Placer uranium deposits

Palaeoplacers are placer concentrations that have occurred in the geological past and which are preserved till the present by a cover of later sedimentary rocks. The Witwatersrand gold-uranium deposits (South Africa) and the uranium deposits at Blind River (Ontario, Canada) and Jacobina (Brazil) are the most important examples of palaeoplacers. 

The bulk of the world s uranium has been produced historically from (1) lower Proterozoic uraninite placer deposits in quartz-pebble conglomerates, (2) epigenetic uranium deposits in sandstones located in many cases at, or near, groundwater oxidation-reduction interfaces and (3) hydrothermal vein uranium deposits. These three distinctly different geologic environments provided most of the uranium that was produced from the 1940s to the early 1970s and they continue to be important exploration targets in the search for new uranium deposits. 

The fossil placer deposits are in fact gold-bearing conglomerates that carry small amounts of PGM, together with gold, uranium and other heavy minerals. However, studies conducted revealed that some of the fossil placer deposits contain about 22 PGM species, including Ir-Os-Ru alloys, sperrylite and isoferroplatinum. 

In the earlier history of the earth, up to perhaps 1.4x 10 years ago, there was almost no oxygen in the atmosphere and uranium oxides could exist at the surface of the earth as grains or nuggets without being oxidized from their 4+ state into the soluble 6+ state. As such, these uranium oxide particles could travel in streams and because of their density they were segregated from less dense materials in streams, in the same way that Au collects in placer deposits. With time these stream deposits were buried, thrust deep into the ground and metamorphosed into the type of accumulation called a quartz... 

Reserves Large resources of yttrium in monazite and xenotime are available worldwide in ancient and recent placer deposits, carbonatites, uranium ores, and weathered clay deposits (ion-adsorption ore). Additional large subeccaiomic resources of yttrium occur in apatite-magnetite rocks, deposits of niobium-tantalum minerals, nonplacer monazite-bearing deposits, sedimentary phosphate deposits, and uranium ores. The world s resources of yttrium are probably very large [39]. 

Coastal and river placer deposits in Guandong and Hainan provinces have been worked for ilmenite and zircon, giving monazite and xenotime as by-products. Owing to the high content of thorium and uranium in the material - giving safety problems - these ores have gradually been replaced by others for the extraction of REMs. 

A large part of the uranium in igneous rocks is contained in heavy chemically resistant minerals the weathering of which is mainly mechanical. Thus, uranium in such form is transported by rivers and streams as clastic particles that are ultimately found in residual soils, in stream sediments and in common sedimentary rocks, either continental or marine. From the viewpoint of ore deposits the previously mentioned particles may accumulate in placer deposits. 

The lanthanides are more abundant in the Earth s crust than many better known metals. The other metals are better known in part because nature has concentrated them in forms that make their recovery economical. The lanthanides are rarely concentrated and even when they are, their chemical beneficiation is usually more costly than for competing metals. In some cases, lanthanides become available as coproducts from mining operations for other metals, e.g., placer tin and titanium deposits, uranium processing liquors. In these operations the burden of mining cost is borne by the other metals. 

---