REE Enrichment in Weathered Carbonatite, Bull Hill: Bear Lodge Mountains, Wyoming

Home , Bear Lodge Mountains, Carbonatite, Manganoan calcite, Nsutite

by Mandi Brooke Hutchinson

A thesis submitted to the Faculty and the Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Master of Science (Geology).

Golden, Colorado

Date ______

Signed: ______Mandi Brooke Hutchinson

Signed: ______Dr. Murray Hitzman Thesis Advisor

Signed: ______Dr. Richard Wendlandt Thesis Advisor

Golden, Colorado

Date ______

Signed: ______Dr. Paul Santi Professor and Head Department of Geology and Geological Engineering

ABSTRACT

Rare earth element (REE)-bearing carbonate, fluorocarbonate, phosphate, and oxide minerals occur within near vertical carbonatite dikes on the western margin of the Paleogene Bull Hill diatreme within the Bear Lodge alkaline complex. The weathering profile displays a mineralogically zoned array of REE-bearing phases. Magmatic burbankite is present as inclusions within manganoan calcite. More abundant REE-bearing minerals include ancylite, bastnäsite with synchysite/parisite, and an unidentified Sr-Ca-REE-phosphate pseudomorhpically replace unidentified hexagonal phenocrysts. These replacive minerals are largely stable in the weakly to moderately weathered carbonatite in the lower portion of the weathering profile. In moderately weathered carbonatite, colloform Sr-Ca-REE-phosphate, and cerianite occur as supergene phases. Weathering of the carbonatite dikes caused oxidation of pyrite to Fe-oxides and Fe-hydroxides, dissolution of calcite and strontianite, and replacement of Mn-calcite with Mn-oxides. These mineralogical changes resulted in an increased porosity. Scanning electron microscope-based automated QEMSCAN® analyses on selected samples from the lower weathering zone yielded an approximate gain of 40% porosity. The volumetric concentration of resistant minerals from the removal of gangue carbonate, mainly calcite, from the carbonatite resulted in relative REE enrichment. Rare earth element concentrations range from an average of 5.4 wt. % total REE (6.3 wt. % total rare earth oxide) in the least weathered carbonatite to an average of 12.6 wt. % total REE (14.8 wt. % total rare earth oxide) in the moderately weathered carbonatite. Chondrite-normalized REE patterns of the weathered carbonatite in the lower weathering zone show no major differentiation or fractionation compared to the REE patterns of the least weathered carbonatite. Isocon plots confirm the increased concentration of REE in the weathered carbonatite and demonstrate that REE, along with the oxides and elements (TiO2, Ta, Nb, Zr, Hf) of resistant minerals, are conserved in the lower weathering zone.

iii

TABLE OF CONTENTS

ABSTRACT ...... iii

LIST OF FIGURES ...... vi

LIST OF TABLES ...... viii

LIST OF ABBREVIATIONS ...... ix

GLOSSARY OF TERMS ...... xi

ACKNOWLEDGEMENTS ...... xii

CHAPTER 1 INTRODUCTION ...... 1

1.1 Overview ...... 1

1.2 Purpose of Study ...... 2

1.3 Previous Research ...... 2

CHAPTER 2 GEOLOGIC SETTING ...... 4

2.1 Local Geology ...... 4

2.2 Alteration of Tertiary Igneous Rocks ...... 11

2.3 Rare Earth Elements and Gold at Bear Lodge ...... 12

CHAPTER 3 METHODS ...... 15

CHAPTER 4 LEAST WEATHERED CARBONATITE ...... 18

4.1 Magmatic Mineralogy ...... 19

4.2 Replacive Mineralogy ...... 23

CHAPTER 5 WEATHERED CARBONATITE ...... 32

5.1 Lower Weathering Zone ...... 32

5.1.1 Weakly Weathered Mineralogy ...... 34

5.1.2 Moderately Weathered Mineralogy ...... 34

5.2 Changes in Porosity with Weathering ...... 38

CHAPTER 6 WHOLE ROCK GEOCHEMISTRY ...... 41

6.1 Geochemical and Mineralogical Changes Due to Weathering ...... 41

6.2 Isocon Analysis ...... 42

6.3 Rare Earth Elements Concentration during Weathering ...... 52

CHAPTER 7 DISCUSSION ...... 54

7.1 Replacive REE Enrichment ...... 54

7.2 REE Enrichment from Weathering ...... 57

7.3 Timing of Weathering ...... 58

7.4 Comparison to Weathered Carbonatites at Araxá and Mt. Weld ...... 60

7.4.1 Bull Hill and Araxá Weathered Carbonatites ...... 61

7.4.2 Bull Hill and Mt. Weld Weathered Carbonatites ...... 63

CHAPTER 8 CONCLUSIONS ...... 65

REFERENCES CITED ...... 66

APPENDIX A SUPPLEMENTAL ELECTRONIC FILES ...... 71

LIST OF FIGURES

FIGURE 2.1 Paleogene Magmatic belt with N40W trend ...... 5

FIGURE 2.2 Geologic Map of Bear Lodge dome...... 7

FIGURE 2.3 Geologic Map of the north lobe of the Bear Lodge dome ...... 10

FIGURE 2.4 Schematic cross-section through the western slope of Bull Hill ...... 14

FIGURE 4.1 Manganoan-calcite textures and relationships in carbonatite...... 21

FIGURE 4.2 Textural relationships of potassium feldspar, biotite, and aegirine-augite in carbonatite ...... 22

FIGURE 4.3 Textural relationships of magmatic ilmenite and hexagonal pseudomorphs ...... 24

FIGURE 4.4 Ancylite texture and habit...... 26

FIGURE 4.5 Rare earth fluorocarbonate textural relationships ...... 27

FIGURE 4.6 Strontianite habit ...... 28

FIGURE 4.7 Backscatter electron images of rare earth phosphate habits ...... 29

FIGURE 4.8 Oxide and sulfide textures and relationships ...... 31

FIGURE 5.1 Carbonatite weathering profile for western slope of Bull Hill with mineral assemblages...... 33

FIGURE 5.2 Manganese- and iron-oxide replacive textures in moderately weathered carbonatite ...... 36

FIGURE 5.3 Ancylite and Sr-Ca-REE-phosphate textures and relationships in moderately weathered carbonatite ...... 37

FIGURE 5.4 False-color porosity maps of the least weathered carbonatite and moderately weathered carbonatite ...... 40

FIGURE 6.1 Isocon plot for average trace element concentrations of Hf, Nb, Rb, Ta, Th, U and Zr ...... 49

FIGURE 6.2 Isocon plot for average major element concentrations ...... 50

FIGURE 6.3 Isocon plot for average rare earth element and yttrium concentrations ...... 51

FIGURE 6.4 Chondrite normalized (Haskin et al., 1968) REE distribution patterns ...... 53

FIGURE 7.1 Schematic model showing three main stages of alteration and REE

enrichment...... 56

FIGURE 7.2 Regional and local geomorphological and climatic events and affecting the

Black Hills, WY...... 59

FIGURE 7.3 Pourbaix diagram illustrating the stability of cerianite...... 61

vii

LIST OF TABLES

TABLE 4.1 Rare earth minerals in sample set with formulas ...... 18

TABLE 5.1 Porosity measurements from QEMSCAN® analyses ...... 39

TABLE 6.1 Major oxide concentrations for moderately and least weathered carbonatite, including dike boundaries with breccia ...... 43

TABLE 6.2 REE and trace element concentrations for moderately and least weathered carbonatite, including dike boundaries with breccia...... 44

TABLE 6.3 Average major oxide concentrations in moderately and least weathered carbonatite ...... 45

TABLE 6.4 Average REE, Y, and trace element concentrations for low-carbonate and high-carbonate moderately weathered and least weathered carbonatite...... 46

TABLE 7.1 Araxá and Bear Lodge REE-bearing mineral assemblages and paleoclimates...... 62

TABLE 7.2 Mt. Weld and Bear Lodge REE-bearing mineral assemblages and paleoclimates...... 64

viii

LIST OF ABBREVIATIONS

Mineral Abbreviations (Whitney and Evans, 2010): aeg-aug aegirine-augite ant anatase anc ancylite bast bastnäsite bt biotite brt barite bur burbankite cal calcite Rfc rare earth fluorocarbonates gth goethite ilm ilmenite Kfs potassium feldspar py pyrite SCRp Sr-Ca-REE-phosphate syn synchysite

Technical Abbreviations: CL cathodoluminescence cpl cross-polarized light BSE backscatter electron EDS electron dispersive X-ray spectra EDX electron dispersive X-ray GRM georeference material ICP-AES inductively coupled plasma-atomic emission spectrometry ICP-MS inductively coupled plasma-mass spectrometry ppl plane polarized light RSD relative standard deviation rl reflected light

SEM scanning electron microscope tl transmitted light

GLOSSARY OF TERMS

Least Weathered Carbonatite Carbonatite exhibiting trace sulfide oxidation (correlates to Rare Element Resources’s “Unoxidized/Sulfide Zone”) Weakly Weathered Carbonatite Carbonatite exhibiting stronger sulfide oxidation and increased amounts of iron oxides (correlates to Rare Element Resources’s “Transitional Zone”) Moderately Weathered Carbonatite Carbonatite exhibiting pervasive Fe-oxide and Fe- hydroxide replacement of sulfides, Mn-oxide replacement of Mn-calcite, and partial calcite dissolution (correlates to Rare Element Resources’s “Oxide-Carbonate Zone”) Highly Weathered Carbonatite Generally lacks igneous carbonates and consists largely of Mn- and Fe- oxides with subsidiary rare earth minerals (correlates to Rare Element Resources’s “FMR” or “Oxide Zone”) Gangue carbonate Non-REE-bearing carbonate mineral, such as calcite and strontianite

ACKNOWLEDGEMENTS

I would like to recognize Rare Element Resources for financial support of this research. Additional funding was provided by the John and Carol Mann Fellowship through the Department of Geology and Geological Engineering and by the Hugh E. McKinstry Fund through the Society of Economic Geologists. I am grateful to Dr. Murray Hitzman and Dr. Richard Wendlandt for their continuous support, guidance, patience, and encouragement through the completion of this thesis. Thank you to Dr. Hitzman for introducing me to this project. I would like to thank my committee members, Dr. Thomas Monecke and Dr. James Clark, for their thoughtful contributions to this research project and their rigorous reviews of the initial draft of this thesis. Your recommendations have greatly added to the final product. Thanks to Dr. Monecke for his time and efforts regarding cathodoluminescence microscopy. Thank you to Dr. Clark for offering his invaluable knowledge of the Bear Lodge Mountains and for the opportunity to work on the Bear Lodge project. I would like to thank Heather Lowers for her assistance with extensive scanning electron microscopy as well as electron microprobe analysis at the USGS Denver Microbeam Laboratory. I would also like to thank Dr. Katharina Pfaff for her time and assistance with automated mineralogical and porosity analyses. I am truly grateful for the support, insight, fieldtrips, and thoughtful discussion from the entire geological team at Rare Element Resources, especially John Ray, Jason Felsman, Danielle Olinger, Joseph Monks, Dr. Allen Andersen, Dr. Adrian van Rythoven, and Mario Mansilla. I am forever grateful to my family and friends for their unwavering support through this entire process. In every capacity, I would not be where I am today without you, William Hutchinson and Teresa Johnson. I value you both more than I could explain here, and I think you know and understand that. I offer a special thank you to Moira and Nataraja for their invaluable support in solidarity during late night writing stints. I would also like to acknowledge Dr. Bruce Geller and Dr. Cynthia Howell for offering a path for expanded understanding of the rare earth elements and a second wind of inspiration in the final reaches of this project.

xii

CHAPTER 1 INTRODUCTION

The majority of the world’s rare earth element (REE) deposits, including some of the largest, are associated with carbonatites and alkaline igneous complexes, as well as their weathering products (Wall and Mariano, 1996; Long et al., 2010; Chakhmouradian and Zaitsev, 2012; Williams-Jones et al., 2012; Verplank et al., 2014). The Bear Lodge carbonatite dike- hosted REE deposit is located in northeastern Wyoming. It is one of the largest rare earth deposits in the United States, with a project-wide measured and indicated resource of 18 million short tons at a grade of 3.05% total rare earth oxide (TREO) (Long et al., 2010; Noble, 2014).

1.1 Overview

Bull Hill marks the central portion of the Bear Lodge deposit and is particularly enriched in LREE. Rare earth element-bearing minerals are concentrated within dominantly northwest- trending, steeply dipping carbonatite dikes, which cut heterolithic diatreme breccia on the western and slope of Bull Hill. The dikes range in thickness from less than 0.3 m to greater than 30.5 m (1 to 100 ft.). The carbonatite dikes at Bull Hill have undergone variable degrees of weathering that typically extends to an average depth of 183 m (600 ft.) below the surface. This study investigates the textural and mineralogical changes that occurred during weathering of the carbonatite in samples from a single drill hole in the western portion of Bull Hill. This study focuses specifically on REE concentration within the lower portion of the weathering profile. The carbonatite and the weathering products are categorized into least weathered, weakly weathered, moderately weathered, and highly weathered. It is important to consider that the textures and mineralogy, especially the REE-bearing assemblages, of the Bull Hill carbonatite appear to display significant heterogeneity (J. Clark, personal communication, 2015). Least weathered carbonatite exhibits only trace oxidation of sulfides and contains minor Fe-oxides. Weakly weathered carbonatite exhibits stronger sulfide oxidation and contains increased amounts of iron oxides. The moderately weathered carbonatite is characterized by pervasive Fe-oxide and Fe-hydroxide replacement of sulfides, Mn-oxide replacement of Mn-

calcite, and partial calcite dissolution. Highly weathered carbonatite generally lacks igneous carbonate minerals and consists largely of Fe- and Mn- oxides with rare earth minerals. Highly weathered carbonatite generally has a granular, unconsolidated texture. The highly weathered carbonatite is locally silicified in the northwest portion of the Bull Hill study area.

1.2 Purpose of Study

This study investigated the mechanisms of REE enrichment in the lower weathering zone of the Bull Hill area at the Bear Lodge deposit through characterization of textural, mineralogical, and geochemical variations in unweathered to moderately weathered carbonatite. Observations and analyses were completed on samples from an uninterrupted carbonatite intercept in diamond drill hole RES09-17 from the western slope of Bull Hill which grades from least weathered to moderately weathered carbonatite. This drill core contained the only continuous interval of unweathered to weathered carbonatite at the time this study was initiated. As many REE deposits appear to display upgrading due to weathering (Morteani and Preinfalk, 1996; Hoatson et al., 2011), this study adds to the understanding of the geochemical changes and the behavior of REE-bearing minerals during carbonatite weathering.

1.3 Previous Research

The publications discussed here represent the more recent or largely notable geologic studies of the Bear Lodge Mountains, subsequent to a lineage of exploration and research completed by many entities as outlined in Staatz (1983), Bird (2005), and Noble (2014). The USGS published the first extensive geologic report and large-scale map of the Bear Lodge Mountains igneous complex (Staatz, 1983). Staatz (1983) provided a detailed geological, mineralogical, and geochemical framework of rare earth element occurrences. Rare Element Resources’s Canadian NI 43-101 compliant Prefeasibility Study Report detailed the deposit exploration history, deposit geology, and REE-occurrences (Noble, 2014). Recent research has advanced the understanding of the geology of the Bear Lodge complex. Duke (2005) places the complex into a broader geochemical and geochronological framework of Paleogene magmatism in the northern Black Hills. The collective work of Staatz

(1983), Jenner (1984), Duke (2005), and Felsman (2009) established a temporal framework for intrusive relationships in the Bear Lodge dome, based on field relationships and K/Ar and 40Ar/39Ar analyses. Three major stages of magmatism were recognized: initial alkaline silicate magmatism occurring 50-46 Ma, followed by carbonatitic magmatism, and a late phase of silicate alkaline magmatism occurring at 40-38 Ma (Duke, 2005). Andersen et al. (2013) constrained the age range of carbonatite magmatism in the Bear Lodge dome from 51.15 Ma to 51.6 Ma based on Ar40/Ar39 analyses of 13 phenocrysts within various REE-bearing carbonatites (A. Andersen, personal communication, 2015). These results indicate the need for additional study of the sequence and absolute timing on a more robust sample set of alkaline silicate rocks in relation to carbonatite. Carbonatite genesis and its relation to the surrounding alkaline silicate rocks of northwestern Bull Hill was investigated by Olinger (2012), who recognized apparently both magmatic and hydrothermally recrystallized calcite in northwestern Bull Hill. Olinger (2012) also identified hydrothermally recrystallized calcite in one sample from western Bull Hill, near the area of the current study. Additional research on carbonatite genesis and REE occurrences within Bull Hill area was completed by Moore et al. (2015) who recognized five different paragenetic stages associated with REE-mineral deposition in unweathered carbonatite, plus two paragenetic stages related to oxidation (or weathering). The parageneses observed in unweathered carbonatite were present in separate veins or dikes (Moore et. al, 2015). Recent work by Rare Element Resources, Ltd. has shed light on district-scale zonation of LREE and HREE within the Bear Lodge dome. The Bull Hill area shows LREE-enrichment; whereas, areas to the north and west of Bull Hill, including Whitetail Ridge, the southern slope of Carbon Hill, and the eastern portion of Taylor Ridge, exhibit elevated HREE concentrations relative to those observed at Bull Hill (Noble, 2013). Preliminary mineralogical studies completed by the company suggest that parisite and synchysite are possible HREE hosts at Whitetail Ridge and the Carbon Hill area; xenotime, rhabdophane, yttrium-vanadium-phosphate, parisite, and synchysite are likely the HREE-bearing minerals in the Taylor Ridge area (Noble, 2014). The company also observed that monazite in these areas is Nd-enriched (Noble, 2013 and 2014). The current study builds upon previous work and is the first to focus with detail on the weathering profile within a single drill core intercept of continuous carbonatite at the Bear Lodge deposit.

