable coal deposits are relatively flat-lying sedimentary rocks. Some examples are illustrated in Figure A-2. As a result, metallic mineral deposits are typically mined in a downward or vertical fashion, while coal deposits are normally mined horizontally. As stated in the COSMAR report (NRC, 1979), backfilling of large open pits for hardrock mines is generally impractical, because the irregular lateral and vertical extents of the metallic deposits normally preclude them from being strip mined as simply as coal deposits, wherein overburden and waste rock can be recycled as backfill while mining proceeds from one area to the next. THE MINING PROCESS The mining process consists of exploration, mine development, mining (extraction), mineral processing (beneficiation), and reclamation (for closure). The first three steps in the mining process are characterized by dramatically increasing costs. Exploration The primary objective of exploration is to find an economic mineral deposit. The initial step in exploration is prospecting, which is the search for indications of a mineral deposit of potential significance. The objective is to define a target that suggests the occurrence of a mineral deposit worthy of subsequent testing with one or more exploration methods. These targets frequently result from the compilation and analysis of data germane to the type of mineral occurrence sought. Target identification may result from relatively unobtrusive and proven approaches, such as detailed surface geologic mapping, sampling of outcrops, and observation of alteration patterns. More typically, modern prospecting embraces various types of geochemical sampling, geophysical techniques, satellite remote sensing, and other sophisticated methods for identifying the often subtle expressions of deeply buried mineral deposits. In general, the environmental impact of prospecting is minimal. Surface access to or near the prospect is the primary requirement. When exploration on federal lands indicates a valuable deposit, the prospector acquires rights, if not previously completed, to develop it by staking claims and recording them with appropriate agencies. The area required for a large mine and its facilities
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HARDROCK MINING ON FEDERAL LANDS FIGURE A-2 Styles of mineralization of Carlin trend gold deposits. (Coope. 1991. p. 14).
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HARDROCK MINING ON FEDERAL LANDS (e.g., waste dumps, tailings ponds) is frequently a few thousand acres. Because of intermixed ownerships, this often involves a combination of federal and private lands for a single mine. Exploration continues with target testing. Target exploration techniques vary; however, they usually focus on confirming the presence of a deposit and delineating its size, shape, and composition. In nearly every case, the primary exploration tool is drilling. Drilling into the subsurface is performed to determine both the lateral and vertical extent of the deposit, as well as other important deposit characteristics (e.g., continuity of mineralization, grade and associated trends, mineralogical relationships and characteristics, rock types, and local hydrologic information). Surface disturbance resulting from exploration activities increases as these programs progress from preliminary to detailed. Most surface disturbance results from the construction of access roads and drill sites. The spacing or distance between drill sites is site-specific, but may be 100 feet or less in some geologic situations. Wider drill spacings may be acceptable for predicting mineral continuity and for deposits that are reasonably well understood and exhibit predictable results over greater intervals. Closer drill spacing is required for other types of deposits containing more variable or erratic mineralization. Surface disturbance also may result from trenching activities across zones of mineralization or from the collection of a larger bulk sample for metallurgical testing. The subsurface environment also may be affected by the drilling program, although the effects are usually minor due to the use of practices such as sealing holes and the proper disposal of drill cuttings. A more significant subsurface impact is often that associated with the collection of a large bulk sample at depth for metallurgical testing. In contrast with most other industries, hardrock mining has few alternatives relative to location, because economic occurrences of minerals are geologically and geographically scarce. Only a very small portion of Earth's continental areas, certainly less than .01%, contains the economic portion of its non-fuel mineral endowment. Thus, one cannot arbitrarily decide to build a mine here or there, but rather one must discover and mine those few places where nature has hidden its minerals (see Sidebar 1–4). Over the last century and a half the western lands owned or formerly owned by the federal government have yielded metals worth hundreds of billions of dollars at today's prices. These lands continue to be major producers of gold (Figure A-3) and many other mineral resources. The U.S. Geological Survey estimates that many new deposits remain to be discovered (U.S. Geological Survey, 1999), but they will be deeper and more costly to find and extract.
