Minerals exploration
Tracking Ozarks elephants
By William Jud,
geologist
A Whale of an opportunity. A 600-pound Gorilla in the room. The Lion’s Share. Big things often are expressed using big-animal metaphors.
Minerals exploration geologists have their own favorite big animal. The search for big mineral deposits is called "Looking For Elephants."
When a real, live elephant passes by, it leaves tracks and other evidence of its passage. If you follow the tracks in the direction the animal was moving, you should eventually find the elephant that made the tracks.
There are metaphorical mineral "Elephant Tracks" in the rocks of the Missouri and Arkansas Ozarks. The mineral elephants are fixed in stone, and are no longer going anywhere. Instead of following where the elephants were going, geologists can follow the tracks back to where they originated, hoping to find really large and useful mineral deposits.
Granite and volcanic rocks, and probably low-grade metamorphic rocks, are the "basement rocks" upon which the Missouri Ozarks’ mostly marine sedimentary rocks were deposited. The "basement rocks" do not contain known, large ore deposits of lead, zinc, copper, cobalt, gold, and related metals, although a low-grade iron/copper deposit was discovered in "basement rocks" near the town of Boss in Dent County. However, mineral deposits containing those metals are known within overlying sedimentary rocks, and have been mined since well before the Civil War. The earliest record of metal mining in Missouri dates from the year 1700. The Missouri “Lead Belt” and Barite District have been mined on a large scale for more than a century, producing enormous mineral wealth.
Commercial metallic minerals are not a normal component of the sedimentary sandstone, shale, limestone, and dolomite rocks which overlie the Ozarks’ basement rocks. Most sedimentary rocks of these types, in most places, are not enriched in those metallic minerals. Actually, this is fortunate, because many metals, such as lead, are toxic within the food chain. If lead, for example, or cadmium, were a common component of limestone, a lot of land overlying limestone bedrock would not be suitable for farming.
If metallic industrial minerals are generally uncommon in sedimentary rocks, why are there such large deposits of those minerals in some Missouri Ozarks sedimentary rocks? The answer is, the metallic minerals were most likely brought in after the rocks formed, and were deposited by a geological mineral relocation mechanism.
There is general agreement among Missouri geologists that mineral relocation was accomplished by hot oilfield brine forced upward through sedimentary rocks of the Ozarks Uplift. Brine, which is another name for salty water, is common in nearby sedimentary "basins" which are bowl-shaped, down-warped geological structures that commonly contain oilfields. Oilfield brine usually contains dissolved hydrogen sulfide, the "rotten egg" gas. Brine becomes more acidic and more capable of dissolving and transporting metals as its temperature rises.
Lead, zinc, and other metals in Ozarks mineral deposits occur as sulfide minerals. There is plenty of sulfide in oilfield brine to create the observed metallic sulfide minerals. But, what was the source of the metals? What conceptual model integrates the data into an exploration plan?
At this point it helps to step back, ignore the details, and look at the regional picture.
Evidence suggests that oilfield brine was injected into Ozarks sedimentary rocks from a southern source. Hot oilfield brine was almost certainly the medium that carried dissolved metals into the sedimentary rocks to create concentrated deposits of metal sulfide minerals. As hot brine got farther from its source it cooled, became less acidic, and less able to transport metals. Dissolved metals precipitated from solution and formed mineral deposits, especially where the cooling brine left basal sandstone and entered overlying limestone or dolomite carbonate rocks, which further reduced acidity. Temporary accumulations of natural gas rich in hydrogen sulfide probably formed at the sandstone/carbonate rock interface, providing concentrated sulfide which assisted in precipitating the metals as sulfide minerals. But, how did dissolved metals get into the brine, and where and what was the metals’ source?
Here’s where tracking elephants begins. In an exercise of geological "reverse engineering," the sedimentary rocks mineral deposits end product is known, and so is the probable northern direction of oilfield brine flow. Presumably, if you follow those "elephant tracks" backward, you will arrive at the source of the metals which the hot brine dissolved and then transported and deposited in Ozarks sedimentary rocks.
Hopefully, on the South side of the Ozarks Uplift the "elephant tracks" will lead back to one or more immense, but probably low-grade, deposits of similar kinds of the metallic minerals that occur in sedimentary rocks to the north around Fredericktown, Park Hills, Potosi, and Viburnum in Missouri. The major differences are that there is likely to be more gold in those source rocks than there is in Missouri Lead Belt mineral deposits, and the uppermost parent metallic minerals are more likely to be oxides and carbonates than sulfides. There probably will be low-grade uranium mineralization as well.
