Retrograde alteration of clay minerals in uranium deposits: Radiation catalyzed or simply low-temperature exchange?

Stable and radiogenic isotopic compositions have been used to trace the origin and age of fluids associated with the formation and alteration of ore deposits such as the unconformity-type uranium deposits in the Athabasca Basin, Saskatchewan, Canada. Since the formation of the U deposits at the unconformity between Aphebian basement rocks and the overlying Athabasca sandstones at ∼ 1200 Ma, the Athabasca Basin has expeienced episodic influxes of meteoric water that permeated large fault structures rooted in the basement graphitic metapelites and gneisses. Because these structures are associated with the initial formation of the U deposits, alteration due to late meteoric water influx is often prominant near the deposits. The development of late-stage kaolinite, lowering of δD-values in all clay minerals in fractures near the ore zones, loss of K₂O and decreasing KAr ages with increasing water contents in illite are indicative of retrograde alteration of clay minerals due to influx of late-stage meteoric water in the Athabasca Basin. Recent proposals that radiation damage and subsequent retrograde alteration of clay minerals near the U deposits have occurred are not consistent with either the data from the McArthur River area in Saskatchewan where kaolinite having low δD-values formed with small quantities of remobilized U at ∼ 400 Ma, or from data throughout the basin where clay minerals devoid of U mineralization also record this retrograde alterations. The results from McArthur River, in conjunction with data from a number of unconformity-type U deposits indicate that radiation does not promote retrograde alteration but do substantiate the simpler process of retrograde alteration of clay minerals in these deposits by influxes of late, low-temperature meteoric water along reactivated shear zones.