Investigation of the Manisa-Salihli-Köprübaşı uranium ore deposit

The Köprübaşı (Manisa) uranium deposit is one of the largest uranium deposits in Turkey. The deposit is hosted in Neogene fluvial sediments which consist of sandstone and conglomerate interlayered with siltstone, claystone and mudstone. Sediments were largely derived from metamorphic rocks adjacent to the basins of deposition. The metamorphic rocks and tuffs containing high U content are considered to be the sources of the uranium ore in the study area. There are six main districts at the Köprübaşı. These are Kasar, Ecinlitaş, Çetinbaş, Topallı, Uğurlu (Tomaşa) and Kayran. The sandstones and conglomerates are the most widespread sediments in these districts and host the majority of the U ore. X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energydispersive X-ray spectroscopy (EDS) analysis were used to identify the ore minerals and associated alteration products. The uranium minerals are torbernite [Cu(UO2)2(PO4)2·10H2O], meta-torbernite [Cu(UO2)2(PO4)2·8H2O] and metaautunite [Ca(UO2)2(PO4)2·6H2O], whereas jarosite and chlorite-kaolinite are the main alteration minerals of the host sedimentary rocks. Macroscopic torbernite and autunite can be easily seen with the naked eye as green and yellow-green crystals within conglomerate and sandstone. In addition, microscopic autunite crystals can be seen under ultraviolet light as disseminated in the pore of conglomerate and sandstone. The uranium ore grade of the sediments varies widely, but is generally low to medium grade (0.01 to 0.4 U3O8 %). However, samples from the iron-rich sediments show elevated U3O8 values as much as 1.06 %. The enrichment of uranium in the sediment seems to be related with increasing Fe content. Presumably, Fe oxides play a significant role in the precipitation of the secondary uranium phosphates. Groundwater samples from well located at shallow depths (2 to 6 m) show near neutral pH values (6.2-6.7) and oxidizing conditions (Eh=119 to 275 mV). They are mostly characterized by Ca-Na-Mg-HCO3. All samples are undersaturated with respect to calcite, anhydrite, and gypsum. On the other hand, they are saturated with respect to hematite, goethite, illite, kaolinite, corundum, diaspor, boehmit, gibbsite and quartz. This indicates that the main hydrogeochemical process is weathering of the silicates in the shallow groundwater system. The ground waters kept in contact with sediments containing uranium phosphates show low U content in the range 1.71 to 23.97 ?g/l. The excellent correlation coefficient between U and P indicates the dissolved uranium in waters would be associated with uranium phosphates. The low U concentrations in groundwater samples is due to low solubility of stable U(VI) phosphate minerals under near neutral conditions in the oxidizing host rock aquifer. Keywords: Köprübaşı, Mineralogy, Geochemistry, Uranium phosphate, Hydrogeology