Summer 2025 has slipped away without so much of a goodbye or a Fall hello. The Fall Equinox was a few days ago on September 22nd and was marked, in my life, by a tough session of physical therapy. October is here, the full moon on October 7 is known as the Hunter’s Moon, and one of my favorite days is scheduled for the end of the month, Halloween. In my younger days October was a time for field trips with camping, smoky fires with sizzling burgers, old collecting stories, a mild overnight frost, and the fabulous smell of coffee in the cool morning.
October, for me, is also a time for evening reads of older authors, especially writers like Washington Irving describing the Catskill Mountains and its people-- Rip Van Winkle was one of those happy mortals, of foolish, well-oiled dispositions, who take the world easy, a simple, good-natured man and a kind neighbor. Next up on the list is one that frightened my children at a young age---"The Legend of Sleepy Hollow." Even today I prefer not to walk in the woods at night while remembering the words: All these, however, were mere terrors of the night, phantoms of the mind that walk in darkness.
On a more serious note this has been an interesting summer, to say the least. Personally, it was the summer where I lost my original left knee, and had it replaced by some sort of metal (?titanium) substitute (hence celebrating the equinox with somewhat painful PT). That project makes my joints more symmetrical since the new replacement joins both hips and the right knee sporting new hardware! They look good lighting up the scanners. On the weird national front there are numerous possibilities that “could take the cake”; however, my choice for spooky happenings in the country---our federal administration deciding that the U.S. should “take over” Greenland, by “hook or crook”. Evidently someone in D.C. woke up, several years too late, and discovered China has a foothold on many Rare Earth Elements (REE) and Critical Minerals (see July 24th Post on antimony). Wow, what do we do now? I got it, we will just nab Greenland and its mineral wealth. Forget about its relationship with NATO friend Denmark. So, it goes on and on!
But back to reality, and perhaps a much better solution than making enemies, has been to watch the progress associated with the Elk Creek REE-Critical Minerals project in far southeastern Nebraska.
The newspapers and Wall Street financiers (at least some of them) have noted the project is the largest niobium and the second largest proven Rare Earth Elements (REE) mine project in the U.S. Someone in Washington, a Rip Van Winkle sort of chap, awoke after 20 years and interpreted his dream that REE were needed for a gazillion high tech products in both civilian and defense applications. The movers and shakers, having been immersed in cultural wars rather than development, suddenly realized that the U.S. must import our supply of REE/Critical Minerals from several different countries although most comes from China. Now, if a REE producing country decides to close the mineral spigot coming to the U.S, well that is serious business. Again, see the July 24th post on antimony.
So, the discovery of this REE/Critical Minerals deposit in rural Nebraska is a very major event for our country. The company operating/owning the deposit is Colorado-based NioCorp, a play on one of the major elements of interest, niobium. According to NioCorp the Elk Creek project is “shovel ready” although they continue to hunt for additional funding needed for a complete move to commercial operations.
The chart below is from a NioCorp prospectus and lists the major minerals, along with their projected tonnage, during a projected mine life span of 30+ years.
Morningstar.com, in accessing the stock value of NioCorp, has a more detailed explanation of the minerals. “Niobium is used to produce specialty alloys as well as High Strength, Low Alloy steel, which is a lighter, stronger steel used in automotive, structural, and pipeline applications. Scandium is a specialty metal that can be combined with aluminum to make alloys with increased strength and improved corrosion resistance. Scandium is also a critical component of advanced solid oxide fuel cells. Titanium is used in various lightweight alloys and is a key component of pigments used in paper, paint and plastics and is also used for aerospace applications, armor, and medical implants. Magnetic rare earths, such as neodymium, praseodymium, terbium, and dysprosium are critical to the making of neodymium-iron-boron magnets, which are used across a wide variety of defense and civilian applications.”
Since this is a science blog, what about the geological setting? OK. The REE/Critical Minerals project is located within a rock unit termed the Elk Creek Carbonatite, a type of interesting, intrusive, igneous rock composed of at least 50% carbonate minerals. At Elk Creek these carbonate minerals are primarily dolomite, calcite, and ankerite with accessory barite, ilmenite, rutile, quartz and others. Blessington and others (2022) described the carbonatite as "a multilithologic carbonatite comprised of an early apatite-dolomite carbonatite, a middle/heavy REE-enriched magnetite-dolomite carbonatite, and a late-stage light REE-enriched, barite-dolomite carbonatite," The carbonatite was intruded into the older Precambrian (Proterozoic) igneous and metamorphic rocks that form the “basement rocks” of southern Nebraska. These country rocks at Elk Creek represent the suturing of Proterozoic island arcs onto the older more stable craton (Laurentia) to the north.
