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The opening of the Gulf of Mexico during the Mesozoic led to the formation of the Sabinas Basin. Large carbonate platforms were developed throughout the Lower and Middle Cretaceous. The basin provided ideal conditions for the formation of a suite of carbonate-hosted, stratabound deposits such as barite, celestine, fluorite, and lead–zinc of Barremian–Aptian age. These deposits resemble Mississippi Valley-type (MVT) and associated deposits. The mining district of Sierra de Santa Rosa is located approximately ~ 7 km SE from Melchor Múzquiz in the state of Coahuila, Mexico. Barite is the economic mineral and the shape of the ore bodies is considered “mantos”, the gangue minerals are calcite, local gypsum, traces of celestine, silica, and iron (oxy) hydroxides. The barite deposits show relict textures such as rhythmic, alternating black and white bands due to the presence of organic matter, and globular clusters similar to the “chicken-wire” anhydrite, typical of evaporites. A fluid inclusion and stable isotope analysis (S from barite, C and O from carbonates) were conducted. The lower manto yielded a melting ice temperature between − 26 °C and − 5 °C (salinities of 7.9 to 27 wt.% NaCl equiv.) and a homogenization temperature ranged between 59 °C to 155 °C. The eutectic temperature was − 51 °C ± 2 °C denoting a primary calcic brine. The upper manto yielded a melting ice temperature between − 22 °C and − 15 °C (salinities of 18.6 and 24 wt.% NaCl equiv.) and a homogenization temperature was ranging from 60 °C to 126 °C. Isotopic analysis of barite showed δ 34 S VCDT ranges from + 14.9‰ to + 19.5‰ (average of 16.9‰). Sulfur isotope data for barite from the Sierra de Santa Rosa is consistent with a sulfur source formed during the Lower Cretaceous, which coincides with the age of the Cupido Formation. The carbon isotope analysis of the host limestone yielded a δ 13 C VPDB range from − 0.01‰ to + 0.11‰. The δ 13 C values for clear and gray calcites ranged from − 0.15‰ to − 1.5‰, and − 1.41‰ to − 2.3‰, respectively. The oxygen isotope analysis showed a range between δ 18 O VSMOW − 4.55‰ and − 10.04‰. Fluid inclusion microthermometry and isotopic measurements lead us to conclude that brines from the Sabinas Basin led to the replacement of the evaporite strata (gypsum) by barite in the Cupido Formation and thus classify these deposits within the category of MVT and associated deposits. [ABSTRACT FROM AUTHOR]

Volcanogenic massive sulfide (VMS) deposits are the most conspicuous type of deposits that formed during the Mesozoic in Mexico. Many Mexican VMS deposits display “classical” Kuroko-type mineral zonation and structure, and some of them, as Cuale and La Minita formed in shallow submarine environments. The most prospective time window for the formation of VMS deposits in Mexico comprises the Late Jurassic and the Early Cretaceous. VMS stopped forming during the progressive continentalization of Mexico, since its metallotectonic processes (dominated by extensive tectonics) changed giving way to compression during the late Early Cretaceous; new VMS deposits did not form until after the opening of the Gulf of California. Mesozoic VMS deposits in Mexico occur in submarine volcano-sedimentary sequences that deposited essentially in association with back-arc basins (now found roughly along the boundaries between tectonostratigraphic terranes) or within juvenile and slightly evolved arcs (at the internal parts of the terranes), while few others occur on the epicontinental seafloor and hinterlands of eastern Mexico. VMS deposits are especially abundant in the Guerrero composite terrane, and are also present in the Alisitos and Parral terranes. However, new evidence referred in this paper indicate the western continental edge of the cratonic block of Oaxaquia as a promising, new prospective region for VMS deposits. Interestingly, no VMS deposits are found in the northern part of the Guerrero composite terrane despite the occurrence of marine volcano-sedimentary sequences similar to those in the south; such