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Review

Geometallurgy of Australian uranium deposits

M.I. Pownceby ⁎, C. JohnsonCSIRO Process Science and Engineering, Bayview Avenue, Clayton, Victoria 3168, Australia

a b s t r a c ta r t i c l e i n f o

 Article history:

Received 12 February 2013Received in revised form 26 June 2013Accepted 5 July 2013

Available online 14 July 2013

Keywords:

GeometallurgyUraniumAustraliaOre deposits

Australian uranium ores are often composed of complex mineral assemblages. Differences in ore compositions and textures are seen between deposits as well as within a single deposit, which can host a range of ore types. Such a wide variety of uranium ores make it impossible for a single extraction or treatment process

to be developed that will accommodate all of the ores. From a mineralogical perspective, key issuesconfronting the Australian uranium mining industry include: the prevalence of low grade ores; a lack of detailed chemical and mineralogical information (uranium speciation, texture, grainsize) for the various ore deposit types; and the presence of refractory uraniumbearing minerals and highly acidconsuming gangueminerals. This paper reviews some of the main controls on uranium geometallurgy by linking concepts relating to ore genesis and the resulting ore mineralogy, with the processing behaviour of specic Australian uranium ore types. Emphasis is placed on the value of detailed ore mineralogical analysis and the insight thisprovides into the factors of importance when considering uranium extraction.

Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

2. Geochemistry of uranium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263. Uranium ore deposit genesis models —  classication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274. Geometallurgical properties of uranium ores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4.1. Uranium mineralogy  —  composition and reactivity during processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294.2. Ore–gangue mineral associations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4.2.1. Quartz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294.2.2. Clays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294.2.3. Carbonates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304.2.4. Gypsum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304.2.5. Salts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304.2.6. Carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304.2.7. Sulphides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314.2.8. Phosphates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314.2.9. Iron oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

4.3. Physical ore properties: grainsize, texture and grade . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315. Uranium deposits in Australia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

6. Geometallurgy of Australian uranium deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336.1. IOCGU (or breccia complex) deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

6.1.1. Processing options for IOCGU (breccia complex) ores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356.2. Unconformityrelated uranium deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

6.2.1 . Processing options for unconformityrelated uranium ores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 386.3. Surcial (calcretehosted) deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

6.3.1. Characterisation of surcial calcrete ores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396.3.2. Processing options for calcretehosted surcial uranium ores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Ore Geology Reviews 56 (2014) 25–44

⁎  Corresponding author. Tel.: +61 3 95458820; fax: +61 3 95628919.E-mail address: mark.pownceby@csiro.au (M.I. Pownceby).

01691368/$ –  see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.oregeorev.2013.07.001

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6.4. Sandstonehosted uranium deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 406.4.1. Characterisation of sandstonehosted uranium ores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 416.4.2. Processing options for sandstonehosted uranium ores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

6.5. Metasomatite uranium deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 416.5.1. Characterisation of metasomatite deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 416.5.2. Processing options for metasomatite uranium ores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42A c k n o w l e d g e m e n t s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

1. Introduction

Geometallurgy is a rapidly expanding area of ore geology involving the detailed characterisation of the geological and mineralogicalcharacteristics of an ore. Such an assessment is crucial for quantifyingthe material properties relevant to optimising processing performance and resource extraction. For example, the amount of uraniumthat can be mined and recovered as a marketable product, and thecosts of mining and processing, are two key factors that determinewhether a uranium deposit can be exploited protably. Both factorsare a complex function of material properties (e.g. ore hardness, de

gree of liberation, susceptibility to leaching and   otation) that areprincipally determined by the uranium ore mineralogy (Bowell etal., 2011; Lottering et al., 2008; Stewart et al., 2000).

Uranium ores often consist of complex assemblages of minerals thatdiffer widely in composition and texture, such that no two deposits arethe same no matter how similar the formation setting may have been(Cuney, 2009; Cuney and Kyser, 2009; Dahlkamp, 1989, 1993). Thesedifferences make it dif cult for a uranium extraction process to beoptimised to accommodate all the possible variations (Merritt, 1971).It is also unlikely that any ore specic treatment can be transferred indetail from one ore to another. The mineralogy of an ore controls, inpart, the metallurgical characteristics exhibited during processing.Therefore, when assessing the viability/potential of an ore to be minedand processed, the following information is usually obtained: (a) the

speciation of uranium in the ore, (b) the associatedgangue mineralogy;(c) the degree of comminution required to affect liberation of theuraniumbearing mineral and the potential for its separation fromgangue minerals and;(d) the nature of the lixiviant required for extraction (e.g. acid, alkali, oxidant) and the potential level of reagent consumption. Of these, variations in the mineralogy (a and b) togetherwith some assessment of the likely behaviour of the different mineralassemblages under specic process environment conditions, are necessary factors to determine when deciding on the best process treatmentfor recovering uranium from an ore (Adams, 2007; Pownceby et al.,2011; Walters, 2011).

This paper briey reviews the mineralogy and geochemistry of uranium, the main types of uranium ore deposits (classication schemes)and some of the more important mineralogical controls on uraniumgeometallurgy. Concepts, relating ore genesis and the resulting mineralogy of specic Australian uranium ore types with their potential impacton processing behaviour, are then discussed through case studies involving selected uranium ore types. Throughout the latter section, emphasis is placed on the value of the information obtained from highresolution, detailed ore mineralogical analysis.

2. Geochemistry of uranium

Uranium is widely distributed in the Earth's crust with estimatesof its abundance varying between 2 and 4 ppm (Fleischer, 1953;

Merritt, 1971), being of similar abundance to molybdenum, arsenic,tungsten and mercury. Table 1 gives approximate orders of magnitude (in ppm) of uranium abundance in some common crustal rocktypes.

Oxidation states of 3+, 4+, 5+, and 6+ are known for uranium(Hanchar, 1999). Of these, the 4+ and 6+ valence states have the required thermochemical characteristics such that pH and Eh conditionswithin the Earth's crust preferentially stabilise one or the other valenceform (Hanchar, 1999). The resulting ions associated with these valencestates have very different properties. For example, the uranous ion(U4+) has low basicity and forms an oxide insoluble in dilute acid andsalts that hydrolyse readily. The U4+ cation with eightfold coordinationand ionic radius ~1.00 Å is similar to that of the likecharged Th 4+

(eightfold coordination, ~1.05 Å) and ~Zr4+ (eightfold coordination,

0.84 Å) ions (Shannon, 1976) resulting in the chemical behaviour between these ions being so alike that uranium is often found associatedwith these elements (Burns, 1999). In comparison, hexavalent uranium(U6+) typically forms the uranyl ion UO2

2+, the oxide of which is highlysoluble in acid and alkaline solutions. It readily forms compounds withother metal anions and cations, thus forming a wide range of complexminerals (Mandarino, 1999). The large size of the uranyl ion alsomeans that it is not readily displaced, or replaced, by isomorphous substitution, resulting in crystalline minerals that are generally unique uranium compounds. For a comprehensive list of the structures andchemical formulas of uranium minerals, the reader is referred to  Burns(1999), Finch and Murakami (1999) and Krivovichev et al. (2006).

Uranium is a lithophile element and therefore has an af nity for silicates. Tetravalent, primary uranium minerals (Table 2) are typically

present in small to trace amounts in igneous rock types although in

 Table 1

Average uranium concentrations in geologic materials.Adapted from data in  Gupta and Singh (2003)   and  Kyser andCuney (2009).

