TIM MERSOI BASIN NIGER

URANIUM PRODUCTION IN NIGER

Niger is currently the fourth largest uranium producer in the world and poised to become the largest. Uranium mineralization closely resembles the deposits of the Colorado Plateau. Since the early 1970s, the primary producers in Niger have been joint ventures dominated by the French utility Areva. Through its joint venture companies, Areva has produced over 100,000 tons of U3O8 from Carbonaceous age mineralization in an open pit and underground mine since 1971. Now, Areva has committed over US$ 1.47 billion to open a mine on what appears to be the largest sandstone type uranium deposit ever found. With inferred reserves of 146,000 tonnes of U3O8, Imouraren, a Jurassic age deposit, over eight km long and two and one half kilometers wide, has an average grade of 0.11% U3O8.

A NEW WAVE OF URANIUM EXPLORATION

As the renewed interest in uranium exploration developed, Niger rewrote its mining laws encouraging foreign companies to apply for exploration permits for reasons that remain obscure but suggest some level of dissatisfaction with the status quo defined by decades of Areva dominance in the national economy. In response, companies capitalized to various degrees by private or public equity and/or state funds from Canada, India, South Africa, Russia, China, Britain, and Australia applied for and received permits to explore blocks from 500 to 4,000 sq kms.

Current exploration activity is conducted by three main actors, Areva, the China National Nuclear Corp, and GoviEx, a subsidy of the Canadian firm Ivanhoe Resources. Each has proven reserves from over 250,000 tonnes U3O8 down to less than 6,000 tonnes. Other active players include the South African companies Niger Uranium and Niger Mine Services, the British group Brinkley Mining and the Canadian companies Uranium International Niger and Global Uranium. The majority of permit holders have yet to make an appearance on their recently granted Saharan real estate.

Tim Mersoi Basin Uranium Exploration Permits, Niger

GEOLOGY OF THE DEPOSITS

General Geologic Observations

The Tim Mersoi Basin is a little explored uranium province where the transition from marine to terrestrial sedimentation is punctuated by at least three significant episodes of alkalic to peralkalic volcanism within a few hundred km of its margins. The earliest volcanic episode in the region, dated at ~407 Ma, lies immediately east of the basin in the Air Massif. Jurassic and Cretaceous alkalic and peralkalic volcanism is well documented a few hundred kilometers to the south. Thick sequences of altered tuffs occur in close stratigraphic proximity to Mesozoic uranium mineralization. Closed basin conditions appear likely during and shortly after the Jurassic episode resulting in exotic pore and surface water chemistry favorable for the mobilization of uranium from highly enriched vitric ash and entrapment of uranium in entombed organic debris. Similar geologic and geochemical conditions prevailed during deposition of the Chinle and Morrison formations on the Colorado Plateau where prior to 1980, 90% of world sandstone-type uranium was produced.

Source Rocks

The Niger-Nigeria younger granite belt is defined by a series of intrusive ring complexes, shallow felsic intrusions, and the eroded remnants of their volcanic cover that run from at least the Air Massif in the north into northern Nigeria in the south. This trend of volcanism is thought by some to be linked to the southeast with the northeast trending Cameroon line. Though calc-alkalic compositions are represented, it is notable that alkalic, peralkalic, and peraluminous felsic intrusions are common. In general, incompatible elements, including uranium, are enriched to highly enriched in these rocks.

There is a well documented younging of this trend from north to south. Igneous complexes in the Air Massif range in age from 470-399 Ma. Similar alkalic and peralkalic intrusions in the Mounio Massif and the intrusives near Zinder in southern Niger are dated at 302 Ma. Just across the border in Nigeria, peralkalic intrusives range in age from 190-153 Ma. Though the alkalic and peralkalic ring complexes in the Air Massif are most proximal to mineralization and have well documented elevated uranium concentrations, they may not be the source of uranium in the Carboniferous deposits. The age relationships suggest a 50 Ma gap between their emplacement and the age of the Madaouela, Guezouman, and Tarat uranium host strata. Similar intrusions and their volcanic equivalents near Zinder, though less well exposed, are much closer in age. Likewise the peralkalic intrusive and volcanic complexes just over the border in Nigeria match the age of the Tchirezrine and Abinky strata that host or immediately underly the Imouraren deposit. Cretaceous age alkalic and peralkalic volcanism is well represented along the northern half of the Cameroon line which terminates at Lake Chad. Though distant from the Tim Mersoi basin, it is not too distant to have deposited volcanic ash in a 50 m interval in the Cretaceous Irahazer formation in the proximity of the Azalik deposit.

Paleoenvironmental Indicators

World-wide, the sedimentary basins that host sandstone uranium deposits have similar geologic histories. The specific host strata share a common fluvial depositional environment. Most are persistent intracratonic basins which alternate between shallow marine and fluvio-lacustrine conditions. In many of these basins evaporite deposits mark the transition from marine to terrestrial depositional environments. Several of these basins display a gradual transition from chemically mature arenite to less mature arkose. Nearly all have some evidence of a strong volcanoclastic input in close proximity to the uranium bearing strata.