absence can be related to differential extensional unroofing, much larger in the southern part of the Guerrero composite terrane than in the northern part. Many VMS deposits occur along or close to terrane boundaries, especially around the Guerrero composite terrane. This distribution reflects the association between VMS deposits and back-arc basins, which represented the frontal part of terranes or sub-terranes that were ultimately accreted to the Oaxaquia cratonic block. As a consequence, VMS deposits usually display strong deformation and thrusting, and their mineral and compositional zonation can be found overturned. Due to such common association between these deposits and terrane boundaries that reactivated during the Cenozoic, VMS deposits are especially susceptible to overprinting by later metallogenic processes, unrelated to VMS-producing environments. This susceptibility may be held accountable for the complex mineral associations found in the Francisco I. Madero deposit. The inclusion fluids in the Cuale, La Minita, El Rubí, Tizapa and Campo Morado deposits have salinities that range from 2.5 to 20.0 wt.% NaCl equiv. and temperatures of homogenization from 110° to 420 °C, although a particular fluid inclusion assemblage (with daughter halite and sylvite) from a stockwork at the Tizapa deposit ranged salinities from 39.7 to 64.7 wt.% NaCl and from 35.9 to 43.5 wt.% KCl, and temperatures of homogenization from 440° to 550 °C. The general characteristics listed above may account for mineralizing fluids from magmatic, marine, and modified marine sources. The entrainment of cool and oxidizing seawater within upwelling fluids increased as the paleohydrothermal systems waned. The waning stages are normally represented by barite-rich mineralizations and, in the case of the La Minita deposit, this notoriously outlasted by Mn oxide mineralization that reflects even more oxidizing conditions. A similar evolution is deduced in Cuale, which is the only known Mesozoic VMS deposit in Mexico that formed in a shallow-submarine environment together with La Minita. The shallower the formation of VMS deposits is, the more quickly mineralizing fluids evolve into highly oxidized end-members, once the paleohydrothermal systems cooled down. The extremely high-salinity and hot inclusion fluids in Tizapa, despite the unavailability of further geochemical data, are likely to represent magmatic brines that did not undergo significant mixing with seawater. As for the temporal evolution of the studied deposits, they generally exhibit (1) an increasing magmatic contribution from early to middle stages for both fluids (as mineralizing fluids grew hotter and more saline) and sulfur, and (2) the waning of hydrothermal activity towards the last stages on mineralization, which is characterized by the prevalence of seawater (as fluids grew cooler, and more diluted and oxidizing), thus reflecting quite a typical evolution for most VMS deposits elsewhere. New δ 34 S values for the same VMS deposits reflect multiple sources of sulfur in the mineralizing fluids: magmatic, sedimentary/metasedimentary, and marine. Different degrees of dominance of either source for sulfur in these deposits and in their stages of mineralization have been inferred, in a way that also responds to a schematic evolution from magmatic- to seawater-dominated paleohydrothermal systems. [ABSTRACT FROM AUTHOR]

The Guaynopa and Guaynopita mineralized areas in central-western Chihuahua are conterminous sets of ore deposits that formed in association with gabbroic and granitic intrusions that correspond to the Lower Volcanic Complex of the Sierra Madre Occidental silicic large igneous province. The Guaynopa IOCG deposit consists of (1) early iron oxide–copper–gold mantos accompanied by potassic (fuchsite, biotite and potassium feldspar) and/or calcic–sodic alteration (tremolite–actinolite) and hosted by marmorized limestones near the contact with intrusive granites, (2) later copper-rich stockworks and gold disseminations, and (3) late gold- and copper-rich quartz-calcite veins. Mantos contain most of