Reservoir/rock type U (ppm)

Igneous rocksPeridotite, dunite 0.003–0.05Eclogite 0.013–0.8Average basalt 0.3MORB basalt 0.07–0.1Continental andesite 0.5–1.0Island arc andesite 0.2–0.4

Average granodiorite 2.0Average granite 3.8Nepheline syenite 200–600

Sedimentary rocksBlack shale 3–1250Sandstone 0.45–3.2Average carbonate 2.2Marine phosphate 50–300Evaporite 0.01–0.43Chert 2.0

Metamorphic rocksAverage quartzite 1.5Average marble 0.5Average slate 2.5Average schist 2.0Average gneiss 3.0

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general, most uranium is present as an impurity element substitutedwithin minor accessory minerals (Table 3). In either form, however,the presence of uranium in igneous rocks seldom occurs in concentrations or tonnages great enough to be exploitable, except in the case of pegmatites (e.g. Lentz, 1996). To generate exploitable deposits, uranium remobilisation mechanisms are necessary. Uranium is transportedin oxidising, acidic or alkaline hydrothermal   uids (e.g.  Romberger,1984). Mobilisation is controlled by oxidative processes as the tetravalent species, being almost insoluble under ambient pH and Eh conditions, requires oxidation to the hexavalent form. The deposition of uranium out of hydrothermal  uids results from redox controlled oxidation–solution–reduction–precipitation processes operating in a variety of environments   —   magmatic, metamorphic, metasomatic andsedimentary (e.g.   Cuney and Kyser, 2009; Fayek and Kyser, 1997;Hoeve and Sibbald, 1978; Romberger, 1984; Skirrow et al., 2009).

There are other sources of uranium where the element is present

as a minor constituent and does not generallyoccur in discrete or specic uranium minerals. For example the occurrence of uranium inseawater (Seko et al., 2003), in phosphate deposits (5–200 ppmMerritt, 1971; Cathcart, 1978; Chernoff and Orris, 2002), in carbonaceous sediments (Eakin and Gize, 1992; Landais and Gize, 1997;Vine, 1962), and porphyrycopper arc systems (Dahlkamp, 2009;Lanier et al., 1978) are all well known. As well, uranium in solubleform in groundwater is present in signicant amounts in some locations (e.g. Hem et al., 1993; Hess et al., 1985).

3. Uranium ore deposit genesis models — classication

Uranium mineralisation occurs in a number of different igneous,hydrothermal and sedimentary environments. The classication of 

uranium ore deposits is therefore subject to a number of complications (Cuney, 2009; Dahlkamp, 1978, 1989, 1993; Heinrich, 1958;Mashkovtsev et al., 1998; McKay and Miezitis, 2001; Mickle andMathews, 1978; Nash et al., 1981; Petrov et al., 1995, 2000; Ruzicka,1971; Skirrow et al., 2009; Stoikov and Bojkov, 1991) chiey due todiffering views on whether to emphasise descriptive features of themineralisation such as host rock type and orebody morphology, genetic aspects, or metallogenic aspects (e.g., see discussions byCuney, 2009; Dahlkamp, 1993). The former approach was adoptedby the IAEA in their widely used classication scheme (OECD/NEAIAEA, 2012) in which  fteen uranium deposit types are distinguished. The classication of each deposit type is based on the geological setting and using a set of criteria that includes; the host rockassemblage, tectonic setting, structural relationships, zoning, alteration, uranium mineral phases and the respective ages of uraniummineralisation compared with that of the host rock. The  fteen de

posit types, designated by the IAEA in order of their approximateworld economic importance, are as follows; (1) unconformity related,(2) sandstone, (3) breccia complex, (4) quartzpebble conglomerate,(5) vein (granite related), (6) intrusive, (7) volcanic and caldera related, (8) metasomatite, (9) surcial, (10) collapse breccia pipe,(11) phosphorite, (12) metamorphic, (13) limestone and palaeokarst,(14) uranium coal, and (15) other types (including black shales andrare metal pegmatites).

Dahlkamp (1993) further subdivided these 15 deposit types intothirty subtypes and thence into classes, while   Plant et al. (1999)regrouped the types listed by the IAEA into three associations in recognition of the shared geological settings among groups of uraniumdeposits i.e. igneous (plutonic and plutonic and volcanic), metamorphic and sediment/sedimentary basin associations. More recently,

Cuney (2009) proposed a genetic classication based on uranium deposit formation conditions through the geological cycle, outlining deposits formed by surface processes, synsedimentary deposits,deposits related to hydrothermal processes, and deposits related topartial melting and crystal fractionation. The Cuney (2009) classication model is illustrated in Fig. 1. A similar classication scheme wasproposed by Skirrow et al. (2009) that instead emphasised the similarities between the processes which form the various uranium deposittypes (Fig. 2). This was dubbed a  ‘mineral systems’ approach, a key feature being that all previous classication schemes could be simpliedtot within and between, three endmember uranium mineralising systems: (1) magmaticrelated, (2) metamorphicrelated, and (3) basinand surfacerelated.

The mode of formation of many deposits is generally well under

stood, and thus their classication is unequivocal within the multitude

 Table 2

Economically important uranium minerals (Edwards and Oliver, 2000; Frondel, 1958; Frondel and Fleischer, 1955; Mandarino, 1999 ).

Mineral type Mineral name Formula Typical % U

Tetravalent uranium minerals (U4+)Oxides Uraninite (U1 − x

4+ ,Ux6+)O2 + x [ideally UO2] 46–88

Pitchblende UO2 amorphous 86–88REE ± Ti ± Feoxides Brannerite (U,Ca,Y,Ce,La)(Ti,Fe)2O6   26–44

Betate (Ca,U)2(Ti,Nb,Ta)2O6(OH) 15–24Davidite (La,Ce)(Y,U,Fe)(Ti,Fe)20(O,OH)38   1–6

Orthobrannerite U2Ti4O12(OH)2   ~53Silicates Cof  nite U(SiO4)1 − x(OH)4x   40–60

Uranothorite (U,Th)SiO4   b10

Hexavalent uranium minerals (U6+)Phosphates Autunite Ca(UO2)2(PO4)2·10–12H2O 48–50

Metaautunite KCa(H3O)3(UO2)7(PO4)4O4·6–8H2O ~57Torbernite Cu(UO2)2(PO4)2·8–12H2O 47

Vanadates Carnotite K2(UO2)2(VO4)2·1–3H2O 53–55Tyuyamunite Ca(UO2)2(VO4)2·5–8H2O ~52

Silicates Boltwoodite HK(UO2)SiO4·1.5H2O ~55Sklodowskite (H3O)2Mg(UO2)2(SiO4)2·4(H2O) 49–54Uranophane Ca(UO2)2(SiO3)(OH)2·5H2O ~41

 Table 3

Typical uranium contents of accessory minerals following isomorphic substitution incrystal lattice.After de Voto (1978).

Mineral ppm U

Allanite  —  (Ca,Ce)2(Fe+2,Fe+3)Al2O.OH[Si2O7][SiO4] 30–1000

Apatite  —  Ca5(PO4)3(OH,F,Cl) 5–100Epidote  —  (CaFe+3)Al2O·OH[Si2O7][SiO4] 20–200Garnet  —  Ca3Al2Si3O12   6–30Ilmenite  —  FeTiO3   1–50Magnetite  —  Fe3O4   1–30Monazite  —  (Ce,La,Th)PO4   500–3000Titanite  — CaTi[SiO4](O,OH,F) 10–700Xenotime  — YPO4   300–35,000Zircon  —  ZrSiO4   100–6000

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of classication schemes which exist. However, a number of deposittypes (e.g. IOCGU) require further investigation and are the subject of continued debate (Cuney and Kyser, 2009). In some cases, agreementregarding genesis may never be reached as questions still remain with

respectto the provenanceof ore formingand relateduids and theconditions of uranium concentration, mobilisation and redeposition.

