I was just starting out with my graduate school studies at the University of Missouri – Columbia when I first introduced to the beauty of the Ozarks. My advisor, Dr. Martin Appold, and his doctoral student, Zach Wenz, were preparing a short field expedition to collect sphalerite (ZnS) samples for fluid inclusion studies on Mississippi Valley Type (MVT) ore deposits. In wanting to be an oh-so-helpful and eager new student, I politely asked if I could go along as an assistant and get some field experience. They obliged and a few days later, we were off southward for my first taste of the Ozarks.
We drove on U.S. 63 south into Jefferson City, and hopped on U.S. 54 southwest past the capitol city for the vast remainder of our short expedition. After a couple short stops into the woods, at two locations, with no luck in sample collecting we headed to our most promising locale, the Gollar Mine just outside of the town of Eugene. After driving 1/3 mile slightly downhill a semi-rough gravel road, we knew we had arrived by the sight of a flooded pit lake to our left, and a wide open field in front of us for parking. After splitting up into teams for a bit, none of us found the scientifically coveted sphalerite but I couldn’t help but notice a massive amount of barite (BaSO4) and some galena (PbS) lying around. When we started on our way back to Columbia, I made sure to take notes on how to get back here on my own for some collecting.
About a year had passed since my initial trip. I also just married someone that loved the outdoors, new adventures, and her childhood hobby of picking rocks was revitalized by virtue of having a geologist beau. It was June 2011 when she asked, “It’s such a gorgeous day out for a picnic! How about we go to that collecting site you told me about with your advisor?” I could never say no to such a request. Upon packing a small cooler with sack lunches and water, and with her dog in tow, we set off southward again being guided by my trusty orange field notebook.
Along the way, my curious wife asked me about the area mineralogy, what to look for, and how everything came to be. I’m not sure which excited me more; talking mineralogy, or going out seeking it! The Gollar Mine is one of the larger sites encompassed in an area known as the central Missouri barite district. The district is, in turn, part of the U.S. midcontinent Mississippi Valley Type (MVT) lead-zinc ore system, named for the remarkable concentration of lead (galena) found in the Mississippi River Valley area of southeast Missouri. The MVT system actually stretches roughly 750 miles from the Tri-State District housed in Oklahoma, Kansas, and southwest Missouri to eastern Tennessee, and also roughly 575 miles from northern Arkansas all the way up to southwestern Wisconsin. The mineralogy of MVT deposits, compared to other ore deposits, is quite simple with the dominant minerals being galena, sphalerite, fluorite (CaF), barite, calcite (CaCO3), and dolomite. What makes this regional system even more remarkable is how the dominant mineralogy can change in such a relatively short distance. Starting in the Tri-State district going east the ore is more zinc rich, transitioning to barite-rich in central Missouri, then lead-rich in southeast Missouri, followed by fluorite rich in southern Illinois and western Kentucky, and going back to zinc-rich in central Tennessee. The variances in mineralogy are probably controlled by varying lithologies serving as metal and sulfur sources, bedrock structures (such as carbonate dissolution, and even deep-seated igneous plutons (serving as a source of fluorine).
A much further understanding of MVT deposit origin has been made over the past 15-20 years with the advent of more advanced analytical instrumentation. One such powerful tool is laser ablation mass spectrometry, coupled with traditional fluid inclusion analysis. Laser ablation has the ability to measure minute concentrations (down to parts per billion) of elements within micro-droplets of fluid trapped within a growing crystal (fluid inclusions). Along with trace element composition, a variety of tests can be performed on inclusions to gain information such as fluid temperature and salinity of hydrothermal fluids during crystallization. Thanks to these techniques, the current prevailing theory on MVT formation is linked to tectonic uplift of several regional plateaus and associated neighboring sedimentary basins. Heated groundwater, ranging from 80-150°C, flows down from the plateaus to the basins that serve as the centers of mineralization. In order for the massive amounts of sulfide ore mineralization to locally occur, separate flows of groundwater are required to transport metals and dissolved sulfur (or sulfate). Fluid inclusion studies reveal that if a single fluid carried all necessary components, a smattering of sub-economic deposits would result, and we wouldn’t come close to the sheer volume of ore available to exploitation. In the case of the Central Missouri barite district, high-grade mineralization is concentrated in open spaces associated with breccia collapse structures in the Lower Orodvician Jefferson City dolomite. As referenced above, the most likely scenario of barite genesis involves mixing separate bodies of groundwater, each being individually saturated in barium and sulfate. Fluid inclusion analysis of barite and trace sphalerite indicates the latter being deposited first with barite being a later stage mineral.
We drove on U.S. 63 south into Jefferson City, and hopped on U.S. 54 southwest past the capitol city for the vast remainder of our short expedition. After a couple short stops into the woods, at two locations, with no luck in sample collecting we headed to our most promising locale, the Gollar Mine just outside of the town of Eugene. After driving 1/3 mile slightly downhill a semi-rough gravel road, we knew we had arrived by the sight of a flooded pit lake to our left, and a wide open field in front of us for parking. After splitting up into teams for a bit, none of us found the scientifically coveted sphalerite but I couldn’t help but notice a massive amount of barite (BaSO4) and some galena (PbS) lying around. When we started on our way back to Columbia, I made sure to take notes on how to get back here on my own for some collecting.
