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quote:Originally posted by Bass I labored many years in uranium exploration.
quote:Originally posted by BassDepends on the rock.(there's a definitive answer for you)Some sedimentary rocks have high concentrations of radioactive elements- including carbonaceous shales, phosphates, coal, muds and sandstones. There is still considerable debate on whether or not the uraninite in the Witwatatesrand is sedimentary.Silicic volcanic rocks (and their plutonic equivilents) commonly are enriched in radioactive isotopes, especially potassium.Subduction causes orogeny.
quote:Originally posted by BassThe three most abundant naturally occurring radioactive elements are uranium, thorium and potassium (along with their daughter products). The radioactive isotope of potassium is K40. While K40 only makes up about .01% all potassium, the abundance of potassium in certain rocks means that there will also be a relative abundance of K40. Potassium has an affinity for more silicic igneous rocks (granites and rhyolites), is elevated in most hydrothermal systems and is common in soils and certain sedimentary rocks. Potassium is a common constituent of feldspars and clays.Uranium and thorium, beause of their large atomic size, do not readily combine with other elements to form common rock forming minerals. As such, they tend to accumulate in the more volatile remnants of (especially silicic) igneous rocks- meaning they tend to accumulate in last parts of the magama to crystalize (pegmatites) and in the veins associated with silicic magmas. So you won't find much uranium and thorium in Hawaii or Iceland- it is much more likely in continental igneous rocks. Granitic rocks and rhyolitic volcanic rocks will contain higher than normal crustal abundances. Uranium minerals include uraninite and pitchblende (uranium oxides) as well as several species of uranium vandates and phosphates.Uranium is also very eH sensitive- it goes into solution in oxidizing conditions and precipitates in reducing conditions. Many of the world's economic uranium deposits form "roll fronts" in sandstones. The sandstones are permeable- allowing water to flow through them. As the water flows down-dip, conditions change from oxidizing to reducing and uranium is precipitated at redox boundary. Any organic matter in the sandstones can cause local reducing conditions- uranium minerals commonly replace fossil logs, stumps etc.Uranium is commonly elevated in coal, shales, phosphate deposits- as these sedimentary rocks tend to be found in reducing conditions.This long post just barely scratches the surface of uranium depostion- I labored many years in uranium exploration- but hope it helps.Subduction causes orogeny.