CHAPTER 2 GEOLOGIC SETTING

The Bear Lodge Mountains are located in Crook County in northeastern Wyoming. They are the northwestern expression of the Black Hills domal uplift, which is a result of the Late Cretaceous-Paleogene Laramide Orogeny (Duke, 2005). The Black Hills magmatic belt is part of a larger regional N40°W trend (Fig. 2.1) that extends approximately 700 km (435 miles) from the Black Hills to the Montana-Alberta border (Duke, 2005). Alkaline igneous intrusions in the Bear Lodge Mountains are associated with the Black Hills Paleogene magmatic belt that extends approximately 109 km (68 miles) in a northwesterly (N70°-80°W) trend from Bear Butte in western South Dakota to the Missouri Buttes in northeastern Wyoming (Staatz, 1983; Duke, 2005). Intrusions within this belt range in age from 58 to 46 Ma and generally decrease in age to the northwest (Duke, 2005). There is also an overall decrease in silica saturation and increase in alkalinity to the northwest (Duke, 2005). The central Bear Lodge dome consists of an alkaline igneous core flanked by Paleozoic and Mesozoic sedimentary rocks (Jenner, 1984; Felsman, 2009). The first phase of alkaline silicate magmatism in the Bear Lodge dome was thought to be contemporaneous (51-46 Ma) with the regional magmatism (Fig. 2.1) (Duke, 2005; Andersen, 2013). However, based on more recent work (Andersen et al., 2013), these rocks may be older than previously thought.

2.1 Local Geology

The oldest rocks in the Bear Lodge Mountains are Precambrian granites that are exposed in the southern portion of the Bear Lodge dome (Fig. 2.2) as irregular and elongated xenoliths within Paleogene intrusives (Staatz, 1983; Felsman, 2009). The Precambrian granite is generally medium-grained, holocrystalline, and leucocratic. Staatz (1983) gives a minimum age of 2.6 Ga. Paleozoic sedimentary rocks surround the Bear Lodge dome and dip radially outward from the igneous core (Fig. 2.2). The oldest sedimentary unit in the Bear Lodge Mountains is the Late Cambrian – Early Ordovician Deadwood Formation. Regionally, it consists of conglomerate, limestone, shaley limestone, shale, siltstone, arkosic sandstone, and quartz arenite.

In the Bear Lodge Mountains the Deadwood Formation is represented primarily by contact metamorphosed sandstones (Staatz, 1983; Felsman, 2009). Younger Paleozoic strata surrounding the Bear Lodge dome include the Late Ordovician Whitewood Limestone, the Early Mississippian Pahasapa Limestone, and the Pennsylvanian-early Permian Minnelusa Sandstone.

Figure 2.1: Paleogene magmatic belt with N40W trend. The Bear Lodge dome is considered part of the Black Hills magmatic belt, but it also shares the larger trend of alkaline magmatism which extends past the Montana-Alberta border. After Duke (2005).

Magmatism within the Bear Lodge dome is thought to have occurred in three episodes. This first included the emplacement of syenite, microsyenite, porphyritic trachyte and megacrystic sanidine trachyte porphyry, and lesser latite, quartz latite, phonolite, and diatreme breccias (Felsman, 2009). Many of these alkaline silicate rocks, especially at Bull Hill, were strongly fenitized (Olinger, 2012). The second magmatic episode resulted in the emplacement of calciocarbonatite and silicocarbonatite dike swarms that crosscut older alkaline silicate rocks, including diatreme breccias. A third magmatic episode included phonolite and trachyte intrusions, as well as lamprophyre dikes. Volcanic breccias were also produced during the latest event (Felsman, 2009). The Bear Lodge dome displays north and south lobes, each of which consists mainly of Paleogene intrusive rocks (Fig. 2.2). Felsman (2009) interpreted the lobes to represent

laccoliths, with syenite grading upward to microsyenite and then hypabyssal trachyte. The North Lobe, in which the Bull Hill deposit is located, contains porphyritic to aphanitic trachyte, phonolite, megacrystic sanidine porphyry, pseudoleucite porphyry, latite, lamprophyre, nepheline syenite, diatreme intrusion and intrusive breccias, and carbonatite. Trachyte intrusions at Bear Lodge occur as stocks, sills, and dikes. Trachyte occurs also as clasts and matrix within intrusive breccia. Texturally, the trachyte ranges from fine-grained aphyric to porphyritic. The trachyte contains sanidine phenocrysts and microphenocrysts with lesser clinopyroxene, biotite, and feldspathoid phenocrysts, dispersed in a fine-grained to aphanitic groundmass. Feldspathoid phenocrysts are rarely unaltered (Jim Clark, personal communication, 2016). The groundmass typically consists of alkali feldspar, devitrified glass, and nepheline with rare sodalite, biotite, augite, alkali amphibole, apatite, and accessory sulfide minerals or oxidized sulfide minerals (Noble, 2013). Phonolite occurs as dikes and as clasts and matrix in intrusion breccia. Phonolite textures range from fine-grained porphyritic to megacrystic (Olinger, 2012; this study). The groundmass is typically felted to pilotaxitic (Felsman, 2009), but also is commonly fine-grained trachytic (Olinger, 2012). The phonolite is mineralogically similar to the trachyte, but contains more feldspathoid (pseudoleucite and nepheline) phenocrysts and microphenocrysts and fewer sanidine phenocrysts. Megacrystic phonolite and trachyte porphyries occur as dikes that cut the intrusive breccia and less commonly as clasts within intrusive breccia (this study). The porphyries are characterized by sanidine feldspar phenocrysts 1 to 5 cm in length (Felsman, 2009; Olinger, 2012). Phenocrysts are surrounded by a dark grey, felted to pilotaxitic or trachytic groundmass (Felsman, 2009; Olinger, 2012). Pseudoleucite porphyry forms narrow dikes which typically cut trachyte and phonolite stocks and dikes. The pseudoleucite phenocrysts in this rock type are fine- to coarse-grained (sometimes larger than 1.5 cm) and are often accompanied by lesser sanidine microphenocrysts in a dark grey to brown groundmass (Noble, 2013; this study). The groundmass consists of devitrified glass, nepheline, sanidine, biotite, sodic pyroxene, and minor sulfide minerals (Noble, 2013).

Figure 2.2: Geologic Map of the Bear Lodge dome. This map shows the north and south lobes of the dome and the surrounding sedimentary rocks. The three diatremes in the north lobe of the dome are highlighted: BH=Bull Hill, WT=Whitetail Ridge, and CH=Carbon Hill. The carbonatite dikes in the area are too small to be observed at this scale map but are concentrated in the diatremes. More detail is shown below in Figure 3.3. After Ray (2013).

Latite dikes and sills contain fine-grained plagioclase and sanidine phenocrysts in a medium grey groundmass (Felsman, 2009; Olinger, 2012; this study). The groundmass is typically felty to trachytic in texture and can be equigranular (Olinger, 2012). The groundmass mineralogy contains plagioclase, sanidine, hornblende, biotite, pyroxene, apatite, zircon, and sometimes nepheline (Felsman, 2009; Olinger, 2012). Syenite is medium- to coarse-grained with grey potassium feldspar and variable amounts of nepheline, biotite, muscovite, pyroxene, alkali amphibole, hornblende, sphene, and olivine (Felsman, 2009; Noble, 2013). Apatite, plagioclase, pyrite, magnetite, allanite, pyrrhotite, and ilmenite occur as accessory phases (Felsman, 2009; Noble, 2013). Drilling by Newmont suggests that the syenite forms a small pluton beneath part of the Bear Lodge dome (Felsman, 2009). Potassium feldspar-rich microsyenite with plagioclase, apatite, zircon, biotite, and muscovite (Felsman, 2009) forms a gradational contact between the syenite and overlying trachyte. Three main intrusive diatreme breccia bodies (Bull Hill, Whitetail Ridge, and Carbon Hill) occur within the north lobe of the Bear Lodge dome (Fig. 2.3). The breccias range from homolithic to heterolithic with well- to poorly-sorted phonolite and trachyte clasts ranging from centimeters to several meters in diameter. The breccias also contain rare carbonatite clasts (Jim Clark, personal communication, 2016). Diatreme intrusive breccias are heterolithic and dominated by recrystallized rock flour matrices of phonolitic to trachytic composition. At Bull Hill, intrusive breccias contain subrounded to subangular clasts in a microcrystalline to fine- grained matrix. The breccias vary from thin zones generally displaying well-sorted clasts to extensive zones of poorly sorted clasts up to boulder size. At Bull Hill, intrusive breccia locally grades into intrusion breccia, typically with a phonolite or trachyte matrix and less commonly with a carbonatitic matrix. Intrusion breccias with syenite and microsyenite clasts and matrices are recognized in the Carbon Hill and Bull Hill areas (Felsman 2009). Carbonatite dikes and surrounding stockworks form a swarm cutting the Bull Hill, Whitetail Ridge and Carbon Hill diatremes (Fig. 2.3). The dikes range in thickness from less than 0.3 m to greater than 30.5 m (1 to 100 ft.), although dikes up to 80 m are known in the area (Noble, 2014). The dikes strike north-northwest and are mostly steeply dipping. Compositionally, these intrusions are predominantly calcitic and display strong Mn, REE and Sr enrichment. When fresh, they are generally white to light grey in color and speckled with dark

biotite/phlogopite, Ti-oxide minerals, brassy sulfide minerals, and light pink-tan hexagonal pseudomorphs. Textures range from fine- to coarse-grained. Typical dikes contain calcite, Mn- calcite, strontianite, dolomite, and ankerite with accessory biotite/phlogopite, potassium feldspar, aegirine-augite, and minor barite, apatite, sulfide minerals (pyrite, pyrrhotite, chalcopyrite, galena, sphalerite, molybdenite), oxide minerals (ilmenite, pyrophanite, anatase/rutile), fluorapatite, and rare earth minerals (ancylite, with lesser synchysite/parisite, bastnäsite, burbankite, carbocernaite, monazite, daqingshanite, and an unidentified Sr-Ca-REE-phosphate) (Noble, 2013 and 2014; J. Clark, personal communication, 2016; this study). Silicocarbonatite dikes are present and are typically fine-grained, though coarse-grained silicocarbonatite with biotite books up to 1 cm in diameter are observed. Silicocarbonatite at Bull Hill contains calcite and 30 to 50 percent silicate minerals, including biotite, phlogopite, and/or potassium feldspar (Olinger, 2012; Noble, 2013 and 2014). Younger trachyte and phonolite dikes crosscut older (51-46 Ma) alkaline silicate intrusive rocks at Bear Lodge (Staatz, 1983; Duke, 2005; Felsman, 2009). These younger dikes are similar in composition to older trachyte and phonolite. The trachytes and phonolites intruded after the carbonatite and do not show evidence of carbonatite-related fenitization/metasomatism (Staatz, 1983). They are considered part of a late, 40-38 Ma magmatic event (Staatz, 1983; Jenner, 1984; Felsman, 2009). Dark grey to black, fine-grained porphyritic to aphyric lamprophyre dikes not affected by fenitization are also considered to have been emplaced around 40-38 Ma (Felsman, 2009). However, Noble (2013) suggested that both early (51-46 Ma) and late (40-38 Ma) lamprophyre dikes occur. Both silica-undersaturated and silica-saturated lamprophyre dikes are recognized (Felsman, 2009). The mineralogical composition of the dikes varies and can include pyroxene, hornblende, biotite, feldspar, and nepheline with minor pyrite, ilmenite, rutile, and magnetite (Olinger, 2012; Noble, 2013). The lamprophyre dikes can contain ocellar zones of carbonate minerals, intergrown with apatite, potassium feldspar, leucite, and lesser biotite (Olinger, 2012; Noble, 2013). The youngest rock units in the Bear Lodge Mountains are the Oligocene White River Formation and the late Miocene-early Pliocene Ogallala Formation. The White River Formation unconformably overlies Phanerozoic sedimentary rocks and Paleogene intrusive rocks (Staatz, 1983). It consists of tan, friable, poorly bedded siltstones that crop out along the sides of flat-

topped ridges in the Bear Lodge Mountains. The Ogallala Formation unconformably overlies the White River Formation. The Ogallala Formation consists of interlayered, locally calcareous sandstones and siltstones with sparse conglomeratic layers containing clasts of Precambrian granite, Paleozoic sedimentary rocks, and trachyte and phonolite (Staatz, 1983).

Figure 2.3: Geologic map of the north lobe of the Bear Lodge dome, including the Bull Hill (BH), Whitetail Ridge (WT), and Carbon Hill (CH) diatremes. The carbonatite outcrops in only one small area on Carbon Hill; a map unit was therefore created to show zones where the weathered carbonatite with greater than 2% TREO is present below the surface. Note the general northwesterly trend of intrusions and weathered carbonatite. After Staatz (1983), Noble (2013), and Ray (2013).

2.2 Alteration of Paleogene Igneous Rocks

Paleozoic sedimentary rocks and early alkaline silicate rocks (51-46 Ma) are hydrothermally altered. The alteration is a form of alkali-ferric iron metasomatism (fenitization) and appears spatially related to carbonatite intrusion. The local fenitization is characterized mesoscopically by the blurring of the phenocryst-groundmass boundaries and a porcelaneous texture (bleached and hardened). The alteration involves the replacement of plagioclase and potassium feldspar by secondary potassium feldspar, which is characterized by a deep red cathodoluminescence (CL) as observed with hot-cathode CL microscopy. The replacive potassium feldspar exhibits a similar red response under unfiltered short wave UV radiation. The red luminescence response reflects the substitution of Fe3+ for Al3+ in the feldspar lattice (Mariano, 1978; Mariano, 2012, personal communication) as determined by electron microprobe analyses (Felsman, 2009). Fenitization may also involve the local replacement of mafic minerals with biotite and pyrite (Felsman, 2009). Early alkaline igneous rocks that are crosscut by carbonatite also commonly contain veins of calcite and sulfides (Felsman, 2009; Olinger, 2012). Multiple episodes of silicification, quartz veining, and opaline or chalcedonic quartz void-filling affected rocks throughout the Bear Lodge dome (Felsman, 2009). Felsman (2009) observed silicified and fenitized trachyte clasts in unfenitized volcanic breccia northeast of Carbon Hill indicating at least some silicification occurred prior to the final pulse of magmatism. Additionally, weathered carbonatite is silicified in places surrounding Bull Hill. Weathering depth in the Bear Lodge dome is variable (Fig. 2.4). In the Bull Hill area, the basal portion of the Moderately Weathered Zone is characterized by weakly weathered carbonatite and is referred to as the Transitional Zone by Rare Element Resources; this zone generally occurs at 152 m to 183 m (500-600 ft) below the surface (Noble, 2013 and 2014. Appendix A includes a diagram of samples obtained from these different zones and a comparison between the terminologies used in this study compared to those of Rare Element Resources. Carbonatite near the present day ground surface has been thoroughly weathered. Rare Element Resources’ geologists have termed this calcite-depleted oxide material as FMR (Fe-Mn- REE-oxide); this material is underlain by granular, calcite-bearing oxide material (Oxide- Carbonate of Rare Element Resources) that is similar to the moderately weathered carbonatite

investigated in this study (Noble, 2013 and 2014). In the Bull Hill area, the boundary between the calcite-bearing and calcite-depleted weathered rocks occurs 61 to 152 m (200-500 ft.) below the surface and marks the boundary between the Upper Weathering Zone and Lower Weathering Zone. This boundary is often transitional, with interlayered calcite-bearing and calcite-depleted intervals (Fig. 2.4). The transition between the calcite-bearing and the overlying calcite-absent zones generally coincides with a shift from ancylite-dominant rare earth mineralogy to bastnäsite group-dominant rare earth mineralogy, respectively (Noble, 2013 and 2014; A. Van Rythoven and J. Clark, personal communication, 2014). Weathered carbonatites rarely and irregularly display textures indicative of post- weathering silicification. Silicified zones to the northwest of Whitetail Ridge and on the southern flank of Whitetail ridge contain chalcedony-lined voids. This late silicification was likely related to the production of silicic acid from the alteration of feldspar to clays and from oxidation (Felsman, 2009). It is unclear if late igneous activity (40-38 Ma) contributed to the late silicification events.