Consequently, the classication of some deposits remains problematic.Nonetheless, a classication method to group the Australian uraniumdeposits considered in this paper is essential. The authors have givenpreference to the IAEA classication system (OECD/NEAIAEA, 2012)

as the IAEA system is widely used and accepted by industry workersand academic researchers (e.g., Cuney, 2009; Cuney and Kyser, 2009;

Fig. 1. A uranium ore deposit classication scheme based on geological formation environments, after the Cuney 2009 model. Uranium deposit types from the IAEA Red Book classication are annotated by number, these correspond with the order of economic importance to Australia (McKay and Miezitis, 2001): 1) breccia complex (IOCGU); 2)unconformityrelated; 3) sandstone; 4) surcial (including calcrete); 5) metasomatite; 6) metamorphic; 7) volcanic; 8) intrusive; 9) vein; 10) quartzpebble conglomerate; 11)collapse breccia pipe; 12) phosphorite; 13) lignite; 14) black shale. Emphasis is placed on the processes involved in the formation of uranium deposits.

Meteoric water, basin brines

Metamorphic fluidsMagma &

magmatic fluids

Magmatic-related

genesis processes

(7, 8, 9)Metamorphic-related

genesis processes

(6)

Metasomatic fluids

(5, 9)

Mixing

   M   i  x   i  n

  g 

M   i   x  i   n   g  

Basin- & surface-related

genesis processes

(10, 12, 13, 14)

(4)

(3)

Connate fluids

Diagenetic fluids

(2)

(7)

Shallow

breccia complex

(1)

Deep

breccia complex

(1)

Hybrid processes

(1, 5, 9, 10, 11)

Meteoric water, basin brines

Metamorphic fluidsMagma &

magmatic fluids

Magmatic-related

genesis processes

(7, 8, 9)Metamorphic-related

genesis processes

(6)

Metasomatic fluids

(5, 9)

Mixing

   M   i  x   i  n

  g 

M   i   x  i   n   g  

Basin- & surface-related

genesis processes

(10, 12, 13, 14)

(4)

(3)

Connate fluids

Diagenetic fluids

(2)

(7)

Shallow

breccia complex

(1)

Deep

breccia complex

(1)

Hybrid processes

(1, 5, 9, 10, 11)

Fig. 2. A uranium classication scheme, after Skirrow et al. (2009). Numbers in parentheses indicate the equivalent uranium deposit types from the IAEA Red Book classication areannotated by number, these correspond with the order of economic importance to Australia (McKay and Miezitis, 2001): 1) breccia complex (IOCGU); 2) unconformityrelated; 3)sandstone; 4) surcial (including calcrete); 5) metasomatite; 6) metamorphic; 7) volcanic; 8) intrusive; 9) vein; 10) quartzpebble conglomerate; 11) collapse breccia pipe; 12)

phosphorite; 13) lignite; 14) black shale.

28   M.I. Pownceby, C. Johnson / Ore Geology Reviews 56 (2014) 25–44

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Plant et al., 1999). As well, the IAEA classication scheme is a descriptive, typological classication with an emphasis on geological settingand ore characteristics  — the latter is of paramount importance whenconsidering metallurgical processing of each ore type.

4. Geometallurgical properties of uranium ores

The mineralogical variability between different uranium ore types

means that the ore processing requirement (e.g. acidic versus alkalineleaching, preconcentration methods, etc.) is a reection of the differentgenesis modes and resulting styles of mineralisation. An understandingof the geological setting in which a deposit has formed, in addition to anindepth knowledge of the ore characteristics, enables ore properties tobe determined and processing dif culties to be foreseen, planned forand potentially circumvented. Furthermore, increased awareness of limitations, which leads to a series of logical, calculated decisionsbeing taken, results in the most cost effective processing method beingimplemented.

A number of independent properties contribute to the overall performance and response of a uranium ore to processing. The main onesto consider in a geometallurgical investigation are; i) the compositionand reactivity in process solutions of the main uranium mineral spe

cies, ii) the ore–

gangue mineral associations, and iii) the deportment(i.e. grainsize and concentration) of the uranium mineralisation.

4.1. Uranium mineralogy— composition and reactivity during processing 

The most common uranium ore minerals encountered during oreprocessing are primary, tetravalent species; uranium oxides (pitchblende and uraninite); uranium silicate (cof nite); and the uranium–REE–Ti ± Feoxide phases (brannerite, davidite, orthobrannerite andbetate) (Table 2). In comparison, secondary uranium minerals arecommonly hydrated and contain uranium in the oxidised, hexavalent(U6+) form. For the most part, uranium deposits are dominated by primary uranium phases and contain lesser amounts of secondary minerals. When present, secondary (U6+) minerals such as autunite,

torbernite, uranophane and boltwoodite are found associated with theoxidised/altered regions of deposits (Kyser and Cuney, 2009). Thereare exceptions where deposits are dominated by secondary minerals,examples include; surcialcalcrete depositswhere carnotite is the principal ore mineral, e.g. Yeelirrie deposit in WA (Carlisle, 1980, 1983;Mann and Deutscher, 1978; Mann and Horwitz, 1979); hydrothermaldeposits of boltwoodite, e.g. the Swakopmund district of Namibia, eastof the Rossing Mine (Kinnaird and Nex, 2007; Marlow, 1981; Nex,1997); and continental phosphorite deposits containing surcial lensformations of concentrated autinite and tobernite, e.g. Bakouma depositin Central African Republic (Bowie, 1979; IAEA, 2009; Notholt, 1980).

The susceptibility of uranium minerals to acid or alkaline processsolutions varies widely and it is essential to characterise the chemistry of the uranium species in order to select the most ef cient method

of processing. The theoretical composition of uraninite, UO2, does notexist naturally, with most natural uraninites containing signicantlevels of cationic impurities (e.g. Ca, Si, Pb, REE) and most having undergone partial oxidation as a result of chemical alteration(Alexandre and Kyser, 2005; Berman, 1957; Fayek and Kyser, 1997;Finch and Ewing, 1991; Finch and Murakami, 1999; Frondel, 1958;

 Janeczek and Ewing, 1992; Ram et al., 2013). Under ambient conditions, uraninite dissolves most ef ciently in an acid solution withthe addition of an oxidant (Laxen, 1971; Merritt, 1971; Nicol et al.,1975); whereas an alkaline leach solution is usually less effective indissolving uraninite unless a strong alkaline reagent combined withhigh temperatures is used (Gupta and Singh, 2003).   Ram et al.(2013) have recently demonstrated that the presence of minor impurities in uraninite has a signicant impact on rates of uranium extrac

tion compared to stoichiometric UO2. By comparison, pitchblende

(amorphous UO2) dissolves readily in both acid and alkaline solutionswith low reagent consumption.

Cof nite is poorly soluble in dilute acid or alkaline solutions andrequires an intermediate oxidising step or a more highly concentrated solution to encourage extraction (Merritt, 1971).

