Mineralogy and Petrogenesis of the California Blue Mine
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MINERALOGY AND PETROGENESIS OF THE CALIFORNIA BLUE MINE AQUAMARINE- AND TOPAZ-BEARING PEGMATITE DEPOSIT, SAN BERNARDINO COUNTY, CALIFORNIA by Carolyn Pauly Copyright by Carolyn Pauly 2019 All Rights Reserved 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: _____________________________ Carolyn Pauly Signed: _____________________________ Dr. Alexander Gysi Thesis Advisor Golden, Colorado Date __________________________ Signed: _____________________________ Dr. M. Stephen Enders Professor and Department Head Department of Geology and Geological Engineering ii ABSTRACT The California Blue Mine Pegmatite is a recently discovered gem aquamarine- and topaz- bearing deposit near Yucca valley in southern California, and is the first significant gem-bearing pegmatite discovered north of the Southern California gem districts (Hunerlach, 2012). Aside from large-scale mapping by the USGS and field work necessary for mining, little research has been conducted on the deposit. The purpose of this thesis is to provide a catalogue of the mineralogy, textures, zoning, and geochemistry of the California Blue Mine pegmatite, and to synthesize these observations into a petrogenetic model. This research combined mineralogical, textural, and geochemical field and hand sample observations with lab analyses, including optical microscopy, field emission scanning electron microscopy (FE-SEM), automated mineralogy, electron microprobe analysis (EMPA), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). A distinct textural and mineralogical zoning has been identified in the pegmatite, including a border zone, wall zone, intermediate zone, and core zone, as well as several miarolitic pockets. The presence of these zones and pockets indicate a transition from magmatic to hydrothermal processes during pegmatite formation, an assertion which is supported by geochemical and textural data in beryl. Beryl shows three types of textures coinciding with three stages of crystallization: Type i) distinct to gradational concentric core zoning, produced in the intermediate zone by magmatic processes; Type ii) transitional patchy rim zoning, produced in the core zone by magmatic processes and mild metasomatism, and Type iii) pervasive alteration rims caused by intense hydrothermal alteration. Beryl cores are enriched in Al, and rims are enriched in Sc, Na, Fe, and Cs; the coupled replacement of Sc3+ and Fe2+ for Al3+ on the octahedral site with Na+ and Cs+ on the beryl channel site is the most important iii substitution mechanism in the beryl and defines the transition from Type i) to Type iii) textures. A combination of this octahedral site substitution and coupled substitution of Li+ for Be2+ on the Be tetrahedral site with alkali cations in the channel site characterizes the Type ii) and Type iii) textures. Metasomatic rims of beryl are characterized by enrichment in Li and Cs. The overall evolution of the pegmatite can be subdivided into three stages based on mineral paragenesis and beryl geochemistry. Stage 1 represents crystallization of the quartz, perthitic K-feldspar, albite, biotite, spessartine, and Type i) Al-enriched beryl in the border, wall, and intermediate zones of the pegmatite, produced by fractional crystallization and subsequent melt enrichment in Li, Be, and Sc. Stage II includes the formation of graphically intergrown quartz and albite and Sc-, Na-, and Fe-enriched Type ii) beryl in the pegmatite core by metasomatism from an aqueous fluid separated from the original melt, and Stage III represents the final crystallization of the miarolitic pockets from ponding of the aqueous fluids, producing gem smoky quartz, topaz, and Type iii) beryl enriched in Li and Cs. We conclude that the California Blue Mine pegmatite contains mineralogy similar to NYF-type pegmatites but metasomatic Li and Cs overprinting similar to LCT-type pegmatites, and thus requires more research to classify as either LCT- or NYF-type. iv TABLE OF CONTENTS ABSTRACT……………………………………………………………………………………...iii LIST OF FIGURES……………………………………………………………………………...vii LIST OF TABLES………………………………………………………………………………..ix LIST OF EQUATIONS…………………………………………………………………………...x ACKNOWLEDGMENTS………………………………………………………………………..xi CHAPTER 1 GENERAL INTRODUCTION………………………………………………...1 CHAPTER 2 MINERALOGY AND PETROGENESIS OF THE CALIFORNIA BLUE MINE AQUAMARINE- AND TOPAZ-BEARING PEGMATITE DEPOSIT, SAN BERNARDINO COUNTY, CALIFORNIA………………………………………………………………….3 2.1 Abstract…………………………………………………………………………3 2.2 Introduction……………………………………………………………………..5 2.3 Geological Setting………………………………………………………………8 2.3.1 Regional Geology……….…………....…………………....