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Review

Towards a Mineral Systems Model for Surficial Uranium Mineralization Based on Deposits in the Erongo District of Namibia

by
Andy Wilde
Centre for Exploration Targeting (CET), The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
Minerals 2023, 13(2), 149; https://doi.org/10.3390/min13020149
Submission received: 21 December 2022 / Revised: 5 January 2023 / Accepted: 17 January 2023 / Published: 19 January 2023
(This article belongs to the Special Issue Towards Mineral Systems Models for Sediment-Hosted and Other Uranium Deposits)
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Abstract

:
Surficial deposits in Namibia’s Erongo district contain substantial but low-grade resources of uranium and vanadium (nearly 500 Mlb U3O8), hosted in palaeochannels. This review attempts to develop a mineral systems model for the deposit type, but it is emphasised that research into this important class of deposit has been minimal since the nineteen eighties, largely as a result of a limited investment in uranium exploration. The deposits are the result of groundwater movement in aquifers developed within Cenozoic palaeochannels. The source of uranium was probably granitic rocks traversed by these palaeodrainages, particularly black-quartz rich pegmatites similar to those that make up the hard-rock alaskite deposits of the region. Transport of uranium is generally assumed to have occurred in aqueous uranium species after palaeochannels became filled with sediment. U-enriched clasts within the palaeochannels have yet to be investigated as a local source of uranium. The localised deposition of uranium occurred after regionally extensive carbonate cementation of the palaeochannel sediments, which was the result of climate change (aridification). Pre-uranium calcite may have acted as a chemical buffer (pH) and probably influenced palaeochannel hydrology, restricting groundwater flow to the deeper portions. Uranium is paragenetically related to Mg clays and dolomite, suggesting that the groundwater evolved to a more Mg-rich composition during uranium deposition, probably as a result of more extreme evaporation. The controls on the localisation of mineralisation remain unclear and unpredictable, as are the controls on uranium grade—the fundamental determinant of economic viability. There are few absolute age determinations for any of the deposits, but none occur in rocks likely to be older than the Miocene. This reflects low preservation potential. For example, the Langer Heinrich deposit is incised by active drainage with attendant erosion and probable removal of mineralised material.

1. Introduction

Growing demand for green energy is manifested in a large number of nuclear reactors both planned under construction. This increase in global nuclear capacity will require the discovery of additional uranium resources to meet the increased demand for and decreasing supply of uranium. In this paper, the surficial type (also known as the calcrete- or palaeochannel-hosted type [1]) of uranium deposit is reviewed with a view to developing a mineral systems model that might aid in future exploration. As with many types of uranium deposit, research on surficial uranium deposits has been negligible in recent years, reflecting a past oversupply of uranium and a reduced investment in uranium exploration. It is hoped that this review might stimulate new research to help discover new examples of this deposit type.
Numerous examples of surficial uranium deposits occur on many continents, but the most widely documented and largest deposits occur in Namibia, Australia and Jordan. This review is confined to the Namibian deposits, which, as will be shown below, are probably fossil deposits and, as such, differ from Australian deposits, which are probably still forming [2]. In the first part of this review, background and descriptive information are presented for the deposits. In the discussion section, key features of the deposits are discussed in the context of the mineral systems approach [3,4,5].
Namibia’s Erongo region contains sixteen surficial uranium–vanadium deposits and several occurrences hosted within unmetamorphosed sediments that fill palaeochannels, developed on Proterozoic and Early Palaeozoic (Damaran) metamorphic and igneous basement rocks. The contained uranium resource of these deposits amounts to nearly half a billion pounds of U3O8, making this region the best-endowed region in the world for this type of deposit (Table 1). Uranium grades are low ranging, from 80 to 540 ppm U3O8 (Table 1).

Mineral Systems Approach to Predictive Exploration

The Mineral Systems Approach considers the origin of mineral deposits in the framework of lithospheric-scale processes from the time-honoured perspective of source, fluids, transport and traps. Applied to exploration strategy, this approach allows for more predictive models. Rather than matching patterns, knowledge of the underlying geological processes and tectonic setting can be used to assess prospectivity. Furthermore, a Mineral Systems Approach can broaden the scope of prospectivity indicators and, therefore, can allow for earlier, more efficient fertility assessments [5,10].
The Mineral Systems Approach can be broken down into five questions; these are geodynamic history and setting, architecture, fluid reservoirs, fluid pathways, and driving forces for transport and deposition. The questions were formulated for hydrothermal mineral systems but are equally applicable to surficial uranium deposits. Translating this theory into a useful tool for exploration involves understanding how critical processes of the mineral system are reflected in the geology and how they are used to define targeting criteria to detect elements directly or by proxy [5,10].