The most dif cult uranium ores to leach are those containing minerals of the multipleoxide type, most commonly found as branneriteand davidite. Brannerite is represented by the chemical formula,

U

4+

Ti2O6. Although the uranium in brannerite is nearly always partlyoxidised and sometimes hydrated, it undergoes extensive substitutionwith other cations (Finch and Murakami, 1999), and is usually metamictdue to the destruction of crystallinity induced by alpha radiation decayfrom theconstituenturanium (Lian etal., 2002; Smith,1984). Thechemistry of natural brannerites has been previously reported by Hess andWells (1920),   Pabst (1954),   Hewett et al. (1957),   Lumpkin et al.(2000), Colella et al. (2005) and Charalambous et al. (2012). Uraniumextraction from brannerite is typically achieved using an intermediateoxidation step with more highly concentrated, hot acid leach solutions.Alkaline solutions are unsuitable as brannerite is poorly soluble in thismedium. Davidite is also refractory and may require   ne grinding,prolonged leaching in hot acid or leaching at elevated temperature inan autoclave to achieve satisfactory extraction (Lunt et al., 2007).

Chemically, the extraction of oxidised, hexavalent uraniummineralsis relatively straightforward, as these minerals are readily soluble inboth acid and alkaline leach solutions under ambient conditions. An oxidant may be needed however, to prevent reduction caused by the presence of other species or ferrous iron introducedthrough grinding mediaor present in the ore (Lunt et al., 2007).

4.2. Ore– gangue mineral associations

Gangue minerals may react with the leach solution(s) used to process uranium ores, obstructing the chemistry and increasing the costof uranium extraction. The following identies some of the commongangue minerals associated with uranium ores and their associatedore processing complications.

4.2.1. Quartz Quartz is a major component in sandstonehosted uranium de

posits but can also be present in signicant amounts in other deposittypes (e.g. vein, metasomatite, quartz pebbleconglomerate). It is generally considered an unreactive mineral phase in uranium extractionprocesses (Merritt, 1971). However, if uranium ore minerals are present as inclusions within quartz, it becomes challenging to extract theuranium from these ores as leach solutions (acid or alkaline) do notreact with quartz. Ores containing this style of mineralisation typically require  ne grinding to liberate the uranium and increase surfacearea exposure to the leach solution.

4.2.2. Clays

Almost every uranium deposittype setting has one or more litholo

gies that contain clay minerals. These largely result from the weatheringof granite terrains and the presence of argillaceous phases in host sediment units. Clay minerals exhibit a tendency to form aggregates producing a cement that commonly encases coexisting micronsized uraniumminerals. Such textures are particularly common in sandstonehostedand surcial calcrete deposits. The ability for a particular leach solutionto access the uranium minerals is dependent on the porosity of the aggregate accumulation and/or whether a pathway exists to act as aleaching conduit (e.g. a microfracture network).

Other problems are caused by clays consuming excessive quantities of leach solution due to solid–liquid reactions (Carroll andStarkey, 1971; Huay and Keller, 1971; Ozdemir and Kipcak, 2004) aswell as the effect of clay mineral surfaces becoming positively or negatively charged (dependent on pH). For example, a positively charged

mineral surface, formed under low pH conditions in an acid leach

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solution system, will attract and adsorb negatively charged uranylcomplexes produced during acid leaching (e.g. UO2(SO4)2

2−). In contrast, a negatively charged clay mineral surface will attract positivelycharged uranyl ions (e.g. UO2

2+), which form in both acid and alkalineleach solution systems. The ability of clay species to adsorb uraniumions or uranyl complexes from the leach system reduces the amountof uranium able to be extracted.

An additional problem for processing clayrich uranium ores, is

that clay mineral surfaces are readily wetted resulting in slurry formation (e.g. Lockhart, 1983). This has a tendency to increase the viscosity of the pregnant solution, which causes poor agitation in stirredtanks, poor solid–liquid separation, clogs pumps and leads to blockages in the processing circuit(s). These factors contribute to metalbeing lost to tailings, slower production rates (Lower et al., 2011;Ritcey, 1980) and ultimately, uranium extraction being a costlyprocess.

4.2.3. Carbonates

Carbonate minerals (calcite, dolomite, magnesite, ankerite andsiderite) are primarily associated with sedimentarybased uraniumore settings (e.g. surcial deposits), skarn deposits (e.g. metasomaticdeposits) and mineralised veins. Carbonate minerals readily reactwith and consume acid, reducing the availability of leachant. It hasbeen estimated (Merritt, 1971) that above about 15% CaCO3   in anore, the cost of using an acid leachant would be prohibitive and an alternative processing route would be required. Furthermore, differentcarbonate species have different solubilities in acid (e.g. some dolomites and siderites react relatively slowly). Identifying the composition of the carbonate phases present in an ore is therefore crucial inorder to model the expected loss of acid and evaluate processingcosts. Ores containing a high carbonate content may, however, be effectively treated using an alkaline leach solution, typically sodiumcarbonate or sodium bicarbonate (Connelly, 2008; Merritt, 1971;Seidel, 1981).

4.2.4. Gypsum

Gypsum may be present in the gangue mineralogy of uranium de

posits that form in sedimentary environments under arid climateconditions (e.g. surcial calcrete deposits). The presence of gypsumis a processing hindrance when an alkaline leach solution is used asit reacts to precipitate calcium carbonate and form sodium sulphatein solution (IAEA, 1990; Kennedy, 1967; Ritcey, 1980). Bowell et al.(2011)  report that a gypsum content of more than 4% in surcialcalcrete uranium ore will lead to excessive reagent consumption

and suf ciently affect the processing costs to rule out the use of sodium carbonate leach solutions.

4.2.5. Salts

Common salt minerals such as halite, bischote, epsomite,hexahydrite and mirabilite,are a frequentmineral component associatedwith many sedimentary basins around the world often forming thinwidespread layers interbedded with sediments (Heard, 1972). They are

often present as a minor component in uranium ores formed in lagoonalor evaporitic sedimentary environments (e.g. cupriferous sandstonetypeuraniumdeposits in theCatskillformation, Pennsylvania— Glaeser, 1974;Rose, 1976), in unconformity related uranium deposits (e.g. AthabascaBasin, Canada   —  Cumming and Krstic, 1992; Fayek and Kyser, 1997)and surcial calcretehosted uranium deposits (e.g. within the YilgarnBlock, Western Australia   —  Mann, 1983; Mann and Deutscher, 1978).Salts such as halite affect the processing of uranium ores as they reactwith theleach solution causing the salt complex to dissociate and releasechloride ions. This reduces the availability of adsorption sites on ion exchange columns or resins, consequently, reducing the ef ciency of uranium recovery (Venter and Boylett, 2009).

4.2.6. Carbon

Carbon (as organic material) may be present in the mineralogy of sedimenthosted and lignite uranium ores, the result of plant materialor/and organic debris incorporated during sediment deposition andsubsequent lithication (Breger, 1974; Ritcey and Wong, 1985;Vine, 1962). Mohan et al. (1991) provided evidence that uranium ina lignite deposit occurred principally (70–90%) in the form of uranylhumates and the rest in the form of poorly crystallised mineral(s).Elsewhere,  Meunier et al. (1990) determined that in carbonrich, bituminous deposits, uranium is mainly   xed in organics asorganouranyl compounds. Carbonaceous uraniumbearing phasesare strongly refractory and dif cult to extract and often require physical beneciation (negrinding) and/or pretreatment (calcining) forextraction to be successful (Hurst, 1976; IAEA, 1980; Lunt et al.,2007; Ritcey, 1980). The removal of carbonaceous material (when

present as a minor or trace component of the gangue mineralogy)has been shown to improve the porosity of the ore (IAEA, 1980).

Depending on solution conditions carbon, in certain forms such asgraphite, has the potential to form a charged surface. The chargedparticles adsorb uraniumbearing ions from the leach solution causing pregrobbing in a similar fashion to clays and thus impede uranium extraction.

