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Germanium Occurrence in the Bornite Deposit, Southwestern Brooks Range, Alaska

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Germanium Occurrence in the Bornite Deposit, Southwestern Brooks Range, Alaska GERMANIUM OCCURRENCE IN THE BORNITE DEPOSIT, SOUTHWESTERN BROOKS RANGE, ALASKA by Alexander Jones A thesis submitted to the Faculty and the Board of Trustees of the Colorado School of Mines in partial fulfilment of the requirements of the degree of Master of Science (Geology). Golden, Colorado Date ____________________________ Signed: ___________________________ Alexander Jones Signed: ____________________________ Dr. Katharina Pfaff Thesis Advisor Golden, Colorado Date _______________________________ Signed: ____________________________ Wendy Bohrson Professor and Department Head Department of Geology and Geological Engineering ii ABSTRACT The carbonate hosted Bornite Cu-Zn-Co(-Ge) deposit is located in the Cosmos Hills just south of the western portion of the Brooks Range of north western Alaska. In 2011, a new orebody referred to as the South Reef was found at greater depth. The South Reef is host to substantial amounts of high-grade Cu, Co, and Zn, but also hosts elevated concentration of Ge. With the increased demand for germanium and the lack of literature on its distribution within primary mineral deposits, understanding known Ge-bearing deposit is critical for future exploration efforts. In this study, the mineralogy and paragenesis of the South Reef has been described through detailed optical petrography, scanning electron microscopy, automated mineralogy and mapping micro-X-ray fluorescence analysis. Five paragenetic stages have been delineated: (I) dolomitization, (II) ferroan alteration, (IIIa) Veining, (IIIb) brecciation, (IV) ore formation event, and (V) a post ore forming event. Germanium was found to exist in renierite, white mica, illite, galena and feldspars within the South Reef. Whole-rock geochemistry of samples revealed highest-grade Ge concentrations correlating to the samples containing the highest abundance of bornite and chalcocite. It is proposed here that reduced acidic basinal brines capable of transporting significant Cu interacted with carbonaceous phyllites at the site of deposition. Upon fluid- rock interaction of the reduced acidic basinal brine with organic matter rich (reduced) carbonates and phyllites, the basinal brine was buffered to an intermediate oxidation state (around or below the hematite-magnetite buffer) at a near-neutral pH of around 5-6. Petrographic observations reveal a distinct spatial and temporal precipitation succession of chalcocite, bornite, followed by chalcopyrite. This temporal and spatial succession is interpreted to reflect the evolving fluid accompanied by a decrease in temperature and concomitant decrease in sulfide activity of the mineralizing fluid. While temperature may not have been the predominant precipitation mechanism (a shift in pH towards more alkaline conditions is thought to be the dominant precipitation mechanism), changes in temperature and a decrease in sulfide activity have contributed to the distinct mineral zonation in space and time. Germanium is interpreted to have precipitated in the form of renierite contemporaneously with chalcocite and bornite when the sulfur activity in the fluid was still high. The Bornite deposit shares many similarities with other Ge-bearing carbonate hosted iii base metal deposits in Africa such as the Tsumeb, Kipushi, Khusib Springs, Kombat, and Kabwe deposits, however it is of lower germanium grade than these deposits. iv TABLE OF CONTENTS ABSTRACT ............................................................................................................................. iii LIST OF FIGURES ................................................................................................................ vii LIST OF TABLES .....................................................................................................................x ACKNOWLEDGEMENTS ..................................................................................................... xi CHAPTER 1: INTRODUCTION ..............................................................................................1 1.1 Overview of Ge distribution in primary mineral deposits .............................................1 1.2 Overview of Ge distribution in carbonate hosted base metal deposits ..........................3 1.3 The Bornite deposit and aim of study ............................................................................6 CHAPTER 2: GEOLOGICAL SETTING .................................................................................8 2.1 Regional Geology ..........................................................................................................8 2.2 Deposit Geology ..........................................................................................................13 CHAPTER 3: SAMPLING AND ANALYTICAL TECHNIQUES .......................................20 3.1 Sampling, Sample Material, and Analytical Approach ..............................................20 3.2 Analytical Techniques ................................................................................................20 CHAPTER 4: RESULTS .........................................................................................................24 4.1 Geochemistry ...............................................................................................................24 4.2 Micro-XRF ...................................................................................................................27 4.3 Petrography ..................................................................................................................32 CHAPTER 5: DISCUSSION ...................................................................................................65 5.1 Mineralogy and geochemistry of the Bornite deposit ..................................................65 5.2 Ore forming processes .................................................................................................77 5.3 Ge - bearing deposits – a global perspective ...............................................................80 CHAPTER 6: CONCLUSION AND OUTLOOK ..................................................................84 6.1 Conclusion ...................................................................................................................84 6.2 Outlook ........................................................................................................................85 v REFERENCES ........................................................................................................................86 APPENDIX A: SAMPLE DATA ............................................................................................90 APPENDIX B: MINERALIZATION......................................................................................97 vi LIST OF FIGURES Figure 2.1 The location of the Brooks Range, Cosmos Hills after USGS, (2020) ..............8 Figure 2.2 Lithologies and geological structures of the Cosmos Hills and the southern portion of the Brooks Range after Till et al. (2008) and Hitzman et al. (1986) ...............................................................................................................11 Figure 2.3 Stratigraphy within the Bornite deposit after Trilogy Metals Inc. (2018) .......12 Figure 2.4 Map and cross section of three main dolostone bodies in the Bornite deposit after Conner (2015) .............................................................................15 Figure 2.5 Paragenesis diagram of the Bornite deposit after Hitzman (1986) ..................16 Figure 2.6 Maps of the Bornite deposit showing ore body zones after Fritts (1970) and Nova copper (2014) ..........................................................................................18 Figure 2.7 Cross section through the Bornite deposit after Trilogy Metals Inc., (2018) ..19 Figure 3.1 Photos of hand samples ....................................................................................22 Figure 4.1 Geochemistry data for Cu, Zn, Fe, and Co plotted against Ge ........................25 Figure 4.2 Geochemistry data for Ag and Pb plotted against Ge ......................................25 Figure 4.3 Geochemistry data of Cu and Ge plotted against depth in different drill holes .................................................................................................................26 Figure 4.4 XRF images for Cu, Fe, Co, Ni, Ba, and K in samples ....................................27 Figure 4.5 XRF images for Ge and K in samples ..............................................................28 Figure 4.6 XRF images for Co, Bi, As, Pb, Hg, Ge, and Sr in sample RC11-0187_465.88 ..........................................................................................30 Figure 4.7 XRF images of Cu, Ge, K, and Pb in two samples ..........................................31 Figure 4.8 XRF correlation data of As, Cu, Fe, K, Pb, and Zn vs Ge for 25 samples ......32 Figure 4.9 Textures of stage 1 dolomitization minerals viewed optically .........................33 Figure 4.10 Automated mineralogy and thin section images showing stage 1 clasts……..34 Figure 4.11 Thin section image, automated mineralogy and element distribution map for stage 2 clasts ...............................................................................................35 Figure 4.12 BSE images of a stage 2 clast ..........................................................................36 vii Figure 4.13 Thin section images showing fossil textures in stage 1 and 2 clasts ................37 Figure 4.14 Textures
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