GOLDHILLITE: A NIFTY GREEN COPPER ARSENATE FROM THE TYPE LOCALITY

 

“Anybody want to head out on a field trip this weekend” was a clarion call during my stay at the University of Utah in the late 1960s. If our faculty members did not have a field excursion scheduled, then our small cadre of grad students could usually find somewhere to explore. As a result, I became decently acquainted with large parts of “rural” Utah--a much less populated state nearly 60 years ago! Couple that with the field work associated with my dissertation research, three years as a summer ranger at Dinosaur National Monument (with weekends to roam), my two sabbatical leaves and my two decades of summer research projects with students during my teaching career, and a large part of Utah became familiar territory. Throw in my many excursions with my close friend James Madsen, he of dinosaur fame, and together they remind me of fun, fun, fun, excitement, learning, digging out of mud holes and snowbanks, running out of gasoline, finding nifty fishing holes, teaching my children to locate fossils (and cow bones) and identify rocks, traveling with my spouse to remote localities as she became the chef for my field crew, having an ice-cold brew around a campfire after work, and having the time of my life. I suppose this is not much different than the lives of other field geologists. Most of us chose geology as a career so we could spend time outdoors and experience hiding from high altitude thunderstorms in the mountains to profusely sweating while hiking through the desert looking for just the right outcrop that could contain fossils. I would not change those experiences for anything.

 

The Deep Creek Mountains of western Utah. Photo courtesy of Qfl247, AKA Matt Affolter.

One of my “experiences” with Jim Madsen was chasing some Miocene vertebrate fossils after a tip from a couple of local rockhounds. This trip took us out to one of the more desolate areas I experienced during my “tromping around” career. First of all, it was an early “up and at em” morning since we needed to head west and cover the 120 miles of Bonneville salt flats before arriving at the oasis of Wendover, Utah, and Wendover, Nevada, a stop for gasoline, coffee and a snack. In those days (life has now changed in Wendover)  the burg was home to a single large establishment, the Stateline Hotel and Casino (currently operating assume other name) out in the middle of the desert. Seems like an entrepreneur by the name of Bill Smith operated a gasoline station in Utah, right east of the state line.  Never one to miss an opportunity,  when gambling was legalized in Nevada in 1931 Bill added a casino across the state line!  During the years that I was stopping in for a refreshment before heading to the desert, the vehicle was parked in Utah, we crossed a white painted line, and voila---we were in a different world with tinkling slot machines, blackjack tables, hard liquor at ten in the morning (a different approach than Utah), and an establishment open 24 hours a day.

But in those early years  we could not afford to succumb to the demons, so we grabbed large coffees and headed south driving  along the west flank of the Deep Creek Mountains.  After a rather unexciting journey we came to the tiny hamlet of Gold Hill, an old mining community that in the 1970s seemed unoccupied except for perhaps ghosts and goblins.

Later in life I developed that earlier hunt into a vertebrate fossil collecting project near the western edge of the southern Deep Creeks but located in Nevada (Nelson and Madsen, 1987).  I remember the area as still being one of the most desolate collecting sites that my students and I worked and made certain that my crew remained in visual site of each other. 

But we could never quite get Gold Hill out of our mind and decided a little exploration was in order. The diggings, both open pit and subsurface, at Gold Hill were mined for gold, copper, zinc, lead, arsenic and tungsten from the mid to late 1800s until the late 1940s.  The peak activity was in the early 1900s when a spur railroad reached the area in 1917.  There was only sporadic mining after World War I although World II brought a mini boom as tungsten was mined for use in the hardening of steel.

Gold Hill, also known as the Clifton District, is located near the northwest end of the Deep Creek Mountains, perhaps Utah’s most isolated and unknown mountain range.   Peaks do reach 12,000 feet—Ibapah Peak at 12,087 and Haystack at 12,020 and are the major topographic feature in western Utah.  The range has a Precambrian core surrounded by Paleozoic sedimentary rocks with later Mesozoic intrusions—mostly quartz monzonite and granite/granodiorite, and later Tertiary volcanics. 

Mineralization began at Gold Hill approximately 152 Ma as magma was intruded into the overlying Paleozoic rocks (Robinson, 1993). This intrusion created three different types of ore deposits: skarn deposits, replacement type bodies, and vein deposits. At Gold Hill skarns are small structures but contain a rich suite of metals that includes copper, iron and tungsten (Ege, 2005).  