 Table 4

Separation criteria and related process equipment ( IAEA, Vienna, 1993).

Separation basis Separation devices Ore property requirements

Radioactivity Radiometric sorting   •  Adequate heterogeneity of uranium minerals within the ore•  Relatively coarse fragmentation•

 Presence of radiometric equilibrium between uraniumand radium in the oreOptical pro perti es Ph otometric sorting   •  Opaque uranium ore and translucent gangue mineral phasesSize/shape Screens, cyclones   •  Differences between the hardness and fragility of the ore and

gangue minerals.•  These differences can correlate with specic size fractions.

Density Dense media separation, tables, jogs, spirals, cones   •  Uranium minerals must be coarse.•  Resist breakage, sliming or association with gangue minerals

that may be separated by a density differential.Paramagnetism Wet high intensity magnetic separators   •  Most uranium minerals are unsuitable for magnetic separation

because the magnetic properties of the ore minerals are notsuf ciently dissimilar from those of the gangue minerals.

•  The particle size of uranium minerals affects the successof this separation technique.

Surface properties Flotation   •   Complications when  oating uranium phases as the tailingsproduced are not discardable.

•  Used to remove sulphide, carbonate or carbonaceousmaterial so to increase overall ore grade.

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4.2.7. Sulphides

The behaviour of sulphide phases during uranium processing is dif cult to predict. Often the presence of sulphides indicates that more oxidant may be required and as a consequence, results in higher reagentconsumption. An advantage of sulphides however, is that the ore is likelyto respond to rapid weathering and bacterial leaching in stockpiles (e.g.Brierley, 1984; Harrison et al., 1996; Lowson, 1975). Lowgrade ores containing sulphides are therefore strong candidates for bacterial heapleaching.

4.2.8. Phosphates

Thepresence of phosphatesin an ore can increase thevolume of acidrequired for leaching, owing to the solubility of certain phosphates inacid. In addition, phosphate that enters solution can complex with anyferric ions present and inhibit the oxidation process. Another effect isthatonce in solution, phosphate ionscan cause reprecipitation of uranium if the pH of the leach system is not maintained below 2.0. The extraction of uranyl ions by ion exchange is hindered by the presence of phosphate as both ions compete for adsorption sites (Kennedy, 1967).

4.2.9. Iron oxides

These are the prevalent gangue minerals in IOCGU deposits but aretypically present to a greater or lesserextent in all uranium ores. The ox

ides, along with ferromagnesium minerals, introduce ferric ions to theleach system which promote the oxidationreactionfavouring theextraction of uranium (e.g. Ragozzini and Sparrow, 1987). The presence of ironoxides can however, lead to pregrobbing, as the charged mineral surfaces attract UO2

2+ ions (Bruno et al., 1995; Ritcey and Wong, 1985).

4.3. Physical ore properties: grainsize, texture and grade

Uranium ore grades are typically dened as: low grade,  b 0.15% U;medium grade, 0.15–0.50% U and high grade,   N0.50% U (Dahlkamp,2009). Despite there being multiple genesis models, one commonality is that uranium mineralisation is often   negrained (~50  μ m tob5  μ m), intimately associated with gangue minerals, and disseminated widely throughout the host rock. Thus, the ability to upgrade the

ore through the separation of uranium species and removal of ganguehas several potential advantages. These are: i) to enhance the ore feedgrade, ii) to remove minerals that will be deleterious to the uraniumleaching and/or recovery and, iii) to produce clean tailings that can berejected without creating environmental hazards. Preconcentrationprocesses often employ differences in mineral properties such as radioactivity, size, shape, density and surface characteristics to separatethe desired minerals. The selection of a process is usually made basedon consideration of the physical properties as outlined in  Table 4.

Physical separation to improve the ore grade and properties for processing has been largely successful, except in cases where the uraniummineralisation is disseminated asnegrained inclusions within gangueminerals (Seidel, 1981). An alternative method to maximise uraniumextraction is to increase theexposure of theore minerals to thelixiviant.This may be achieved by processing the ore to improve mineral liberation. As with any ore, coarsegrained mineralisation is easier andmore cost effective to process. However, for uranium ores coarsemineralisation is uncommon and grinding is required to increase thesurfacearea exposure of theuranium mineral(s). The degreeof grindingrequired is a function of the ore mineral grainsize, as well as the oreand gangue mineralogy. Ores containing   negrained uraniummineralisation will need ner grinding while the hardness of the associated gangue will inuence the grinding time and energy requirement. Ores processed using alkaline leach solutions (e.g. surcialdeposit ores), also require  ne grinding because of the slower kineticsof the chemical system. Examples of the preferred grind size and theleach solution used to extract uranium for some common uranium oretypes (andthe host deposit types), arepresented in Table 5. Autogenousor semiautogenous grinding are the more favourable methods for ura

nium mineral liberation, as crushing, particularlyne crushing, is energy intensive, costly and produces dust and releases radon that havenegative environmental impacts (Edwards and Oliver, 2000). The suitability of an ore to physical/mechanical methods of beneciation andupgrading, may be inferred during the preliminary ore examination.

 Table 5

The degree of grinding required for each uranium ore type (IAEA, Vienna, 1993).

Ore type and associated deposit types Grind range Leach solution

Sandstonee.g., Sandstone-hosted deposits,unconformity related deposits

−0.6 mm to −0.4 mm Agitationacid leach

Carbonatee.g., Sur  cial deposits

−0.2 mm Alkaline leach

Conglomerate

e.g., IOCG-U andbreccia complex deposits

Finer grinding required,

50% passing 0.074 mm

Acid leach

Intrusivee.g., Magmatic deposits

−1.7 mm Acid leach

0

200

400

600

800

1,000

1,200

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

tonnes of uranium Aus$ million

   E  x  p  o  r   t  s   (   t  o  n  n  e  s   U   3   O   8   )

Ann u a l   e x p o

r  t   e  a r ni  n g s  (  A u s  $ mi  l  l  i   on )  

Fig. 3. Annual exports, tonnes of U3O8 and earnings (Australian dollar) between 2000 and 2012. The dashed line indicates the estimated forecast for total earnings from exports of 

U3O8 in 2012 given an average annual U3O8 spot price of 92.32 Aus$/kg.

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5. Uranium deposits in Australia

Australia possesses approximately 31% of the world's uranium resources (WNA, 2013) and uranium is a major export earner for thecountry generating revenue up to $1B annually (e.g., Fig. 3, year 2009),from the export of 8000 tpa to 10,000 tpa of uranium oxide, U 3O8(WNA, 2013). Major resourcesinclude theOlympic DamIOCGU deposit(South Australia), the Ranger, Jabiluka, Nabarlek, Koongarra (NorthernTerritory) and Kintyre (Western Australia) unconformityrelated

deposits, the Yeelirrie surcial (calcrete) deposit (Western Australia)and the Beverley, Four Mile and Honeymoon sandstonehosted deposits(South Australia). Australia is the third largest uranium producer in theworld after Kazakhstan and Canada, and uranium production withinAustralia is forecast to grow signicantly over coming decades (WNA,2013).

Australia's uranium ore resources are comprised of a diverse rangeof deposit types with just under 100 known uranium depositsscattered across the Australian continent, varying in size from small

Fig. 4. The locations of major uranium mines (past and presently active) and deposits. Symbols correspond with different styles of mineralisation.