……....8 2.3.2 Local Geology……………………………………………………..9 2.4 Methods..............................................................................................................14 2.4.1 Field Work and Sampling..............................................................14 2.4.2 Analytical.......................................................................................14 2.5 Results................................................................................................................17 2.5.1 Pegmatite Zoning and Textures.....................................................17 2.5.2 Miarolitic Pockets..........................................................................23 2.5.3 Automated Mineralogy and Mineral Zoning in the Pegmatite......27 2.5.4 Beryl Zoning and Textures............................................................31 v 2.5.5 Beryl Chemistry.............................................................................34 2.5.5.1 EMPA Element Maps............................................34 2.5.5.2 EMPA Beryl Compositions...................................38 2.5.5.3 LA-ICP-MS Analyses ............................................47 2.6 Discussion..........................................................................................................57 2.6.1 Magmatic to Hydrothermal Transition Recorded in Beryl............57 2.6.2 Comparison to Other Pegmatites...................................................58 2.6.3 Classification of the California Blue Mine Pegmatite...................65 2.6.4 Origin and Petrogenetic History....................................................69 2.6.5 Conclusions....................................................................................72 CHAPTER 3 GENERAL CONCLUSIONS............................................................................76 REFERENCES..............................................................................................................................78 APPENDIX A SAMPLING LOCATION AND TEXTURAL INFORMATION FOR BERYL SAMPLES..........................................................................................82 APPENDIX B FULL EMPA DATA OF BERYL.....................................................................83 APPENDIX C FULL LA-ICP-MS DATA OF BERYL............................................................91 APPENDIX D STEP-BY-STEP OUTLINE OF RECALCULATION METHOD FOR EMPA DATA...................................................................................................99 APPENDIX E COMPARISON PLOTS OF BERYL EMPA AND LA-ICP-MS DATA IN APFU.........................................................................................................100 vi LIST OF FIGURES Figure 2.1 Geological map of the Yucca Valley region, California, showing the surrounding geology of the California Blue Mine pegmatite................................10 Figure 2.2 Field relations of the California Blue Mine and surrounding geology..................13 Figure 2.3 A summary of pegmatite zoning............................................................................18 Figure 2.4 Thin section textures in the California Blue Mine pegmatite................................21 Figure 2.5 Fluid inclusions in thin section..............................................................................22 Figure 2.6 Slab of hand sample CBM-M2 showing graphic texture changing to cleavelandite in the miarolitic pocket....................................................................24 Figure 2.7 BSE image of the beryl replacement minerals noted in Figure 2.6.......................25 Figure 2.8 Textures of feldspar in the miarolitic pockets in thin section................................26 Figure 2.9 TIMA mineral maps showing overall mineralogy and textures of pieces of Sample CBM-12....................................................................................................28 Figure 2.10 TIMA element maps of a section from the host rock to the intermediate zone of Sample CBM-12................................................................................................30 Figure 2.11 Epoxy mounts of beryl grains, cut perpendicular to the c-axis, from pegmatite and float samples...................................................................................32 Figure 2.12 BSE photomicrographs of beryl grains..................................................................33 Figure 2.13 EMPA element maps showing depletion of Al and enrichment of Na, Cs, Sc, and Fe from the core to the rim of Beryl 3............................................................36 Figure 2.14 EMPA element maps showing depletion of Al and enrichment of Na, Cs, Sc, and Fe from the core to the rim of Beryl