the copper and gold ores in this deposit, and their hypogene mineralogy consists of magnetite, fuchsite, chalcopyrite, Ag-rich gold, cuprite, and late hematite. The Guaynopita porphyry copper deposit consists mainly of stockworks within potassic alteration zones, and includes ancillary sulfide copper- and lead-rich skarn deposits. Microthermometric studies of fluid inclusions were carried out on most mineral associations of the Guaynopa IOCG and in the Guaynopita porphyry copper deposits. In IOCG mantos, temperatures of homogenization (Th) in calcite and quartz vary between 152° and 310 °C, and apparent salinities between 10.7 and 24.2 wt.% NaCl equiv. In IOCG veins, Th in quartz vary between 310° and 400 °C, and apparent salinities between 11.1 and 21.0 wt.% NaCl equiv. In IOCG stockworks, Th in tremolite vary between 330° and 410 °C, and the apparent salinity is 16.0 wt.% NaCl equiv. In porphyry-copper stockworks, Th in quartz vary between 205° and 448 °C, and apparent salinities between 8.1 and 21.3 wt.% NaCl equiv. δ 34 S values for sulfides (mostly chalcopyrite) in IOCG associations range between − 15.1 and 7.0‰ in mantos, between − 3.7 and − 0.2‰ in veins, and between − 1.6 and 0.2‰ in stockworks. δ 13 C VPDB values from calcite in IOCG mantos range between − 5.22 and 3.45‰, and δ 18 O VSMOW values between 9.61 and 17.23‰, which correspond to the interaction of magmatic fluids with host limestones at a broad range of temperatures and water to rock volume ratios. Likely ore-formation processes are isothermal mixing, conductive cooling, and boiling. In the case of the Guaynopita porphyry copper deposit, similar possible magmatic-dominated fluids underwent progressive dilution and cooling with time, with the occurrence of boiling at some extent. New geochronological data from hydrothermal minerals and host rocks for these deposits suggest a minimum ~ 14 m.yr. span of conjoined magmatic and hydrothermal activity: biotite and chromian muscovite (fuchsite) from mantos at the Guaynopa IOCG deposits yielded 40 Ar/ 39 Ar plateau ages at 98.12 ± 0.37 and 95.42 ± 0.71 Ma (Cenomanian), respectively, whereas U–Pb dating in zircons from diorite and granodiorite host intrusives, and a potassic alteration assemblage at Guaynopita yielded ages at 92.4 ± 0.5 Ma, 89.1 ± 0.7 Ma (Turonian to Coniacian), and 84.4 ± 1.0 Ma (Santonian), respectively. These ages place the Guaynopa–Guaynopita ensemble within the Mesozoic metallogenic provinces and epochs in northwestern Mexico. Unlike the majority of Mesozoic generic magmatic-hydrothermal iron oxide deposits in Mexico, Guaynopa in Chihuahua and Cerro del Oro in Sonora formed at a distal position from the paleo-Pacific margin (~ 500 km inland) and therefore it cannot be ascribed to a general intra-arc “Coastal Andes Cordillera-type” tectonomagmatic setting. Therefore, this region may constitute a part of a previously unnoticed metallogenetic region, and petrogenetic studies should follow in order to properly evaluate the hypothetical possibility for a back-arc setting there during the Late Cretaceous. [ABSTRACT FROM AUTHOR]

Abstract: The most important deposit in the Plomosas–Rosario district, Sinaloa, is the vein tract named Plomositas–Plomosas–Los Arcos. These are NNW–SSE striking veins hosted in rocks of the Lower Volcanic Supergroup (LVS), and also in rocks at the bottom of the Upper Volcanic Supergroup (UVS). Both supergroups belong to the Sierra Madre Occidental. These veins evolved from an early intermediate sulfidation stage (1), rich in base metal sulfides, to a low sulfidation stage (2), rich in silver sulfides and sulfosalts. There is also a 45 m-wide stockwork with native silver and gold. Stage 1 is found in the deeper portion of the veins whereas stage 2 is found in the most shallow portion of the deposit. These stages record fluid inclusion salinities ranging from 7 to 12 wt.% NaCl equiv., and from 0.2 to 3.5 wt.% NaCl equiv., respectively. Homogenization temperatures range from 120 °C for surface samples to ∼200 °C at a depth of 320 m. The low homogenization temperatures recorded, and the dispersion of veins within host rocks as veinlets, suggest that this deposit formed at shallow depths and was probably blind. [Copyright &y& Elsevier]