If the source deposits exist, they will likely be found within ancient rock conglomerates and rubble layers made of volcanic and low-grade metamorphic rock, unlike the younger limestone/dolomite marine sedimentary rocks of the Lead Belt. A source deposit is most likely to be very large and low-grade, iron-rich, and similar to the big mineral discovery near Olympic Dam in Australia. One such deposit could produce a significant amount of metals for a hundred or more years.
Once a deposit of mineralized source rock is found, work shifts from exploration to evaluation. You may have found a mineral deposit, but not an ore deposit. "Ore" is an economic term that describes a mineral deposit that can be mined profitably. Many non-geological features affect profitability.
The discovered mineral deposit may not be ore because the minerals are too dispersed, or the deposit is too deep for practical mining, or because the deposit lies under a place such as the city of Poplar Bluff, Missouri, and mining would conflict with existing land uses. The mineral deposit may drift in and out of the "ore" category as the prices of metals fluctuate. There may be geological problems such as hot water or residual hydrogen sulfide poison gas that make mining impractical. Count on environmentalists to find some rare or endangered critter that lives only in Critical Habitat located over the mineral discovery, no matter where the discovery is.
Even a mineral deposit made of bars of pure, refined gold, would not be "ore" if you had to go to the moon to get it.
Minerals exploration always is a gamble. The conceptual model may be wrong, the expected mineral deposits may not exist, and the known mineral deposits in sedimentary rocks may have been created by an altogether different geological mechanism. Luck enters exploration, and even incorrect conceptual models sometimes accidentally result in mineral discoveries. The conceptual model offered in this article is a starting point, to be adjusted to fit geological reality as new data arrive.
Mining companies, with good reason, do not like to gamble on being first to test a new concept or exploration area. Mining companies really hate being second. The company that makes the initial discovery usually gets the "lion’s share" of the mineral elephant, and everybody else tries to make-do with scraps.
If a mineral "elephant" is discovered in Southern Missouri and/or Northern Arkansas, expect an exploration stampede to develop. It’s probably just a matter of time before that happens.
Part 2:
Minerals exploration
Elephant's spoor
One conceptual model for minerals exploration in Madison and nearby Counties in Southeastern Missouri states that metallic minerals in the known lead deposits mined from marine carbonate rock at Park Hills, Viburnum, and vicinity, were relocated from a pre-Cambrian source into overlying carbonate rocks by hot oilfield brine migrating up-dip from the South.
That conceptual model is Exploration 101, a very basic tool used for centuries, to look for mineral deposits based on outwash from those deposits. It is exactly the same as tracking down a barbeque by following the odor of cooking meat wafting on the wind.
The geological situation in Madison County is a special case that lends credence to the conceptual model.
The lead/cobalt mine at Fredericktown is notable for the high concentration of cobalt in its mineral deposit. Mineral Area lead mineral deposits generally contain cobalt in small concentration, usually about 1/10 of one percent of the mineralized rock. The deposit at Fredericktown contains cobalt at a concentration of one percent and even higher, a tenfold increase in cobalt concentration, when compared with other deposits in the Mineral Area.
Lead deposits in Southeastern Missouri are all pretty much similar in composition and metal content, including the deposit at Fredericktown. The increased cobalt concentration at Fredericktown is anomalous, and probably reflects something unusual in the local geology.
Cottoner Mountain is located between Fredericktown and Marquand. Cottoner Mountain is the remnant of an ancient volcano. Some of the exposed rocks on Cottoner Mountain are rubble formation created by collapse of the volcano wall.
What is interesting, in terms of the conceptual model, is that Cottoner Mountain has both a strong positive magnetic anomaly, and a strong positive gravity anomaly. In addition, drilling on Cottoner Mountain brought up rock with iron oxide mineralization.
The aeromagnetic map, which includes Cottoner Mountain, indicates that the ancient volcano extends Eastward from Cottoner. The gravity map shows that the positive gravity anomaly accompanies the magnetic anomaly. The combined gravity and magnetic signals, and the drilling, suggest that important commercial mineralization, including cobalt and nickel, may be present at depth in the preCambrian rocks.