2.3 Rare Earth Elements and Gold at Bear Lodge

Rare earth elements are concentrated within calciocarbonatites and silicocarbonatite dikes and associated stockwork veins. Within carbonatite dikes, rare earth mineral assemblages vary with depth. Ancylite is the main rare earth-bearing mineral in unweathered carbonatite. Additional rare earth phases in carbonatite include synchysite/parisite, bastnäsite, an unidentified Sr-Ca-REE-phosphate, monazite, carbocernaite, and burbankite (Noble, 2014; this study). Ancylite remains stable in the lowermost portion of the weathering profile, which is mainly calcite bearing. Other, generally minor, REE-bearing minerals in the lower weathering zone are rare earth fluorocarbonates, monazite, Sr-Ca-REE-phosphate, and cerianite. Rare earth fluorocarbonates, especially bastnäsite, are dominant higher in the weathering profile. The rare earth fluorocarbonates are accompanied by lesser monazite, cerianite, and very minor rare earth aluminum phosphates (Noble, 2014; A. Van Rythoven and J. Clark, personal communication, 2014). The Bull Hill area shows strong LREE-enrichment, which is typical of carbonatites. Areas to the north and west of Bull Hill, including Whitetail Ridge, the southern slope of Carbon

Hill, and the eastern portion of Taylor Ridge, also exhibit REE-enrichment but with elevated HREE abundances relative to those observed at Bull Hill (Noble, 2014; J. Clark, personal communication, 2016). Preliminary mineralogical studies completed by Rare Element Resources indicate that parisite and synchysite appear to be HREE hosts at Whitetail Ridge and the Carbon Hill area. Xenotime, Y-V-phosphate minerals, parisite, and synchysite are likely the HREE-bearing minerals in the Taylor Ridge area (Noble, 2014). Gold is found on the periphery of the LREE-enriched core of the Bear Lodge alkaline complex, specifically in the Smith Ridge area southeast of Bull Hill, the Taylor Ridge area west of Bull Hill, and in an area occupying the southeastern flank of Carbon Hill leading southeastward to Whitetail Ridge (northwest of Bull Hill). Gold is spatially associated with potassic fenitization and occurs within faults and breccias as well as along lithologic contacts (Felsman, 2009; Noble, 2013). Disseminated native gold and gold-tellurides are observed in phonolite, trachyte, microsyenite, latite, intrusion breccia, sandstone and siltstone of the Paleozoic Deadwood Formation, and Precambrian granite (Felsman, 2009; Noble, 2013 and 2014). Pseudoleucite porphyry hosts low-grade, disseminated native gold and tellurides in the Smith Ridge area southeast of Bull Hill (Felsman, 2009). Gold and HREE occurrences are generally coincident in the East Taylor Ridge, Carbon Hill, and Whitetail Ridge areas (Felsman, 2009; Noble, 2013 and 2014).

Figure 2.4: Schematic ross-section through the western slope of Bull Hill. The section shows igneous carbonatite dikes crosscutting the Bull Hill diatreme. This study focused on the Lower Weathering Zone, with samples derived from diamond drill hole RES09-17, interval 133 m to 540 m. Variation of carbonate (mainly calcite and strontianite) content in the Lower Weathering Zone is shown with darker background colors depicting low-to no carbonate regions. The Lower Weathering Zone contains both moderately and weakly weathered carbonatite.

CHAPTER 3 METHODS

This study utilized diamond drill hole RES09-17 from the western slope of Bull Hill to examine an uninterrupted transition from least weathered to highly weathered carbonatite. The drill core was logged in detail and quarter core samples taken for analysis (Appendix A). Forty- five polished thin sections were prepared from the samples. Representative thin sections were selected for petrographic analysis using standard transmitted and reflected light techniques (Appendix A). Selected thin sections, non-epoxy grain mounts of granular weathered carbonatite samples, and epoxy mounts of intact and granular moderately weathered carbonatite samples were analyzed on three different scanning electron microscopes: an FEI Quanta 600, a JEOL JSM-5800LV, and a JEOL JSM840-A. Energy dispersive X-ray spectra (EDS) were collected on the FEI Quanta 600 and JEOL JSM-5800LV using either 10 or 15 kV accelerating voltage, a 0.26 nA beam current, and a 30 second collection time. For the JEOL JSM840-A, EDS were collected with a 20 kV accelerating voltage, a 0.08mA beam current, and a 60 second collection time. Optical cathodoluminescence microscopy was performed on four least weathered carbonatite samples for observing the zoning in manganoan calcite and calcite (Appendix A). The cathodoluminescence microscopy was conducted on polished, carbon-coated thin sections using an HC5-LM hot-cathode CL microscope from Lumic Special Microscopes, Germany operated at 14 kV with a current density of 10 μA mm-2. Photomicrographs were taken with a high sensitivity, double-stage Peltier cooled Kappa DX40C CCD camera with exposure times ranging from approximately 320 milliseconds to 950 milliseconds. Areas containing feldspar required longer exposure times of approximately 1.4 to 3.0 seconds. Sixteen samples underwent compositional and textural analysis using SEM-based automated mineral technology at Colorado School of Mines. These analyses were completed with an FEI QEMSCAN® system and the control program iDiscover to acquire spectra. For this study, spectra were acquired using an accelerating voltage of 25 keV, a beam current of 5 nA, and a beam stepping interval based on desired pixel resolution. The mineral phase assignment was made based on acquired electron dispersive X-ray (EDX) spectra and backscatter electron (BSE) values and comparison of the spectra and BSE values with those in a reference mineral

library. The assignment is made without differentiation between mineral species and amorphous grains of similar composition. Results include a false-color mineral map and quantitative area percent (modal abundance). Scans were completed at 1 pixel = 40 μm resolution for three of the least weathered carbonatite samples and five moderately weathered carbonatite samples. Higher resolution (1 pixel = 20 μm or 1 pixel = 2 μm) was utilized for detailed analyses of three of the least weathered carbonatite samples and six of the weathered samples. The porosity of the least weathered carbonatite and the moderately weathered carbonatite was also measured using the QEMSCAN® system and iDiscover software. The methodology utilized for the porosity measurements was based on laboratory specific protocols and Jobe (2013). Scans were completed with a 1 pixel = 2 μm resolution. Pore spaces, minerals, and overlapping areas were identified using predetermined backscatter electron (BSE) values. Modifications were made to the parameters to represent better the samples in this study and to accommodate for the presence of bubbles in the epoxy samples. A total of 17 quarter-split core samples plus 8 quality control samples were submitted to Analytical Laboratory Services (ALS) in Reno, Nevada for analyses of rare earth elements, major and trace elements, loss on ignition, total C, total S, and ferrous Fe. Additional sample information, including core interval and sample weight, is included in Appendix A. Rare earth element and trace element concentrations were determined using inductively coupled plasma- mass spectrometry (ICP-MS) on samples prepared with a lithium metaborate fusion method. Major oxide components were determined using inductively coupled plasma – atomic emission spectrometry (ICP-AES) for samples decomposed by lithium metaborate/lithium tetraborate fusion. Loss on ignition was determined on samples that were subjected to a thermal decomposition furnace at 1000°C for one hour. Ferrous Fe concentration was determined by titration with a potassium dichromate solution on samples prepared with a four-acid digestion

(HCl-HNO3-H2SO4-HF), ammonium hydroxide Fe-precipitation, and SnCl2 Fe-reduction. Total S and total C were determined on samples using the Leco method. Two duplicates, two quartz sand georeference materials, and two high and two low REE georeference materials (GRM) RE_9004x and RE_9003x provided by Rare Element Resources, Ltd. (Noble, 2013) were analyzed with the sample set for quality control. Precision for major element and rare earth element data was calculated using GRM RE_9003X and GRM RE_9004X. The relative standard deviation (RSD) was less than 2% for all REE, except erbium

(2.54 max % RSD), thulium (6.41 max % RSD), and ytterbium (5.34 max % RSD). Precision- based errors for trace elements, including Th, were below 2% RSD, except for Ta. Georeference material RE_9003X had 2.89% RSD for Ta. Georeference material RE_9004X contained Ta concentrations near the 0.5 ppm detection limit, resulting in 10.88% RSD. The error was below 4% RSD for all major elements analyses, including loss on ignition, C, and S and except for ferrous Fe (30.55 max % RSD) and Na (6.73 max % RSD). The source of the high error for ferrous Fe is unknown; the error for Na is explained by the concentration in the samples approaching the lower detection limit of 0.01 wt. %. Additional quality control information, including analytical accuracy, is outlined in Appendix A.

CHAPTER 4 LEAST WEATHERED CARBONATITE

The carbonatite dikes within the Bear Lodge dome are predominantly calciocarbonatitic in composition. These dikes display moderate to strong heterogeneity in texture and mineralogy. When fresh, they are generally white to light grey in color and speckled with dark biotite/phlogopite books, Ti-oxide clusters, brassy sulfide mineral clusters, and light pink-tan hexagonal pseudomorphs. Textures range from fine- to coarse-grained. Typical dikes contain calcite, Mn-calcite, strontianite, dolomite, and ankerite with accessory biotite/phlogopite, potassium feldspar, aegirine-augite, and minor barite, apatite, sulfide minerals (pyrite, pyrrhotite, chalcopyrite, galena, sphalerite, rare molybdenite), oxide minerals (ilmenite, pyrophanite, anatase/rutile), fluorapatite, and rare earth minerals (ancylite, synchysite/parisite, bastnäsite, burbankite, daqingshanite, and an unidentified Sr-Ca-REE-phosphate) (Table 4.1) (Noble, 2013 and 2014; J. Clark, personal communication, 2016; this study). The only visible signs of weathering in the least weathered carbonatite observed in this study are the presence of trace Fe- and Mn-oxide minerals, which are only visible microscopically, infilling voids within REE- bearing minerals.

Table 4.1: Rare earth minerals in sample set with formulas.

Mineral Chemical Formula

Ancylite SrLREE(CO3)2(OH) • H2O

Bastnäsite LREE(CO3)F

Synchysite CaLREE(CO3)2F * Unidentified Sr-Ca-REE-phosphate (Sr,Ca, LREE, Th)PO4

Daqingshanite (Sr,Ca,Ba)3LREE(CO3)3-x(PO4)(OH,F)2x

Burbankite (Na,Ca)3(Sr,Ba,LREE)3(CO3)5

Britholite Ca2(Ce,Ca)3(SiO4 ,PO4 )3(OH,F) 4+ Cerianite (Ce ,Th)O2 * The unidentified mineral has a hypothetical formula based on SEM EDX data (Appendix A).

Textures observed in thin section indicate two main paragenetic stages of rare earth mineral growth: magmatic and replacive. Moore et al. (2015), however, recognized three paragenetic stages of rare earth mineralization in various carbonatite samples from the Bull Hill area. The magmatic rare earth mineralization stage of this study may correspond to the “burbankite” paragenetic stage of Moore et al. (2015), while the replacive stage of the current study may correlate to Moore et al.’s (2015) “ancylite” and “ancylite-fluorocarbonate” paragenetic stages.

4.1 Magmatic Mineralogy

The carbonatite displays early crystallization of phenocrystic and microphenocrystic potassium feldspar, biotite, aegirine-augite, ilmenite, and an unidentified hexagonal mineral. Most phenocrysts are moderately to strongly embayed, exhibit resorption textures, and display alteration rims (Fig. 4.1a). These phenocrysts are set in a matrix composed dominantly of manganoan calcite that constitutes approximately 45-60 average volume percent of the rocks. Manganoan calcite has grain sizes ranging from 0.02 to 5 mm and typically displays a bimodal size distribution. Smaller grains are anhedral, mostly sub-equant, and contain few inclusions, mainly burbankite (Fig. 4.1b). Larger grains are commonly anhedral with a consertal texture (serrated edges). They contain abundant one- and two-phase fluid inclusions and have inclusions of burbankite and possibly fluorite (Fig. 4.1c). Large and small manganoan calcite grains are intergrown. Based on EDX analyses, Mn-calcite contains up to 8 wt. % Mn and up to 1.5 wt. % combined Sr, Fe, and Mg. Smaller grains exhibit zoned cathodoluminescence (CL) which is expressed as quenched, dark cores with bright red rims (Fig. 4.1d, e), as well as zoning in BSE images (Fig. 4.1b). Larger, consertal grains exhibit quenched luminescence (Fig. 4.2e) and may encapsulate relict smaller grains (Fig. 4.1f). The occurrence of this relict pattern in the BSE images agrees with the finding of Olinger (2012), who suggested from REE patterns that Mn-calcite at northwestern Bull Hill may have recrystallized through interaction with a hydrothermal fluid. Potassium feldspar occurs as large subhedral phenocrysts that display a strongly resorbed and partially embayed texture with Mn-calcite and rare earth minerals mainly infilling the interstices (Fig. 4.2a). Grain sizes generally range from 0.1 mm to 2.0 mm, though the feldspar

phenocrysts up to 4.0 mm in length have been observed. Some of the potassium feldspar grains exhibit patchy zoning in cross-polarized light. Resorbed potassium feldspar phenocrysts display altered rims (Fig. 4.2a) that contain up to 1.5 wt. % Ba (based on EDS). Both unaltered and altered rims on potassium feldspar grains can contain trace amounts of Na (up to 0.3 wt. % Na, based on EDS). Limited observations from interference figures in this study suggest that the feldspar is orthoclase. Olinger (2012) also observed orthoclase in carbonatite from Bull Hill; however J. Clark (personal communication, 2015) observed mostly sanidine in the carbonatite, based on observations of very small 2Vα in the feldspar phenocrysts. The differences may be due to sample location. Owing to this uncertainty, the feldspar will be referred to as potassium feldspar in this study. Biotite is commonly intergrown with potassium feldspar phenocrysts and may in places be replaced by potassium feldspar (Fig. 4.2a, b). Biotite commonly exhibits multiple resorption and regrowth boundaries (Fig. 4.2c, d) and variable degrees of complex compositional zoning (Fig. 4.2c, d, and e), suggesting open system processes, and possibly magma mixing (Appendix A). Biotite rims are most commonly subhedral. Aegirine-augite was not present in the least weathered samples studied but was observed in several moderately weathered samples. It typically occurs in clusters as euhedral prismatic crystals (Fig. 4.2f), which are generally sub-parallel to slightly radiating. It exhibits green-pink pleochroism in transmitted light and can contain up to 13.4 wt. % Na, up to 10 wt. % Fe, up to 0.7 wt. % Ca, and up to 0.5 wt. % Al (based on SEM EDX data). Aegirine-augite is intergrown with calcite and biotite, suggesting concurrent magmatic crystallization (Fig. 4.2f) and is apparently intergrown with the REE minerals synchysite/parisite, and Sr-Ca-REE-phosphate, which may replace original magmatic burbankite. Ilmenite occurs as subhedral grains, ranging in size from 0.02 mm to 2.0 mm. It is Mn- bearing, containing up to 16.6 wt. % Mn (based on SEM EDS), and is therefore more of an ilmenite-pyrophanite solid solution. Magmatic ilmenite within the weakly weathered samples is anhedral and commonly displays partially resorbed edges (Fig. 4.3a, b) with discontinuous rims of niobian anatase (Fig. 4.3a, b). Ilmenite rims hexagonal pseudomorphs together with biotite (Fig. 4.3c, d). Ilmenite rarely can occur intergrown with galena, chalcopyrite, and pyrite.