Abstract: Mineralizing fluids at the San Martín skarn show an evolution characterized by prograde and retrograde associations. The prograde mineral associations consist of (1) a massive garnet zone, (2) a tremolite±garnet zone, and (3) a late association of quartz, sphalerite, calcite and fluorite lining the vugs in the garnet zone. The fluids of the prograde associations exhibit decreasing temperatures of homogenization (Th) and variable salinities. The fluids of the massive garnet zone have salinities of ∼36 wt.% NaCl equiv. and Th of 645 to 570 °C, corresponding to pressures of ∼1055 bar. At the tremolite±garnet zone, Th range from 438 to 354 °C. In the late association at the endoskarn, the following evolution can be drawn: (a) salinities of 50 to 42 wt.% NaCl equiv., and Th of 455 to 346 °C in quartz, (b) salinities of ∼46 wt.% NaCl equiv., and Th of 415 to 410 °C in sphalerite, (c) salinities of 50 to 37 wt.% NaCl equiv., and Th of 479 to 310 °C in calcite, (d) salinities of 33 to 28 wt.% NaCl equiv. and of 24 to 22 wt.% KCl in fluorite, and (e) two types of fluids with salinities of ∼2 and 39 wt.% NaCl equiv. and Th ∼344 and 300 °C, respectively, in later saccharoidal quartz segregations. The retrograde mineral associations comprise pervasive propylitic alteration to carbonization, and mantos with sulfides. Fluids in epidote have salinities of ∼7.6 wt.% NaCl equiv. and Th of 287 to 252 °C, and in calcite have salinities of 9.2 to 1 wt.% NaCl equiv. and Th of 188 to 112 °C. Fluids in the sulfide assemblages in the mantos have salinities of 8 to 3 wt.% NaCl equiv. and Th ∼300 °C, with corresponding pressures of ∼94 bar. Fluids in late epithermal veins close to the intrusive body have salinities of 10 to 5 wt.% NaCl equiv. and Th of 275 to 200 °C, and distal veins show salinities of 2 to 1 wt.% NaCl equiv. and Th of ∼160 °C. [Copyright &y& Elsevier]

Abstract: The El Barqueño district, Jalisco, consists of several polymetallic (Au–Ag–Cu–Zn–Pb) epithermal veins hosted by a volcanoplutonic assemblage of andesitic rocks, granites and granodiorites that belong to the Lower Volcanic Supergroup of the Sierra Madre Occidental. The paragenetic sequence indicates two main mineralization stages. Stage I occurs as breccias with base-metal sulfides and virtually no gold and silver. Stage II is made up of several crustiform quartz bands that form symmetrical vein fillings and contain the precious-metal mineralization. Rhombohedral adularia crystals and quartz pseudomorphs of bladed calcite are widespread in stage II, providing evidence for boiling. Au- and Ag-bearing metallic minerals locally precipitated directly on adularia crystals, an association indicating boiling as the main mechanism of metal deposition. Temperature and salinity of mineralizing fluids range from 101 to 252 °C and 0.2 to 19.1 wt.% NaCl equiv., respectively. While temperatures cluster in the range from 120 to 160 °C, salinities have a bimodal distribution, with most of them falling either below 3 wt.% NaCl equiv. or higher than 10 wt.% NaCl equiv. These two arrays of salinity values describe mixing trends only locally, and may reflect different sources of mineralizing fluids, possibly dominated by meteoric and magmatic sources, respectively. An adularia separate from stage II was dated by 40Ar/ 39Ar geochronology at 57.91±0.44 Ma (isochron age). Thus, El Barqueño is the oldest known epithermal deposit in Mexico, and suggests the existence of a previously unrecognized Paleocene metallogenic event in southwestern Mexico. [Copyright &y& Elsevier]

