AusGeo News March 2010 Issue No. 97
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.
Eocene to Miocene paleochannels and paleovalleys host several uranium deposits in the Lake Frome region (figure 1). 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 kilometre to 2.1 metres per kilometre in the Billerroo channel (Ellis 1980).
The sediments in these channels were sourced from the Proterozoic basement in the Curnamona Province (Ellis 1980). They comprise an interlayered sequence (70 to 80 metres thick) of sands, silts and clays with most of the organic material concentrated in the basal part of the lower sands filling scours in the basement rocks. These sands also contain abundant pyrite.
Uranium occurrences and deposits often occur at bends (Honeymoon and East Kalkaroo) and/or the site of confluences with tributaries (such as Goulds Dam and Oban; figures 1 and 2). The general shape and orientation of the channels is controlled by basement rocks and structures. According to Skidmore (2005), mineralisation at the Honeymoon deposit is located close to a bend where the channel breached a ridge along a fault zone (figure 2).
The Four Mile East deposit and Pepegoona prospect in the northern Lake Frome region, are also hosted by the Eocene Eyre Formation (Heathgate Resources 2009). Although a paleochannel setting for them is not clear at this stage, the location of Four Mile East deposit in a northeast trending valley-like embayment indicates possible similarities with a paleovalley setting.
Mineralisation at the Beverley deposit (figure 3) is hosted by the Beverley Sands unit of the Miocene Namba Formation. The mineralised sequence of sand and mudstone fill a channel into the organic-rich Alpha Mudstone (figure 3). Although anomalous uranium is found throughout the channel, ore zones are located at the bends of the main north-south channel and/or near the points of confluence with tributaries (figure 3). The main channel is up to one kilometre wide and filled with 90 to 170 metres of Miocene sediments overlain by 80 to 150 metres thick cover of Quaternary sediments of the Willawortina Formation.
The shape of the paleochannel is determined by a series of faults, such as the Poontana Fault, and the movement of sediments along it has created a system of valleys and rises. The provenance of infill sediments is not clear, although a general north-south trend of the main channel indicates that the sediments could have been derived from the Proterozoic basement to the south of the channel. Palynological studies record a Late Oligocene to Pliocene age (possibly between around 25 and 6 to 4 Ma or million years; Wulser 2009).
Tabular mineralisation is located predominantly at or near the contact with the underlying Alpha Mudstone, primarily because the mudstone is rich in organic material (plant fragments and carbonised wood). Uranium-Lead dating of coffinite and secondary carnotite defines the age of mineralisation between 5.3 and 3.1 Ma (Wulser 2009). Mineralisation thus seems to have occurred just after the deposition of sediments infilling the Beverley paleochannel.
The Monument Valley and White Canyon districts near the southeastern border of Utah have produced around 3900 tonnes of uranium oxide (U3O8) and 4500 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).
Mineralisation 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 carbonised and locally silicified wood. Uranium deposits are commonly localised 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).
According to Young (1964), radiometric ages indicate that mineralisation was initiated at around 180 Ma and was remobilised several times by ground water flowing through the channel.
This brief comparison of two areas with sandstone-hosted uranium deposits in paleovalleys and paleochannels shows that mineralisation is often located at the bends and/or sites of confluences of the main channel with tributaries, frequently occupying basement scours filled with coarser sediments. In some deposits tabular mineralisation is located at the contact with organic-rich underlying silt and mudstone. This preferred location could be the result of several inter-related factors which include:
The shape and gradients of paleochannels can be used to ascertain the location of the source of sediments and the direction of sediment transport. They can also assist in determining the location of possible source rocks of uranium and the direction of fluid flow of uranium-bearing fluids. In general, uranium-rich fluid will flow along the channel, however if the sandstone uranium system is generated after the channel is filled and covered by younger sediments, fluid can flow across the channel system. It is important to establish the direction of fluid flow at the time of mineralisation because it can help to determine the location of oxidation-reduction front as well as the ore zone within the aquifer.
This summary of sandstone-hosted uranium deposits in paleovalleys and paleochannels shows that mineralisation is often located at a number of specific sites within the paleochannels. Consequently exploration of such deposits will benefit by focusing on the following features of the system:
Roger Skirrow, Ian Roach and Dean Hoatson reviewed the paper. Theo Chiotis provided cartographic and drafting support.
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