摘要：The Nasirabad manganese deposit is located 5 km south of Nasirabad, 8 km SW of Neyriz in the Fars province. Structurally, the area is placed in the southeastern part of Zagros thrust belt. In this area, the manganese mineralization occurred as ore layers and nodules, interlayered with Pichakun radiolarite chert deposits. In this study, mineralogy and geochemistry of uranium, thorium and lead isotopes were used to investigate the primary and secondary processes. In this way, in addition to petrographic and XRD studies, ICP-MS analysis was carried out in order to measure the U, Th and Pb isotopes. The strong fractionation of Fe and Mn phases and also the absence of Fe-bearing minerals in the XRD results, presence of syngenetic todorokite and quartz crystals, high U/Th ratios in some samples and Th versus U diagrams, all indicate entrance of Mn-bearing hydrothermal fluids into the sedimentary basin of the Nasirabad manganese deposit. The pyrolusites in radiolarites tests as replacement textures, host rock space filling and fracture filling pyrolusites, indicates the influence of secondary exogenic processes on primary hydrothermal mineralization. Non-homogenous 206Pb/Pb204, 207Pb/Pb204 and 208Pb/Pb204 values show non-steady hydrothermal processes in the sedimentary basin and indicate mixing of hydrothermal lead isotopes with another secondary source. Strong positive correlation between absolute values of radiogenic lead isotopes and insoluble High Field Strength Elements (HFSE) such as 207Pb vs Nb (r=0.81), 207Pb vs TiO2 (r=0.93), 207Pb vs Th (r=0.79) and strong correlation between these elements and some mafic components like 208Pb vs Fe2O3 (r=0.94) and Th vs MgO (r=0.86) represent entrance of radiogenic lead with mafic detrital materials into the sedimentary basin. Similar linear trend among 206Pb/Pb204 vs 208Pb/Pb204 and 207Pb/Pb204 ratios in nodules and manganese layers show the same geochemical condition in Mn-nodules and layers formation and indicate that isotopic ratios were unaffected by secondary processes including leaching and redeposition of manganese.
