Process Calcrete Classification
Nonp«dogenic Superficial Calcrete Subsurface Water Zones X LwwnM crutu
/ ullv ta«d c«m«nt«l(on
C.IC.M.S _3&j'-j( , f^rfff wit>_ ^-Vr Gravitational Zone Calcrete
V - "V"" -"•" x^
.- 7 V^ "•,'*-' !-•- T,*n.p»n ——— p. NONPEDOGENIC GROUNOWATER ' s. *• CALCRETE
Iji '*" -jL-4, -^ ^A&&^£ • VALLEY (CHANNEL) CALCRETE ' —— •—— J^' '^"^^^r^K W«rt»m Au«ti*li*n
N»irob D.««rt
• DELTAIC CALCRETE
• LAKE M ARG IN CALCRETE
• Alluvial fan. Cienaga, fault-trace,
w,.,r~.ns DETRITAL AND RECONSTITUTED CALCRETE
"nd EfOM°" Transported OCCASIONAL of pr«..»t«o URANIUM
„ , Brecciated and recemented in SITU
""*'"• FA VOR ABI LI TV Aftv Orlitl* (1980 19831
Figure 1
A genetic classification of calcretes and their uranium favourability based upon relations to subsurface water, depositional process and geomorphic setting. A given calcrete may result from multiple processes as suggested by the arrows but not all varieties form conjointly or under similar conditions [22]. (Reproduced b y permission of
the Geological Society of London.)
cementing materials. Gypsum is subordinate to absent except under special conditions such as on the margins of or downwind from salinas or as on the Namib Desert, where marine mists provide sulphate. Calcretes may be eroded and the transported fragments reconstituted into detrital calcrete.
The role of climate and other regional controls of calcrete uranium mineralization is tied to physical, chemical, and genetic differences between two major categories of calcrete (Figures 1 and 2). In calcrete uranium exploration, a critical distinction must be made between these:
1. Pedogenic calcrete, i.e., soil caliche, kunkar, crote calcaire, nari or caprock, which is generated in the soil moisture zone by ordinary soil-forming processes and is not favourable for uranium mineralization and, 2. Nonpedogenic = groundwater calcrete [4,6,7], i.e., "valley calcrete" or simply "calcrete" in earlier Australian
terminology [8,9] is generated mainly near the water table from groundwater moving along extremely low gradients and which, under favourable circumstances, may contain recoverable uranium.
2.1 Distinguishing Characteristics
Pedogenic calcrete, then, is simply a calcic soil horizon or, if indurated, a"petrocalcic" horizon. It is typically only a few centimetres to a metre or two thick, and laterally extensive. Other things being equal, the thickness and maturity of a pedogenic calcrete are a function of the age and stability of the surface beneath which it forms. The calcrete in turn tends to reinforce the surface. Almost invariably, pedogenic calcretes display some sort of
"calcrete soil profile" consisting, for example, of calcified soil perhaps overlain by a completely cemented (plugged) or hardpan horizon, and possibly by laminated or solution-brecciated calcrete or by more soil. Multiple or stacked pedogenic calcretes are common. The calcium, magnesium, carbonate, and trace elements are derived from adjacent soil or from the air and are merely redistributed vertically within the profile. Only if soils are unexpectedly rich in uranium or enriched from the air, from below, or by deep residual concentration, are pedogenic calcretes likely to reach ore grade. Though small-scale examples are known, the favourability of pedogenic calcretes as uranium ore, is very low to negligible [10].
Nonpedogenic groundwater calcretes, on the other hand, result from lateral transport of soluble ions toward favourable sites of deposition, making it possible for ores to develop by concentration of uranium from a large source area into a relatively compact calcrete body. The calcrete-free drainage catchment above the ore zone at Yeelirrie for example, is 3 000 km2, which is 20 times the area occupied by calcrete and 1 000 times the area underlain by ore. Because water tables fluctuate and because flowing groundwater brings a continuing supply of constituents, such calcretes can be much thicker than a single pedogenic calcrete. In arid regions of low relief, concentrations of calcium, magnesium, uranium, vanadium, and potassium are increased downdrainage by evaporation, but not that of dissolved CO2. As groundwaters converge, typically along the axes of stable near-surface trunk drainages, or where they encounter bedrock constrictions, lower gradients, less permeable clays or
"NEW PLATEAU" "OLD PLATEAU"
VALLEY CALCRETE WESTERN AUSTRALIA
BREAKAWAY
LATERITIC PROFILE AND WEATHERED
ARCHEAN.