2. Geological Setting

2.1. Namib Unconformity Surface (NUS)

Palaeochannels that host Namibia’s surficial uranium deposits are developed on a basement of various igneous and metamorphic rocks of the ca 540 Ma Damaran orogen [11,12]. Protracted erosion of the orogen during the Late Cretaceous resulted in the development of a prominent palaeosurface, referred to as the Namib Unconformity Surface [13]. The morphology of this surface and the extent to which it has been modified (incised) during later erosional and tectonic events remains uncertain, although the palaeochannels themselves have been reasonably well defined by a combination of airborne electro-magnetic surveys and drilling by several exploration companies since the 1970s (Figure 1 and Figure 2). The palaeochannels are generally oriented NE–SW and are probably drained via outcropping basement rocks to the E and NE (Figure 1).

2.2. Basement Rocks & Weathering

The lithologies of Damaran basement rocks, traversed by mineralised palaeochannels, are quite varied and include gneisses, marbles, skarns and multiple generations of felsic intrusions, along with prominent Karoo-aged dolerite dykes, typically oriented north–south. The latter outcrop is rather better than the older lithologies and may have formed resistant ridge-like outcrops along the floors of the palaeochannels [15].
There is little evidence of substantial weathering in the basement rocks below the NUS, probably as a consequence of predominantly arid conditions during and since the Cretaceous period [14]. Alternatively, weathered rocks may have been partially to completely stripped during the erosive event that formed the NUS. A breccia of angular quartz vein fragments up to 3 m thick (Gomkaeb Conglomerate or basal breccia) occurs at the basement contact along the Kuiseb River and has been interpreted as a palaeoregolith [14]. Dating of the weathering-related secondary minerals at the Skorpion and Wolkenhauben base-metal deposits (to the north of the area of interest) indicates that there was at least some localised chemical weathering (and therefore a relatively humid climate) during the Late Cretaceous period, between 50 and 85 Ma [16].

2.3. Palaeovalley Fill of the Kuiseb Valley and Elsewhere

Approximately 240 m of Miocene (23 to 5.3 Ma) to Pleistocene (2.6–0.01 Ma) sediments are exposed on the flanks of the Kuiseb valley and represent erosional remnants of a sedimentary sequence that probably once filled the valley (Figure 2) [14]. The oldest unit, which directly overlies the possible regolith of the Gomakaeb basal breccia, is the Tsondab Sandstone (Figure 2). The Tsondab Sandstone is largely of aeolian origin, but contains layers of dolomite, halite, sylvite and gypsum deposited in playa lakes [14].
The deposition of the overlying Karpfenkliff Conglomerate probably reflects a change in the climate, with increased rainfall permitting the deposition of coarse fluvial debris in the ancestral Kuiseb River [14]. A prominent unconformity at the top of the Karpfenkliff Conglomerate is indicative of a neotectonic event at this time, which rejuvenated the Kuiseb River and resulted in its incision into the Miocene rocks [14].
The Pleistocene Oswater Conglomerate unconformably overlies these older rocks and in places rests directly on basement rocks. Incision by the Gawib River into the palaeochannel sediments of the Langer Heinrich deposit and diversion of the Gawib River to the north may have occurred at this time [15]. Figure 3 shows the relationship of the current ephemeral drainage of the Gawib to the remnant calcretised conglomerate.
Overlying the Oswater Conglomerate is approximately 40 m of silt and clay of the Homeb Silt, interpreted as the product of periodic flooding, indicating a return to wetter climates and also a periodic incursion by seawater [14]. The sequence is completed by recent fluvial gravels and carbonate deposits of present-day interdunal regions.

2.4. Carbonate Cementation, Tufa, Travertine and Sinter

Carbonate cementation is common throughout the Cenozoic sequence but is particularly intense in the 5-m-thick Kamberg Calcrete, which forms prominent scarps along the present-day Kuiseb valley. The Kamberg Calcrete appears to be developed mainly (though not exclusively) at the top of the Karpfenkliff Conglomerate and is regarded as a stratigraphic marker [14]. The basal contact is gradational as the calcrete grades downwards into weakly cemented conglomerate. The prominent scarp deposits, immediately to the east of the Langer Heinrich, may correlate with the Kamberg Calcrete (Figure 3A).
Carbonate cementation of the upper part of the Karpfenkliff Conglomerate must have occurred prior to the deposition of the Oswater Conglomerate, since the latter unit contains clasts of calcrete [14]. The Oswater Conglomerate is also cemented by carbonate, suggesting that there were at least two carbonate-cementing events, one of which occurred during the Miocene or Pliocene, and the other during the Pleistocene or more recently.
Carbonate cements from various stratigraphic levels have been dated using the 14C method at between 35 and 10 Ka. Further evidence of a second carbonate-forming event or events includes the isolated outcrops of tufa and travertine of the Hudoab tufa unit in the Kuiseb Valley, dated at between 31 and 10.9 Ka [17]. Caves near Rössing Mountain contain carbonate “sinter”, dated at between 42 and 23 Ka, possibly indicating relatively humid conditions at this time [18].

