MINNESOTA GEOLOGICAL SURVEY D.L. SOUTHWICK, Director IRON·FORMATION PROTO LITH AND GENESIS, CUYUNA RANGE, MINNESOTA Peter L. McSwiggen, G.B. Morey, and Jane M. Cleland Report of Investigations 45 ISSN 0076-9177 UNIVERSITY OF MINNESOTA Saint Paul - 1995 IRON·FORMATION PROTOLITH AND GENESIS, CUYUNA RANGE, MINNESOTA The University of Minnesota is committed to the policy that all persons shall have equal access to its programs, facilities, and employment without regard to race, color, creed, religion, national origin, sex, age, marital status, disability, public assistance status, veteran status, or sexual orientation. CONTENTS Page Abstract ........................................................................................................................................................... 1 Introduction .................................................................................................................................................... 1 Regional geologic setting .............................................................................................................................. 1 Local geologic setting ................................................................................................................................... 2 Rabbit Lake Formation .......................................................................................................................... 2 Trommald Formation .............................................................................................................................. 3 Upper member ................................................................................................................................... 3 Middle and lower members ............................................................................................................. 5 Mahnomen Formation .......................................................................................................................... 12 Geochemistry ................................................................................................................................................ 12 Range of major elements ..................................................................................................................... 12 Minor elements ..................................................................................................................................... 12 Rare earth elements .............................................................................................................................. 20 Stratigraphic distribution ..................................................................................................................... 20 Mineral chemistry of the Trommald Formation ....................................................................................... 21 Aegirine ................................................................................................................................................. 22 Rhodonite .............................................................................................................................................. 22 Hyalophane ............................................................................................................................................ 22 Carbonates ............................................................................................................................................. 26 Minnesotaite, chamosite, and stilpnomelane ..................................................................................... 36 Oxides .................................................................................................................................................... 41 Jacobsite ................................................................................................................................................ 41 Tourmaline ............................................................................................................................................ 41 Barite ...................................................................................................................................................... 45 Discussion ..................................................................................................................................................... 45 Conclusions .................................................................................................................................................. 52 References cited ........................................................................................................................................... 52 ILLUSTRATIONS Figure 1. Map showing generalized geologic framework of the Penokean orogen ............................. 2 2. Map showing locations of Cuyuna North range mines and drill holes ................................. 3 3. Stratigraphy of the Merritt drill hole ........................................................................................ 4 4. Stratigraphy of the Arko drill hole ............................................................................................ 4 5. Stratigraphy of the Northland drill hole ................................................................................... 4 6. Stratigraphy of the North Hillcrest drill hole .......................................................................... 5 7. Lithofacies map of the Trommald Formation, Cuyuna North range ..................................... 6 8. Map of the dominant rock types of the host rock in Cuyuna North range mine pits .......... 6 9. Mineralogical variations in micronodules in the upper member of the Trommald Fm ...... 7 9a. Photomicrograph of rhodonite core surrounded sequentially by hematite, Ba-feldspar, and rhodochrosite ......................................................................................... 7 iii 9b. Photomicrograph of rhodonite core grain surrounded by intergrown cryptocrystalline rhodonite and hematite ......................................................................... 7 9c. Composite image of photomicrograph and backscattered electron image of micronodule cored by hematite and calcite surrounded by rhodochrosite ................... 8 9d. Backscattered electron image of aegirine and rhodonite core grain surrounded by bands of hematite and rhodonite ................................................................................. 9 10. Internal stuctures in micronodules in the upper member of the Trommald Fm ................. 10 lOa. Internal banding within the core grain of rhodonite and hematite surrounded by rhodochrosite .............................................................................................................. 10 lOb. Undeformed core grain surrounded by deformed layering ......................................... 10 10c. Shrinkage cracks in a hematite core grain .................................................................... 11 IOd. Spherical microstructures in a hematite core grain ..................................................... 11 11. Geochemistry of the middle and lower members of the Trommald Fm ............................. 20 12. Geochemistry of the upper member of the Trommald Fm ................................................... 21 13. Rare earth element plots for the Trommald Fm .................................................................... 22 14. Compositional profiles from drill core at the Merritt mine ................................................. 26 15. Compositional profiles from drill core at the North Hillcrest mine .................................... 35 16. Compositional profiles from drill core at the Northland mine ............................................ 35 17. Compositional profiles from drill core at the Arko mine ..................................................... 36 18. Mn vs. C02 plots for the Trommald Fm ............................................................................... 36 19. Phase diagrams representing the system CaC03-MnC03-FeC03-MgC03' ........................ 40 20. Phase diagram for carbonates in the Trommald Fm. from the Merritt drill hole ............... 40 21. Phase diagram for carbonates in the Trommald Fm. from the Arko drill hole .................. 40 22. Phase diagram for vein carbonates in the Trommald Fm. from the Merritt drill hole ...... 41 23. CaC03-MnC03-FeC03-MgC03 plots of coexisting carbonate compositions .................... 41 24. Variation in composition of R3 (dolomite-like) carbonates in the Trommald Fm ........... .44 25. Compositional variation of minnesotaite and stilpnomenlane ............................................. 44 26. Ternary plots showing compositional fields of tourmalines ................................................ 45 27. Ratio of Si/AI in the Trommald Fm. compared to ratios in marine sediments .................. 51 28. Ratio of Fe/Mn/Ni+Co+Cu in the Trommald Fm .................................................................. 51 29. Ratio of U/Th in the Trommald Fm ........................................................................................ 52 30. Model for sediment-hosted, submarine exhalative sulfide deposits .................................... 52 TABLES Table 1. Chemical data from the Merritt hole ...................................................................................... 13 2. Chemical data from the Arko hole .........................................................................................