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Overview

Stillwater River canyon
The east side of the Stillwater River canyon, near Nye, Montana. Photograph by Michael L. Zientek.

Why are we doing this research?

Platinum group elements, also known as PGEs, are a group of elements that have specific properties which make them useful for various applications in industry. Even though industry within the U.S. is heavily reliant on platinum group elements, most of the supply comes from areas outside of the U.S. Platinum group elements are generally rare in the earth’s crust, but in specific geologic settings under specific conditions they may be concentrated in amounts high enough to be economically mined. One geologic setting that contains large concentrations of platinum group elements is layered mafic intrusions. Studying the internal structure of these intrusions along with the structure and chemistry of the rocks they intrude provides a better understanding of the mineral resource potential of the intrusion.

Layered mafic intrusions form as magma rises, cools, and crystallizes in the earth’s crust. They have specific chemistry and pattern of mineral crystallization and are often associated with continental rifting events. The Midcontinent Rift system found in the United States has a number of these intrusions; however, the intrusions in this area are not as well studied or understood as other systems around the world.

What research is USGS going to do?

This project will use new and preexisting geophysical datasets to characterize the internal structure of layered intrusions. Datasets include magnetic, gravity, magnetotellurics, and seismic tomography. Analyses completed for better exposed and more thoroughly studied intrusions around the world (Great Dyke, Zimbabawe, Muskox, Canada, and Skaergaard, Greenland) will be used as a starting point to identify key geophysical signatures that correspond with high concentration areas of critical minerals. The same analyses will be completed and applied to the less studied Duluth and Beaver Bay Complex, Minnesota, Mellen Complex, Wisconsin, and Glen Mountains Complex, Oklahoma.

This project will focus on three objectives:

  1. Characterization of the internal architecture of intrusions with specific attention on the feeder zones.
  2. Investigation of how proximity to feeder zones may affect both distribution and properties of magmatic deposits.
  3. Exploration of how crustal structure and geochemistry of the surrounding basement rocks influences the emplacement and geochemistry of the intrusions.

What is the potential result of our research?

The potential for certain magmatic ore deposits is directly related to the internal structure of intrusions; the results of the study will be used to constrain mineral resource potential. Work will focus on intrusions of the Duluth, Beaver Bay, Mellen along with the the Glen Mountains, Oklahoma. The proposed work will also consider how sedimentary basins localize the emplacement of intrusions in the crust and provide sulfur sources leading to deposits within an intrusion.

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Project Activities

Characterize known and suspected feeder zones for large layered intrusions

The character, mineralogy, and chemistry of the large layered intrusions vary based on location within the intrusion. Platinum group element concentrations increase near feeder zones, therefore it is beneficial to understand this zone as much as possible. Three dimensional (3D) inversion and advanced geophysical analysis will be used to constrain the physical state, tectonic history, and geodynamic processes of the regions underlying large igneous provinces. 3D views of electrical conductivity variations in the crust and upper mantle will be developed from magnetotellurics studies using existing regional and new detailed data.

Characterize layering of layered mafic intrusions

The potential for certain magmatic ore deposits is directly related to the internal structure of the intrusions. One way to better map the internal structure is to look at the magmatic layering and its relation to feeder zones. For this task, interpretation techniques of magnetic and electromagnetic data previously applied to the Stillwater and Bushveld intrusions will be applied to the Duluth Complex, Minnesota, the Mellen Complex, Wisconsin, and the Glen Mountains Complex, Oklahoma.

Mapping sedimentary basement rocks beneath layered mafic intrusions

Sulfur isotope studies of other magmatic ore deposits indicate that external sources of sulfur are necessary for their formation. However, little attention has been given to mapping the rocks that may be the source of the sulfur or to considering mass balance relations. This task will consider how the sedimentary basin rocks beneath the intrusions localize the emplacement of intrusions in the crust and provide sulfur sources leading to deposits within an intrusion. The Animikie Basin and the Duluth Complex provide an opportunity to model this interaction using preexisting and new electromagnetic and magnetotelluric data.

Advanced Geophysical Characterization of Layered Mafic and Ultramafic Intrusions (2013-2015)

Initial studies were conducted to characterize and model the geology and mineral resource potential of layered intrusions in the U.S. based on advanced analysis of geophysical data typically used in exploration (gravity, magnetic, and shallow electromagnetic methods) as well as unconventional data sets (seismic, magnetotelluric) and comparison with the world's largest layered mafic intrusion and source of platinum group elements, the Bushveld Complex in South Africa. Mafic and ultramafic layered intrusions in the U.S. have potential for mineralization that may contain platinum group elements, notably the Stillwater Complex in Montana and the Duluth Complex in Minnesota.

