Yesterday, we went through the basics of unconformity-hosted uranium deposits. And why these are the big prize for the uranium industry.And yet, despite decades of study, we’ve still only found unconformity-type mineralization in a handful of basins worldwide. Athabasca, Thelon, Kombolgie, Kintyre, Hornby Bay, Otish, Karku.
This is largely a function of exploration. There is no shortage of potential targets. Uranium gurus Kurt Kyser and Michel Cuney note in a recent paper there are over 200 sedimentary basins globally that fit the general bill for unconformity mineralization. That being (as we discussed yesterday), the presence of old Archean rocks overlain by younger Proterozoic rocks.
Something happened around the Archean/Proterozoic transition. Almost certainly related to the first appearance of large amounts of oxygen in Earth’s atmosphere.
The formation of uranium deposits is controlled by “oxidation state” (generally, the presence or absence of oxygen) to a much greater degree than other metals. And Archean/Proterozoic basins appear to have been the setting where oxidation conditions were optimal for the formation of large, high-grade uranium deposits.
But it’s highly unlikely that all 200 of the world’s Proterozoic basins will be significantly mineralized. We know that a number of special geological and geochemical conditions blessed places like Athabasca. Playing a critical role in uranium ore formation.
The trick to finding the next major uranium discovery is to survey the world’s basins. And figure out which ones might also have these special ore-enhancing features.
Here’s a checklist of things to look for in a good basin.
1) Uranium source. As we discussed yesterday, unconformity deposits form when older uranium is dissolved in oxygen-rich waters, concentrated, and then re-deposited. This means a basin must have an identifiable source of old uranium.
There’s debate in uranium circles about sources for unconformity mineralization. Some workers believe U is sourced from granites in the Archean rocks that underlie Proterozoic basins. Some believe the overlying sediments are the source, with uranium coming from minerals like zircon and monazite. It’s also possible both these sources come into play.
Candidate basins for the next big uranium find should have indications of uranium enrichment in underlying granites. And the presence of uranium-bearing minerals in sediments.
They should also show evidence that uranium has been leached from some pockets of granites and/or sediments. In the Athabasca, large sections of sediment show uranium-bearing minerals have been dissolved away. Creating uranium-rich fluids that formed major ore deposits.
2) Dissolving more uranium. I mentioned last week some special factors that make Namibia’s Rossing uranium deposit a success. Minerals at Rossing are highly fractured. Among other things, this allowed fluids to percolate deep into the Rossing granite. Turning the primary mineral uraninite into the softer, more fragile mineral uranophane.
The increased circulation of fluids in Rossing probably dissolved a lot of uranium. In some places it appears this was re-deposited, enriching the granite.
It’s possible that somewhere in the world a Proterozoic basin is underlain by fractured granite similar to Rossing. This would be a boon for unconformity mineralization. Such a granite would likely allow basin fluids to dissolve more uranium. Creating bigger deposits once this metal is finally dumped out.
Other factors may also help increase dissolved uranium. Evidence suggests that rocks called evaporites may help uranium transport, by adding sulfur to basin fluids. When high levels of sulfur are present, it appears more uranium will dissolve in fluids. And more uranium equals bigger mines. Rocks called carbonates, or rocks rich in chlorine, may also help in the same way.
A final factor that may dissolve more uranium is heat. There is evidence that uranium-rich basins like the Athabasca were unusually hot at depth.
Good basins will have features that help uranium dissolve, such as unusual fracturing or weathering. This may happen in basins deposited in tropical environments (tropical 2.5 billion years ago, not necessarily today), where weathering and alteration would have been particularly intense. Evaporites and evidence of high geothermal gradients are also a bonus.
3) Get those fluids flowing. One of the requirements for unconformity-style deposits is fluid flow. Water must dissolve uranium in one place, and then flow to another where it can deposit the metal.
This can only happen in basins with high porosity and permeability. Sandstones in the Athabasca Basin appear to have been very good pathways for fluid flow. In other basins like the Thelon, large sections of sediments are relatively impermeable. Restricting the formation of uranium deposits.
Good basins will show high porosity and permeability across much of the overlying sedimentary package. Brittle faults may be especially important pathways for fluid flow, as appears to be the case in the southeastern Athabasca. It’s even better if these faults were active for long periods of time, keeping space open. If it doesn’t flow, it’s a no-go!
4) Dumping the metal. The final requirement for a big uranium deposit is a place where dissolved uranium precipitates as ore.
This is mostly controlled by oxidation. Oxygen-rich fluids transport dissolved uranium. When the oxygen is taken away, uranium gets deposited in solid form.
This happens when fluids run into a “reducing agent”. In places like the Athabasca, natural graphite (carbon) in the rocks underlying the basin appears to have acted as the reducing agent. Many Athabasca deposits are centered around graphite lenses.
In other uranium deposits globally, iron acted as the reducing agent. At places like Lagoa Real in Brazil, uranium-rich fluids flowed into rocks rich in the iron mineral hematite. The iron removed oxygen from the fluids, causing uranium to dump out at grades up to 0.35%.
Prospective basins must have reducing agents. Identifiable graphite or iron mineralization is a big plus for the creation of rich uranium deposits.
That’s the scoop. The good thing is much of this information is already available (if you know where to look). It’s just no one has ever taken the time to put it together systematically.
A little legwork along these lines will quickly winnow down hundreds of basins worldwide to a handful that have the “right stuff” for hosting high-grade unconformity deposits. A powerful tool that can be used to generate some “next generation” uranium projects.
I’ll be embarking on this study over the next few months, with the resulting report to follow. Project U rolls on.
Copyright 2009 Resource Publishers Inc.
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