 Replacement deposits occur along the contact between the cooled magma (granodiorite) and limestone. The host rocks for the ore deposits are Mississippian to Pennsylvanian (360 to 330 Ma) limestones (Ege, 2005; Robinson, 1993).  These deposits are rich in silver, gold, arsenic, copper, and lead.

 Vein deposits are found within the intruding igneous rocks. All of the veins are small and represent only a small portion of the ore mined from the district (Ege, 2005). 

 A second stage of mineralization commenced in the early to middle Tertiary (perhaps ~38 Ma) when magma again intruded into Mississippian to Pennsylvanian sedimentary rocks.  These deposits are rich in silver, lead, copper, gold, and other metals.  In addition, extrusive volcanic rocks, ~8 Ma, created some low-grade beryllium mineralization in veins associated within the granodiorite (Ege, 2005; Robinson, 1993).

The primary minerals (hypogene) in and near the cooked limestone include galena, sphalerite, chalcopyrite, pyrite, pyrrhotite, tetrahedrite, and arsenopyrite.  The overlying oxidized supergene includes all of those beautiful arsenates: adamite, austenite, arsenosiderite, beudanite, conichalcite, clinoclase, mimetite, olivenite, pharmacosiderite, scorodite, and veszelyite  (listed by El-Shoutoury and Whelan, 1970).  Since that publication, there have been other supergene minerals identified—see MinDat.com.  The arsenic in the arsenates probably was derived from the primary (hypogene) mineral arsenopyrite (FeAsS).  Shoutoury and Whelan (1970) noted the arsenates of zinc, lead, copper, iron and calcium were produced when primary chalcopyrite, galena, sphalerite and pyrite were oxidized in close proximity to arsenopyrite. 

Goldhillite in a nest of a Mixite Group Mineral, probably Mixite. With of emerald green goldhillite is ~0.8 mm.

A closeup of goldhillite showing the flattened tabular crystals in the rosettes. The photo is somewhat fuzzy due to magnification by camera. Again width of specimen ~0.8 mm.

One quite rare copper arsenate mineral from Gold Hill is goldhillite that was named and defined in 2022 (Ismagilova and others, 2022). Celestian (2022), while describing new mineral names in the American Mineralogist, noted that this new Ismagilova study redefines the structure and chemical composition of philipsburgite, Cu₅Zn[(AsO₄)(PO₄)](OH)₆⋅H₂O,  to show that it has ordered phosphate-arsenate sites in its structure and then defines the arsenate end member (no PO₄) of the goldhillite-phillipsburgite-kipushiote solid solution series to be goldhillite, Cu₅Zn (AsO₄ )₂ (OH)₆ ·H₂ O.  In other words, phillipsburgite was restudied and “split into” two different members in the solid solution series since new classification schemes [now] open the door to new mineral descriptions [and names].

Although usually a micromineral, goldhillite is beautiful under the scope and is a bright  emerald green in color with a vitreous luster. The crystals are usually tiny flattened tabs forming small rosettes. Crystals are soft (~3.5 Mohs) and brittle with an irregular fracture. Goldhillite is often found with cornwallite, mixite, and/or conichalcite and my specimen exhibits a neat bed of mixite surround the goldhillite rosette. One of the problems associated with goldhillite splitting off from phillipsburgite is that the two minerals are very difficult to visually distinguish between, and many localities that originally listed phillipsburgite in their mineral list must now use an electronic gizmo to determine if their tiny green mineral has phosphate (PO₄) substituting for some of the arsenic (note chemical formulae in above paragraph).  Ain’t life a bowl of cherries. My specimen was located in the Middle Pit at Gold Hill and that locality is the Type Locality for the mineral so goldhillite it is. 

You can't fight the desert.. you have to ride with it.    Louis L'Amour  

 REFERENCES CITED    

 

Ege, C.L., 2005, Selected mining districts in Utah: Utah Geological Survey, Miscellaneous Publication 05-5.

Celestian, A.J., 2022, New mineral names: American Mineralogist,. vol. 107. 

El-Shatoury, H.M., and Whelan, J.A., 1970, Mineralization in the Gold Hill mining district, Tooele County, Utah: Utah Geological and Mineralogical Survey Bulletin 83.