2.5. Gypcrete

Surface and near-surface sediments are often cemented by gypsum and bassanite, as well as carbonate [19]. Figure 4 shows a typical cross-section from the Tumas deposit in which gypsiferous sediments (sand and gravel) form a continuous layer at about 1–2 m below the surface. The layer is overlain by unconsolidated sands and gravels related to the present-day ephemeral drainage. Elsewhere, the gypsiferous layers form small scarps where they are eroded by current drainage (see Figure 3B). Locally, the gypcrete is radioactive, containing similar uranium grades to the calcrete-hosted deposits. This type of mineralisation has minimal economic potential and has not been studied in detail.

3. The Mineralised Palaeochannels

The contemporary ephemeral drainage basins south of the Swakop River host two major palaeochannel systems and four of the larger surficial uranium deposits of the region, at Langer Heinrich, Tumas, Tubas and Aussinanis (Figure 1).
The northern palaeochannel network contains the major uranium deposits of Langer Heinrich, Tubas and Tumas, which occupy 50 and 15 km of the palaeochannel, respectively. The southern palaeochannel system contains the Aussinanis deposit, which extends over 16 km of the palaeochannel and the Namib IV deposit (also known as Aussinanis), which extends over approximately 10 km of the palaeochannel. Hence, a key feature of this style of mineralisation is its lateral extent, a feature which has important implications for genesis (see below). The depth of the palaeochannels (below surface) increases from a few metres in the east to over 100 m at the western end of the Tumas palaeochannel.
The geology of the Langer Heinrich Valley is shown in Figure 5 and Figure 6. Unusually for the district, palaeochannel sediments (mainly carbonate-cemented conglomerates and sands of the Langer Heinrich Formation) outcrop. A prominent scarp, formed by a carbonate-cemented conglomerate to the east of the Langer Heinrich deposits (Figure 3A and Figure 5), is a probable lateral equivalent of the Kamberg Calcrete of the Kuiseb Valley. This would mean that the Langer Heinrich Formation is a likely lateral equivalent of the Miocene Karpfenkliff Conglomerate. The exposure of these palaeochannel sediments is the result of substantial erosion caused by the incision of the present-day Gawib ephemeral drainage (Figure 3A and Figure 5) [15,21]. It was this incision and consequent exposure of the deeper levels of the palaeochannel that facilitated the discovery of the deposits using airborne radiometric surveys [15].
The palaeochannel fill is the dominant host to uranium mineralisation, typically a weakly stratified and poorly sorted polymict conglomerate, with clasts up to 20 cm, as well as sand-dominant layers (Figure 7A,B). Clay-rich layers are rare. Bedding can be correlated only for short distances, indicating transient high energy flows, such as flash floods typical of desert climates [22,23]. The angular habit of detrital feldspar suggests deposition close to the source [15,22]. Clasts within the mineralised conglomerates usually include abundant monomineralic black quartz, which is very probably derived from leucogranite pegmatites.
The conglomerate and sand are cemented to varying degrees by calcite (and sometimes dolomite), clay (illite, smectite, palygorskite, sepiolite etc—see below) and, rarely, cryptocrystalline silica The degree of carbonate cementation varies substantially from none to over 50% calcite. There is no correlation, however, between uranium grade and calcite abundance either at the deposit scale or in thin sections. Indeed, all palaeochannels are cemented to various degrees by carbonate minerals and clays, but the occurrence of uranium mineralisation is much more restricted. There is little or no published information on the distribution of carbonate cement within palaeochannels, but the author’s observations at Tumas are that the most intense cementation typically occurs in the central portion (of a vertical section), persisting upwards to within a few metres of the surface.
In some parts of the palaeochannels, notably at Tubas, the dominant conglomerate/sand unit is overlain by a distinctive red sand unit. The red sand is poorly consolidated, coarse and rusty red in colour and of probable aeolian origin. This material hosts the Tubas Red Sand mineralisation. Relatively thin sandstone layers occur with the Karpfenkliff Conglomerate, but the most likely correlative of the Tubas Red Sand is the cross-bedded aeolian sand unit of the Pleistocene Oswater Conglomerate, which can reach thicknesses of 14 m [14].
A thin veneer of sediment related to contemporary ephemeral drainage, such as the Gawib and sheet wash, overlies the calcretised conglomerate or aeolian sand at all the deposits, typically reaching a few metres in thickness. This material often obscures the courses of the palaeochannels and suppresses the radiometric response of the mineralisation. At Tumas, this surficial layer overlies a gypsum–bassanite rich layer (Figure 3).

4. Uranium Mineralisation

4.1. Geometry of Mineralisation

The uranium mineralisation of the Kuiseb Basin is relatively poorly documented, despite intense exploration during the 1970s and 1980s, and again in the years after 2000. Even the mine at Langer Heinrich has yielded relatively little public domain information. It is possible to establish, however, that mineralisation typically extends over many kilometres of the palaeochannel and is somewhat irregular in form, with a tendency to occur as quasi-tabular bodies elongated in the flow direction of the host palaeochannel (Figure 1). This simple situation is complicated by the occurrence of uranium along fractures, including fractures in the underlying basement rocks. A significant but unquantified amount of uranium occurs in underlying basement rocks. This included, for example, carnotite-rich seams along open fractures in the schist of the Tinkas Formation at Langer Heinrich (Figure 7D).