Figure 4.1: Manganoan-calcite textures and relationships in carbonatite. A) Manganoan calcite cutting and surrounding fractured, embayed and partially resorbed biotite with chlorite cores (BSE). B) Fine-grained Mn-calcite exhibiting compositional zoning and small burbankite inclusions (BSE). C) Coarse-grained, interstitial manganoan calcite with anhedral burbankite (white) and fluid (black) inclusions (BSE). D) Cathodoluminescent (CL) image of carbonatite showing zoning in fine-grained calcite and quenching in coarse-grained calcite. The edge of a rare earth-bearing pseudomorph is outlined at the top of the image. E) Photomicrograph (cpl, tl) of the image in (D). F) BSE image of course-grained calcite recrystallized from fine-grained calcite (partially outlined in yellow dashed lines). bur=burbankite, bt=biotite, chl=chlorite, Mn-cal=manganoan-calcite, 21

Figure 4.2: Textural relationships of potassium feldspar, biotite, and aegirine-augite in carbonatite. A) BSE image of strongly resorbed and embayed potassium feldspar, intergrown with biotite and displaying reaction rims which contain barium. B) BSE image of embayed potassium feldspar phenocryst, intergrown with biotite and surrounded by Mn-calcite. C) Photomicrograph (tl, ppl) of biotite displaying complex zoning and multiple resorption and regrowth boundaries, surrounded by REE-bearing minerals and calcite. D) Biotite displaying complex zoning (tl, ppl) and surrounded by Mn-calcite. E) Biotite displaying weak zoning and alteration in a matrix of Mn-calcite (tl, ppl). F) Weakly zoned biotite intergrown with aegirine-augite (dark pale green; tl, ppl). aeg-aug=aegirine- augite, bt=biotite, Kfs=potassium feldspar, Mn-cal=manganoan-calcite, Ti-bt=titanian biotite.

Hexagonal-shaped pseudomorphs after an unidentified phenocryst make up approximately 10 vol. % of the primary magmatic mineralogy. These hexagonal pseudomorphs comprise a polymineralic assemblage dominated by REE-bearing minerals (Fig. 4.3e, f). The hexagonal-shaped pseudomorphs range in size from 1 mm to 1 cm with rare examples to 2 cm in diameter. The pseudomorphs have been observed to contain rare inclusions of burbankite (Moore et. al, 2015). Olinger (2012), Moore et al. (2015), and Clark (personal communication, 2012) consider that burbankite was the original hexagonal phenocrystic phase. Burbankite also occurs as rare, very fine-grained (< 10 μm) inclusions in magmatic Mn-calcite (Fig. 4.1b, c). The burbankite inclusions contain a range of LREE, from 5.3 wt. % to 17.6 wt. % (based on EDX data). Minor magmatic fluorapatite occurs as single crystals and clusters of anhedral to subhedral grains intergrown with biotite, Mn-calcite, biotite, and potassium feldspar. Fluorapatite is also observed within or adjacent to the rare earth pseudomorphs. It can host up to 1.7 wt. % Na and up to 1.6 wt. % Sr (based on SEM EDX data).

4.2 Replacive Mineralogy

The early magmatic hexagonal phenocrysts were replaced with multimineralic pseudomorphs consisting of prismatic to anhedral REE-bearing minerals, strontianite, and barite surrounded by interstitial calcite. The original mineral could have been one of several hexagonal minerals, including apatite, bastnäsite, or burbankite. The pseudomorphing mineral assemblage includes ancylite and RE-fluorocarbonates containing higher concentrations (typically 40-50 wt. %) of REO than burbankite (typically around 7.0 wt. % REO). The dominant REE-bearing mineral assemblage observed today consists of ancylite, rare earth fluorocarbonates (synchysite/parisite, rarely with bastnäsite cores), Sr-Ca-REE-phosphate, and daqingshanite. Ancylite and strontianite are the main constituents, each accounting for approximately 5 to 30 vol. % of the total pseudomorphic assemblage. The assemblage also contains up to 20 to 30 vol. % calcite, and up to 5 vol. % each of barite and rare earth fluorocarbonates. The unidentified Ca-Sr-REE-phosphate and daqingshanite constitute less than 2 vol. % of the REE-pseudomorph assemblage.

Figure 4.3: Textural relationships of magmatic ilmenite and hexagonal pseudomorphs. A) BSE image of ilmenite phenocryst replaced by anatase, surrounded by secondary assemblage. B) BSE image of ilmenite exhibiting resorbed edges and secondary anatase. C) Thin section scan (tl, plane light) of hexagonal phenocrysts (outlined with dashed red lines) which are pseudomorphed by strontianite, ancylite, calcite, and rare earth fluorocarbonates and rimmed by ilmenite, biotite, and anatase. D) False color mineral map from automated mineral analyses. Mineralogy of the same area is shown in (C). E) Hexagonal pseudomorph surrounded by magmatic Mn- calcite (tl, ppl). Orange blebs are epoxy-filled voids. F) Photomicrograph (tl, ppl) of pseudomorph which is surrounded by magmatic Mn-calcite and replaced by mainly by ancylite and barite. ant=anatase, anc=ancylite, brt=barite, bt=biotite, ilm=ilmenite, Mn-cal=manganoan-calcite, Rfc=rare earth fluorocarbonate, str=strontianite, vd=void.

The hexagonal pseudomorphs contain a variable distribution of cavities, which generally surround the secondary minerals and, in rare cases, can account for up to 40 vol. % of the original, hexagonal mineral shape. Much of this void space may be attributed to the loss of loosely bound minerals during core extraction and sample preparation. However, based on observations made during core logging and with comparison to other carbonatite samples (Olinger, 2012; Moore et al., 2015) from Bear Lodge, many of the cavities within the pseudomorphs appear to have formed during replacement of the original hexagonal mineral by other mineral phases. Ancylite is the predominant rare earth mineral in the least weathered carbonatite, ranging in abundance from 5 to 30 vol. % (15 vol. % average). It occurs in the original sites of the magmatic hexagonal REE-bearing mineral (Fig. 4.3e, f). Ancylite is commonly intergrown with strontianite, barite, (Fig. 4.4a, b) calcite, and less commonly with synchysite/parisite. It is commonly replaced by unidentified Sr-Ca-REE-phosphate and can, in rare cases, be intergrown with this mineral. Ancylite occurs as euhedral to anhedral grains that range in size from 1 μm to 0.2 mm; it often forms a chain of individual linked euhedral to subhedral crystals (Fig. 4.4a, c). Ancylite commonly contains fluid inclusions or mineral inclusions, such as goethite or hematite. Sprays of goethite have been observed to radiate from the ancylite mineral grain boundary (Fig. 4.4d). Ancylite compositions are enriched in the light rare earth elements, including cerium, lanthanum, neodymium, and praseodymium, as detected with SEM EDS data. Ancylite may contain up to 0.7 wt. % Th based on EDS data. Calcium commonly substitutes for strontium forming calcioancylite. No more than 2 wt. % Ca on average was found in calcioancylite based on EDX data. Synchysite/parisite forms euhedral radiating sprays 0.01mm to 0.2 mm in length and occasionally contains anhedral bastnäsite cores (Fig. 4.5a). Synchysite/parisite is commonly intergrown with ancylite and barite (Fig. 4.5b) and is more rarely intergrown with anatase, pyrite, and calcite (Fig. 4.5c, d, e). Brookite may also occur within the interstices of synchysite/parisite needles though grain sizes are too small for absolute confirmation. Synchysite/parisite rarely contains up to 1.3 wt. % Th. Bastnäsite within synchysite/parisite also may contain Th (up to 1.0 wt. %), Ca (up to 1.0 wt. %), and Sr (0.5 wt. %) (Appendix A). Synchysite and parisite are generally grouped together in this study as synchysite/parisite, due to potential intergrowths of the two compositions on a finer scale than is recognizable with

the methods used. Since the two are distinctly described as separate by other researchers, the terminology used in those instances will remain the same as described by those researchers. The term rare earth fluorocarbonate (Rfc) is used in this study to denote the grouping of synchysite/parisite and bastnäsite in situations when the compositions of these minerals are difficult to determine as separate in optical microscopy or in SEM analyses.

Figure 4.4: Ancylite texture and habit. A) BSE image of a corner of a pseudomorphed hexagonal grain, exhibiting ancylite intergrown with strontianite and barite. B) BSE image of ancylite, barite, and strontianite intergrowth texture within a hexagonal pseudomorph. C) Subhedral ancylite, forming a chain-link pattern of crystals within a calcite matrix (tl, cpl). D) Photomicrograph of ancylite, murky with inclusions (tl, ppl). Some of the inclusions are iron- oxide, including goethite. anc=ancylite, brt=barite, cal=calcite, gth=goethite, str=strontianite.

Figure 4.5: Rare earth fluorocarbonate textural relationships. A) BSE image of synchysite/parisite (grey) with bastnäsite cores. B) BSE image of rare earth fluorocarbonates (mainly synchysite/parisite) intergrown with barite. C) BSE image of rare earth fluorocarbonates intergrown with pyrite and partially intergrown with niobian anatase and calcite. D) Rare earth fluorocarbonates (likely synchysite) intergrown with pyrite in association with strontianite and ancylite (rl and tl, ppl) ant=anatase, anc=ancylite, bast=bastnäsite, brt=barite, cal=calcite, Rfc=rare earth fluorocarbonate, py=pyrite, str=strontianite, syn=synchysite/parisite.

Strontianite is subhedral to euhedral and can display acicular to smooth edges (Fig. 4.6a). Strontianite ranges in size from 0.01 mm to 1.0 mm. It is commonly intergrown with ancylite, barite, and rare earth fluorocarbonates but sometimes replaces those minerals (Fig. 4.6b). Strontianite may host up to 2.0 wt. % Ca (based on EDX data). Barite occurs as subhedral to anhedral, 0.03 to 1.5 mm grains. It is intergrown with ancylite, strontianite, and

synchysite/parisite. Barite rarely contains strontium with maximum observed values of 2 wt. % (based on EDS data). Textures suggest that rare earth phosphates crystallized later in the replacive sequence. Daqingshanite (Appendix A) occurs in trace amounts as subhedral grains with ancylite and strontianite and is sometimes replaced by an unidentified anhedral Sr-Ca-REE-phosphate (Fig. 4.7a). The unidentified Sr-Ca-REE-phosphate is also present as small, anhedral grains within strontianite and replacing ancylite (Fig. 4.7b, c). It can also occur as euhedral, cigar-shaped forms (Fig. 4.7d). On average, the Sr-Ca-REE-phosphate contains up to 55 wt. % LREE and up to 4.5 wt. % combined Ca and Sr (based on EDS data). It is commonly compositionally zoned (Fig. 4.7c, d) with subtle variations in REE and P compared to Ca and Sr contents. In BSE images brighter zones have higher REE and P contents. The mineral typically lacks Th but rarely contains up to 1.6 wt. % Th (based on EDX data). This mineral may be a strontium- and calcium-bearing rhabdophane ((LREE) PO4∙H2O). Calcite commonly constitutes 20-30 vol. % of the mineral assemblage replacing the early magmatic hexagonal REE-bearing mineral. This calcite commonly occurs as anhedral grains, which exhibit zonation under cathodoluminescence. More rarely this calcite contains dolomite cores and is intergrown with euhedral pyrite. This replacive calcite contains up to 1.5 wt. % Mg, Sr, and Fe (based on EDS data).

Figure 4.6: Strontianite habit. A) BSE image of acicular strontianite intergrown with calcite. B) BSE image of strontianite replacing ancylite and rare earth fluorocarbonates. anc=ancylite, brt=barite, cal=calcite, Rfc=rare earth fluorocarbonate, str=strontianite.

Figure 4.7: Backscatter electron images of rare earth phosphate minerals. A) Replacement of daqingshanite by Sr-Ca-REE-phosphate in association with ancylite and strontianite within a rare earth pseudomorph. B) Rare earth pseudomorph with Sr-Ca-REE-phosphate exhibiting small, anhedral texture and replacing ancylite. The two minerals have very close BSE intensities, and are therefore difficult to discern in this image. C) Evidence of Sr-Ca-REE-phosphate replacement of ancylite. D) Subhedral, cigar-shaped and compositionally zoned Sr-Ca-REE- phosphate. Similar textures were observed by Jenner (1984). anc=ancylite, brt=barite, dq=daqingshanite, str=strontianite, SCRp=Sr-Ca-RE-phosphate.

Niobian anatase texturally appears to have crystallized contemporaneously with the rare earth pseudomorph assemblage. The anatase grains are subhedral to euhedral and are clear to murky transparent exhibiting an indigo to brown hue in transmitted light (Fig. 4.8a). The anatase generally contains less than 2.0 wt. % Nb with less than 0.5% of Al, Si, and Ca (based on EDX data). Niobian anatase commonly replaces ilmenite-pyrophanite along cleavage planes and edges (Fig. 4.3a, b). It also replaces biotite and is intergrown with synchysite/parisite (Fig. 4.4c), ancylite, and calcite.

Pyrite forms fine, anhedral grains in some pseudomorph mineral assemblages with niobian anatase, REE- fluorocarbonates (Fig. 4.5c), and with ancylite, barite, and calcite (Fig. 4.8b). Pyrite outside of pseudomorphic replacements occurs as medium to coarse, subhedral and often skeletal grains that display both cubic (Fig. 4.8c) and octahedral morphologies (Fig. 4.8d). This pyrite may enclose potassium feldspar, calcite, biotite, and REE-bearing minerals. It also replaces biotite and chlorite. Fine- to very fine-grained pyrite forms a symplectic intergrowth with magnetite at the edge of large, subhedral pyrite grains (Fig. 4.8d). These intergrowths may also contain pyrrhotite and hematite (Appendix A). Other sulfide minerals present within the least weathered carbonatite include galena, chalcopyrite, and sphalerite. Both galena and chalcopyrite occur as anhedral inclusions within pyrite and anhedral grains intergrown with pyrite. Trace anhedral sphalerite is present replacing symplectic pyrite and magnetite.

Figure 4.8: Oxide and sulfide textures and relationships. A) Blue Niobian-anatase in, intergrown with rare earth fluorocarbonates, and calcite (ppl, tl). The orange-brown interstitial mineral is likely goethite. B) Hexagonal pseudomorph (outlined in orange) including fine-grained pyrite (rl). C) Octahedral pyrite exhibiting euhedral rims (rl). D) Euhedral cubic pyrite (with corner pointed away from view) partially to fully enclosing calcite (or Mn-calcite), potassium feldspar, and hexagonal pseudomorph (view of elongated c-axis parallel to slide) (tl, cpl).. E) Intergrowths of pyrite and magnetite and pyrite with a possible combination of pyrrhotite and magnetite or hematite. Intergrowths rim edge of pyrite (rl). F) Galena is partially to fully enclosed by pyrite (rl). ant=anatase, cal=calcite, gn=galena, gth=goethite, hem=hematite, Kfs=potassium feldspar, Mn-cal=manganese calcite, mt=magnetite, po=pyrrhotite, pseudo=hexagonal pseudomorph, py=pyrite, vd=void.

CHAPTER 5 WEATHERED CARBONATITE

Progressive weathering of carbonatite resulted in its change from a white-grey holocrystalline rock to brownish-black, highly porous and saprolitic material. Intensely weathered carbonatite has the consistency of coffee grounds. Weathering resulted in the partial to complete dissolution of calcite and strontianite, replacement of magmatic Mn-calcite by Mn- oxides, and the oxidation of sulfides to Fe-oxides and Fe-hydroxides. Potassium feldspar is variably replaced by clay minerals at shallow depths. Rare earth element mineralogy is dominated by ancylite in both the least weathered carbonatite and the moderately weathered carbonatite, which occupy the lower weathering zone (Fig. 5.1). Bastnäsite is the dominant REE mineral in the upper weathering zone. Weathered carbonatite also contains secondary cerianite, monazite, and (rare earth) aluminum phosphates; these increase in abundance upwards in the weathering profile. In the drill hole examined, the carbonatite is weakly but pervasively weathered at 147.8 m (485 ft) and becomes progressively more weathered up hole. This study characterized weathering intensity in the drill hole into two domains: weakly weathered and moderately weathered (Fig. 5.1). The carbonatite dike bends away from the drill hole above 132.9 m (436 ft). Thus, the uppermost, most highly weathered carbonatite was not studied. However, this type of material is observed in a number of other drill holes and has been studied in detail by Rare Element Resources geologists.

5.1 Lower Weathering Zone

Weakly weathered carbonatite generally forms an irregular zone up to 3 m in width above unweathered carbonatite and occurs in the drill hole examined at down-hole depths of 144.8- 147.8 m (475-485 ft.). Above this, the carbonatite is moderately weathered to the top of the carbonatite interval in the drill hole at 132.9 m (436 ft.).

Figure 5.1: Schematic representation of the mineral assemblages within the weathering profile of carbonatite from the western slope of Bull Hill as represented by drill hole RES09-17 that intersected carbonatite from 131- 171 m. Mineralogy of the strongly weathered zone from is derived from megascopic observations of other drill holes and Rare Element Resources data. The mineral assemblages shown here demonstrate a shift from Mn- calcite, ancylite, and sulfide-bearing assemblage to an oxide-rich, bastnäsite, and phosphate-bearing assemblage.

