20. Geoenvironmental Features By Robert R. Seal II and Nadine Piatak 20 of 21 Volcanogenic Massive Sulfide Occurrence Model Scientific Investigations Report 2010–5070–C U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior KEN SALAZAR, Secretary U.S. Geological Survey Marcia K. McNutt, Director U.S. Geological Survey, Reston, Virginia: 2012 For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment, visit http://www.usgs.gov or call 1–888–ASK–USGS. For an overview of USGS information products, including maps, imagery, and publications, visit http://www.usgs.gov/pubprod To order this and other USGS information products, visit http://store.usgs.gov Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. 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Suggested citation: Seal II, R.R. and Piatak, Nadine, 2012, Geoenvironmental features in volcanogenic massive sulfide occurrence model: U.S. Geological Survey Scientific Investigations Report 2010–5070 –C, chap. 20, 18 p. 321 Contents Weathering Processes .............................................................................................................................323 Sulfide Oxidation, Acid Generation, and Acid Neutralization Processes ........................................323 Metal Cycling Associated with Efflorescent Sulfate Salts .................................................................324 Secondary Precipitation of Hydroxides and Hydroxysulfates ...........................................................324 Pre-Mining Baseline Signatures in Soil, Sediment, and Water .........................................................324 Past and Future Mining Methods and Ore Treatment .........................................................................325 Volume of Mine Waste and Tailings .......................................................................................................327 Mine Waste Characteristics ....................................................................................................................327 Mineralogy .........................................................................................................................................327 Acid-Base Accounting ............................................................................................................328 Element Mobility Related to Mining in Groundwater and Surface Water ......................330 Pit Lakes .............................................................................................................................................334 Ecosystem Issues ......................................................................................................................................335 Human Health Issues ................................................................................................................................335 Climate Effects on Geoenvironmental Signatures ...............................................................................335 References Cited........................................................................................................................................336 Figures 20–1. Graphs showing geochemical data for waters associated with unmined massive sulfide deposits .........................................................................................................326 20–2. Graphs showing geochemical data for major constituents in mine drainage associated with massive sulfide deposits ...........................................................332 20–3. Graphs showing geochemical data for minor constituents in mine drainage associated with massive sulfide deposits ...........................................................333 20–4. Graphs showing geochemical data for dissolved metals in drainage associated with massive sulfide deposits ............................................................................334 Tables 20–1. Selected common mineralogical characteristics of volcanogenic massive sulfide deposits with a comparison to sedimentary-exhalative deposits .......................329 20–2. Environmental guidelines relevant to mineral deposits .....................................................331 20. Geoenvironmental Features By Robert R. Seal II and Nadine Piatak Weathering Processes acid generated by the oxidative weathering of sulfide miner- als. Thus, the chemistry of drainage from a mine site is the Modern weathering processes associated with mine result of the competing processes of acid generation and acid wastes from massive sulfide deposits are similar, in many neutralization. respects, to those operating in the supergene environment The oxidation of pyrite or pyrrhotite and other sulfide after the initial formation of the mineral deposit. Nevertheless, minerals proceeds with either dissolved oxygen (O2) or dis- 3+ some important differences exist. Acid-mine drainage is one of solved ferric iron (Fe ) as the oxidizing agent. Dissolved oxy- the most significant challenges associated with these deposits gen is the most important oxidant at pH values above approxi- due to the abundance of pyrite, pyrrhotite, or both iron sulfides mately 4, whereas ferric iron dominates below approximately and the general lack of any significant neutralizing potential 4 (Williamson and others, 2006). The aqueous oxidation of or alkalinity. The geochemistry of acid-mine drainage has pyrite by dissolved oxygen is described by reaction 1: been reviewed by Nordstrom and Alpers (1999a), and addi- 2+ 2- + tional aspects of the weathering of a variety of ore and gangue FeS2 + 7/2 O2 + H2O → Fe + 2 SO4 + 2 H (1) minerals were discussed by Plumlee (1999). Seal and others (2001a), Seal and Hammarstrom (2003), and Seal (2004) have The aqueous oxidation of pyrrhotite by dissolved oxygen is reviewed the geoenvironmental characteristics of volcanic- described by reaction 2: hosted massive sulfide deposits. 2+ Geochemical aspects of the formation of acid-mine drain- Fe1-xS + (2-x/2) O2 + x H2O → (1-x) Fe + 2- + age and its burden of metals and other elements of concern SO4 +2x H (2) can be divided into three broad topics: (1) sulfide oxidation, acid generation, and acid neutralization processes; (2) metal where x ranges from 0.000 to 0.125. Both reactions 1 and 2 cycling associated with secondary efflorescent sulfate salts; actually represent the mass action of numerous intermediate and (3) secondary precipitation of hydroxides and hydroxysul- reactions. In the oxidative weathering of pyrrhotite, a common fates and associated sorption of metals. initial reaction is the oxidation of pyrrhotite to either pyrite or marcasite as described by the reaction: Sulfide Oxidation, Acid Generation, + 2 Fe1-xS + (1/2-x) O2 + (2-4x) H → FeS2 + (1-2x) Fe2+ + (1-2x) H O (3) and Acid Neutralization Processes 2 The abundance of pyrite and pyrrhotite in massive sulfide Textural evidence of marcasite replacement of pyrrhotite is deposits dominates most aspects of the environmental behav- common in pyrrhotitic mine wastes (Jambor, 1994, 2003; ior of these deposits and their mine wastes. The acid generated Hammarstrom and others, 2001). by their oxidative weathering can aggressively attack other ore The aqueous oxidation of pyrite by ferric iron is and gangue minerals, thereby liberating a variety of potentially described by reaction 4: toxic elements including aluminum and manganese, which are 3+ 2+ 2- + not part of the “typical” ore assemblage of metals but instead FeS2 + 14 Fe + 8 H2O → 15 Fe + 2 SO4 + 16 H (4) are found in silicate and carbonate gangue minerals. These acidic, metal-laden acid-sulfate waters can adversely affect the The aqueous oxidation of pyrrhotite by ferric iron is described surrounding surface- and groundwaters. Within the hydro- by reaction 5: logic system of mine workings or mine wastes, minerals and 3+ 2+ other compounds, such as lime used in flotation circuits, and Fe1-xS + (8-2x) Fe + 4 H2O → (9-3x) Fe + 2- + even monosulfide minerals, such as sphalerite, can neutralize SO4 + 8 H (5) 324 20. Geoenvironmental Features For reactions 4 and 5, where ferric iron is the oxidant, ferrous Evaporative processes can operate during hot arid conditions, iron must be oxidized to ferric iron to perpetuate the reaction within mine workings or other sheltered areas, or in tailings as described by the reaction: piles beneath snow packs. Common secondary sulfate salts in mining environments include melanterite, rozenite, halotrich- 2+ + 3+ Fe + ¼ O2 + H → Fe + ½ H2O (6) ite, alunogen, copiapite, goslarite, and chalcanthite, among numerous others (Jambor, 1994; Jambor and others, 2000; The rate of the oxidation of ferrous iron to ferric iron is greatly Hammarstrom and others, 2001). Gypsum is another common enhanced by the iron oxidizing bacterium Acidithiobacil- secondary sulfate that can contribute dissolved solids to drain- lus ferrooxidans. Singer and Stumm (1970) observed that A. age but does not store acidity or metals. Metal-sulfate salts ferrooxidans increased the rate of oxidation of ferrous iron to offer a means of temporarily sequestering acidity and metals ferric iron by a factor of 100,000 compared to the abiotic rate. for later dissolution during rain events