Uranium ores from saline arid-zone groundwaters
In the arid desert zone that is the northern Yilgarn region of West Australia, uranium deposits occur in playa and paleochannel calcretes, with an intermediary third type occurring geomorphologically between the two settings, typically designated as platform or delta calcretes, often located where channel drainages enter playas (A). All deposits are at or near the land surface and have a similar capillary-driven genesis and ore mineralogy. They are responses to concentrative changes in shallow groundwater redox interfaces, with circulations driven by evaporation, and U-precipitates typically dominated by carnotite (K2(UO2)2(VO4)2·3H2O). Uranium-rich secondary silica is also important locally in the mineralised zones (B). Carnotite deposits occur along the drainage systems throughout the Yilgarn region and include the world class Yeelirrie deposit (A). Other potential U-resources on the Yilgarn craton are the Lake Maitland, Centipede and Lake Way prospects.
Uranium in the Yilgarn groundwater is sourced from granite weathering, and vanadium from greenstones and alluvial sediments. Groundwater moves down gradient into the channels (A), where capillary evaporation occurs (both in channels and playas) and, as a result, carbonate and apatite (if PO4 is present) precipitates. This is important for two reasons\: (i) carbonate-rich zones will have a high-pH groundwater; and (ii) the formation of carbonate and apatite will result in the loss of P and HCO3 from the groundwater. Both of these conditions force the reaction of the aqueous species of U, V and K and so form carnotite.
Although apatite precipitation could drive this reaction, it is a minor contributor to the carbonate formation in the Yilgarn. The role of PO4 in the solubilization of U is hypothesized to be the more important mechanisms in arid zones where groundwater pH is more acidic, such as the active weathering uplands (closer to the breakaway crests). Apatite precipitation is more likely near these zones and away from the carnotite mineralization. This process will shift the pH to more alkaline and change the dominant U-bearing aqueous species from UO2HPO4 to UO2(CO3)2.
The arid association of the uraniferous calcretes in Australia means gypsum is often present as a gangue mineral and this can create processing problems). The presence of gypsum is a processing hindrance when an alkaline leach solution is used as it reacts to precipitate calcium carbonate and forms sodium sulphate in solution. A gypsum content of more than 4% in a surficial calcrete uranium ore leads to excessive reagent consumption and sufficiently affects processing costs to rule out the use of relatively cheap sodium carbonate-based leach solutions.
To form a significant volume of uraniferous salts in this semi-arid setting requires a combination of groundwater and aridity to drive capillary concentration of the uraniferous salts, and the co-precipitation of evaporitic capillary salts such as gypsum and various sodium sulphate salts. Lateral transport of uranium in brackish to increasingly saline groundwater in the outflow sump is essential to ore deposition (B), as is the presence of bedrock barriers or constrictions. These narrow the channel and act as sills with respect to the sluggish groundwaters moving down-dip within the channel confines. Constriction forces the groundwater to pond and so move closer to the land surface, where it is subject to evaporation and loss of dissolved CO2. A lack of constriction and groundwater ponding to focus mineralization explains why the larger part of all valley calcretes and gypcretes are unmineralized in the Yilgarn. Ore-grade mineralization is roughly horizontal in attitude and related to capillary concentration of shallow past or present groundwater tables. Mineralization also requires a uranium source, which are typically weathered Proterozoic to Archaean migmatites, pegmatites, alaskites, and granites. Uraniferous "calcretes" are typically found in regions with nearby“hot” sources, that are are already appreciably enriched in uranium. Deposits cannot form without evaporitic ground conditions, so the uranium salts are true evaporite salts, driven by solar evaporation, in the same way that capillary anhydrite and capillary halite are true evaporite salts forming in a sabkha mudflat.
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