Petrologic studies of alteration associated with uranium mineralization in unconformity-type uranium deposits have established a detailed paragenetic sequence of the preore and synore alteration. In addition, fluid inclusion studies have determined the presence of two distinct fluids, and thermodynamic analysis of the alteration assemblages has defined their basic characteristics. These fluids were probably basinal brines derived from the overlying sandstones and have been distinguished by their different salinities.Fluids responsible for alteration and ore formation were Na-Ca-Cl brines. Speciation calculations suggest that the hydrothermal fluids responsible for the formation of unconformity-type uranium deposits were distinguished by their high oxygen fugacity and relatively low pH. Uranium was transported mainly as uranyl chloride complexes.Reconstruction of the reactions associated with the formation of the deposits, employing chemical mass transfer calculations, helps unravel the geochemical processes operating during their genesis. Reaction path calculations indicate that Fe present in aluminosilicates in the host rocks is the principal reductant of oxidized uranium present in the ore-forming fluids. The use of variable host-rock lithologies results in the reproduction of mineralogical features unique to each deposit, along with the characteristics common to all unconformity-related uranium deposits. The calculations show that the observed paragenesis is a function not only of the fluid chemistry but also of the host-rock lithologies. The modeling indicates that although an orebody can be produced with or without graphite as a reactant, its presence leads to a better agreement with the observed alteration paragenesis.