Carlson and Treves (2005) described the carbonatite as lower Paleozoic in age (Cambrian). In addition, Peterman (Z. E. Peterman, personal communication to Carlson, 1985) dated a biotite from the carbonatite as ~544 Ma (Cambrian). The intrusion and the basement rocks are now covered with about 200 m of Pennsylvanian marine sedimentary rocks and lesser amounts of glacial drift. That fact indicates that post-Cambrian sedimentary rocks are absent from the intrusion and the basement rocks until the Pennsylvanian marine rocks appear. There is no surficial representation of the carbonatite intrusion.
However, the question now becomes, were pre-Pennsylvanian rocks once present in the Elk Creek area? That is a type of question that I encouraged my students to ask. Don't let the instructor just stop with that positive statement about the presence of later Paleozoic rocks. In my olden days of grad school (pre-plate tectonics days) we often referred to a Paleozoic, high platform area (a positive area) trending southwest to northeast through the center of the country as the Transcontinental Arch. Here Paleozoic rocks were thin or absent due to periodic rejuvenation of basement highs. Students assumed that a section of Paleozoic sediments were deposited in the central part of the U.S. but were thinned or destroyed by erosion during uplift. One of my favorite grad school classes was taught by the the famous (at least to geologists in the 1940s through 1970s) geologist Armand J. Eardley using his book Structural Geology of North America. Later seismic work associated with exploratory drilling indicates that smaller scale transverse (to the Transcontinental Arch), basement controlled, tectonic features probably had significant control on depositional patterns (Carlson, 1999). In addition, later Paleozoic uplift of the Nemaha Ridge was most likely responsible for the destruction of earlier Paleozoic rocks from the Elk Creek area (see later discussion on Nemaha Ridge).
A little prior history.
Back in the late 1960s (my undergrad days) and early 1970s (my early teaching days) surficial igneous rocks were virtually unknown in Kansas with the exception of a few exposures of kimberlite diatremes in northeastern part of the state near Manhatten, and the strange occurrence of lamproite (ultrapotassic, mantle-derived, volcanic or subvolcanic rocks) intrusions in southeastern Kansas—the Rose and Silver City Domes. The diatremes are pipe-like (shape) structures containing igneous rocks that originated deep in the earth, maybe 100-400 miles, and exploded to the surface in a very short time. They are of great interest to geologists for a number of reasons, not the least of which is that most diamonds in the world are found in these structures—i.e. those in South Africa. So, I always took my students on field trips to Riley County, near Manhattan, to examine these features and allow them to collect small garnets from the known (at that time) exposures.
The Kansas diatremes are part of the Central North American Kimberlites Field stretching about 3000 miles from Somerset Island in the Canadian Arctic Archipelago (Nunavut) south through the Saskatchewan fields and ending in Riley And Marshall Counties in northeast Kansas. The kimberlites in this corridor are mid-Cretaceous in age (~105-95 Ma). All structures are located at the highly attenuated lithospheric edge of the North American Craton, facilitating edge-driven convection (Kjarsgaard and others, 2017).
NOTE: I can assure you that in 1971, or in 2025, I was not familiar with Edge Driven Convection :)
At about the same time as we were collecting garnets, the Nebraska Conservation and Survey Division, along with the USGS, were flying aeromagnetic surveys (pulling a magnetometer behind the plane) over the southeastern part of the state trying to see if the emplacements of the Kansas diatremes, now known to number 13 in 2025, continued into Nebraska. The igneous kimberlites contain ultrabasic rocks that produce a positive magnetic high. Their quest was unsuccessful; however, they did discover a “vertical gravity gradient high” (Drenth, 2014) over Elk Creek where later drilling confirmed the presence of the carbonatite intrusion.
Early on MolyCorp and Cominco American both investigated the economic potential of the carbonatite. In 2010 Quantum (now NioCorp, name changed in 2013) acquired the mineral rights and started drilling, that continues today, with plans to construct an underground mine and harvest REE and Critical Minerals.
The mineralization of the REE (https://miningdataonline.com/ ) is within the Rare Earth Minerals:
• Bastnäsite ([Ce,La,Y]CO3F);
• Parisite (Ca[Ce,La]2[CO3]3F2);
• Synchysite (Ca(Ce,La)(CO3)2 F);
• Monazite ([Ce,La]PO4).
The niobium, titanium, and scandium mineralization zones are found scattered throughout the carbonatite although most niobium comes from niobium-rich Pyrochlore Supergroup minerals, disseminated and included, in ilmenite and magnetite. (open access papers at PorterGeo.com).
