SETTING OF VOLCANOGENIC MASSIVE SULFIDE DEPOSITS IN THE PENOKEAN VOLCANIC BELT, GREAT LAKES REGION, USA by Ashley Kaye Quigley 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: ________________________ Ashley Kaye Quigley Signed: ________________________ Dr. Thomas Monecke Thesis Advisor Golden, Colorado Date ________________ Signed: ________________________ Dr. Paul Santi Professor and Head Department of Geology and Geological Engineering ii ABSTRACT The Paleoproterozoic (ca. 1875 Ma) Penokean volcanic belt represents one of the world’s most significant orogens hosting volcanogenic massive sulfide (VMS) deposits. Sporadic exploration from 1970-1995 has identified a large number of VMS deposits and prospects throughout the belt, including the world-class Crandon deposit that comprises an estimated 61 million tonnes of polymetallic massive sulfide ore. Despite successful exploration and the significant economic potential in the Penokean volcanic belt, only limited academic research has been conducted focusing on constraining the tectonic, structural, and volcanic setting of the VMS deposits. Many key aspects of the regional geology are not well understood, which is in part due to extensive glacial cover of the Paleoproterozoic bedrocks. As part of the present study, aeromagnetic and gravity data were used to study the geological make-up of the Penokean volcanic belt. The data were integrated with existing mapping to constrain the bedrock geology under the glacial cover. Interpretation of the geophysical data showed that several distinct geological domains could be distinguished. Whole- rock major and trace element geochemical data were used to identify magmatic affinities of volcanic rocks within the geophysically defined domains. The results of the geochemical analyses reveal subtle differences between the geophysical domains. The majority of the volcanic rocks sampled have a tholeiitic affinity with fewer calc alkaline and transitional rocks. The geochemical evidence suggests that volcanism was arc-related, with the massive sulfide deposits being presumably formed in zones of intra-arc or back-arc extension. High-precision chemical abrasion ID-TIMS U-Pb dating was performed on zircon grains separated from felsic volcanic samples collected from the host rock successions of some of the major VMS deposits and prospects within the Penokean volcanic belt. Results of the geochronological investigations showed that four of the deposits, namely Bend, Horseshoe, Lynne, and Pelican River, were all formed at about 1874 Ma. This suggests that several of the deposits of the Penokean volcanic belt formed during a major, but short-lived, period of rapid extension. iii The Back Forty massive sulfide deposit, located at the east end of the Penokean volcanic belt, is hosted by rhyolite that yielded an apparent age of about 1833 Ma. This is approximately 50 million years younger than the host rock successions of the other deposits of the belt. There are two possible explanations for this apparent age. The first explanation is that this represents a crystallization age and the host rocks to the Back Forty are part of a distinctly younger volcanic succession. Alternatively, a thermal event at 1833 Ma may have reset the U-Pb isotopic system. A crystallization age at around 1833 Ma is potentially consistent with an age recorded for a felsic volcanic rock from the Mountain area to the southwest. The rhyolite sampled from the host rock successions of the Lynne and Back Forty deposits were found to contain rare Archean-aged zircon grains. These zircon grains yielded U-Pb ages of approximately 2700 Ma and are presumably inherited from an Archean basement. The existence of inherited ages supports the model that volcanism of the Pembine-Wausau terrane occurred, at least in part, on older Archean basement. iv TABLE OF CONTENTS ABSTRACT ................................................................................................................................... iii LIST OF FIGURES ...................................................................................................................... vii LIST OF TABLES ...................................................................................................................... .viii ACKNOWLEDGEMENTS ........................................................................................................... ix CHAPTER 1: INTRODUCTION ................................................................................................... 1 1.1. The Penokean Volcanic Belt ................................................................................... 1 1.2. Previous Research ................................................................................................... 4 1.3. Research Approach and Methods ............................................................................ 5 1.3.1. Compilation of Existing Data ...................................................................... 7 1.3.2. Acquisition of New Data ............................................................................. 7 1.4. Thesis Organization ................................................................................................. 8 1.5. References ............................................................................................................. 10 CHAPTER 2: VOLCANOGENIC MASSIVE SULFIDE DEPOSITS IN THE 1.8 GA PENOKEAN VOLCANIC BELT, MICHIGAN AND WISCONSIN: GEOLOGICAL FRAMEWORK AND PALEOTECTONIC SETTING ............ .13 2.1. Introduction ........................................................................................................... 13 2.2. Geological Background ......................................................................................... 14 2.3. Magnetic and Gravity Data ................................................................................... 18 2.3.1. Methodology.............................................................................................. 18 2.3.2. Geophysical Domains ................................................................................ 23 2.3.2. Distribution of Volcanic and Plutonic Rocks ............................................ 26 2.3.3. Lineaments ................................................................................................ 28 2.3.4. VMS Deposits ........................................................................................... 30 2.4. Geochemical Data ................................................................................................. 30 2.4.1. Methodology.............................................................................................. 31 2.4.2. Geochemistry of Volcanic Rocks .............................................................. 32 2.5. Discussion and Conclusions .................................................................................. 38 2.6. Acknowledgements ............................................................................................... 40 2.7. References ............................................................................................................. 42 v CHAPTER 3: ID-TIMS U-PB GEOCHRONOLOGY OF THE PALEOPROTEROZOIC PENOKEAN VOLCANIC BELT, MICHIGAN AND WISCONSIN: TIMING OF VOLCANOGENIC MASSIVE SULFIDE FORMATION AND EXPLORATION IMPLICATIONS ..................................................................... 45 3.1. Introduction ........................................................................................................... 45 3.2. Geological Setting ................................................................................................. 47 3.3. Previous Geochronology ....................................................................................... 52 3.4. Materials and Methods .......................................................................................... 53 3.5. Results ................................................................................................................... 57 3.6. Discussion and Conclusions ................................................................................. .66 3.7. Acknowledgments ................................................................................................. 74 3.8. References ............................................................................................................. 75 CHAPTER 4: CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE WORK .................................................................................................................. 81 APPENDIX A: SUPPLEMENTAL ELECTRONIC FILES ........................................................ 85 vi LIST OF FIGURES FIGURE 1-1. Simplified Geology map of the Penokean volcanic belt ......................................... 2 FIGURE 1-2. Location of the Penokean orogeny .......................................................................... 3 FIGURE 1-3. Schematic cross-sections of the tectonic evolution of the Penokean orogeny .................................................................................................................... 6 FIGURE 2-1. Simplified Geology map of the Penokean volcanic belt ....................................... 15 FIGURE 2-2. Magnetic TMI-RTP and Gravity maps of the Penokean volcanic belt ................. 24 FIGURE 2-3. Detailed Geology map of