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Planes looking for underground mineral deposits (01/06/2013)
By Chris Rogers


     
Frac sand might not be the only valuable resource under Southeastern Minnesota soil. The US Geological Survey (USGS) is in the middle of an $800,000 aerial survey of Southeastern Minnesota and Northeastern Iowa, which will help determine whether valuable minerals are in rocks deep beneath the surface.

On Christmas Eve, a plane outfitted with a gravity spectrometer began flying over the survey area, which roughly spans an area defined on the north by Spring Grove, Minn. and Decorah, Iowa, on the south. The gravity spectrometer measures the gravity of rocks below the surface (all objects have some gravitational pull, not just planets). The plane finished its survey on Wednesday, January 3, and a helicopter dragging a string of sensors will measure magnetic and electromagnetic fields in the area later this month.

The project's aim is to give geologists a better picture of what sort of rock lies beneath the limestone and sandstone layers in that area, and how it is arranged. "[The area] is a hole in our academic knowledge," said USGS geophysicist Ben Drenth, the leader of the project.

Geologically speaking, the region is part of a feature called the Mid-continental Rift, where the crust of the continent started to split apart more than a billion years ago. The rift runs from the shores of Lake Superior to Central Kansas. Geologists believe that about 1,500 feet down, beneath the sedimentary rocks, there may be deposits of iron-rich rock similar to mineral-bearing deposits in Northeastern Minnesota. One of those deposits, known as the Duluth Formation or Duluth Complex, contains copper, nickel, and "platinum group elements," a group of six elements including platinum and iridium. Explorations of the Duluth Formation have been going on for years, and other sections of the rift have been mined for copper and nickel, according to Drenth.

"We think that [the survey area] might be similar to the Duluth Complex except that it's buried [under sedimentary rock]," Drenth said. He also said that the intent of the survey is to provide general information about the geology of the region, and that the USGS does not promote mining of specific deposits.

Minnesota Geological Survey (MGS) Director Harvey Thorleifson said there will be lots of information coming out of this study, including a better understanding of groundwater in the area, but that "certainly part of this work is considering the potential for mineral resources in this region."

Both Drenth and Thorleifson said the survey is just an early stage of research on geology in the area. Thorleifson said that if the survey yields promising results regarding minerals, "that could lead to follow-up. But it would be a multi-year, intricate process of many steps before anything tangible would be realized."

If mining companies were interested in the deposit, they would first have to develop relationships with the local communities, get approval from several levels of government, and negotiate with private land owners, Thorleifson said. Then the companies would conduct ground geophysical surveys and experimental drilling. "Ultimately, if successful in respect to mineral resources, it could be a mine," he said.

Thorleifson and Drenth said it was unclear whether mining resources so far beneath the surface would be economically feasible. "I would think it's possible," Thorleifson said.

There are active mining operations of the sedimentary rocks in Southeast Minnesota and Northeast Iowa, including mining for frac sand. Presumably, the deposits being studied would be more accessible in areas where some of the bedrock has already been removed, but both Thorleifson and Drenth said there was no connection between the survey and current mining operations in the region.

Initial conclusions from the study will be released this spring or summer, said Drenth. More conclusive results will be released over the next year or two, and will be published in scientific journals.

 

 

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