4.2. Mineralogy & Paragenesis

The main ore mineral is considered to be the vanadate hydrate carnotite [K(UO2)(VO4)2.1-3H2O]; however, there have been few published studies focusing on uranium mineralisation using modern mineralogical tools. Tyuyamunite [Ca(UO2)2(VO4)2.7-10H2O] has been described at Langer Heinrich and appears to be at least as abundant as carnotite in the studied samples [24]. Carnotite and tyuyamunite occur disseminated in the host sand and conglomerates, and coating clasts and fractures (Figure 7B and Figure 9A,B).
Gangue minerals include detrital quartz, feldspar, mica, magnetite, ilmenite, titanite, zoisite and apatite [15,22]. This assemblage is consistent with derivation from a predominantly granitic source region. Post-depositional minerals are dominated by interstitial carbonate, chiefly calcite, and clays [21,22,24]. Some calcite, however, is found as rounded clasts (as in the Oswater conglomerate) suggesting the periodic emergence and erosion of carbonate-cemented rocks. Dolomite is apparently uncommon, but has been described at Langer Heinrich [21,22]. Clay minerals include palygorskite, vermiculite, sepiolite, illite, smectite and kaolinite [21,24] (Figure 8). Amorphous silica has been described at Langer Heinrich, but its abundance and distribution is poorly understood and also occurs in minor amounts at Trekkopje (Figure 7C) [21].
The paragenesis of uranium deposition at the Tumas deposit Is illustrated in Figure 9A,B. Most carnotite is associated with Mg-rich silicates that enclose irregular calcite. The calcite is frequently heavily fractured and has a serrated contact with the clay matrix minerals. Occasional inclusions of carnotite can be observed within the calcite as in Figure 9B; however, in these cases, carnotite is almost always related to fractures in the calcite, confirming that it was introduced after formation of the calcite. Recent studies at Langer Heinrich also suggest that uranium mineralisation is paragenetically later than calcite cement, and is associated with the magnesian clay palygorskite, amorphous Mg silicate and dolomite (Figure 9C,D) [21,25].

4.3. Absolute Age of Uranium Deposition

The absolute age or ages of uranium deposition remain uncertain. There has been one attempt to date the Langer Heinrich deposit using an ion microprobe. Dating was carried out on grains which were found to be mixtures of the carnotite and tyuyamunite endmembers [24]. This resulted in a U-Th age of 68 ± 2 Ka [24], i.e., the Pleistocene. The authors noted the need to analyse additional samples to validate this result.

5. Discussion: The Surficial Uranium Mineral System

5.1. Architecture

The significance of terrain architecture in mineral systems analysis is that it establishes fundamental controls on the distribution of high permeability sites within the crust. Mineral systems analysis typically considers the structural control of hydrothermal mineral deposits, but the distribution of permeability is also a crucial factor in the development of surficial uranium deposits. Existing genetic models for surficial uranium posit that uranium was introduced to its depositional sites in groundwater moving downstream within palaeochannel aquifers [2,26,27,28,29]. Hence, understanding the evolution of permeability and porosity within the palaeochannels is crucial to understanding deposit genesis.
A relatively wet climate was required to scour the Damaran orogen and produce the system of palaeochannels, the substantial remnants of which we can map today. A shift to a more arid climate and a reduction in the volume and velocity of water flowing down the palaeodrainage would have induced the precipitation of carbonate, probably via evaporation (evapo-transpiration) from slow-moving groundwater [9]. Thus, climate change is an essential element of the Mineral Systems model, but the temporal relationship between uranium mineralisation and the many fluctuations of Cenozoic climate in Namibia remain poorly resolved.
The paragenetic relationship of uranium phases to calcite indicates that the cementation of the palaeochannels was well advanced prior to most uranium deposition. Much, if not all, the uranium post-dated calcite cementation. It is important to emphasise that carbonate cementation is extremely widespread, whereas the occurrence of economic levels of uranium is much less extensive.
The intensity of carbonate cementation might be expected to influence both porosity and permeability within the palaeochannels. Basement lithologies at the Trekoppje deposit have measured porosity between 1.8 and 4.2%, whereas carbonate-cemented conglomerates have porosities ranging from 10 to 30% [30]. There is a negative correlation between porosity and carbonate content in the conglomerates (Figure 10), while porosity does not correlate with uranium content [30]. The potential importance of fractures to providing porosity and permeability has not been considered, despite the fact that fractures in basement rocks clearly were the locus of uranium precipitation. Indeed, carnotite fracture coatings have also been observed by the author in core samples of carbonate-cemented conglomerates, although these appear to be relatively uncommon.
The transmissivity and hydraulic conductivity (K) of part of the Tumas palaeodrainage were measured at 1–341 m2/day and 0.1–17 m/day, respectively, using standard pump tests [31]. To provide some perspective, the in situ recovery of uranium in sandstone aquifers (“roll-front” type deposits) generally requires K > 0.1 m/day and porosity in excess of 15% [32]. Thus, at least parts of the palaeochannels can be regarded as having good porosity and permeability. More research is required to better understand the hydrology of the palaeochannels. It seems likely that calcite cementation reduced porosity and probably permeability. The upshot of this would have been to channel groundwater flow into the deeper, less well-cemented parts of the palaeochannels.