The Tim Mersoi Basin stratigraphic sequence is consistent with this pattern of basin evolution, depositional environments, arid climate, and proximity of volcanoclastic sediments to the strata that host uranium mineralization. The conventional interpretation of the stratigraphic hosts and the surrounding strata link the mudstone and sandstone to a fluvial or fluvio-lacustrine environment. Gross stratigraphic architecture and sedimentary structures suggest a braided or anastomosing fluvial regime. Alternating red and green mudstones attest to variable redox conditions during deposition and after shallow burial. Additional early diagenetic evidence reveals even more specific paleo-environmental conditions.

Bedded zeolites are good indicators of the alteration of volcanic glass in closed basin conditions. Sedimentary basins where evaporation exceeds precipitation evolve surface and shallow groundwater whose compositions are driven by the lithology of the basin bedrock. Closed basins dominated by igneous rocks evolve toward sodium carbonate brine by evaporative concentration. Substantial inputs of felsic vitric ash can drive this brine to exceptionally high pH and the formation of K-Na-HCO3 water. The extensive analcime cement in the Triassic age Teloua and bedded analcime in the Jurassic Abinky formations are strong evidence of a closed basin.

Structure

Are Niger's uranium deposits related to fault structures? Before considering the specific case of Niger, a look at this class of uranium deposits around the world can add a wider perspective to the question of the genetic influence of structure. The most thoroughly studied examples of sandstone deposits are the Mesozoic age uranium deposits of the Colorado Plateau. Other deposits are described from Iran, India, Kazakhstan, South Africa, and the Late Cretaceous through Eocene of Wyoming.

It is widely thought and in some instances supported by substantial evidence that this class of ore deposit forms shortly after deposition of the sandstone host or during the very earliest stages of diagenesis. Examples of ore zones truncated by scoured channels, low temperature mineral assemblages, stable isotope ratios and in some cases poorly constrained radiometric dating all point to the near or syndepositional origin of uranium ore. In those deposits which have undergone deep burial or exposure to later oxidizing groundwater regimes, the original textures, mineral assemblages, fluid inclusions, and formerly closed radiometric systems have been disturbed or overprinted.

Major fault zones are by no means a common feature of sandstone uranium deposits. Some are proximal to fault zones, even cut by faults, but many have no local association with major faulting. Basin tectonics play a major role at least in the sense that the patterns of sedimentation are largely controlled by changes in base level, differential subsidence, or formation of fault bounded depositional centers. The most common structural attribute of sandstone uranium deposits is a link to host sandstone thickening sometimes associated with broad, gentle synclines. These "synclines" are generally not created in compressive stress fields but indicate zones of subsidence that are preferentially filled by channel sands. In most sandstone uranium districts, good isopach maps and to a lesser extent structural contour maps are valuable exploration tools.

To return to Niger, much has been said about the relationship of ore zones to the Arlit and other fault systems. The link between ore and increases in host rock thickness are also well documented. There is reason to think that some but not all of the faulting in the basin has been intermittently active prior too and certainly after deposition of the host sandstones. To the extent that this has influenced depositional patterns, exploration in the vicinity of those faults that clearly pre-date or are penecontemporaneous with host sandstone deposition should be part of any larger exploration strategy. The hypothesis that the fault zones are feeders of reduced fluids that mixed with oxidized uranium bearing groundwater to form the deposits is plausible but far from proven. Similarly the converse that the fault zones are the source of the mineralizing fluids though less likely is possible. The evidence for either of these mechanisms is weak at best and strongly contradicted by the absence of a clear gradient in ore grade that can be spatially linked to the Arlit fault at either Akouta or Somair. There is even less evidence of a relationship between major faulting and ore at Imouraren other than the role faulting plays in fluvial facies and sandstone thickness.

There is no easy answer to the significance of large faults in relation to uranium mineralization in the Tim Mersoi Basin. The historic bias toward fault based exploration models is currently being tested. Some of the exploration now underway will take place far from known fault zones and may clarify just how important proximity to faults in this basin actually is.

STRATAMODEL'S ROLE

Stratamodel develops uranium exploration strategies, performs field traverses, conducts geologic mapping, down-hole radiometric logging, and geochemical and radiometric soil surveys in sandstone-type uranium provinces including the Tim Mersoi Basin of Niger.

Stratamodel has worked on sixteen permit areas in the Tim Mersoi Basin with a combined area of 11,000 square kilometers. Our primary focus has been systematic radiometric soil surveys at small scale (>1 point per hectare) and detailed coverage at large scale. Our small scale reconnaissance surveys have covered 5,000 sq km. Large scale detailed surveys have focused on fault zones and as follow-up to dispersed airborne radiometric anomalies. Stratamodel geologic traverses have been formal in some cases and a derivative of covering the area during the course of a radiometric survey in others. In both cases these traverses presented us with the chance to observe surface geology over a wide swath of the basin. This widening exposure enables Stratamodel to bring greater experience to each new client without compromising the sensitive project specific information of our client base.


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