Western Kentucky University TopSCHOLAR® Honors College Capstone Experience/Thesis Honors College at WKU Projects 5-13-2015 Geothermobarometry and Petrographic Interpretations of Christensen Ranch Meta- Banded Iron Formation from the Ruby Range, Montana Jacob Hughes Western Kentucky University, [email protected] Follow this and additional works at: http://digitalcommons.wku.edu/stu_hon_theses Part of the Geology Commons Recommended Citation Hughes, Jacob, "Geothermobarometry and Petrographic Interpretations of Christensen Ranch Meta- Banded Iron Formation from the Ruby Range, Montana" (2015). Honors College Capstone Experience/Thesis Projects. Paper 580. http://digitalcommons.wku.edu/stu_hon_theses/580 This Thesis is brought to you for free and open access by TopSCHOLAR®. It has been accepted for inclusion in Honors College Capstone Experience/ Thesis Projects by an authorized administrator of TopSCHOLAR®. For more information, please contact [email protected]. GEOTHERMOBAROMETRY AND PETROGRAPHIC INTERPRETATIONS OF CHRISTENSEN RANCH META- BANDED IRON FORMATION FROM THE RUBY RANGE, MONTANA A Capstone Experience/Thesis Project Presented in Partial Fulfillment of the Requirements for the Bachelor of Science Degree with Honors College Graduate Distinction at Western Kentucky University By Jacob Hughes ***** Western Kentucky University 2015 CE/T Committee: Approved by Professor Andrew Wulff Ph. D, Advisor Professor Fredrick Siewers Ph. D ________________________ Professor Clay Motley Ph. D Advisor Department of Geography & Geology Copyright by Jacob Hughes 2015 ABSTRACT The Ruby Range in southwestern Montana was tectonically uplifted during the mountain-building event of the Big Sky orogeny. Contained within the Christensen Ranch Formation, made up of sedimentary units metamorphosed during the Big Sky orogeny, are a small number of metamorphosed banded iron formations (BIFs). It is the presence and composition of these BIFs which is the focus of this research, principally their depositional origin, relationship to surrounding sedimentary packages, elemental and mineral compositions, and lastly, peak metamorphic conditions. Samples were collected, cut into thin sections, and powdered for textural and compositional analyses employing polarized light microscopy, Raman microscopy, scanning electron microscopy, X-ray diffraction and electron microprobe analyses. Mineral phase assemblages and compositional data suggest differences in protoliths between Stone Creek and Iron mine metamorphosed BIFs in addition to greater activity of fluid phases during the metamorphism of Iron Mine formations. Keywords: Geology, Montana, Ruby Range, Big Sky orogeny, Metamorphosed Banded Iron Formation, Geothermobarometry ii ACKNOWLEDGMENTS The author would like to thank the Keck Geology Consortium for the incredible opportunity, Julie Baldwin (University of Montana) and Tekla Harms (Amherst College) for their guidance and patience throughout the course of the project, the other members of the “Ruby Rangers” for their help and camaraderie, Andrew Wulff (Western Kentucky University) for his tremendous instruction and continued support, as well as John Andersland (Western Kentucky University) and Dave Moecher (University of Kentucky) for their expertise and kindness. iii VITA 2015…………………………………….. Bachelor of Science, Western Kentucky University, Bowling Green, Kentucky 2014…………………………………….. Research Experience for Undergraduates, Keck Geology Consortium 2010…………………………………….. Greenwood High School, Bowling Green, Kentucky PUBLICATIONS AND PRESENTATIONS Hughes, J.M., Wulff, A.H., 2015 Geothermobarometry and Petrographic Interpretations of Christensen ranch Meta- Banded Iron Formation from the Ruby Range, Montana: Keck Geology Consortium Spring Symposium Journal. Hughes, J.M., Wulff, A.H., 2015 Geothermobarometry and Petrographic Interpretations of Christensen ranch Meta- Banded Iron Formation from the Ruby Range, Montana: WKU Student Research Conference, Bowling Green, KY, Hughes, J.M., Wulff, A.H., Garmon, W. T., Higgins, B., 2014, A Comparison of Kentucky Mississippi Valley Type Deposits: Geological Society of America Annual Meeting, Vancouver, BC, Hughes, J.M., Wulff, A.H., Garmon, W. T., Higgins, B., 2014, A Comparison of Kentucky Mississippi Valley Type Deposits: WKU Student Research Conference, Bowling Green, KY, FIELD OF STUDY Major Field: Geology iv TABLE OF CONTENTS Page Abstract…………………………………………………………………………………... ii Acknowledgments………………………………………………………………………. iii Vita………………………………………………………………………………………. iv List of Figures…………………………………………………………………………… vi List of Tables………………………………………………………………………….... vii Chapters: 1. Introduction……………………………………………………………………….. 1 2. Literature Review…………………………………………………………………. 6 3. Methods………………………………………………………………………….. 