Exposed upper Proterozoic and Cambrian rocks in the Caborca region, Sonora, Mexico, consists of a shallow-water miogeoclinal sequence of quartzite, siltstone, dolomite, limestone, and minor amounts of conglomerate and greenstone. A revised stratigraphic sequence is divided into 14 formations, 11 of which have been previously named and 3 of which are named here. The sequence is as much as 3,300 m thick and rests unconformably on a basement terrane of 1,600-to l,750-m.y.-old metamorphic and igneous rocks, intruded by l,400-m.y.-old porphyritic granite and l,100-m.y.-old granite. The sequence contains a wide assortment of fossils that include algallike filaments, possible trace fossils, and conical stromatolites (Conophyton and related forms) in the upper Proterozoic rocks; a primitive shelly fauna in the lowermost Cambrian rocks; and archaeocyathids, trilobites, Salterella, Hyolithes, Gimanella, gastropods, and brachiopods in the overlying Cambrian rocks. Paleocurrent measurements in six different formations reveal no dominant trend, although individual studies characteristically show a single dominant direction or oppositely directed paleocurrents suggestive of the ebb and flow of tides. The revised stratigraphic sequence in the Caborca region is correlated, much of it unit for unit, with stratigraphic sequences in the southern Great Basin region of eastern California and southern Nevada, in the San Bernardino Mountains of southern California, and in the Sierra Agua Verde of central Sonora, Mexico. The upper Proterozoic and Cambrian rocks of the Caborca region have long been recognized as a southward extension of the Cordilleran miogeocline, but interpretations have varied as to whether or not many of these rocks have been tectonically displaced. The position of the Caborca rocks to the southeast of correlative rocks in the Southwestern United States may be due to an eastward curvature of the Cordilleran miogeocline into northern Mexico, to major left-lateral offset along the Mojave-Sonora megashear, or to a combination of both these factors. A complex pattern of tectonic disruption involving leftlateral and subsequent right-lateral offset is also possible.

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The Ixtacamaxtitlá n hydrothermal deposit is made up of a succession, from bottom to top, of: (1) a porphyritic subvolcanic body, crosscut by quartz veins, and a stockwork with subordinate sulfides (pyrite and chalcopyrite), showing propylitic alteration haloes overprinting a previous potassic alteration event (biotitization); (2) an overlying, kaolinized lithic-rich rhyolitic tuff; and (3) a layered opal deposit with preserved sedimentary structures. This vertical zonation, coupled with the distribution of the alteration assemblages, lead us to the interpretation of the whole as a porphyry-type deposit grading upwards to a barren, steam-heated, acid-leached, kaolinite blanket with a partially preserved silica sinter on top. Both the fluid inclusion study carried out on the veins and stockwork, and the stable isotopic analyses of the kaolinized bodies, suggest the presence of two major hydrothermal events. The older event is characterized by the occurrence of hot hypersaline fluids (up to 320°C and 36 wt% NaCl equivalents), likely of magmatic origin, closely associated with the emplacement of the underlying early Miocene porphyry-type deposit. The later event is characterized by the presence of cooler and dilute fluids (up to 140°C and 4 wt% NaCl equivalents) and by advanced argillic alteration close to the paleosurface. The calculated isotopic composition of water in equilibrium with the kaolinitic sequence plots close to and underneath the meteoric water line, partially overlapping the Los Humeros present-day geothermal fluids. This evidence coupled with the petrographic observations suggests that steam-heated phreatic waters altered the lithic-rich rhyolitic tuff. This would have occurred when acid vapors, exsolved from deeper hydrothermal fluids by boiling, reached the local paleowater table and condensed, after a sector collapse that changed the system from lithostatic to hydrostatic conditions.