5.1.1 Weakly Weathered Mineralogy

Unweathered carbonatite is generally separated from moderately weathered carbonatite by an approximately 3 m thick “Weakly Weathered Zone”. This zone corresponds to the “Transition Zone” identified Rare Element Resources’ geologists. The most apparent visual effect of weak weathering of carbonatite is the oxidation of Fe-bearing minerals and the formation of hematite, which can compose up to 4 wt. % by volume of the rock. Iron sulfides are moderately to strongly oxidized, primarily to hematite. Other sulfides appear to be less affected by weathering, though both chalcopyrite and galena commonly display partial or complete hematite rims. Biotite proximal to oxidized iron sulfides commonly exhibits a strong red coloration, probably the result of either higher titanium or iron content (Deer, Howie and Zussman, 1992; Lalonde and Bernarnd; 1993). Hematite also rims and replaces non-sulfide minerals, such as ilmenite and anatase. Iron-oxide minerals, mainly hematite and goethite, partially infill pores within hexagonal pseudomorphs after the early rare earth element mineral and infill fractures in potassium feldspar.

5.1.2 Moderately Weathered Mineralogy

Moderately weathered carbonatite exhibits significant textural and mineralogical variation ranging from a rock with manganoan calcite in which original igneous textures are still discernable to dark brown, granular material with abundant Mn- and Fe-oxides. The primary changes observed upwards in the weathering profile within the moderately weathered carbonatite are the increasing destruction of Mn-calcite and calcite and a progressively increasing granularity of texture. Near the top of this zone, carbonatite observed in this study consists of approximately 50 vol. % Fe- and Mn-oxide minerals and 18 vol. % ancylite. It is important to note that mineral abundances within the small scale of this study may not reflect the entire zone, due to the heterogeneity of the carbonatite. Moderately weathered carbonatite contains abundant Mn- and Fe-oxide minerals derived from the replacement of Mn-calcite with Mn-oxide minerals and the replacement of sulfides with Fe-oxide minerals. Mn-oxide minerals initially replaced the cores of anhedral, granular Mn-

calcite (Fig. 5.2a) grains as well as larger interstitial Mn-calcite grains. The Mn-oxide minerals are cryptocrystalline to amorphous (Fig. 5.2b) and commonly exhibit bladed, sheet-like, and botryoidal textures (Fig. 5.2c). These minerals often contain up to 2.7 wt. % Na, 5.5 wt. % Ca, and 1.9 wt. % Si and can contain up to 4.0 wt. % Ba, 12.0 wt. % Pb, and 7.5 wt. % Fe (Appendix A). Depending on the structure type of the oxide, Mn-oxides commonly host minor amounts of cations, such as Na, Ca, Fe, Mg, K, Al, Si, Pb, and Ba (Post, 1999). Manganese oxide minerals with framework tunnel structures, such as nsutite, ramsdellite, and hollandite, are the most likely Mn-oxide minerals to host additional cations due to the large site availability within the tunnels and the cooperation of Mn to reduce valence state to counter-balance the cationic change (Post, 1999). Iron sulfides were replaced by Fe-oxides (Fig. 5.2d), which petrographically appear to be hematite and goethite. Occasional masses of goethite occur without indication of the primary mineral being replaced. Fe-oxides can contain up to 5.5 wt. % Si, 4.4 wt. % Na, 3.0 wt. % Zn, and less than 1 wt. % each of Mn, Al, and Ca (Based on SEM EDS data) (Appendix A). Biotite in moderately weathered carbonatite displays tan or yellowish orange to reddish orange coloration in thin section. It exhibits weak to strong alteration to Fe-oxide (mainly hematite and goethite). Higher in the section studied, biotite is weakly altered to a variety of mica with expanded cleavage (possibly vermiculite?); it is also weakly to strongly replaced by Fe-oxide minerals within this zone. Calcite and strontianite were progressively leached with increased weathering. Calcite is present in variable amounts within moderately weathered carbonatite. More granular and oxide- enriched samples typically contain less calcite. Minor calcite is commonly present within the rare earth pseudomorphs. Most pseudomorphs exhibit increased porosity due to calcite dissolution during weathering. This is demonstrated texturally with the occurrence of skeletal, partially dissolved calcite surrounding ancylite and barite. Strontianite was strongly depleted by the weathering process. Like calcite, strontianite is reactive to acidic fluids and likely dissolved during the weathering process. It is present in only trace amounts (less than 0.5 vol. %) or is completely absent. Ancylite is the most abundant rare earth mineral in the moderately weathered carbonatite and may compose 6 to 40 vol. % of the rock. It commonly exhibits a stained yellow-green coloration in thin section but can be reddish-orange when adjacent to Fe-oxides. With

progressive weathering, ancylite tends to occur as loosely bound, subhedral to sometimes euhedral grains (Fig. 5.3a) that commonly exhibit etched crystal faces. Ancylite is commonly intergrown with or replaced by Sr-Ca-REE phosphate (Fig. 5.3b, c), and it is also commonly encapsulated by Fe-oxide and/ or Mn-oxides (Fig. 5.3d).

Figure 5.2: Replacive textures of manganese- and iron-oxide minerals in moderately weathered carbonatite. A) Mn-oxide replaced cores of Mn-calcite, with secondary interstitial Pb-bearing Mn-oxide from 143 m (BSE). B) Lead-bearing Mn-oxide with amorphous, slightly crustiform texture. The Mn-oxide replaces Mn-calcite and occurs interstitial to Mn-calcite grains (BSE). C) Sheet-like and botryoidal Mn-oxide from 133 m (BSE). D) Iron-oxide replaced cubic pyrite and spread beyond the pyrite boundary (144m, BSE). anc= ancylite, Mn-cal, Mn-ox=Manganese- oxide, Mn-ox (Pb)= Lead bearing Mn-oxide, Fe-ox=Iron-oxide,

The unidentified Sr-Ca-REE-phosphate mineral is the next most abundant REE-bearing phase in moderately weathered carbonatite and may make up 0.5 to 2.6 vol. % of the rock. The mineral is commonly iron-stained (Fig. 5.3d) and may exhibit colloform textures (Fig. 5.3e). It contains similar amounts of LREE (~50 wt. %) to unidentified Sr-Ca-REE-phosphate in the least weathered carbonatite (55 wt. %) (based on SEM EDX data).

Figure 5.3: Textures of ancylite and the unidentified Sr-Ca-REE-phosphate mineral in moderately weathered carbonatite. A) Subhedral ancylite that is loosely bound to other minerals (from 133 m down hole depth, BSE). B) Ancylite intergrown with and partially replaced by the unidentified Sr-Ca-REE-phosphate. The unidentified Sr-Ca-REE-phosphate is strongly stained by and replaced with Fe-oxide (tl, cpl). C) Ancylite intergrown with the unidentified Sr-Ca- REE-phosphate, which is replaced by Fe-oxide. These minerals are surrounded by pore space (automated mineral analysis image). D) Iron- and manganese-oxide minerals replace ancylite and the unidentified Sr-Ca-REE-phosphate (automated mineral analysis image). anc= ancylite, Mn-ox=Manganese-oxide, Fe-ox=Iron-oxide, SCRp= unidentified Sr-Ca-REE-phosphate.

An unidentified mineral or combination of minerals, likely cerianite with britholite, is observed in the moderately weathered carbonatite. The mineral(s) occurs in trace amounts as very fine-grained orbicular to anhedral grains within the pores surrounding Mn-oxide minerals. The unidentified mineral(s) appear to contain phosphorous and silicon; they can contain up to 65 wt. % Ce, 13 wt. % Nd, and 6 wt. % Pr (based on SEM EDX data). Bastnäsite and synchysite/parisite are rare within the moderately weathered zone. When present they are strongly Fe-stained and generally partially to completely replaced by Fe-oxide minerals. It is unclear whether these rare earth fluorocarbonates are less abundant due to weathering or heterogeneous distribution during post-magmatic crystallization. Burbankite is also rare in the moderately weathered carbonatite and was only identified using automated mineral analyses. Maximum concentrations of 0.01 vol. % were present. The effects of weathering on burbankite remain unclear; it may have altered during weathering or been originally distributed heterogeneously. Several gangue minerals appear to be resistant to weathering in the moderately weathered samples. These minerals include ilmenite, potassium feldspar, fluorapatite, and barite. Barite and fluorapatite appear stable in moderately weathered carbonatite but may display crystal breakage.

5.2 Changes in Porosity with Weathering

Petrographic observations indicate that porosity strongly increased during weathering of carbonatite due to the dissolution of calcite and strontianite, as well of the replacement of Mn- calcite by Mn-oxides, which appear to occupy less volume than Mn-calcite. Quantitative measurement of porosity was obtained using 2 μm resolution QEMSCAN® analyses for three of the least weathered carbonatite samples and three moderately weathered samples from the upper portion of the lower weathering zone. The weathered samples are less granular than the majority of the most weathered carbonatite and therefore provide conservative porosity values. More information on the technique used for measuring porosity in this study is available in Jobe (2013). Porosity values, especially those of the least weathered carbonatite, may be maximum values if any additional void space was created during sample extraction and preparation.

The least weathered carbonatite has an average porosity of 7-8%, and the moderately weathered carbonatite has an average porosity value of approximately 50% (Table 5.1). Porosity in the weathered carbonatite (Fig. 5.4) is concentrated at the sites of igneous carbonate minerals and REE-bearing mineral pseudomorphs.

Table 5.1: Porosity measurements from QEMSCAN® analyses, which were completed on thin section and epoxy mount samples of carbonatite and moderately weathered carbonatite.

Sample Type Sample Porosity Mineral Total RBL11-42B 6.21 93.79 100.00 Least RBL11-45A 8.61 91.39 100.00 Weathered RBL11-45B 8.65 91.35 100.00 Carbonatite Average 7.82 92.18 100.00

RBL11-25Q1 41.39 58.61 100.00 Moderately RBL11-27Q1 61.23 38.77 100.00 Weathered RBL11-27Q2 47.53 52.47 100.00 Carbonatite Average 50.05 49.95 100.00

. Figure 5.4: False-color porosity maps of the least weathered carbonatite and moderately weathered carbonatite. Samples RBL11-42B (A) and RBL11-45A (B) exhibit lower porosity (shown in blue and green) than the moderately weathered samples RBL11-25Q2 (C) and RBL11-27Q1 (D).

CHAPTER 6 WHOLE ROCK GEOCHEMISTRY

Seventeen samples of carbonatite representing various degrees of weathering were analyzed for major and trace element compositions (Tables 6.1, 6.2). The results were utilized to compare chemical variations throughout the lower weathering profile to what is observed petrographically. Average compositions for the least weathered carbonatite and two subsets of moderately weathered carbonatite, those containing significant remnant carbonate minerals and those without, were calculated for comparison (Tables 6.3, 6.4).

6.1 Geochemical Changes Due to Weathering

The most abundant mineral constituents of the carbonatite, Mn-calcite and calcite, were either lost or altered during the weathering process as were strontianite and iron sulfides. Mineral dissolution during weathering resulted in rock volume loss that increased both porosity and the concentration of more weathering resistant minerals. Calcium, strontium, and carbon are present primarily in minerals that underwent dissolution during weathering, so tracking their abundance in the samples analyzed serves to indicate the degree of weathering. The least weathered carbonatite contains on average 27.9 wt. % CaO and 7.68 wt. % SrO (Table 6.3), while moderately weathered carbonatite on average contains 2.57 wt. % CaO and 4.55 wt. % SrO (Table 6.3). Carbon concentrations also decrease, from 8.37 avg. wt. % in the least weathered carbonatite to 4.59 avg. wt. % in the moderately weathered samples. Manganese from primary Mn-calcite was progressively concentrated in residual Mn- oxides during weathering. Manganese increases from 2.62 avg. wt. % in the least weathered carbonatite to 7.13 avg. wt. % MnO in moderately weathered carbonatite (Table 6.3). This represents a 172% increase in MnO content or an increase by a factor of 2.72. The concentration of sulfur decreases in the weathered carbonatite samples due to the oxidation of pyrite and other sulfides. Average sulfur concentration in carbonatite decreases from 6.63 avg. wt. % in the least weathered to 0.49 avg. wt. % in the low-carbonate moderately

weathered carbonatite (Table 6.3); the decreased concentration suggests that most of the sulfur was transported out of the local system during weathering. The remaining sulfur is likely hosted by barite, which remained stable in the weathering environment. Iron shows a complex behavior with weathering. Ferrous iron averages 3.24 wt. % in the least weathered carbonatite but is below detection limits in the most of the moderately weathered carbonatite (Table 6.3, 6.4). Ferric iron increases from 1.86 avg. wt. % in the least weathered carbonatite to 13.70 avg. wt. % in the low-carbonate moderately weathered zone (Table 6.3). The shift in iron oxidation states indicates a progressively oxidizing weathering environment, in 2+ 3+ which Fe from pyrite is converted into Fe and was precipitated as hematite and other Fe- oxides. It is unclear if any Fe2+ in biotite was converted to Fe3+. The dissolution of carbonate, the main constituent of the carbonatite, led to significant rock volume decrease during weathering. This decrease in volume yielded an increased concentration of resistant minerals. These resistant minerals, including ilmenite and anatase, concentrate the high field strength elements (HFSE) Hf, Nb, Ti, Ta, and Zr. Ilmenite and anatase, however, do not appear to increase in abundance based on petrographic observations; this may be due to their heterogeneous distribution (Appendix A) and generally low total percentages in the assemblage. Ti is also present in biotite that weathers in place. Hafnium and Zr increases (Fig. 6.1) are difficult to explain given that no apparent mineral hosts for these elements are observed. However, given the small total amounts of these elements (approximately 10 and 20 ppm, respectively) it is likely that they are present within very minor zircon that has not been observed petrographically.

6.2 Isocon Analysis

Isocon plots based on Grant’s (1986, 2005) modifications of Gresen’s (1966) approach demonstrate element redistribution in carbonatite during weathering (Fig.’s 6.1, 6.2, 6.3, and 6.4). A “conservative best fit elements” line (Fig. 6.1) depicts the concentration trend for the high field strength elements Hf, Zr, Nb, and Ta that should have been conserved during weathering (Morteani and Preinfalk, 1996; Lottermoser, 1990). The slope of the line is m = 1.67. A conserved element would neither be considerably less than nor more abundant than a hypothetical concentration of the element in a more porous weathered medium.

Table 6.1: Major oxide concentrations for moderately and least weathered carbonatite, including dike boundaries with breccia. Moderately weathered samples are grouped based on gangue carbonate (calcite, Mn-calcite, and strontianite) content.

