3D fracture patterns and present-day stress fields are important parameters to consider when modelling fluid flow in fractured rock. However, the exact relationship between the fracture geometry and flow is unclear. This study was undertaken in order to characterize the fracture pattern within 2700 million year old Archean rocks and to evaluate if the tectonic paleo-fractures participate in present-day fluid flow and how they influence the flow pattern.

         The studied rock mass is located in the CANMET Experimental Mine at Val d’Or (Québec) within the Bourlamaque granodioritic batholith. The batholith is a syn- to late-tectonic intrusion within the Abitibi greenstone belt of the archean Superior province. The site is located less than 3 kilometers north of the Cadillac-Larder Lake fault system, an important regional structure of Archean age.

         Fractures in the underground workings were measured and mapped by multidirectional sampling along 4 vertical faces of a pillar. Fractures are usually tightly closed although some are filled with hydrothermal quartz and/or chlorite. Approximately 10% of the fractures participate in present-day fluid flow. The data was then incorporated into a numerical simulator, based on the rock mass fragmentation (Villemin et al., 1998), and used to reconstruct the 3D geometry of the fracture array within the rock body. With this model, we can actually visualize the fracture pattern either outside or inside the rock body or along specified cross sections.

         Structural analysis of fracture network combined with fluid flow observation and present-day stress analysis demonstrate that the Archean fracture network controls present-day flow within the mine.The fracture data shows a main E-trending fracture set, dipping 65º to the south. This fracture pattern is structurally compatible with the Cadillac Larder Lake fault system, a major E-trending shear zone, and with the regional Archean structural trend. In situ stress measurements by over-coring in several mines in the Abitibi belt (Herget and Arjang, 1990 ; Corthesy et al., 1996) show a present-day stress field characterized by a N70ºE compression with an average horizontal s1 and s2 striking N250ºE and N330ºE respectively, and a vertical s3. The fractures participating in present-day fluid flow strike between N60ºE and N110ºE and are thus favourably oriented within the present-day stress conditions to promote fluid circulation within the pre-existing fracture network.

          This study will help provide important geological constraints towards an integrated model of fluid flow within a fractured rock mass.

Villemin T. ; Empereur-Mot L., Dezayes C. and Genter A. (1998) - 3-D geometrical simulation of a rock mass fragmentation based on drillhole data. Application to the geothermal site of Soultz-sous-Forêts (NE-France), Draft proceeding, 4th HDR forum, Strasbourg 26-30 Sept.1998.

Corthésy R., Leite M.-H. and Gill D. E. (1996) - Mesures des contraintes in situ à la mine Louvicourt. Projet IRSST. Rapport de l'Ecole Polytechnique de Montréal, Département de Génie Minéral et Génie Civil, EPM/RT-96-07, 37p.

Herget G. and Arjang B. (1990) - Update on ground stresses in the canadian shield, in Stresses in underground structures, edited by G. Herget, B. Arjang, M. Bétournay, M. Gyenge, S. Vongpaisal and Y.S. Yu, pp. 33-47, CANMET-Canada Centre for Mineral and Energy Technology, Venture Inn, Ottawa, Ontario.