NONPEDOGENIC VALLEY CALCRETE/DOLOCRETE.
0 10 Km Approximate Scale
B. VALLEY CALCRETE/GYPCRETE AND
SURFICIAL CALCRETE/GYPCRETE, NAMIB DESERT
PEDOGENIC SHEET CALCRETE / NONPEDOGENIC
OR GYPCRETE VALLEY CALCRETE
AND GYPCRETE
Figure 2
Valley calcrete morphology, a) valley calcrete. Western Australia, (b) valley calcrete/gycrete and surficial calcrete/gypcrete, Namib Desert [22]. (Reproduced by permission of the Geological Society of London.) dense hypersaline waters on the edge of evaporative basins, water tables tend to rise toward the surface, loss of C02 and H2O is accelerated, and authigenic carbonate precipitates within the regolith. Carnotite may precipitate with it.
Groundwater calcretes are diagnostically free of any pedogenic soil profile or vertical sequence of pedogenic calcrete horizons unless modified by later pedogenic processes. They tend simply to preserve the original sedimentary structures and textures of the host materials or to develop new structures related to groundwater flow. Groundwater calcretes which develop within the phreatic zone or in relatively unweathered sands or gravels tend to contain clean sparry carbonate as opposed to the clayey, impure, and amorphous mixtures characteristic of pedogenic calcrete. Even in a clay-rich host, they do not contain a distinct horizon of clay accumulation.
Groundwater calcrete generated in swampy or lake margin environments may develop primary sedimentary—not pedogenic — layering, however, and may contain organic structures perhaps of algal, bacterial or macrobiotic origin. This latter variety is in essence a transition toward fresh-water limestone.
On a large scale, groundwater calcretes may develop systematic changes in morphology and composition along their length, reflecting lateral flow in an evaporative regime. Thus in Western Australia, calcretes are part of an orderly sequence from silicified non-calcareous soils on the flanks of the valleys, to siliceous calcrete
115° 120°
15°
300 200 100
• CALCRETE URANIUM DEPOSIT A« AN
ARIDIC-WILUNA NULLAGINE KALGOORLIE
AK TYPES f1
U Us U STIC 300F
20°
SUBUSTIC (MONSOONAL) 200 XERIC
x Xs SUBXERIC (MEDITERRANEAN)
*" -—-.a/vît——
VALLEY CALCRETE REGION
•AW
WILUNA 300
40oFKalgoorlie, PEDOGENIC
*S CALCRETE
\
400\EVAPORATION -160 2Û300|-TEMPERATURE^40 °C
20o-PRECIPITATION20
0 50 100 150 20O 250 0 100 200 300 400
Kilometer
J. S. N. J M. M Month
Figure 3
Valley calcrete, Wiluna Hardpan and soil moisture regimes of Western Australia inferred from climadiagrams.
Nonpedogenic valley, deltaic and lake-margin calcretes, siliceous Wiluna Hardpan and Mulga-dominant plant community all fall within the tropical cyclone belt and predominantly within a specific aridic regime (Aw) characterized by erratic late summer storms. Boundaries of valley calcrete region and Wiluna Hardpan largely after Sanders [9] and Bett enay and Churchward [23] respectively. Climadiagrams assembled from data supplied by Australian Bureau of Meteorology [22]. (Reproduced by permission of the Geological Society of London.)
increasingly magnesian m the mam channels, to lake-margin gypsite and finally the salt pan itself The calcrete portion, up to hundreds of kilometres in length, several kilometres m width and tens of metres thick (Figure 2) commonly contains domal structures ("mounds") apparently developed around rising groundwater plumes Valley calcrete usually terminates in a calcrete delta or lake-margin calcrete, and at Lake Maitland [11], uramferous groundwater calcrete appears to have formed within the mam body of the playa itself.