Breccia + Moderately Weathered Low Carbonate Moderately High Carbonate Moderately Least Weathered Breccia + Least Carbonatite Weathered Carbonatite Weathered Carbonatite Carbonatite Weathered Carbonatite Sample no. 34518 34519 34520 34521 34522 34523 34528 34529 34530 34531 34532 34533 34538 34539 34540 34541 from (m) 127.4 131.1 133.2 133.5 133.8 134.1 137.2 141.7 143.2 146.3 149.3 152.4 158.5 161.5 164.6 167.6 to (m) 131.1 133.2 133.5 133.8 134.1 137.2 141.7 143.2 146.3 149.3 152.4 158.5 161.5 164.6 167.6 170.7 Wt. %

SiO2 40.4 43.7 3.88 9.11 11.7 9.51 5.01 5.83 3.98 2.14 1.9 2.26 1.67 5.74 20.2 12.25

TiO2 1.96 1.56 0.08 0.33 0.48 0.44 0.36 0.2 0.38 0.12 0.34 0.09 0.12 0.38 1.2 0.97

Al2O3 11.25 12.6 0.82 1.78 3.12 1.88 0.96 1.04 0.77 0.45 0.45 0.54 0.36 1.5 5.65 3.35

Fe2O3* 14.55 12.3 5.88 23.4 12.35 13.15 6.86 6.86 3.43 2.43 7.88 5.58 6.43 5.35 5.71 5.63 FeO <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 1.35 2.71 2.9 3.08 3.75 2.96 4.76 4.07 MnO 2.6 3.31 8.17 9.45 6.3 4.59 5.07 4.73 3.06 3.12 2.78 2.97 2.12 1.5 1.76 1.67 MgO 1.6 1.24 0.38 0.78 0.94 0.84 0.71 0.6 0.66 0.7 0.62 0.78 0.71 1.33 2.24 1.52 CaO 1.71 2.41 20.8 2.57 15.1 16.5 34.1 27.5 32.8 31.2 27.3 31.3 25.1 20.9 16.2 17.2

Na2O 0.27 0.24 0.24 0.3 0.21 0.5 0.25 0.24 0.28 0.13 0.12 0.09 0.1 0.14 0.15 0.16

K2O 9.04 9.85 0.72 1.33 2.35 1.38 0.64 0.79 0.57 0.45 0.4 0.49 0.35 1.36 5.04 2.91

P2O5 0.82 0.76 0.18 1.3 1.31 0.41 0.15 0.09 0.39 0.39 0.1 0.13 0.1 0.12 0.82 0.36 SrO 1.16 0.83 4.96 4.55 4.01 4.36 3.44 3.64 5.81 7.61 7.73 4.82 10.5 11.8 6.18 9.07 BaO 1.52 0.92 3.44 3.66 2.23 2.05 0.95 1.14 0.65 0.61 0.93 0.68 0.82 1.15 0.6 1.21 LOI 6.54 6.2 28.5 16.65 22.3 24.6 33.6 31.6 31.9 29.2 18.8 25.0 22.4 20.6 14.05 16.55 C 0.59 0.6 6.34 1.91 4.66 5.44 8.53 8 8.63 8.87 8.1 8.83 8.19 7.36 4.86 5.78 S 0.13 0.04 0.57 0.64 0.38 0.36 0.08 0.19 1.29 3.08 8.2 5.36 6.32 5.2 4.15 5.45 Total 94.1 96.6 85.0 77.8 87.4 86.0 100.7 92.5 95.9 93.2 88.5 92.0 89.0 87.4 93.6 88.1

Table 6.2: REE and trace element concentrations for moderately and least weathered carbonatite, including dike boundaries with breccia. Moderately weathered samples are grouped based on gangue carbonate (calcite, Mn-calcite, and strontianite) content. Breccia + Mod. Weathered Low Carbonate Moderately High Carbonate Moderately Least Weathered Breccia + Least Carbonatite Weathered Carbonatite Weathered Carbonatite Carbonatite Weathered Carbonatite Sample no. 34518 34519 34520 34521 34522 34523 34528 34529 34530 34531 34532 34533 34538 34539 34540 34541 from (m) 127.4 131.1 133.2 133.5 133.8 134.1 137.2 141.7 143.2 146.3 149.3 152.4 158.5 161.5 164.6 167.6 to (m) 131.1 133.2 133.5 133.8 134.1 137.2 141.7 143.2 146.3 149.3 152.4 158.5 161.5 164.6 167.6 170.7 ppm Y 145 132 435 489 347 277 210 209 255 218 164 209 169 169 128 134 La 10250 6100 40800 42800 31200 38500 20500 30000 16600 14600 14100 13200 20400 22900 11950 18050 Ce 16600 10800 62800 66700 48700 57500 31800 45600 26300 24400 23700 21700 31400 35400 18250 29400 Pr 1840 1185 5212 5725 4800 4529 3040 4280 2610 2500 2460 2220 3090 3460 1770 2960 Nd 5900 3980 20100 23100 14800 15450 8700 12350 7880 7810 7660 6920 9200 10450 5270 9110 Sm 787 583 2750 3340 2000 1870 927 1215 942 966 933 833 1035 1175 613 1135 Eu 154.5 117.5 576 687 401 369 176.5 218 182.5 183 174.5 158.5 187 216 118.5 212 Gd 310 235 1165 1365 816 726 345 419 371 360 341 322 356 412 239 409 Tb 19.5 15.65 74.1 84.7 53.4 45 23.3 26.3 26.2 23.9 21 22.2 22 25.7 16.35 24 Dy 56.1 46.6 202 234 151 124.5 73.4 79.8 87.1 77.2 63.9 73.9 66.9 75.1 50.9 65.8 Ho 6.03 5.32 20.3 23.1 15.55 12.15 8.56 8.91 10.5 9.41 6.93 8.86 7.41 7.54 5.49 6.2 Er 9.8 9.5 32.9 38.3 25.4 20 17.3 16.5 20.4 17.4 12.6 16.5 12.6 12.2 9.5 9.4 Tm 1.1 1.25 3.72 4.44 3.11 2.34 2.25 2.1 2.39 2.04 1.51 2.04 1.43 1.23 0.99 0.98 Yb 6.3 6.8 21.5 25.2 17.1 13.5 14.2 12.8 13.9 12 8.8 11.9 8 6.6 5.2 4.8 Lu 0.9 1.02 3.33 3.79 2.62 2.11 2.13 1.97 1.9 1.74 1.33 1.63 1.18 0.88 0.77 0.69 Σ REE 36086 23219 134196 144619 103332 119440 65840 94439 55303 51181 49649 45700 65957 74311 38428 61522

(La/Lu)cn 1115 585.43 1199 1105 1166 1786 942.1 1491 855.3 821.4 1038 792.7 1692 2547 1519 2561 Th 348 226 1375 1565 924 876 390 486 249 243 298 258 330 427 231 515 U 58.4 46.4 17.3 70.3 51.9 68.8 29.6 54.6 43.4 91.2 27.5 45.2 1 158 290 332 Zr 50 40 10 20 40 50 20 20 40 10 10 20 30 40 50 30 Hf 2 1 1 2 2 3 1 1 2 1 1 1 1 1 1 1 Nb 406 332 47 184 167 200 99 109 115 32 84 50 135 306 543 637 Ta 1.4 1.5 <0.5 0.8 0.8 0.8 <0.5 <0.5 0.7 <0.5 <0.5 <0.5 0.5 0.9 1.3 0.7 Rb 249 307 28 63 86 54 29 37 32 19 20 21 23 53 168 93 Sn 16 8 6 10 12 26 11 7 12 <5 <5 <5 <5 <5 <5 11

Table 6.3: Average major oxide concentrations in moderately and least weathered carbonatite. Moderately weathered samples are grouped based on gangue carbonate (calcite, Mn-calcite, and strontianite) content.

Low Carbonate High Carbonate Moderately Moderately Least Weathered Weathered Weathered Carbonatite Carbonatite Carbonatite Percent Sample Average (n=4) Average (n=4) Average (n=3) Increase from from Least (m) 133.2 137.2 149.3 to Most Weathered to (m) 137.2 149.3 161.5 % Wt. % 340 SiO2 8.55 4.24 1.94 81 TiO2 0.33 0.27 0.18 322 Al2O3 1.90 0.81 0.45 636 Fe2O3 13.70 4.89 1.86 na FeO na 2.03 3.24 172 MnO 7.13 4.00 2.62 5 MgO 0.74 0.67 0.70 -51 CaO 13.74 31.40 27.90 202 Na2O 0.31 0.23 0.10 250 K2O 1.45 0.61 0.41 627 P2O5 0.80 0.26 0.11 -42 SrO 4.47 5.13 7.68 251 BaO 2.85 0.84 0.81 4 LOI 23.01 31.58 22.07 -45 C 4.59 8.51 8.37 -93 S 0.49 1.16 6.63

Table 6.4: Average REE, Y, and trace element concentrations for low-carbonate and high- carbonate moderately weathered and least weathered carbonatite. Total REE (Σ REE) and chondrite normalized La to Lu ratios are also shown for each group. The percent increase in concentration of each element, sum, and ratio from the least weathered to the most weathered (the Low Carbonate Moderately Weathered) is shown. Moderately weathered samples are grouped based on gangue carbonate (calcite, Mn-calcite, and strontianite) content.

Low Carbonate High Carbonate Moderately Moderately Weathered Weathered Least Weathered Carbonatite Carbonatite Carbonatite Percent Sample Average (n=4) Average (n=4) Average (n=3) Increase from from (m) 133.2 137.2 149.3 Least to Most Weathered to (m) 137.2 149.3 161.5 % ppm 114 Y 387 223 180.67 141 La 38325 20425 15900 130 Ce 58925 32025 25600 96 Pr 5066 3108 2590 132 Nd 18363 9185 7927 167 Sm 2490 1013 934 193 Eu 508 190 173 200 Gd 1018 374 340 196 Tb 64.30 24.93 21.73 161 Dy 177.88 79.38 68.23

130 Ho 17.78 9.35 7.73 110 Er 29.15 17.90 13.90 105 Tm 3.40 2.20 1.66 102 Yb 19.33 13.23 9.57 115 Lu 2.96 1.94 1.38 133 Σ REE 125397 66691 53768 -16 (La/Lu)cn 1314 1027 1174 301 Th 1185 342 295.33 112 U 52.075 54.7 24.57 50 Zr 30 22.5 20.00 100 Hf 2 1.25 1.00 67 Nb 149.5 88.75 89.67 60 Ta 0.8 0.7 0.50 171 Rb 57.75 29.25 21.33 na Sn 13.5 10 na

Based on porosity measurements (Table 5.1), a hypothetical concentration of a conserved element can be calculated with the following equations:

C1 V1 = C2 V2 [1]

C1 92.18%× = xC1× 49.95% [2] (92.18× 49.95) C×1 = xC1 [3] ÷1.845 = x [4]

The factors C1 and C2 denote the concentration of the conserved element in the least and moderately weathered carbonatite, respectively. The numeric value, V1 = 92.18%, in lines [3] and [4] is based on porosity measurements (Table 5.1) of the total rock volume percent in the mildly porous least weathered carbonatite. The value, V2 = 49.95%, represents the average rock volume of the highly porous moderately weathered carbonatite (Table 5.1). The “x” component is the factor by which the original concentration hypothetically increases due to weathering. This calculation does not take into account heterogeneity in mineral distribution and conservative porosity measurements. Higher porosity in the most weathered carbonatite would yield a higher concentration factor. The following calculations demonstrate how a higher porosity in the weathered carbonatite could result in a greater concentration of a conserved element. Thus, the value x = 1.845 is likely conservative.

C1 V1 = C2 V2 [5]

C1 92.18%× = xC1× 30.00% [6] (92.18× 30.00) C×1 = xC1 [7] 3.073 = x [8] ÷

To account for the possible effects of different porosities, two trend lines, y = 1.845x and y = 3x, are shown on the isocon plots in addition to the trend line (with slope m = 1.67) for the conservative elements Zr, Hf, Nb, Ta, and TiO2 (Fig. 6.1, 6.2). A y = x trend line (Fig. 6.1) depicts no change in concentration with weathering. Elements or oxide concentrations plotting next to or within the bounds of the conservative element and 3x trends likely are to have increased due to rock volume loss or original heterogeneous mineral distribution. Elements

plotting near or below the line (y = x) should indicate elements or oxides that were lost to the system during weathering. Oxides or elements plotting above the y = 3x trend line, such as

Fe2O3, Th, P2O5, SiO2, and BaO, indicate elements concentrated by weathering or could signify an influx into the system (Fig. 6.2).

Al2O3 and K2O also plot above the y = 3x line (Fig. 6.2) and show a strong increase with weathering. These oxides, together with SiO2, occurred within magmatic feldspar and biotite. However, the error bars on the isocon plot for these elements are large. Based on automated mineral analyses and petrographic observations, the heterogeneous distribution of potassium feldspar and biotite are likely responsible for their observed highly variable abundance. Silicon is a highly mobile element in this system as evidenced by the silicification of weathered carbonatite in various areas within the north lobe of the Bear Lodge dome. It is likely that SiO2 locally accumulated in the weathered carbonatite. Fe-oxides contain up to 2.7 wt. % Si (based on SEM EDS). The isocon plots show that iron was essentially completely oxidized during weathering.

Ferrous iron is depleted in the low-carbonate moderately weathered zone and Fe2O3 increases well above the y = 3x trend (Fig. 6.2). This indicates ferric iron substitution for ferrous iron in mica, the oxidation of Fe2+ to Fe3+ during sulfide alteration to Fe-oxides, and the influx of additional iron into the local system, precipitating as Fe3+-oxide.

Phosphorous also shows a strong increase with weathering. Like Fe2O3, P2O5 plots well above the y = 3x trend line. The unidentified Sr-Ca-REE-phosphate is the main host for phosphorous with apatite a minor host. Petrographic information from automated mineral analyses (Appendix A) shows that the unidentified Sr-Ca-REE-phosphate increased from approximately 1.0 wt. % on average in the least weathered carbonatite to approximately 2.4 avg. wt. % in the moderately weathered carbonatite. Apatite showed a similar level of increase. It is unclear how P2O5 was concentrated beyond the capacity of unidentified Sr-Ca-REE-phosphate mineral and apatite. While thorium is typically grouped with the other HFSE, it does not appear to have behaved conservatively during weathering of the carbonatite. Thorium plots above the y = 3x line (Fig. 7.1). The trace amounts of Th detected (by SEM EDS) in ancylite and the unidentified Sr-REE-phosphate grains suggest that thorium increased during weathering because of its incorporation in these minerals.

1300

1200 Th 1100

1000

900

800 100Ta 5Nb 700

600 10Rb

500 10U

400

300 10Zr 100Hf 200 -Low Carbonate Moderately Weathered Carbonatite (ppm) Carbonatite Weathered Moderately Carbonate -Low 100 2 C 0 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300

C1 - Least Weathered Carbonatite (ppm)

Figure 6.1: Isocon plot for average trace element concentrations of Hf, Nb, Rb, Ta, Th, U and Zr within the low-carbonate moderately weathered vs. least weathered carbonatite. Concentration values were modified by a given factor, shown as a coefficient by the oxide name, in order to scale elements to one plot. Error bars demonstrate the highest standard deviation in precision for the element. After Grant (1986, 2005).

10K2O Fe2O3 14 5BaO

12 0.5LOI

10 5Al2O3

SiO2 8 10MgO 10P2O5 MnO 0.5CaO 6 SrO C 4 10TiO2

-Low Carbonate Moderately Weathered Carbonatite (%) Carbonatite Weathered Moderately Carbonate -Low 10Na2O 2 2 C

FeO S 0 0 2 4 6 8 10 12 14 16 18 20

C1 - Least Weathered Carbonatite (%)

Figure 6.2: Isocon plot for average major element concentrations within the low-carbonate moderately weathered vs. least weathered carbonatite. Concentration values were modified by a given factor, shown as a coefficient by the oxide name, in order to scale elements to one plot. Error bars demonstrate the highest standard deviation (% RSD) in precision for each element. After Grant (1986, 2005).

800

700

10Tb 600 0.01Ce 0.5Gd 500 Eu 0.1Pr

400 0.01La Y 100Tm 300 100Lu 0.1Sm 10Er

200 0.1Nd 10Yb Dy

100 -Low Carbonate Moderately Weathered Carbonatite (ppm) Carbonatite Weathered Moderately Carbonate -Low

2 C 0 0 100 200 300 400 500 600 700 800

C1 - Least Weathered Carbonatite (ppm)

Figure 6.3: Isocon plot for average rare earth element and yttrium concentrations within the low- carbonate moderately weathered vs. least weathered carbonatite. Concentration values were modified by a given factor, shown as a coefficient by the oxide name, in order to scale elements to one plot. Error bars demonstrate the highest standard deviation (% RSD) in precision for each element. After Grant (1986, 2005).

The high increase in BaO concentration from the least weathered to the low-carbonate moderately weathered carbonatite indicates that Ba was conserved in barite. However, BaO increased by a factor of 3.5, from 0.81 avg. wt. % to 2.85 avg. wt. %. The strong increase in BaO suggests that additional Ba was transported, accumulated locally, or a result of strong mineral heterogeneity. Manganese-oxides can contain up to 4.03 wt. % Ba (based on SEM EDS data) and may account for the additional barium.

6.3 Rare Earth Element Concentration during Weathering

Rare earth oxide concentrations are known to increase with weathering, although the REO all plot within the conserved element field (Fig. 6.3). While the rare supergene REE- minerals (cerianite and britholite) were observed in moderately weathered carbonatite, the major REE-bearing mineral phases, ancylite and Sr-Ca-REE-phosphate, appear petrographically to have been resistant to alteration in the lower weathering zone environment. Total average REE concentration is significantly higher in the low-carbonate moderately weathered carbonatite compared to the least weathered carbonatite (Table 6.4). Each of the individual rare earth elements increases in concentration during weathering. Most importantly, a group referred to as the middle rare earth elements (MREE), including Sm, Eu, Gd, Tb, and Dy, show the highest increase in concentration (Table 6.4). Europium and gadolinium plot directly on the y = 3x trend line (Fig. 6.3). This is significant when comparing the trend to the lanthanide electron configurations. The MREE have the greatest number (5-7) of unpaired electrons in the 4ƒ subshell compared to the remainder of the lanthanides. Chondrite-normalized plots (Fig. 6.4) display little difference in light to heavy REE abundances with weathering because of log scaling. However, La to Gd slopes of the low- carbonate moderately weathered group are less steep on average than the La to Gd slopes of the high-carbonate moderately and least weathered samples. Chondrite-normalized patterns of the high-carbonate moderately weathered group exhibit lower relative concentrations of LREE (La to Gd) and higher relative concentrations of HREE (Gd to Lu) when compared to the patterns of the least weathered samples. Average (La/Lu) values (Table 6.4) demonstrate that the overall slope of the chondrite normalized pattern decreases with weathering. It is unclear from available data whether the LREE were transported during weathering or if the observed trends simply mark an originally heterogeneous distribution of REE minerals within the carbonatite. Overall, the total REE contents of weathered carbonatite are higher in weathered rock due to dissolution and alteration of other, non-REE-bearing, mineral phases. Other than the slight increase in concentration of MREE within expected concentration limits; the geochemical data do not indicate differential mobilization of different rare earth elements beyond what is expected to be conserved and therefore concentrated during weathering.