Ismagilova, R.M., Rieck, B., Kampf, A.R., Giester, G., Zhitova, E.S., Lengauer, C.L., Krivovichev, S.V., Zolotarev, A.A., Ciesielczuk, J., Mikhailova, J.A., Belakovsky, D.I., Bocharov, V.N., Shilovskikh, V.V., Vlasenko, N.S., Nash, B.P., and Adams, P.M., 2022, Goldhillite, a new mineral species, and redefinition of philipsburgite, as an As-P ordered species: Mineralogical Magazine, vol. 1, issue11. 

Nolan, T.B., 1935, The Gold Hill Mining District Utah: USGS Professional Paper 117.

Staples, L.W., 1935, Austinite, a new arsenate mineral, from Gold Hill, Utah: American Mineralogist, v. 20.

 Robinson, J.P., 1993, Provisional geologic map of the Gold Hill quadrangle, Tooele County, Utah: Utah Geological Survey Map 140, scale 1:24,000.

 

SURPRISES FROM THE BASEMENT: IOWAITE, NORTHEAST IOWA

 

My last posting described the major economic discovery of REE/Critical Minerals located in a fairly nondescript (surficial description) area of southeastern Nebraska called Elk Creek. At this locality, ~600 feet of Pennsylvanian marine rocks, topped with varying small amounts of glacial debris, cover some very complex subcrops of Precambrian (Proterozoic) rocks. Most geologists working with Phanerozoic (post- Precambrian) rocks of the Plains and Midwest simply refer to these subcrops as the “basement.” I certainly did during my stints of teaching Historical Geology. Most of the class time was spent studying fossiliferous sedimentary rocks unless those pesky mountains were being emplaced (life became easier with the emerging knowledge of plate tectonics). In places like Nebraska and Kansas, surficial outcrops of Precambrian rocks were essentially unknown. The good news was that field trips to the Colorado Precambrian outcrops were always enjoyed.

My friends in the field of geophysics seemed to be the only people who really enjoyed the “basement.”  They could look at a page of squiggly lines and come up with ideas and interpretations about anomalies, faults, basins, etc. I believed these were secret codes that geophysicists used to communicate with each other and tease the paleontologists and soft rockers. I admit that interpretations of squiggly lines easily confuse me; therefore, I rely on my friends, and professional journals, to locate interpretations. 

Several decades ago, I was a student in the graduate program at the University of South Dakota in Vermillion. Today USD is a well-known university, especially in the Plains and Midwest, due to their top-notch academics, and their winning athletic teams. During my two-year tenure in Vermillion USD was sort of lost among the schools of the Big 10 and Big 8. The academics were good and the athletic teams excellent/OK; however, in those days before widespread TV coverage the “Dakotas” were cold and somewhere near Canada and maybe just a few miles from the Arctic Circle. While traveling out of the state, especially to my home state of Kansas, I got many opportunities to answer questions about the small-town kid who disappeared from home. Normally, questions centered around, “where are you working”, or “where did you say you were going to school.”  My answers usually brought blank stares and finally they recovered with “is that near the Black Hills.”  But I enjoyed my stay, learned much, met many “really nice” people, and always looked forward to field trips to the Hills, about 400 miles to the west. USD was closer to my home in Kansas than to the Hills (but a lot less interesting). At any rate, I took a course in geophysics trying to help calm my fear of squiggly lines—no such luck. But one thing I do remember were assignments trying to interpret those early gravity and magnetic maps. One assignment was for students (group project) to interpretate a magnetic map with an anomaly situated in a sedimentary rock section out near the Hills. Oh, the students dreamed up a variety of scenarios except the easy one. Turns out this anomaly was centered on a “gravel pit” of Pleistocene age where many high-content iron minerals, like biotite, tourmaline, magnetite and hematite, accumulated after their weathering from igneous and metamorphic rocks and stream transportation from the Hills. An embarrassing situation for ?smart students. Oh well, we learned.

The Department, or the SD Geological Survey, also had a magnetometer that was cumbersome, old, cranky, and tough to haul around and use, but we “sort of” learned the principles of use. I was then amazed to find out that a senior South Dakota Survey geologist had published, in 1962, the Magnetometer map of southeastern South Dakota.  Our class discussions then moved to this map and the source of magnetic highs in the far southeast corner of the state. The only possibility that we could support was fault movement or doming of the Precambrian basement rocks.  Close but no cookie! My fellow group members were also confused by gravity and magnetic maps! This map revealed the presence, not of gravel pits nor outcrops of faulted igneous rocks, but of a band of mafic intrusive igneous rocks subcropping in Union County, next door to Sioux County, Iowa (and very near Vermillion). 

