GeochemistryGeochemistry ofof thethe BerkeleyBerkeley pitpit lake,lake, Butte,Butte, MontanaMontana Chris Gammons, Ph.D. Dept. of Geological Engineering Montana Tech of The University of Montana Talk overview •• SiteSite orientationorientation •• HistoryHistory ofof minemine floodingflooding •• GeochemicalGeochemical profilesprofiles inin spacespace andand timetime •• WhyWhy isis waterwater qualityquality soso bad?bad? – Geological controls – Evapoconcentration – Bad influent groundwater – Subaqueous pyrite oxidation •• ConceptualConceptual limnolimno--biobio-- hydrohydro--geogeo--chemicalchemical modelmodel Extent of Butte mine workings (as of 1980) Over 15,000 km of underground mine workings Continental Pit old leach Tailings Pond pads U D HSB Berkeley Pit Image courtesy of NASA History of mine flooding Water balance Water inputs Water outputs • Direct precipitation • Evaporation – Only ~ 12”/yr – Roughly 24”/yr • Pitwall runoff – Not quantified, believed to be minor • Groundwater seepage ~ 10 million L/day • Surface water inputs – Horseshoe Bend – Diverted storm water deep Berkeley Pit chemistry… changes with time 100000 B-Pit 30 to 60 m depth Cu recovery 10000 SO4 1000 Fe Zn Cu 100 dissolved concentration, mg/L 10 Jan-82 Jan-84 Jan-86 Jan-88 Jan-90 Jan-92 Jan-94 Jan-96 Jan-98 Jan-00 Jan-02 Jan-04 Jan-06 Shaded regions show when HSB dumped into pit Shallow Berkeley Pit chemistry… changes with time 100000 B-Pit surface Cu recovery 10000 SO4 Fe 1000 Zn Cu 100 dissolved concentration, mg/L 10 Jan-82 Jan-84 Jan-86 Jan-88 Jan-90 Jan-92 Jan-94 Jan-96 Jan-98 Jan-00 Jan-02 Jan-04 Jan-06 Shaded regions show when HSB dumped into pit Horseshoe Bend Springs • 5 to 10 million L/day (more during active mining) • pH ~ 3.1, SC = 4.6 mS/cm, Cu = 62 mg/L, Fe = 200 mg/L • Lower SC, so floats on top of deep pit lake • Origin is disputed (almost certainly from tailings dam) Resource Recovery: Copper Cementation ~ 150 ppm Cu Berkeley Pit-lake ~ 30 ppm Cu Cu2+ + Fe → Fe2+ + Cu Over 40M lbs Cu dissolved in B-Pit! Process is 75 - 90% efficient Sludge (from HSB treatment) Cementation return flow Copper cementation circuit and lime treatment sludge returning to surface of pit lake Berkeley Pit: pH and Cu profiles over time precip. plant pH return flow Cu2+ 2.0 2.2 2.4 2.6 2.8 3.0 0 50 100 150 200 250 0 50 100 copper cementation 150 May, 1998 Depth, m Depth, May 2003 200 May, 2005 250 Berkeley Pit: SC and Temp profiles over time SC, mS/cm Temperature, C 0.0 2.0 4.0 6.0 8.0 10.0 2 4 6 8 10 12 14 16 0 50 May 1998 100 May 2003 May, 2005 May, 1998 150 Depth, m Depth, May 2003 May, 2005 200 250 warming trend with time since last top-to-bottom turnover Why is Berkeley pit lake so bad? • Geology – Leaching of acid and metals from wallrock • Evapoconcentration – Stable isotopes to quantify E • Input of deep groundwater from undetermined source with poor water quality • Subaqueous pyrite oxidation Geologic cross-section Central Zone Continental Pit Mark Reed, Univ. of Oregon (Newbrough & Gammons, 2002) Berkeley Pit Continental Pit Sericite after feldspar Pyrite + musc after biotite Humidity cell experiments on crushed wallrock (Newbrough & Gammons, 2002 Environmental Geology 41, 705-719) 9 Phase 1 Phase 2 Phase 3 8 7 6 Continental Pit pH 5 Supergene/Central Zone 4 3 Berkeley Pit 2 6/29/99 8/18/99 10/7/99 11/26/99 1/15/00 3/5/00 4/24/00 Date Evaporation calculations: Butte mine waters δ18O -30 -17 -14 -11 F = 90% Butte 80% 0 LMWL 0.5 70% -50 h = 0.6 0.2 100 h = relative humidity 60% 0.7 depth, m -70 200 50% 0.8 VSMOW -90 D, ‰ D, 0.9 40% δ Tailings Pond -110 30% Mine shafts F = % of initial water lost 20% to evaporation -130 10% Berkeley pit-lake -150 -20 -15 -10 -5 0 5 18 δ O, ‰ VSMOW Gammons et al., 2006, J. of Hydrology Where is groundwater coming from? Parrot Tailings Continental Pit Tailings Pond Berkeley Pit 800´ head drop, dh/dl ~ 15% Aerial view of Butte, looking East Subaqueous pyrite oxidation Above water line 1)FeS2 + 7/2O2 + H2O Æ 2+ 2- + Fe + 2SO4 + 2H Below water line 3+ 2) FeS2 + 14Fe + 8H2O Æ 2+ 2- + 15Fe + 2SO4 + 16H Berkeley Pit: Fe profiles over time Fe2+ Fe3+ 0 200 400 600 800 1000 0 200 400 600 800 0 50 100 May 1998 May 1998 May 2003 May 2003 150 May 2005 May 2005 depth, m depth, 200 250 downwards shift with time: subaqueous pyrite oxidation? Iron Cycling Model: Berkeley Pit Air column: Oxygen diffusion Fe-oxidizing Fe3+ ferric precipitates bacteria Lake turnover Fe2+ Particles settle by pyrite gravity pyrite pyrite 3+ oxidation Fe pyrite pyrite Pit sediments Berkeley pit-lake minerals SEM photos taken by Dick Berg at ICAL, Montana State Univ. jarosite schwertmannite III III KFe3 (SO4)2(OH)6 Fe8 O8(SO4)(OH)6 Pit lake sediment 0 20 40 60 depth in core, cm core, in depth 80 -2 -1 0 1 2 3 saturation index schw ertmannite ferrihydrite ferrihydrite K-jarosite H-jaros ite goethite • Pit lake sediment is oxidized to depth of 1 m • Possible transformation of schwertmannite to “aged” ferrihydrite and K-jarosite Twidwell et al., 2006 Berkeley pit lake: Conceptual Model surface inputs Cu cementation landslides ▪ Horseshoe Bend Spring evaporation ▪ lime treatment sludge photochemical reactions? ▪ Cu recovery return flow rain, O diffusion water 2 snow ▪ storm runoff table Fe2+ → Fe3+ → schwertmannite, jarosite epilimnion Fe-oxidizing seasonal leaching of bacteria overturn mixolimnion soluble salts gravitational from weathered Cu recovery settling bedrock deep intake H+, Fe2+ groundwater influx adsorption, Fe3+ monimo- subaqueous limnion pyrite oxidation pit sediment Flooded underground mine workings Acknowledgments • Montana Bureau of Mines and Geology • Montana Resources • BP-ARCO • US-EPA, US-DOE • Montana Tech Mine Waste Technology Program • Montana Tech graduate students Mike Kerschen, MBMG References Cameron D., Willett M., Hammer L. (2006) Distribution of organic carbon in the Berkeley Pit Lake, Butte, Montana. Mine Water and the Environment 25(2), 93-99. Gammons C. H., Wood S. A., Jonas J. P. and Madison J. P. (2003) Geochemistry of rare earth elements and uranium in the acidic Berkeley Pit lake, Butte, Montana. Chemical Geology, 198, 269-288. Gammons C. H., Metesh J. J. and Duaime T. E. (2006) An overview of the mining history and geology of Butte, Montana. Mine Water and the Environment 25(2), 70-75. Gammons C. H. and Duaime T. E. (2006) Long-term changes in the geochemistry and limnology of the Berkeley pit-lake, Butte, Montana. Mine Water and the Environment 25(2), 76-85. Gammons C. H., Metesh J. J., and Snyder, D. M. (2006) A survey of the geochemistry of flooded mine shaft water in the Butte District, Montana. Mine Water and the Environment 25(2), 100-107. Gammons C. H., Poulson S. R., Pellicori D. A., Roesler A., Reed P. J., Petrescu E. M. (2006) The hydrogen and oxygen isotopic composition of precipitation, evaporated mine water, and river water in Montana, USA. Journal of Hydrology 328, 319-330. Madison J. P., Gammons C. H., Poulson S. R. and Jonas J. P. (2003) Oxidation of pyrite by ferric iron in the acidic Berkeley pit lake, Montana, USA. Proc. 6th International Conf. on Acid Rock Drainage, Cairns, Australia, July 14-17, 2003, Australian Inst. of Mining & Metall., Publ. Series 3/2003, 1073-1078. Newbrough P. and Gammons C. H. (2002) Experimental investigation of water-rock interaction and acid mine drainage at Butte, Montana. Environmental Geology 41(6), 705-719. Pellicori D. A., Gammons C. H., and Poulson S. R. (2005) Geochemistry and stable isotope composition of the Berkeley pit lake and surrounding mine waters, Butte, Montana. Applied Geochemistry 20, 2116-2137. Twidwell L., Gammons C. H., Young C., and Berg R. (2006) Deepwater sediment/pore water characterization of the metal-laden Berkeley pit lake in Butte, Montana. Mine Water and the Environment 25(2), 86-92. References in bold are included on the MEND workshop CD Questions? Frozen lake surface with strange gypsum (?) concretions.