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

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

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