the importance of geology - usgs · the importance of geology sharon f. diehl, usgs billings...
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The Importanceof Geology
Sharon F. Diehl, USGSBillings Symposium / ASMR Annual Meeting
Assessing the Toxicity Potentialof Mine-Waste Piles Workshop
June 1, 2003
U.S. Department of the InteriorU.S. Geological Survey
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Flow Chart for Ranking and Prioritization
Characterization
Regulatory
Detailed
ReconnaissanceGeology
Reports by Licensed geologists
Geologic map / geologic settingMining methods / processingHand lens examinationMineral deposit type
Microscopic Analysis:PetrographicMineralogy (XRD)SEM / microprobeLaser ablation
Scale
Microscopic Site Watershed Regional
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Potential Environmental Impact
A complex function of:
GeologyGeochemical and biogeochemical processes Climate TopographyThe mining and mineral processing methods used
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Environmental Geology of Mineral Deposits
Acid generation:Iron sulfide contentOther sulfide content
Acid consumption:Host rockWallrock alterationGangue mineralogy
Trace element release:Abundance (deposit, host rocks)Access of weathering agentsSusceptibility of source mineral phases to weathering
Many sulfides (not all)generate acid when oxidized
Minerals and weathering products may consume or generate acid
Often results in a characteristicgeochemicalsignature (depending upon type of mineral deposit)
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Acid-Generating MineralsPyrite (FeS2) Marcasite (FeS2)Pyrrhotite (Fe1-xS) Arsenopyrite (FeAsS)Enargite (Cu3AsS4) Tennantite (Cu12As4S13)Realgar (As2S3) Orpiment (AsS) Others
If ferric iron is oxidant, above minerals plus:Chalcopyrite (CuFeS2) Covellite (CuS) Sphalerite (ZnS)Chalcocite (Cu2S) Acanthite (Ag2S) Galena (PbS)
If metal hydroxides (solid or aqueous) form, above minerals plus:Siderite (FeCO3) Rhodochrosite (MnCO3)
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Euhedralpyrite
Massive pyrite
Colloformpyrite
Sulfide Texture andResistance to Weathering
Less easily weathered
More easily weathered
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Acid-Consuming Minerals
Carbonate minerals and some other minerals (some silicates, volcanic glasses) in mineral deposits, their host rocks, and watershed rocks:
Can help consume acid generated by sulfide oxidation
Can generate alkalinity in ground and surface waters, thereby increasing the waters’ ability to buffer acid
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Acid-Consuming Minerals(after Sverdup, 1990; Kwong, 1993)
Most Effective:aragonite, calcite
Other Carbonates (may consume acid):dolomite, rhodochrosite, magnesite, ankerite,
brucite
Rapidly Weathering Minerals:anorthite, nepheline, olivine, garnet, jadeite,
leucite, spodumene, diopside, wollastonite, poorly-welded volcanic glass
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Less-Effective Acid-Consuming Minerals(after Sverdup, 1990; Kwong, 1993)
Intermediate weathering:Epidote, zoisite, enstatite, hypersthene, augite,
hedenbergite, hornblende, glaucophane, talc, chlorite,biotite , welded “volcanic glass”
Slow weathering:Albite, oligoclase, labradorite, vermiculite, montmorillonite,
gibbsite, kaoliniteVery slow weathering:K-feldspar, muscoviteInert:Quartz, rutile, zircon
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Host Rock Characteristics
May consume or generate acid
May contribute trace elements to the deposit’s environmental signature
Their physical characteristics (i.e., porosity, permeability, fractures) control access of weathering agents to the deposit (e.g. water, oxygen, CO2, acid)
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Hydrologic Characteristics
Potential interactions of ground water with the mineral deposit during and after mining
Potential impacts of the mineral deposit on water quality down gradient from a mine
Must be considered when determining remediation approaches
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Influence of Alteration Zones/Characteristicsof Ore Deposits
Mineralizing processes modify an ore deposit’s host-rock mineral assemblage to a new mineral assemblage
• Results in different acid-generating and acid-consuming capacities
Can strongly influence the environmental signature of a mineral deposit
Can modify the physical characteristics of the host rocks (porosity, permeability, fractures, strength)
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Alteration Zones
Bove, D.J. et al., in press
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TRACE METAL ENRICHMENT IN WATERSDRAINING FROM ALTERATION ZONES
[Bove, D.J., in press]
Acid buffering capacity: Propylitic >Weak Sericite Pyrite >Qtz-Pyrite
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Mining and Processing Methods
Influenced by the geologic characteristics of the mineral deposit
Dictates the amount of rock surface exposed to weathering • Accessibility of weathering agents• Opportunities for evaporative
concentration
In general, for the same geologic characteristics, degradation of mine-water quality decreases from:open-pit > mine dumps > underground mine
workings
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Climate
Weathering Rates • Weathering is faster and more intense
in wetter, warmer climates
Acid-Buffering Capacity of Soils, Alluvium, and Waters• Carbonate-rich soils and rock coatings
in dry climates• Surface and ground waters have higher
acid-buffering capacity in drier climates• Organic acids in high-vegetation areas
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Climate, cont.
