The Sierra Madre Occidental is the result of Cretaceous-Cenozoic magmatic and tectonic episodes related to the subduction of the Farallon plate beneath North America and to the opening of the Gulf of California. The stratigraphy of the Sierra Madre Occidental consists of five main igneous complexes: (1) Late Cretaceous to Paleocene plutonic and volcanic rocks; (2) Eocene andesites and lesser rhyolites, traditionally grouped into the so-called Lower Volcanic Complex; (3) silicic ignimbrites mainly emplaced during two pulses in the Oligocene (ca. 32–28 Ma) and Early Miocene (ca. 24–20 Ma), and grouped into the “Upper Volcanic Supergroup”; (4) transitional basaltic-andesitic lavas that erupted toward the end of, and after, each ignimbrite pulse, which have been correlated with the Southern Cordillera Basaltic Andesite Province of the southwestern United States; and (5) postsubduction volcanism consisting of alkaline basalts and ignimbrites emplaced in the Late Miocene, Pliocene, and Pleistocene, directly related to the separation of Baja California from the Mexican mainland. The products of all these magmatic episodes, partially overlapping in space and time, cover a poorly exposed, heterogeneous basement with Precambrian to Paleozoic ages in the northern part (Sonora and Chihuahua) and Mesozoic ages beneath the rest of the Sierra Madre Occidental. The oldest intrusive rocks of the Lower Volcanic Complex (ca. 101 to ca. 89 Ma) in Sinaloa, and Maastrichtian volcanics of the Lower Volcanic Complex in central Chihuahua, were affected by moderate contractile deformation during the Laramide orogeny. In the final stages of this deformation cycle, during the Paleocene and Early Eocene, ∼E-W to ENE-WSW–trending extensional structures formed within the Lower Volcanic Complex, along which the world-class porphyry copper deposits of the Sierra Madre Occidental were emplaced. Extensional tectonics began as early as the Oligocene along the entire eastern half of the Sierra Madre Occidental, forming grabens bounded by high-angle normal faults, which have traditionally been referred to as the southern (or Mexican) Basin and Range Province. In the Early to Middle Miocene, extension migrated westward. In northern Sonora, the deformation was sufficiently intense to exhume lower crustal rocks, whereas in the rest of the Sierra Madre Occidental, crustal extension did not exceed 20%. By the Late Miocene, extension became focused in the westernmost part of the Sierra Madre Occidental, adjacent to the Gulf of California, where NNW-striking normal fault systems produced both ENE and WSW tilt domains separated by transverse accommodation zones. It is worth noting that most of the extension occurred when subduction of the Farallon plate was still active off Baja California. Geochemical data show that the Sierra Madre Occidental rocks form a typical calcalkaline rhyolite suite with intermediate to high K and relatively low Fe contents. Late Eocene to Miocene volcanism is clearly bimodal, but silicic compositions are volumetrically dominant. Initial 87 Sr/ 86 Sr ratios mostly range between 0.7041 and 0.7070, and initial ϵNd values are generally intermediate between crust and mantle values (+2.3 and -3.2). Based on isotopic data of volcanic rocks and crustal xenoliths from a few sites in the Sierra Madre Occidental, contrasting models for the genesis of the silicic volcanism have been proposed. A considerable body of work led by Ken Cameron and others considered the mid-Tertiary Sierra Madre Occidental silicic magmas to have formed by fractional crystallization of mantle-derived mafic magmas with little (<15%) or no crustal involvement. In contrast, other workers have suggested the rhyolites, taken to the extreme case, could be entirely the result of partial melting of the crust in response to thermal and material input from basaltic underplating. Several lines of evidence suggest that Sierra Madre Occidental ignimbrite petrogenesis involved large-scale mixing and assimilation-fractional crystallization processes of crustal and mantle-derived melts. Geophysical data indicate that the crust in the unextended core of the northern Sierra Madre Occidental is ∼55 km thick, but thins to ∼40 km to the east. The anomalous thickness in the core of the Sierra Madre Occidental suggests that the lower crust was largely intruded by mafic magmas. In