The Potential Impacts of Gold Mining in Virginia (2023) / Chapter Skim
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3 Modern Gold Mining Operations
3 Modern Gold Mining Operations
Pages 47-90
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From page 47... ...
Because high-grade ore deposits exposed at the surface have largely been discovered and depleted by mining, mining companies are increasingly looking deeper in the Earth or at lower-grade deposits to maintain supply. Recent improvements in technology have also led to profitable production from previously uneconomic, mineralized deposits, or from remining of material that was historically considered "waste." For example, remining of waste rock is currently being considered at the Brewer Gold Mine Superfund Site in South Carolina.
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From page 48... ...
48 THE POTENTIAL IMPACTS OF GOLD MINING IN VIRGINIA FIGURE 3-1 The life cycle of a gold mine. Stakeholder engagement is a necessary component during all stages of the life cycle and is shown in red, whereas technical, regulatory, and economic considerations by the industry are shown in black/gray.
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From page 49... ...
The methods employed for gold exploration are very similar to exploration for other metallic and nonmetallic mineral resources. Typically, exploration efforts for new mineable prospects are extensions of currently operating mines or nearby regions with similar geologic conditions.
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From page 50... ...
Near-surface and lower-grade deposits are mined with surface mining methods while deeper and/or higher-grade deposits are generally mined with underground mining methods. The quantity and quality of analytical data (i.e., the level of confidence in the size and grade of the deposits)
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From page 51... ...
The two main categories of mining methods are surface and underground mining, with different mining methods within each category. Mines use surface mining methods for near-surface deposits and underground mining methods for deposits that are too deep to access with surface mining methods.
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From page 52... ...
. Virginia and the geology described in Chapter 2, the likely surface mining method is open pit mining while the likely underground mining methods are those suitable for vein-type gold deposits (see Figure 3-4)
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From page 53... ...
, with the ore transported by mining haul trucks or conveyor belts from the mine to the process plant or leach dump piles. Waste rock or any non-ore material that is encountered through mining is also transported out of the pit to waste dumps.
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From page 54... ...
Cut-and-fill mining is more expensive than other mining methods because mined-out areas are completely backfilled. The backfill may simply be broken rock, but more likely it is a mix of cement and waste rock, a hydraulic sandfill, or a cemented paste fill (possibly made with spent mine tailings)
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From page 55... ...
In underground mining methods, the rock is fragmented during drilling and blasting. Ammonium nitrate-based blasting agents, similar to open pit blasting, are used underground.
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From page 56... ...
. As described in the exploration section, the primary dust control method used by modern mines is water sprays in different forms to suppress dust emissions, but mines can also limit dust emissions by covering conveyor belts and promptly revegetating disturbed areas.
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From page 57... ...
The impacts of underground mines on biodiversity and stream sediment loading are similar to those of surface mines, although the scale tends to be smaller because the amount of disturbed surface land is less. Surface and underground mining activities can also affect water quantity and quality.
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From page 58... ...
Because, as discussed in Box 1-2, the committee considered "gold deposits" to be those deposits where gold is a primary metal of material economic interest, the following discussion of gold processing does not delve into the metallurgical approaches that may be adopted to treat volcanic massive sulfide deposits. Figures 3-9 and 3-10 are schematic, simplified flowsheets depicting two different processing routes deemed most likely for treating gold ores in Virginia.
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From page 59... ...
It is also possible, given the relatively small scale of many of the gold ore deposits in Virginia and the capital costs associated with building a processing plant, that a mining company would only produce a concentrate on site, using either gravity separation methods, flotation, or both, and then ship it elsewhere, including to another state or country, for leaching and gold recovery. This approach was adopted at the Jamestown Mine in northern California, with concentrates being shipped to Nevada for processing, as well as in the Melville gold mine in Orange County, Virginia, in the 1930s, with concentrates being shipped to the American Metal Company smelter in Carteret, New Jersey, for processing.
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From page 60... ...
Several modern mines in South Carolina are described below, but because of the differences in shape and size, these mining methods used are unlikely to be adopted at new gold mines in Virginia. Haile Gold Mine • Mining methods: Gold ore is currently mined in open pits.
