Rich, Attractive, and Extremely Shallow

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No, it’s not a title for a new reality-dating TV show, but it is real science! It also describes the ideal mineral deposit.

So how do you find a potential mineral deposit when it’s buried underground?

Perhaps it's a geo-physical attraction...

Just as someone's personality characteristics might be attractive, certain geophysical characteristics are appealing to scientists searching for potential mineral deposits. USGS scientists are using geophysical characteristics and techniques to map the geology of large areas in the middle part of the U.S. This region has the potential to host iron deposits.

Image shows a map of the Mid-Continent of the United States
Map showing large part of the United States referred to as the southern mid-continent region.(Public domain.)

Looking beyond the surface...

The majority of rocks that host the iron mineralization are buried under meters to kilometers of sedimentary rock. This means we can't see how deep the mineralization is or if it even exists; its true nature is concealed. Scientists approach this problem using geophysical techniques to help "see" below the surface and uncover the underlying geology.

Iron deposits are great geophysical targets for magnetic and gravity surveying because they are typically magnetic and dense; both properties are associated with the increased amount of iron minerals in the rocks. Rocks with high density change the local pull of gravity enough to be sensed by a gravimeter. Importantly, these types of deposits can be also very rich in other significant metals such as copper, gold, and rare earth elements. 

Some of the most important strategic deposits provide valuable metals that are essential for domestic and high technology and military industries. These critical metal deposits include iron-oxide-copper-gold, igneous rare earth element, and platinum group element ore bodies. The iron deposits in southeast Missouri originated from magmas that formed in the mantle over 1.4 billion years ago. Using regional magnetic and gravity data, we are studying how the magmas that produced the rocks that host the deposits traveled from the mantle upward to the shallow crust where we see them today. We are modeling the data to image, in 3-D, the earth’s crust across depths that go from the surface to as deep as 30 to 45 kilometers.  

Image shows graphs showing magnetic, gravimetric, and integrated analysis of the St. Francois Mountain terrane
Gravity, magnetic, and integrated analysis maps of the St. Francois Mountain terrane of the Mid-Continent of the United States.(Public domain.)

Did you say Mr. Terrific or magnetotelluric?

No successful relationships are one-sided. Likewise, we have many partners that have helped gather and collect data. Earthscope, a program of the National Science Foundation that has deployed thousands of geophysical instruments, allows us to map the earth’s subsurface electrical conductivity using magnetotelluric data. Magnetotellurics (MT) is a geophysical technique that images resistivity, or how well rocks conduct electricity, on depth scales ranging from hundreds of meters to hundreds of kilometers. MT is an electromagnetic method in which naturally-occurring electromagnetic signals induce tiny electrical currents in the Earth, similar to how an induction stove induces currents in a pot in order to heat it.

Image shows a diagram of a cross-section of the Earth
A cross-section of the Earth, showing the sub-surface layers that are being mapped.(Public domain.)

What’s our contribution to this relationship?

Along with the magnetic and gravity data, the MT data provide one more geophysical characteristic that helps reveal the subsurface geology. This type of data doesn’t cover the entire U.S., so, as part of our project, the USGS is collecting new MT data to fill in missing areas.

Members of the public, including the mining industry and academia, may be interested in large scale data from our projects because of its wide uses. The kinds of geophysical data we collect and use contribute to mapping the architecture of Earth’s crust. In our research, these data are important to map deep crustal and mantle structures, some of which may control where mineral deposits form.

Image shows a map of the Mid-Continent with accompanying geophysical models
Side-by-side comparisons of magnetic and density models across iron oxide deposits in the Mid-Continent region of the United States.(Public domain.)

After the Honeymoon

Just like with all relationships, it’s good to have a plan for the long-term. We are leveraging these data, along with state-of-the art in-house 3D modeling approaches, to better understand the deep plumbing of these important critical metal ore systems.  

Ultimately, our goals are twofold. First, to advance the current understanding of North America’s tectonic evolution. Second, to improve our understanding of how heat, magma, and fluids interact at shallow and sometimes great depths in order to form large mineralized systems. Our work can be used by the private sector to define prospective critical mineral deposits in the southern mid-continent of the United States.

Read more about this project here. Stay up-to-date with our other attractive projects by following us on Twitter.