Abstract The importance of the La Caridad disseminated copper deposits is not only the history of its exploration and discovery by modern methods and its location as the southernmost occurrence known to date of the copper belt running from Utah through Arizona in the United States to Mexico, but also its example of Mexico’s copper potential. References on the detailed geology and geologic history of the deposit are included here. However, for purposes of a knowledgeable general abstract, this chapter is summarized from unpublished work in the files of the Consejo de Recursos Minerales by Richard H. Sillitoe, who was invited in 1975 to research Mexico’s copper ores. The La Caridad exploration program was described by Coolbaugh (1971) and the general geology by Saegart and others (1974); Echavarri (1971,1975) carried out detailed petrographic and alteration studies. Consequently, Sillitoe restricted his work to certain outstanding geologic features.

Abstract Mexico is widely known to be a richly endowed country in both metallic and industrial mineral deposits, the exploitation of which has constituted an economic activity of paramount importance for centuries. This paper presents an analysis of the time and space distribution of over 200 mineral deposits, which is based on the available absolute and relative ages of mineralization and constitutes a modified and updated version of the analysis of Camprubí (2009). Pre-Jurassic ore deposits are relatively scarce and of subordinate economic significance. These include Ti-bearing anorthosites and rare element pegmatites in intracratonic environments, barite sedimentary-exhalative (sedex) deposits, and ultramafic-mafic Cr-Cu-Ni(-platinum group element [PGE]) deposits in oceanic environments. Since the Jurassic, the metallogenic evolution of Mexico can be understood as a product of the evolution of two major regions: the Pacific margin and the Gulf of Mexico. The Mesozoic evolution of the Pacific margin is characterized by rifting and separation of the Guerrero composite terrane from the North American continent and the initiation of arc magmatism in an extensional continental margin setting. The ore deposits emplaced in this period are mostly polymetallic volcanogenic massive sulfide (VMS) and Cr-Cu-Ni(-PGE) deposits associated with ultramafic-mafic complexes. These occur dominantly near the boundaries of the Guerrero composite terrane. Porphyry-type deposits emplaced in the mid- Cretaceous are subordinate and, apparently, small. These likely formed in island arcs that were later accreted to the mainland. A shift from extensional to compressional tectonics resulted in the accretion of the Pacific terranes, most importantly the Guerrero composite terrane, to the Mexican mainland by the Late Cretaceous. This tectonic shift gave rise to the initial stages of the Paleocene boom in porphyry-type and sulfide skarn deposits. The continental arcs in these epochs represent the earliest stages for the Sierra Madre Occidental silicic large igneous province. The earliest known examples of epithermal deposits in Mexico are Paleocene and include, besides intermediate to low sulfidation deposits, the La Caridad Antigua high sulfidation deposit, in association with the giant La Caridad porphyry copper deposit. The Late Cretaceous iron oxide copper-gold (IOCG) deposits formed in northern Baja California and along the Pacific margin in southwestern and southern Mexico, and continued forming in the latter regions into the Paleocene. Contrastingly, some Late Cretaceous IOCG deposits formed several hundreds of km inland in northwestern Mexico, and are suspected cases for emplacement in back-arc environments. The formation of orogenic Au deposits began in the Late Cretaceous, and they kept forming into the Eocene as compressional tectonics progressed. The formation of porphyry-type, sulfide skarn, and epithermal deposits continued during the Eocene, and followed the eastward progression of the magmatism of the Sierra Madre Occidental. The number of known examples of epithermal deposits relative to porphyry-type and sulfide skarn deposits increases with time. The formation of IOCG deposits along the Pacific margin seemingly dwindled during the Eocene, although they began to form close to the Chihuahua-Coahuila border, possibly in association with the earliest stages of mineralization in the Eastern Mexican alkaline province. Also, a group of U-Au vein deposits in Chihuahua, in association with felsic volcanic rocks, is apparently restricted to the Eocene. The maximum geographic extension and climactic events of the Sierra Madre Occidental (for both magmatic and ore-forming events) were attained during the Oligocene, as the arc kept migrating eastward and southward. As magmatism reached the Mesa Central, epithermal and subepithermal, sulfide skarn, Sn veins associated with F-rich rhyolites, IOCG, and Sn-W greisen deposits formed around the main reactivated fault zones, generating the highest concentration of ore deposits known in Mexico. The focus of magmatism and mineralizing processes shifted progressively southward in the Eastern Mexican alkaline province between the Oligocene and the Miocene, and intensified significantly in northern Coahuila and Chihuahua in the Oligocene. This province also includes alkaline porphyry Cu-Mo deposits, REE-bearing carbonatites, and polymetallic skarns. During the Miocene, the magmatism of the Sierra Madre Occidental retracted dramatically southward and began concentrating in an E-W arrangement that corresponds to the Trans-Mexican volcanic belt, while continental extension evolved into the opening of the Gulf of California. During this time, metallogenic processes associated with the Sierra Madre Occidental virtually ceased. From the late Miocene, the formation of epithermal deposits, sulfide skarns, and porphyry-type deposits resumed in the Trans-Mexican volcanic belt and the Eastern Mexican alkaline province, whereas IOCG deposits seem restricted to the latter. The opening of the Gulf of California represents the beginning of a new cycle in metallogenesis, with the formation of shallow analogues of sedex deposits and sedimentary phosphorites along the Baja California peninsula, epithermal deposits near the cul-de-sac of the Gulf, and recent VMS deposits in passive continental margins and mid-ocean ridges. The sedimentary-diagenetic history of the Gulf of Mexico includes the formation of Mississippi Valley-type (MVT) and associated industrial mineral, red bed-hosted U and Cu-Co-Ni, sedimentary phosphorite, and sedex deposits. The emplacement of MVT and red bed-hosted deposits was associated with the emplacement of basinal brines through reactivated faults that controlled basin inversion. These faults also played a significant role as channelways for magmas and associated magmatic-hydrothermal ore deposits of the Eastern Mexican alkaline province.