Nevertheless, differentiating between groundwater and pedogenic calcretes at the local scale is not easy, and hand specimens of the two varieties may appear physically and chemically identical Moreover, upper surfaces of exposed groundwater calcretes are usually modified by pedogenic processes including soloution-brecciation and recementation Groundwater calcretes may be overlain by younger pedogenic calcretes or, if the water table is very shallow, carbonate deposition from the capillary fringe or the phreatic zone may overlap pedogenic deposition in the soil moisture zone On alluvial fans, pedogenesis may combine with lateral transport, producing hybrid calcretes However, as discussed next, uramferous groundwater calcretes tend to develop optimally under conditions which are much less favourable for pedogenic calcretes and vice versa.
3. THE UNIQUE DISTRIBUTION OF URANIFEROUS CALCRETE IN AUSTRALIA
On geological evidence [6] and by inference from isotopic studies [7, 12, 13, 14, 15] the well-exposed uramferous calcretes of Western Australia are less than 700 000, perhaps less than 25 000 years in age Some are forming and some are being reconstituted today Their genesis is in essence directly observable
With few exceptions, none of which have been shown to be economic, all of the important uranium occurrences are in nonpedogenic valley, deltaic and lacustrine calcrete and dolocrete in the most arid parts of Western Australia ( Figure 3), the Northern Territory, and South Australia. They are inland from the active coastal streams of the Indian Ocean, south of the monsoonal ram belt and entirely north of the "Menzies Line", a curving boundary approximately along latitude 30 °S Over sixty significant prospect areas have been found, all within an area of one-quarter million km2 in the northern part of the Archaean Yilgarn Block The main region of valley calcrete extends as far again to the north above proterozoic granitic, metavolcanic and metasedimentary rocks and even above Palaeozoic and Cretaceous sedimentary rocks but contains few prospects outside the Yilgarn Block Yilgarn granites with uranium contents of from less than 2 to 25 ppm are apparently the main uranium source rock [1 6] By contrast, Yilgarn source rocks are found and much the same geological and geomorphic history pertains to the area south of latitude 30 °S But here there are only pedogenic calcretes and no significant uranium mineralization
The uranium-bearing calcretes occupy minor portion of alluviated valleys on the "new plateau,' which is a Cenozoic surface of extremely low relief within a pre-Tertiary deeply latenzed surface of the old plateau' (Figure 2) [1 7] Drainage gradients range from roughly 1 5 % near the headwaters to as little as 0 02 % m the vicinity of, for example, the Yeelirne orebody. The thickest and widest calcrete bodies tend to occur where bedrock morphology or some other feature has caused a decrease in gradient and where the groundwater table approaches the surface They are entirely within the uppermost portion of the valley-fill, at Yeelirne roughly the upper one-fifth As suggested above, therefore, they postdate considerably the onset of arid alluviation some 2 5 Ma ago and perhaps may correspond with the major period of aridity beginning 25 000 years ago [18]
4. CLIMATE AND SOIL MOISTURE REGIME IN RELATION TO CALCRETE URANIUM MINERALIZATION IN AUSTRALIA
The contrast between regions with uramferous nonpedogenic calcrete and those with nonuramferous pedogenic calcrete is clearly reflected m the Soil Map of Australia North of latitude 30 °S and over an area almost precisely congruent with that of the valley calcretes, the dominant soils within the new plateau, exclusive of the calcretes themselves, are noncalcareous earthy loams with a red-brown siliceous hardpan (see Wiluna Hardpan below and Figure 3, not to be confused with silcrete on the old plateau) None of these soils occur in the semi-arid region south of latitude 30 °S Instead, one finds great areas of alkaline and calcareous red earths and grey-brown calcareous earths, many with visible pedogenic calcrete
An almost exactly correlative and equally striking difference is shown by the dominance of the Mulga tree (Acacia aneura) m the plant communities of the north and of the Mallees, eucalyptus trees with a multi-stemmed growth habit, in the south The Mallees are characteristic of drier areas with winter rainfall Mulgas favour the more arid summer storm belt These and other contrasts between climate and consequent soil moisture regimes north and south of latitude 30 °S are summarized in Table 1 Mam climatic factors which influence soil moisture regimes and calcrete genesis are shown on Figures 4 and 5
1. Inferred Xeric (X, Xs) - Characteristic of a Mediterranean climate where winters are moist and cool and summers are warm and dry The moisture coming m winter when potential evaporation is at a minimum, is particularly effective for leaching
2. Inferred Aridic (Ak, Aw, An) — Characteristic of an arid climate, less commonly semi-arid The soils are hot and dry on average or never moist for long periods Potential evaporation and temperature are usually high during rainy periods Calcretes in general form predominantly in andic regimes
Table 1
Contrasts Between Uraniferous and Nonuraniferous Calcrete Regions, Western Australia [6,22] (Reproduced by permission of the
Geological Society of London)
VALLEY CALCRETE REGION GENERALLY SOUTH OF VALLEY CALCRETE
ANNUAL RAINFALL (Pa):
170-250 mm. Highly variable, episodic late summer thunderstorms sporadic tropical cyclones.