5.2. Source of Uranium & Vanadium

The source of uranium in these deposits is usually regarded as granitic rocks [22]. It has been suggested that uranium at Langer Heinrich was sourced from the Bloedkoppie Granite, which outcrops immediately upstream (Figure 5) and which apparently has a higher mean and median uranium content, compared to the other granitic intrusions (Table 2) [15,22,33]. Uranium in the Tumas palaeochannel is very unlikely to have been sourced from the Bloedkoppie granite (Figure 1 and Figure 11). Recent drilling shows that the Tumas palaeochannel extends at least as far east as the Donkerhoek batholith. Indeed, the large Donkerhoek batholith is a plausible source for all the deposits south of the Swakop River, based on the fact that most of the uraniferous palaeodrainages apparently emanate from this batholith (Figure 11). Conversely, samples of the batholith reveal relatively low uranium content (Table 2).
A study of the U and Th content of the Bloedkoppie Granite, and the pegmatites that intruded it, found that the pegmatites have nearly three times the uranium content of the granite [34]. Thus, the pegmatites could be a source of more uranium than the host, but this would also depend on the relative abundance of the pegmatites within the palaeochannel catchment area. The abundance of black quartz clasts in sediments of the Langer Heinrich and Tumas palaeochannels is indicative that a significant portion of the palaeochannel sediment is derived from pegmatite, the only known source of black quartz in the region. Indeed, the Langer Heinrich open pits correspond in part to where mapped pegmatites underlie the palaeochannel (Figure 6), which adds weight to the concept that pegmatites are the principal source of uranium.
Some genetic models require separate sources of uranium and vanadium based on the assumption that significant quantities of uranium and vanadium could not be carried in the same groundwater. The geochemical modelling of contemporary groundwaters, discussed in the next section, shows that this is not necessarily the case [35]. In any case, it should be noted that many potential source granites also have much more V than U (Table 2).

5.3. Transport

It is generally assumed that uranium was introduced into its depositional site and dissolved in groundwater moving at depth through palaeochannel aquifers [2,15,23,26,36,37], probably as carbonate complexes [35]. Today, much of the hyperarid Erongo area receives <100 mm of rainfall annually. The Kuiseb River flows, on average, for only 16 days each year, with water reaching the Atlantic ocean only on rare occasions [38]. It is likely that much of the rainfall evaporates before reaching the Kuiseb and there are a few small active playas, such as Tsondab Vlei and Zebra Pan [39]. Nevertheless, the palaeochannels are charged with groundwater, as is amply evidenced by drilling [30,33,40].
Present day groundwaters from the Trekkopje deposit have uranium contents ranging from 0.1 to 0.5 mg/L, pH between 6.5 and 7.5 and eH between −50 and +150 mV; these are saturated with respect to carnotite. Therefore, these waters may be representative of mineralising groundwater [30,41]. Unfortunately, the total dissolved solids (TDS), carbonate and anion content of these samples were not analysed, so it is not possible to comment on likely uranium speciation.
Groundwaters recovered from boreholes at the Tubas deposit have a salinity (TDS) close to that of seawater, ranging from 0.7 to 4.2% and a pH range of 7.3 to 8.0; however, unlike Trekkopje, uranium was not detected [40]. The near seawater salinity may indicate that the Tubas palaeochannel was once charged with seawater. Groundwaters from the Langer Heinrich mine have a pH range from 6.9 to 7.2, a TDS ranging from 0.6 to 1.6 mg/L, and a very low uranium concentration (<180 µg/L) [33]. The V concentration in the Langer Heinrich groundwater greatly exceeds the U content, and ranges between 50 to 340 µg/L. Langer Heinrich and Tumas groundwaters are undersaturated with respect to carnotite and, therefore, are probably poor analogues for the mineralising fluids.
Many analysed groundwaters in the Erongo district have relatively high conductivity, due to high chloride content. This is reflected in airborne and ground electromagnetic (EM) surveys, which effectively map the extent of these brines and make EM a valuable exploration tool [20]. It is noteworthy that a reduced EM response can often be observed in the headwaters of palaeochannels, which can be interpreted to be the result of periodic incursion of, and dilution by, rainwater.
There has been little consideration as to whether any uranium was introduced in solid form, i.e., as detrital clasts during initial sedimentation of the palaeochannels. The amount of uranium contained in detrital phases within the palaeochannels (if any) is completely unknown. At the Beverley and Bigryli sandstone-hosted deposits in Australia, the dissolution and reprecipitation of uranium in detrital phases contributed significantly to uranium endowment [42,43]. Mining and exploration geologists in the Erongo district have noted an apparent association between the occurrence of higher uranium grades and abundant black quartz, probably derived from pegmatite. This raises the possibility that uranium-rich phases, derived from pegmatite, were introduced as clasts during the initial stages of sedimentation of the palaeochannels; uranium was subsequently dissolved and reprecipitated more or less in situ. Indeed, the black quartz itself may be a source of uranium, as suggested by various uranium phases included in black quartz from the sub-economic Iguana alaskite-type deposit [44].

