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