The Environmental Geology of Mineral Deposits - 2001

Mining & Minerals - Evaluations / Critical Mineral Resources

This is Chapter 3 by Plumlee (USGS) who has summarized many key geologic characteristicsof mineral deposits that control their environmental signatures and their effects on the surrounding environment, both prior to mining and that may result from mining and mineral processing. Careful attention to the environmental geology of mineral deposits  is crucial to all aspects of mineral-resource development, from grass-roots exploration through mine development, closure, and remediation:

• Establishment of pre-mining baseline conditions: It is more cost-effective, technologically feasible, and realistic to remediate mine sites to the background conditions that existed in a mineralized area prior to mining, rather than to remediate to conditions found in unmineralized areas. Geologically constrained baseline data are crucial to establish reasonable pre-mining conditions for diverse deposit types in various climates. These baseline models can then be used to establish analogues for pre-mining conditions in districts where historic mining activities obscure pre-existing baseline conditions.
  
  
• Mineral exploration: Knowledge of the likely environmental effects associated with development of particular deposit types can be integrated into grass-roots exploration efforts. For example, development of deposit types with high geologic potential to generate very acidic mine drainage with extreme metal contents will have the lowest environmental mitigation expenses in arid climates, in deposits with extensive to complete pre-mining oxidation, or in geologic terranes with abundant carbonate rocks. Conversely, development of such acid-generating deposits in wet climates may require such expensive environmental mitigation measures as to either require high ore grades or make development non-profitable.
  
  
• Mine planning and development: Improved predictive capabilities provided by geologic knowledge enable mine planners to better anticipate, plan for, and mitigate potential environmental problems, rather than to try to treat (with much greatertechnical difficulties and costs) the environmental problems after they occur. Similarly, geologic characteristics of a particular deposit can be exploited to help mitigate potential environmental problems. For example, carbonate-bearing wallrock alteration that is often present on the fringes of deposits, or carbonate sediments that occur in the proximity of some deposits, may be useful in helping to mitigate acid drainage from sulfide-rich ore zones.
  
  
• Remediation: Accurate geological, geochemical, and hydrologic information (such as geologic controls on ground-water flow and ore mineralogy) is needed by engineers to develop the most effective remediation plans at mine sites. Many remedial plans implemented in the past ignored or dangerously oversimplified important geologic information. For example, adit plugging has been used or is proposed to reduce acid drainage from a number mine sites. Detailed geologic characterization is crucial to identify the location of faults or other hydrologic conduits that might reduce the effectiveness of adit plugging as a remedial solutions.
  
  
• Abandoned mine lands issues: Although mineral resource extraction has been carried out for several millennia, relatively little attention has been given until the last several decades to minimizing the effects of such extraction on the environment. As a result, a number of historic (meaning they were operated prior to the last several decades) mining and mineral processing sites that were abandoned once the profitable ores ran out are now potential or ongoing sources of environmental contamination. In the United States, land management agencies are currently faced with the daunting task of identifying andprioritizing for remediation all abandoned mine sites on public lands; although a large majority of these sites will not require
  remediation, the total number of sites to be investigated and prioritized for remediation is likely in excess of several hundred thousand. Geologic information provides land managers with a low-cost screening technique to help identify, prioritize for study, and remediate potentially hazardous mine sites on public lands.
• Mineral deposits in a holistic context: In the future, as metal extraction technologies improve (such as techniques to remove metals from waters) mineral deposits may be treated in a more holistic context. For example, acid-mine waters may be viewed as economic resources rather than as environmental liabilities. Techniques to extract metals in situ, or to extract environmentally problematic metals from solid mine wastes or mineral processing wastes may reduce the need for mining with physical extraction techniques, and may turn former wastes into ores. Geologic characterization and mineral-environmental deposit models will help identify mineral deposits or mineral deposit types amenable to such a holistic treatment. Quite simply, geologic knowledge is a fundamental and critical component of environmentally friendly mineral-resource development.