GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 80-13
Presentation Time: 11:30 AM


LACROIX, Brice J.1, HUGHES, Jacob2, LAHFID, Abdeltif3, SPANGENBERG, Jorge4, PUTLITZ, Benita5, KEMPTON, Pamela D.2 and WARD, Christine1, (1)Geology, Kansas State University, 110 Anderson Hall, 919 Mid Campus Drive North, Manhattan, KS 66506, (2)Department of Geology, Kansas State University, 108 Thompson Hall, Manhattan, KS 66506, (3)BRGM, Orléans, 45000, France, (4)Institut des dynamiques de la surface terrestre, Université de Lausanne, Quartier UNIL-Mouline, Bâtiment Géopolis, Lausanne, 1015, Switzerland, (5)Institute of Earth Sciences, University of Lausanne, Lausanne, CH-1015, Switzerland

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 19th and early 20th 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.