Abstract: The Cobre–Babilonia vein system formed during a single major hydrothermal stage and is part of the Taxco district in Guerrero, southern Mexico. Homogenization and ice melting temperatures range from 160 to 290 °C and from −11.6 to −0.5 °C, respectively. We determined an approximate thermal gradient of 17 to 20 °C per 100 m using fluid inclusions. A thermal peak marked by the 290 °C isotherm is interpreted as a major feeder channel to the veins. The highest content of Zn+Pb in ore coincides with the 220 and 240 °C isotherms. Salinities of mineralizing fluids range from 0.8 to 15.6 wt.% NaCl equiv, and are distributed in two populations that can be related with barren or ore-bearing vein sections, with 0.8 to 6 wt.% NaCl equiv and 7 to 15.6 wt.% NaCl equiv, respectively. δ 13C and δ 18O water values from calcite from the Cobre–Babilonia vein system and the Esperanza Vieja and Guadalupe mantos range −5.4‰ to −10.4‰ and 9.9‰ to 13.4‰, respectively. δ 34S values range from 0‰ to 3.2‰ and −0.7‰ to −4.3‰ in sphalerite, −4‰ to 0.9‰ in pyrite, and −1.4‰ to −5.5‰ in galena. Both fluid inclusion and stable isotope data are compatible with magmatic and meteoric sources for mineralizing fluids. Also, sulfur isotope compositions suggest both magmatic and sedimentary sources for sulfur. [Copyright &y& Elsevier]

The Montaña de Manganeso is a manganese vein-type deposit spatially associated with back-arc basin remnants of the Guerrero tectonostratigraphic terrane. The study of major- and trace-element geochemical characteristics of the deposit provides insight into the controls on ore-forming processes within the area. The deposit is characterized by low Co, Cu and Ni abundances, and high Ba (>10,000 ppm) contents and Mn/Fe ratios (<500), typical of hydrothermal Mn deposits. In addition, the low ∑REE abundances (18.7 to 103 ppm), negative Ce anomalies (0.2 to 0.6), and positive Y (1.00 to 2.34) and Eu anomalies (0.6 to 4.4) also suggest a hydrothermal source for the deposit. Discrimination plots involving ∑REE and Zr vs. (Cu + Ni + Co) and Ce/Ce* vs. Nd and Y/Ho further indicate a hydrothermal source in an oxidizing depositional environment. The Mn-Fe-(Ni + Cu + Co), MnO 2 -MgO-Fe 2 O 3 , (Cu/Zn)/Fe 2 O 3 vs. (Zn/Ni)/MnO 2 and Na/Mg diagrams display intermediate signatures between marine and terrestrial environments. This suggest that the Montaña de Manganeso deposit is the result of two metallogenic stages: (I) the earliest stage, which involved the formation of Mn oxides by hydrothermal/diagenetic processes in the Arperos back-arc basin during the Cretaceous; and (II) the latest stage took place subsequent to accretion the Guerrero tectonostratigraphic terrane onto the continent and involved the remobilization of the Cretaceous submarine Mn oxides (and associated trace elements) and subsequent redepositation by Tertiary continental hydrothermal activity. • The Montaña de Manganeso deposit is associated with the composite Guerrero tectonostratigraphic terrane. • Manganese deposition occurred initially in the back arc Arperos basin. • Mn was remobilized by Tertiary volcanism of the Central Plateau. • The deposit formed in a two-stage hydrothermal process. [ABSTRACT FROM AUTHOR]

The formation of most fluorite deposits in northern Coahuila (NE Mexico) is explained by MVT models, and is a part of the metallogenic province of northeastern Mexico. However, fluorite skarn deposits also occur in the same region, and there is evidence for late hydrothermal manifestations with no clear origin and evolution. The latter are the main focus of this study; in particular, F–Be–U–Mo–V–P stringers in the Aguachile-Cuatro Palmas area that overprint preexisting fluorite mantos. The region experienced the emplacement of several intrusives during the Eocene and the Oligocene that are collectively grouped into the East Mexico Alkaline Province (EMAP) and postdate MVT-like deposits. Some of these intrusives have associated skarn deposits; most of them are polymetallic, but the unusual El Pilote deposit contains fluorite mineralisation that was remobilised from MVT-like deposits. The formation of the Aguachile deposit (and, collectively, part of the Cuatro Palmas deposit) has been attributed to a shallow retrograde skarn model. The Cuatro Palmas and Las Alicias fluorite deposits consist of MVT-like deposits overprinted by late hydrothermal fluorite mineralisation rich in Be–U–Mo–V–P, and the Aguachile deposit consists entirely of the latter type. The systematic fluid inclusion study of MVT-like, skarn, and late hydrothermal fluorite deposits reveals a very different distribution of temperature and salinity data that allows the discrimination of mineralising fluids for the type of deposit. MVT-like deposits were formed by fluids with temperatures of homogenisation that range between 50 °C and 152 °C and salinities between 5 and 15.5 wt.