The recognition of hydrothermal processes in the trace element geochemistry of uranium- and thorium-enriched tuffaceous rocks, Northwest Iran

Radiometrically anomalous tuffaceous rocks occur as conformable lenses in a volcano-sedimentary sequence of Neogene age near the village of Sarkhanlu, northwest Iran. The anomalous rocks have been extensively altered by hydrothermal fluids, and kaolinization and silicification are widespread. 250 samples of the anomalous and background tuffs and other sediments have been analyzed for Fe, Mn and 18 trace elements. Enhanced levels of Ag, As, Mo, Pb, Sn, Sr, Th, U, W and Zr indicate an alkaline volcanic origin of the tuffaceous material, and the elements of this association dominate the statistical analysis of the geochemical data. Maximum U and Th values are among the highest ever recorded in extrusive volcanic rocks. A zone of kaolinization contains increased levels of U, Th, Pb and Sr relative to the unaltered country rock, although nine other elements are depleted in the zone. This association of elements is recognized in the radiometrically anomalous lithologies, particularly in the kaolinized tuffs, and indicates the availability of U in the hydrothermal fluids causing the alteration. Although U mineralization has not yet been found at Sarkhanlu, comparisons with the similar geological environment in central Italy and elsewhere suggest possible exploration criteria for the future. Distinct sedimentary U associations have been defined in tuffs with only background radioactivity and in dark, calcareous shales.