A quantitative evaluation of the solubility of uraninite (UO2) in aqueous solutions under hydrothermal conditions was made using previously reported thermodynamic data, so as to inquire into the controlling factors for Canadian unconformity-type ore mineralization as observed in the Athabasca uranium field. The results of solubility calculations suggest that uranyl carbonate complexes, such as UO2CO~, UO2(CO3) 2-and UO2(CO3)~-, predominate under relatively oxidizing and slightly acidic-alkaline conditions and that the uranyl chloride complex, UO2 C1 § is dominant under acidic conditions. These features are predicted at temperatures up to 200 ~ over reasonable ranges of CO 2 pressure (Pco 2) and salinity. Consequently, the physico-chemical parameters, such as oxygen activity (ao2), and pH are regarded as the most important factors controlling uraninite solubility. Judging from the paragenetic sequences observed in most unconformity-type uranium deposits in the Athabasca district, appreciable decreases in the above variables are postulated to have occurred in the stage of principal uranium deposition. Such changes would be due to fluidmixing phenomenon accompanied by the 'diagenetic-hydrothermal' activity (Hoeve and Quirt 1987).

No abstract

No abstract

Formation conditions of hydrothermal uranium deposits associ ated with organic matter, with special reference to the unconformity-related and sandstone-type deposits This study summarizes geologic and geochemical features of the "unconformity-related type" (Athabasca and Alligator Rivers districts), "Oklo-Mounana type" (Franceville district) and "sandstone-type" (Grants district) uranium deposits , in order to compare mechanisms of ore formation and the role of organic matters in uranium mineralizations. The unconformity-related type and Oklo-Mounana type uranium deposits occur mostly in Early Proterozoic sedimentary rocks, and are controlled frequently by basal unconformity of the overlying unmetamorphosed sediments and/or steeply dipping faults. Principal mineralization in these types of deposits is considered to have been caused by reactions of uraniferous oxidizing solution with hydrocarbon-bearing reducing fluid derived from carbonaceous materials (graphite and bitumen). The uranium deposits in the Grants district are located in felsic sandstone members within the Upper Jurassic Morrison Formation, and uranium enrichment can be regarded as a result of interactions of diagenetic uraniferous solution with amorphous organic matter (humic substance). These phenomena clearly indicate that all of the three types of deposits are "epigenetic" , although these deposits passed through distinct thermal histories in the diagenetic-hydrothermal metallogenic processes. In the processes of uranium fixation, such carbonaceous and humic materials are inferred to have played an important role as actual reductants.