GEOLOGY AND STRUCTURAL EVOLUTION OF THE SAN FRANCISCO OROGENIC GOLD DEPOSIT, SONORA, MEXICO

Multiple large Au occurrences along northwest Sonora to southern California have been categorized as structurally controlled or orogenic gold deposits. Beside the Caborca Orogenic Gold Belt, Proterozoic metamorphic rocks host gold-quartz veins mainly emplaced along shear zones. The San Francisco gold deposit, located in the southeastern end of that belt in north-central Sonora, records a long-term geologic evolution. New U-Pb geochronologic data of complexly zoned zircons provide spatial-temporal constraints on magmatism and deformation. Magmatic crystallization ages from 42 core zircon grains range from 1800 to 1700 Ma with a TuffZirc age equal to 1737.46 +5.23 -8.86 Ma (95% conf.), relating the metamorphic rocks to the Paleoproterozoic basement of the area. Eight data on overgrowths range from 1700 to 1470 Ma indicating recrystallization during a thermal event related to small leucogranite bodies, sills, and dykes. The Paleoproterozoic metamorphic complex thrust over a Jurassic meta-volcano-sedimentary sequence, probably during a Late Cretaceous compressive event. The main structure in the San Francisco deposit is a pervasive foliation interpreted as a mylonitic foliation, which transpose the previous regional foliation, which is interpreted as formed by compressional stress based on pervasive slickenlines and minor asymmetric porphyroclasts developed along the foliation planes. Au ore is hosted in quartz-tourmaline-pyrite veins occurring both along the foliation planes and tension veins. Previous Ar-Ar dating in muscovite bearing ore veins indicate Paleocene ages. Four sets of faulting occurred after the main mineralization event: Set 1) conjugate normal faults striking NW-SE; Set 2) normal faults striking E-W and north dipping; Set 3) steep dip right lateral faults striking NE-SW; and Set 4) conjugate normal faults striking N-S. A late hydrothermal system is developed also along these normal faults as well as multiple fracture planes. At last, numerous non-mineralized lamprophyric dikes are emplaced mainly along the normal faults and foliation surfaces.