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

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

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  1. NEUTRAL DRAINAGE AND AVAILABLE ACID-NEUTRALIZATION CAPACITY FROM SILICATES IN A MAGMATIC SULFIDE DEPOSIT Mark Logsdon and Lucy Logsdon Geochimica, Inc.

  2. 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

  3. Do Silicates Neutralize Acidity? YES

  4. 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

  5. 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

  6. Cross Section: Magmatic Segregation Low S Peridotite High S Peridotite

  7. Po Pn Pn Sulfide Mineralogy Cpy Ilm Ilm

  8. Gangue Mineralogy Cpx Bio Ilm T Ol S Ol Plg

  9. 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

  10. Low S pH

  11. 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]

  12. Results – Titrated Cation Eq/m2/wk Low S Peridotite High S Peridotite

  13. Ratios

  14. 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

  15. 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

  16. Thank YouQuestions?

  17. 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

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