ABSTRACT

Calcrete and gypsite lithofacies, recorded from inland drainage systems of Australia, are chemical sedimentary rocks which have been deposited under arid to semi-arid environmental conditions irrespective of age and/or stratigraphic position. Reconstruction of the depositional environments of these deposits in inland Australia indicates that, since early Tertiary times, silting up of inland drainages developed internal salt lake systems with gypsite as the dominant evaporitic lithotype. Elongate bodies of phreatic calcrete formed in paleochannels leading into the drainage sumps. Textures, structures, and compositions of almost all primary calcretes and gypsites have been modified by phreatic and vadose diagenetic processes as hydrogeologic units were redistributed. Because the diagene ic processes are complex, stratigraphic analysis is difficult; this particularly applies to determination of the primary lithofacies in hydro-geochemically active systems.

Carnotite mineralization of the calcrete profiles is probably not contemporaneous with calcretization. Assuming that waters from fertile (uraniferous) sources are flowing through the calcrete/gypsite sediments, several controls will govern the presence, and extent of, any carnotite mineralization, including a) pending by physical or chemical barriers, b) solution chemistry (P_CO2; CO₃/HCO₃, V, U, and K concentrations; Eh, pH), and c) postdepositional physical or chemical changes which may cause remobilization and removal of the carnotite. For significant carnotite bodies to remain, primary deposition must have been followed by a "freezing" mechanism to preserve the deposit. Despite the similarities in the primary depositional mechanism for most carnotit deposits in calcretes, 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.