Search

Hydrothermal activity at Bahía Concepción, on the western coast of the Gulf of California, is not linked to present volcanic activity. This site is a potential energy source; however, geothermal modelling of the system is needed in order to determine the processes that generated this activity. Two processes might lead to the formation of the secondary fluids that were sampled in this submarine hydrothermal system: the thermal endmember might be undergoing either boiling or mixing with a fluid trapped within the sedimentary strata. However, boiling was ruled out as this process would not produce a highly saline fluid within the temperature range determined for the Bahía Concepción system (∼87°C). We modelled the mixing of a highly saline fluid with a geothermal fluid of meteoric origin. Our results suggest that the composition of the thermal water representing the hot endmember at Bahía Concepción consists of 20–30% highly saline fluid and 70–80% aqueous fluid of meteoric origin. The computed fluid has a chemical composition similar to that determined for the submarine vents. Its derived contents of calcium, chloride, and silica are similar to those we measured for the vent discharge. Our hypothesis of saline water addition is supported by the mineralizing fluid characteristics determined from fluid inclusion microthermometric study of a fossil intertidal system that discharges in the vicinity of the vent areas. This intertidal spring fluid at ∼40°C, according to the geochemical model, would be supersaturated in opaline silica and calcite, consistent with the mineralogy of the stromatolitic sinter encircling the discharges. [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]

The El Barqueño epithermal vein district is located in the southwestern part of the Sierra Madre Occidental of Mexico. It is made up of several polymetallic (Au-Ag-Cu-Zn-Pb) epithermal veins hosted by a volcanoplutonic assemblage of andesitic rock s, granites, and granodiorites, which belong to the Lower Volcanic Supergroup of the Sierra Madre Occidental, within the Jalisco block. The veins trend from 45° to 100°, dipping to the southeast and south, and steepening with depth. The maximum strike length of the veins at El Barqueño is about 5 kin, and the maximum vein thickness is about 10 m (avg 2-3 m), narrowing with depth. The paragenetic sequence indicates two main mineralization stages. Stage I occurs as breccias with base metal sulfides and virtually no gold or silver. Stage II consists of crustiform quartz bands that form symmetrical vein fillings and contain the precious metal mineralization. Rhombohedral adularia crystals and quartz pseudomorphs after bladed calcite ar e widespread in stage II, providing evidence for boiling. Gold- and silver-bearing metallic minerals locally precipitated directly on adularia crystals, an association indicating boiling as the main mechanism of metal deposition. Conductive cooling and m ixing also may have been important. The temperature and salinity of the mineralizing fluids ranged from 101° to 252°C and 0.2 to 19.1 wt percent NaCl equiv, respectively. Whereas temperatures cluster in the range of 120° to 160°C, salinities have a bimodal distribution, mos t of them falling below 3 or higher than 10 wt percent NaCl equiv. These two arrays of salinity values only locally reflect mixing and may represent different sources of mineralizing fluids, possibly dominated by meteoric and magmatic sources, respective ly. An adularia separate from crustiform quartz bands of stage II was dated by 40Ar/39Ar geochronology at 57.91 ± 0.44 Ma (isochron age). Thus, El Barqueño has the oldest absolute age of any epithermal deposit in Mexico an d suggests the existence of a previously unrecognized Paleocene metallogenic event in southwestern Mexico associated with several types of ore deposits, including iron oxide-copper-gold, porphyry-type skarn and epithermal deposits. This further suggests that older rocks beneath the Upper Volcanic Supergroup of the Sierra Madre Occidental may be prospects for other epithermal veins and that exploration for epithermal deposits and related mineral deposit types should be focused in the oldest outcrop porti ons of the Sierra Madre Occidental. [ABSTRACT FROM AUTHOR]

