But why is the uranium confined to the streambed?
The uranium is concentrated by accumulations of organic matter which create a reducing environment within the ground water which carries the uranium salts. This causes the uranium salts to precipitate out. The plant material may be replaced by the uranium bearing minerals or may act as a nucleus to concentrate the ore.
These accumulations of organic material can be found in stream deposits, especially in the cresent shaped deposits formed in the inner curve of a stream, where it is shallow and the stream slows. It is here that the heavier material will settle out into the deposits called "point-bar deposits" because of their shape.
In the region near Grants, these organic deposits are found in the Morrison Formation, the youngest Jurassic Formation in the area. The formation can be dated from the fossils found in the rock. The uranium can be dated using the radioactive dating techniques. The uranium has proven to be much younger than the rock layers in which it is found.
How does this happen?
The formations in which the organic material is concentrated are very porous, and groundwater moves through them easily. The uranium is believed to have come with the intrusive igneous masses that formed the mountains such as Mt. Taylor and with the other lava flows seen in the area. These uranium salts were then dissolved in the groundwater and moved through the subsurface layers. When the organic deposits were encountered, the uranium salts were concentrated into ores of sufficient richness to allow mining if the price was right.
It is this combination of circumstances that has been a strong socio-economic force in this region since World War II.
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