Carbonatite-Related Rare-Earth Mineralization in the Bear Lodge Alkaline Complex, Wyoming

258 Pages·2014·13.02 MB·English

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) and (La/Ce) values. The early minerals are replaced with polycrystalline
cn cn
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) , and chondrite-normalized profiles of the rare-earth minerals and calcite record
cn
multiple stages of hydrothermal deposition involving fluids of different chemistry. A single
sample of primary calcite shows mantle-like δ18O and δ13C values, whereas most
V-SMOW V-PDB
other samples are somewhat depleted in 13C (δ13C ≈ –8 to –10‰) and show a small positive
V-PDB
shift in δ18O due to degassing and wall-rock interaction. Isotopic re-equilibration is more
V-SMOW
pronounced in the transitional and oxidized zones; large shifts in δ18O (to ~ 18‰) reflect
V-SMOW
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
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are remarkably uniform in their Nd, Sr and Pb isotopic composition: (143Nd/144Nd)=0.512591-
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0.512608; εNd=0.2-0.6; (87Sr/86Sr)=0.704555-0.704639; εSr=-1.5-2.7; (206Pb/204Pb)=18.071-
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18.320; (207Pb/204Pb)=15.543-15.593; (208Pb/204Pb)=38.045-39.165. These isotopic
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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.

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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.
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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.
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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
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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 parameters .................................................. 53
4.4 Mass Spectrometry .................................................................................... 54
4.4.1 Laser-ablation inductively-coupled-plasma mass spectrometry 55
4.4.2 Thermal ionization mass spectrometry ........................................ 57
4.4.3 Gas-bench isotope-ratio mass spectrometry ................................ 58
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4.5 Whole-rock Geochemistry......................................................................... 58
4.6 Raman Microspectroscopy ........................................................................ 59
Chapter 5. Petrography of Carbonatites and their Alteration Products .............................. 60
5.1 Unoxidized Zone......................................................................................... 60
5.1.1 Burbankite paragenesis ................................................................. 61
5.1.2 Rare-earth fluorocarbonate paragenesis ..................................... 65
5.1.3 REE fluorocarbonate-fluorite paragenesis .................................. 68
5.1.4 Ancylite paragenesis ...................................................................... 69
5.1.5 Ancylite-REE fluorocarbonate paragenesis ................................ 73
5.2 Transitional Zone ....................................................................................... 78
5.3 Oxidized Zone............................................................................................. 84
Chapter 6. Mineral Chemistry ................................................................................................ 100
6.1 Burbankite and Carbocernaite ............................................................... 100
6.1.1 Introduction to the crystal chemistry of burbankite-group
minerals and carbocernaite ......................................................... 100
6.1.2 Burbankite .................................................................................... 101
6.1.3 Carbocernaite ............................................................................... 105
6.2 Rare-Earth Fluorocarbonates................................................................. 106
6.2.1 Synchysite ..................................................................................... 110
6.2.2 Parisite .......................................................................................... 114
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6.2.3 Bastnäsite ...................................................................................... 116
6.3 Ancylite ..................................................................................................... 118
6.4 Monazite.................................................................................................... 125
6.5 Cerianite.................................................................................................... 133
6.6 Apatite ....................................................................................................... 134
6.7 Fluorite ...................................................................................................... 139
6.8. Calcite........................................................................................................ 140
6.8.1 Unoxidized zone ........................................................................... 141
6.8.2 Transitional zone .......................................................................... 148
6.8.3 “Ox-Ca” subzone ......................................................................... 149
6.8.4 Five generations of calcite: comparative analysis ..................... 152
6.8.5 Calcite zonation ............................................................................ 153
Chapter 7. Geochemistry ......................................................................................................... 158
Chapter 8. Stable Isotopic Compositions ............................................................................... 167
Chapter 9. Radiogenic Isotopic Compositions ....................................................................... 170
9.1 Sr, Nd and Pb Isotopic Compositions of the BLAC Carbonatites ...... 170
9.2 Isotopic Characteristics of Mantle Reservoirs with Relation to
Carbonatites ............................................................................................. 177
9.3 Isotopic Variations of Carbonatites Worldwide: Key Examples ........ 178
Chapter 10. Discussion ............................................................................................................. 181
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10.1 REE Mineralization: Paragenetic Evolution and Compositional
Variation ................................................................................................... 181
10.2 Compositional Variation of REE Minerals and its Relation to the
Evolution of Bull Hill Carbonatites ........................................................ 194
10.3 Compositional Variation of Calcite in Fresh and
Oxidized Carbonatites ............................................................................. 197
10.4 Trace-element Composition of Fluorite ................................................. 202
10.5 Stable Isotopic Variations ....................................................................... 203
10.6 Sources of Bear Lodge Carbonatites: Evidence from Radiogenic
Isotopes...................................................................................................... 207
Chapter 11. Conclusions ........................................................................................................... 212
References ................................................................................................................................. 215
Appendix A .............................................................................................................................. 229
Samples and Sample Analyses.............................................................................. 229
Appendix B .............................................................................................................................. 233
Analytical Instrumentation Parameters .............................................................. 233
Appendix C .............................................................................................................................. 239
Electron Microprobe WDS Data .......................................................................... 239
Appendix D .............................................................................................................................. 240
LA-ICP-MS Data ................................................................................................... 240
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