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Stratiform Chromite Deposit Model Stratiform Chromite Deposit Model Chapter E of Mineral Deposit Models for Resource Assessment Scientific Investigations Report 2010–5070–E U.S. Department of the Interior U.S. Geological Survey COVER: Photograph showing outcropping of the Bushveld LG6 chromitite seam. Photograph courtesy of Klaus J. Schulz, U.S. Geological Survey. Stratiform Chromite Deposit Model By Ruth F. Schulte, Ryan D. Taylor, Nadine M. Piatak, and Robert R. Seal II Chapter E of Mineral Deposit Model for Resource Assessment Scientific Investigations Report 2010–5070–E U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior KEN SALAZAR, Secretary U.S. Geological Survey Marcia K. McNutt, Director U.S. Geological Survey, Reston, Virginia: 2012 For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment, visit http://www.usgs.gov or call 1–888–ASK–USGS. For an overview of USGS information products, including maps, imagery, and publications, visit http://www.usgs.gov/pubprod To order this and other USGS information products, visit http://store.usgs.gov Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report. Suggested citation: Schulte, R.F., Taylor, R.D., Piatak, N.M., and Seal, R.R., II, 2012, Stratiform chromite deposit model, chap. E of Mineral deposit models for resource assessment: U.S. Geological Survey Scientific Investigations Report 2010–5070–E, 131 p. iii Acknowledgments This report has benefited greatly from the reviews of several scientists, among them Dr. Hugh V. Eales, Rhodes University. Grahamstown, South Africa; and Dr. Anthony J. Naldrett, University of Witwatersrand, South Africa. The authors would like to thank Edward A. du Bray, Richard Goldfarb, Nora K. Foley, and Michael L. Zientek of the U.S. Geological Survey (USGS) for their assistance, guidance, and expertise in completing this manuscript. In addition, the authors gratefully appreciate the reviews and comments of Bruce R. Lipin, emeritus of the USGS, as well as the knowledge of the geophysical characteristics of stratiform chromite deposits provided by Carol Finn of the USGS. iv v Contents Acknowledgments ........................................................................................................................................iii Abstract ...........................................................................................................................................................1 Introduction.....................................................................................................................................................1 Purpose...................................................................................................................................................3 Scope ......................................................................................................................................................3 Deposit Type and Associated Commodities ..............................................................................................4 Name ......................................................................................................................................................4 Synonyms ...............................................................................................................................................4 Brief Description ...................................................................................................................................4 Associated Deposit Types ...................................................................................................................4 Primary Commodities ...........................................................................................................................4 Byproduct Commodities ......................................................................................................................4 Example Deposits..................................................................................................................................5 Historical Evolution of Descriptive and Genetic Concepts .....................................................................5 Regional Environment ...................................................................................................................................7 Geotectonic Environment ....................................................................................................................7 Temporal Relations ...............................................................................................................................7 Duration of Magmatic-Hydrothermal System and (or) Mineralizing Processes ........................9 Relations to Igneous Rocks .................................................................................................................9 Relations to Sedimentary Rocks ........................................................................................................9 Relations to Metamorphic Rocks .......................................................................................................9 Physical Description of Deposit ..................................................................................................................9 Dimensions in Plan View .....................................................................................................................9 Size of Magmatic System Relative to Extent of Economically Mineralized Rock ....................10 Vertical Extent .....................................................................................................................................10 Form/Shape ..........................................................................................................................................12 Host Rocks ...........................................................................................................................................12 Bushveld Complex .....................................................................................................................14 Stillwater Complex .....................................................................................................................16 Great Dyke...................................................................................................................................18 Muskox Intrusion .......................................................................................................................21 Kemi Intrusion.............................................................................................................................22 Rum Intrusion..............................................................................................................................23 Burakovsky Intrusion.................................................................................................................24 Niquelândia Complex ................................................................................................................24 Ipueira-Medrado Sill .................................................................................................................26 Campo Formoso Complex .........................................................................................................29 Fiskenæsset Anorthosite Complex .........................................................................................30 Bird River Sill ..............................................................................................................................31 Structural Settings and Controls ......................................................................................................32 Geophysical Characteristics ......................................................................................................................32 Magnetic Signature ............................................................................................................................32 Gravity Signature ................................................................................................................................33 Electrical Signature ............................................................................................................................36 Seismic Data ........................................................................................................................................37 vi Hypogene Ore Characteristics ..................................................................................................................37 Mineralogy ...........................................................................................................................................37 Chromite ......................................................................................................................................37 Bushveld Complex ............................................................................................................38 Stillwater Complex ............................................................................................................39 Great Dyke..........................................................................................................................40
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