% NaCl equivalent. The El Pilote fluorite skarn was formed by fluids with temperatures of homogenisation that range between 78 °C and 394 °C and salinities between 5 and 34 wt.% NaCl equivalent, and include CaCl2-rich brines with salinities that range between 24.5 and 29.1 wt.% CaCl2. Late shallow fluorite–Be–U–Mo–V–P hydrothermal deposits were formed by fluids with temperatures of homogenisation that range between 70 °C and 180 °C and salinities between 0.9 and 3.4 wt.% NaCl equivalent; the sole exception to the above is the La Fácil deposit, with salinities that range between 7.9 and 8.8 wt.% NaCl equivalent. While temperatures of homogenisation are similar between MVT-like and late hydrothermal deposits, and both even have hydrocarbon-rich fluid inclusion associations, the salinity of late deposits is similar to that of retrograde skarn fluids, although further diluted. However, homogenisation temperatures tend to be higher in late hydrothermal than in MVT-like deposits, thus making them more similar to retrograde skarn fluids. Although this characteristic cannot solely establish a genetic link between a retrograde skarn model and late hydrothermal deposits in the study area, the characteristics of fluids associated with the latter separate these deposits from those ascribed to an MVT-like model. Assuming that mineralising fluids for late fluorite–Be–U–Mo–V–P hydrothermal deposits may correspond to a retrograde skarn (or "epithermal") deposit, the source for fluorine may be either from (A) the dissolution of earlier formed MVT-like deposits, (B) the entrainment of remaining F-rich basinal brines, or (C) hydrothermal fluids exsolved from highly evolved magmas. Possibilities A and B are feasible due to a hypothetical situation similar to the El Pilote skarn, and due to the occurrence of hydrocarbon-rich fluid inclusions at the La Fácil deposit. Possibility C is feasible because intrusive bodies related to highly evolved magmas would have provided other highly lithophile elements like Be, U and Mo upon the exsolution of their hydrothermal fluids. Such intrusive bodies occur in both study areas, and are particularly conspicuous at the Aguachile collapse structure. [ABSTRACT FROM AUTHOR]

The state of Oaxaca is one of the most geologically complex and diverse in Mexico. It comprises five tectonostratigraphic terranes of contrasting nature that form a sort of tectonic collage where several magmatic pulses were developed. The different lithological assemblages of Oaxaca favored the generation of a variety of ore deposit types, some of which have been a prime source of several commodities for centuries. This paper addresses the time-space relationship between ore deposits and tectono-magmatic events, based upon the available descriptions and radiometric ages. Most Meso-to Neoproterozoic ore deposits of Mexico occur in this region, and were formed during the regional granulite-facies metamorphic stage(s) recorded in the Oaxacan Complex. Such deposits are rare element-bearing pegmatites, nelsonites (ilmenite-apatite rich rocks), Ti-rich anorthosites, and pegmatites. The Oaxacan Complex also hosts a number of orogenic gold lodes that were likely formed in more than a single metamorphic episode and still require additional research to clarify their age and geological affiliation. The onset of arc-related magmatism in the Late Carboniferous-Permian is recorded by the intrusion of the calc-alkaline Zaniza, Cuanana and Honduras batholiths, along with several minor deformed and undeformed bodies enclosed in the Oaxacan Complex. Little is known about the metallogeny associated with such magmatism. Although supra-subduction magmatism re-initiated in the Jurassic and is probably responsible