Epithermal ore deposits have traditionally been the most economically important in México, with renowned world-class deposits like those in the Pachuca–Real del Monte, Guanajuato, Fresnillo, Taxco, Tayoltita, and Zacatecas districts. Whereas in certain areas (like the Great Basin in Nevada) intermediate and low sulfidation deposits have been found to be mutually exclusive in time and space; in the case of epi thermal deposits in México, the intermediate and low sulfidation types do not appear to be mutually exclusive and, to the contrary, they coexist in the same regions, formed during the same time spans, and even occur together within a single deposit. These deposits are all Tertiary in age, ranging from middle Eocene to early Miocene, with the possible sole exception of a Paleocene deposit. Their space and time distribution follows the evolution of the continental arc volcanism of the Sierra Madre Occidental and Sierra Madre del Sur. The vast majority of epithermal deposits in México belong to the intermediate (IS) or low (LS) sulfidation types; only a few high sulfidation (HS) deposits have been described in the NW part of the country (e.g., El Sauzal, Mulatos, Santo Niño, La Caridad Antigua, all of them in Sonora and Chihuahua). Because most epithermal deposits in México exhibit composite characteristics of both IS and LS mineralization styles (as well as scarce characteristics of HS), they cannot be simply characterized as IS (polymetallic deposits associated with the most saline brines) or LS deposits (mainly Ag and Au deposits associated with lower salinity brines). Thus, in this paper we propose to use an empirical classification for IS + LS deposits (that is, alkaline/neutral epithermal deposits) into three types of mineralization; namely, A, B, and C. Type A (or IS type) comprises those deposits that generally formed at greater depths from highly saline but unsaturated brines and contain exclusively from top to bottom IS styles of mineralization with a consistent poly-metallic character. Type B (or LS-IS type) comprises those deposits that exhibit dominant LS characteristics but have polymetallic IS roots (Zn-Pb-Cu); this is the most widespread type of epithermal mineralization in México. Types A and B generally exhibit mineralogic and/or fluid inclusion evidence for boiling. Type C (or LS type) comprises those deposits that exhibit only LS styles of mineralization, formed generally by shallow boiling of low salinity fluids, and have relatively high precious metal and low base metal contents. In this paper, we also review other known or attributable aspects of Mexican epithermal deposits, including ore and gangue mineralogy and their evolution in time and space, structure, geothermometry, stable iso topic composition of mineralizing fluids and other components of the deposits, chemistry and sources for mineralizing fluids, and the plausible mechanisms for the mobilization of deep fluid reservoirs and for mineral deposition in the epithermal environment.

