GEOMETRICAL AND FRACTAL ANALYSIS OF A 3-DIMENSIONAL HYDROTHERMAL VEIN NETWORK IN A FRACTURED GRANITE

A three-dimensional study of a fracture network has been performed on a 0.66-m3 block of fine-grained granite from La Peyratte (Deux-Sevres, France). The block was sawn in nine parallel plates. The fractures associated with altered haloes were mapped for each plate using a photographic method. The nine fracture sets were subjected to an analysis of the percolated network based on a measurement of the width of altered haloes developed around fractures. Hydrothermal alteration is created by circulation of a certain amount of fluid flow in the rock. We assume that the alteration zone width is proportional to fluid mass flow. The width of alteration zones will give a good idea of the path preferentially followed by fluids. The relation between fracture length and width of the alteration zone shows that the total fracture network consists in fact of two sorts. The largest fractures (surface > 0.1 m2) create a-loose and poorly interconnected pattern in the block. However, they have developed wide alteration zones ( > 3 mm) and have then had a great role in fluid conduction. They represent only 10% of the fractures but are responsible for 47% of alteration (in volume). Approximately 80% of these fractures with wide alteration zones are oriented in the same direction. This main network is linked to a more dense, highly interconnected system made of shorter fractures with narrower altered zones. Fractal analysis has also been performed on pictures of the fracture patterns using the linear Cantor's dust method. Fractal dimension values (D) indicate an anisotropy of fracture distribution on each plane. Moreover, when the planes are compared to one another in succession, variations of D values can be noticed indicating an inhomogeneity in the third dimension. The study of such a three-dimensional fracture set shows that fluids follow preferential paths within a fracture network and helps the understanding of fracture distribution.