5.4. Deposition

Published genetic models for surficial uranium deposits in general posit a range of chemical depositional mechanisms. These include the mixing of U and V-rich groundwaters, pH change, evaporation, a local increase in K activity, a change in the ƒCO2, the dissociation of uranyl carbonate complexes, and redox-controlled precipitation [22,23,29,30,37,41,45].
Evaporation is clearly an important process in the formation of Australian surficial uranium deposits, which typically occur in active salt lakes [2,46]. While there are some minor salt pans in the Erongo district [39,47], these do not have any spatial association with uranium deposits. Palaeo-evaporitic facies occur within the Tsondab Sandstone, but the tentative stratigraphic correlation of the palaeochannel filled with younger rocks would indicate that these evaporites predate the formation of the palaeochannel fill.
The role of evaporation in forming surficial uranium deposits in Texas was modelled using the USGS software PHREEQC and current groundwater compositions [35]. This modelling found that most modelled groundwaters precipitated calcite upon evaporation. Carnotite precipitation, however, was predicted in relatively few groundwaters and was dependent upon the major ion composition of the groundwater, specifically the ratio of Ca2+ to carbonate species (alkalinity) [35]. Evaporation concentrates uranium and removes CO32– from the solution by precipitating calcite and/or dolomite. The removal of the carbonate ion from the solution, and the resultant decrease in the activity of CO32– also decreases the activity of uranyl carbonate complexes and can lead to the precipitation of carnotite [35]. The process of evaporation may drive several other chemical changes cited as depositional mechanisms above, including pH change, a change in the ƒCO2 and the dissociation of uranyl carbonate complexes. The modelling incidentally demonstrated that U and V can both be carried in natural groundwaters, meaning that the mixing of a U-rich and a V-rich fluid is not required to saturate a groundwater in uranyl vanadates [35].
The association of carnotite with Mg-rich clay minerals and dolomite at Langer Heinrich (Figure 9C,D) suggests that deposition was from a groundwater that was much richer in Mg than that in equilibrium with early calcite cement [25]. Similar mineralogy and paragenesis in the deposits of Western Australia were interpreted as the result of the “desiccation of shallow groundwater aquifers through progressive aridification” [2]. Early calcite may have played an indirect role in the formation of uranium deposits by buffering pH in the near neutral range, since the transport of uranium (as carbonate complexes) requires a neutral to alkaline pH [2].
A critical question, and one of prime importance for exploration, concerns the factors that localized uranium deposition. It has been suggested that constrictions in the palaeochannels, for example resistant dykes forming ridges perpendicular to the palaeochannel axis, lead to uranium precipitation by “ponding” groundwaters and forcing groundwater upwards towards the zone of evaporation [36]. This hypothesis seems implausible given that uranium mineralisation extends over 50 km of the Tumas palaeochannel. While considerable irregularity in the palaeochannel floor has been identified by drilling, there does not appear to be any constrictions of sufficient magnitude to cause the stagnation of groundwater flow for 50 km upstream.
Redox-controlled uranium deposition is unlikely given the absence of evidence of redox “fronts”, such as those encountered in roll-front type deposits. Neither is there evidence of any relationship between mineralisation and vertical changes in redox. Furthermore, uranium in surficial deposits occurs in the oxidised 6+ state (i.e., in carnotite and tyuyamunite), rather than the reduced 4+ state.

5.5. Preservation

All known examples of surficial uranium deposits are Cenozoic in age. This is no doubt a reflection of the ease with which the mineralisation can be eroded and dispersed downstream and eventually transferred into the ocean, as at Langer Heinrich. This type of deposit, therefore, has a low preservation potential.