About a year had passed since my initial trip. I also just married someone that loved the outdoors, new adventures, and her childhood hobby of picking rocks was revitalized by virtue of having a geologist beau. It was June 2011 when she asked, “It’s such a gorgeous day out for a picnic! How about we go to that collecting site you told me about with your advisor?” I could never say no to such a request. Upon packing a small cooler with sack lunches and water, and with her dog in tow, we set off southward again being guided by my trusty orange field notebook.
Along the way, my curious wife asked me about the area mineralogy, what to look for, and how everything came to be. I’m not sure which excited me more; talking mineralogy, or going out seeking it! The Gollar Mine is one of the larger sites encompassed in an area known as the central Missouri barite district. The district is, in turn, part of the U.S. midcontinent Mississippi Valley Type (MVT) lead-zinc ore system, named for the remarkable concentration of lead (galena) found in the Mississippi River Valley area of southeast Missouri. The MVT system actually stretches roughly 750 miles from the Tri-State District housed in Oklahoma, Kansas, and southwest Missouri to eastern Tennessee, and also roughly 575 miles from northern Arkansas all the way up to southwestern Wisconsin. The mineralogy of MVT deposits, compared to other ore deposits, is quite simple with the dominant minerals being galena, sphalerite, fluorite (CaF), barite, calcite (CaCO3), and dolomite. What makes this regional system even more remarkable is how the dominant mineralogy can change in such a relatively short distance. Starting in the Tri-State district going east the ore is more zinc rich, transitioning to barite-rich in central Missouri, then lead-rich in southeast Missouri, followed by fluorite rich in southern Illinois and western Kentucky, and going back to zinc-rich in central Tennessee. The variances in mineralogy are probably controlled by varying lithologies serving as metal and sulfur sources, bedrock structures (such as carbonate dissolution, and even deep-seated igneous plutons (serving as a source of fluorine).
A much further understanding of MVT deposit origin has been made over the past 15-20 years with the advent of more advanced analytical instrumentation. One such powerful tool is laser ablation mass spectrometry, coupled with traditional fluid inclusion analysis. Laser ablation has the ability to measure minute concentrations (down to parts per billion) of elements within micro-droplets of fluid trapped within a growing crystal (fluid inclusions). Along with trace element composition, a variety of tests can be performed on inclusions to gain information such as fluid temperature and salinity of hydrothermal fluids during crystallization. Thanks to these techniques, the current prevailing theory on MVT formation is linked to tectonic uplift of several regional plateaus and associated neighboring sedimentary basins. Heated groundwater, ranging from 80-150°C, flows down from the plateaus to the basins that serve as the centers of mineralization. In order for the massive amounts of sulfide ore mineralization to locally occur, separate flows of groundwater are required to transport metals and dissolved sulfur (or sulfate). Fluid inclusion studies reveal that if a single fluid carried all necessary components, a smattering of sub-economic deposits would result, and we wouldn’t come close to the sheer volume of ore available to exploitation. In the case of the Central Missouri barite district, high-grade mineralization is concentrated in open spaces associated with breccia collapse structures in the Lower Orodvician Jefferson City dolomite. As referenced above, the most likely scenario of barite genesis involves mixing separate bodies of groundwater, each being individually saturated in barium and sulfate. Fluid inclusion analysis of barite and trace sphalerite indicates the latter being deposited first with barite being a later stage mineral.
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Upon arriving again at the open gravel parking area, we decided to satisfy our appetites first in our car. After making sure we were equipped with water and our own hammer sets, I briefly showed her the pit lake and a few spots that I found desirable during my first go-around. Some of the initial hotspots included some large boulders in the open field, a small spoils pile just inside the vegetation line north of where we parked, and a small ground outcrop off of a four-way dirt-road intersection (roughly 600 feet to the southwest). Being the fast learner that she is, she elected to go off for some self-exploring right from the get-go. While she decided to check out some open field boulders first, I started at the small spoils pile just a few paces from our car. There are cobbles and boulders of anhedral, opaque, snow-white barite everywhere for the taking, which definitely made deciding what (and how much) to keep and throw out quite a “heavy” task, no pun intended! After one trip of starting a keep pile by the car, I did a second once over of the pile and discovered a small, but pleasant surprise I didn’t see my first time here; small, but water clear hexagonal barite crystals on a messy snow white and carbonate matrix. The crystals were millimeter-scale, but it still provided excitement as to if more like these could be found. After some more searching turned up fruitless, I took my clear cluster to the car pile wherein I almost stepped on a trio of inch scale translucent calcite rhombs! The biggest one was roughly 2 by 1.25 inches, but was marred with milky weathered surfaces, while the two smaller rhombs showed more transparent surfaces.