15 4. Results…………………………………………………………………………… 20 5. Conclusion………………………………………………………………………. 32 References………………………………………………………………………………. 35 v LIST OF FIGURES Figure Page Figure 1. Map of the Ruby Range and Surrounding Ranges…………………………….. 2 Figure 2. MMT and BBMT within the Wyoming Craton……………………………….. 7 Figure 3. Tectonic Evolution Model for the MMT……………………………………... 10 Figure 4. General Stratigraphy and Structure of the Ruby Range……………………… 12 Figure 5. Reference map of sample 14-JH-14………………………………………….. 18 Figure 6. SC BIF field photo…………………………………………………………… 21 Figure 7. Actinolite exsolution/intergrowth lamellae…………………………………... 23 Figure 8. Grunerite exsolution/intergrowth lamellae…………………………………… 23 Figure 9. SEM traverse of amphibole exsolution from sample 14-JH-6……………….. 24 Figure 10. SEM back-scatter image of sample 14-JH-6………………………………... 24 Figure 11. IM BIF field photo #1……………………………………………………….. 27 Figure 12. IM BIF field photo #2……………………………………………………….. 28 Figure 13. Photomicrographs of representative mineral species……………………….. 30 Figure 14. EDS spectrum from 14-JH-6 magnetite…………………………………….. 31 Figure 15. EDS spectrum from 14-JH-13d pyroxene…………………………………... 31 Figure 16. EDS spectrum from 14-JH-18b garnet……………………………………… 31 vi LIST OF TABLES Table Page Table 1. XRD results of select samples..……………………………………………….. 25 Table 2. Estimated modal percents from sample thin sections.………………………… 30 vii CHAPTER 1 INTRODUCTION Iron formation is defined as: a chemical sediment, typically thin bedded or laminated, whose principal chemical characteristic is an anomalously high content of iron, commonly but not necessarily containing layers of chert (Klein, 2005; James, 1954; Trendall, 1983). The metamorphosed Banded Iron Formations (BIFs) found in the Ruby Range and the surrounding Tobacco Root and Highland Mountains account for a volumetrically small portion of a metasupracrustal (metamorphosed rocks deposited directly on basement rocks) package (herein referred to as the Christensen Ranch suite) in the Ruby Range. The presence and composition of these BIFs is the focus of this research, principally their depositional origin, relationship to surrounding sedimentary packages, elemental and mineral compositions, and lastly, peak metamorphic conditions. Located in southwestern Montana near the town of Dillon (Figure 1), the Ruby Range was formed by a mountain-building event in the Proterozoic which has been termed the Big Sky or Trans-Montana orogeny (Brady et al., 2004; Cheney et al., 2004; Alcock and Muller, 2012; Harms, et al., 2004; Roberts et al., 2002). The Ruby Range and surrounding ranges comprise the Montana Metasedimentary Terrane (MMT) in the northwestern portion of the Wyoming Province (Mogk et al., 1992, Mueller et al., 1993) and are associated with Proterozoic orogenic activity. Farther to the east is the Beartooth- Bighorn Magmatic Terrane (BBMT) believed to be arc-granitoids intruded 1 Figure 1. Map of the Ruby Range and Surrounding Ranges. Taken from Immega and Klein, 1976. 2 into Archean crust (Roberts et al., 2002; Wooden and Muller, 1988). Three metasedimentary units have been identified within the MMT (James, 1990). The criteria for this categorization is largely predicated upon the presence of marble and iron formation, as exhibited by the younger Christensen Ranch suite, or the absence thereof, identified in regional basement as the Pre-Cherry Creek suite (Alcock and Muller, 2012). The third group found between the two is the Dillon Gneiss which, while poorly characterized, is known to include abundant biotite gneiss, quartzofeldspathic gneiss, metagranite, as well as localized marble and amphibolite (James, 1990). These general unit definitions were established based on structural relationships in the Madison and Gravelly ranges where they were first recorded (Alcock and Muller, 2012; Erslev, 1983; Erslev and Sutter, 1990; Hadley, 1969), but have been adapted and correlated throughout the region. Metamorphosed Banded Iron Formations (meta-BIFs) in the region are found interspersed throughout inferred metasupracrustal sequences containing amphibolite, metapelite, calc-silicate, marble, and quartzite. This Christensen Ranch metasupracrustal suite has been identified as a part of the Cherry Creek sequence (Dahl, 1979; Garihan, 1973; James, 1990; Karasevich et al., 1981), while the Pony Group of the Tobacco Root Mountains is roughly correlative with the Pre-Cherry Creek Group in the Ruby Range (Wilson and Hyndman, 1990). The exact paleo-environmental setting has been contested within the literature for some time. Disagreement between an accretionary wedge, fore- arc depositional model and a passive margin, back-arc or intra-arc basin model has fueled debate on the subject (Wilson and Hyndman,