DYNAMIC ARSENIC CYCLING IN SCORODITE-BEARING HARDPAN CEMENTS, MONTAGUE GOLD MINES, NOVA SCOTIA by Stephanie Leigh DeSisto A thesis submitted to the Department of Geological Sciences and Geological Engineering In conformity with the requirements for the degree of Master of Science Queen’s University Kingston, Ontario, Canada (December, 2008) Copyright ©Stephanie Leigh DeSisto, 2008 Abstract Hardpans, or cemented layers, form from precipitation and subsequent cementation of secondary minerals in mine tailings and can act as both physical and chemical barriers. During precipitation, metals in the tailings are sequestered, making hardpan a potentially viable method of natural attenuation. At Montague Gold Mines, Nova Scotia, tailings are partially cemented by the iron (Fe) arsenate mineral scorodite (FeAsO4·2H2O). Scorodite is known as a phase that can effectively limit aqueous arsenic (As) concentrations due to its relatively low solubility (<1 ppm, pH 5) and high As content (~30 wt.%). However, scorodite will not lower As concentrations from waters to below the Canadian drinking water guideline (0.010 ppm). To identify current field conditions influencing scorodite precipitation and dissolution and to better understand the mineralogical and chemical relationship between hardpan and tailings, coexisting waters and solids were sampled to provide information on tailings-water interactions. Hardpan cement compositions were found to include Fe arsenate and Fe oxyhydroxide in addition to scorodite. End-member pore water chemistry was identified based on pH and dissolved concentration extremes (e.g. pH 3.78, As(aq) 35.8 ppm) compared to most other samples (avg. pH 6.41, As(aq) 2.07 ppm). These end- member characteristics coincide with the most extensive and dispersed areas of hardpan. Nearly all hardpan is associated with historical arsenopyrite-bearing concentrate which provides a source of acidity and dissolved As+5 and Fe+3 for scorodite precipitation. A proposed model of progressive arsenopyrite oxidation suggests localized ii As cycling involving scorodite is occurring but is dependent on sulfide persistence. Therefore, permanent As sequestration is not expected. Remediation efforts would have to consider the possibility of scorodite dissolution after complete sulfide consumption or as a consequence of applying certain technologies, such as a cover. Conversely, if scorodite stability were maintained, the hardpan could be considered as a component in remediating the tailings at Montague. iii Co-Authorship This project was developed by Dr. Michael Parsons and Dr. Heather Jamieson with support from the NSERC Metals in the Human Environment Strategic Network (MITHE-SN). Two chapters from this thesis are intended for journal publication. Chapter 4 will be submitted to Geochemistry: Exploration, Environment, Analysis and Chapter 5 to Applied Geochemistry. Both manuscripts are co-authored with Dr. H.E. Jamieson and Dr. M.B. Parsons who acted in a supervisory and review capacity. iv Acknowledgements I extend sincere gratitude to my advisors, Dr. Heather Jamieson and Dr. Michael Parsons, for the opportunity to work with you on this project. This has turned into an ideal combination of my interests in environmental geochemistry and mineralogy. All that I have learned over the last two years has only strengthened my choice to continue to pursue this as a career. The enthusiasm and care you both display for what you do is admirable and made it a pleasure to learn from you. To Heather especially, I deeply appreciate all your support in continuing on at Queen’s and am looking forward to the future. Thanks also go to Steve Walker, Lori Wrye and Megan Little for field assistance. Steve, thank you for making a first field season a little less daunting! And for your always thoughtful advice and suggestions. Lori, thank you for being an excellent travel companion and for your dear friendship. Skya Fawcett also deserves thanks for introducing me to the highs and lows of the synchrotron, which I’m sure will be of great use in the future! Warmest thanks to Dianne Hyde, Linda Brown and Joan Charbonneau for all you do that gets things done. Dave Kempson and Al Grant (Geology Department, Queen’s University); Dirk Wallschläger, Jacqueline London and Britta Planer-Friedrich (Trent University); and Kalam Mir, Mary Andrews and Allison Rutter (ASU, Queen’s University) are thanked for sharing their technical expertise and help with analyses. v Very special mention goes to Drs. Christopher Cirmo, Robert Darling and Gayle Gleason from SUNY Cortland. You are all inspiring individuals, teachers and scientists and have had a great influence on my life. I thank you for taking an interest when I had just begun in geology as your guidance and encouragement have undoubtedly brought me to where I am today. And, greatest thanks goes to my family; for your love and prayers that are so meaningful and for your constant encouragement to keep working at whatever I find worthwhile in life. vi Table of Contents Abstract ............................................................................................................................................ ii Co-Authorship ................................................................................................................................ iv Acknowledgements .......................................................................................................................... v Table of Contents ........................................................................................................................... vii List of Figures ................................................................................................................................. xi List of Tables ................................................................................................................................ xiii List of Equations ........................................................................................................................... xiv List of Abbreviations ..................................................................................................................... xv Chapter 1: INTRODUCTION ...................................................................................................... 1 1.1 THESIS ORGANIZATION AND PURPOSE ...................................................................... 1 1.2 GEOLOGY AND GOLD DEPOSITS OF NOVA SCOTIA ................................................. 2 1.3 MONTAGUE GOLD MINES ............................................................................................... 5 1.4 PROJECT DESCRIPTION .................................................................................................. 12 1.4.1 Research Objectives ...................................................................................................... 12 1.4.2 Research Rationale ........................................................................................................ 13 Chapter 2: BACKGROUND INFORMATION ........................................................................ 14 2.1 CHARACTERIZATION AND OCCURRENCE OF ARSENIC ........................................ 14 2.1.1 Aqueous Speciation ...................................................................................................... 14 2.1.2 Oxidation-Reduction and pH ........................................................................................ 15 2.1.3 Sources and Mineralogy of Arsenic .............................................................................. 17 2.1.4 Arsenic in Mine Environments ..................................................................................... 18 2.2 ARSENIC CYCLING .......................................................................................................... 19 2.2.1 Mechanisms .................................................................................................................. 19 2.2.2 Natural Attenuation ....................................................................................................... 20 2.2.3 Focus on Scorodite and Iron Arsenate .......................................................................... 22 2.2.3.1 Precipitation and Dissolution ................................................................................ 22 2.2.3.2 Solubility and Arsenic Control ............................................................................... 26 2.3 HARDPAN AND MINING ENVIRONMENTS ................................................................ 28 2.4 SIGNIFICANCE TO HUMAN HEALTH .......................................................................... 30 Chapter 3: SAMPLING AND ANALYTICAL METHODOLOGY ....................................... 33 vii 3.1 FIELD METHODS .............................................................................................................. 33 3.1.1 Sampling Strategy ......................................................................................................... 33 3.1.2 Site Locations ................................................................................................................ 34 3.1.3 Sample Collection ......................................................................................................... 39 3.1.3.1 Waters .................................................................................................................... 39 3.1.3.2 Solids .....................................................................................................................