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THE MINERALOGICAL FATE OF ARSENIC DURING WEATHERING OF SULFIDES IN GOLD-QUARTZ VEINS: A MICROBEAM ANALYTICAL STUDY A Thesis Presented to the faculty of the Department of Geology California State University, Sacramento Submitted in partial satisfaction of the requirements for the degree of MASTER OF SCIENCE in Geology by Tamsen Leigh Burlak SPRING 2012 © 2012 Tamsen Leigh Burlak ALL RIGHTS RESERVED ii THE MINERALOGICAL FATE OF ARSENIC DURING WEATHERING OF SULFIDES IN GOLD-QUARTZ VEINS: A MICROBEAM ANALYTICAL STUDY A Thesis by Tamsen Leigh Burlak Approved by: __________________________________, Committee Chair Dr. Charles Alpers __________________________________, Second Reader Dr. Lisa Hammersley __________________________________, Third Reader Dr. Dave Evans ____________________________ Date iii Student: Tamsen Leigh Burlak I certify that this student has met the requirements for format contained in the University format manual, and that this thesis is suitable for shelving in the Library and credit is to be awarded for the project. _______________________, Graduate Coordinator ___________________ Dr. Dave Evans Date Department of Geology iv Abstract of THE MINERALOGICAL FATE OF ARSENIC DURING WEATHERING OF SULFIDES IN GOLD-QUARTZ VEINS: A MICROBEAM ANALYTICAL STUDY by Tamsen Leigh Burlak Mine waste piles within the historic gold mining site, Empire Mine State Historic Park (EMSHP) in Grass Valley, California, contain various amounts of arsenic and are the current subject of remedial investigations to characterize the arsenic present. In this study, electron microprobe, QEMSCAN (Quantitative Evaluation of Minerals by SCANning electron microscopy), and X-ray absorption spectroscopy (XAS) were used collectively to locate and identify the mineralogical composition of primary and secondary arsenic-bearing minerals at EMSHP. Primary arsenic-bearing minerals identified include the following sulfoarsenides: arsenian pyrite (Fe(S,As)2), arsenopyrite (FeAsS), and cobaltite ((Co,Fe)AsS). Subaerial weathering of these primary sulfoarsenide minerals within mine waste piles has led to oxidation of As(-I) to As(V), allowing for the formation of several arsenic-bearing secondary minerals including the hydrous ferric v oxides (HFO) ferrihydrite (5Fe2O3•9H2O) and goethite (FeOOH), scorodite (FeAsO4•2H2O), and various other hydrous ferric arsenates (HFA) and Ca-Fe arsenates. Some of the secondary oxide and arsenate minerals contained more arsenic on a weight basis than the primary sulfide minerals, up to a maximum of 48.1 wt. % arsenic compared to a maximum 44.8 wt. % arsenic in primary minerals. This trend of higher concentrations of arsenic in the secondary minerals than in the primary minerals may be caused by multiple factors, including preferential weathering of arsenic-rich regions in zoned arsenian pyrite, weathering of higher arsenic arsenopyrite, and incorporation of arsenate in HFO and HFA by adsorption or coprecipitation. According to other studies, secondary minerals such as arsenic-bearing Fe-oxides and Ca-Fe arsenates are more soluble in the human gut than the primary As-bearing sulfide minerals, leading to higher bioaccessibility and bioavailability. Results from studies conducted in this thesis may have implications for improving the understanding of arsenic bioaccessibility of mine waste within the EMSHP, and possibly at other historic gold mine sites in California and elsewhere that have similar mine waste undergoing subaerial weathering involving oxidation of arsenic-bearing primary sulfoarsenide minerals and formation of secondary oxide and arsenate minerals. _____________________, Committee Chair Dr. Charles Alpers ______________________ Date vi FOREWARD This thesis is part of a multi-disciplinary investigation into arsenic bioavailability in mine waste focused on the Empire Mine State Historic Park (EMSHP) in Grass Valley, California. The purpose of the overall investigation is to better understand the nature and chemical speciation of arsenic in mine waste and at what level exposure to it becomes a concern to human health. Funding for the overall investigation was provided to the California Department of Toxic Substances Control (DTSC) through a Brownfields Training, Research, and Technical Assistance Grant from the U.S. Environmental Protection Agency. A goal of the investigation for DTSC is to develop an assessment tool that would allow the prediction of arsenic bioavailability in soil samples from mine sites in a sound, defensible, and cost-efficient manner (California DTSC, 2010). DTSC initiated a partnership involving several other institutions to carry out the multi-disciplinary studies of bioavailability and bioaccessibility of arsenic in mine waste. The partners in this research include Prof. Nicholas Basta (The Ohio State University) who is doing bioaccessibility studies with simulated gastric and intestinal fluids (in vitro testing), Prof. Stan Casteel (University of Missouri) who is doing bioavailability studies using juvenile swine (in vivo testing), Prof. Christopher Kim (Chapman University) who is analyzing the concentration of arsenic and other metals in various grain-size fractions, Dr. Andrea Foster (U.S. Geological Survey, USGS) who is analyzing the speciation of iron and arsenic in mine waste samples using X-ray absorption spectroscopy (XAS) with vii synchrotron radiation, and Dr. Alex Blum (USGS), who is characterizing soil and rock samples using powder x-ray diffraction (XRD). The work in this thesis is focused on characterizing the mineralogy and geochemistry of primary and secondary (weathering) minerals from mine waste piles at the EMSHP. The thesis is designed to complement the work being done by others on the multi-disciplinary research team with the goal of improving the understanding of mineralogy and chemical speciation and their relation to arsenic bioavailability and bioaccessibility. viii DEDICATION I lovingly dedicate this thesis to my family and future husband, who together supported me each step of the way. ix ACKNOWLEDGEMENTS I am grateful to my primary advisor, Charles Alpers, whose guidance and support from the beginning to the end enabled me to appreciate this thesis project and to develop a more comprehensive understanding of the subject. I am also grateful to Lisa Hammersley and Dave Evans, whose encouragement, editing assistance, and support allowed me to stay on course during this process. I offer my regards to Andrea Foster who provided assistance with XAS, Sarah Roeske and Nick Botto who provided assistance on the electron microprobe, and Erich Petersen who provided assistance on QEMSCAN. I would also like to thank DTSC and Holdrege and Kull for assistance in sample collection, and the USEPA and USGS for funding. In addition, I would like to show my gratitude to my future husband, Andrew Regnier, and my close friend, Maia Kostlan, for providing your love and undying support in a number of ways through the writing process. Lastly, I offer my regards to all of those not mentioned who supported me in any respect during the completion of this thesis. x TABLE OF CONTENTS Page Foreward ........................................................................................................................... vii Dedication .......................................................................................................................... ix Acknowledgements ............................................................................................................. x List of Tables ................................................................................................................... xiii List of Figures .................................................................................................................. xiv Chapter 1. INTRODUCTION .......................................................................................................... 1 Geologic Setting...................................................................................................... 5 Units and Rock Types Present .................................................................... 6 2. METHODS ..................................................................................................................... 8 Reconnaisance Sampling ........................................................................................ 9 Trench Sampling ..................................................................................................... 9 X-Ray Absorption Spectroscopy using Synchrotron Radiation ........................... 12 Beamline 10-2 ........................................................................................... 12 Beamline 2-3 ............................................................................................. 13 QEMSCAN ........................................................................................................... 14 Electron Microprobe ............................................................................................. 14 Methods Summary ................................................................................................ 20 3. RESULTS ..................................................................................................................... 21 Sulfide Composition from Electron Microprobe Analysis ................................... 21 Oxide Composition from Electron Microprobe Analysis ..................................... 29 Comparison of Sulfide and Oxide Compositions ................................................
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