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Geochemistry and Genesis of Late Paleoproterozoic Banded Iron

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Geochemistry and Genesis of Late Paleoproterozoic Banded Iron Geochemistry and Genesis of Late Paleoproterozoic Banded Iron Formations and Metamorphosed Chemical Precipitates Spatially Associated with Pb-Zn Broken Hill-type Mineralization near the Broken Hill Deposit, Curnamona Province, Australia by Erica Serna December, 2014 Director of Thesis: Adriana Heimann Major Department: Geological Sciences Banded and massive iron formations (IFs) and garnet- and gahnite-quartz rocks are spatially associated with the giant, late Paleoproterozoic, Pb-Zn-Ag Broken Hill sulfide deposit and hundreds of minor Broken Hill-type deposits in the southern Curnamona Province, Australia. The sulfide deposits, host rocks, and a variety of chemical precipitates that include IFs and garnet- and gahnite-rich rocks were metamorphosed to the granulite facies. The IFs are found above, below, and along strike with the ore deposits and are among the youngest IFs that formed in the late Paleoproterozoic at ~ 1.69 Ga, after the Great Oxidation Event (GOE; ~2.35 Ga). Here the petrology and geochemistry of banded and massive IFs and garnet- and gahnite- quartz rocks from near the Broken Hill, Little Broken Hill, Wild Dog, Pinnacles, and Nine Mile Pb-Zn-Ag sulfide deposits are investigated to determine the genesis of the rocks and the physicochemical conditions of the fluids in the intracontinental Broken Hill rift basin at ~1.69 Ga, and ultimately the redox state of the water in this basin at the time of deposition. Banded and massive IFs mainly consist of magnetite-hematite, quartz, and ± garnet, while the rocks from Nine Mile consist of garnet and quartz, and gahnite and quartz. Bulk-rock major and trace element compositions show that the rocks are enriched in Si and Fe with moderate but variable amounts of Mn and Al, which reflect variations of hydrothermal (up to 80%) and detrital contribution in the precursor phases. The rocks formed as a mixture of Fe or Fe-Mn oxyhydroxides and minor clays similar to sediments from the modern redox-stratified rift basin at the Red Sea. Post-Archean Australian Shale (PAAS) normalized rare-earth element (REE) patterns of banded and massive IFs and garnet-quartz rocks exhibit a variety of REE contents and signatures with absent, very small, and moderately strong positive Eu anomalies suggesting that the precursor minerals formed from source hydrothermal fluids of variable temperatures. Strong positive Eu anomalies reflect source fluids with T > 250 °C and are associated with Mn- rich compositions that favor the incorporation of Eu. Small Eu anomalies reflect dilution by the addition of detritus, whereas strong negative Eu anomalies in gahnite-quartz rocks from Nine Mile likely reflect the discrimination of Eu by Fe-rich minerals in the precursor phases. Bulk-rock PAAS-normalized REE patterns range from flat, to light-REE (LREE) enriched, and slightly heavy-REE enriched, which reflect an oxygen-stratified water body. True negative Ce anomalies are only present in a few Mn-bearing banded IFs that have flat REE patterns and slightly higher Al contents than the rest, which reflects precipitation from relatively shallow and oxygenated water. Most rocks lack Ce anomalies, have variable LREE trends, some have LREE enrichment, and are Al-poor and Fe-rich, reflecting precipitation under suboxic conditions from deeper water than the other IFs, the existence of a redoxcline, and reductive dissolution of Fe-Mn oxyhydroxides that provided additional LREEs. Overall, the variety of REE patterns and LREE enrichment and the negative and absent Ce anomalies suggest that at ~ 1.7 Ga the water column in the intracontinental Broken Hill rift basin was stratified and supported the existence of an oxide shuttle that cycled LREEs, which is consistent with results from IFs of similar ages in other basins. Geochemistry and Genesis of Late Paleoproterozoic Banded Iron Formations and Metamorphosed Chemical Precipitates Spatially Associated with Pb-Zn Broken Hill-type Mineralization near the Broken Hill Deposit, Curnamona Province, Australia A Thesis Presented to the Faculty of the Department of Geological Sciences Thomas Harriot College of Arts and Sciences East Carolina University In Partial Fulfillment of the Requirements for the Degree Masters of Science Geology by Erica Serna December, 2014 © Erica Serna, 2014 Geochemistry and Genesis of Late Paleoproterozoic Banded Iron Formations and Metamorphosed Chemical Precipitates Spatially Associated with Pb-Zn Broken Hill-type Mineralization near the Broken Hill Deposit, Curnamona Province, Australia by Erica Serna APPROVED BY: DIRECTOR OF THESIS: _______________________________________________________ Adriana Heimann, PhD COMMITTEE MEMBER: ________________________________________________________ Terri L. Woods, PhD COMMITTEE MEMBER: _______________________________________________________ Richard L. Mauger, PhD COMMITTEE MEMBER: _______________________________________________________ Richard K. Spruill, PhD CHAIR OF THE DEPARTMENT OF GEOLOGICAL SCIENCES: ___________________________________________________ Stephen J. Culver, PhD DEAN OF THE GRADUATE SCHOOL: _________________________________________________________ Paul J. Gemperline, PhD ACKNOWLEDGEMENTS I owe my deepest gratitude to my advisor, Adriana Heimann Ríos, for her guidance and support in concluding my Master’s thesis. I am grateful for the opportunity to work with you and all of your unconditional support you have provided me. I would also like to thank my committee members Drs. Eric Horsman, Richard Spruill, Richard Mauger, and Terri Woods. Special thanks go to Dr. Paul Spry from Iowa State University and Mr. Wolfgang Leyh (Eaglehawk Geological Consulting) for collecting and providing the rock samples for this research and for providing information about the local geology of the studied area and about iron formations from Broken Hill. Thanks to Josh O’Brien for the geologic map. I would also like to thank Mike Wise for doing electron microprobe analysis at the Smithsonian Institution in Washington, DC, and Nick Foster for helping with electron probe microanalysis at Fayetteville State University, NC. Also, thanks to Tom Fink for helping me with the scanning electron microscope, and Dr. Mauger for taking the time for helping me with XRD analysis. Last but not least, I thank Jim Watson and John Woods for helping with the equipment to prepare my rock samples. This research was funded by the College of Arts and Sciences and the Division of Research and Graduate Studies at East Carolina University, and a North Carolina Space Grant New Investigator Grant (to Dr. Heimann), as well as a Sigma Xi (the Scientific Research Society) Grant-in-Aid of Research. The inspiration to pursue a Master’s thesis was given to me by Drs. David Bish and Robert Wintch, two of my professors at Indiana University. They taught me how to hit the ground running and to never give up. Thank you for caring, thanks for EVERYTHING; this thesis is dedicated to you both. I am indebted to many of my ECU colleagues: Leatha Moretz, Heather Lancaster, Jason Yonts, Sarah Harrison, David Hawkins, David Young, Katie Cummings, and Jessica Strand, among many others. Thank you for always being there for me and for the unconditional support you extended over the past two years at East Carolina University. Last but not least, I deeply thank my family. Mom I truly appreciate all of the sacrifices you have made. This accomplishment would not have been possible without you. Thanks to my sister, and all my family in Mexico for their kind words of encouragement. TABLE OF CONTENTS ACKNOWLEDGEMENTS ................................................................................................... iv LIST OF TABLES ............................................................................................................ viii LIST OF FIGURES ............................................................................................................ ix 1. Introduction ............................................................................................................ 1 1.1 Abstract ............................................................................................................ 3 2. Regional Geologic Setting ................................................................................................. 4 3. Sampling and Analytical Methods ..................................................................................... 8 4. Petrography ............................................................................................................ 10 5. Mineral Chemistry ............................................................................................................ 15 5.1 Major Element Composition of Garnet................................................................ 15 5.2 Compositional Zoning of Garnet ......................................................................... 16 5.3 Major Element Composition of Magnetite, Hematite, and Gahnite .................... 17 6. Whole-Rock Geochemistry ................................................................................................ 18 6.1 Major Element Compositions .............................................................................. 18 6.2 Minor and Trace Element Compositions ............................................................. 20 6.3 Rare Earth Elements and Yttrium Compositions ................................................. 21 7. Discussion ...........................................................................................................
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