Arc-related volcanism results from the interplay of magmatic processes occurring in trans-crustal systems that consolidate over time in a given location. Trans-crustal systems comprise extensive networks of magma reservoirs and conduits at different depths, wherein melts cyclically differentiate and segregate before reaching the surface. The study of these systems provides valuable insights into crustal-scale phenomena, such as the evolution of the continental crust and the formation of metallogenic regions. In this study, we address the trans-crustal magmatic evolution of lower Miocene magmatism parental to several intermediate-sulfidation epithermal deposits in the eastern Sierra Madre del Sur igneous province, southern Mexico. Using a multi-methodological approach, we document changes over ca. 1.1 Myr in the magmatic system that fed andesitic-to-felsic volcanism in this region. We employ whole-rock REE ratios and λ parameters—which are used to quantify the shape of a REE pattern—to track the involvement of pressure-sensitive minerals in the deep-crustal magmatic evolution. The andesitic rocks consist of lava flows, porphyries, and dikes that collectively show REE patterns suggestive of control by fractionated or residual (i.e. in crustal melting) amphibole and/or clinopyroxene. In contrast, the felsic rocks consist of rhyolitic–dacitic ignimbrites, domes, and dikes that show contrasting REE patterns suggestive of control by plagioclase, clinopyroxene, amphibole, and/or garnet. The distinct pressure-sensitive mineral assemblages in the andesitic and felsic rocks indicate that the locus of deep-crustal magma evolution varied within the middle–lower crust. These magmas were episodically injected into ephemeral shallow crustal reservoirs (shortly?) before being erupted, inducing a progressive thermomechanical maturation of the middle–upper crust. Meanwhile, low degrees of crustal assimilation occurred as recorded by Mesozoic inherited zircon ages and Sr–Nd–Pb radiogenic isotopes. An extensive middle–lower crustal magma evolution has been linked to the formation of porphyry Cu deposits (i.e. 'fertile' magmatism). Given that intermediate-sulfidation epithermal deposits may be genetically linked with porphyry Cu deposits, the documented processes could have contributed to the formation of epithermal deposits in the region. However, magmatic fertility proxies resemble those from infertile magmas worldwide. Since these proxies have been exclusively applied to porphyry-type deposits, our results highlight the importance of developing new geochemical exploration tools applicable to a wider range of ore deposits. [ABSTRACT FROM AUTHOR]

The Zn-Pb-Ag(-W) skarn deposits at Zacatepec (or Mɨɨygɨxy) in Oaxaca are associated with rocks of the Sierra Madre del Sur (SMS) magmatic province, and located in its easternmost part, in southern Mexico. This region consists of Eocene to Miocene calc-alkaline hypabyssal and volcanic rocks, with intermediate to felsic compositions that intrude or overlie Cretaceous carbonate sequences. Prograde mineralization in the Zacatepec skarn deposits was dated by means of the 40Ar/39Ar method in a sample of the granitic hypabyssal body, with which the formation of these deposits is directly associated. The isochron ages thus yielded are 17.52 ± 0.14 Ma for biotite and 17.33 ± 0.40 Ma for hornblende. Such ages are in accordance with several other magmatic-hydrothermal deposits (epithermal, skarn, and porphyry-type deposits) that are hosted by Miocene rocks in central Oaxaca state. Deposits in the region with known ages for Miocene hypabyssal rocks with which they are closely related are Taviche, Lachigalla, Cobre Grande, Aurena, Natividad, Altagracia-Águila-Arista, Santa Margarita-Azucena, and Guielavazar. The plausible ages for these ore deposits determine a time bracket between < 23.8 and < 13.01 Ma (early to middle Miocene) that can be considered as favorable for the finding of magmatic-hydrothermal ore deposits. The latter can be tentatively used to define a new metallogenic period that corresponds to a new metallogenic province, for which the rocks derived from the Miocene magmatism of the SMS constitute the metallotect. Although such metallogenic activity has not been altogether overlooked, no previous papers have focused on its relevance and extent in space and time. Therefore, this province constitutes the fifth Miocene mineralized region known in Mexico, besides the southernmost Sierra Madre Occidental, the Trans-Mexican Volcanic Belt, the Gulf of California, and the alkaline province in Chiapas. [ABSTRACT FROM AUTHOR]

Epithermal bonanza-type ores, characterized by weight-percent contents of e.g., gold and silver in a few mm to cm, are generated by mixtures of magmatic-derived hydrothermal brines and external fluids (e.g., meteoric) that transport a variety of metals to the site of deposition. However, the low solubilities of precious metals in hydrothermal fluids cannot justify the high concentrations necessary to produce such type of hyper-enriched metal ore. Here we show that boiling metal-bearing brines can produce, aggregate, and accumulate metal nanomaterials, ultimately leading to focused gold + silver ± copper over-enrichments. We found direct nano-scale evidence of nanoparticulate gold- and/or silver-bearing ores formed via nonclassical growth (i.e., nanomaterial attachment) during boiling in an intermediate-sulfidation epithermal bonanza. The documented processes may explain the generation of bonanzas in metal-rich brines from a range of mineral deposit types. [ABSTRACT FROM AUTHOR]

