Let me begin with a short account of our survey progress and local news. I will follow this with some observations and ideas about the local geology and the formation of our target uranium deposits. When your eyes begin to glaze over reading about geochemistry and the evolution of this sedimentary basin, give it up and wait for the next blog and something a little less esoteric.
This survey is nearly done. We are closing in on our goal of 2000 points even as our equipment is failing under the severe demands of the hard drilling. The crew has been tearing the auger motors apart and cannibalizing parts to keep as many of the chain saw motors running as possible. Despite this we are down to one working machine. We will receive two replacements late Sunday which will boost us over the top early next week. In about ten days we will have it wrapped up and head south to Niamey to catch our flight home about a week earlier than scheduled.
The security situation seems to have improved and the US embassy has relaxed some of its former travelers warnings. We hope the convoy system has also been relaxed so that we might make better time back to the capital. The areas we work are nearly deserted with the exception of a few scattered Touareg encampments were the crews have been trading plastic bags and a little money for bricks of sheep cheese and the occasional sip of camel milk.
Each day gets a little hotter and the morning chill dissipates a little earlier. We will avoid the blistering days ahead this trip but when we return in late March, the heat will be on. Still no signs of our February wind storms so we might be lucky enough to miss them.
Every day brings in a new crop of interesting rocks and artifacts from the desert. This area has been inhabited for tens of thousands of years and the debris left by generations is still out there for the taking. One of the geologists brought in a bluish glassy mass shot full of gas cavities. Small bits of copper where still stuck in the glass. This slag could be several thousand years old and mark the site of an old furnace where people smelted malachite to produce copper tools.
SOME THOUGHTS ON THE EVOLUTION OF THE TIM MERSOI BASIN
The rocks that keep arriving in camp tell a story of the evolution of this large inland sedimentary basin and are giving us clues to the development of the uranium deposits. We know from past work that sometime in the middle Paleozoic, sediment began to accumulate in a shallow sea or large lake. As the basin grew, sometime in the later Paleozoic, the land to the east began to rise even as the basin continued to subside. If the first sediments to accumulate where marine, by the late Paleozoic or early Mesozoic, the basin was isolated from the sea and became an inland sea or large lake.
As the basin became even more isolated the surface and pore waters began to undergo their unusual chemical evolution. This is evident from the gross chemical stratigraphy of the basin. Sometime in the late Paleozoic, carbonate began to cement the sandstones and even more telling, carbonate and perhaps gypsum nodules grew in the mudstones. In isolation, this is not so unusual but the next chemical step, a signal event in this evolution, puts these early chemical steps in a different light and may be a valuable clue to the origin of the uranium deposits within the basin. A rare mineral, analcime (a sodium rich feldspathoid), cements some of the sandstone, formed thin beds, and grew within the mudstones.
Sedimentary analcime is currently forming in abundance in only a few places around the world in a specific type of physiographic, chemical, and geologic environment. Our geologic understanding of the process of analcime formation dates from the late sixties and early seventies and is certainly not widely appreciated within the broader geologic community. Sedimentary analcime forms at low temperature in the presence of very high concentrations of silica and sodium in water with a pH of 9-10 or higher. Geologic conditions that match these criteria currently exist in some of the rift lakes of East Africa and playa lakes in volcanic terrains. Climatic conditions and watershed geology are a key part of the analcime story. Before speculating on the significance to uranium mineralization, a bit of geologic background must be explained. Bear with me if you are a geologist.
Lake water with highly alkaline chemistry develops from a combination of relatively rare geologic and climatic events. Climatic conditions can be summarized by the simple statement that evaporation must exceed rainfall. This simple condition sets the stage for the development of a physiographic feature called a closed basin (a lake with inflow but no outlet). Water entering a closed basin undergoes evaporative concentration which over time produces a brine. Seawater, a familiar brine, contains sodium and chlorine. Lake water in closed basins can evolve into far more complex and unusual brines that are determined by the chemistry of the rocks within the watershed, distance from the sea, and ratio of rainfall to evaporation.
Many such saline lakes exist in closed basins around the world. For example, The Great Salt Lake in Utah, the Dead Sea in Jordon and Israel, and the Aral Sea in Central Asia are all closed basins. Each of these three, while highly saline, does not have a particularly high pH. Saline water contains the dissolved alkalies lithium, sodium, and potassium and the alkali earth elements calcium, magnesium, and barium. These form positive ions (cations) that must be balanced by an equal charge equivalent of anions since water is electrically neutral (charge balance). The typical anions in natural waters are chlorine, sulphate and carbonate and indeed these are the dominant anions in most saline lakes as well as seawater. A garden variety brine would have elemental abundances Na>Ca>>Mg>>>K>>>>Ba>>>>>>Li and Cl>SO4>>>HCO3. Chloride reaches most bodies of water through wind born chlorine from the ocean or the chemical weathering of chloride rich rocks. Sulphate is derived from sulphide or sulphate minerals during the weathering of bedrock.
