This document provides an overview of uranium roll-front deposits found in sandstone. It discusses the key components required to form these deposits, including a uranium source, transportation system, suitable host rock, and sufficient time. Roll-fronts are redox fronts where oxidizing and reducing groundwaters mix, concentrating uranium. They form sinuous bands within sandstone layers and can stack in complex patterns over multiple sandstone units. Drilling is needed to fully characterize the 3D geometry and grade distribution of roll-front systems for mining purposes.

1. Sandstone Uranium Deposits –
Roll-Fronts, Solution Fronts, Redox Fronts
IAEA Technical Meeting on the
Origin of Sandstone Uranium
Deposits: A Global Perspective
28 May – 1 June 2012
Vienna, Austria
W. William Boberg, Boberg GeoTech International Ltd., Denver, Colorado USA 1

2. Summary Review of Roll-Front History
 Gruner in 1956 described the multiple migration-
accretion process to concentrate uranium in
sediments
 Harshman in 1962 recognized alteration as a guide
to uranium deposits in the Shirley Basin and
published pictures and diagrams of roll fronts being
mined in open pit mines at the time
 Hans Adler in 1964 describes a concept of genesis of
“ore-rolls” for sandstone-type uranium deposits
 Shawe & Granger in 1965 summarize “ore-rolls”
 Bruce Rubin in 1970 describes roll-front zonation
using a diagram that is still in wide use today
2

3. Roll-Front Deposits
 Have been called roll-fronts, “ore-rolls”, solution
fronts, geochemical cells, reduction-oxidation
(redox) fronts
 In reality the process is the mixing of groundwater
fluids of varying chemistry (for example oxidizing
groundwater interfacing with/mixing with a
reducing groundwater)
 May contain a variety of accessory metals (such as
molybdenum, vanadium and selenium)
 Other metals appear to be able to be concentrated
by similar processes but not always by a redox
reaction
3

4. General Roll Front Diagram
Bounding Clay
Altered or Oxidized Sand
Roll-Front Deposit
Altered Tongue
Reduced or
Unoxidized Sand
Bounding Clay
4
Adapted From Harshman (USGS) 1962

5. What does it take to form a Uranium
Roll-Front Deposit?
 A Source of Uranium
 A Transportation System
 Surficial Water Flow (Transports Uranium to Ground Water
Recharge)
 Ground Water Flow
 Regionally Transmissive Host Sandstone (Good Porosity and
Permeability)
 Oxygenated Ground Water
 Focused Ground Water Flux
 Aquacludes (bounding shales / clays)
 Paleochannel systems
 A Trap (A Suitable Host Rock)
 Fluvial or Marine Sandstones
 Regional Chemically Reducing Environment
 Development of Reduction-Oxidation Interface
 Time (Continuity of Favorable Conditions to Build Deposit)
5

6. Postulated Eh-pH Conditions During
Transportation and Deposition of Uranium
Eh
+200 pH
8
0
6
-200
4
-400 6

7. General Characteristics of Uranium
Roll-Front Deposits
 Host Rock - Sandstones with good porosity and permeability
 Genesis – Epigenetic (introduced later than host rock)
 Mineralization
 Mineral is commonly uraninite - UO2 or coffinite – U(SiO4)1-x(OH)4x
 Commonly as amorphous coatings on sand grains and fillings in interstitial
spaces
 Deposits
 Average Grade: 0.05% - 0.25% U3O8
 Size: Economic deposits may contain 2-25+ million pounds U3O8
 Depth: 60 – 600 meters
 Roll-Front Geometry
 Map View
 Narrow: Commonly 3 meters to 40 meters wide
 Long: Continuous for long distances (kilometers to 10’s of kilometers),
not all ore quality along length
 Sinuous: Extreme, often complex sinuousity
 Cross-Sectional View
 Crescent (convex downdip) shape, commonly 2 meters to 8 meters thick
 Commonly in a complex stacked system of multiple roll-fronts 7

8. Model of Formation of a
Roll Front Deposit In Sandstone
Modified from Granger & Warren (USGS), 1978
 Ground Water
 Bulk flow rate of ground water = 58m3/yr thru 1m2
 Ground water velocity = 290 m/year
 Oxygen content = 5 ppm
 Uranium content = 50 ppb
 Sedimentary Unit
 Porosity = 20%
 Hydraulic conductivity = 40m/day
 Rate of advance of roll-front = 1.4 cm/year
 Time Required to Form
 10 km long oxidized tongue = 700,000 years
 10 m wide, 0.28% U3O8 grade deposit = 50,000 years
8

9. Source of the Uranium for
Roll-Front Deposits In Sandstone
The source of uranium for roll-front deposits
in sandstones has been postulated to be the
following:
 Hydrothermal Fluids
 Precambrian Vein Deposits
 Precambrian Granites and Derived Arkose
 Uraniferous Volcanic Tuffs
The last two on the list, individually or in
combination, are the most likely sources for most
roll front deposits in sandstone
9

10. Basic Roll-Front Characteristics
10

11. General Formation of a Roll-Front System
 Uranium source – U-enriched granitic rocks or volcanic tuffs
 Uranium carried in oxidizing surface and ground waters
 Uranium precipitated at reduction oxidation (redox) interface
 Redox interface migrates downdip with continued oxidation and
 precipitation
 The uranium mineralization at the redox interface
 is commonly called a roll-front
From Van Holland, 2010, After Boberg, 1981 11