If a pre-Cambrian metallic mineral deposit is present at or near Cottoner Mountain, and if the deposit is exposed on the buried pre-Cambrian surface and was accessable to the migrating brine which is thought to have moved minerals into the overlying carbonate rocks, then the high concentration of cobalt in the mine at Fredericktown may be the direct result of relocation of metallic minerals from a cobalt-rich mineral deposit exposed on the buried pre-Cambrian surface, at or near Cottoner Mountain.
If the conceptual model is correct, and migrating brine dissolved metals from preCambrian mineral deposits and then carried those metals upward and deposited them into overlying carbonate rocks to form secondary mineral deposits, that suggests that a primary mineral deposit containing lead, zinc, copper, cobalt, nickel, and other metallic minerals may exist in buried pre-Cambrian rocks in the Cottoner Mountain vicinity.
The Cottoner Mountain preCambrian primary mineral deposit, if it exists, should be similar in composition with the mineral deposits in overlying carbonate rocks, but should contain low-grade uranium and gold in addition to lead, zinc, and other common metals. The weathered upper part of the primary pre-Cambrian mineral deposit exposed to migrating brine is likely to contain metallic minerals as oxides, carbonates, and perhaps silicates, unlike deeper parts of the primary deposit and the secondary deposits in carbonate rocks, where metals are present as sulfide minerals.
Mineral deposits in carbonate rocks are an indication that there may be primary deposits nearby in buried pre-Cambrian igneous rocks, but the size and richness of the secondary mineral deposit are not a firm indication of the size and richness of the primary mineral deposit which provided the metals. A big mineral deposit such as the lead deposits around Park Hills may indicate a big primary deposit as the metals source, or it may mainly indicate that there was just considerable exposure of a primary mineral deposit to the migrating brine.
Size and richness of the secondary deposits are definitely factors, but so is the amount of exposure of the pre-Cambrian source deposit to the brine. A large and rich primary deposit that is poorly exposed on the preCambrian surface would provide only a small amount of metals to the migrating brine, and result in a relatively small secondary minerals deposit such as the deposit at Fredericktown. A large primary mineral deposit that is not exposed at all on the buried preCambrian surface would not be a source for relocation of metals by migrating brine and would not create a secondary minerals deposit. Many primary mineral deposits are likely to be completely buried below the preCambrian surface and would not provide metals for secondary mineral deposits.
Of course, at this time, all this is speculation. Geology is complex. The conceptual model may be correct, or it may be wrong. Luck is a factor, and valuable mineral deposits have been discovered by accident by following an exploration concept that was later found to be wrong.
Mineral Elephants are a term that exploration geologists use to describe really large and valuable mineral deposits. There is evidence of Mineral Elephants' spoor in Madison County and the rest of the Mineral Area. Maybe the big, historically mined areas hereabouts are just a teaser, and the region’s mining glory days still lie mainly in the future.
Part 3:
Minerals exploration -
Prospecting ancient Ozarks rivers
One Conceptual Model for exploring for huge Olympic Dam-type mineral deposits in the Missouri Ozarks is based upon logic instead of geology.
Olympic Dam-type mineral deposits are named for the large ore deposit mined near Olympic Dam at Roxby Downs in Australia. Olympic Dam-type mineral deposits tend to be low-grade, but really big. They may contain iron, copper, cobalt, uranium, gold, rare-earths, and other useful materials. They do exist in the Missouri Ozarks. The iron mine at Pea Ridge is a known Olympic Dam-type ore deposit. So is the copper deposit at Boss-Bixby.
The lead deposits at Fredericktown, Park Hills, Viburnum, and elsewhere in Missouri’s Old and New Lead Belts are called Mississippi Valley-type mineral deposits. The standard Conceptual Model says that known Mississippi Valley-type (MVT) lead/zinc/copper/cobalt/iron sulfide mineral deposits were created by regional-scale activity. Regional-scale activity ought to have created fairly uniform, regional-scale mineralization at all suitable mineral deposition sites.
However, known MVT mineral deposits are not uniform, and are not regional in size. Known MVT mineral deposits vary widely in metal content and richness, are small in size on the regional scale, and are widely separated from each other by large areas of non-mineralized ground.
Logic says that if mineralizing activity was regional in scope, but known mineral deposits are only local, widely separated, and vary considerably in size and richness of metals content, then some additional factor or factors must have been involved in creating known MVT mineral deposits and ore deposits.
Mineral Deposits are just that - deposits of minerals. Ore Deposits are mineral deposits that can be mined profitably. “Ore” is an economic term, in addition to being a geological term. Whether a particular mineral deposit is an ore deposit or a mineral deposit, depends on many non-geological factors such as location, metals prices, transportation cost, mining difficulty, and other items. Rock mineralized with bars of pure, refined gold would not be an ore deposit if you had to go to the moon to get it.
