Bulletin 141 2020 GENESIS AND EXPLORATION POTENTIAL FOR LATE CRETACEOUS VEINS OF THE BIG FOOT MINING DISTRICT, JEFFERSON COUNTY, MONTANA Stanley L. Korzeb Montana Bureau of Mines and Geology, Butte, Montana Cover photo: The Big Four mine, the largest producer in the district, located on State Creek road. Bulletin 141 2020 GENESIS AND EXPLORATION POTENTIAL FOR LATE CRETACEOUS VEINS OF THE BIG FOOT MINING DISTRICT, JEFFERSON COUNTY, MONTANA Stanley L. Korzeb Montana Bureau of Mines and Geology, Butte, Montana TABLE OF CONTENTS Abstract .......................................................................................................................................................... 1 Introduction ..................................................................................................................................................... 3 Methods .......................................................................................................................................................... 5 Previous Investigations................................................................................................................................... 5 History ............................................................................................................................................................ 5 Regional Geology ........................................................................................................................................... 5 District Geology .............................................................................................................................................. 7 Geophysics ..................................................................................................................................................... 7 Infrared Spectrometry ..................................................................................................................................... 7 Results............................................................................................................................................................ 9 Wall-Rock Alteration Types ........................................................................................................................... 10 Propylitic ................................................................................................................................................. 10 Carbonate ................................................................................................................................................11 Silicic–Illite .............................................................................................................................................. 12 Argillic–Illite ............................................................................................................................................. 12 Alteration and Litho-Geochemistry ............................................................................................................... 13 Alteration Trace Elements ...................................................................................................................... 19 Vein Mineralogy ............................................................................................................................................ 24 Host Rock Alteration Minerals ................................................................................................................ 26 Base Metal Stage Minerals ..................................................................................................................... 26 Supergene Stage Minerals ..................................................................................................................... 26 Vein Trace Elements..................................................................................................................................... 26 Quartz Cathodoluminescence ...................................................................................................................... 29 Fluid Inclusions ............................................................................................................................................. 34 Sulfur Isotopes.............................................................................................................................................. 39 Oxygen Isotopes........................................................................................................................................... 41 Discussion .................................................................................................................................................... 42 Wall-Rock Alteration ............................................................................................................................... 43 Pressure and Depth of Mineralization .................................................................................................... 43 Vein Formation Temperature .................................................................................................................. 44 Trace Elements ....................................................................................................................................... 44 Sulfi de-Quartz Vein Genesis .................................................................................................................. 44 Geologic Model............................................................................................................................................. 47 Exploration Potential..................................................................................................................................... 48 Acknowledgments ........................................................................................................................................ 49 References ................................................................................................................................................... 49 Appendix A.................................................................................................................................................... 55 Appendix B ................................................................................................................................................... 61 Appendix C ................................................................................................................................................... 65 Appendix D ................................................................................................................................................... 69 Appendix E ................................................................................................................................................... 73 Appendix F ................................................................................................................................................... 79 Appendix G ................................................................................................................................................... 83 FIGURES Figure 1. Generalized geologic map of the Boulder Batholith showing location of the Big Foot mining district. ....................................................................................................................................................... 3 Figure 2. Geologic map of the Big Foot mining district showing sample and major mine locations and aeromagnetic low anomalies. .................................................................................................................... 4 Figure 3. Geologic map of a portion of the Ratio Mountain 7.5’ quadrangle EDMAP showing relationship of aplite and alaskite intrusions to area of aeromagnetic low anomaly. ......................................................... 8 Figure 4. Histogram showing the distribution of illite spectral maturity (ISM) values indicating high temperature for silicic–illite alteration and low temperature for argillic–illite alteration. ............................. 9 Figure 5. Histogram showing distribution of Al-OH shortwave infrared wavelengths for silicic–illite alteration. ................................................................................................................................................. 10 Figure 6. Histogram showing distribution for Al-OH shortwave infrared wavelengths for argillic–illite alteration. ..................................................................................................................................................11 Figure 7. Propylitic alteration. ........................................................................................................................11 Figure 8. Carbonate alteration, microphotograph of carbonate altered granite showing calcite fi lling open spaces. .................................................................................................................................................... 12 Figure 9. Silicic–illite alteration microphotographs showing microbrecciated granite with schorl fi lling fractures and open spaces, and replacement of granite with fi ne- to coarse-grained quartz. ................13 Figure 10. Argillic–illite alteration. ................................................................................................................ 14 Figure