TECTONIC AND STRUCTURAL SETTING OF GOLD MINERALIZATION IN THE SONORA-MOJAVE BELT: EXAMPLE FROM THE JUAREZ-TAJITOS AREA

Active or historical gold mining districts within the Sonora-Mojave megashear belt include La Choya, La Herradura, Noche Buena, El Chanate, San Francisco, Santa Gertrudis in Mexico and Mesquite, Picacho in California. Gold mineralization typically occurs as high grade pods and low grade disseminations within shear zones where high angle faults and fractures are overprinted by NW-striking thrusts. Typical ore grades are ≤1 g/T, usually mined by bulk tonnage, open pit methods. Oxidation of pyrite is complete in most localities. Similarities in the structure, age and style of mineralization within and between deposits in this belt indicate regional tectonic controls for gold occurrence. Gold-bearing quartz veins near La Choya and San Francisco mines preserve white mica with ⁴⁰Ar/³⁹Ar ages between ~58 Ma and 41 Ma (Iriondo, 2005; Perez-Segura, 1996), coincident with the Laramide orogeny in Sonora.

One well-exposed gold district ~35 km northwest of Caborca includes the old Juarez and Tajitos mine camps. Geological mapping and assay results from surface sampling and drilling reveal strong correspondence of gold with two generations of faults. Host rocks are typical components of Sonora’s Middle Jurassic arc, including rhyolite porphyry intrusives and a distinctive section of rhyolite and andesite flows and breccias interstratified with quartz arenite. The stratigraphy is broken along NW and NE trends by an early system of Late Jurassic(?) conjugate strike-slip faults. Superimposed Laramide(?) contraction is manifested by a SW-vergent fold-thrust belt in which the NW-striking sinistral faults record reverse reactivation and NE-striking dextral faults form transverse tear structures. Synorogenic gold-bearing quartz veins were emplaced along the thrusts and tear faults; fault movements coincident with hydrothermal fluid circulation enhanced permeability and resulted in a network of high grade quartz veins (1 to 35 g/T Au) within broad zones of brecciated, hematite-silica altered wall rock (0.3 to 3 g/T Au). Exploration focuses on intersections of high angle and low angle faults believed to represent superposition of Late Jurassic transtensional and Laramide convergent tectonic driving mechanisms. Detachment faults related to Late Tertiary extension may have entrained or remobilized gold mineralization.