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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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Presentation on theme: "NEUTRAL DRAINAGE AND AVAILABLE ACID-NEUTRALIZATION CAPACITY FROM SILICATES IN A MAGMATIC SULFIDE DEPOSIT Mark Logsdon and Lucy Logsdon Geochimica, Inc."— Presentation transcript:

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

2 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

3 Do Silicates Neutralize Acidity? YES

4 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

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 Sulfide Mineralogy Po Pn Cpy Ilm

8 Gangue Mineralogy S Plg Ol T Cpx Bio Ilm

9 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 Weeks % 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 Weeks

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12 Low S pH

13 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]

14 Results – Titrated Cation Eq/m 2 /wk m^2/kg eq/m^2Weeks 0-80Weeks Weeks Total Major Cations1.44E E E-04 Total Cations1.45E E E m^2/kg eq/m^2Weeks 0-46Weeks Weeks Total Major Cations1.13E E E-03 Total Cations1.13E E E-03 Low S Peridotite High S Peridotite

15 Ratios Low S PeridotiteHigh S Peridotite Molar ratio (Si) Ca Mg Na K 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)AlSi3O

16 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

17 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

18 Thank You Questions?

19 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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