Sistema Regional de Información
en línea para Revistas Científicas de América Latina,
el Caribe, España y Portugal

The La Joya prospect is found in the Mezcala district, Guerrero, Mexico. The gold-bearing mineralization is associated to jasperoids that form a small lenticular body near the surface. The ore is related to a granodioritic intrusive with a skarn aureola with grossular formed on recrystallized limestones from the Morelos Formation. An argillic alteration zone located between the skarn and the recrystallized limestones contains small copper sulfide lenses, that predate the formation of Au-bearing jasperoids. The formation of presently mineable skarns at the Mezcala district is related with adakite intrusions during the Paleocene. However, the area contains several skarn deposits associated with calc-alkaline intrusives. Most of these, such as La Joya, have a subeconomic character. We performed a microthermometric study of the fluid inclusions contained in samples from (1) late-magmatic early-hydrothermal stringers in the granodiorite, (2) the argillic alteration zone and skarn, (3) jasperoids, and (4) recrystallized limestones. The fluid inclusions from late-magmatic stringers, jasperoids and recrystallized limestones are biphasic (liquid+vapor) at room temperature, whereas the fluid inclusions from the skarn and argillic alteration zones are multiphasic (liquid+NaCl+vapor). The fluid inclusions in quartz from late-magmatic stringers in the granodiorite display calculated salinities between 15.5 and 17.3 wt. % NaCl equiv., and homogenization temperatures (Th) between 360 and 413 ºC. The fluid inclusions in quartz from the skarn and argillic alteration zones display higher salinities, between 32 and 42 wt. % NaCl equiv., and Th between 306 and 348 ºC. The fluid inclusions in calcite from recystallized limestones display salinities between 5 and 15 wt. % NaCl equiv., and Th between 172 and 205 ºC. Finally, the fluid inclusions in quartz from the gold-bearing jasperoids display the lowest salinities and Th of the deposit, between 0.3 and 1 wt. % NaCl equiv., and between 160 and 170 ºC, respectively. We interpret that boiling occurred in the copper sulfide-rich zone, and early gold transport by chloride complexes, considered as “moderately” saline initial magmatic fluids. The brine diluted towards the surface in the jasperoid zone, allowing gold to precipitate. The characteristics of the initial mineralizing fluids would partially explain the relatively low gold content of the La Joya deposit, in contrast with the deposits associated to Paleocene adakitic magmas of the Mezcala district, formed by oxidizing and hypersaline initial mineralizing fluids.

Laguna de Metztitlán (Hidalgo State, east-central Mexico) is a natural lake dammed by an unbreached, large-scale rock avalanche (sturzstrom) deposit (area: 2.5 km²; volume: ~0.6 km³; up to~400 m thick in the valley axis; horizontal runout distance: 2,600 m; vertical fall height: 860 m) that impounds the Metztitlán River. The natural outflow of the lake is by seepage, the difference between the lake level and the level of resurgence being ~250 m; this value also corresponds approximately to the maximum thickness of the lake deposits. Based on the earlier extent of the lake beyond the town of Metztitlán, indicated in the «Relación Geográfica de Metztitlán» (A.D. 1579), the delta progradation rate of the Metztitlán River is estimated at ~30 m/yr. Based on this rate and the maximum extent of the paleolake, the rock avalanche damming Laguna de Metztitlán must have occurred ~500–1,100 yr B.P. There are general observations that support a seismic origin of the rock avalanche. Two regional-scale, late Cenozoic, east-west striking, and south-dipping normal faults at the northeastern margin of this rock avalanche feature pronounced scarps, up to ~250 m high. These faults have the same orientation as other normal faults with Quaternary activity farther west in the stress and deformation province of the Trans-Mexican Volcanic Belt, which is characterized by north-south oriented extension. Thick colluvium layers have been cut off and rotated along the northern fault. The southern fault forms a scarp with a height of 275–305 cm where it crosses the debris mound of the rock avalanche, which suggests the occurrence of a surface-rupturing earthquake on this fault in the past 1,100 yr.