So, my stay in Vermillion was 1965-67, the SD map was published in 1962, and somewhere in that period of time Iowa had also identified several of these magnetic anomalies since an exploratory drilling program and aeromagnetic surveys had been conducted by the New Jersey Zinc Company starting somewhere around 1963. The company was looking for possible iron ore of economic significance when in 1966 they drilled through about a thousand feet of glacial debris and sedimentary rocks and then cored, with a diamond bit, as much as 500 feet of Precambrian igneous/metamorphic rocks. Since the drill site was near the small town of Matlock, Iowa, one often sees the term Matlock Drill Core (see MinDat). In fact, the Company drilled 12 different wells into and around the anomaly.

In addition, MinDat lists 30 known minerals from these cores. One of the cores came from an ultrabasic serpentinite created by metamorphism of an igneous rock originally composed of olivine and pyroxene that was altered  (minerals replaced) by serpentine, brucite, and magnesite with veinlets in the serpentine filled with dolomite, calcite, pyrite, brucite, magnesite, and other unknowns (Cordua, 1990). Two geologists from the New Jersey Zinc Company, in examining the core, noted “a  material giving an unidentified X-ray diffraction pattern …coming from a bluish green, translucent, platy, soapy mineral…a previously undescribed hydrous magnesium hydroxide-ferric oxychloride” they named Iowaite (Mg₆Fe³⁺₂(OH)₁₆Cl₂ · 4H₂O) to honor the State of Iowa (Kohls and Rodda, 1967).

But there is another story associated with the naming of this new mineral, and that is the extent of these magnetic highs and the composition of these anomalies, and the reason for their existence. So now the story switches over to a great paper by Windom, Seifert, and Anderson (1991). What I find amazing, with regards to this work, is reading about the new amount of information that came forth in the previous 25 years (1966-1991).

I believe that the original SD magnetic map was created from information provided by lugging that old magnetometer (Askania Vertical Ground Magnetometer—maybe) over county roads and pasture lanes to acquire readings about every five miles. My confidence in this call is based on the 1961 land survey of Tood and Mellette Counties by Survey geologist Bruno Petch along the Nebraska-South Dakota state line in the south-central part of the state. I am less familiar with Iowa so I remain uncertain if hand magnetometers were used in northwest Iowa or if aeromagnetic surveys were the choice of the day.

At any rate, after completion of drilling the 12 wells, and core examination at Matlock, information delineated a very complex assortment of layered igneous and metamorphic rocks that were now tilted and dipping to the northwest about 25 degrees. Their total stratigraphic thickness may exceed ~8000 feet (Windom and others, 1991) and are Precambrian Archean (2890 +- 90 Ma) in age (Van Schmus and Wallin, 1991). These units were given the name of Otter Creek Layered Igneous Complex and Otter Creek Magnetic Anomaly.

Additional magnetic surveys (probably aeromagnetic) has shown the Otter Creek Anomaly is related to a string of nearby magnetic anomalies trending southwest-northeast from South Dakota through Iowa and into southwestern Minnesota.

 

This location map, courtesy of Windom and others (1991), shows the relationship of the Otter Creek Magnetic Anomaly (*) to the other known associated magnetic anomalies associated with the southern boundary of the Superior Province.

These anomalies lie parallel to, but northwest of, the Spirit Lake Trend (SLT) (Windom and others, 1991). The SLT has been described as the boundary between Precambrian Archean rocks (2.5 Ga or older) of the Superior Province and the Precambrian Proterozoic rocks of the Penokean Province (2.5 Ga to 600 Ma) (Windom and others, 1991). Van Schmus and Wallin (1991), as noted above, defined an Archean age (2.39+- Ga) to the Otter Creek Layered Complex just northwest of the SLT, and dates of 2.5 to 3.0 Ga throughout the Superior Province. Southeast of the SLT, core samples from Nebraska and South Dakota are from ~1.76 Ga to ~1.80 Ga, or Proterozoic in age. The contact between the terranes seem sharp.

Map from Whitmeyer and Karlstrom (2007) showing location of Yavapai terrain. Black butterfly is hovering over the tall grass prairie at Matlock, Iowa, along the Spirit Lake Trend. Compare with previous map.

