Piégut-Pluviers Granodiorite
Its outline is in the shape of an inverted comma pointing north with a nearly squarish main southern part turned into the NE-SW direction.
This section continues farther north in an almost ten kilometer long appendix that is separated from the main body only by a very thin layer of migmatitic gneiss.
The Piégut-Pluviers Granodiorite is surrounded in the north, northeast and east by the Saint-Mathieu Leucogranite and its equivalents (SML on the geological map), which has a slightly younger age of 315 ± 17 million years BP (Pennsylvanian, Bashkirian).
Along the southwestern and western border the granodiorite is transgressed by liassic arkoses belonging to the sedimentary infill of the Aquitaine Basin.
It comprises the following minerals: Accessories are zoned allanite, apatite, epidote, occasionally green hornblende, zircon and zoisite.
The coarse-grained facies sometimes contains dark, fine-grained rounded to subrounded inclusions that cover the centimeter to decimeter size range.
Major outcrops are centered on Lacaujamet near Piégut (old abandoned quarry for lintels and facing stones) and on Puybégout near Augignac.
The fine-grained facies can be found mainly along the Bandiat river southeast of Nontron at the southern edge of the Piégut-Pluviers Granodiorite.
These fine- to medium-grained rocks have a rather dark appearance, they are quite rich in green hornblende but nearly devoid in alkali feldspar (less than 10 volume percent).
Besides baryte, calcite, chalcedony, dolomite, galena, marcasite, pyrite and sphalerite occur very rare minerals like anglesite, cerussite, crocoite, dundasite, embreyite, hisingerite, leadhillite, mimetite, ozokerite (pseudomineral), pyromorphite, native silver, vauquelinite and wulfenite.
Rocks compositionally very similar (but texturally different) to the light grey facies can also be found in isolated kilometer-sized enclaves within the main granodiorite (near Saint-Barthélemy-de-Bussière and south of Marval.
Associated with the pink aplite dikes is a coarser-grained red facies occurring in two larger outcrops near Ballerand and near Fargeas (commune of Abjat-sur-Bandiat).
Microgranites and lamprophyres show similarities and belong to the I-type; they are hypaluminous to normal aluminous rocks and most likely have originated from an independent magma pulse or batch.
Despite a rather homogenous appearance the Piégut-Pluviers Granodiorite carries a foliation of tectonic origin, that is clearly evident in more weathered superficial outcrops.
At the end of the Pennsylvanian hot hydrothermal solutions deposited lead, zinc and silver in veins, and in a second pulse a suite of rare arsenic-molybdenum mineralization was created.
Modally the rocks of the massif plot as granodiorite in the QAPF diagram yet are in close vicinity of the granite field.
Chemical analyses indicate the existence of SiO2-poorer rock types, i.e., the fine-grained hornblende-bearing border facies, the microgranites of the northern apophysis, the lamprophyres and the dark inclusions.
Even after solidification deformations didn't stop but kept on transforming the rocks in a brittle fashion and allowing the still very hot metasomatic fluids to perform their alterations on the massif.
Due to its mineralization at the boundary fault the metals lead, zinc and silver were mined in the Cantonnier Lode and its prolongations towards Saint-Pardoux-la-Rivière.
Today one last quarry is operational near Abjat-sur-Bandiat, where the rare decorative red facies is mined and mainly used as road surfacing.
After that research activities shifted to the granitoid of Civray-Charroux in the Vienne, which is covered by Jurassic sediments of the Seuil du Poitou.
At the moment ANDRA seems to favour the Bure site in the Meuse in eastern France as the final repository − in Mesozoic clays 500 meter below the surface.