Rarely, a watershed has little sulphate and is far removed from the ocean so wind borne chloride is in short supply. Since water must be charge balanced and chemical weathering continues in the watershed supplying a steady stream of cations to the water, the only other source of anions is carbon dioxide from the atmosphere itself.
The bedrock in these somewhat unusual circumstances is frequently dominated by felsic volcanic or plutonic rocks (granite-rhyolite). The paucity of a source of chloride and sulphate is complimented by a high ratio of alkali to alkali earth cations (K + Na / Ca + Mg) in these same types of rocks.
Closed basins are frequently structurally controlled. Early stage continental rifting e.g. the Dead Sea and orogenic uplifting produce horst and graben structures e.g. the numerous playa lakes of Nevada. These settings are predisposed to becoming closed basins under the right climatic conditions. Broader closed basins can develop under the right climatic conditions in areas of crustal downwarping such as the Lake Eyre basin of south central Australia and Lake Chad of north central Africa.
The setting for the formation of sedimentary analcime is well established in the geologic literature and its presence in a basin can lead us to reach some general conclusions about the chemical evolution of the basin waters and give some clues to the likely tectonic setting. In short, evaporation must exceed rainfall, a highly alkaline brine must evolve, and the basin must be dominated by felsic volcanic or plutonic rocks. Moreover, these geologic conditions are often met in tectonically active areas undergoing crustal extension in an arid or semi-arid climate.
To return briefly to the Tim Mersoi basin, the gross chemical evolution can be read from the sedimentary cements and authigenic growth of minerals associated with brine evolution. Early stage carbonate nodules and cement pass to the more exotic mineral analcime in the Jurassic and return to carbonates during the Cretaceous.
If your eyes are glazed from this discourse on brine evolution and geochemistry, how about a little volcanology to add some drama to this tale?
Imagine you are a lowly creature trying to scratch out a living beside a bad water lake and suddenly your day goes really bad. About a hundred klicks east, a volcanic eruption sends a Plinian cloud 10,000 meters up into the atmosphere and it begins to rain ash and lapilli. Some of you may remember the video of a horse in eastern Washington state shaking off a blanket of vitric ash from Mt St Helens when it erupted in the 80s. Or, photos of the houses and cars covered with meters of vitric ash from Mt Pinatubo when it erupted in the Philippines a decade ago. Our poor little creature scurries for the nearest bush but he is destined to end up like the citizens of Pompeii.
There is another piece of the puzzle that fits in nicely but does not necessarily prove anything with regard to the uranium mineralization. By the middle Mesozoic, a critical event took place just east of the Tim Mersoi basin that may have introduced vast amounts of uranium into the local environment. Volcanic eruptions of a somewhat unusual chemical composition began along a north south line about 100 km east of the basin margin. Eight or nine volcanic complexes up to 70 km across are still preserved in the Air Massif to our east. Their chemical composition is highly alkaline to peralkaline (high proportion of potassium and sodium relative to aluminum). Volcanic rocks of this type from around the Earth are known to carry relatively high proportions of incompatible elements and metals (uranium, thorium, rare earth elements, tin, molybdenum, etc). Incompatible elements are the dregs of magmatic evolution. Loners who don’t readily form minerals of their own and misfits that aren’t easily incorporated into more common mineral structures because of their size, charge, or molecular orbital symmetry.
Repeated eruptions of vitric ash raining in the Tim Mersoi basin would have had a significant impact on the evolution of the brine. At first, the very fine grained vitric ash is inert but with a little time it begins to dissolve. As it dissolves it starts to release its load of silica, alkalies and even more important its payload of incompatible elements including uranium. The rising load of cations sucks carbon dioxide out of the air to balance the charge jacking up the alkalinity. Bear in mind we are not speaking of typical rhyolitic vitric ash but material derived from a highly alkaline and even peralkaline magma. As more ash dissolves the pH begins to climb rapidly. Additonal eruptions add vitric ash that starts to dissolve like alka seltzer in the steadily rising pH of the lake. Zeolites and ultimately analcime begin to grow in the place of the dissolving ash. The highly alkaline brine can not only dissolve ash and carry over 100 ppm silica, it can carry very high concentrations of uranium in solution as uranyl-carbonate complexes.
