Jaireth, Clarke and Cross reported in 2010 that significant sandstone-uranium deposits are located in fluvial (or river) sediments filling ancient river channels known as paleochannels. These often occur within buried valley systems cut into bedrock which are known as paleovalleys. This article briefly outlines the geology of sandstone-hosted uranium deposits in paleovalleys and paleochannels in the Lake Frome region (Australia) and Mountain Valley and White Canyon districts (USA). Uranium deposits in the two areas are commonly localized at the confluences and intersections of channels and/or near bends. It is possible that the location of deposits at these sites is caused by several inter-related factors such as the presence of basement scours, predominance of coarse-grained sediments, and abundance of organic material.
The Lake Frome Region
Eocene to Miocene paleochannels and paleovalleys host several uranium deposits in the Lake Frome region. The Eocene paleochannels running generally south-north are filled with Eyre Formation sediments. They incise the Proterozoic to Cambrian basement and are covered by Miocene lacustrine (or lake) and fluvial sediments. The main valleys with channels are generally five to ten kilometres wide and extend for more than 200 kilometres, sometimes joined by smaller tributaries. The general gradient of the channels in the south to north direction varies between 1.3 metres per kilometer to 2.1 metres per kilometer in the Billerroo channel (Ellis 1980).
Monument Valley and White Canyon Districts, USA
The Monument Valley and White Canyon districts near the southeastern border of Utah have produced around 3,900 tonnes of uranium oxide (U3O8) and 4,500 tonnes of vanadium (V2O5) at average grades of 0.32 per cent and 0.25 per cent U3O8 and 0.23 per cent and 0.94 per cent V2O5 (Dahlkamp 1993).
Mineralization is hosted by the Late Triassic Chinle Formation which is 50 to 600 metres thick and consists of fluvial sediments deposited in braided and meandering river channels. The mineralised channels were incised into the Moenkopi Formation sediments by streams flowing generally northward from a highland area in southern or central Arizona and southern New Mexico (figure 4). The sediment infill was derived from the exposed granitic and felsic volcanics in the highlands. An increase in the volcanic activity in the highland area coincided with the deposition of younger sediments in the Chinle Formation (Malan 1968). Prior to the deposition of the overlying Monitor Butte Member, the earlier flood plain and channel sediments were thinned or completely removed by erosion (figure 4).
All major deposits are confined to the thin Shinarump Member of the Chinle Formation with a few extending downwards in the underlying Moenkopi Formation. The ore-bearing Shinarump Member is composed of lenticular beds of sandstone, conglomerate, siltstone and mudstone with abundant fragments of carbonized and locally silicified wood. Uranium deposits are commonly localized at confluences and intersections of channels and/or near bends. This is interpreted to be caused by stronger currents able to cut deep scours in the basement (Young 1964). Within braided channels coarser sediments (sandstone and conglomerate) were deposited where the channel was narrow and gradients high, whereas carbonaceous-rich mudstone was deposited in channels which were broad, meandering and of low gradient. As the gradients lowered the filled scours were covered by layers of silt and carbonaceous mudstone (Malan 1968).
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