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.
Figures & Tables
Kaolin is an important industrial mineral in several world markets including uses in paper coating and filling, ceramics, paint, plastics, rubber, ink, fiberglass, cracking catalysts and many other uses (Murray, 1991). The kaolin minerals kaolinite, halloysite, dickite, and nacrite have essentially similar chemical composition but each has important structural and stacking differences. The most common kaolin mineral and the one that is the most important industrially is kaolinite [Al2Si205(OH)4]. Kaolinite can be formed as a residual weathering product, by hydrothermal alteration, and as an authigenic sedimentary mineral. The residual and hydrothermal occurrences are classed as primary and the sedimentary occurrences as secondary. Primary kaolins are those that have formed in situ usually by the alteration of crystalline rocks such as granites and rhyolites. The alteration results from surface weathering, groundwater movement below the surface or action of hydrothermal fluids. Secondary kaolins are sedimentary which were eroded, transported and deposited as beds or lenses associated with other sedimentary rocks. Most kaolin deposits of secondary origin were formed by the deposition of kaolinite which had been formed elsewhere. Some secondary deposits were formed from arkosic sediments that were altered after deposition, primarily by groundwater. There are far more deposits of primary kaolins in the world than secondary kaolin deposits because special geologic conditions are necessary for both the deposition and preservation of secondary kaolins.