Nuclear waste repositories in clays: The Orciatico metamorphic aureole analogy

Data on physical, chemical and mineralogical transformations caused in argillaceous rocks by heat and chemical sources are of great interest for problems concerning nuclear waste repositories in clays. A small magmatic laccolith, emplaced in Pliocene clays very close to the original topographic surface at Orciatico in Tuscany, Central Italy, presents features qualifying it as a good natural model of such heat and chemical sources. Around the laccolith the sediments have been transformed depending upon distance from the intrusion.

• 1.(a) Physical transformations, caused by a conspicuous re-crystallization, mainly consist of a sharp loss of plasticity and formation of very hard rocks closest to the contact and of indurated fissile shales farther away. Stresses in the metamorphosed zone caused a diffuse micro-cracking, thus sharply increasing the permeability of rocks.
• 2.(b) Chemical transformations are mainly an important migration of highly mobile elements such as alkalis (Na > K = Rb) and alkaline earths (Ca > Ba > Sr) resulting from hydrothermal circulation. In the Orciatico model the zones affected by cation migration appear to extend for distances up to 15 m.
• 3.(c) Mineralogical transformations are a K-feldspar-plagioclase-pyroxene-biotite assemblage crystallization in zones 0.5 to 1.5 m thick next to the laccolith and a destabilization of the original clay minerals farther out, leading to crystallization of smectite and feldspars.