6. Conclusions

Since the Mineral Systems Approach considers the origin of deposits in the framework of large-scale processes from the perspective of source, fluids, transport, and trap, it has been clearly demonstrated herein that a definitive mineral systems model for surficial uranium deposits of the Erongo district requires much further research. It is possible, however, to discern an intimate linkage between uranium mineralisation and changing climate. The massive erosion of the Damaran orogen during the Cretaceous and early Cenozoic resulted in a land surface incised by a network of immature drainages. Rainfall during the early development of these drainages must have been substantial, in order to drive the necessary processes of erosion and material transport. Aridification would have resulted in filling of the drainage by sediment derived mainly from granites (sensu lato) and uraniferous pegmatites.
It is not clear what proportion of the uranium was transported in solution from source regions in the headwaters of the palaeochannels and what portion, if any, was derived from the dissolution and reprecipitation of local accumulations of clasts derived from the same source regions.
With aridification, the flow regime would have transitioned to ephemeral surface flows and a low velocity groundwater flow within the palaeochannel aquifers. As the water flow velocity continued to decline, the evaporation or evapo-transpiration near the surface drove the widespread precipitation of calcite in pore spaces of sand and conglomerate in the upper parts of the palaeochannels.
Uranium minerals probably precipitated in response to more extreme levels of evaporation of surface and/or groundwaters. Extreme evaporation would have driven the groundwater composition to higher Mg/Ca ratios as Ca was removed via calcite precipitation and to the dissociation of uranium carbonate complexes, as proposed by several previous studies, providing that the initial groundwater composition was suitable. The higher Mg/Ca ratios of the evaporated groundwater would have favoured the precipitation of Mg phases, such as dolomite and palygorskite.
A major shortcoming of the current Mineral System model is the lack of an adequate explanation for deposit localization. Uranium precipitation occurred over substantial lengths of the palaeochannel (50 km in the case of Tumas), which argues against fluid mixing and local barriers to flow as contributors to uranium localisation. The controls on uranium localisation and, therefore, the ability to predict further undiscovered uranium deposits, should be a focus of further research, as well as the factors that control uranium grade.
The Langer Heinrich mine provides evidence that recent active drainage has incised into the older palaeodrainage, probably resulting in the removal of some mineralisation. This illustrates that surficial deposits have a low preservation potential.

Funding

This research received no external funding.

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Acknowledgments

The author wishes to acknowledge many discussions with former colleagues at Paladin Energy and Deep Yellow, including Ed Becker, Jean-Christophe Corbin and Dave Princep, and others too numerous to mention that have shaped the views contained herein. More recently, I acknowledge discussions with Justin Drummond. Three anonymous reviewers provided positive and constructive reviews that improved the manuscript.

Conflicts of Interest

The author declares no conflict of interest.

