NEUTRAL DRAINAGE AND AVAILABLE ACID-NEUTRALIZATION CAPACITY FROM SILICATES IN A MAGMATIC SULFIDE DEPOSIT

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NEUTRAL DRAINAGE AND AVAILABLE ACID-NEUTRALIZATION
CAPACITY FROM SILICATES IN A MAGMATIC SULFIDE
DEPOSIT

• Mark Logsdon and Lucy Logsdon
• Geochimica, Inc.

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Acknowledgments

• Rio Tinto Kennecott Eagle Minerals Company
• SGS-CEMI / UBC XRD Lab
• John Jambor
• H.D. Holland
• The authors were compensated by KEMC for the
  original field and laboratory work and for
  advising KEMC on mine-waste management.
• The authors work with permission but without
  compensation on this geochemical interpretation

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Do Silicates Neutralize Acidity?
YES
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Geochemical Premise and the Problem

• Weathering involves titration of bases (minerals)
  by acids (H2CO3, H2SO4)
• Mineral dissolution provides Acid Neutralization
  Capacity (ANC)
• pH of solutions is a response variable during
  weathering of rocks
• WHAT IS THE AVAILABILITY OF ANC FROM SILICATE
  MINERALS?
• Acid Neutralization Capacity per unit time

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Background for This Study

• Long term (between 288 and 317 weeks) column
  leaching
• Data for 2 samples from a small, high-grade,
  Ni-Cu magmatic sulfide deposit
• The ore is pentlanditegtgtchalcopyrite in
  pyrrhotite-dominated, massive to disseminated
  sulfides hosted by peridotite

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Cross Section Magmatic Segregation
Low S Peridotite
High S Peridotite
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Sulfide Mineralogy
Cpy
Ilm
Ilm
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Gangue Mineralogy
Cpx
Bio
Ilm
T
Ol
S
Ol
Plg
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Two Peridotite Samples

• Low S Peridotite

• High S Peridotite

• lt 0.2 Stot
• NP 42
• The long-term weathering acid is H2CO3
• Data broken up into 3 groups
• Weeks 0-80
• Weeks 81-229
• Weeks 230-317

• 8.13 Stot
• NP 23.9
• The long-term weathering acid is H2SO4
• Data broken up into 3 groups
• Weeks 0-46
• Weeks 47-190
• Weeks 200-288

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Low S pH
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Methods of Data Analysis

• Compile analytical data/Check ion balance/convert
  to Molality/Check Ratios
• Analyze statistics focus on Medians by period
• Convert mg/L Solution to Eq/m2 of charge (per
  week)
• Compile Sum Major Cations Ca,Mg,Na,K and Sum
  All Cations
• Work with Sum CaMgNaK

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Results Titrated Cation Eq/m2/wk
Low S Peridotite
1.034 m2/kg
eq/m2 Weeks 0-80 Weeks 81-229 Weeks 230 - 317
Total Major Cations 1.44E-03 5.67E-04 1.21E-04
Total Cations 1.45E-03 5.69E-04 1.22E-04
High S Peridotite
0.762 m2/kg
eq/m2 Weeks 0-46 Weeks 47-184 Weeks 203-288
Total Major Cations 1.13E-03 9.98E-04 1.58E-03
Total Cations 1.13E-03 1.48E-03 2.95E-03
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Ratios
  Molar Ratio Mg/Si (Ca,Na,K)/Si
Forsterite Mg2SiO4 2  
Lizardite Mg3(Si2O5)(OH)4 1.5  
Talc Mg3Si4O10(OH)2 0.75  
Augite (Ca,Na,Mg,Fe)2(Si,Al)2O6 lt1 lt1
Anorthite CaAl2Si2O8   0.5
Alkali feldspar (Na,K)AlSi3O8   0.333
Low S Peridotite High S Peridotite
  Molar ratio (Si) Molar ratio (Si)
Ca 0.62 0.21
Mg 1.81 6.50
Na 0.98 0.21
K 0.69 0.22
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Mineralogical Changes Post Leaching

• VR71213 Low S

• VR90125 High S

• Effectively no change in mineralogy post leaching

• Significant decreases seen in
• Lizardite
• Pyrrhotite
• Pentlandite
• Development of
• Clinochlore
• Vermiculite
• Goethite
• Lepidocrocite

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Conclusions

• r-ANC Increases (Weakly) with Strength of
  Weathering Acid
• pKa H2SO4 - 3 r-ANC 1.6 E-03
• pKa H2CO3 6.35 r-ANC 1.2 E-04
• Increases with H Flux from Sulfide Oxidation
• Long-term, available ANC in Peridotite from
  Olivine and Lizardite
• These silicate minerals dissolve fast enough to
  control the longevity of neutral drainage in rocks

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Thank YouQuestions?
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Control Longevity of Neutral Drainage NOT
Prevent Acidification

• r-ANC 1.6E-03 Eq H consumed/m2/wk
• 1 mol H2SO4 delivers 2 mol (eq) H
• ? Sufficient to neutralize H from 8.0E-04 mol
  H2SO4 8.0E-04 mol SO42- in solution
• 77 mg/L SO4 as average release rate
• Observed 525 mg/L 5.5E-03 mol SO4