Whitmeyer and Karlstrom (2007) and Van Schmus and others (1991, 2007) also believed the Penokean crustal rocks are limited to the central and northern parts of Wisconsin, Minnesota, and Michigan, They noted the Yavapai Province crustal rocks continue from Arizona eastward through Colorado south of the Cheyenne Belt, Nebraska, the mid-continent region, eastward further into Ontario and then further east into the protolith of the Grenville Province. However, dates associated with the Yavapai Orogen/Province do overlap with dates of the Penokean crustal rocks; interestingly, these two provinces are interpreted in terms of subduction flip from south dipping in the Penokean orogeny to north dipping along the southern border of the Superior Province/Laurentia.  In addition, Windom and others (1991) used the term Central Plains Province to describe the eastward extensions of Colorado Proterozoic rocks into the mid-continent region. In today’s language these rocks are now part of the Yavapai Province (Whitmeyer and Karlstrom, 2007). The Yavapai Orogeny is now defined  in terms of a long-lived convergent plate margin orogen along a southward-growing Laurentia. Most of this new crust is the result of a series of separate oceanic arcs that developed diachronously outboard of Laurentia and became welded together and to Laurentia (Whitmeyer and Karlstrom, 2007).

But back to iowaite and Iowa. The mineral from the Matlock core was never abundant and any/all studies on iowaite had to come from tiny specimens retrieved from the drill core. In fact, Cordua (1990) stated, “iowaite has, to my knowledge, only been found in this one drill core in this one spot.” That all changed in 1983 when Jon Gliddon, the Manager of Mining at the large Palabora Mine, Limpopo, South Africa, discovered a mineral he provisionally identified as pyroaurite (Mg₆Fe³⁺₂(OH)₁₆[CO₃] · 4H₂O). To confirm his identification, Gliddon sent a sample to Richard Braithwaite, a well-known mineralogist at the University of Manchester in the UK. Using microprobe, carbon analyses, thermal analyses, w-ray diffraction, mass spectrometer and optical studies Braitwaite and his colleagues (1994) stated  “this new material have shown that it is indeed similar to pyroaurite, but with chloride taking the place of most of the interlayer carbonate in the latter, and despite some differences in analyses and physical properties, seems to be identical with iowaite” (Mg₆Fe³⁺₂(OH)₁₆Cl₂ · 4H₂O). And, it turns out, that the Palabora specimens are far superior to the Matlock Core specimens in purity, crystal distinction, size and greater availability. The Palabora specimens are well crystalized and are similar in habit to their relatives in the Chlorite Group. For a much better description of iowaite see: Cairncross (2018),  Braithwaite and others (1994), Gliddon and Braithwaite (1991), and Southwood and Cairncross (2017). In addition, MinDat (assessed 16 Oct 25) noted that iowaite “is bluish green, becoming pale green with a rusty red tint on exposure to air (alteration to pyroaurite).”

Since the cat came out of the bag in Palabora, iowaite has shown up in Australia, three Canadian provinces, China, France, Oman, Poland, four localities in Russia, Spain, Uzbekistan, Sterling Mine in New Jersey, and deep ocean sediments in both the Atlantic and Pacific Oceans.

However, the iowaite story continues. I certainly did not have the slightest idea about these fascinating stories about iowaite until three years ago. I had heard of iowaite since minerals named after states are rare and at one time, I wrote a little story about coloradoite (a mercury telluride) and discovered the other three minerals. I knew about the Matlock Drill Core but do not remember why it was stuck into the back recesses of my mind. I tend to gravitate to strange and weird minerals and so when I saw a specimen of iowaite for sale in Denver 2022, I nabbed it. After the purchase I grabbed a coffee, rested my ole body, pulled out my phone and dialed up MinDat to examine iowaite. Wow, the specimen I purchased certainly did not resemble, or even seem related to, any mineral found in Iowa. What I had purchased was a nice specimen of chromium-bearing iowaite (Mg₆(Fe³⁺,Cr³⁺)₂(OH)₁₆Cl₂·4H₂O ) where there is significant replacement of Fe³⁺ by Cr³⁺ that perhaps leads to a transition to woodallite (Mg₆Cr₂(OH)₁₆Cl₂ · 4H₂O). This colorful variety of iowaite is collected from a single locality in an ultrabasic massif in the Altai Republic, Russia, somewhere in southern Siberia! Specimens of this variety are beautiful purple (of various shades) that is platy with a greasy feel. Unfortunately, I can locate very littles information about the discovery, and more importunately, the source of the chromium. Also, I assumed that such a seemly rare mineral (only the single locality I think) would be “pricey” on the mineral market. However, I noticed that on Etsy and  Ebay that prices were quite modest, even ”cheap”, with thumbnails starting in the single digits. Digging a little deeper in MinDat I found that at the main collecting site, chromium minerals of the hydrotalcite group [including iowaite], are confined to linear zones in serpentinites, sometimes stretching for tens of meters, are represented by massive fine-scaled aggregates of purple/lilac color in different shades — from light pink-lilac to bright deep violet-lilac. They form lenses and nests up to 30 cm in diameter, as well as veins in chrysotile-lizardite serpentinites. They also form pseudomorphs along rounded chrome spinel grains in serpentinite. So, there seems to be a good source of material but getting it collected and out of Siberia might be difficult in today’s world? Presumably this information in MinDat is from a Russian publication, unavailable here in the Village Library in Holmen, WI.

