STRUCTURAL STUDY OF A SEMIDUCTILE STRIKE-SLIP SYSTEM IN THE CENTRAL IBERIAN ZONE (VARISCAN FOLD BELT, SPAIN) - STRUCTURAL CONTROLS ON GOLD DEPOSITS

The Villalcampo shear system is a regional dextral strike-slip fault z one that affects Late Variscan granites and their metamorphic country rocks over an area of about 150 km2. The detailed geometry of this sub vertical north-west-south-east shear zone is outlined. The system form s an extensional fan to the northwest and extends to the south-east as a broad extensional duplex. Particular attention is focused on the di stribution of fault rocks and associated veins in its north-west splay . A structural study of the shear bands (encompassing both geometric a nd kinematic criteria) and a microscopic study of the fault rocks has led to the interpretation of the system as a brittle - ductile shear z one. Calculations give a shear strain value of gamma = 1.5 and a minim um displacement of s = 3 700 m. The localization of gold mineralizatio n in mylonite-filled subvertical extensional veins is a product of the formation of the Villalcampo shear system. The subvertical faults and veins underwent a process of cyclical sealing and reopening. As such they acted as valves controlled by fluid pressure regulating fluid - r ock interactions and gold deposition. Conditions favouring these proce sses occur near the base of the seismogenic zone in the vicinity of th e frictional - quasi-plastic transition at mid-greenschist metamorphic conditions (T = 350-degrees-C and 10 - 15 km depth).