Masters Thesis

Geology and ore deposits of the Cosumnes Copper Mine skarn deposit, El Dorado County, California.

The Cosunnes Copper Mine is a contact-metasomatic copper deposit located in the Sierra Nevada Mountains approximately fifteen miles southeast of Placerville, California. The deposit is located near the western margin of a large marble and biotite schist body surrounded by granitic rocks. A small lens of this marble occurs in diorite along the western edge of these metamorphics. The marble lens was totally replaced by skarn due to metasomatic reaction between the marble and post-magmatic hydrothernal fluids under initially hypothermal conditions. Cooling of the fluid,combined with reaction of the fluid with the wall rocks, resulted in a series of retrograde replacement reactions. There were three zones of skarn formed at the Cosumnes Copper Mine: a sericite zone formed in the diorite, a pyroxene skarn zone formed between the altered diorite and the garnet skarn, and the garnet skarn zone which has replaced the original marble. The skarn was formed as a primary result of the diffusion process of skarn formation and to a lesser degree, the infiltration process. The initial skarn forming fluids were moderately iron rich. The hydrothernal fluids became progressively more iron rich as they cooled through the mesothermal range. As the fluids cooled into the epithermal range they became iron deficient. V/hen mesothermal conditions were reached by the cooling hydrotherr.-al solutions, extensive bomite-clalcopyrite and molybdenite mineralization occurred as vein fillings, open space filling and replacements in the skarn zone itself. The final hydrothernal process was emplacement of massive quartz veins with minor sulfide mineralization.