STRUCTURE AND ORIGIN OF THE GOLD MINERALIZATION IN THE NACIMIENTO BLOCK: THE LOS BURROS DEPOSITS (CENTRAL CALIFORNIA)

The Au- Los Burros deposit is located within the Franciscan complex from the Nacimiento block, California. Although this deposit has been mined during the late 19^th and early 20^th centuries, its origin and its relationship with the regional metamorphism and deformation is unclear. Based on both vitrinite reflectance and illite cristallinity methods, Underwood et al. (1995) suggest that the Nacimiento block is locally perturbed by a thermal anomaly (up to ~300ºC). Although both thermal anomaly and deposit seem spatially correlated, their relationship is still poorly constrained. In order to better understand the mode of formation of the deposit and its possible link with the post-subduction thermal anomaly, we performed a detailed geological and structural mapping within the Los Burros district coupled to a thermal study. The maximum temperature reached by the ore-host metasediments from the Nacimiento block have been investigated using Raman Spectroscopy of Carbonaceous Materials (RSCM) method. In addition, through a careful fluid-inclusion and stable isotopes (O and C) studies on the ore deposit, the temperature and the potential source of the fluid responsible of the Los Burros Au-deposit emplacement are investigated. The Los Burros Au-deposit formed from a metamorphic aqueous CH₄ fluid at temperature of about 300˚C. RSCM results confirm the previous temperatures and the presence of the thermal anomaly in the range 260-320ºC near LBMD. Our structural analysis shows that the Los Burros Au-deposit consists to a serie of meter-wide, quartz-calcite-sulfide “en-echelon” veins, formed during a late transpressive event, likely related to the San Andreas fault activity. Finally, we propose that the thermal anomaly is not associated with a post metamorphic hydrothermal event but rather with a late transpressive deformation, which uplifted deeply buried metamorphic rocks.