Short guide to uranium deposits - Part 1 (Athabasca basin)

This is a simple description of geology of Athabasca Basin and uranium deposits with aim at investors with limited geology background. Warning: geologists may find it overly simplistic!

What you will learn in this blog post?

  • Simple geology of Athabasca basin

  • What is the unconformity and why it matters?

  • Why faults and sandstone are important?

  • How uranium deposits form?

  • Where does uranium come from?

  • Why are all deposits around edges of the basin?

Geology of Athabasca basin

Athabasca basin is located in the northern Saskatchewan. It is a world class location for uranium deposits, with average grade 20x higher (2% U3O8) than global average grade (0.2% U3O8).

Athabasca basin is filled with with Paleo- to Mesoproterozoic sediments of continental and marine origin, which are 10 to 1000 m thick. Most of Athabasca sediments are red in colour and are often referred to as “red beds”.

How are rock distributed across Athabasca and which rock types are important?

Fig.1 Geological map of Athabasca basin.

Geological map above (Fig 1.) shows how different rock types and uranium deposits are distributed across the basin. Let's dive into different colours and look for correlations to uranium deposits. Yellow colours represent sedimentary rocks of Athabasca Group. For simplicity we will refer to them as sandstones going forward.

Grey colours are the basement rocks. Purple colours are “graphitic” rocks – these are the rocks that are detected by electromagnetic (EM) surveys and referred to as “conductors” in company press releases. They are often associated with uranium deposits. Nevertheless, many of the graphitic “conductors” are barren and host no uranium deposits. In a summary, graphitic rocks increase probability of finding uranium deposit, but are not critical!

Interestingly, there are significant amounts of graphitic rocks in the east part of the basin (Fig.1). These rocks continue underneath sandstone cover and are associated with uranium mines such as McArthur river or Cigar Lake. Another graphitic rich area is located in the NW of the basin around Uranium City.

If you need more geological background on rock types mentioned above check this small guide.

Why unconformity matters?

In Athabasca, the unconformity is a gap in a geological time that separates the basement rocks from sandstones (Fig.2). Uranium deposits tend to form in a close proximity to the unconformity surface. Why is that?

Fig.2 Importance of unconformity in formation of uranium deposits.

Ore metals, including uranium, are transported in a fluid solution. They will precipitate metals, when the right conditions are encountered. This conditions can be set up by: 1) pressure and temperature changes, 2) chemical changes in surrounding rocks, 3) presence of an impermeable cap rock.

In Athabasca basin change in chemistry of the surrounding rocks is the most important mechanism. Unconformity surface facilitates this contrast in rock chemistry between sandstone and basement rocks. The red sandstones of Athabasca basin are oxidizing vs the basement rocks, especially the graphitic rocks are reducing. This sets up chemical contrast for uranium metals to precipitate and form ore deposit.

(Fig.2) In Athabasca basin uranium deposits form along the unconformity surface (A), up to few hundred ms above the unconformity surface (B) and in the basement faults (C).

Why faults and sandstones are important?

We need space in the rock to precipitate uranium ore. This space can be provided by faults and fractures or pores within the sandstone. In case of sandstones this is very similar to hydrocarbon accumulations.

Fig.3 Fractured basement and porous sandstone.

Where does uranium come from?

The most commonly accepted model is that uranium comes from red sandstones of Athabasca basin. Sedimentary basins have active fluid circulation (Fig. 4). Fluids circulating in the Athabasca basin had been leaching uranium from the sandstone what resulted in a concentrated uranium solution. When the fluids encountered right chemical conditions and open space, uranium ore is formed. As a simple rule of thumb you want large, heavily faulted structures to find large uranium deposits.

Fig.4 Schematic profile of conceptual basin illustrating fluid circulation (blue arrows).

Uranium deposits types

Three different uranium deposit classes are present in the Athabasca basin (Fig.5):

1) Engress are located above the unconformity and hosted by sandstone e.g. Cigar Lake.

2) Ingress are located only within the basement rocks. Examples include Arrow and PLS discoveries.

3) Mixed deposits are hosted both within the basement and overlying sandstone e.g. McArthur River.

Fig 5. Uranium deposits types.

Why are all deposits around edges of the basin?

Note how almost all mines and discoveries lie along the margins of the basin.

What controls that?

The depth to unconformity.

Fig 6. Depth to unconformity in Athabasca basin.

The map above (Fig 6.) shows the depth to unconformity. Red colours are shallow parts, blue colours are the deepest parts. You do not want to be deeper than the red-yellow colours (0-300m depths). Note, McArthur River and Cigar Lake mines lie at 500-700 m. These are expensive to operate mines and only justified due to the size of deposits.

In summary: deeper parts will need larger deposits, shallow parts can justify cheaper, even open pit development.

Arrow and PLS discovery proved the concept of exploring outside of the edges of Athabasca basin to find large uranium discoveries. There are also significant uranium occurrences outside of the basin on the map (yellow dots). But don’t you need sandstone to form uranium deposits?

The Athabasca basin was most likely much larger than it is today and sandstones were eroded by glacial processes. Therefore, there is significant potential for uranium deposits outside of Athabasca basin within the basement rocks.

Hope this guide was helpful. In part 2 we will focus on exploration techniques for uranium deposits in Athabasca basin. Part 3 will cover how to interpret the drilling results.

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