the westernmost Sierra Madre Occidental adjacent to the Gulf of California, crustal thickness is ∼25 km, implying over 100% of extension. However, structures at the surface indicate no more than ∼50% extension. The upper mantle beneath the Sierra Madre Occidental is characterized by a low-velocity anomaly, typical of the asthenosphere, which also occurs beneath the Basin and Range Province of the western United States. The review of the magmatic and tectonic history presented in this work suggests that the Sierra Madre Occidental has been strongly influenced by the Cretaceous-Cenozoic evolution of the western North America subduction system. In particular, the Oligo-Miocene Sierra Madre Occidental is viewed as a silicic large igneous province formed as the precursor to the opening of the Gulf of California during and immediately following the final stages of the subduction of the Farallon plate. The mechanism responsible for the generation of the ignimbrite pulses seems related to the removal of the Farallon plate from the base of the North American plate after the end of the Laramide orogeny. The rapid increase in the subduction angle due to slab roll-back and, possibly, the detachment of the deeper part of the subducted slab as younger and buoyant oceanic lithosphere arrived at the paleotrench, resulted in extension of the continental margin, eventually leading to direct interaction between the Pacific and North American plates.

Abstract The Mulatos district is a volcanic-hosted, advanced argillic, gold enargite system of late Oligocene age, located in the northern Sierra Madre Occidental volcanic province of Sonora, Mexico. Hypogene mineralization is associated with rhyodacite domes and major faults. Gold is associated with pyrite ± enargite in distinct pods of vuggy silica-pyrophyllite-diaspore-dickite in altered dacite-rhyodacite volcanic rock. Past production of more than 300,000 oz Au and reserves of more than 2.3 Moz make the district one of the largest gold systems in northern Mexico and one of the larger advanced argillic gold systems in the world. To the west of Mulatos, five other similarly altered systems are present, and these systems provide additional insight into the genesis and possible variations in mineralization, level of exposure, and physio-chemical conditions of formation. Unlike many acid-sulfate systems, hypogene alunite is uncommon at Mulatos and instead the main alteration mineral is pyrophyllite. The district was tilted ˜15° to 25° NE after mineralization, exposing >1 km of a mineralized and variably altered section. Advanced argillic alteration (>3 km 2 ) can be traced laterally outward through intermediate argillic (>5 km 2 ) into chlorite-montmorillonite ± epidote. Prominent silicified ridges and red (oxidation of pyrite) hills with kaolinite and scattered barite veinlets characterize the surface expression above ore zones. The age of mineralization is bracketed between 31.6 Ma mineralized tuffs and 25 Ma crosscutting and overlying unaltered basaltic andesites. Ore minerals include free gold, Au-rich pyrite, enargite, sphalerite, and less commonly, tennantite, Au telluride, covellite, and chalcopyrite. Elevated concentrations of Ag, As, Au, Ba, Cu, Hg, Mo, Sb, and Te are common in a 2-km 2 alteration zone surrounding the mineralized centers. Mass balance calculations based on whole-rock studies of progressively altered samples show decreasing Ca, K, and Na and increasing Si and Al associated with intensifying acid leaching. The apparent increase in Si and Al is likely a consequence of cation leaching related to the low-pH hydrothermal fluids rather than element addition. Early Au with pyrite, followed by auriferous pyrite + enargite ± Ag sulfosalts, and late Au-containing barite make up the three principal ore stages. Stratigraphic reconstructions show that the tops of the shallowest orebodies are structurally controlled, thin, high-grade pyrite-barite, Au telluride, and Au pyrite + quartz veins formed at a depth of <200 m, whereas the top of the main Mulatos orebody (Cerro Estrella) formed at ˜600 m and continues downward for >400 m. Deep mineralization