So now that we have uranium soup, where does it go? The abundance of silicified sandstones, wood, bones, carbonate nodules and the like are evidence that silica dissolved in the brine, saturated and replaced every suitable host in its path. Uranium having an enormous hydrated ionic radius is not so easily precipitated. A suitable reductant is necessary to pop it back out of solution. Buried organic debris like wood chips, logs, branches, humate cemented sandstones (Arlit) etc are just what is needed.
Here is where the trail grows cold and murky. My tidy little geofantasy becomes far more speculative and as such less helpful in the discovery of a uranium deposit than I would like. Never the less, lets blunder forward and see where it takes us.
Several observations about the structure, lithology, and timing of events are important. The age of alkaline/peralkaline volcanism is Jurassic according to mapping done in the 60s and 70s. The age of the sedimentary uranium hosts range from Carboniferous to Cretaceous. The pre-volcanic uranium deposits discovered to date all lie along a large fault that has presumably been active since early in the basin’s history. The mineralization occurs as clusters of humate cemented sandstone and sparse organic debris in paleochannels. The only(?) syn-volcanic deposit to date is the large blanket of sandstone directly overlying massive analcime beds at Imouraren. Post-volcanic mineralization is known from limestones in the Cretaceous Irhazer formation and in sandstone at the Teguma-Irahazer contact.
How are these three types of deposits that span over a hundred million years of geologic time related? At Imouraren, organic debris in stream channels overlying the pregnant brine was perfectly situated to adsorb uranium from the dewatering of the playa muds during compaction driven by sediment loading. The mineralized Cretaceous sandstones and limestones of the Irhazer might have seen the remnants of this brine and they are not so separated in geologic time from Imouraren.
How to account for the Carboniferous deposits though? I was an early skeptic of the local wisdom that these deposits are structurally controlled. Now I am not so sure but still not convinced. It may not be such a coincidence that Imouraren lies along the same structure as the deposits at Arlit (Arlit fault). It is worth noting that the volcanic complexes to the east are aligned north-south, sub-parallel to the Arlit fault. It is not hard to imagine movement along this fault during the volcanic events. Fluid transport along this potentially transmissive structure could conceivably bring Jurassic age, alkaline, uranium bearing brine into contact with buried organic debris. Indeed, the Carboniferous has analcime cemented sandstone beds and the zeolites clinoptilolite and huelandite have been identified in the mines thus evidence for alkaline brine is present in them as well. I also can’t tell how well dated the volcanic complexes really are or how constrained the dates are so it is possible that volcanic events occured during the Carboniferous as well.
How is any of this helpful to the discovery of ore? In the end what difference does an understanding of the process matter if it can’t provide the basis for concrete exploration strategies? At the very least, if these observations and ideas have any validity at all, the practical implications for Imouraren and Arlit style mineralization are obvious.
Identification of the main axis of post playa sedimentation where it directly overlies the massive altered tuff (analcime tuff) should be a high priority in any exploration program here. In the field, there are well defined pebble and cobble trains of welded unaltered tuff eroded from the volcanic complexes. Clear flow banding, fiame, broken crystals of sanidine, in an aphanitic groundmass make identification of the original emplacement of these tuffs from ash flows unmistakable. Their distribution appears to be confined to linear trends where they comprise up to 90% of all cobbles present. It is very likely that these are weathering out of former stream channels. The coarse grain size (cobbles) speaks to significant stream power and the implication is that this represents a major axis of sedimentation. The timing of the erosion of the welded tuff must be post eruption thus the air fall phase which yielded uranium to solution must lie at a lower stratigraphic horizon. It is worth mentioning that the derivation of significant uranium to the system from the welded tuff at the site of eruption is highly unlikely. These cobbles are unaltered, unweathered and highly impermeable. They no doubt still carry their original charge of uranium. It would certainly be interesting, though perhaps academic, to perform a chemical assay to assess the potency of the original magmatic source.
Arlit style mineralization remains more problematic. If one accepts local wisdom, identification of favorable structures is key to their discovery. Proximity to the main phase of playa development may or may not be important. The relative size of these targets is also far less favorable than Imouraren type mineralization. In the Arlit case, one is exploring for beads on a string where there are many strings without beads. In the case of an Imouraren style target one must identify a flat lying blanket that covers many square kilometers if not more.
