Field work in Minnesota

Me examining rock in drill core using my hand lens to get a good look at the minerals. We took careful notes for every sample we took: the depth, rock type, sample number, and lots of other details.

Since March, I had been busy working from home writing up the results of my first project. But after getting married on September 5th, my husband and I drove to northeastern Minnesota with our two dogs to spend 2.5 weeks up in Orr, Minnesota, so I could do field work to collect the rocks for my next PhD project. As I’m now in my 3rd year of my PhD, this is a great time for me to start project #2, and it’s extra exciting because it’s on rocks from Minnesota! I’ve wanted to get a project going in northeastern Minnesota since my first year of grad school, and the idea for this project was born out of conversations with folks at the Polymet Mine in Minnesota and during the 2019 Institute on Lake Superior Geology (ILSG) conference that I attended in Terrace Bay, Ontario.

The project’s goal is to understand how a set of titanium deposits formed in Minnesota. From Duluth to Hoyt Lakes there is a series of ~14 smaller-scale intrusions that cut through the intrusive rocks of the Duluth Complex that contain sizeable amounts of titanium. A couple of these have been drilled recently and are great prospects for domestically-sourced titanium. We use titanium in a variety of ways, most commonly as a white pigment for paint or powdered donuts! Titanium is also commonly alloyed with steel to make stronger, lighter materials that are corrosion resistant. As the United States only produces 4% of titanium globally, we are highly dependent on imports for this element essential to modern society. Therefore, if we can better understand our domestic sources, we can more easily figure out economic ways to mine titanium and find more deposits by knowing where to look for more!

Dean standing on top an outcrop of Biwabik Iron Formation

I was invited to the field by two collaborators at the Natural Resources Research Institute (NRRI), an applied research arm of the University of Minnesota that’s located in Duluth, MN. Dean Peterson and George Hudak are the two economic geologists at the NRRI and are also working on the same rocks I am, but for slightly different purposes. This makes our collaboration very fruitful for both sides as we will share samples and data to get the most out of rocks we collect. Dean has worked most of his career in Minnesota and has a ton of experience looking at rocks in the field and in drill core. George has been involved with a Canadian lab over the last few years to develop a more efficient way of extracting the impurities that exist in the titanium ore of these deposits, and in 2017 they published a report that showed they were successful in purifying the titanium concentrate! This is big news because the purification process was the stumbling block for previous mining companies, the latest of whom went bankrupt in 2012 after finding out how impossible it was to remove the impurities.

My project is focused on two of the 14 deposits: the Titac Deposit in the southern portion and the Longnose Deposit in the north. We chose these two deposits because they have the most drill core available to study and represent the two end-members of the series of intrusions: the north section and the south section. Previous geologists studying these deposits had put forth 2 main models for how they formed:

  • Magmatic model – here the titanium-rich rocks crystallized from a hot, molten magma and the dense oxide minerals that host the titanium segregated from the rest of the magma to form these deposits.
  • Iron formation assimilation model – underlying the deposits in the north is the Biwabik Iron Formation, a sedimentary rock that contains a lot of iron. It has been proposed that as the magma intruded it heated up and melted parts of this iron formation, which increased the iron content of the magma and potentially triggered the crystallization of iron-titanium minerals of these deposits.
Typical Minnesota forest: finding rocks in this is not easy! Dean’s advice: look for changes in slope or small hills where rock could be just at the surface.

The thing is that neither of these models has been tested for these deposits, so in this study we hope to provide evidence for or against these models and potentially suggest a new model if the evidence doesn’t fit with either. To do this we will use geochemical and petrological methods that investigate what minerals are present, how the minerals fit together (rock texture), measure the composition of minerals, and use isotopes to fingerprint the source of the iron and titanium for these rocks. We will compare the results from both the Titac and Longnose deposits to see how similar or different the rocks are and see if one model can describe how both of them formed.

Dean and I walking in the abandoned Mary Ellen iron mine to collect a sample of Biwabik Iron Formation.

The first step of this project was to collect rocks, and that’s exactly what I did with Dean, George, and Adam during the second half of September. The first week Dean took me around to some of the better spots to see rocks in outcrop, as seeing what the rocks look like exposed at the surface is super important for having a visual for when you’re looking at a tiny piece of rock in drill core and trying to figure out what it is. We camped at the South Kawishiwi River Campground near Ely, MN, and spend 4 days driving around, walking through the brush, and banging on hard rocks to collect smaller chunks I could take back to the lab. The following week Adam arrived and I showed him what we had seen the week before and also got to go into the abandoned Mary Ellen iron mine from the 1950’s near Biwabik, MN. We collected samples of the Biwabik Iron Formation so we can compare the iron isotope values from the titanium deposits to it and determine if the iron was indeed sourced from this formation.

One of many boxes of drill core. Read like a book, each box contained about 10 feet of rock.

The last two days were spent looking at drill core in a core shed in Ely. Drill core is obtained by drilling deep into the Earth and bringing up a long, skinny tube of hard rock to the surface so geologists can inspect it and try to make maps of the subsurface. It’s basically like sticking a 1,500 foot straw into the Earth! George, Dean, Adam, and I spent two long days looking at the core, taking notes, and cutting samples to take back. As my first time logging core, I learned a ton from George and Dean who had done this many times in the past. It is a completely different thing to look at a small circle of rock and try to think about it in a larger setting underground. We ended up taking over 100 rock samples from the drill core alone.

Now that I’m back in Michigan and have started to work on getting the rocks ready for analyses, I’m realizing how much material I have to work with! Part of the process is to sort the samples into which analyses will be done and then to further limit the number of samples to analyze for more expensive methods, like isotopes. Next I’ll need to spend some time just looking at the minerals and textures that are in these rocks to ‘get to know them’ and visually compare the Titac and Longnose deposits. In the coming months I’ll determine which samples are the priority and will do more analyses on those to try to figure out how these deposits formed. Starting a new project can be daunting with a massive to-do list, but I’m excited to see what these rocks tell us and to put in the hours to understand it!