Characteristics of gold deposits on Nortern Sonora, Mexico: a preliminary report

The geology of northwestern Mexico is complex and is similar in many respects to that of southeastern California and southern Arizona. The region (Fig. 1), typical of the southern basin-and-range physiographic province of which it is a part, is characterized by elongate, northwest-trending ranges separated by wide alluvial valleys. Basement rocks in the area include Precambrian gneisses, metamorphosed andes-ites, and granites. These rocks are overlain by younger Proterozoic quartzites and limestones, Paleozoic and Mesozoic carbonate rocks, and Mesozoic volcanic, clastic, and carbonate sedimentary rocks. Mesozoic plutonic rocks and Tertiary extrusive and intrusive rocks related to volcanic activity of the Sierra Madre Occidental are widely distributed. Broad areas are underlain by plutonic and associated volcanic rocks of the Sonora-Sinaloabatholith of Cretaceous to early Tertiary (Laramide) age. The outcrop areas of the plutonic rocks are smaller in northwestern Sonora, west of Magdalena de Kino where many of the gold deposits are concentrated, than they are farther to the east and south (Fig. 2).

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The most common structural features are northwest-trending range front faults and numerous low-angle shear zones related to thrust or detachment faults. The Mojave-Sonora megashear defined by Silver and Anderson (1974) and discussed in detail by Anderson and Silver (1979) is a principal regional feature that passes near Quitovac, Caborca, and just south of San Francisco (Figs. 2 and 3). The megashear is a wide zone separating Precambrian basement rocks of slightly different age. The megashear is occupied by a Jurassic magmatic arc composed of volcanic, sedimentary, and plutonic rocks. The volcanic units within the zone are strongly deformed along low-angle thrust faults; the associated sedimentary rocks have been tightly folded. The southwestern boundary of the megashear appears to be a major fault which juxtaposes the Precambrian basement against the Jurassic magmatic terrane (Anderson and Silver, 1979). The nature of movement along the shear is uncertain, but Anderson and Silver (1979) suggest that as much as 800 km of left-lateral movement occurred along it after Middle Jurassic time. Earlier movement is possible. Many of the gold prospects in Sonora occur within or adjacent to the southwestern boundary of the megashear in Precambrian, Mesozoic, and Tertiary rocks.