Classification and genesis of calcrete and gypsite lithofacies in paleodrainage systems of inland Australia and their relationship to carnotite mineralization

Calcrete and gypsite lithofacies are chemical sedimentary rocks irre spective of age and/or stratigraphic position. The depositional environments of these deposits indicate that, since early Tertiary silting up of inland drainage developed internal salt lake systems with gypsite as the dominant evaporitic lithotype. Elongate bodies of phreatic calcrete formed in paleochannels. Almost all primary calcretes and gypsites have been modified by phreatic and vadose diagenetic processes. Because the diagenetic processes are complex, stratigraphic analysis is difficult. Carnotite mineralization is probably not contemporaneous with calcretization. Assuming that waters from fertile (uraniferous) sources are flowing through the sediments, several controls will govern carnotite mineralization; ponding by physical or chemical barriers, solution chemistry (P CO2 ; CO 3 /HCO 3 , V, U, and K concentrations; Eh, pH), and postdepositional physical or chemical changes. For significant carnotite bodies to remain, primary deposition must have been followed by a "freezing" mechanism to preserve the deposit. Diagenetic conditions which may cause remobilization vary between deposits; therefore it may be unwise to use a single deposit as the basis for a widely applicable exploration model.--Modified journal abstract.