STRUCTURAL GEOLOGY OF THE SAN FRANCISCO GOLD DEPOSIT, SONORA, MEXICO

The San Francisco gold deposit is located in the Santa Ana Region, Sonora, Mexico, 151 miles south of Tucson, Arizona. This deposit is part of a larger group of Au occurrences forming the Caborca Orogenic Gold Belt (COGB) that trends northwest and extends from west central Sonora to southern California along the Sonoran Plains physiographic province. All deposits in this belt are categorized as “structurally controlled”. The geology comprises a purportedly Mesoproterozoic metamorphic complex, a Jurassic meta-volcano-sedimentary sequence and small bodies of leuogranite emplaced as sills and isolated lenses. The main structure in the mine area is a pervasive foliation interpreted as a mylonitic foliation, probably superposed on previous regional foliation. The mylonitic foliation is interpreted as formed by compressional stresses based on pervasive slickenlines and minor asymmetric porphyroclasts developed along the foliation planes. Au ore is hosted in quartz-tourmaline-pyrite veins occurring either as lenses along the foliation and as tension veins. Various sets of normal faults are superposed after the main mineralization event, which are arranged in at least three groups oriented about N-S±5°, N40°-75°W both dipping to NE and a lesser group striking N15°-45°E. Secondary hydrothermal alteration is developed along these normal faults as well as along multiple fracture planes. At last, numerous non-mineralized lamprophyric dikes are emplaced following the foliation surfaces and along the normal faults. Previous Ar-Ar dating in muscovite bearing veins indicate Paleocene ages. U-Pb dating in progress will yield basis to relate the inception of mineralization with igneous or metamorphic events in the area. With the structural analysis and the geochronological data a model for the formation and evolution of the San Francisco gold deposit will be developed.