ANNUAL POTENTIAL EVAPORATION (Ea )-3 )-300-4 200 mm
REGIME-Strongly andic and distinctive Moderate to severe drought incidence.
SOILS:
Acidic to earthy loams with siliceous Wiluna Hardpan. Calcareous on or near calcrete.
VEGETATION-Mulga (Acacia aneura) dominant.
GROUNDWATER:
Potable high on drainages, saline downdramage
200-500+ mm. Predominantly winter rainfall from anti-cyclonic frontal rams or indefinite ram season.
3 300 mm
6-16
3 000 mm 19°C
Andic to Xenc. Moderate to low drought incidence.
Alkaline calcareous earths common. Neutral, alkaline, to acidic m humid southwest.
Mallee habit of eucalyptus dominant.
Extensively saline and nonpotable in wells or bores.
3. Inferred UsticfU, Us) —I n this case, characteristic of the monsoonal areas of the north where rainfall reaches a decided peak in the summer and is accompanied by a decline in evapotranspiration.
All of the major calcrete uranium occurrences are m region Aw, the subdivision characterized not only by the most extreme water deficiency but also by brief, intense, highly variable storms which occur during the summer and autumn while soil temperatures and evaporation rates are near their highest It lies directly in the belt of hightly erratic tropical storms and unlike the andic Kalgoorlie region (Ak), it lacks year-round precipitation and frontal rains which would cause the soil moisture zone to become saturated for long periods. The boundary between Aw and Ak closely approximates latitude 30 °S
One more striking coincidence between the andic region Aw and the region of uraniferous calcrete is the distribution of the siliceous Wiluna Hardpan (Figure 3). Wiluna Hardpan is an authigenic deposit consisting of soil, colluvium, and alluvium up to 15 m thick, variably cemented and replaced by opaline or chalcedomc silica.
Part of the orderly but less conspicuous sequence mentioned earlier is just as characteristic of the valley calcrete region as in the calcrete itself and essentially contemporaneous with it It is found universally beneath alluvial and sand plains of the new plateau down to valley axes where it may alternate with or give way to calcrete. Soils above the hardpan are porous earthy loams which a re freely permeable and well drained. They are nonsalme, acidic, and low in organic matter and in base exchange capacity. In addition to the distribution shown on Figure 3, hardpan is also known to occur near the Western Australia - South Australia border m that area of nonpedogenic calcrete.
The writer suggests that siliceous hardpan, valley calcrete, and carnotite mineralization are related genetically to climate, including analogous climates of the last 25 000 years or more m Western Australia
North of latitude 30 °S, rain from the sporadic mid-summer and late-summer storms falls almost invariably on hot, dry surfaces. A large part of it evaporates directly or runs off initially as sheet flood. The bulk infiltrates deeply enough into open-textured earthy loams above the hardpan to be more or less protected from evaporation and loss of C02 The permeability of hardpan is apparently sufficient when wet to facilitate downward movement of the water, and the fact that soil temperatures decrease with depth m summer, results in downward transport of soil water vapour as well. In essence, a relatively small fraction of the storm rain remains m the soil moisture zone, where it is subject almost immediately to evapotranspiration The soil does not remain wet for any long period, and there is very little opportunity for precipitation of pedogenic carbonate by C02 loss prior to evaporative
115° 120° 125°