Low Carbonate Moderately A Weathered

B High Carbonate Moderately Weathered

C Least Weathered

Figure 6.4: Chondrite normalized (Haskin et al., 1968) REE distribution patterns for low carbonate moderately weathered (A), high carbonate moderately weathered (B), and least weathered (C) carbonatite whole rock samples.

CHAPTER 7 DISCUSSION

Rare earth element concentrations in the carbonatite in the western portion of Bull Hill at Bear Lodge may have been enhanced through REE-mineral replacement during late-magmatic, post-magmatic, or possibly hydrothermal events. Petrographic observations show that REE- concentrations were enriched by the pseudomorphic replacement of unidentified magmatic hexagonal minerals. Weathering certainly resulted in enhanced rare earth element concentrations in the carbonatite studied (Fig. 7.1). Data from several different techniques, including geochemical analyses, petrographic analyses, and porosity measurements, demonstrate that carbonatite at Bull Hill underwent REE enrichment during weathering processes. Weathering of the carbonatite at Bear Lodge is attributed to groundwater infiltration. During initial weathering, the oxidation of sulfides would have lowered the pH of the system speeding gangue carbonate dissolution. The dissolution was likely an open system process, as indicated by geochemical results.

7.1 Replacive REE Enrichment

Economic REE occurrences commonly result from secondary late-magmatic, post- magmatic, or hydrothermal enrichment of originally REE-enriched igneous material (Mariano, 1989; Gieré, 1996; Chakhmouradian and Wall, 2012; Williams-Jones et al. 2012). At Bear Lodge, an unidentified hexagonal mineral is a prominent phenocrystic phase within carbonatite. These phenocrysts were replaced by a multimineralic assemblage, including ancylite, strontianite, barite, synchysite/parisite with bastnäsite cores, an unidentified Sr-Ca-REE- phosphate mineral, calcite, and trace daqingshanite. Given the shape of these phenocrysts and the mineral assemblage replacing them, they are thought to have originally been burbankite, (Na, Ca)3(Sr, Ba, Ce)3(CO3)5, (Moore, et al., 2015). Burbankite is a common early crystallizing phase in a number of carbonatites (Zaitsev et al., 2002; Belovitskaya and Pekov, 2004). Burbankite was observed in this study as small anhedral inclusions within magmatic Mn-calcite, indicating it was present as an early magmatic

phase. These inclusions are similar to those observed in the Khibina carbonatites (Zaitsev et al. 1998). Burbankite in the Kola Peninsula carbonatites commonly displays pseudomorphic replacement by strontianite, synchysite, ancylite, and barite (Zaitsev et al., 1998; Zaitsev et al., 2015). This assemblage is similar to the replacive mineral assemblage observed in the hexagonal pseudomorphs at Bear Lodge and in the replacements of magmatic burbankite in a number of other carbonatites, an event that is assumed to have occurred during an open system, late magmatic or a magmatic-related hydrothermal event (Wall and Mariano, 1996; Zaitsev et al., 1998, 2002; Belovitskaya and Pekov, 2004; Zaitsev et al., 2015). While the original phenocrystic phase appears to have been magmatic, the conditions of the replacement event remain unclear. It is possible that burbankite became unstable in the Bear Lodge carbonatite due to decreasing alkalinity in the melt or with late magmatic or hydrothermal fluids, similar to what is observed in other carbonatites (Belovitskaya and Pekov, 2004). The lack of sodium in the replacive minerals within the hexagonal pseudomorphs indicates that Na was removed during the replacement event. Wall and Mariano (1996) and Zaitsev et al. (2002) consider this to be an open-system process. Bastnäsite is another hexagonal rare earth mineral that could have been a precursor for the pseudomorphs at Bear Lodge. Primary igneous bastnäsite is recognized in the Mountain Pass carbonatite (Mariano, 1989; Castor, 2008) and in the Arshan carbonatites (Doroshkevich et al., 2008). Bastnäsite also commonly precipitates from hydrothermal fluids during a late or post- magmatic stage (Mariano, 1989). Given that burbankite is a known primary phase in the Bear Lodge carbonatite (Olinger, 2012; Moore et al., 2015; this study), it appears more likely that burbankite was the parental hexagonal mineral to the pseudomorphs in the Bear Lodge carbonatites. The hexagonal REE-bearing pseudomorphs constitute an average 10 vol. % of the least weathered carbonatite at Bear Lodge. The REE grade from magmatic burbankite occupying 10 vol. % of carbonatite would be approximately 0.53 to 1.7 wt. %, based on the range of measured 5.3 to 17 wt. % REE in the magmatic burbankite inclusions within calcite in the current sample set (as determined by BSE EDX). The actual average concentration of REE (not including Y) in the least weathered carbonatites is 5.38 wt. % as determined from whole rock geochemical analyses. Thus, it appears that rare earth elements were added to the rock during the replacive event. It is important to note that the REE concentration was not measured in all the minerals

Figure 7.1: Schematic model showing three main stages of alteration and REE-enrichment within the Bear Lodge Carbonatite.

within the unweathered carbonatite. Olinger (2012) measured up to approximately 1000 ppm REE concentration in magmatic calcite within carbonatite from northwestern Bull Hill, which indicates that the potential for calcite in samples from this study may have also contributed to the overall REE concentration in the rock.

7.2 REE Enrichment from Weathering

Enrichment of REEs in carbonatite systems can occur due to transport and accumulation of resistant REE-phases, such as monazite, into placers (Long et al., 2010). In place weathering can also result in REE enrichment of carbonatites, as has been demonstrated at Araxá, Brazil and Mt. Weld, Australia (Lottermoser, 1990; Morteani and Preinfalk, 1996; Chakhmouradian and Wall, 2012; Verplank et al., 2014). This study shows significant REE upgrading due to weathering at western Bull Hill. Petrographic observations show that the carbonatite displays mineralogical variability over a range of scales, from millimeter to meter, within the drill core interval examined. Whole rock geochemical data representing average values over approximately 10 m demonstrate that the REE’s behaved conservatively with no major fractionation or mobilization (Fig. 6.3). The TREO concentration increased by 133% in the least weathered carbonatite (Table 6.4),primarily due to dissolution of non-REE-bearing mineral phases during weathering and subsequent concentration of the more resistant REE-bearing minerals. While the REE concentration in the calcite and manganoan calcite was not measured in this study, it could prove useful to pursue such quantification in future studies. As stated earlier, Olinger (2102) measured up to 1000 ppm REE in magmatic calcite from Bull Hill. The contribution of liberated calcite-hosted REE during weathering to the overall REE-concentration in the weathered carbonatite was not explored in this study. Ancylite, much of it apparently replacing magmatic burbankite, is the dominant REE- bearing mineral in the carbonatite and remains the dominant REE-host in the Lower Weathering Zone. Bastnäsite group minerals, and to a lesser extent monazite, cerianite, and Al-phosphates, host the majority of REE in the Upper Weathering Zone at Bull Hill (A. Van Rythoven and J. Clark, personal communication, 2014). The process by which the bastnäsite group minerals become the dominant REE-host in the Upper Weathering Zone remains unclear.

7.3 Timing of Weathering

Given that weathering was critical to the formation of a high grade REE deposit at Bear Lodge, it is important to consider the timing of carbonatite weathering. Geological evidence suggests that the alkaline rocks at Bull Hill could have experienced weathering from the Late Eocene to the Holocene; although, weathering may have been episodic. At the time of initial magmatic activity in the Bear Lodge Mountains at 51-46 Ma the Western Interior Seaway had receded and the Black Hills were being uplifted (Lisenbee and Dewitt, 1993; Duke, 2005; Andersen, 2013). Although environmental conditions in the Bear Lodge Mountains at the time of early alkaline silicate and carbonatite magmatism (Early to Middle Eocene) are uncertain, nearby areas of southwestern, central, and much of northern Wyoming contain extensive lacustrine deposits (Snoke, 1993) (Fig. 7.2) suggesting at least limited areas of subaerial exposure. Lillegraven (1993) has suggested that this portion of Wyoming experienced widespread and intense erosion between 42 Ma and 37 Ma (Fig. 7.2), which is partially contemporaneous with the late alkaline magmatic activity (40-38 Ma) in the 18 Bear Lodge dome (Duke, 2005). Interestingly, Moore et al. (2015) demonstrated high δ OV- o SMOW values (approx. 18 /oo) from the Bull Hill carbonatite samples, which suggest mixing of meteoric water with late magmatic-hydrothermal fluids during emplacement of the carbonatite. The unconformity between the Late Eocene-Oligocene White River Formation and the underlying Paleogene intrusive rocks and pre-Cenozoic rocks, including the Spearfish Formation, the Minnelusa Sandstone, and the Sundance Formation, indicates that these rocks were exposed at the surface prior to deposition of the White River Formation (Lillegraven, 1993; Staatz, 1983). Thus, the Late Eocene to Oligocene transition probably represents the first period of significant weathering that could have affected the carbonatites. The Eocene to Oligocene transition was marked in the Bear Lodge Mountains by a fluvial depositional environment represented by the White River Formation (Staatz, 1983).

Figure 7.2: Regional and local geomorphological and climatic events and affecting the Black Hills, WY.

Since the Oligocene, the Bear Lodge area has apparently been subaerial and had the potential for weathering. The Oligocene Ogallala Formation, preserved in a graben adjacent to the Bull Hill diatreme and referred to by some as the Redwater Creek Graben, (Staatz, 1983; J. Ray, personal communication, 2015), contains clasts of igneous rocks that could have been derived from the deposit area, suggesting active erosion. Weathering of the rocks at Bull Hill has certainly been occurring for at least the last 12 million years. The precipitation of carbonate minerals, such as bastnäsite, is problematic in a lateritic environment that may involve low pH groundwater (Mariano, 1989). Weathered carbonatite REE-deposits such as Mt. Weld, and Araxá in lateritic environments contain REE-phosphates as the main REE-phases in subsoil layers; REE-fluorocarbonates are rarely observed. The supergene fluorocarbonate REE mineral assemblage at Bear Lodge may have formed in a semi- arid environment within which a calcitic soil would have developed (Machette, 1985). Some additional insight into the weathering environment at Bull Hill may be given by work completed by Pan and Stauffer (2000) on the Ce-anomaly in the Flin Flon paleosol and also by Loges et al. (2012) on the stability relationships of manganese and cerium in the Clara Fluorite Mine waters. The stability fields for cerianite from both studies overlap, as shown in Figure 7.3, under modern atmospheric conditions. Cerianite appears to be stable in conditions with pH 4 to pH 12; however, cerianite stability is highly dependent on redox conditions (Fig. 7.3). Cerianite appears to be stable in an alkaline environment with reduced conditions. As the environment becomes more acidic, it must also become more oxidizing in order for cerianite to remain stable. Cerianite appears to be less abundant in the Lower Weathering Zone of this study than what is reported for the Upper Weathering Zone (J. Clark, personal communication, 2016). Thus the Lower Weathering Zone at Bull Hill could have ranged from reduced and alkaline to oxidized and acidic during weathering.

7.4 Comparison of Weathered Carbonatite at Bull Hill, Araxá, and Mt. Weld

Although several weathered alkaline complex-carbonatite type deposits are known to exist, the Araxá, Brazil and Mt. Weld, Australia REE deposits are two of the best studied. Comparison of the Bull Hill carbonatite and its weathering products to those of other weathered carbonatite REE-deposits indicates that supergene mineralogy is primarily the result of the

geochemical environment of weathering. Weathering in a tropical or semi-tropical environment results in a dominance of REE-phosphates (Araxá, Mt. Weld), while weathering in a semi-arid environment results in a dominance of REE-carbonates (Bull Hill, Bear Lodge).

Figure 7.3: Pourbaix diagram illustrating the stability of cerianite. Cerianite stability field is shown in green for the system Ce-P-C-H-O ( Ce = 1.4 x 10-3 ppm) at modern atmospheric conditions, 25°C and 1 bar (diagram after Pan and Stauffer, 2000). Cerianite stability field shown in blue is based on calculations for low,� median, and high Ce mine water concentrations (5.9 x 10-7ppm to 2.7 x 10-5 ppm), at 25°C and 1 bar (after Loges et al., 2012)

7.4.1 Bull Hill and Araxá Weathered Carbonatites

The carbonatite ring complex at Araxá consists of a sövite and rauhaugite core surrounded by metasomatically altered pyroxenites (glimmerites) and peridotites (Mariano 1979; Morteani and Preinfalk, 1996). Morteani and Preinfalk (1996) recognized magmatic apatite and calcite as the main REE-bearing minerals in the fresh carbonatite, while Mariano (1979) identified monazite and ancylite as hydrothermal REE-minerals; secondary apatite is also present

(Morteani and Preinfalk, 1996). Ancylite and monazite do not occur together. Ancylite is associated with quartz, pyrite and other sulfide minerals but contains turbid hematite inclusions suggesting precipitation during at least weak oxidation of sulfide (Mariano, 1979). This is similar to the iron-oxide inclusions observed within ancylite at Bull Hill. Minor bastnäsite and parisite were also recognized at Araxá as late stage minerals formed by a hydrothermal or mixed hydrothermal-meteoric fluid (Mariano, 1979). A well-developed weathering profile consisting of saprolite, laterite, and soil overlies the Araxá carbonatite. The predominant REE-bearing minerals in the weathered zones are monazite and gorceixite (Mariano, 1979; Morteani and Preinfalk, 1996). Even though gorceixite is a hydrated Ba-Al-phosphate and not classified as a REE-bearing mineral, it can contain up to 6 wt. % REE (Mariano, 1979). According to Morteani and Preinfalk (1996), the average REE concentration in the fresh carbonatite is 0.5 wt. % while the average concentration of REE in the laterite is 1.8 wt. %. In the lower residual portion of both the Araxá and Bull Hill weathering profile, primary REE-bearing minerals maintain dominance as REE hosts. What is apparent from the two deposits is that different primary magmatic and hydrothermal mineralogy yields different supergene mineralogy, especially when subjected to dissimilar weathering environments (Table 7.1).

Table 7.1: Araxá and Bear Lodge REE-bearing mineral assemblages and paleoclimates.

REE-bearing Mineral assemblage Deposit Magmatic Hydrothermal Supergene Climate Araxá, Brazil apatite, calcite monazite, monazite, Humid- ancylite, apatite gorceixite tropical (?), bastnäsite, parisite Bear Lodge, burbankite, ancylite, Sr-Ca- bastnäsite, Warm-humid Wyoming USA ancylite (?) REE-phosphate, parisite, monazite, to Cool, (Bull Hill) bastnäsite, cerianite florencite, Semi-arid synchysite/parisite goyazite

At Araxá, weathered carbonatite (La/Yb)cn ratios range between 1500 and 1900 (Morteani and Preinfalk,1996), slightly higher than those in Bull Hill weathered carbonatite (avg.

1498). The (La/Yb)cn ratios of the Araxá laterite average approximately 80, while Araxá saprolite has ratios of approximately 670. These values are significantly lower than that of the low-carbonate moderately weathered zone at Bull Hill (1266) and indicate that the Araxá carbonatite experienced more pronounced HREE enrichment during weathering. This is reflected in the mineralogy of the deposits. At Araxá, monazite in the weathered horizons probably contains appreciable HREE. There are no high HREE mineralogical hosts in the supergene mineral suite at Bull Hill.

7.4.2 Bull Hill and Mt. Weld Weathered Carbonatites

The Mt. Weld alkaline complex consists of a central carbonatite core, surrounded by a brecciated zone. Sövite is the predominant component of the carbonatite core, with lesser rauhaugite and beforsite (Middlemost, 1990). The complex is capped with a lateritic profile. The Mt. Weld laterite cap developed during the Mesozoic to Cenozoic (Lottermoser and England, 1988; Middlemost, 1990). It contains late stage carbonate and silica at the top of the weathering profile, which may indicate a change from the tropical, humid climate to a more arid environment (Lottermoser and England, 1988; Lottermoser, 1990). The upper weathering zone at Mt. Weld contains abundant secondary phosphates and Al- phosphates, clays, cerianite, and monazite (Lottermoser, 1990; Middlemost, 1990). Primary and secondary monazite occur in the weathering profile but ancylite, bastnäsite and parisite are not recognized (Lottermoser, 1990). The predominant REE-bearing minerals at Mt. Weld are phosphate minerals, such as monazite, apatite, crandallite, gorceixite, goyazite, and florencite (Lottermoser, 1990). In contrast, the REE-bearing minerals of the weathered Bull Hill carbonatite are mainly carbonate minerals, such as ancylite, bastnäsite, and parisite/synchysite. The REE content of the residual zone directly above the fresh carbonatite at Mt. Weld ranges from 0.1 wt. % to 1.0 wt. % whereas the upper supergene zone ranges in REE concentration from 0.5 wt % to 30 wt. % (Lottermoser, 1990). Fresh carbonatite at Mt. Weld contains (La/Lu)cn ratios of 45 to 55 while laterite (La/Lu)cn ratios exhibit a broad range from 8.7 to 395.3 (Lottermoser, 1990). HREE are concentrated near the upper weathering zone and the

lower-accumulation zone. This vertical HREE zonation differs from the lateral HREE zonation within the Bear Lodge deposit, where the HREE are enriched peripheral to the central core of the deposit.