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HARDROCK MINING ON FEDERAL LANDS Mine Development When a deposit identified and defined from the exploration program has been judged economic and the required permits have been obtained, the next activity is mine development to prepare the deposit for extraction or mining. Infrastructure (e.g., power, roads, and water) is put in place throughout the mine and physical facilities. Planning and construction of support facilities (e.g., offices, maintenance shops, fuel bays, and materials handling systems) and the mineral processing facility is completed. Surface locations are delineated and prepared for waste or overburden material placement, heap leach piles (the chemical leaching of low-grade ores), stockpiles, tailings impoundments, and so on. Near-surface deposits in open-pit mines are prepared initially for production by removing the overlying waste material or overburden for placement at surface waste dumps. These dumps are engineered structures that continue to grow both laterally and vertically until mining ceases; occasionally it may be possible to return waste to exhausted portions of the pit as mining progresses. Deeper deposits designed for underground mining are developed initially by gaining access to the mineralization through vertical or inclined shafts or horizontal adits. Underground drifts, crosscuts, raises, and ramps are excavated to provide the access needed to mine the ore. At this stage in the mining process the nature of the environmental impacts is different and their magnitude is greater than those associated with exploration activities. The footprint of the mining operation is essentially defined at this point. Mining (Extraction) As mine development nears completion, there is a transition to the mining component, which refers to extraction of the in-place mineralized material, as well as associated waste rock as the mine becomes deeper and grows laterally. Even though every ore deposit is different, generalizations can be made about the basic functioning of mining. Whether an ore deposit is mined by surface (open pit) or underground, most mines use the same basic operations:
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HARDROCK MINING ON FEDERAL LANDS FIGURE A-3 Historic gold production in the United States Modified from Dobra, 1999.
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HARDROCK MINING ON FEDERAL LANDS drilling, blasting, mucking (loading), and transporting (hauling). At times a fifth operation of extending services, supplies, and ground control is needed. Drilling and blasting refer to the drilling of holes for placement of explosive materials and detonation of the explosives contained in the drill holes. After blasting, the fragmented rock (muck) is typically loaded into some form of transportation system, in which it begins its journey to the mineral processing facility. As mining progresses, open pits are excavated on the surface or a maze of underground openings or voids are created where the in-place ore was removed. In some cases, these relatively large underground voids (slopes) in the deposits may be backfilled with waste material, either for convenience or to enhance structural support, provide safety, or improve ore recovery. Continued mining activities necessarily result in larger mines, along with growing waste dumps, heap leach piles, tailings ponds, etc. Many of the environmental impacts become obvious as one scans the surface; other impacts (i.e., impacts on the associated hydrologic regime) are more subtle and perhaps not so apparent. Placer mining, in which gold is extracted from stream or beach sediments by gravity separation, is different from typical hardrock mining and mineral processing in several respects. Two types of placer mining are common on federal lands: (1) mining with mechanized earth-moving equipment and (2) suction dredging in streams. Placer mining with mechanized earth-moving equipment typically involves relocating a short (on the order of 2,000 feet) stretch of a stream, removal of the vegetative mat or soil, mining of gravels, removal of gold with sluices that separate dense from light minerals, and reclamation by replacement of gravel and the vegetative mat or soil. Suction dredging is a technique that uses a pump to suction sediment from the stream bottom and process it in a floating sluice. Suction dredges can be small enough to be “recreational” one-person operations. States regulate mechanized placer mining both with suction dredges and with earth-moving equipment. The practice of hydraulic mining, wherein massive jets of water erode mineral-bearing gravels and wash them through an extraction facility, is practiced in a few places in the United States today, and these operations must comply with state and federal water quality discharge requirements. Mineral Processing (Beneficiation) Mineral processing, or beneficiation, consists of upgrading or concentrating the ore material before the concentrate is transported to a smelter or refinery. It begins by crushing and grinding the ore into small particles, thereby releasing interlocked ore grains from each other and from waste mineral