for the San Juan Mazatlán Cu porphyry, most of the metallogeny of Oaxaca is tightly linked to the Sierra Madre del Sur (SMS) igneous province, which began in Jalisco by the Late Cretaceous and migrated to the SE until it reached the Isthmus of Tehuantepec by the Middle Miocene. In Oaxaca, the Oligocene-Miocene coastal belt of granitic-granodioritic intrusions accounts for some IOCG deposits, especially in its easternmost portion, which then continues to the Coastal Chiapanecan Neogene batholithic belt. In W-NW Oaxaca, the intracontinental igneous province of the SMS marked the beginning of Cenozoic magmatism and is represented by thick volcanic-hypabyssal successions along with minor intrusions, with which copious magmatic-hydrothermal deposits are associated. Early Eocene and mid-Oligocene volcanism is restricted to the western half of the state, where basic-to intermediate-composition effusive products occur in the Mixteca and western Oaxaca tectonostratigraphic terranes. During this time span, many low-to intermediate-sulfidation epithermal, Fe-rich (IOCG) and polymetallic skarn deposits were produced. However, the "boom" in magmatic-hydrothermal mineralizations occurred during the terminal pulses of the SMS volcanism (Miocene), which comprise profuse intermediate volcanic rocks capped by the outburst of silicic magmatism. The early-to mid-Miocene event constitutes the most prominent in terms of metal endowment and diversity of ore deposit types, accounting for about 250 deposits and occurrences in the central and near-eastern portions of the state, which are ascribed to low-to intermediate-sulfidation epithermal, sulfide skarns, IOCG "clan" and Cu(–Mo) porphyry types. Among these, epithermal deposits are, by far, the most abundant type. Recent Ti-rich beach deposits and Au alluvial placers are present in southern and easternmost Oaxaca, respectively. In addition, supergene enrichment zones and uneconomic laterites occurrences are found in some localities. Oaxacan ore deposits can be grouped into five major metallogenic epochs: 1) Meso-Neoproterozoic REE-Th-U-Ti deposits and, possibly, orogenic gold lodes during the Olmecan-Zapotecan events (assembly of Rodinia); 2) feasible late Carboniferous-Permian (?) orogenic gold deposits; 3) early Eocene Ag–Au–Pb–Zn–Fe deposits during the onset of the SMS volcanic activity; 4) mid-Oligocene Ag–Pb–Zn–Sb–Hg–Fe(-Au) deposits during effusive-dominated pulses of the SMS volcanism; and 5) early-to mid-Miocene Au–Ag–Pb–Zn–Cu–Fe(-Mo-Sb) deposits during the termination of the SMS magmatism. The existence of a wealth of ore deposits with varying features underlines the economic potential of metallogenetic provinces and epochs in Oaxaca and exalts the necessity to expand the knowledge regarding the tectonic-magmatic-metallogenic evolution of the state. • The state of Oaxaca records a complex geological and metallogenic history. • Ore deposits in Oaxaca are magmatic-hydrothermal, orthomagmatic, metamorphic and sedimentary. • Oaxacan ore deposits comprise different typologies with contrasting natures and ages. • The boom in deposit formation is coeval with the Sierra Madre del Sur magmatism. • Basement-related deposits are mostly linked to the evolution of the Oaxacan Complex. [ABSTRACT FROM AUTHOR]

Abstract: The Tiámaro deposit in Michoacán state has been dated as Lower Cretaceous (Valanginian), though most of the porphyry deposits in central Mexico were dated or have an attributed Eocene–Oligocene age. The host rocks belong to a volcanoplutonic complex overlain by red conglomerates. These rocks were intruded by pre-Valanginian plutonic and hypabissal rocks. Propylitic, phyllic, and argillic alteration assemblages developed, and their superimposition draws the evolution of the deposit. Stage I is represented by propylitic assemblages, stage II contains the main ore forming stockworks and both phyllic and argillic assemblages, and stage III contains late carbonatization assemblages. The obtained temperatures and salinities from inclusion fluids are low for a porphyry-type deposit, but we interpret that the known part of the deposit represents the shallow portion of a bigger deposit. The evolution of mineralizing fluids draws a dilution trend of brines from “porphyry-like” to “epithermal-like” stages. The richest ore zone is roughly located between the 300 and 350 °C isotherms, though unnoticed resources may occur at depth. [Copyright &y& Elsevier]

