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The Measurement of the Se/S Ratios in Sulphide Minerals and Their Application to Ore Deposit Studies

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The Measurement of the Se/S Ratios in Sulphide Minerals and Their Application to Ore Deposit Studies The measurement of the Se/S ratios in sulphide minerals and their application to ore deposit studies By Alexander John Fitzpatrick A thesis submitted to the Department of Geological Sciences and Geological Engineering in conformity with the requirements for the degree of Doctor of Philosophy Queen’s University Kingston, Ontario, Canada February, 2008 Copyright © Alexander John Fitzpatrick, 2008 Abstract New analytical techniques have been developed for the determination of selenium concentrations in sulphide minerals to assess the utility of the Se/S concentration ratios in tracing fluids associated with ore deposit formation. This has been accomplished via a new hydride generation (HG) sample introduction method for the determination of selenium contents in sulphide minerals, development of solid calibration standards by a sol-gel process, and the establishment of protocols for the measurement of selenium/sulphur ratios in sulphides using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). The low concentrations of selenium in sulphides require the use of hydride generation, which also requires the removal of metals to be effective. A process for the determination of selenium in sulphide minerals (~ 50 mg sample weight) wherein metals are removed by precipitation under alkaline conditions, followed by further removal by chelating resin, was developed. Determinations by HG-ICPMS on reference materials showed quantitative recoveries (100±5 %). Precision is 10 % relative standard deviation and the detection limit is 4 g g-1 in a sulphide mineral. A sol-gel method for the fabrication of multi-element calibration standards, suitable for laser ablation, was developed. The addition of selenium and sulphur to a normal sol-gel method does not introduce detectable heterogeneity. Xerogel heterogeneity is less than that of the NIST glass standards. Calculated sulphur contents in the NIST glass SRMs are comparable to published data. Xerogels are potentially useful as standards in studies of glasses, minerals, and other materials. ii The development of a laser ablation technique allowed measurement of Se/S ratios in sulphide minerals. Sulphide minerals were sampled from volcanic hosted massive sulphide (VHMS) (Flin Flon area, Canada), high-sulphidation epithermal (Pierina, Peru), and iron oxide copper gold (IOCG) (Mantoverde, Chile) deposits. Thermodynamic data, mineral assemblages, and isotopic compositions are used to define depositional conditions and Se/S ratios of ore generating fluids. Although the elements can fractionate from each other, high Se/S ratios and low 34S values generally reflect magmatic fluids, typical of VHMS and epithermal deposits, whereas the opposite is true for basinal or evaporitic sources, such as recorded by sulphides at the Mantoverde deposits. The Se/S ratios aid in the identification of ore generating fluid sources and can indicate mixing of fluids. iii Acknowledgments This project would not have been possible without the help of many people from whom I received support, intellectual and otherwise. First, I would like to thank my supervisor Dr. Kurt Kyser for his infinite patience and positive reinforcement. Dr. Diane Beauchemin is thanked for her particular support on the development of the xerogel technique. The staff of the Queen’s Facility for Isotope Research are thanked for their support in the development of analytical techniques and particularly Dr. Don Chipley for his instruction in the mystical art of tuning and operating the high resolution ICPMS and Kerry Klassen for instruction in sulphur isotope ratio determinations. Discussions with Dr. Dan Layton-Matthews proved particularly useful in clarifying some of the mysteries of selenium thermodynamic data interpretation. Dr. Stephen Wilson of the U.S.G.S. is thanked for providing the PS-1 provisional reference material. This research project benefited tremendously from discussions with and assistance from my fellow graduate students and other colleagues. In particular, Paul Polito, Amelia Rainbow, and Jorge Benavides took time from their own research to provide samples, isotope data, and useful discussions on the deposits they were studying. Peter Jones and Lew Ling of Carleton University are gratefully acknowledged for their assistance with microprobe and SEM analyses. The staff at the Centre for Neutron Activation at McMaster University is thanked for rapid turnaround on samples. Finally, I would like to thank my family for their continued support throughout this and all my academic endeavours. iv Statement of Originality The major contributions of this study are based on the author’s development of analytical techniques, and interpretation of data obtained from determinations utilizing the created techniques. However, through discussions with academic supervisors and colleagues, the author received constructive criticism and insightful observations, both of which were beneficial to the project. The project has two main components: analytical method development, and exploration of the utility of sulphide Se/S ratios in ore deposit studies, in particular the identification of mineralizing fluids and their sources. Selenium measurement in sulphides has received prior attention, but analytical difficulties such as spectral and isobaric interferences have generally limited wide-scale studies. The availability of analytical facilities at Queen’s University prompted the development of techniques for the measurement of selenium in sulphides, selenium being an element of geochemical interest with limited knowledge of its occurrence and behaviour on an ore deposit scale. The need for a method to allow more precise and accurate selenium determinations in sulphides prompted development of an autochthonous precipitation and column separation technique for the preparation of sulphides for hydride generation inductively coupled plasma mass spectrometry. Precipitation as a technique to concentrate selenium has been used before (Plotnikov 1960), while column separation is a widely employed technique. However, the present technique is the first time, to the author’s knowledge, that autochthonous precipitation and column separation have been combined in a manner suitable to prepare sulphides for hydride generation techniques. v The acquisition of a new laser ablation system over the course of this study prompted an investigation of the possibilities of using laser ablation ICPMS as an analytical method for selenium determination in sulphides. Preliminary investigations indicated that there were an inadequate number of suitable sulphide reference standards. Consequently, sulphides measured in the first part of this study were adopted as provisional calibration standards. These were combined with two reference materials and a synthetic galena-clausthalite series to form the basis of a calibration suite. Compositional gaps in this suite prompted investigations into methods for additional solid calibration standards. The use of a sol-gel process to produce a solid calibration standard suitable for laser ablation was suggested by Dr. Diane Beauchemin and this study is the first attempt, to the author’s knowledge, to produce a selenium calibration standard by this method. The utility of Se/S ratios in conjunction with sulphur isotope data in ore deposit studies has been investigated previously for VHMS (Yamamoto 1976; Yamamoto et al. 1983; Yamamoto et al. 1984; Huston et al. 1995) and magmatic nickel deposits (Eckstrand et al. 1989). Samples collected by others provide the working materials used for the Se/S determinations performed by the author. Others have performed the sulphur isotope determinations. To the author’s knowledge, this is the first assessment of the utility of Se/S ratios in combination with 34S value for the study of ore deposits other than VHMS and magmatic nickel and PGE deposits. vi Table of Contents Abstract ........................................................................................................... ii Acknowledgments .......................................................................................... iv Statement of Originality ................................................................................. v Table of Contents.......................................................................................... vii List Of Figures .............................................................................................. xii List of Tables................................................................................................ xiv Chapter 1. Introduction.................................................................................. 1 1.01 Selenium Determination..............................................................................................2 1.02 Selenium and Ore Deposits .........................................................................................4 1.03 Scope of Investigations ................................................................................................6 1.04 The Scientific Contributions of this Study..................................................................7 1.04.1 GEOCHEMICAL ANALYSIS ................................................................................................................8 1.04.2 ORE DEPOSIT STUDIES ......................................................................................................................8 1.05 Structure of Thesis.......................................................................................................9
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