GEOCHEMICAL MODELING OF THE FORMATION OF AN UNCONFORMITY-TYPE URANIUMDEPOSIT

Petrologic studies of alteration associated with uranium mineralizatio n in unconformity-type uranium deposits have established a detailed pa ragenetic sequence of the preore and synore alteration. In addition, f luid inclusion studies Have determined the presence of two distinct fl uids, and thermodynamic analysis of the alteration assemblages has def ined their basic characteristics. These fluids were probably basinal b rines derived from the overlaying sandstones and have been distinguish ed by their different salinities. Fluids responsible for alteration an d ore formation were Na-Ca-Cl brines. Speciation calculations suggest that the hydrothermal fluids responsible for the formation of unconfor mity-type uranium deposits were distinguished by their high oxygen fug acity and relatively low pH. Uranium was transported mainly as uranyl chloride complexes. Reconstruction of the reactions associated with th e formation of the deposits, employing chemical mass transfer calculat ions, helps unravel the geochemical processes operating during their g enesis. Reaction path calculations indicate that Fe present in alumino silicates in the host rocks is the principal reductant of oxidized ura nium present in the ore-forming fluids. The use of variable host-rock lithologies results in the reproduction of mineralogical features uniq ue to each deposit, along with the characteristics common to all uncon formity-related uranium deposits. The calculations shows that the obse rved paragenesis is a function not only of the fluid chemistry but als o of the host-rock lithologies. The modeling indicates that although a n orebody can be produced with or without graphite as a reactant, its presence leads to a better agreement with the observed alteration para genesis.