Mobility of U and Granite Kaolinization in Southwest England
Kaolinization of Hercynian granites in southwest England has resulted in the extensive development of china clay deposits. These deposits typically have a funnel-shaped form often more than 200m in diameter and extending to depths greater than 300m. They occur along major northwest - southeast trending faults and fractures, and have formed at sites where high permeability channelways control descending limbs of radiogenically-driven hydrothermal circulation cells. The presence of U in this environment is important for three reasons:
U is the main source of radiogenic heat,
changes in the abundance of U and its mineralogical distribution with degree of alteration provide evidence of the nature of the alteration processes, and
any disequilibrium in the U decay series can be used to date the occurrence of the alteration processes.
The reduction in density from about 2650 kg/m3 for fresh granite to about 1900 kg/m3 for highly-altered granite provides the basis for a suitable Kaolinization Index to quantify the degree of alteration. Results of gamma-ray, geochemical, autoradiographic and radioactive disequilibrium studies of U in relation to Kaolinization Index show that:
an early stage of high temperature alteration preceded kaolinization,
the kaolinization process is still taking place, and
recystallization of kaolinite appears to be on-going.
The overall picture is one of an active system which, at the present day, is of low energy. Evidence of more vigorous activity in the past suggests other geothermal and seismic phenomena have reinforced the system on several occasions.