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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 (H 2 CO 3, H 2 SO 4 ) 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 pentlandite>>chalcopyrite 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 Po Pn Cpy Ilm

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Gangue Mineralogy S Plg Ol T Cpx Bio Ilm

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Two Peridotite Samples Low S PeridotiteHigh S Peridotite < 0.2 % S tot NP = 42 The long-term weathering acid is H 2 CO 3 Data broken up into 3 groups: Weeks 0-80 Weeks 81-229 Weeks 230-317 8.13 % S tot NP= 23.9 The long-term weathering acid is H 2 SO 4 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 [Ca+Mg+Na+K]

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Results – Titrated Cation Eq/m 2 /wk 1.034 m^2/kg eq/m^2Weeks 0-80Weeks 81-229Weeks 230 - 317 Total Major Cations1.44E-035.67E-041.21E-04 Total Cations1.45E-035.69E-041.22E-04 0.762 m^2/kg eq/m^2Weeks 0-46Weeks 47-184Weeks 203-288 Total Major Cations1.13E-039.98E-041.58E-03 Total Cations1.13E-031.48E-032.95E-03 Low S Peridotite High S Peridotite

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Ratios Low S PeridotiteHigh S Peridotite Molar ratio (Si) Ca0.620.21 Mg1.816.50 Na0.980.21 K0.690.22 Molar RatioMg/Si(Ca,Na,K)/Si ForsteriteMg2SiO42 LizarditeMg3(Si2O5)(OH)41.5 TalcMg3Si4O10(OH)20.75 Augite(Ca,Na,Mg,Fe)2(Si,Al)2O6<1 AnorthiteCaAl2Si2O8 0.5 Alkali feldspar(Na,K)AlSi3O8 0.333

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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 pK a H 2 SO 4 : - 3r-ANC: 1.6 E-03 pK a H 2 CO 3 : +6.35r-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 You Questions?

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Control Longevity of Neutral Drainage – NOT Prevent Acidification r-ANC = 1.6E-03 Eq H + consumed/m 2 /wk 1 mol H 2 SO 4 delivers 2 mol (eq) H + Sufficient to neutralize H + from 8.0E-04 mol H 2 SO 4 [8.0E-04 mol SO 4 2- in solution] 77 mg/L SO 4 as average release rate Observed: 525 mg/L = 5.5E-03 mol SO 4

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