Depth of Oxidation• Water table is deeper in drier climates,
thus deeper oxidation
Evaporation • Somewhat increases acidity and metal
content of acid waters• Dry periods lead to formation of
soluble salts; wet periods lead to flushing of soluble salts
Metal Transport• Dilution greatest in wet climates
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Geoenvironmental Models(Plumlee and Nash, 1995; du Bray, 1995)
A compilation of geologic, geochemical, geophysical, hydrologic, and engineering information pertaining to the environmental behavior of geologically similar mineral deposits (1) prior to mining, and (2) resulting from mining, mineral processing, and smelting.
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Types of Massive Sulfide Deposits(Seal and Hammarstrom, 2003)
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$55 for the 2$55 for the 2--volume set. For ordering info: www.segweb.orgvolume set. For ordering info: www.segweb.org
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Microscopic Analytical Methods
Method: Result:Parameter:
Mineralogy Petrographic MicroscopeScanning Electron Microscope
Mineral Species;acid or non-acidgenerating;
Mineral Textures; particle size,cleavage
Structure
Exact Residence of trace metals
Spatial Distributionof trace metals
Quantitative data
Trace Metals MicroprobeLaser Ablation Mass Spectrometry
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Case Studies
1. Hard-Rock Mine Waste in Humidity Cell Tests(Lapakko, 1999; Lapakko and White, 2000; White and Lapakko, 2000)
2. Pyrite-Rich Coal Samples
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Elemental Residence Phases(from mineral separates, 4 Colorado mine-waste sites)
Jarosite [ KFe3(SO4)2(OH)6 ] Pb, Ag, Cu, Bi
Pyrite [ FeS2 ] Cu, Bi, Ag, As
Sphalerite [ ZnS ] Cd, Cu, Mn, Ag
Galena [ PbS ] Ag, Bi
Anglesite [ PbSO4 ] Zn, Cd, Bi, Cu
Mineralogical CharacterizationMineralogical Characterization
Tennantite-Tetrahedrite Cu, Zn, Sb, As[(Ag, Cu, Fe)12 (Sb, As)4S13
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Case Study 1:Bulk Mineralogy of Mine Waste Samples
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pH
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Microstructure;Sample 99.1
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Microstructure; Sample 99.1
Etched carbonate-filled vein
Silica-filled vein
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Microprobe Element Distribution Maps
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Pyrite-bearing Sample 99.1
fractures
matrix material
pyritepyrite
qtz
Pyrite Before Leaching
Pyrite After Leaching
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Cu and Zn from sulfides
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Sample 81196
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TexturesCleavage/Shrinkage Cracking (?)
in Mica Zoned Jarosite
Jarosite crystals exhibit a chemical zonation evidenced by dissolution of the crystal cores.
Cleavage in mica is a highly porous structure and a viable pathway for fluid infiltration and migration.
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0.56 wt. % Cu X
Trace elements in Jarosite
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Case Study 2:
Trace Elements in Coal/Pyrite from the Lost Creek Mine, Warrior Basin, Alabama
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Framboidal Pyrite
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Lost CreekMine
Pyrite fills woodycell structures.
Arsenic-rich pyritereplaces early arsenic-poor pyrite in lumens, occurs as overgrowths, and along microfaults.
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Element Maps of Arsenic in Microstructures
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Arsenic Element Distribution Map; Framboidal vs. Coarse-Grained Pyrite
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Coal
Pyrite
Arsenic-rich Pyrite and Clay-filled
Fractures
Coal
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Arsenic-rich Pyrite-filled Veins
Arsenic-richpyrite goes into solution more readily than arsenic-poor pyrite.
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Summary
Trace metals are associated with characteristic
geologic settings and their mineral assemblages.
Therefore, trace metal release and acid mine
drainage can be predicted from the mineralogy in
mine waste.