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HARDROCK MINING ON FEDERAL LANDS grains. These particles are then subjected to various physical or chemical processes to separate and concentrate the valuable minerals from the unwanted waste and deleterious substances in the ore. Separation is designed specifically to take advantage of the unique physical or chemical properties of the valuable mineral(s). In many instances, various chemicals and reagents are used in the separation process. Most notably with certain copper ores, metal can be produced on site directly through treatment of the ores by a process called solvent extraction/electrowinning. This process avoids the need to produce a concentrate prior to smelting. The waste or unwanted minerals (tailings) separated from the valuable minerals, now in concentrated form, are routinely disposed of in a tailings pond near the mine site; the associated water is typically recycled, treated, and used in subsequent mining or processing activities. Tailings generally contain small amounts of the valuable mineral(s) not completely recovered during beneficiation; some unwanted or undesirable deleterious minerals; waste rock minerals; and some of the chemicals that may have been used in the course of the separation process. Underground mines may use these tailings to backfill mining-created underground voids. Leaching is an increasingly common alternative extraction practice, almost universally applied (as cyanide solution) to the ore mined from some gold mines and (as ferric sulfate/sulfuric acid solution) to low-grade copper ore. In heap leaching, run-of-mine ore can be leached without crushing, or it is crushed only fine enough to allow the lixiviant (leaching solution) access to most of the mineral grains. The solution percolates through the piled ore, and the pregnant solutions are pumped to a processing facility for extraction. Then the fluids are treated and recycled. In some cases, uranium ores are leached in situ without mineralized material being removed or mined in a conventional manner (see following section on the uranium industry). While tailings dams and ponds and leach pads are carefully designed to high standards, the potential impacts resulting from release or discharge of tailings, leached rock, or pregnant leach solutions can be substantial. Reclamation Reclamation returns the mining and processing site to beneficial use after mining. A mine should not be closed until reclamation is complete, but as discussed extensively in this report, in some circumstances reclamation may never be fully accomplished, and long-term monitoring will be necessary. Some common reclamation practices include lessening the slopes on the edges of waste rock dumps and heaps (to minimize erosion); capping these
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HARDROCK MINING ON FEDERAL LANDS piles and tailings piles with soil; planting grasses or other plants that will benefit wildlife or grazing stock and help prevent erosion; directing water flow with French drains and other means to minimize the contact of meteoric water with potentially acid-generating sulfides in the waste rock dumps, heaps, and tailings piles; removing buildings; and eliminating roads to minimize unnecessary future entry by vehicles. THE URANIUM INDUSTRY The General Mining Law of 1872 classifies uranium as a hardrock mineral. Because special provisions apply to uranium, a brief discussion of regulatory issues related to uranium mining is given in this appendix. During the uranium “rush” of the 1950s and 1960s, a large number of mining claims were staked, and many uranium mines were developed on federal lands in Utah, Colorado, Wyoming, and other western states. However, because of the low price of uranium in the late 1990s, few uranium mines remain operating in the United States, primarily on BLM, private, and Indian lands. The need for uranium, principally as a fuel for commercial power plants that generate approximately 20% of U.S. electric power, is increasingly being met by imports. Uranium is also recovered as a by-product of other mineral mining activities, such as the production of phosphoric acid (from phosphate rock). In the United States uranium is also recovered from a variety of low-grade uranium-bearing rocks, from the mineral wastes of various processing facilities, and from groundwater that issues from an abandoned underground mine in New Mexico. According to the EPA (1993), approximately 4 billion tons of mine-related wastes have been generated from surface and underground uranium mines. From the 1970s through the early 1990s open-pit mining produced almost 90% of that waste. According to the EPA, of the more than 1,300 open-pit uranium mines that have operated in the United States, most