Abstract
In the the Armorican Massif (NW France), the formation of kaolinite in surficial conditions is demonstrated by the existence of alteration profiles enriched in kaolinite at the top of metamorphic and sedimentary formations. For some of the largest deposits located in granitic massifs, the origin of kaolinization (hydrothermal or supergene) is debatable because these deposits are often spatially associated with networks of quartz veins. O and H isotopic compositions of kaolinitic rocks are very similar in all types of deposits (from alteration profiles, from sedimentary deposits and from those after granites) and are consistent with a supergene origin: δ 18O kaol=20.2±1‰, δD kaol=−55‰ to −67‰. In the Plœmeur Variscan leucogranite, some of the kaolinite rocks located in the vicinity of massive quartz veins may contain a minor hydrothermal component on the basis of slightly lower bulk isotopic compositions. Some of the late quartz veins with peculiar shapes (“draperies” or “wires” made of coalescent submillimetric euhedral quartz) may be evidence of this hydrothermal event at temperatures of the order of 100–150°C ( δ 18O Qtz=23±1‰). We conclude that, in the light of the present data, kaolinite of the Armorican Massif is essentially of supergene origin. The local association of kaolinite with quartz veins in deposits after granites is likely due to an increase of permeability induced by the former event of hydrothermal alteration.
Published Version
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