EMPLACEMENT OF AN AU-DEPOSIT WITHIN THE FRANCISCAN COMPLEX: EXAMPLE FROM THE LOS BURROS MINING DISTRICT

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 emplacement and its relationship with the regional metamorphism and deformation is unclear. Based on both vitrinite reflectance (Rm) 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 explain the anomalous temperatures recorded in the vicinity of the deposit and their possible link with mineralization processes, we first performed a detailed geological and structural mapping within the Los Burros district coupled to a thermal study. The maximum temperature reached by the host metasediments 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 deposit, the potential source and the temperature of the fluid responsible of the Los Burros Au-deposit emplacement are investigated.

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. Here we propose that the Los Burros deposit is not associated with a post metamorphic hydrothermal thermal anomaly but rather with a late transpressive deformation uplifting deeply buried metamorphic rocks.