Carlson and Treves (2005), in describing the tectonics of the area in relationship to the carbonatite intrusion, noted the following framework (see diagram below). The country rocks are composed of low to medium grade metamorphic gneiss and schist of island arc origin with ages ~1.84 to1.71 Ga. Igneous granitic intrusions came along about 1.78 to 1.35 Ma. The intrusion of the ~544 Ma carbonatite likely was influenced by several (probably all related) nearby features (see figure below): 1) the Mid Continent Rift System (MRZ ~1.2 Ga), a failed continental rift extending from Oklahoma northeast to Lake Superior and beyond (see Posting October 31, 2013); 2) the buried boundary sutures of two Precambrian terrains (mountain building events) termed the Penokean Orogen (1.84) (with exposures in Wisconsin and Minnesota and subsurface accretions to the south)), and the Central Plains Orogen (1.78 Ma), Proterozoic mountains now preserved in the subsurface of Kansas, Nebraska, and Missouri; 3) the Nemaha Ridge or Anticline, a buried granite high associated with a larger tectonic/structural zone trending almost north-south from Oklahoma City to Omaha. The Ridge is faulted along the sides and runs parallel to the MCZ located about 40 miles to the west. Its association with the MRZ might indicate isostatic uplift was responsible for the elevation of this strange ridge. Parallel to the Ridge is the large Humbolt Fault System, the source for most of the small Kansas earthquakes, The Elk Creek Carbonatite was intruded very close to the crest of the Anticline which was then responsible for destroying the early and middle Paleozoic rocks. The age of the Nemaha uplift seems centered at around 300 Ma.
This map shows the intersections of Penokean Orogen rocks in the northeast, Central Plains Orogen rocks in the southwest, dark gray Nemaha Ridge, and light gray Mid Continent Rift Zone. All of these tectonic zones are related (my understanding) to/with a multitude of basement tectonics.
Simplified cross section of Nemaha Uplift after Steeples, 1982. According to Carlson and Treves (2005) early and middle Paleozoic rocks are absent at the Elk Creek Carbonatite since the intrusion is very near the center of the uplift. As Don Steeples, my old friend from long ago college days, would say,"who would expect a granite mountain range not far from the surface in Kansas?
The Elk Creek Carbonatite also seems related to other carbonatites emplaced across North America ~550 Ma. All seem related to deep seated suture zones similar to those in the Elk Creek accretionary material along the southern extension of the Penokean Orogen near its boundary with the Central States Orogen. Evidently a rejuvenation of these suture zones during the Cambrian allowed for the emplacement of the carbonatites (Carlson and Treves, 2005).
So, with all the news about a lack of REE/Critical Minerals being mined in the U.S., and our dangerous reliance with somewhat unfriendly foreign entities, it would seem that the Elk Creek project has a better than average change of succeeding. I guess only time will tell.
Meanwhile, as I think about dumb stuff, the situation is getting worse (Hirukuma). If I think about good stuff my mind goes on a whirlwind and that is why I tried to tackle this difficult (for me) writing adventure. I am "way outta" my comfort zone with Ridge Driven Convection and carbonatites. But, this has been a tremendous learning experience for an old plugger like me. At times my mind does a few cartwheels trying to assimilate some of the info. However, cartwheels keep the ole mind in gear and rumbling down the road and not stagnating. I hope readers can understand what I am trying to explain, and understanding that the discovery of Elk creek is exciting. Not only exciting for what Elk Creek may produce, but exciting to think about the decades long perseverance of generations of geologists who brought the project to fruition (we hope).
I also hope that readers will examine some of the references listed below. I could not have written a complete paragraph without consulting numerous professional papers written by geologists "much smarter than me." So, I offer them my sincere thanks for their fantastic presentations.
REFERENCES CITED
Blessington, M., Johnson, C., Koenig, A., Farmer, G., Kettler, R., Verplanck, P., & Lowers, H., 2022, Petrogenesis and rare earth element mineralization of the Elk Creek carbonatite, Nebraska, USA: Ore Geology Reviews, vol. 146.
Carlson, M.P., 1999, Transcontinental Arch--a pattern formed by rejuvenation of local features across central North America: Tectonophysics, vol. 305, issues. 1-3.
Carlson, M.P. and S.B. Treves, 2005, The Elk Creek Carbonatite, southeast Nebraska--an overview: Natural Resources Research, vol. 14, no.1.
Drenth, B.J., 2014, Geophysical expression of a buried niobium and rare earth element deposit: The Elk Creek carbonatite, Nebraska, USA: Interpretation, vol. 2, issue 4.
Kjarsgaard, B.A., L.M. Heaman, C Sarkar, and D.G. Pearson, 2017, The North America mid-Cretaceous kimberlite corridor: Wet, edge-driven decompression melting of an OIB-type deep mantle source: Geochemistry, Geophysics, Geosystems, vol. 18, issue 7.
Steeples, D. W., 1989, Geophysics in Kansas: Kansas Geological Survey, Bulletin 226.