Gold deposits in the La Rumorosa mining area (Baja California), located in the eastern Peninsular Ranges batholith (PRB), consist of auriferous quartz-(carbonate) veins hosted by brittle-ductile shear zones within metamorphic and synorogenic plutonic rocks. A combination of two geochronology methods, U–Pb zircon dating of country rocks and 40Ar/39Ar dating of hydrothermal and metamorphic white micas, along with the structural characterization of the main orebodies, has allowed the differentiation of two gold mineralization events in the study area. The oldest deposit (>93 Ma) is related to the main compressive phase of the PRB construction (∼110‒90 Ma) and is part of the yet poorly known Peninsular Ranges orogenic gold belt. Two other gold deposits were dated at ∼77 and 75 Ma, an age span that approximately corresponds to the beginning of the Laramide tectonism instead. Therefore, these deposits can be considered the oldest and westernmost constituents of the Caborca orogenic gold belt (∼69‒36 Ma), formerly recognized in the neighboring states of Sonora, Arizona, and the southeastern tip of California. Furthermore, the Laramide gold deposits at La Rumorosa are contemporaneous with the first generation (∼76‒74 Ma) of copper porphyries in the southern North American Cordillera belt, which are also situated on the opposite margin of the Gulf of California-San Andreas fault system. Both gold and copper deposits show an eastward migration during the early Upper Cretaceous, similar to the well-documented shift in magmatism and deformation in the region, albeit occurring asynchronously. The apparent continuous formation of orogenic gold deposits in southwestern North America from the middle Cretaceous to the Eocene, resulting from the strong convergence between the subducted Farallon plate and the continent, suggests a connection between the Peninsular Ranges and Caborca gold belts. We surmise that both of these orogenic gold belts may form part of a large metallogenic province in western Mexico. [Display omitted] • Two gold mineralization events occur at La Rumorosa, which are related to the Peninsular Ranges and the early Laramide orogenies. • ∼77‒75 Ma Au deposits at La Rumorosa are coeval with the earliest Laramide Cu porphyries in the region. • The Caborca orogenic gold belt extends westward into the eastern margin of the Peninsular Ranges batholith. • During the Upper Cretaceous-early Paleogene, magmatism, deformation, metamorphism, and gold deposits migrated eastward. • The Peninsular Ranges and Caborca orogenic gold belts may form part of a larger metallogenic province. [ABSTRACT FROM AUTHOR]

The gold deposits in the northern Sierra San Pedro Mártir region of the Peninsular Ranges batholith (PRB) occur within a middle-Cretaceous suture zone that juxtaposes the Alisitos island arc against the continental margin of North America. Mineralization shares affinities with orogenic-type gold deposits and typically consists of shear zone-related gold-bearing quartz-carbonate veins, with minor amounts of sulfides, white mica and native gold. Host rock types encompass varied synorogenic plutonic rocks and greenschist to amphibolite facies metamorphic rocks. In this paper, we present four 40Ar/39Ar ages of hydrothermal white micas for representative gold deposits: 106.2 ± 0.2 Ma (Valladares), 100.5 ± 0.2 Ma (La Fortuna-San José), 94.2 ± 0.2 Ma (El Socorro), and 92.2 ± 0.2 Ma (Corazón de Oro de Jesús). The first two ages correspond to highly accurate determinations of the timing of the orogenic gold event, whereas we attribute the younger ages to thermal resetting caused by the emplacement of a nearby large La Posta-type pluton (∼97‒90 Ma). Additionally, in order to establish the maximum age of mineralization for the granitoid-hosted deposits, three new U–Pb zircon ages were obtained for plutonic rocks (136.7 ± 0.9 Ma, Corazón de Oro de Jesús quartz diorite orthogneiss; 107.0 ± 0.5 Ma, Valladares tonalite pluton; 105.0 ± 2.0 Ma, Santa Cruz diorite pluton) Furthermore, fluid inclusions were analyzed from six gold deposits. Four different types of ore fluids were distinguished, with likely temperatures of trapping between ∼300° and >600 °C, and salinities between ∼1 and 17 wt.% NaCl equiv. Gold mineralization formed subsequent to the initial collisional phase, occurring simultaneously with regional metamorphism and pluton emplacement. At least some synorogenic granitoid-hosted deposits were formed under a transient dextral transpressive regime, intercalated between the predominant regional orthogonal compression. A connection is established between these gold deposits and others found in historical mining districts along the exhumed axis of the PRB (e.g., El Álamo), and we propose that all form part of the 'Peninsular Ranges orogenic gold belt'. [Display omitted] • Shear-zone hosted Au-bearing quartz-carbonate veins emplaced in a mid-Cretaceous arc-continent suture zone. • Hydrothermal white micas from four gold deposits yielded 40Ar/39Ar ages of ∼106‒92 Ma. • Mineralization formed during regional metamorphism and concomitant magmatism in the southern Peninsular Ranges batholith. • Studied deposits are formally described as part of the orogenic gold type. • This mining area supports the existence of the Peninsular Ranges orogenic gold belt. [ABSTRACT FROM AUTHOR]

