RARE-EARTH ELEMENT MINERALIZATION AND FORMATION OF THE IRON HILL CARBONATITE COMPLEX, GUNNISON COUNTY, COLORADO, USA by Jae Erickson A thesis submitted to the Faculty and 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: ________________________ Jae Erickson Signed: ________________________ Dr. Katharina Pfaff Thesis Advisor Golden, Colorado Date ________________ Signed: ________________________ Dr. Paul Santi Professor and Head Department of Geology and Geological Engineering ii ABSTRACT The Iron Hill carbonatite complex is located in the Powderhorn district in Gunnison County, Colorado, USA, roughly 35 km south-southwest of the city of Gunnison. The complex consists of a ringed series of silica-deficient alkalic igneous rocks with a carbonatite stock at the center. The genesis of Rare Earth Element (REE)-enriched igneous complexes like Iron Hill is contentious, and the processes involved with REE enrichment at these complexes are poorly understood. The complex is one of the largest known niobium and titanium endowments in the United States and contains significant amounts of light REEs and thorium. The igneous rocks of the Iron Hill complex intruded Proterozoic granite, syenite, and metamorphic country rock roughly 600 million years ago. From oldest to youngest, the major rock types found at the complex are pyroxenite, uncompahgrite, ijolite, nepheline syenite, and an ankeritic dolomite carbonatite stock. Carbonatite dikes, as well as jasper, limonite-barite, orthoclase-quartz-thorite, and fluorite-quartz veins are present. All major lithological units at Iron Hill are enriched in REEs, including a halo of intense fenitization within the surrounding country rock. This study aims to further our understanding of the geologic processes involved with carbonatite genesis and the formation of REE-bearing minerals at complexes like Iron Hill. The pyroxenite of the Iron Hill complex consists primarily of augite, with lesser amounts of biotite, phlogopite, perovskite, ilmenite, aegirine, fluorapatite, sphene, and magnetite. At least three subunits have been interpreted to occur, including a coarse-grained biotite and magnetite- rich pyroxenite, a perovskite and apatite-rich pyroxenite, and a melanite garnet (Ti-andradite)- rich pyroxenite. The uncompahgrite is composed of melilite-group minerals and diopside, with lesser amounts of melanite garnet, magnetite, apatite, and perovskite. The ijolite unit consists primarily of medium-grained augite and nepheline, with lesser amounts of biotite, apatite, magnetite, ilmenite, and melanite garnet. The nepheline syenite is dominantly composed of perthitic microcline, with lesser nepheline, aegirine-augite, biotite, magnetite, and ilmenite. The carbonatite stock ranges from coarse to fine grained, and is dominantly composed of ankeritic dolomite, with lesser aegirine, potassium feldspar, hematite, biotite, fluorapatite, rutile, barite, calcite, and quartz. iii On the basis of textural relationships, pyroxene is interpreted to have formed very early in the crystallization sequence and is therefore appropriate for recording the parental melt evolution. The mineral chemistry of pyroxene reveals a continuous chemical trend within the uncompahgrite, ijolite, nepheline syenite, and carbonatite units. This trend is visible in decreasing Ca/Na and Mg/Fe ratios, as well as a continuous trend in end-member composition. However, the pyroxenite unit does not fit this trend, implying that the major intrusive units, except for the pyroxenite, may have originated from a single parental magma body and formed due to crystal fractionation. The trace element chemistry of early pyroxenes and the presence of carbonate melt inclusions within the nepheline syenite suggest liquid-liquid immiscibility likely played a role in the later stages of the complex’s genesis. Six species of REE-bearing minerals have been identified: apatite, bastnaesite, parisite, synchysite, monazite/rhabdophane, and ancylite. The distribution of REE-bearing mineral phases, alteration mineral associations, and complex cathodoluminescence properties of early apatite crystals reveals a complex alteration history within the intrusive units as well as the surrounding fenite. iv TABLE OF CONTENTS ABSTRACT ................................................................................................................................... iii LIST OF FIGURES ...................................................................................................................... vii LIST OF TABLES ......................................................................................................................... ix ACKNOWLEDGEMENTS ............................................................................................................ x CHAPTER 1 INTRODUCTION .................................................................................................. 1 1.1 Overview of carbonatites ........................................................................................ 1 1.2 Rare Earth Elements ............................................................................................... 6 CHAPTER 2 RARE-EARTH ELEMENT MINERALIZATION AND FORMATION OF THE IRON HILL CARBONATITE COMPLEX, GUNNISON COUNTY, COLORADO ........................................................................................................... 8 2.1 Introduction ............................................................................................................ 8 2.2 Geological setting and previous work .................................................................. 10 2.3 Analytical methods.. ............................................................................................ 16 2.3.1 Quantitative Evaluation of Minerals by Scanning Electron Microscopy (QEMSCAN) ........................................................................................................ 16 2.3.2 Cathodoluminescence and backscattered electron imaging ....................... 17 2.3.3 Electron microprobe analysis ..................................................................... 18 2.3.4 Laser ablation inductively coupled plasma mass spectrometry ................. 19 2.4 Results ................................................................................................................... 19 2.4.1 Petrography ................................................................................................ 19 2.4.2 Mineral chemistry ...................................................................................... 35 v 2.5 Discussion ............................................................................................................. 47 2.5.1 Chemical evolution trends observed in pyroxene and apatite.. ................. 47 2.5.2 Distribution of REE-bearing minerals.. ..................................................... 52 2.5.3 Constraining the source of the Iron Hill melt(s) ........................................ 55 2.5.4 Genetic relationship between carbonatite and silicate units.. .................... 58 2.6 Summary and conclusions.. .................................................................................. 64 CHAPTER 3 CONCLUSIONS AND OUTLOOK .................................................................... 67 3.1 Conclusions ........................................................................................................... 67 3.2 Outlook and future work ....................................................................................... 69 REFERENCES CITED ................................................................................................................. 71 APPENDIX A FALSE-COLOR MINERAL AND ELEMENT MAPS ............................. 76 vi LIST OF FIGURES Figure 1.1 IUGS non-genetic carbonatite classification system ............................................... 2 Figure 2.1 Simplified geologic map of the Iron Hill carbonatite complex (IHCC) with sample locations .................................................................................................... 11 Figure 2.2 Photograph of the pyroxenite unit showing enhanced weathering proximal to vertical carbonatite dike ........................................................................................ 13 Figure 2.3 Photograph of carbonatite dike crosscutting the Iron Hill carbonatite stock ........ 15 Figure 2.4 Photomicrographs showing characteristic textures of each major lithology present at the Iron Hill carbonatite complex ........................................................ 21 Figure 2.5 Crystallization sequence for each major lithology present at the Iron Hill carbonatite complex .............................................................................................. 23 Figure 2.6 QEMSCAN image of coarse biotite and magnetite-rich pyroxenite ..................... 24 Figure 2.7 Pyroxenite sample showing early pale yellow-green diopside being replaced by magnetite .......................................................................................................... 25 Figure 2.8 Backscattered electron images showing REE-bearing minerals hosted in various lithologies at the Iron Hill carbonatite complex ....................................... 26 Figure 2.9 QEMSCAN image of perovskite and apatite-rich subunit of the pyroxenite ........ 27 Figure 2.10 QEMSCAN image of least-altered uncompahgrite