Carbonatite-Related Rare-Earth Mineralization in the Bear
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Carbonatite-Related Rare-Earth Mineralization in the Bear Lodge Alkaline Complex, Wyoming: Paragenesis, Geochemical and Isotopic Characteristics by Meghan Moore A Thesis Submitted to the Faculty of Graduate Studies of The University of Manitoba in partial fulfillment of the requirements of the degree of MASTER OF SCIENCE Department of Geological Sciences University of Manitoba Winnipeg Copyright 2014 by Meghan Moore Abstract The Bear Lodge alkaline complex in northeastern Wyoming (USA) is host to potentially economic rare-earth mineralization in carbonatite and carbonatite-related veins and dikes that intrude heterolithic diatreme breccias in the Bull Hill area of the Bear Lodge Mountains. The deposit is zoned and consists of pervasively oxidized material at and near the surface, which passes through a thin transitional zone at a depth of ~120-183m, and grades into unaltered carbonatites at depths greater than ~183-190m. Carbonatites in the unoxidized zone consist of coarse and fine-grained calcite that is Sr-, Mn- and inclusion-rich and are characterized by the presence of primary burbankite, early-stage parisite and synchysite with minor bastnäsite that have high (La/Nd)cn and (La/Ce)cn values. The early minerals are replaced with polycrystalline pseudomorphs consisting of secondary rare-earth fluorocarbonates and ancylite with minor monazite. Different secondary parageneses can be distinguished on the basis of the relative abundances and composition of individual minerals. Variations in key element ratios, such as (La/Nd)cn, and chondrite-normalized profiles of the rare-earth minerals and calcite record multiple stages of hydrothermal deposition involving fluids of different chemistry. A single 18 13 sample of primary calcite shows mantle-like δ OV-SMOW and δ CV-PDB values, whereas most 13 13 other samples are somewhat depleted in C (δ CV-PDB≈ –8 to –10‰) and show a small positive 18 shift in δ OV-SMOW due to degassing and wall-rock interaction. Isotopic re-equilibration is more 18 pronounced in the transitional and oxidized zones; large shifts in δ OV-SMOW (to ~ 18‰) reflect input of meteoric water during pervasive hydrothermal and supergene oxidation. The textural relations, mineral chemistry, and C and O stable-isotopic variations record a polygenetic sequence of rare-earth mineralization in the deposit. With the exception of one Pb-poor sample showing an appreciable positive shift in 208Pb/204Pb value (~39.2), the Bear Lodge carbonatites i 143 144 are remarkably uniform in their Nd, Sr and Pb isotopic composition: ( Nd/ Nd)i=0.512591- 87 86 206 204 0.512608; εNd=0.2-0.6; ( Sr/ Sr)i=0.704555-0.704639; εSr=-1.5-2.7; ( Pb/ Pb)i=18.071- 207 204 208 204 18.320; ( Pb/ Pb)i=15.543-15.593; ( Pb/ Pb)i=38.045-39.165. These isotopic characteristics indicate that the source of the carbonatitic magma was in the subcontinental lithospheric mantle, and modified by subduction-related metasomatism. Carbonatites are interpreted to be generated from small degrees of partial melt that may have been produced via interaction of upwelling asthenosphere giving a small depleted MORB component, with an EM1 component likely derived from subducted Farallon crust. ii Acknowledgements First of all, I would like to thank my advisor, Dr. Anton Chakhmouradian, for your encouragement, patience and guidance. I have learned so much from you in the field, in the lab, and in the classroom. Thank you for your time and energy, and for sharing your expertise of not only carbonatites, but also of writing and culture, she-she. Thanks to my family and friends for encouraging me to finish, when it would have been easier to give up. I am especially thankful for the unconditional love and support of my husband, Jonathan, for providing a platform in which I could pursue this goal. Thank you for all you are to me and our family. Thank you to my children, Jonah and Sophie, who unknowingly have provided me with endless inspiration. Thank you for forgiving the time I have dedicated to this work, for your laughter and love, and the ever- present reminder of what is really important in life. Thank you to my Dad for your constant emphasis of how important it is to finish what I started, and to do it well. I would like to thank Dr. Tony Mariano for sharing your passion, and for many field excursions and conversations involving wine, poetry, birds and rocks. You are a constant inspiration, chiend-an. I would like to thank Clint Cox for investing in my future. Thank you for your time, advice and patience. I hope to one day repay your kindness. I would like to thank all the folks from Rare Element Resources, particularly Jim Clark and John Ray, for giving me the opportunity to study the Bull Hill deposit. I am grateful for the conversations and questions we have shared. I would like to acknowledge Ravi Sidhu for all your time and patience in the EMPA lab. My data would not be of the caliber it is without your help. Thank you to Panseok Yang, Misuk Yun and Lizzie Ann Spencer for your guidance in collecting the data on which this thesis is built. Finally, I would like to acknowledge the University of Manitoba for funding through the UMGF. iii Dedication I am proud to dedicate this thesis to the memory of my mother, Mary Ann Marsters, who taught me to “be the best I can be.” I wish you were here to celebrate this moment in time, but I know that you are celebrating where you are now. Thanks for teaching me the value of education, a good work ethic, how to take care of myself, how to smile in the face of adversity, and the innumerable things that simply cannot be described with words. Sometimes life takes you on a different route than you had planned on the path to follow your dreams, you showed me how to take it all in stride and keep going, never giving up but sometimes taking a necessary reprieve. There were days when I relied on the memory of the sound of your voice telling me that I can do anything I set my mind to. I did it, Mom. iv Table of Contents Chapter 1. Introduction ............................................................................................................... 1 1.1 Introduction to Rare Earth Elements ........................................................ 1 1.1.1 Chemical properties of the REE ..................................................... 2 1.1.2 Partitioning of the REE ................................................................... 5 1.1.3 Natural abundances of the REE ..................................................... 6 1.2 Uses of the REE .......................................................................................... 12 1.3 Geological Environments that Concentrate Rare Earth Elements ....... 13 1.4 Objectives and Purpose of the Present Work.......................................... 15 1.5 Sample Collection and Selection ............................................................... 16 Chapter 2. Geological Setting ................................................................................................... 18 2.1 Archean Wyoming Province ..................................................................... 18 2.2 Cordilleran Tectonics ................................................................................ 20 2.2.1 Sevier tectonics ............................................................................... 21 2.2.2 Laramide tectonics ......................................................................... 23 2.3 Tectonic Models ......................................................................................... 27 2.3.1 Flat-slab subduction....................................................................... 27 2.3.2 Mantle upwelling ............................................................................ 29 2.3.3 Post-orogenic collapse and extension related to lithospheric delamination ................................................................................... 30 v 2.3.4 Slab window hypothesis ................................................................. 31 2.4 Black Hills ................................................................................................... 33 2.4.1 Structure and timing...................................................................... 33 2.4.2 Igneous rocks .................................................................................. 34 Chapter 3. Bear Lodge Alkaline Complex (BLAC)................................................................. 37 3.1 History of Exploration and Previous Work............................................. 37 3.2 Current Exploration and Research .......................................................... 39 3.3 Local Geological Setting ............................................................................ 42 3.3.1 Igneous rocks .................................................................................. 42 3.3.2 Structural geology .......................................................................... 48 3.3.3 Alteration ........................................................................................ 48 Chapter 4. Methodology ............................................................................................................. 51 4.1 Microscopy.................................................................................................. 51 4.2 Cathodoluminescence ................................................................................ 51 4.3 Electron-microprobe Analysis .................................................................. 53 4.3.1 EMPA instrumental