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NEUTRAL DRAINAGE AND AVAILABLE ACID-NEUTRALIZATION CAPACITY FROM SILICATES IN A MAGMATIC SULFIDE DEPOSIT PowerPoint Presentation
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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. . Rio Tinto – Kennecott Eagle Minerals Company SGS-CEMI / UBC XRD Lab John Jambor H.D. Holland

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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.

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

Acknowledgments
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

Geochemical Premise and the Problem
background for this study
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
Cross Section: Magmatic Segregation

Low S Peridotite

High S Peridotite

gangue mineralogy
Gangue Mineralogy

Cpx

Bio

Ilm

T

Ol

S

Ol

Plg

two peridotite samples
Two Peridotite Samples

Low S Peridotite

High S Peridotite

  • < 0.2 % Stot
  • NP = 42
  • The long-term weathering acid is H2CO3
  • Data broken up into 3 groups:

Weeks 0-80

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

methods of data analysis
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 m 2 wk
Results – Titrated Cation Eq/m2/wk

Low S Peridotite

High S Peridotite

mineralogical changes post leaching
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

conclusions
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

control longevity of neutral drainage not prevent acidification
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