0.01

0.10

1.00

10.00

0.1 1 10 100 1000

   G  r  a   d  e   %    U

Million Tonnes Ore

IOCG-U - Olympic Dam

IOCG-U - Mt Gee

Metasomatite - Valhalla

Sandstone Hosted - Beverley

Sandstone Hosted - Four Mile

Sandstone Hosted - Mulga Rock 

Sandstone Hosted - Westmoreland

Surficial calcrete - Yeelirrie

Unconformity - Jabiluka

Unconformity - Ranger

Unconformity - Kintyre

Fig. 5. Uranium grade against deposit tonnage (reserve and measured resource values) for the largest uranium resources in Australia.

Adapted from McKay et al. (2009).

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metasomatite), independent chemical and mineralogical data wasobtained for samples sourced from known Australian deposits or prospects. Resultsare discussed in view of theimplicationsfor potential processing of the ore types. Ores from the two other prevalent uraniumdeposit types; IOCGU (breccia complex), and unconformityrelated,also offer some challenging characterisation issues (e.g. extremelyne, submicron grainsize mineralisation, particularly in the IOCGUores). However, in general, the processing of these ores is consideredmore straightforward. Results from these deposit types are notdiscussed in this paper but the reader is referred to discussions and

case studies previously reported in IAEA Technical Report 196 (IAEA,1980) and also in Cuney and Kyser (2009).

6.1. IOCG-U (or breccia complex) deposits

IOCGU deposits form in association with alkalirich volcanics andintrusives. The genesis of this deposit type was most prevalent duringthe Proterozoic. Deposits are characterised by hydrothermal breccia

tion that forms close to surface at sites affected by phreatic activity.The precipitation and concentration of metallic mineral phases results from  uidmixing redox reactions between upwelling, hot, reducing, magmatic  uids (introduced locally by volcanic activity) andlow temperature, oxidising, meteoric and lacustrine nearsurface waters, which are drawn downwards as the magmatic activity ceasesand the system cools. Hence, the uranium minerals crystallise closeto the breccia complex zone (Haynes et al., 1995; Oreskes andEinaudi, 1990; Reeve et al., 1990). A schematic diagram showing theformation of breccia complex deposits is provided in Fig. 7.

The main deposit of this type in Australia is the Olympic DamIOCGU ore deposit located in South Australia. It is unique, being theonly IOCG breccia complextype deposit in the world known to contain recoverable uranium credits. It is also the largest, single resource

of uranium in Australia (and the world). The deposit is associatedwith a hematiterich granite breccia complex that is situated in theRoxby Downs Granite of the Gawler Craton. The host granite terrain,the Roxby Downs granite, is an Atype granite enriched in K, U andTh, of Mesoproterozoic age (Creaser and Cooper, 1993). Circulationof the mineralising   uids caused potassicalteration of the granitehost rock and resulted in an abundance of quartz and clays whichare the dominant gangue minerals (Neumann et al., 2000).

Compared to other styles of uranium mineralisation, IOCGU deposits are of very low grade (diamond symbols,  Fig. 5) and wouldnot normally be considered exploitable uranium deposits, were it

Unconformity-related, 20.4%

Sandstone-hosted, 6.7%

BrecciaComplex,

65.5%

Surficial,Calcrete-

hosted, 4.9%

Metasomatite,1.5%

Other *, 1.0%

Fig. 6. Style of uranium mineralization and percentage abundance found within Australiafollowing the IAEA uranium deposit type classication system (OECD/NEAIAEA, 2012).The classication  “Other” comprises; vein, volcanic, pegmatite and intrusivetype uranium deposits.

Playa Lake

Dykes

Oxidising meteoric

fluid

Hotmagmaticfluid

Ore precipitation

Ore leaching

Olympic Dam

breccia complex

Basalt

Roxby Downs

Granite

Fig. 7. Simplied genesis model for the formation of the Olympic Dam IOCGU breccia complex deposit. The geological setting and associated mixing of hot magmatic  uids withcooler, oxidising, surface meteoric waters.

Adapted from Robb (2005), after Haynes et al. (1995).

34   M.I. Pownceby, C. Johnson / Ore Geology Reviews 56 (2014) 25–44

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not for the polymetallic mineralogy that typies these deposits. TheOlympic Dam deposit has a measured resource of 1474 Mt at an average uranium grade of 0.03% U3O8 with an additional indicated resourceof 4843 Mt at 0.027% U3O8. Probable ore reserves are reported to be469 Mt at 0.056% U3O8 (BHP Billiton Annual Report, 2012). Uraniniteis the most abundant ore mineral, with lesser amounts of brannerite,cof nite and complex REE–Ti–U–Th oxides also present (Hitzman andValenta, 2005; Hitzman et al., 1992; Reeve et al., 1990). These mineralsare most abundant in the hematite breccia zone and are associated withCu, Au, Ag and CaF2 mineralisation (Hitzman and Valenta, 2005).

Other ironrich breccia deposits with varying amounts of uranium,copper and rare earth elements, formed by similar processes to Olympic Dam, have been identied in the Gawler Craton, e.g., Acropolis,Wirrda Well, Oak Dam, Emmie Bluff and Murdie deposits (Cross,

1993) and Mount Painter areas of South Australia, e.g., Mt Gee,31,300 t U3O8, average grade of 0.1% U3O8   (Goldstream, 1999,2000). The mineralisation features of these deposits, specically theextent of brecciation, hematite/magnetite ratios, and the intensityand grade of uranium, copper, gold, silver and rare earth elementmineralisation, suggest that the IOCGU deposit style forms an arrayof ore types and are probably best referred to as a subset of the breccia complex deposits (Gow et al., 1994; Hitzman, 2000). For example,

the Olympic Dam deposit contains strongly mineralised, hematiterich ore, whereas the Murdie deposit is comparatively magnetiterich and poorly mineralised (McKay and Miezitis, 2001). Similarly,the Mount Gee deposit does not have the same Cu/Ag mineral associations that are present at Olympic Dam. Smaller hematite brecciacomplex deposits that have uranium credits associated with copper,gold, silver and REE mineralisation, include the Ernest Henry, Starra,Mount Elliot and Osborne deposits in northwest Queensland(Haynes, 2000; Hitzman, 2000; Pollard, 2000; Porter, 2000). Presently, none of these deposits recover uranium.

6.1.1. Processing options for IOCG-U (breccia complex) ores

The uranium component of the Olympic Dam breccia complexore partitions into the copper concentrate during ore processing

(copper being the primary commodity). Uranium is subsequentlyextracted from the copper concentrate using an acid leach solution(MacNaughton et al., 1999, 2000; Ring, 1979). The presence of Fe3+

ions derived from the acid dissolution of hematite (the dominantgangue mineral), as well as  uoride ions from the dissolution of associated   uorite, oxidise the U4+ mineral phases (uraninite, cof niteand brannerite) and aid in improving overall uranium recovery(Ragozzini and Sparrow, 1987; Ring, 1979).

Granite

Al l u v i u m  /  C o l l u v i u m 

Channel fill

Watertable

P ot a ssium U r a nium

U r a ni umV a na d i um

Potassium

CalcreteCarnotite mineralisation

Al l u v i u m  /  C o l l u v i u m  s  l o  p e 

M   a  f   i   c  

M   a  f    i   c  

pH 6.0-7.0

PH 4.5-7.0 pH 7.0-8.5Dune

radiogenic source rocks

Vanadium

Fig. 9. Idealised model of calcrete uranium mineralisation.

Adapted from Mann and Deutscher (1978) and Hou et al. (2007).

