Oliver, and 9 co-authors, reported in 2020 that the Eureka deposit in Castell-estaó in the Catalan Pyrenees is a Cu–U–V deposit, hosted by Triassic red-bed sandstones, and classified here as a low-temperature, sandstone-hosted stratabound U deposit. The main mineralization is stratabound, related to coal-bearing units and formed during the Alpine deformation by migration of hydrothermal fluids [aka groundwater likely in this case]. In this stage, the original sedimentary and diagenetic components (quartz and calcite, micas, hematite and locally apatite) were replaced by a complex sequence of roscoelite, fine-grained REE phosphates, sulfides and Ni–Co arsenides and sulpharsenides, Ag–Pb selenides, bismuth phases, sulphosalts and uraninite.
The black shales of the Silurian sediments underlying the deposit and the nearby Carboniferous volcanoclastic rocks are interpreted as the source of the redox-sensitive elements concentrated in Eureka. The sulfur source is related to leaching of the evaporitic Keuper facies. The REE transport would be facilitated by SO4-rich solutions. The reduction of these solutions by interaction with organic matter resulted in the widespread precipitation of REE and redox-sensitive elements, including many critical metals (V, Bi, Sb, Co.....also Se and Mo would also be expected), whereas barite precipitated in the oxidized domains. The occurrence of similar enrichments in critical elements can be expected in other similar large uranium deposits, which could be a source of these elements as by-products.
The authors concluded that the petrographic characterization of the Eureka deposit revealed a unique mineralogical complexity and diversity encompassing characteristics previously observed in a variety of IAEA deposit types. Moreover, the Eureka deposit contains significant concentrations of critical elements in discrete minerals. In particular, REE minerals are widespread as fine-grained phases intergrown with roscoelite. Cobalt occurs as arsenides and sulpharsenides; bismuth, as sulphosalts and native bismuth; vanadium, mainly as roscoelite and secondary vanadates. Therefore, these elements could be a by-product of the beneficiation of Cu or U in this style of deposit, although the fine grain size of the ores can complicate their extraction. The source of the HFSE could be the underlying Silurian black shales (the detachment level of many of the thrusts in the basement), whereas the source of U and REE is probably associated with the volcanic rocks of the area. The source of S is likely related to the nearby evaporitic sulphates of the Keuper facies, which are also the detachment level of many thrusts in the Pyrenees.
The authors also concluded that deposit was formed during the Alpine orogeny, favored by hydrothermal fluid (aka groundwater) movement during thrusting. Most metals were transported as chloride complexes, and REE as sulphate complexes. The arrival of these hydrothermal fluids to the coal-bearing Triassic unit resulted in the reduction of the SO4 in solution to S2−, triggering the precipitation of the metals as sulfides, uraninite and roscoelite by interaction with the original cements. Reduction of the SO4 complex to sulphide, combined with destabilization of fluorapatite, also forced the precipitation of REE as phosphates. The low temperature of the mineralizing fluids was most likely controlled by the prehnite-pumpellyite facies regional metamorphic conditions. The irregular distribution of mineral phases is here related to pH and chemical gradients in the deposit. These gradients are explained by the movement of the fluids (aka best termed groundwater) along the slaty cleavage in rocks of different grain sizes and parageneses, resulting in diverse reactivity.
Textural observations also suggest lack of equilibrium between the minerals and the fluids. Supergene alteration resulted in the dispersion of some critical elements, such as nickel and cobalt, while vanadium was fixed as vanadates, and bismuth, as oxides. REE phosphates as monazite and xenotime are stable in this context. Regardless of the origin of the fluids, the deposition of critical elements in Eureka was most likely controlled by the occurrence of organic matter. Therefore, other sandstone-hosted deposits in non-metamorphosed sedimentary basins could also be explored for the same critical elements found in Eureka if similar coal levels are present.
Updated: http://www.i2mconsultants.com/downloads/corp/minerals-10-00034-v2.pdf
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