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REE Mineralogy and Resources Chapter 279 199 4.1.5 Sediment-Hosted Hydrothermal Deposits This deposit type is typically associated with unconformity-related uranium mineralization, which is found in the Athabasca basin, Saskatchewan, Canada, and Gordon Downs region in Western Australia. The Athabasca basin in Saskatchewan, Canada, 350 km200 km in size, hosts the largest uranium resources in the world. The uranium deposits in the basin, which are categorized to unconformity-associated deposits, are distri- buted along the margin of the basin near the unconformity between the Pro- terozoic basin-fill sedimentary rocks and the basement gneiss (Jefferson et al., 2007). In this basin, there are some REE prospects which are characterized by the presence of xenotime with goyazite, hematite, and clay minerals disseminated in sandstone. Such REE mineralization, ie, at Maw Zone, appar- ently formed from the similar saline oxidizing basin fluids that were responsible for hydrothermal alteration and uranium mineralization (Quirt et al., 1991). In the Brown Ranges prospect in Western Australia, xenotime-(Y) and subordinate florencite-(Ce) are disseminated and/or included in quartz vein- lets with REE-bearing zircon, goyazite, and hematite, forming several ore bodies a few hundred meters in size sporadically distributed in the Archean to Paleoproterozoic metasedimentary rocks. Detectable uranium and thorium are found in xenotime-(Y) (Cook et al., 2013). Vallini et al. (2007) proposed that the xenotime mineralization at the Killi Hills prospect, 60 km south of Browns Ranges, is related to an unconformity between the Killi Formation and the overlying Cardiner Sandstone and have formed from fluids released during diagenesis of the later. However, the REE mineralization at Browns Ranges is clearly ascribed to percolation of a volatile-rich, acidic fluid, possi- bly granite-derived, through permeable arkose units. Late hematite may sug- gest mixing with meteoric water and subsequent oxidation (Cook et al., 2013). 4.1.6 Ion-Adsorption Type Deposits Ion-adsorption type deposits are formed by weathering of igneous rocks (typ- ically granites) that contain certain REE-bearing minerals. Due to surface weathering, REE minerals are decomposed, and ionized REEs are absorbed on clay minerals such as halloysite and kaolinite. The absorbed REEs are recovered by a dilute electrolyte solution such as ammonium sulfate solution and precipitated by oxalic acid solution. Because the mining and recovery processes are simple and inexpensive, REEs are commercially produced from extremely low grade (typically up to 2000 ppm REE) weathered clay in south- ern China. On the other hand, developing of ion-adsorption type deposits includes environmental issues related to in situ leaching, large leach residue, overexploitation, and environmental restoration costs (Ding, 2012; Yang et al., 2013). Although ion-adsorption type deposits were developed only in southern China (Xie et al., 2016), several projects have been also known in thePDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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