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I, then, started my way across the open flat to see how the wife was doing. She was busy hammering on the biggest boulder in the field, hoping to find a hidden lucky sulfide treasure (lucky as in we did not have a 15 pound sledgehammer, nor a long crowbar to effectively work with monster boulders)! Her efforts did result in a petite 1/3 inch galena cube, perched in a mixed carbonate-barite matrix, complete with two triangular corners. While I admired the mineralogical prize, she bent down to a small bush behind me and presented a handful of larger, freeform galena cubes measuring in the two-inch range. The cubes were of a more matte grey, and could definitely tell they were not extracted from any boulder interiors. Still in my opinion, any galena crystals found in any form is fascinating!
After helping haul her finds back to the car, and now having two piles, we decided to rejoin forces and explore near the four-way trail intersection, with all the off-paths covered in foliage. Like a dart she took off for one of the corners, almost like she knew something hidden was near. While hammering was going on, I started to walk briefly down the three paths searching for potential outcrop targets. I had just barely started down the second path when I heard my name rather loudly in excitement. I rushed over to my wife to find that she had broken open a smaller boulder, and revealed the most lustrous galena cube I have ever encountered in the field! We then took turns to chip out a reasonable matrix piece, while leaving the shining cube undamaged. Our mission was accomplished as we chiseled out an intermixed carbonate-barite matrix piece with a 1.25” cube on an edge. It still resides on a bookshelf as a field trophy piece.
It became clear that this was the potential spot to hit good sulfide mineralization. A few more freeform galena crystals were removed from the “trophy boulder”, as well as the ground outcrop we were sitting on. After some more smashing, my wife then asked me if my thesis advisor found any sphalerite from our first visit. I said that we could not find any, to which she replied “you said that sphalerite can be a root beer/dark red color right?” I concurred and then she points to a spot saying “is it this he was looking for?” After a second I was taken quite aback for a couple reasons. The first being that she found what couldn’t be found during the first trip with my advisor and I. The second thought that came to mind is that she must have quite an affinity for zinc. It was not long ago that we traveled to the Keokuk, Iowa area for geode hunting. During that trip we found out that she seemed to find all the geodes containing “black-jack” sphalerite. After our trip here I presented the small, anhedral sphalerites to my advisor in which he showed quite a bit of surprise. It was then he said “if I ever have a need to go out to find sphalerite, I’ll be sure to ask your wife to come out with us to help sniff them out!”
It became clear that this was the potential spot to hit good sulfide mineralization. A few more freeform galena crystals were removed from the “trophy boulder”, as well as the ground outcrop we were sitting on. After some more smashing, my wife then asked me if my thesis advisor found any sphalerite from our first visit. I said that we could not find any, to which she replied “you said that sphalerite can be a root beer/dark red color right?” I concurred and then she points to a spot saying “is it this he was looking for?” After a second I was taken quite aback for a couple reasons. The first being that she found what couldn’t be found during the first trip with my advisor and I. The second thought that came to mind is that she must have quite an affinity for zinc. It was not long ago that we traveled to the Keokuk, Iowa area for geode hunting. During that trip we found out that she seemed to find all the geodes containing “black-jack” sphalerite. After our trip here I presented the small, anhedral sphalerites to my advisor in which he showed quite a bit of surprise. It was then he said “if I ever have a need to go out to find sphalerite, I’ll be sure to ask your wife to come out with us to help sniff them out!”
After spending the rest of the day around this corner spot, it was time to start calling it a day. As we traversed the open field back, I noticed another sizeable boulder nearly at the opposite end where Rebecca was initially. Thinking to myself “why didn’t I notice this before”, I had to take a quick peek. The first thing I noticed was a small blackish streak coming out of a fracture. Upon closer inspection, it seemed to have some faint glimmer to it. Naturally, I decided trying to pry to crack open. Given the soft nature of mineralization, it definitely did not take long to reveal a nice-sized mass of drusy chalcopyrite. The specimen consisted of millimeter-scale granular crystals, consisting of shimmering blue and green hues contrasting very well with a snow white barite matrix. Since we did not possess any bigger tools to work the boulder, and a noticeable lack of other easy fractures to pry open, we decided to call it a day with the peacock ore. With the appreciable amount of galena and barite collected, I was somewhat surprised that our little sedan did not bottom out!
The Gollar Mine is a wonderful place for both the seasoned and beginning rockhounder alike. My advisor )I believe) still takes his introductory-level earth resources classes here, and from what I have heard his students always have a blast. Although the entrance to the site can be a little rough, access is still easy with a two-wheel drive vehicle. In order to reach the Gollar Mine site from Columbia, take U.S. Route 63 south to Jefferson City. From there, get on U.S. 54 towards Eldon for roughly 20 minutes to the exit for Missouri 17 (toward Eugene/Fort Leonard Wood). Make a left off the exit and go for a mile, and then make a left on road BB. Continue on BB for roughly 2.5 miles, and Swift Road will be on the right. As you turn to get on Swift, there will be a dirt road at an angle behind you, as of the roads make a “wide V”. Continue down the dirt road for approximately 0.35 miles until you reach the pit lake to the left and the open collecting area in front. |