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Products

Journal Articles, Reports, and Book Chapters

Bedrosian, P.A., 2016, Making it and breaking it in the Midwest: Continental assembly and rifting from modeling of EarthScope magnetotelluric data: Precambrian Research, 278, p. 337-361, doi:10.1016/j.precamres.2016.03.009.

Cole, Janine, Finn, C.A., and Webb, S.J., 2013, Overview of the magnetic signatures of the Palaeoproterozoic Rustenburg Layered Suite, Bushveld Complex, South Africa: Precambrian Research, 236, p. 193-213, doi:10.1016/j.precamres.2013.07.017.

Cole, Janine, Webb, S.J., and Finn, C.A., 2014, Gravity models of the Bushveld Complex – Have we come full circle?: Journal of African Earth Sciences, 92, p. 97-118, doi:10.1016/j.jafrearsci.2014.01.012.

Finn, C.A., Bedrosian, P.A., Cole, J.C., Khoza, T.D., and Webb, S.J., 2015, Mapping the 3D extent of the Northern Lobe of the Bushveld layered mafic intrusion from geophysical data: Precambrian Research, 268, p. 279–294, doi:10.1016/j.precamres.2015.07.003.

Taylor, C.D., Finn, C.A., Anderson, E.D., Bradley, D.C., Joud, M.Y., Taleb, M.A., and Horton, J.D., 2016, The F’derik-Zouérate Iron District: Mesoarchean and Paleoproterozoic Iron Formation of the Tiris Complex, Islamic Republic of Mauritania, in Bouabdellah, M., and Slack, J.F., eds., Mineral Deposits of North Africa: Springer International Publishing, Switzerland, p. 529-573, doi:10.1007/978-3-319-31733-5_21.

Zientek, M.L., and Parks, H.L., 2014, A geologic and mineral exploration spatial database for the Stillwater Complex, Montana: U.S. Geological Survey Scientific Investigations Report 2014-5183, 28 p., http://dx.doi.org/10.3133/sir20145183.

Abstracts

Bedrosian, P., 2016, Making it and breaking it in the upper Midwest: Constraints on continental assembly and rifting from EarthScope: Institute on Lake Superior Geology Proceedings Volume 62, p. 12. View ILSG 2016 Proceedings Part 1 [PDF file, 13.8 MB].

Bloss, B.R., Finn, C., Zientek, M.L., and Minsley, B.J., 2014, Old data, New tricks: New insights into the Stillwater Complex, Abstract NS43A-3878 presented at 2014 Fall Meeting, AGU, San Francisco, Calif., 15-19 Dec. View Bloss 2014 AGU abstract.

Finn, C.A., Bedrosian, P.A., Zientek, M. L., Cole, J.C., Webb, S.J., and B. Bloss, 2015, Geophysical Imaging of the Stillwater and Bushveld Complexes and Relation to Platinum-group Element Exploration, Abstract NS42A-01 presented at 2015 Fall Meeting, AGU, San Francisco, Calif., 14-18 Dec. View Finn 2015 AGU abstract.

Finn, C.A., Cole, Janine, Khoza, T.D., Webb, S.J., and Bedrosian, P.A., 2014, The geometry of the Northern Lobe of the Bushveld Complex—Implications for intrusion volumes and feeder locations: Geological Society of America Abstracts with Programs. Vol. 46, No. 6, p. 636. View Finn 2014 GSA abstract.

Finn, C.A., Zientek, M., Bedrosian, P., Bloss, B., Burton, B., Peterson, D., and Parks, H., 2016, Geophysical Imaging of Layered Mafic Complexes and Relation to Platinum Group Element Exploration: Institute on Lake Superior Geology Proceedings Volume 62, p. 43-44. View ILSG 2016 Proceedings Part 1 [PDF file, 13.8 MB].

Finn, C.A., Zientek, M.L., Bloss, B.R., Wintzer, N.E., and Parks, H.L., 2013, Geophysical imaging of the Stillwater Complex and relation to platinum group element exploration: Geological Society of America Abstracts with Programs, Vol. 45, No. 7, p. 277. View Finn 2013 GSA abstract.

Hennings, Cassandra, Thomson, J.A., and Zientek, M.L., 2015, The Origin of Xenoliths with Cumulate Textures Found Above the Subsurface Extension of the Stillwater Complex, MT: Proceedings of the 86th Annual Meeting Northwest Scientific Association, April 1-4, 2015, Pacso, WA, p. 61. View 2015 NSA proceedings [PDF file, 2.6 MB]

Jenkins, M.C., Thomson, J.A., and Zientek, M.L., 2015, Origin of Enigmatic Rocks Located North of the Stillwater Complex, Montana: Proceedings of the 86th Annual Meeting Northwest Scientific Association, April 1-4, 2015, Pacso, WA, p. 63. View 2015 NSA [PDF file, 2.6 MB]

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