The Miocene skarn deposits at Tatatila–Las Minas are found in central-eastern Veracruz state, east Mexico, near the Palma Sola massif. These deposits are geologically associated with the early stages of the Trans-Mexican Volcanic Belt (TMVB) intruded Mesozoic carbonate rocks of the Sierra Madre Oriental. Skarn associations are distributed in the classic evolution from prograde to retrograde mineralization stage. Prograde associations consist mainly of grossular-andradite, clinopyroxene, quartz, wollastonite, clinopyroxene, potassium feldspar, quartz, epidote, chromian muscovite, and are rich in magnetite, whereas retrograde associations are richer in hematite than magnetite, chlorite, fuchsite, hornblende and additionally contain chalcopyrite, pyrite, bornite, and native gold. The systematic study of fluid inclusions shows a broad variety of petrographic and microthermometric features. Fluid inclusion associations in early prograde minerals show evidence for boiling and effervescence, which are hereby interpreted as deep boiling in the -magmatic environment (or "first boiling") with no evidence for supercritic fluids. Such a process would account for the generation of two distinct brines at ≤650 °C (a) up to ∼20 wt% NaCl equiv. and (b) up to ∼70 wt% NaCl. Both brines have cooled down independently until the brittle-ductile transition (∼400 °C) bypassed it and were able to interact with other fluids, and perhaps between with each other. Later on, deep isothermal mixing would have occurred between both brines and deeply evolved meteoric water (thermally equilibrated with host rocks). REE geochemistry of garnet suggests that mineralizing fluids at prograde stage evolved from near neutral pH and oxidizing conditions to mildly acidic and reducing conditions, perhaps reflecting the entrainment of shallow fluids. Both brines or their mixed products have experienced further dilution leading to the precipitation of retrograde associations. The most noticeable features in the retrograde stages, however, are conductive cooling and shallow isothermal mixing, this time between the products of the two pathways that originated as two distinct types of magmatic brines. The occurrence of magmatic fluids is corroborated by C and O isotope geochemistry, in which such source presents different types of water/rock interaction with sedimentary C and O from local limestones (δ13C VPDB between 0.9 and 3.7‰ and δ18O VPDB between −7.5 and −3.7‰), thus drawing an isotopic zonation that parallels the marble front associated with skarns (endoskarn calcite, δ13C VPDB between −8.5 and −3.5‰, and δ18O VPDB between −22.4 and −15.0‰). δ34S VCDT values from pyrite, chalcopyrite and galena in retrograde associations (between −3 and 4.2‰) also indicate a dominant magmatic source for sulfur, with a minor sedimentary/metasedimentary contribution. Low H 2 S concentrations enhanced magnetite deposition in prograde stages, and the progressive increase in H 2 S concentrations, coupled with a decrease in temperature by conductive cooling, led to sulfide and gold precipitation during retrograde stages. These deposits are hereby classified as IOCG skarn that were developed in association with the regional adakitic stage of the TMVB. • The Miocene skarn deposits at Tatatila–Las Minas in the Trans-Mexican Volcanic Belt are classified as IOCG skarn deposits. • Massive magnetite deposits occur in the prograde stage, and Cu–Au mineralization in the retrograde stage. • Ore-bearing fluids show a complex evolution from the magmatic scant deep domains. • Massive magnetite would have formed by boiling and/or effervescence. [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]

