NEUTRAL DRAINAGE AND AVAILABLE ACID-
NEUTRALIZATION CAPACITY FROM SILICATES IN A MAGMATIC
SULFIDE DEPOSIT
Mark Logsdon and Lucy Logsdon Geochimica, Inc.
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
Do Silicates Neutralize Acidity?
YES
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
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 pentlandite>>chalcopyrite in pyrrhotite-dominated, massive to disseminated sulfides hosted by peridotite
Cross Section: Magmatic Segregation
Low S Peridotite
High S Peridotite
Sulfide Mineralogy
Po
Pn
Pn
Cpy
Ilm
Ilm
Gangue Mineralogy
S
Plg
Ol
T
Ol
Cpx
Bio
Ilm
Two Peridotite SamplesLow S Peridotite High S Peridotite
• < 0.2 % Stot
• NP = 42• The long-term
weathering acid is H2CO3
• Data broken up into 3 groups:
Weeks 0-80Weeks 81-229Weeks 230-317
• 8.13 % Stot
• NP= 23.9 • The long-term
weathering acid is H2SO4
• Data broken up into 3 groups:
Weeks 0-46Weeks 47-190Weeks 200-288
Low S pH
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 [Ca+Mg+Na+K]
Results – Titrated Cation Eq/m2/wk
1.034 m^2/kg
eq/m^2 Weeks 0-80 Weeks 81-229Weeks 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
0.762 m^2/kg
eq/m^2 Weeks 0-46 Weeks 47-184 Weeks 203-288Total Major Cations 1.13E-03 9.98E-04 1.58E-03
Total Cations 1.13E-03 1.48E-03 2.95E-03
Low S Peridotite
High S Peridotite
Ratios
Low S Peridotite
High S Peridotite
Molar ratio
(Si)Molar ratio
(Si)Ca 0.62 0.21
Mg 1.81 6.50Na 0.98 0.21K 0.69 0.22
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,A
l)2O6 <1 <1
Anorthite CaAl2Si2O8 0.5Alkali
feldspar (Na,K)AlSi3O8 0.333
Mineralogical Changes – Post LeachingVR71213 Low S VR90125 High S
• Effectively no change in mineralogy post leaching
• Significant decreases seen in:LizarditePyrrhotitePentlandite
• Development of:ClinochloreVermiculiteGoethiteLepidocrocite
Conclusions• r-ANC Increases (Weakly) with Strength of
Weathering AcidpKa H2SO4: - 3 r-ANC: 1.6 E-03pKa 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
Thank YouQuestions?
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 SO4
2- in solution]
•77 mg/L SO4 as average release rate
•Observed: 525 mg/L = 5.5E-03 mol SO4