Fracture-controlled

U mineralisation

(perched)

Lake

U mineralisation

at unconformity

(basin-hosted) Unconformity

Granitic gneiss Pelitic gneiss

Graphitic

pelitic gneiss

Vein & breccia

U mineralisation

asement-hosted)

Quartzite

Arkosic gneiss

Pelitic

gneiss

Unconform ty

Arkos c neiss

Fracture-controlled

U mineralisation

(perched)

Lake

U mineralisation

at unconformity

(basin- osted)

U mineralisation

at unconformity

(basin-hosted) Unconformity

Granitic gneiss Pelitic gneiss

Graphitic

pelitic gneiss

Vein &

U mineral

as ent-ho

Vein & breccia

U mineralisation

(basement-hosted)

Quartzite

Arkosic gneiss

Pelitic

gneiss

Fig. 8. Schematic genesis model for Proterozoic unconformityrelated uranium mineralisaiton.Adapted from Kyser and Cuney (2009).

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C   e  n  t  i    p  e  d   e   Lake Way

Lake Maitland

Thatcher So ak 

Dawson-Hinkler Well

  Y e e  l i r r i

 e

Hillview

WindimurraLake Mason

 Perth

Kalgoorlie

Carnarvon

Esperance

Lake Raeside

Nowthana

WESTERN AUSTRALIA

 Indian

Ocean

 G r e a t  A u s

 t r a l i a n  B i g

 h t

28o

32o

120o

126o

0 200

kilometres

Yilgarn Craton

Palaeochannel

Jailor Bore

Munabal lya Well

Gascoyne Province

 N 

Fig. 10. Location of major calcretetype uranium occurrences in the Yilgarn and Gascoyne districts, Western Australia.

Fig. 11. Mineral phase maps showing the disseminated nature of carnotite (white) in a northern Yilgarn calcrete ore. Data was collected using a JEOL 8500F EPMA equipped withve wavelength dispersive spectrometers and two solidstate energy dispersive detectors.

After Aral et al. (2010).

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For other uraniumbearing hematite breccia complex ores, whereuranium is the principal ore mineral extracted, e.g. the Mount Geedeposit and Mount Gee East prospect (located to the east of OlympicDamin the Mount Painterregion), processing can be more complicated.The low grade,  negrained, quartzhematiterich hosted mineralisation, characteristic of IOCGU breccia complex ores typically requiresne grinding and preconcentration prior to processing. Fine grindingis energy intensive owing to the high Bond working index of quartz.In addition, the presence of refractory, complex U ± Th ± REEoxidesincreases the quantity of oxidising agent needed to obtain optimal uranium recovery. In the event that polymetallic sulphide phases are

present, additional oxidant is needed, which results in higher acid consumption. Alternatively, an alkaline leach approachmay be taken to improve the separation of uranium from base metals.

6.2. Unconformity-related uranium deposits

Large, unconformityrelated uranium deposits formed in Australiaduring the Proterozoic within intracontinental basins at the interfacebetween altered Paleoproterozoic metasediments and overlying LaterPaleo to MesoProterozoic, continental clastic sediments (Hegge andRowntree, 1978; Maas, 1989). Ore bodies are stratastructure bound,

Dol

Qtz

Clay Clay

500 µm

a) b)

c)

20 µm

10 µm

Ct

Ct

Fig. 12. Backscattered electron images showing typical negrained textures of carnotite grains in calcrete ores. Image (a) shows small carnotite grains (bright phase) as inclusionsin dolomite while images (b) and (c) show high mag. views of carnotite grains. Dol = dolomite, Qtz = quartz and Ct = carnotite ( Aral et al., 2010).

Unaltered sandstone

(reduced) aquifer

Ore zoneAlteration halo

Oxidised

sandstone

Impermeable

sediment (e.g. shale)

Impermeable sediment (e.g. shale)

Groundwater

flow

Infiltration of uranium-bearingsurface fluids

Water table

Redox front

Una tere san sto

(r duc d aquif r

Unaltered sandstone

(reduced) aquifer

Ore zonOre zoneat aAlteration halo

O idisedOxidised

sandstone

Im ermeable

s dime (e g sha e)

Impermeable

sediment (e.g. shale)

Im ermeab e sedi ent (e.g. sha e)Impermeable sediment (e.g. shale)

roundwater

f o

Groundwater

flow

tr tsurface f ui sInfiltration of uranium-bearingsurface fluids

Water table

R dox rontRedox front

Fig. 13. Generalised conceptual model of a uranium rollfront sandstone deposit; modied from published sources (e.g. De Voto, 1978; Harshman, 1962, 1972; Kyser and Cuney,2009; Nash et al., 1981; Rubin, 1970; Spirakis, 1996). A crescentshaped uranium ore body typically forms at the dynamic reaction front in the sandstone aquifer between oxidised

and reduced lithologies. The redox front moves with meteoric water  ow more and more inside the original reduced rock unit, driven by hydraulic head.

37M.I. Pownceby, C. Johnson / Ore Geology Reviews 56 (2014) 25–44

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hosted in argillaceous, faulted sandstones located immediately aboveand below the unconformity. Mineralisation commonly extends intothe basement (Fig. 8). Elsewhere worldwide, smaller deposits arerecognised to have formed during the Phanerozoic, however, nonehave been located in Australia ( Jefferson et al., 2007).

Studies of the unconformity uranium deposits from around theworld have resulted in a consensus that these deposits resultfrom the mixing of sandstonesourced diagenetic brines with upwell

ing, basementcirculating, slightly acidic, hot, oxidised, Na, Carichdiagenetic brines (Hoeve et al., 1980; Hoeve and Sibbald, 1978;Kotzer and Kyser, 1995; Pagel and Jaffrezic, 1977; Sibbald et al.,1976). These circulate downwards through underlying basementrocks (Derome et al., 2003; Kyser and Cuney, 2009) dissolving quartzand simultaneously reacting with graphite and sulphides (reductants), which results in the precipitation of tetravalent uranium minerals. Mineralisation is stratabound, present along the unconformitycontact and associated with faults, fractures and breccias present inthe underlying basement Proterozoic metasediment horizons (Kyserand Cuney, 2009; Wilde, 1988; Wilde et al., 1989).

Uranium minerals associated with this style of deposit are mainlypitchblende and uraninite, with lesser amounts of cof nite, branneriteand thucholite. Uranium mineralisation is typically cogenetic withminor sulphides such as Fe, Pb, and Cu and commonly hematite(e.g., Fayek and Kyser, 1997). The gangue mineralogy, determined bythe metasedimentary host units, typically includes chlorite, quartz,sericite, argillaceous and carbonate minerals.

Within Australia, unconformity deposits account for approximately20% of the uranium resources. The second and third largest uranium deposits in Australia, Jabiluka and Ranger, are located within the AlligatorRivers ore eld (Northern Territory) and Kintyre, the  fth largest uranium resource in Australia, is hosted in the Rudall Province (WesternAustralia). Resources are typically medium to large (40,000–180,000 tU3O8) and grades are low to medium, 0.2–1.0% U3O8   (www.wiseuranium.org, 2013). Smaller unconformity deposits include; Hades Flat(~726 t U3O8) and Caramal (~2500 t U3O8) in the Alligator Rivers Uranium Field; Mount Fitch (~1500 t U3O8, grade 0.042% U3O8) in the Rum

 Jungle Field; and Coronation Hill (~1850 t U3O8, grade 0.537% U3O8) in

the South Alligator Valley  eld. Detailed descriptions of the characteristics(structure, age,genesis,uraniummineralisation,gangue mineralogy)for each of these deposits are discussed by: Ryan (1972)  —  Hades Flat;Ewers et al. (1984),   Fraser (1980),  Berkman and Fraser (1980)  andFoster et al. (1990)   —   Mount Fitch; and  Hills and Richards (1972),Cooper (1973),   Needham (1987, 1988),   Valenta (1991),   Wyborn(1990, 1992) and Mernagh et al. (1994)  —  Coronation Hill.