have been small, with only about 300 having a total ore production of more than 900 metric tons. The Nuclear Regulatory Commission regulates uranium processing on public and private lands. It does not regulate uranium mining by traditional mechanical methods, but it does regulate in situ solution mining, because that extraction method is considered a form of processing. Traditional uranium ore mining is regulated by the usual federal and state agencies. Uranium mill tailings are regulated under the Uranium Mill Tailings Radiation Control Act (UMTRCA), which mandates special closure designs for uranium mill tailings ponds to prevent unacceptable release of radon gas to the environment. Currently, there are only a few small underground uranium mines, and no active open-pit mines, operating in Colorado and Utah. Since the mid-1970s, in situ solution mining has been a commercially viable method for extracting
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HARDROCK MINING ON FEDERAL LANDS uranium from subsurface ore bodies in porous sandstone. In situ uranium mining involves the injection through a borehole of an alkaline, oxidizing leaching agent (lixiviant) into the uranium-bearing sandstone at depth. The uranium is recovered, through production wells adjacent to the injection wells, from the pregnant lixiviant by passing the solution through ion-exchange columns; afterwards, the solution is chemically reconstituted and re-injected into the subsurface. The lixiviant consists of an aqueous solution of sodium carbonate-bicarbonate and/or carbon dioxide, and oxygen. In situ mining occurs in Wyoming, Texas, and Nebraska. As mentioned earlier, the Nuclear Regulatory Commission is involved in the permitting and regulation of in situ uranium mines because the leaching of the ore is considered a form of chemical processing. The injection wells are considered by the EPA to be Class 3 underground injection wells, subject to regulation in the Underground Injection Control Program (40 CFR 143–147). Industry personnel report a significant degree of overlap between the regulatory requirements of the Nuclear Regulatory Commission and the EPA for injection wells. Several states have primacy over the Nuclear Regulatory Commission and the EPA for regulating radiation issues and underground injection. The primary risk associated with in situ uranium mining is the potential for contamination of adjacent groundwater. If the system of injection and production wells is not properly designed and constructed, the pregnant lixiviant may escape into the sandstone aquifer, carrying with it dissolved uranium and radium. Small amounts of several trace metals are also present in the lixiviant, including lead, selenium, molybdenum, and arsenic. The requirements of the Underground Injection Control Program include restoration of the aquifer after completion of in situ mining. In situ mining also has been applied to other metals, especially copper, but not to the extent it has been used for uranium extraction. Reclamation of uranium mines includes the typical steps considered at most other mine sites. Waste rock piles are regraded and revegetated, with the major objectives usually being control of erosion, physical stability, and restoration of the land for such uses as grazing and wildlife habitat. Open pits are evaluated relative to the physical stability of high walls and land use objectives, including pit lake water quality. Depending on the situation, the reclamation of a uranium mine may also include radiation-specific objectives. For example, reclamation of the large Jackpile mine in New Mexico used objectives for radiation control similar to the mill tailings program. At the Jackpile mine, a cover was constructed to limit the radiation exposure from the waste rock piles. Overburden enriched in uranium was placed in the bottom of the mine pit prior to backfilling. This selective burial of higher-grade overburden is consistent with reclamation practices used elsewhere, as in the
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HARDROCK MINING ON FEDERAL LANDS Wyoming and Texas abandoned mines reclamation programs (under the Wyoming Environmental Quality Act of 1973 and the Texas Surface Mining Act of 1975). Most western states have regulations requiring the submission of a reclamation plan and bond prior to the issuance of a mining permit. Although these programs are not designed to mitigate potential radiological risks, such measures do reduce potential exposure to direct radiation, radon gas, fugitive dust emissions, and the possibility that waste materials will be removed and put to other uses, such as construction. Many mines developed prior to implementation of regulations requiring reclamation are not yet reclaimed. Finally, there are examples of uranium mine reclamation in Superfund environments that follow the very stringent guidelines for reclamation specified in the UMTRCA.
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