Purple resinous mass of chromium-bearing iowaite with inclusions of greenish yellow/brown serpentine. Some of the lighter colored, lilac shade, may be closely related stitchtite. Width of specimen 2 cm.   

This story of iowaite, at the time of discovery, seemly would restrict the rather non-descript mineral to the single drill core brought up from a serpentine-rich metamorphic rock about 1500 feet below the surface of a tall grass prairie in rural northeast Iowa near Matlock (population 74).  Little additional work was completed on the mineral due to a lack of material in the core. But the 12 different drill cores in the area provided radiometric dates for the basement rock and helped build the foundation for mapping the Archean and Proterozoic boundaries.

So, iowaite was sort of moved to the back burner until an observant mine manager in South Africa sent samples of a mineral to a mineralogist in the UK who might have said, “wow this sample is the same as long forgotten iowaite from the colonies.”  The sample from the Palabora opened up a wealth of information about iowaite and soon new localities were popping up across several continents. And somewhere in southern Siberia Russia miners/mineralogists opened a seam that would provide chromium-rich iowaite to collectors and researchers around the world. And now, in this century, synthetic iowaite had been cooked up in a chemical lab and today some deep hunting on the WEB will locate a plethora of research articles searching for industrial uses of, no kidding, iowaite. For example, see Molecules. 2021 May 20;26(10):3052. doi: 10.3390/molecules26103052: Synthetic Iowaite Can Effectively Remove Inorganic Ar.

Who woulda thought??

REFERENCES CITED

Browning, S.A. and K.E. Karlstrom, 1990, Growth, stabilization, and reactivation of Proterozoic lithosphere in the southwestern United States: Geology (USA), vol. 18, no. 12.   

Braithwaite, R.S.W. and J.P. Gliddon, 1991, Zeolites and associated minerals from the Palabora Mine, Transvaal [South Africa]: Mineralogical Record, vol. 22, no. 4.

Browning, S.A. and K.E. Karlstrom, 1990, Growth, stabilization, and reactivation of Proterozoic lithosphere in the southwestern United States: Geology (USA), vol. 18, no. 12. 

Cairncross, B., 2018, Iowaite, Sioux County, Iowa: Rocks and Minerals, vol. 93, no. 3.    

Cordua, W.S., 1990, A mineral named for Iowa: Leaverite News, v. 15, no. 8, p. 2.

Kohls, D. W. and J.L. Rodda, 1967, Iowaite, a new hydrous magnesium hydroxide ferric oxychloride from the Precambrian of Iowa: The American Mineralogist, vol. 52, nos. 9 and 10.

Southwood, M. and B. Cairncross, 2017, The minerals of Palabora, Limpopo Province, South Africa: Rocks and Minerals, vol. 92, no. 5.

Van Schmus, W.D. and E.T. Wallin, 1991, Studies of the Precambrian Geology of Iowa: Part 3. Geochronologic data for the Matlock drill holes: Journal of the Iowa Academy of Science, vol.98, no. 4.

Van Schmus, W., D. Schneider, D. Holm, S. Dodson, S. and B. Nelson, 2007, New insights into the southern margin of the Archean–Proterozoic boundary in the North-Central United States based on U–Pb, Sm–Nd, and Ar–Ar geochronology: Precambrian Research, vol. 157, issues 1-4.

Windom, K.E., K.E. Seifert, and R.R. Anderson, 1991, Studies of the Precambrian geology of Iowa: Part 1. The Otter Creek layered igneous complex. Journal of the Iowa Academy of Science, vol. 98, no. 4.

Whitmeyer, S. and Karlstrom, K. E., 2007, Tectonic model for the Proterozoic growth of North America; Geosphere vol. 3, no 4.