Origin of kaolinization in Brittany (NW France) with emphasis on deposits over granite: stable isotopes (O, H) constraints

In the the Armorican Massif (NW France), the formation of kaolinite in surf icial conditions is demonstrated by the existence of alteration profiles en riched in kaolinite at the top of metamorphic and sedimentary formations. F or some of the largest deposits located in granitic massifs, the origin of kaolinization (hydrothermal or supergene) is debatable because these deposi ts are often spatially associated with networks of quartz veins. O and H is otopic compositions of kaolinitic rocks are very similar in all types of de posits (from alteration profiles, from sedimentary deposits and from those after granites) and are consistent with a supergene origin: delta(18)O(kaol ) = 20.2 +/- 1 parts per thousand, delta D-kaol = - 55 parts per thousand t o - 67 parts per thousand. In the Ploemeur Variscan leucogranite, some of t he kaolinite rocks located in the vicinity of massive quartz veins may cont ain a minor hydrothermal component on the basis of slightly lower bulk isot opic compositions. Some of the late quartz veins with peculiar shapes (''dr aperies'' or ''wires'' made of coalescent submillimetric euhedral quartz) m ay be evidence of this hydrothermal event at temperatures of the order of 1 00-150 degrees C (delta(18)O(Qtz) = 23 +/- 1 parts per thousand). We conclu de that, in the light of the present data, kaolinite of the Armorican Massi f is essentially of supergene origin. The local association of kaolinite wi th quartz veins in deposits after granites is likely due to an increase of permeability induced by the former event of hydrothermal alteration. (C) 20 00 Elsevier Science B.V. All rights reserved.