Orogenic gold belt overlap in the eastern Peninsular Ranges batholith: La Rumorosa mining area, Baja California, Mexico

Abstract

Gold deposits in the La Rumorosa mining area (Baja California), located in the eastern Peninsular Ranges batholith (PRB), consist of auriferous quartz-(carbonate) veins hosted by brittle-ductile shear zones within metamorphic and synorogenic plutonic rocks. A combination of two geochronology methods, U–Pb zircon dating of country rocks and 40Ar/39Ar dating of hydrothermal and metamorphic white micas, along with the structural characterization of the main orebodies, has allowed the differentiation of two gold mineralization events in the study area. The oldest deposit (>93 Ma) is related to the main compressive phase of the PRB construction (∼110‒90 Ma) and is part of the yet poorly known Peninsular Ranges orogenic gold belt. Two other gold deposits were dated at ∼77 and 75 Ma, an age span that approximately corresponds to the beginning of the Laramide tectonism instead. Therefore, these deposits can be considered the oldest and westernmost constituents of the Caborca orogenic gold belt (∼69‒36 Ma), formerly recognized in the neighboring states of Sonora, Arizona, and the southeastern tip of California. Furthermore, the Laramide gold deposits at La Rumorosa are contemporaneous with the first generation (∼76‒74 Ma) of copper porphyries in the southern North American Cordillera belt, which are also situated on the opposite margin of the Gulf of California-San Andreas fault system. Both gold and copper deposits show an eastward migration during the early Upper Cretaceous, similar to the well-documented shift in magmatism and deformation in the region, albeit occurring asynchronously. The apparent continuous formation of orogenic gold deposits in southwestern North America from the middle Cretaceous to the Eocene, resulting from the strong convergence between the subducted Farallon plate and the continent, suggests a connection between the Peninsular Ranges and Caborca gold belts. We surmise that both of these orogenic gold belts may form part of a large metallogenic province in western Mexico.