6.2.1. Processing options for unconformity-related uranium ores

The mineralogy of unconformityrelated uranium ores is oftensimple, dominated by uraninite, pitchblende and cof nite withminor brannerite and U–REEoxides. The uranium minerals are relatively coarsegrained and can be readily preconcentrated by radiometric sorting and processed using an acid leach solution. Theaddition of an oxidant is required to aid the uranium extraction, particularly of the refractory U–REEoxide phases. Acid consumption for

some deposits is high, re

ecting the presence of carbonate and swelling clays in the gangue. The presence of pyrite can lead to an increasein the requirement for an oxidant and consequently increase acidconsumption. The unconformity contact controls the deposit morphology and orientation, and in certain cases, if the basement andoverlying lithological units have low porosity and the deposits are located below the water table, insitu leaching can be implemented.

6.3. Sur  cial (calcrete-hosted) deposits

Surcial (calcretehosted) deposits are syngeneticto early epigenetic, nearsurface uranium concentrations that form from intracratonicsedimentation and weathering processes. Uniquely, this style of deposithosts hexavalent uranium minerals, most commonly in the form of carnotite and/or tyuyamunite. Compared with the other styles of uraniumdeposits, surcial deposits are geologically much younger, forming between the Tertiary to the present.

Deposits tend to be laterally extensive, stratabound horizons present within indurated sedimentary formations or unconsolidated sediments located in surface depressions (e.g., calcretised uvial drainagechannels   —  Carlisle, 1983). These represent fossilised river channelsformed during periods of higher rainfall when the water table washigher. Uranium enters the hydrological system when a river drains auraniumfertile source (e.g. granite), while vanadium, also present inthe system, is likely to be sourced from nearby mac units. Uranyl–carbonate complexes (UO2(CO3)2

2−) form and are transported in solution until ground water evaporation decreases the complex solubility,causing the destabilisation and subsequent precipitation of uranium,vanadium and carbonate minerals (Fig. 9).

Surcialdeposit uranium ores exhibit texturalevidence for repeatedprecipitation, dissolution and reprecipitation of uranium and gangue(carbonate) minerals. Mineralisation is typically  negrained and disseminated. Concentrations may occur in small fractures and vug cavities, forming a powdery coating or accrete by adsorption onto thesurface of clay–quartz sediments that are intimately associated withthe predominantly carbonate host mineral assemblage.

Fig. 14. a) Back Scattered Electron (BSE) image, and b) phasepatched mineral map showing the mineralogy and textures of key phases in a sandstonehosted uranium sample.

Scale bar indicates 500  μ m. Data was collected using a JEOL 8500F EPMA equipped with  ve wavelength dispersive spectrometers and two solidstate energy dispersive detectors.

38   M.I. Pownceby, C. Johnson / Ore Geology Reviews 56 (2014) 25–44

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Surcial uranium deposits account for approximately 5% of Australia's uranium resources. Within Australia these deposits arefound mainly in the northern region of the Yilgarn Craton, WesternAustralia. Deposits are located either within Tertiary to Quaternaryaged palaeodrainage channels or playa lake sediments, which drainthe uraniumrich Archaean granitoids and greenstone basement(Arakel, 1988; Langford, 1974; Mannand Deutscher, 1978). Differencesin environmentalcharacteristics suchas soil type, groundwater chemis

try and climate causethe sur

cialcalcrete deposits to be con

ned tothenorthern portion of the Yilgarn Block (Fig. 10). Deposits are typicallysmall (5000 t to   b10,000 t U3O8), low grade (0.02%–0.07% U3O8) andoften challenging to exploit, as the hostgangue mineralogy complicatesore processing and uranium extraction. The exception is the Yeelirriedeposit (Cameron, 1990), the world's largest surcial deposit containing 65,810 t U3O8 grading 0.15% U3O8. Examples of other smaller surcial deposits that formed contemporaneously, under similar conditionsto the Yeelirrie deposit include; Lake Raeside (~1700 t U3O8, ore grade0.025% U3O8), Lake Mason (2700 t U3O8, ore grade 0.035% U3O8) andCentipede (~3800 t U3O8, ore grade 0.07% U3O8).

Aside from theYilgarn region, surcialcalcrete uranium deposits arealso present in Tertiary calcrete overlying Proterozoic granite and

metamorphics in the Gascoyne Province of northwest Western Australia. These deposits occur in a cluster approximately 200 km to 250 kmeast northeast of Carnarvon and include, Minindi Creek–Wabli Creek,

 Jailor Bore, Lamil Hills, Munaballya Well South, and Red Hill Well(Fig. 10) (Arakel, 1988).

6.3.1. Characterisation of sur  cial calcrete ores

Four samples were examined from surcial deposits in the northern

Yilgarn and two from deposits in the Gascoyne Province. Uraniumgrades of the four northern Yilgarn samples varied from 60 to560 ppm uranium (0.007–0.066% U3O8), which in general, representsthe extremes in grade variations of the surcial calcrete ores in thenorthern Yilgarn (except for the anomalously highgrade Yeelirrie deposit). The vanadium content also varied accordingly and ranged from70 to 530 ppm V 2O5 indicating probable carnotite mineralisation. Theuranium grades fromthe two Gascoyne Province sampleswere approximately 400 ppm U (0.0470% U3O8) while the vanadium contentrangedfrom 270 to 410 ppm V 2O5.

EPMA mapping of the Yilgarn samples conrmed the mainuraniumbearing mineral in the deposit to be carnotite (Fig. 11). Although the carnotite particles shown in Fig. 11 appear coarsegrained,

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Mary Kathleen

Glen Isla &

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Glen Isla &

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Kuridala

Duchess

Rita, Rary

Mothers Day

Turpentine

Spear Creek 

Mt. IsaCitation

Anderson’s

Lode

Gorge Creek Flat Tyre

SkalValhallau

ParooCreek 

Warwai

Watta

Calton

Hills Kajabbi

Gunpowder

Georgina Basin

u

Closed Mine

Deposit

Prospect

50 km0

Mt. Isa Inlier

20°00’

22°00’

        1        3        9        °        3        0        ’

Carpentaria Basin

uu

Mary Kathleen

Glen Isla &

Malakoff 

Elizabeth

Anne

Elaine

Kuridala

Duchess

Rita, Rary

Mothers Day

Turpentine

Spear Creek 

Mt. IsaCitation

Anderson’s

Lode

Gorge Creek Flat Tyre

SkalValhallauu

ParooCreek 

Warwai

Watta

Calton

Hills Kajabbi

Gunpowder

Georgina Basin

u

Closed Mine

Deposit

Prospect

uu

Closed Mine

Deposit

Prospect

50 km0 50 km0

Cloncurry – Selwyn zone

Mt. Isa Inlier

Western succession

Lawn Hill Platform

Leichhardt River Fault Trough

MyallyShelf 

Kalkadoon – LeichhhardtBelt

EwenBlock 

Kalkadoon-Leichhardt Block 

Eastern Succession

Mary Kathleen zone

Quamby – Malbonzone

Mt. Isa Inlier

20°00’

22°00’

        1        3        9        °        3        0        ’

NT

QLD

NSW

VIC

WA

SA

NT

QLD

NSW

VIC

WA

SA

Fig. 15. Regional geology of the Mount Isa Inlier and principal uranium deposits and prospects.

Adapted from McKay and Miezitis (2001).

39M.I. Pownceby, C. Johnson / Ore Geology Reviews 56 (2014) 25–44

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