The El Álamo is one of the most important historic gold mining districts in the Peninsular Ranges batholith. Gold mineralizations at El Álamo consist in fault-fill laminated quartz veins with small amounts of sulfides and oxides, and lesser amounts of gold. Veins are mainly emplaced along compressional brittle-plastic shear zones within the El Álamo quartz diorite (146.6 ± 1.6 Ma, U–Pb zircon), which is intruded by the San Marcos dike swarm (126.8 ± 1.7 Ma, U–Pb zircon). Several lines of evidence indicate that the dike swarm is older than quartz veins, and the formation of the later is related to the Peninsular Ranges orogeny (∼110−98 Ma). Gold bearing quartz veins formed during regional prograde metamorphism (greenschist facies), and the veins subsequently underwent plastic deformation, dynamic recrystallization and metamorphism. Gold remobilization and concentration seems to have been largely enhanced by metamorphism, probably accounting for very high grade, albeit sporadic, bonanza ores, typical of the district. 40Ar/39Ar dating of hydrothermal white micas from the auriferous veins yielded ages of 97.86 ± 0.17 Ma at the Ulises mine and 97.46 ± 0.17 Ma at the La Americana mine. These ages are interpreted as the minimum age of the regional metamorphism in the area. Based on their primary characteristics, the gold deposits from El Álamo district can be ascribed to the orogenic gold type. These deposits are similar to other known deposits in the same orogenic context (e.g., Julian district; southern California), and they probably conform a new Cordilleran orogenic gold belt that could extend over much of the exhumed axis of the Peninsular Ranges batholith. [Display omitted] • The El Álamo deposits are formally described as part of the orogenic gold type. • These deposits may constitute a part of a new Cordilleran orogenic gold belt. • Gold-bearing quartz veins formed during prograde phase of the mid-Cretaceous metamorphic event in the Peninsular Ranges batholith. • Gold deposits underwent plastic deformation, dynamic recrystallization and metamorphism (upper-greenschist facies) affecting gold concentration. [ABSTRACT FROM AUTHOR]

The mineralogy, bulk geochemical composition, chemistry and U–Pb ages of detrital zircons in sediments from the La Pesca (LP) and Tesoro Altamira (TA) beaches, NW Gulf of Mexico are analyzed. The aim of this study is to infer the weathering history and provenance of sediments, and to identify the potential source terranes that are contributing sediments to the LP and TA beach areas. The beach sediments are rich in quartz and aluminosilicates. The Chemical Index of Weathering (CIW') and trace elements such as Th, Sr, U, and Ba reveal high weathering intensity for the LP and TA beach sediments. Major-element based diagrams and trace elemental ratios based on Co, Cr, Sc, La, and Th contents in sediments indicate felsic provenance, which is further supported by a negative europium anomaly and rare earth element (REE) patterns. The Th/U ratios (>0.3) together with positive cerium and negative europium anomalies of zircons in the LP and TA beaches indicate igneous origin. The comparison of zircon U–Pb ages of this study with ages reported from the adjacent terranes revealed that the Proterozoic (Paleoproterozoic: 1607.07–1943.45 Ma and Mesoproterozoic: 1021.19–1586.52 Ma) zircons are derived from the Oaxaquia, Mesa Central, Sierra Madre Oriental Provinces of Mexico and Mazatzal–Yavapai Province of the USA. On the other hand, Mesozoic (Jurassic 146.49–199.43 Ma and Cretaceous 68.46–136.82 Ma) and Cenozoic (Eocene 33.97–51.46 Ma and Oligocene 23.39–33.86 Ma) zircons are contributed by the Mexican volcanic rocks, Mesa Central and Sierra Madre Oriental Provinces in Mexico, and Mogollon-Datil Volcanic field and Colorado Plateau in USA. The rivers and their tributaries draining from the source areas are considered as a carrier and agent of distributing sediments along the northwestern Gulf of Mexico coastal areas, which are subsequently mixed by littoral currents. Simplified diagram showing river drainage basins, source terranes and seasonal surface current transport directions in the northwestern Gulf of Mexico. [Display omitted] • Mineralogy, bulk sediment geochemical composition, and U–Pb ages of detrital zircons are investigated from two beaches. • Chemical Index of Weathering (CIW') and trace element ratios indicate high intensity of weathering in source region. • Felsic provenance is characterized based on bulk sediment geochemical and detrital zircon age data. • Detrital zircons are brought by the rivers draining from the source terranes and are later mixed by the littoral currents. [ABSTRACT FROM AUTHOR]

