Van Tuyl Lecture: Phil Brown, Wisconsin University
April 2 @ 4:00 pm - 5:00 pm
Phil Brown, Wisconsin University
Berthoud 241, 4-5PM
Iron Ores in North America – Past Importance, Future Promise
Abstract: Iron ores of the upper Midwest and the research done on them and the rest of the Lake Superior region between 1870 and 1945 drove the Industrial Revolution, facilitated westward expansion and provided seminal work in many subfields of the geosciences.
Approximately 3.4 billion tons (Gt) of iron ores containing >50 percent Fe were produced from U.S. mines in the Lake Superior region from 1848 until they were exhausted 30+ years ago. The Vermilion Range in Minnesota produced nearly 100 million tons (Mt) of this ore from Archean greenstone belt hosted iron formation. The remaining production came from Proterozoic strata including 2.3 Gt from the Mesabi and 100 Mt from the Cuyuna Ranges in Minnesota while Michigan and Wisconsin contributed 230 Mt from the Marquette Range, 290 Mt from the Menominee Range and 325 Mt from the Gogebic (Penokee) Range. The low-grade precursors to these high-grade direct-shipping ores are carbonate- or oxide-facies banded iron formations that contained 25 to 35 percent Fe prior to undergoing leaching (de-silicification), oxidation, and volume loss. The conventional model ascribing these changes to supergene processes (downward flow of meteoric waters) has been challenged by research showing that hypogene fluids (upward or lateral flow of metamorphic or basinal fluids), channeled by faults into structurally favorable horizons and settings, has played a dominant role in producing some of the high-grade (>60% Fe) ores that are presently providing much of the world’s iron ore. Descriptions of the North American iron ores, generally starting with the U.S.G.S. monographs published at the beginning of the 20th century provide many tantalizing clues suggesting that hypogene fluids have indeed played an important role in the evolution of some of these districts.
Present day iron mines in the U.S. process banded chert-iron oxide ores that require grinding, separation and pelletizing before shipping. Such mines are highly susceptible to small changes in the global price of iron ore and changes in the technologies used to convert ore to steel. The only known high-grade (>60% Fe) ores in North America occur on Baffin Island in the Canadian Arctic(?) and are under development at the present time.