Abstract Interacting fractures enhance and localize permeability in the Earth's crust and are, therefore, important phenomena in localizing magmatic and hydrothermal systems. The ability to identify where such interactions are present is useful in evaluating likely areas of mineralized rock, particularly in covered terrains. Regardless of map scale, the interpretation of gravity and magnetic data can define deep-seated crustal fractures and faults that may have guided emplacement of igneous rocks and large ore deposits. Here we emphasize recurring regional-scale structural relationships mainly from the western United States based on the interpretation of potential-field data, which can elucidate areas of past and present fluid flow in the crust. In particular, we explore the utility of regional gravity and magnetic data to aid in understanding the distribution of large Mesozoic and Cenozoic ore deposits (primarily epithermal and pluton-related precious and base metal deposits, and sediment-hosted gold deposits) in the western United States cordillera. On the broadest scale, most ore deposits lie within areas characterized by low magnetization. The Mesozoic Mother Lode gold belt displays characteristic geophysical signatures (regional gravity high, regional low-to-moderate background magnetic field anomaly, long curvilinear magnetic highs) that might serve as an exploration guide. Geophysical lineaments characterize the Idaho-Montana porphyry belt and the La Caridad-Mineral Park belt (from northern Mexico to western Arizona) and, thus, indicate deep-seated control for these mineral belts. At a more local scale, in Nevada, geophysical data define deep-rooted faults and magmatic zones that correspond to patterns of epithermal precious-metal deposits, and that may relate to the Carlin gold trend and the Battle Mountain-Eureka mineral belt. One recurring structural model evolving from this study is that mineralization in the western United States may be localized along strike-slip fault zones where pull-apart basins or releasing bends provided the increased fracture permeability for the migrating ore-forming fluids (e.g., the Butte, Tombstone, Bagdad, and Battle Mountain districts). Many deposits discussed in the paper appear, at least in part, to be associated with reactivated older faults as well as with faulting contemporaneous with ore deposition. We conclude that at a local scale, structural elements work together to localize mineral deposits within regional zones or belts. Perhaps the greatest utility of regional geophysical data is the identification of structural relationships that help narrow the study area, where more intensive multidisciplinary team studies can be carried out in a concerted effort to evaluate the mineral potential.

The sustained magmatic activity along the North American Cordillera during late Mesozoic and Paleogene times produced the emplacement of numerous porphyry copper deposits. This activity extended by most of western México, particularly along the northwestern part of the country. This region, along with Arizona and New Mexico in the United States, contains one of the most important centers of copper mineralization on Earth. Most of the Mexican deposits lie in the eastern part of the Laramide magmatic belt (90–40 Ma) and were formed predominantly between 75 and 50 Ma. The largest deposits occur in northeastern Sonora and are represented by Cananea (∼30 Mt Cu) and La Caridad (∼8 Mt Cu). The copper ores are locally accompanied by molybdenum, tungsten, gold, and other metals. However, the metal distribution is apparently coupled with major changes in the basement of emplacement, which can be roughly separated into three domains: a northern domain characterized by Proterozoic crystalline rocks of North American affinity; a central domain composed of Paleozoic deep-marine basin rocks underlain by the Proterozoic North American rocks; and a southern domain, represented by Mesozoic island-arc–related sequences of the Guerrero terrane. Sr and Nd isotopic data from Laramide plutons along these domains suggest that the basement modified the final composition of the Laramide magmas. Also, the basement seems to have partly controlled the metal commodities along the porphyry copper belt, with relatively larger deposits characterized by Cu-Mo-W mineralization in the northern and central domains, and smaller and more Cu-Au dominated systems in the southern (more oceanic) domain.

Abstract Several porphyry copper deposits are located in the State of Sonora, Mexico. These deposits are similar to those of the copper province of the southwestern United States in Arizona and New Mexico. Physiographic, mineral, and geologic conditions can be traced from the southwestern United States southward and parallel with the Pacific Mexican coast. This paper describes the geologic characteristics, mineralization, and alteration of the main deposits in Sonora—Cananea, Pilares de Nacozari, and La Caridad (Fig. 1).