Arc-related plutonic rocks in southern Mexico occur along the Pacific coast in the Sierra Madre del Sur (SMS) igneous province. These rocks record the migration of the Caribbean–North America–Farallon/Cocos junction towards the east between the Late Cretaceous and Miocene. However, little is known about the petrogenesis of igneous rocks that were emplaced during the latest expressions of magmatism in the SMS (i.e., early–middle Miocene). Based on fieldwork, petrography, whole-rock and mineral geochemistry, thermobarometric calculations, and zircon U Pb dating, we document the petrogenetic evolution of the Jalapa del Marqués pluton and adjoining intrusions, which are the youngest so far dated in the SMS and are located ∼150 km inland from the present-day trench. This subduction-related magmatic complex consists of granites, granodiorites, diorites, dacite porphyries, and intermediate-to-felsic dikes, whose geochemical features suggest early evolutionary processes driven by fractionation of amphibole ± clinopyroxene (moderate–high pressures) or plagioclase (low pressures). Pulses of evolved magmas were episodically injected into upper-crustal reservoirs (7–14 km) over ∼9 Myr, starting at 22.14 ± 0.65 Ma (2σ)—earlier than previously thought (∼15.7 Ma)—and producing subsequent pulses at 19.45 ± 0.36 Ma, from 16.36 ± 0.37 to 15.8 ± 0.34 Ma, and from 13.9 ± 0.5 to 13.3 ± 0.2 Ma. These ages correlate with the onset and early stages of volcanic centers in the Trans-Mexican Volcanic Belt (TMVB), some of which were located up to 500 km landward from the present-day trench. The large difference in the distance-to-trench between coeval magmatism in the Jalapa del Marqués pluton and the TMVB, indicates a variation in the subduction angle between two segments of the Farallon/Cocos Plate that were likely separated by a slab-tear. The eastward migration of this slab-tear (along with the triple junction) terminated the magmatic activity in the SMS by switching the locus of magmatism to the TMVB. This probably occurred shortly after the studied magmatic complex was assembled. • The Jalapa del Marqués pluton records the amalgamation of episodic magma pulses. • Magma injections from deep-crustal reservoirs consolidated upper-crustal mushes. • Subduction-related magmatism evolved in a normal-to-thickened crust. • The late Sierra Madre del Sur was coeval with the early Trans-Mexican Volcanic Belt. • Extinction of the Sierra Madre del Sur was caused by the migration of a slab-tear. [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]

• Vein-type Mn deposit formed in the epithermal environment. • Aqueous fluids derived from magmatic fluids and meteoric waters. • A new genetic model proposed for the Central Plateau deposits. • Age of deposit coincides with the ignimbrite flare-up event in the Central Plateau. Manganese mineralization at Montaña de Manganeso, San Luis Potosí state, consists of oxide ores that form sharp contacts with volcanic host rocks. The orebodies are generally in the form of veins and irregular masses, and locally as mineralized breccias. Petrographic analyses indicate that the mineralization is multi-episodic, with colloform and crustiform textures predominating. The X-ray diffraction and electron microscopy show that manganese oxides (todorokite, birnessite, pyrolusite, romanechite and cryptomelane) are the main ore minerals, while iron oxides (goethite and hematite) are accessory. The most common gangue minerals are calcite and quartz with subordinate amounts of barite. According to fluid inclusion microthermometry, the mineralization is associated with aqueous solutions of intermediate salinity (8–16 wt% NaCl equivalent) and temperatures between 101 and 140 °C. Stable isotope analysis of calcite (δ13C PDB : −7.76 to −6.32‰; δ18O PDB : −8.01 to −4.71‰) and barite (δ34S: 7.9–13.6‰) shows a contribution of magmatic volatiles to hydrothermal fluids dominated by meteoric water (calculated δ18O fluid : 6.58–13.14‰, relative to SMOW). Argillic alteration is the most widespread hydrothermal alteration at Montaña de Manganeso and indicates fluid temperature below ~150 °C and near neutral pH. Much more local, advanced argillic alteration, revealed by the occurrence of kaolinite, is interpreted as a steam-heated overprint, which in turn suggests a boiling process that must have happened at greater depths. The Mn ores formed through a process of mixing of: (a) boiled-off hydrothermal fluids, with (b) cold, diluted meteoric water. Our results support a hot-spring deposit model, according to which Montaña de Manganeso corresponds to the shallowest portion of an epithermal system formed in a continental volcanic arc setting. [ABSTRACT FROM AUTHOR]

