Abstract

The Caborca orogenic gold belt is a large gold metallogenic province located in the North American Cordillera that hosts orogenic gold-bearing quartz veins and extends from northwestern Mexico into the southwestern United States. The Caborca orogenic gold belt includes a number of deposits currently in production (e.g., La Herradura, San Francisco-Llano, El Chanate, Soledad-Dipolos, and Noche Buena) that, combined, contain gold reserves and resources of over 10 million ounces.

Extensive new 40Ar/39Ar geochronology of white micas associated with gold-bearing quartz veins along the Caborca orogenic gold belt suggests that mineralization developed rapidly after ~69 Ma, peaked at ~61 Ma, and decreased more slowly from 58 to 36 Ma, when it ceased. Triggering of mineralization is closely related to an increase in the rate of convergence and the subsequent shallowing of the subducted Farallon slab, which resulted in rapid eastward migration of the magmatic arc toward the continent during the Late Cretaceous-Eocene Laramide orogeny. Mineralization within the Caborca orogenic gold belt formed contemporaneously with some other orogenic gold deposits along the North American Cordillera, including districts in Alaska (e.g., Chichagof, Juneau, and Port Wells) and Canada (e.g., Bridge River).

After a process of crustal shortening/thickening during the early compressive stage (~90–70 Ma) of the Laramide orogeny, which was also a time for generation of mineralizing fluids, a process of synconvergent crustal extension started at ~70 Ma and affected the Cordillera forearc region in northwestern Mexico as a response to the flat subduction and the eastward Laramide arc migration. This Laramide synconvergent extension produced a marked cooling of the thinned crust, which, in turn, generated regional brittle conditions that facilitated the ascent and precipitation of metal-laden metamorphic fluids that formed the quartz-gold vein mineralization along the Caborca orogenic gold belt.

Another consequence of the eastward arc migration during the Laramide orogeny was the generation of contemporaneous Late Cretaceous-Eocene Cu-Mo porphyry deposits in northwestern Mexico and the southwestern United States. In this scenario, orogenic gold deposits would have formed in the amagmatic forearc region, whereas the porphyries would have been generated farther to the east in the magmatic arc region.

In this mineralization model, we interpret that the Caborca orogenic gold belt area was more uplifted and suffered higher degrees of denudation (veins formed at depths of ~4–6 km) than the region of Cu-Mo porphyries that formed at shallower crustal depths (~1–2 km). In other words, both coeval mineral belts were exposed next to each other, even though they represent very different crustal levels. This crustal level disparity could be explained by a period of intense Eocene to Oligocene tectonic/crustal denudation that preferentially affected the previously overthickened forearc region. Perhaps due to isostatic rebound, at least 4 to 6 km of crustal material from the forearc region was removed until the final exhumation of the Caborca orogenic gold belt veins occurred, most likely by the Oligocene.

Soon after this period of intense crustal denudation, Oligocene-Miocene epithermal mineralization overprinted both the Caborca orogenic gold belt and the porphyry Cu-Mo belt. In addition, supergene alteration of hypogene orogenic mineralization and supergene enrichment of the porphyry deposits (enriched layers of secondary copper minerals) increased mineral concentrations due to groundwater level fluctuations (oxidation-reduction cycles) in an arid desert environment that prevailed since the Miocene. The combination of all these tectonic and climatic factors makes northwestern Mexico a unique and attractive place in the North American Cordillera to undertake mineral exploration endeavors.

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