12. Roll-Front Oxidized Tongue in Plan View
1 – 20 km
After Van Holland, 2010 12

13. Typical Gamma – Resistivity Log Used in
Geophysical Logging of Bore Holes
Mineral Intercepts are
Commonly defined as:
Thickness – Average Grade – Depth / GT
(GT = Grade x Thickness)
Example:
3.1m – 0.12% eU3O8 – 162m / 0.37
GT = 0.37 (3.1m x 0.12%)
13
After Van Holland, 2010

14. Calculation of Equivalent Uranium (eU3O8)
Downhole probe records natural gamma radiation
which is then used to calculate eU3O8
Spontaneous
Gamma – grade calculation curve Single Point Resistivity
2000 cps/10 units calibrated Potential (SP) (Lithology indicator)
Lithology indicator
Depth
Shale
143
0.9m of 0.04% eU3O8 Sand
146 Siltstone
Gamma
Lithology
2.9m of 0.05% eU3O8 Sand Indicator
149 100 cps/10 units
Shale
151 Sand
Shale
153 Shale
Sand
14
From Boberg, 2006

15. Mapping of a Roll-Front
3m – 10m
From Van Holland, 2010 Contour of Ore Grade GT’s 15

16. “Stacking” of Roll-Fronts in Multiple Sands
It is not uncommon for multiple sand layers in a sedimentary
sequence to contain roll fronts.
The complexity of the three-dimensional view of changes in a roll
front combined with the variations in the shape and dimensions of
the altered tongue make mapping of each individual roll front a
challenge, let alone multiple stacked fronts.
After Boberg, 1981 16

17. Mapping of Roll-Fronts in an Upper Sand
Modified From Van Holland, 2010 17

18. Mapping of Roll-Fronts in a Middle Sand
Modified From Van Holland, 2010 18

19. Mapping of Roll-Fronts in a Lower Sand
Modified From Van Holland, 2010 19

20. Composite Mapping Showing the
Complexity of Roll-Fronts in all 3 Sands
Modified From Van Holland, 2010 20

21. Drill Pattern to Test Roll-Front Complexity
Delineating the composite
deposit on a 30m grid
will require 383 holes.
Modified From Van Holland, 2010 21

22. Radioactive Decay Series of Uranium 238
Uranium Equilibrium and Disequilibrium
 The downhole gamma probe measures
natural gamma radiation released by
uranium daughter products – mainly
Bismuth 214, not Uranium
 When uranium and its daughter
products are in secular equilibrium,
chemical uranium will be equal to
gamma equivalent uranium
 Roll front uranium deposits in
sandstones are dynamic systems with
active groundwater flow
 Varying chemical nature of decay
products (Example – Radon is a short-
lived gas easily moved by ground
water) results in each decay product
being dissolved and moved
differentially, resulting in
disequilibrium in the uranium deposit
Modified From Boberg, 2006 and Van Holland, 2010 22

23. Uranium Disequilibrium Evaluations
Graph of multiple sample results
from a deposit comparing
chemical uranium (cU3O8) to
gamma equivalent uranium
(eU3O8) for each sample pair.
Deposit is out of equilibrium in
favor of chemical at higher
grades. Gamma commonly
under-represents high values.
Note: It has been found to be common for higher
grades to be in positive disequilibrium and lower
grades (~<0.04%) to be in negative
disequilibrium
Plot comparing chemical uranium
(cU3O8) to gamma equivalent
uranium (eU3O8) for each sample pair
for a core interval through a deposit.
From Boberg, 2006 23

24. Basic Roll-Front Characteristics
From Van Holland, 2010 (adapted from Rubin, 1970) 24

25. Roll-Front Remobilization
Results in Actual Ore Body Being in a Different
Location than Projected by Gamma Probe Data
Previous redox Current redox
front location front location
Roll-front by gamma log
interpretation (eU3O8)
Roll-front by chemical assay
interpretation (cU3O8)
From Van Holland, 2010 25

26. In Situ Recovery Mining Considerations
Different considerations are necessary to assess amenability
of the deposit for ISL mining than for conventional mining
High grade “limb ore” tied up in mudstones or with organics will not be ISL
mined but will often be considered in conventional mine planning.
Open Pit Zone
0 0
ISL Mine Zone
5 5
10 10
15 15
 Cutoff grade of 0.02% U3O8 (or 0.03% U3O8), minimum GT of 0.09 for depths of
less than 300 meters or minimum GT of 0.15 for depths greater than 1000 feet
(i.e., 5 meters of 0.02% U3O8 above 300 meters could be considered an ISL
addressable resource – depending on deposit disequilibrium studies and
economic sensitivity evaluations and specific deposit characteristics)
 Sand unit must be saturated, porous and permeable
 Uranium must be in porous and permeable sand, not organics or mudstones
 Each production pattern (one production well and four injection wells) covering
a surface area of approximately 450 to 900 square meters, will commonly
address a resource of approximately 5,000 to 10,000 pounds U3O8.
26

27. Shirley Basin Converging Roll-Fronts
27
Photo by Nick Ferris – 1960’s

28. Roll-Front in Pit Wall
From Wyoming Mining Association Website - http://www.wma-minelife.com/uranium/mining/rllfrnt1.html
28

29. Roll-Fronts in Outcrop
Photo from Bill Boberg Collection 29