Table 7.2: Mt. Weld and Bear Lodge REE-bearing mineral assemblages and paleoclimates.

REE-bearing Mineral assemblage Deposit Magmatic Hydrothermal Supergene Climate Mt. Weld, apatite, ? monazite, churchite, Humid- Australia monazite, crandallite, goyazite, tropical to synchysite florencite, gorceixite, arid apatite (?), cerianite Bear Lodge, burbankite, ancylite, Sr-Ca- bastnäsite, parisite, Warm- Wyoming ancylite (?) REE-phosphate, monazite, cerianite humid to USA bastnäsite, florencite, goyazite Cool, Semi- (Bull Hill) synchysite/parisite arid

CHAPTER 8 CONCLUSIONS

This study of weakly to moderately weathered REE-bearing carbonatite in the Lower Weathering Zone of western Bull Hill demonstrates:

 Rare earth element concentration increased in the carbonatite by both late- to post- magmatic/hydrothermal and weathering processes. Both appear to have been open system processes.

 Ancylite is the predominant REE-bearing mineral in the least weathered carbonatite. It appears to replace magmatic burbankite. Ancylite remains the predominant REE-host in the moderately weathered carbonatite within the Lower Weathering Zone. Weathering of carbonatite resulted in progressive leaching of gangue carbonate minerals and an increase in porosity. Rare earth element concentrations, along with the concentration of the more resistant gangue minerals, increased due to volume loss from calcite and strontianite dissolution.

 Supergene mineralogy is primarily the result of the geochemical environment of weathering. Comparison of the carbonatite and its weathering products to those of other weathered carbonatite REE-deposits indicates that weathering in a tropical or semi- tropical environment results in a dominance of REE-phosphates (Araxá, Mt. Weld) and that weathering in a semi-arid environment results in a dominance of REE-carbonates (Bull Hill, Bear Lodge).

REFERENCES CITED

Andersen, A.K., Cosca, M.A., Larson, P.B., 2013, Timing of carbonatite magmatism in the Bear Lodge alkaline complex, Geological Society of America Abstracts with Programs, v. 45, p. 499.

Belovitskaya,Y.V., Pekov,I.V., 2004, Genetic Mineralogy of the Burbankite Group, New Data on Minerals. Moscow, v. 39, p. 50-64.

Bird, W., 2005, Bear Lodge Rare-Earth Project: Rare Element Resources Ltd. Retrieved December 2, 2010, from http://www.rareelementresources.com/s/BearLodge.asp

Castor, S.B., 2008, The Mountain Pass Rare-Earth Carbonatite and Associated Ultrapotassic Rocks, California: The Canadian Mineralogist, v. 46, p. 779-806.

Chakhmouradian, A.R., Wall, F., 2012, Rare Earth Elements: Minerals, Mines, Magnets (and More): Elements, v. 8, no. 5, p. 333-340.

Chakhmouradian, A.R., Zaitsev, A.N., 2012, Rare Earth Element Mineralization in Igneous Rocks: Sources and Processes: Elements, v. 8, no. 5, p. 347-353.

Deer, W.A., Howie, R.A., and Zussman, J., 1992, An Introduction to the Rock-Forming Minerals, 2nd ed.: Pearson, London, 696 p.

Doroshkevich, A.G., Ripp, G.S., Viladkar, S.G., Vladykin, N.V., 2008, The Arshan REE Carbonatites, Southwestern Transbaikalia, Russia: Mineralogy, Paragenesis and Evolution: The Canadian Mineralogist, v. 46, p. 807-823.

Duke, G. I., 2005, Geochemistry and Geochronology of Paleocene-Eocene Alkalic Igneous Rocks, Northern Black Hills, South Dakota and Wyoming. Doctoral Dissertation, South Dakota School of Mines and Technology, Rapid City, SD.

Felsman, J., 2009, Geology, Hydrothermal Alteration, and Gold – Telluride Mineralization in the vicinity of Carbon Hill and Taylor Ridge, Bear Lodge Mountains, Crook County, Wyoming. M.S.Thesis, University of Idaho, Moscow, ID.

Flanagan, K.M., Montagne, J., 1993, Neogene Stratigraphy and Tectonics of Wyoming, in Snoke, A.W., Steidtmann, J.R., and Roberts, S.M., eds. Geology of Wyoming: Geological Survey of Wyoming Memoir No. 5, p. 572-607.

Gieré, R., 1996, Formation of Rare Earth Minerals in Hydrothermal Systems, in Jones, A.P., Wall, F., and Williams, C.T., eds., Rare Earth Minerals - Chemistry, Origin and Ore Deposits: London, United Kingdom, Chapman and Hall, The Mineralogical Society, p. 105-150.

Grant, J.A., 1986, The Isocon Diagram – A Simple Solution to Gresens’ Equation for Metasomatic Alteration: Economic Geology, v. 81, p. 1976-1982.

Grant, J.A., 2005, Isocon analysis: A Brief Review of the Method and Applications: Physics and Chemistry of the Earth, v. 30, p. 997-1004.

Gresens, R.G, 1966, Composition-Volume Relationships of Metasomatism: Chemical Geology, v. 2, p. 47-55.

Hembree, D.I., Hasiotis, S.T., 2007, Paleosols and Ichnofossils of the White River Formation of Colorado: Insight into Soil Ecosystems of the North American Midcontinent During the Eocene-Oligocene Transition: Palaios, v. 22, p.123-142.

Hoatson, D.M., Jaireth, S., Miezitis, Y., 2011, The Major Rare-Earth-Element Deposits of Australia: Geological Setting, Exploration, and Resources, Geoscience Australia, 204 p.

Jenner, G.A., 1984, Tertiary Alkalic Igneous Activity, Potassic Fenitization, Carbonatite Magmatism, and Hydrothermal Activity in the Central and Southeastern Bear Lodge Mountains, Crook County, Wyoming. M.S. Thesis, University of North Dakota, Grand Forks, ND.

Jobe, T.D., 2013, Sedimentology, Chemostratigraphy and Quantitative Pore Architecture in Microporous Carbonates: Examples from a Giant Oil Field Offshore Abu Dhabi, U.A.E, Doctoral Dissertation, Colorado School of Mines, Golden, CO.

Lalonde, A.E., Bernard, P., 1993, Composition and Color of Biotite from Granites: Two Useful Properties in the Characterization of Plutonic Suites from the Hepbrun Internal Zone of Wopmay Orogen, Northwest Territories: Canadian Mineralogist, v. 31, p. 203-217.

Lillegraven, J.A, 1993, Correlation of Paleogene Strata Across Wyoming – A User’s Guide, in Snoke, A.W., Steidtmann, J.R. and Roberts, S.M., eds., Geology of Wyoming: Geological Survey of Wyoming Memoir No. 5, p. 414-477.

Lisenbee, A.L., DeWitt, E., 1993, Laramide evolution of the Black Hills uplift, in Snoke, A.W., Steidtmann, J.R. and Roberts, S.M., eds., Geology of Wyoming : Geological Survey of Wyoming Memoir No. 5, p. 374-412.

Loges, A., Wagner, T., Barth, M., Bau, M., Göb, S., Markl, G., 2012, Negative Ce Anomalies in Mn oxides: The role of Ce4+ mobility during water-mineral interaction: Geochicimica et Cosmochimica Acta, v. 86, p. 296-317.

Long, K., Van Gosen, B., Foley, N., Cordier, D., 2010, The Principal Rare Earth Elements Deposits of the United States—A Summary of Domestic Deposits and a Global Perspective: U.S. Geological Survey Scientific Investigations Report 2010–5220, 96 p.

Lottermoser, B.G., 1990, Rare Earth Element Mineralization within the Mt. Weld Carbonatite Laterite, Western Australia: Lithos, v. 24, p. 151-167.

Lottermoser, B.G., England, B.M, 1988, Compositional Variation in Pyrochlores from the Mt. Weld Carbonatite Laterite, Western Australia: Mineralogy and Petrology, v. 38, p. 37-51.

Machette, M.N., 1985, Calcic Soils of the Southwestern United States, in Soils and Quaternary Geology of the Southwestern United States. D.L. Weide and M.L. Faber, eds. Geological Society of America Special Paper v. 203, p. 1-21.

Mariano, A.N., 1979, Rare Earth Mineralization at Araxá Minas Gerais, Brasil, Unpublished company report, 95 p.

Mariano, A.N., 1978, The application of cathodoluminescence for carbonatite exploration and characterization in Braga, C.P. et al., eds., Proceedings of the International Symposium on Carbonatites, 1st, Pocos de Caldas, Minas Gerais, Brasil, June, 1976, p. 39-57.

Mariano, A.N., 1989, Economic geology of rare earth elements: Reviews in Mineralogy, v. 21, p. 309–337.

Meyer, B. H., 2002, Geological Exploration Report of the Bear Lodge Rare-Earth Property Crook County, Wyoming – USA. Prepared for: Paso Rico Resources Ltd., Vancouver, BC.

Middlemost, E., 1990, Mineralogy and Petrology of the Rauhaugites of the Mt. Weld Carbonatite Complex of Western Australia: Mineralogy and Petrology, v. 41, p. 145-161.

Moore, M., Chakhmouradian, A.R., Mariano, A.N., Sidhu, R., 2015, Evolution of Rare- Earth Mineralization in the Bear Lodge Carbonatite, Wyoming: Mineralogical and Isotopic Evidence: Ore Geology Reviews, v. 64, p. 499-521.

Morteani, G., Preinfalk, C., 1996, REE Distribution and REE Carriers in Laterites formed on the Alkaline Complexes of Araxá and Catalão (Brazil) in Jones, A.P., Wall, Frances, and Williams, C.T., eds., Rare earth minerals - Chemistry, origin and ore deposits: London, United Kingdom, Chapman and Hall, The Mineralogical Society, Series 7, p. 227–255.

Noble, A.C., 2013, Bear Lodge Project Canadian NI 43-101Technical Report on the Mineral Reserves and Resources and Development of the Bull Hill Mine. Prepared for Rare Element Resources, May. Amended June, 278 p.

Noble, A.C., 2014, Bear Lodge Project Canadian NI 43-101 Pre-Feasibility Study Report on the Reserves and Development of the Bull Hill Mine, Wyoming. Prepared for Rare Element Resources, October, 516 p.

Olinger, D.A., 2012, Characterization and Genetic Relation of Carbonatite and Associated Alkaline Silicate Rocks of Northwest Bull Hill, Bear Lodge Mountains, Northwest Wyoming, M.S. Thesis, Texas Tech University, Lubbock, TX.

Pan, Y., Stauffer, M.R., 2000, Cerium anomaly and Th/U fractionation in the1.85Ga Flin Flon Paleosol: Clues from REE- and U-rich accessory mineral and implications for paleoatmospheric reconstruction: American Mineralogist, v. 85, p.898-911.

Post, J.E., 1999, Manganese Oxide Minerals: Crystal Structures and Economic and Environmental Significance, Proceedings of the National Academy of Science, USA, v. 96, p. 3447–3454.

Ray, J., 2013, Exploration for REE and gold at the Tertiary Bear Lodge Alkaline Complex, Crook County, Wyoming. Oral Presentation, Technical Session: Geology: Entering the Realm of the Rare-Earths. SME Annual Meeting, Denver, CO.

Snoke, A.W., 1993, Geologic history of Wyoming within the tectonic framework of the North American Cordillera, in Snoke, A.W., Steidtmann, J.R., and Roberts, S.M., eds., Geology of Wyoming: Geological Survey of Wyoming Memoir No. 5, v. 2, p. 2-56.

Staatz, M.H., 1983, Geology and Description of Thorium and Rare-Earth Deposits in the Southern Bear Lodge Mountains, Northeastern Wyoming: U.S. Geological Survey Professional Paper 1049-D, United States Government Printing Office, Washington, D.C.

Verplank, P.L., Van Gosen, B.S., Seal, R.R., McCafferty, A.E., 2014, A Deposit Model for Carbonatite and Peralkaline Intrusion-Related Rare Earth Element Deposits, U.S. Geological Survey Scientific Investigations Report 2010–5070–J.

Wall, F., Mariano, A.N., 1996, Rare Earth Minerals in Carbonatites: A Discussion Centered on the Kangakunde Carbonatite, Malawi, in Jones, A.P., Wall, F., and Williams, C.T., eds., Rare Earth Minerals - Chemistry, Origin and Ore Deposits: London, United Kingdom, Chapman and Hall, The Mineralogical Society, Series 7, p. 193–225.

Whitney, D.L., Evans, B.W., 2010, Abbreviations for names of rock-forming minerals, American Mineralogist, v. 95, p. 185–187.

Williams-Jones, A.E., Migdisov, A.A., Samson, I.M., 2012, Hydrothermal Mobilisation of the Rare Earth Elements: A Tale of “Ceria” and “Yttria”: Elements, v. 8, no. 5, p. 355-360.

Zaitsev, A.N., Chakhmouradian, A.R, 2002, Calcite-Amphibole-Clinopyroxene Rock from the Afrikanda Complex, Kola Peninsula, Russia: Mineralogy and a Possible Link to Carbonatites. II. Oxysalt Minerals: The Canadian Mineralogist, v. 40, p. 103-120.

Zaitsev, A.N., Demény, A., Sindern, S., Wall, F., 2002, Burbankite Group Minerals and their Alteration in Rare Earth Carbonatites – Source of Elements and Fluids (Evidence from C-O and Sr-Nd isotopic data): Lithos, v. 62, p. 15-33.

Zaitsev, A.N., Wall, F., Le Bas, M.J., 1998, REE-Sr-Ba Minerals from the Khibina Carbonatites, Kola Peninsula, Russia: their Mineralogy, Paragenesis and Evolution: Mineralogical Magazine, v. 62, no. 2, p. 225-250.

Zaitsev, A.N., Williams, T.C., Jeffries, T.E., Strekopytov, S., Moutte, J., Ivashchenkova, O.V., Spratt, J., Petrov, S.V., Wall, F., Seltmann, R., Borozdin, A.P., 2015, Reprint of “Rare Earth Elements in Phoscorites and Carbonatites of the Devonian Kola Alkaline Province, Russia: Examples from Kovdor, Khibina, Vuoriyarvi and Turiy Mys Complexes”: Ore Geology Reviews, v. 64, p. 477-498.

APPENDIX A SUPPLEMENTAL ELECTRONIC FILES

These supplemental electronic files provide additional data collected, methods used, and error calculations regarding the samples examined in this study. The files are outlined with description in the table below. They are listed in the order for which they occur throughout the thesis text.

RES09-17_430ft-555ftLog.pdf Drill core log completed for this study, which detailed the drill core interval, 430ft to 555ft of RES09-17. PetroSampleList_Location.pdf Petrography sample list and location schematic for samples used in this study. The list includes depth down hole from which the sample was taken and weathering classification and zone correlation. Optical Microscopy.pdf This file contains individual thin section descriptions with select descriptions for granular, weathered carbonatite for which thin sections were not made. SEMdata.pdf This file contains raw data collected with scanning electron microscopy, including spectra and compositional information for mineral species. CLdata.pdf Images and image collection parameters from the optical cathodoluminescence microscopy of thin sections described in this study.

QEMSCAN_Mineraldata.xlsx Mineral maps and modal abundance of minerals examined with QEMSCAN® automated mineral analyses. Results from a resolution variability study are included. QEMSCAN_Porositydata.xlsx Results and false-color maps from QEMSCAN® automated porosity measurements. Whole-RockGeochem_Methods.pdf This file contains the sample list, sample preparation, methods for analysis, and error analysis for all whole rock geochemical analyses. Micas_EMPAdata.xlsx This file contains raw data as well as error analyses from electron microprobe analyses of micas in the least weathered carbonatite. Additional data, images of spots, and graphs are shown for each area. of the thin sections which were analyzed.