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Figure 1. Location of palaeochannel-hosted uranium deposits of the Erongo region of Namibia.
Figure 1. Location of palaeochannel-hosted uranium deposits of the Erongo region of Namibia.
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Figure 2. Stratigraphy of the Kuiseb Valley based on outcrop locations, as shown in Figure 1. Mainly after [14]. Figures to right are 14C age ranges in Ka. Sources in text.
Figure 2. Stratigraphy of the Kuiseb Valley based on outcrop locations, as shown in Figure 1. Mainly after [14]. Figures to right are 14C age ranges in Ka. Sources in text.
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Figure 3. (A)—Looking westwards down the Langer Heinrich Valley showing relict calcrete deposits above the present level of the Gawib River and the uranium deposits. Hills in distance are formed by outcrops of Tinkas Formation schist. (B)—Rare outcrop of Tubas Red Sand overlain by gypcrete at the Tumas River. The scarp in this image is due to incision by the active Tumas River and shows that the gypcrete formation predates the Tumas River.
Figure 3. (A)—Looking westwards down the Langer Heinrich Valley showing relict calcrete deposits above the present level of the Gawib River and the uranium deposits. Hills in distance are formed by outcrops of Tinkas Formation schist. (B)—Rare outcrop of Tubas Red Sand overlain by gypcrete at the Tumas River. The scarp in this image is due to incision by the active Tumas River and shows that the gypcrete formation predates the Tumas River.
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Figure 4. Typical cross section from Tumas [20].
Figure 4. Typical cross section from Tumas [20].
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Figure 5. Geology of the Langer Heinrich Valley. Dashed red line is the approximate extent of the mineralised palaeochannel. Geology after [21]. Note ten times vertical exaggeration.
Figure 5. Geology of the Langer Heinrich Valley. Dashed red line is the approximate extent of the mineralised palaeochannel. Geology after [21]. Note ten times vertical exaggeration.
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Figure 6. Simplified geological map of the Langer Heinrich area (after [21]). Recent sediments are related to the present day Gawib ephemeral drainage (see Figure 3A). White areas are outcropping Damaran metamorphic rocks, notably the schist of the Tinkas Formation. Letters refer to open pit designation.
Figure 6. Simplified geological map of the Langer Heinrich area (after [21]). Recent sediments are related to the present day Gawib ephemeral drainage (see Figure 3A). White areas are outcropping Damaran metamorphic rocks, notably the schist of the Tinkas Formation. Letters refer to open pit designation.
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Figure 7. (A)—carbonate-cemented conglomerate from Tumas. Note the irregular distribution of pale calcite-cemented areas. The dominant cement is noted. (B)—Mineralised and calcite-cemented polymictic conglomerate from Langer Heinrich. Yellow colour is carnotite. Note the erratic distribution of the carnotite. (C)—Silicified conglomerate, Trekoppje open pit. (D)—Carnotite crusting sub-vertical fractures in schist of Tinkas formation at Langer Heinrich.
Figure 7. (A)—carbonate-cemented conglomerate from Tumas. Note the irregular distribution of pale calcite-cemented areas. The dominant cement is noted. (B)—Mineralised and calcite-cemented polymictic conglomerate from Langer Heinrich. Yellow colour is carnotite. Note the erratic distribution of the carnotite. (C)—Silicified conglomerate, Trekoppje open pit. (D)—Carnotite crusting sub-vertical fractures in schist of Tinkas formation at Langer Heinrich.
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Figure 8. Interstitial clay minerals. (A) Back-scattered electron image of fibrous palygorskite from Langer Heinrich. Platy clay mineral not identified [21]. (B) TEM image of vermiculite, Langer Heinrich. Black phase unidentified [24].
Figure 8. Interstitial clay minerals. (A) Back-scattered electron image of fibrous palygorskite from Langer Heinrich. Platy clay mineral not identified [21]. (B) TEM image of vermiculite, Langer Heinrich. Black phase unidentified [24].
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Figure 9. Back-scattered electron images showing the relationship of carnotite to detrital grains and cements. (A,B) Association of carnotite (light grey, Car) with authigenic Mg-Al silicates (Mg-Clay) and calcite (Cal). Detrital quartz (Qtz) fully enclosed in calcite cement. In addition, albite (Alb); Tumas. (C,D) Association of carnotite with dolomite, crystalline Mg silicate (Mg-Clay) and amorphous Mg silicate (AMS); Langer Heinrich [25].
Figure 9. Back-scattered electron images showing the relationship of carnotite to detrital grains and cements. (A,B) Association of carnotite (light grey, Car) with authigenic Mg-Al silicates (Mg-Clay) and calcite (Cal). Detrital quartz (Qtz) fully enclosed in calcite cement. In addition, albite (Alb); Tumas. (C,D) Association of carnotite with dolomite, crystalline Mg silicate (Mg-Clay) and amorphous Mg silicate (AMS); Langer Heinrich [25].
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Figure 10. Relationship between measured porosity and carbonate (measured CO2) content in conglomerates of the Trekoppje palaeochannel [30].
Figure 10. Relationship between measured porosity and carbonate (measured CO2) content in conglomerates of the Trekoppje palaeochannel [30].
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Figure 11. Simplified geological map showing the distribution of felsic intrusions north of the Kuiseb Valley.
Figure 11. Simplified geological map showing the distribution of felsic intrusions north of the Kuiseb Valley.
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Table
Table 1. Uranium resources of palaeochannel type deposits in the Erongo region of Namibia [6,7,8].
Table 1. Uranium resources of palaeochannel type deposits in the Erongo region of Namibia [6,7,8].
Deposit NameOre (Mt)Grade (ppm U3O8)Contained U3O8 (Mlb)
Langer Heinrich145.9535171.9
Tumas 1, 2 and 3200.0258114.1
Klein Trekkopje25.010557.9
Marenica276.09457.3
Aussinanis34.623718.0
Koppies (Tumas)37.120817.0
Trekkopje46.012913.1
Tubas Red Sand 34.017012.7
Klein Spitzkoppe25.015011.2
Welwitschia Flats35.01209.2
Tubas Calcrete7.43746.1
MA 722.8804.1
Brandberg3.02121.4
Hakskeen0.43310.3
TOTAL855.0 494.3
The term “calcrete” is used extensively in this paper. There is no universally accepted definition of the term, which is synonymous with kankar, kunkar, caliche and others [9]. There is no strict definition of the amount of carbonate minerals that must be present in order for the rock to qualify as a calcrete. Informally, a calcrete should contain over 40% carbonate [9], but the amount of carbonate is seldom accurately determined. The usage below is adopted from the various source publications.
Table
Table 2. Mean and median abundance of U, Th and V in various granites of the Erongo region.
Table 2. Mean and median abundance of U, Th and V in various granites of the Erongo region.
U ppmTh ppmV ppm
Intrusion#MeanMedian#MeanMedian#MeanMedian
Bloedkoppie1615141831311044
Bloedkoppie Pegmatite [34]2525 2530
Bloedkoppie Granite [34]339 3333
Donkerhoek1932381311382017
Gawib2655262119233126
Vogelfederberg1284125437126148
Gray62155635628214935
Red1954197070112316
Salem28815252464544
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Wilde, A. Towards a Mineral Systems Model for Surficial Uranium Mineralization Based on Deposits in the Erongo District of Namibia. Minerals 2023, 13, 149. https://doi.org/10.3390/min13020149

AMA Style

Wilde A. Towards a Mineral Systems Model for Surficial Uranium Mineralization Based on Deposits in the Erongo District of Namibia. Minerals. 2023; 13(2):149. https://doi.org/10.3390/min13020149

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Wilde, Andy. 2023. "Towards a Mineral Systems Model for Surficial Uranium Mineralization Based on Deposits in the Erongo District of Namibia" Minerals 13, no. 2: 149. https://doi.org/10.3390/min13020149

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