Abstract: Mineral alteration and fluid inclusion studies of drill cuttings and core samples indicate that the sedimentary basement rocks and the volcanic rocks associated with Tulancingo–Acoculco Caldera Complex have been the site of two distinct and major hydrothermal events. The complex, located in the eastern portion of the Trans-Mexican Volcanic Belt, is formed by the Pliocene Tulancingo Caldera and the younger (Pleistocene) Acoculco Caldera, which developed within the older depression. The volcanic rocks are underlain by Cretaceous sedimentary rocks of the Sierra Madre Oriental. The earliest important hydrothermal event occurred during the emplacement of Mid-Tertiary granitic intrusions that metamorphosed the sedimentary rocks; these intrusives are not exposed at the surface. However, granitic rocks were encountered at the bottom of exploratory borehole EAC-1, drilled within the Caldera Complex. The second main event occurred during the formation of the Tulancingo and Acoculco Calderas. Both episodes lead to secondary mineralization that reduced the permeability of the reservoir rocks. A possible third hydrothermal event may be associated with the recent magmatic activity within the Acoculco Caldera.Thermal logs from well EAC-1 display a conductive thermal gradient with maximum temperatures exceeding 300°C at 2000m depth. Although there are no active thermal springs in the area, there is extensive fossil surface hydrothermal alteration and cold gas discharges with high He3/He4 ratios. [Copyright &y& Elsevier]

Abstract: Generation of electricity at the Las Tres Vírgenes (LTV) geothermal field, Mexico, began in 2001. There are currently nine geothermal wells in the field, which has an installed electricity generating capacity of 10MWe. The chemical and temperature conditions prevailing in the field prior to its exploitation have been estimated, including their central tendency and dispersion parameters. These conditions were computed on the basis of: (i) geochemical data on waters from springs and domestic wells, and on geothermal well fluids (waters and gases); most of the sampling took place between 1995 and 1999; (ii) fluid inclusion studies; (iii) geothermometric data; and (iv) static formation temperatures computed using a modified quadratic regression Horner method. Fluid inclusion homogenization temperatures (in the 100–290°C range) suggest that there is a high-temperature fluid upflow zone near wells LV3 and LV4 in the southern part of the field. Computed average chemical equilibrium temperatures for the geothermal fluids are ∼260°C, based on the Na/K and SiO2 geothermometers, and ∼265°C, based on the H2/Ar, and CO2/Ar geothermometers. In general, the fluid inclusion homogenization temperatures are consistent with geothermometric data, as well as with static formation temperatures. Some of the observed differences could be related to well interference effects and different fluid production/sampling depths. The deeper geothermal waters show higher concentrations of Cl, Na, K, B, Ba, but lower concentrations of SO4, Ca, and Mg than the shallower waters. Fluid inclusion salinities are also higher in the deeper rocks. The measured Na/Cl ratios of the geothermal well waters are more or less uniform throughout the field and are very similar to that of seawater, strongly suggesting a seawater component in the fluid of the LTV system. The heat stored in the LTV geothermal system was estimated to be at least 9×1012 MJ, of which some 4×1011 MJ (equivalent to about 148MWe for 30 years of operation, assuming a conversion efficiency of ∼35%) might be extracted using wells. These results indicate that the installed capacity at LTV could be safely increased from the current 10MWe. [Copyright &y& Elsevier]