2011 年 61 巻 1 号 p. 13-28
The characteristics of three sub-types of the unconformity-type uranium deposits in the Athabasca Basin, Thelon Basin, Kombolgie Basin, and Rudall River area are summarized. A new ore fluids flow model is proposed to explain the mechanisms that produce such deposits. It is proposed that these deposits are formed by the combination of basinal brines lateral flows driven by hydraulic gradients, and the downward flow of heavy brines through brittle faults which were reactivated by continent-wide tectonic events that postdated the basin deposition. The role of basinal brines was to leach uranium from basin sediments and transfer uranium to the fault planes. The role of heavy brines with high oxygen fugacity and high salinity, derived from evaporite-hosted surficial sediments, was to capture uranium along the fault planes and to transfer it down to the unconformity and farther down into the basement rocks. Ore fluids that flowed into the basement and reacted with rocks rich in graphite and/or mafic minerals, formed monometallic type deposits along the faults, or formed sub-unconformity post-metamorphic type deposits within specific strata-structures. In the case of basement rocks with thick and deep seated graphitic fault zones, the thermally-driven basement brines in reduced condition and rich in Ni-Co-As flowed upwards to the unconformity and reacted with oxidizing ore fluids to form polymetallic type deposits.
This model was investigated quantitatively by calculating the uranium balance in the simplified ore fluids flow system. It was found that uranium deposits with 10's to 100's of thousands tU can be formed within 1 Ma when basinal brines bearing 1-10 ppmU and flowing at a rate of 0.1m/y cross the fault planes.