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From page 61... ...
Referring to Figures 3-9 and 3-10, if the ore is to be treated by heap leaching, comminution would generally end with secondary crushing. This achieves particles small enough that the leach solution and air can access all parts of the ore particle, and transport gold away, over the duration of treatment in the heap.
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From page 62... ...
Concentration Depending on the mineralogy and several site-specific factors, some gold ores would proceed directly to leaching after grinding. Others would be amenable to concentration, or could be leached more effectively as a concentrate.
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From page 63... ...
These tailings go to solid-liquid separation in "thickeners" where some process water is recycled and the remaining solid-water mixture goes to an impoundment. As noted above, it is possible that the processing at small mines ends with the production of a gold concentrate, which is transported elsewhere for leaching and gold recovery.
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From page 64... ...
These ores contain carbonaceous material that readsorbs gold after leaching, thereby significantly reducing gold recoveries, and oxidation of these ores is aimed at destroying the carbonaceous material. Such carbonaceous-rich gold ores are not known in Virginia, and chlorine/hypochlorite leaching is no longer used, hence this approach is not discussed further here.
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From page 65... ...
Sulfidic gold ores may be biooxidized in heaps (the engineering and operation of which is discussed below) , in which case the ore is agglomerated with an acidic solution containing appropriate microorganisms selected to withstand the operating temperature, which in turn would depend on the sulfide content of the ore (Brierley, 2016)
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From page 66... ...
Any gold ores mined in Virginia would almost certainly be treated by leaching, although, as mentioned above, this need not be done at the mine site. While mercury was previously used in historic mines for gold recovery, it is no longer employed in modern commercial gold mining operations because of its toxicity, propensity to bioaccumulate in food webs, and persistence in the environment (see Box 3-2)
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From page 67... ...
In the United States, and globally, stirred tank leaching produces significantly more gold than heap leaching. Nevertheless, because two gold mines in South Carolina (Brewer and Barite; see Box 3-1)
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From page 68... ...
Instead, leach piles are constructed within valleys, with an engineered dam to retain ore during leaching along with the liners needed to retain gold-laden leach solutions. Given the typical landscape in the Virginia Piedmont, any heap leaching that may be used would most likely be on flat leach pads or valley fill, depending on the local topography at the site.
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From page 69... ...
(B) Valley-fill heap leach operation, with impoundment dam toward the bottom of the image.
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From page 70... ...
In contrast with heap leaching, tank leaching has higher capital and operating costs, but is much faster, on the order of a day or so, and may yield higher recovery. As such, it may be economically comparable to heap leaching, depending on site-specific considerations.
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From page 71... ...
In a carbon-in-leach process, carbon is present in all leach tanks. After heap leaching, clarified rich leach solution flows through columns loaded with carbon, which adsorbs gold from the leach solution.
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From page 72... ...
Environmental Risks Associated with Modern Processing The potential impacts associated with gold processing include noise generated by machinery, the emission of air pollution, and the potential release of chemicals to surface or groundwater. A properly designed and operating plant will control such emissions.
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From page 73... ...
Virginia hosts diverse wildlife including many species that are dependent on surface water, such as pond breeding amphibians, that might require modified types of deterrents to ensure their efficacy. Tank leaching has less potential than heap leaching to accidentally harm wildlife, because the leach solutions are stored in tanks in operating plants that are less accessible/attractive to wildlife.
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From page 74... ...
Waste Rock In open pit mining, the volume of waste rock can be considerable, while in underground mining the volume is relatively limited. Waste rock typically is piled and stored on the mine property in nonmineralized rock facilities, also known as waste dumps or waste rock facilities.
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From page 75... ...
. In the first two cases, the tailings can be mixed with waste rock in various proportions prior to disposal.
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From page 76... ...
Environmental Risks Associated with Waste Management The potential impacts associated with waste material include the production of fugitive dust from the transport of waste material, the potential failure of tailings dams releasing toxic material downstream, and the failure to capture acidic, nitrate-laden, and metal-laden drainage from waste material. Of these, the potential failure of tailings dams is likely the least common, but most serious.
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From page 77... ...