[Display omitted] • Latest Oligocene–Miocene arc magmatism in central Oaxaca developed in a thick continental crust. • Early–mid-Miocene metallogenic epoch in southern Mexico comprises Au-Ag rich epithermal deposits. • Precious metal-rich intermediate-sulfidation epithermal deposits occur at Natividad. • Detailed vein stratigraphy reveals multi-episodic fluid pulses and interaction with wall rocks. • First formal report of argyrodite (Ag 8 GeS 6) in Mexico. The final magmatic arc—Miocene in age—of the long-lived Sierra Madre del Sur igneous province in southern Mexico hosts several epithermal deposits that have hitherto received little attention. The Natividad Au-Ag(-Ge) epithermal deposit is one of them and holds a mining record of > 200 years. Here, we present the first petrogenetic and metallogenic study on the Natividad deposit, in which we use a multi-methodological approach to assess the genesis and evolution of the deposit and its associated igneous rocks. Natividad is mostly hosted by Oligocene–Miocene arc-related dacites (U-Pb zircon dates of 22.8–24.7 Ma), whose geochemical features suggest lower-crustal differentiation (i.e., involvement of garnet) at the base of a thickened continental crust. The mineralization occurs in three multi-stage quartz + carbonate veins and consists of Fe-poor sphalerite (<5 mol. % FeS), galena, chalcopyrite, acanthite, and marcasite (sulfide-dominated stage) overprinted by pearceite-polybasite, pyrargyrite-proustite, tetrahedrite-group minerals, electrum, and argyrodite (Ag 8 GeS 6 ; Ag-sulfosalt-dominated stage). Geothermometry and fluid inclusion data for ore-bearing assemblages reveal fluid temperatures of 170°–400 °C and salinities of 14.6–19.5 wt% NaCl equiv. δ18O from 0.8 ‰ to 7.5 ‰ in mineralizing fluids suggests mixtures of magmatic brines and meteoric waters, while δ13C from –8.4 ‰ to 2.2 ‰ indicates the recycling of organic carbon from the meta -sedimentary basement. Further, δ34S from –3.2 ‰ to –0.3 ‰ in sulfides attest to magmatic S. These results align with an intermediate-sulfidation epithermal model, whereby upwelling magmatic ore-bearing brines precipitated the ores due to conductive cooling, dilution, and, locally, boiling. Our study highlights the metallogenic potential of Miocene magmatism in the eastern Sierra Madre de Sur. [ABSTRACT FROM AUTHOR]

Hot springs with associated Mn-Ag mineralization were identified in the intertidal zone of the Bahia Concepcion Bay of the central-eastern Baja California Peninsula. Hot spring activity is related to a system of NW-SE faults that are associated with the opening of the Gulf of California. The hot springs are hosted by Miocene volcanic and volcaniclastic units of the Comondu Group. The maximum discharge temperature recorded is 72 °C, with the pH ranging from 6.2 to 6.5 and salinity values reaching up to 27 g/kg. Veins, stockworks, breccias, crusts, and organosedimentary stromatolites of silica-carbonate and Mn oxides are associated with the hot springs. The Mn oxide and silica deposits show an association with Ag, As, Ba, Ca, Cu, Mg, Pb, Sb, Sr, Tl, V, W, and Zn. Acanthite and native silver occur along with pyrite in a mineral assemblage rich in opal-A, todorokite, romanechite, aragonite, high-Mg calcite, and barite. Three distinct mineral associations characterized by sulfides, sulfates, and oxides are recognized. The silver minerals formed late in the paragenesis. Textural evidence suggests a microbial mediation for the Mn mineral deposition. Pourbaix diagrams for Mn and Ag species were constructed for the physicochemical conditions of the coastal hot springs. These diagrams show that both Mn and Ag minerals are stable at low temperature (65 °C), and near neutral pH, under slightly reducing or oxidizing conditions. The characteristics of the subaerial to shallow submarine hot springs and the results of the geochemical modeling suggest that the hydrothermal system in the Bahia Conception Bay is formed through convection of meteoric waters, intermixed with some saline fluids or seawater. [ABSTRACT FROM AUTHOR]