Modellazione 3D di filoni quarzo-auriferi a letto di una grande faglia normale post-metamorfica (Faglia Aosta-Ranzola, Brusson, Valle d’Aosta)

Structural data collected in the field and mine plans have been integrated in a gOcad® 3D structural model, representing the fracture and fault network of the Fenillia gold mine, one of the most important gold deposits in the Monte Rosa district, in the core of the NW Alps. The 3D model provides a deeper insight into the complex fracture and fault network, clarifying its architecture. The deposit was emplaced at the footwall of a major post-metamorphic oligocenic normal fault. Gold-bearing quartz hybrid faultveins are kinematically and spatially linked to the master fault. Larger quartz masses occur along N-dipping hybrid fault-veins (or dilatant faults), which characterize the ca. 500 m thick footwall damage zone of the master fault. They are composite veins, emplaced due to repeated cycles of fracture opening, fluid emplacement, and precipitation, as is demonstrated by the common occurrence of crack-and-seal textures. The fault and fracture network related to the Oligocene activity of the master fault is crosscut by later NW-SE Miocene faults. These 100-metre-scale faults show moderate offsets, which can be neglected at common mapping scales (e.g. 1:10.000). Nevertheless, when implementing a detailed 3D model, the effect of these mesoscopic features is very important. From a mining point of view, a correct reconstruction and restoration of 3D relationships between Oligocene and Miocene faults would have been of the greatest relevance, since the younger faults crosscut the mineralized structures, preventing from following the ore deposits. Vein thickness data have been included in the model as spatial properties of the larger polycyclic gold-bearing veins. They reveal a tapered profile at the tipline, followed by a sudden increase in thickness and then a wide central area characterised by irregular thickness variations. A simple mechanical model, based on linear elastic fracture mechanics, has been successfully developed in order to explain the observed vein thickness distribution.