If a local feature determined the location, richness, and size of an MVT mineral deposit, that local feature may be an Olympic Dam-type mineral deposit nearby in the buried preCambrian igneous rocks.
Known MVT mineral deposits in surface rocks may be up-dip outwash of metals relocated from a nearby deeper Olympic Dam-type metals deposit. An example is the unusually high concentration of cobalt in the Fredericktown lead/cobalt deposit. Fredericktown ore may represent outwash of metals originating in a nearby preCambrian cobalt mineral deposit in the collapsed pre-Cambrian Cottoner Mountain volcano between Fredericktown and Marquand.
The currently accepted geological theory for emplacement of Ozarks MVT mineral deposits begins with a heat source in a nearby, down-warped geological "basin."
Heat warms salty water, or "oilfield brine," that rises through a sandstone formation. Hot, buoyant brine migrates laterally and upward through the sandstone formation, perhaps for miles or tens of miles, until the sandstone formation ends against a hill on the preCambrian surface. Dissolved minerals carried by the rising brine are deposited in the sandstone and particularly in overlying limestone or dolomite along the "sand pinch-out" line where the sandstone terminates against the hill. Old algal reefs in porous and permeable limestone or dolomite rock are the best depositional sites.
Why is the entire sand pinch-out line not uniformly mineralized? One reason may be that the brine itself was not uniformly mineralized. Perhaps the biggest and richest MVT ore deposits were created by brine that crossed a buried pre-Cambrian Olympic Dam-type mineral deposit and moved some of the metals up into the overlying sedimentary rocks. The Conceptual Model says that a known MVT ore deposit might indicate that an Olympic Dam-type deposit is present in nearby buried preCambrian rocks.
The "additional factor" may be buried stream channels.
During geological field work I did at Fredericktown, I noticed that buried stream channels on the pre-Cambrian surface are usually mineralized where the channel intersects the sandstone pinch-out line.
Buried stream channels are filled with coarse gravel. Gravel transmits water much better than either sandstone or limestone/dolomite. Buried stream channels full of gravel, cutting across the buried pre-Cambrian landscape for tens of miles, served as pipelines to gather and carry rising mineralized brine and to concentrate the discharge of the mineralized brine into a small volume of rock at the sandstone pinch-out, creating mineral deposits and ore deposits.
In most places, there was not a lot of mineral content in the brine rising through the buried stream channels, so there was not a lot of mineral deposition where the channel intersected the sandstone pinch-out. But at some places, the buried stream channel may have crossed an Olympic Dam-type mineral deposit exposed on the buried preCambrian surface. Hot brine would have dissolved a large quantity of metals from the oxidized upper couple of hundred feet of the Olympic Dam-type mineral deposit. That highly enriched brine would have created a large and rich MVT ore deposit at the stream channel/sandstone pinch-out intersection.
Knowing that an exploration target is likely to exist, the next step is deciding where to go explore. Again, buried stream channels define the initial exploration area and greatly reduce the area that must be searched.
In any stream system, flow channels begin at the ridge around individual watersheds. Channels merge downstream and join channels coming from other watersheds.
Buried channels can be mapped. Much of that information is already available on existing maps.
You begin at a known MVT ore deposit and go downstream along the buried river channel. Where that channel intersects another channel, look upstream on the intersecting channel. If the intersecting channel does not have an ore deposit at its upstream end, then the intersecting channel marks the downstream limit of exploration drilling along the channel that does have the ore deposit. If the intersecting channel does have an ore deposit at its upstream termination, then continue the process downstream with subsequent intersecting channels until a non-mineralized channel is intersected.
In this way, the presence of an Olympic Dam-type mineral deposit is indicated and an initial drilling area of manageable size is defined. Do the initial drilling along the ancient gravel-filled stream channel, starting at a known MVT ore deposit, and ending at the intersecting non-mineralized stream channel.
As an exploration bonus, once a buried Olympic Dam-type preCambrian mineral deposit is found, all the other places in that ancient watershed, where stream channels terminate at the sandstone pinchout under carbonate rocks suitable for hosting mineral deposits, immediately become additional exploration targets for MVT ore deposits that may have previously been overlooked due to lack of surface indication.
If there are discoveries of buried Olympic Dam-type metals deposits, the Ozarks’ glory days of mining will just be starting.
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