MODERN GOLD MINING OPERATIONS 77 FIGURE 3-21 Above-ground tailings storage facility construction methods: (A) downstream, (B)
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From page 78... ...
. For example, two tailings dam failures at the Mochikoshi Gold Mine were associated with the 1978 Izu-Ohshima-Kinkai magnitude 7.0 earthquake in Japan (Ishihara, 1984)
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From page 79... ...
Naturally Occurring Water and Precipitation The diversion of naturally occurring springs, seeps, and surface water features around mine disturbance areas is sometimes necessary during development and operation. Other management techniques for naturally occurring surface water, groundwater, and precipitation may involve the removal of water from within or near pits, as well as from tailings storage facilities or underground mines.
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From page 80... ...
Environmental Risks Associated with Water Management The diversion or depletion of surface water sources or removal of groundwater from mine areas could have impacts on downstream or down-gradient areas (see Chapter 4) unless flows are returned to the systems from which
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From page 81... ...
Water quality impacts can occur from seepage from impoundments or ponds, storm runoff or infiltration on disturbed surfaces or waste disposal areas, reactive mineral surfaces within underground mines, or open pits that affect adjacent groundwater. Additionally, water quality impact may occasionally occur from loss of containment from impoundments, ponds, tanks, or other retaining structures, which may be triggered by seismic or storm events.
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From page 82... ...
constructed in nonreactive bedrock are likely much more stable than unconsolidated waste rock or fill materials described above. If the approved postmining land use would allow highwalls to remain at closure, then the highwalls may remain uncontoured at a steeper angle than unconsolidated material.
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From page 83... ...
Wet closure could include a permanent pond or wetland feature within an impoundment or other disposal area, or the in-pit storage of waste rock or tailings beneath a pit lake. The goal of this method is to keep the waste rock, highwall or underground mine surfaces, and/or tailings in a saturated, lowoxygen environment.
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From page 84... ...
Wet closure methods for tailings impoundments are not consistent with current best practices for geotechnical stability, so a thorough analysis of site-specific conditions and facility designs should be conducted prior to implementing such a closure system. Water Management During Reclamation Similar to during mine operations, water quantity management during reclamation and closure may include dewatering of pits, tailings storage facilities or underground mines to control water levels and flow paths.
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From page 85... ...
The Brewer pit was then backfilled with rinsed heap leach tailings, wastewater treatment sludge, demolition debris, and waste rock, and the B-6 pit was backfilled with sulfide-rich waste rock. A geosynthetic liner was installed over the pit area and a limestone-filled drainage conduit was constructed within the backfill to passively adjust the pH of the water while it drained from the pits.
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From page 86... ...
Closure of Portals Reclamation for underground mines include closing or securing entrances to underground workings in order to prevent access by the public, as well as to manage the potential release of water from underground excavations due to rising groundwater elevations or during periods of flooding. This might include installing a secure gate or similar barrier across the portal opening, or may include using backfill (e.g., mine waste such as waste rock or tailings)
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From page 87... ...
Environmental Risks Associated with Reclamation, Closure, and Long-Term Stewardship There are a number of potential impacts during reclamation and closure that are similar to the impacts of mine operation. These include the production of fugitive dust during grading, emissions from vehicles, and the production of waste by-products from water treatment as described in operational water management.
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From page 88... ...
Here, the committee highlights mine sites that are thought to broadly reflect the scale and type of mining operations that could occur in Virginia, while cautioning that engineering design is influenced by many factors beyond those considered here, including site-specific considerations such as climate, proximity to population centers, current use of the site, and local infrastructure. The currently active Kensington Mine in southeast Alaska broadly reflects the underground mining and processing methods that could be utilized in Virginia.
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From page 89... ...
This style of surface mining may be more comparable to the Harvard Pit at the Jamestown Mine in the Mother Lode district of California (Savage et al., 2009) , which started as small open pits to recover near-surface gold, followed by underground mining to access deeper ores.
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From page 90... ...
If underground mining is used, subsurface storage of tailings and waste rock could reduce surface storage and related impacts to air and water quality. Processing methods for gold ores in Virginia would likely include some on-site methods (e.g., crushing, grinding, flotation)
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