This study presents new field, structural, petrographic, and geochemical data on the Cenozoic volcanic rocks of the northwestern Sierra Madre Occidental (SMO). Since the Mesozoic, this region has been the site of three successive magmatic events: (1) A calcalkaline sequence (90 to 40 Ma), which in Sonora is characterized by the emplacement of the granitoids of the Cordilleran coastal batholiths coeval with the subduction of the East Pacific plate under the North American continent. (2) An Eocene-Oligocene (around 35 Ma) volcanic sequence represented by calcalkaline andesites and high-K ignimbrites. The plagioclase-biotite phenocrystic assemblage is typical of the ignimbrites of this sequence in the northern SMO. Some of these rocks are caldera-related but they are much thinner and restricted in exposure than those in the southernmost areas. This magmatic activity probably accounts for much of the mineralization in the region. (3) A bimodal Oligocene-Miocene sequence (30 to 17 Ma) consisting of successive eruptions of basalt and basaltic andesite associated with acidic calcalkaline tuffs and lavas. Basic rocks form the predominant outcrops in the northern area of the SMO. The trap-forming flows were extruded from fissures and small calderas. These mafic rocks are transitional between the calcalkaline and tholeiitic series and show a progressive change from orogenic to anorogenic affinities with time. This volcanic event is related to pre-Basin and Range normal-faulting tectonism. Extensional tectonism at 17 Ma accounts for the modern basin-range morphology, which is well developed in the northern SMO. The volcanism progressively disappeared along the central axis of the SMO during this time. Quaternary volcanism is rare in Sonora; there are scattered basaltic fields such as the Quaternary lavas of Moctezuma, which crop out in central Sonora. Basalts are typically anorogenic and coeval with strike-slip faults, which affected the whole western margin of the North American continent, and with the opening of the Gulf of California. Based on the age and geochemical characteristics of the studied rocks, it is possible to conclude that, since Mesozoic time, the magmatism in the northern SMO probably reflects successive changes in the geodynamic regime.

Igneous rocks of the Sierra Madre Occidental have been studied along two traverses across the range. One is at lat 24°N between Mazatlán and Durango City, where contiguous mapped areas extend across the Sierra; the other is near lat 28°N, where several separate areas west and north of Chihuahua City have been mapped. In these regions the Sierra contains two vast and largely coextensive igneous sequences, both calc-alkalic and both including ignimbrites. The older sequence of rocks, which ranges in age from 45 m.y. to at least 100 m.y., is characterized by abundant batholithic as well as volcanic rocks and is dominantly intermediate in composition. The younger sequence is dominated by rhyodacitic to rhyolitic ignimbrites erupted from large caldera complexes, generally accompanied by rhyolite flows and domes and small outpourings of mafic lavas. Intermediate rocks are rare, and volcanism was largely confined to the interval 34 to 27 m.y. ago, although some activity persisted until 23 m.y. ago. Mid-Tertiary volcanic rocks are exposed eastward across Chihuahua between the Sierra Madre Occidental and the alkalic volcanic province of Trans-Pecos Texas. Volcanic rocks of this region are chiefly calc-alkalic in chemistry, but their alkalinity is intermediate between that of the Sierra Madre Occidental and of Trans-Pecos Texas. Ages are both similar to and older than those in the Sierra. The chronology of both igneous sequences of the Sierra Madre Occidental fits some of the known sea-floor–spreading patterns. The older sequence is similar in age and composition to batholithic terranes of the western United States that were emplaced during Cretaceous–early Tertiary convergence along the western margin of the North American plate. The younger sequence was also emplaced during a period of convergence. Volcanism declined 27 m.y. ago at about the time of reorientation of an east Pacific spreading center and early ridge-trench interaction at the nearest continental margin. The gap in magmatism between 45 and 34 m.y. and the abrupt onset of volcanic activity 34 m.y. ago, however, do not match known sea-floor–spreading events. Furthermore, the brief age span and the bimodal calc-alkalic composition of the younger sequence are not typical for an igneous province at a continental margin.