In southern Mexico, the ~1 Ga Oaxacan Complex is in fault contact with the Mesozoic–Cenozoic Chatino terrane. 40Ar/39Ar dating of minerals from hornblende gneiss and quartz monzonitic gneiss in the southern Oaxacan Complex collected, 10 km and 1 km north of the Oaxaca–Xolapa boundary, yielded the following data, respectively: (1) a plateau age of 584 ± 10 Ma in hornblende and a pseudoplateau age of 23 ± 3 Ma in biotite; and (2) a plateau age of 42 ± 3 Ma in biotite, and a maximum age of 36 ± 1 Ma in K-feldspar. These are inferred to date cooling through ~500–550° C for hornblende,~280° C for biotite, ~220° C for plagioclase, and ~310–270° C for K-feldspar. The ~582 Ma age is much younger than cooling ages from the northern Oaxacan Complex, suggesting that it records resetting during a Neoproterozoic Brasiliano tectonothermal event. On the other hand, the Tertiary cooling ages suggest reheating adjacent to either ~40 Ma or 35–25 Ma Tertiary plutons: ~42 Ma and ~23 Ma biotite ages, respectively. The former was followed by rapid cooling through ~310–270° C by ~36 Ma and exhumation before deposition of Miocene volcanic rocks. We relate these cooling ages to northward migration of the magmatic arc during the Oligocene–Miocene as a consequence of flattening of the subduction zone due to subduction erosion.

The use of storm water presents a wide range of benefits to achieve a sustainable management of the resource. The accumulation of water during sporadic rain events in zones with arid climates for its subsequent recovery and utilization is not usual in Latin America. In other regions with similar climatic conditions, for example Australia and the Southwest of the United States, the storm water runoff of urban areas is frequently recovered for use. Nevertheless, storm water is considered a possible contaminantion source for subterranean and superficial water bodies. Depending on the urbanization of the area, the surface water runoff presents site specific characteristics. Nevertheless, the major contaminant in the runoff consists in particulate material. Giving a well sufficient retention time, 80– 85% of the particulate material will be able to settle. Giving the hydraulic characteristics of common retention structures, the occurring sedimentation process is of Type I, where the sedimentation velocity depends on the diameter y density of the particle. Due to the variable size and density of particles in storm waters, the mean sedimentation velocity can only be estimated if the size distributions of the particles is known. The data regarding suspended particles and its removal suggest that the sedimentation velocities are directly proportional to the level of urbanization of the studied sites. Additionally, it is shown that a lesser content of suspended solids and higher sedimentation velocities correlate with the increase in urbanization. This could indicate that the particles in urbanized areas present a higher density, which could cause partial precipitation in the case of low turbulences in the runoff. The sedimentation velocities based on time steps (g/m3/min) indicate high sedimentation rates at the beginning for the samples taken in Cd. Juárez following a significant decrease at 32 and 128 minutes for the sites PEMEX and MORELIA, respectively. In the other sites, the measured precipitation velocities are low, which indicates the presence of particles of low density, which have to be treated with an agent to initiate the precipitation of the suspended solids.

The ~1 Ga Oaxacan Complex is the largest exposure of the Oaxaquia terrane of Mexico. Microprobe analysis of co-existing ortho- and clino-pyroxene, garnet, ilmenite and plagioclase indicate metamorphic conditions of 735 ± 5° C and 7.7 ± 0.1 kbar in the granulite facies, which was dated between ~998 and ~979 Ma using U–Pb isotopic analyses of zircon populations. Peak temperatures >800° C and isobaric cooling are indicated by the sporadic presence of sapphirine in metasediment and garnet–cordierite– sillimanite–K-feldspar–rutile assemblages, and garnet coronas around pyroxene and titanomagnetite. Common Pb isotopes from acid-leached whole-rock samples define an errorchron that intersects the Stacey/Kramers crustal growth curve at 1,187 ± 63 Ma indicating a major crust-forming event at this time, and suggesting that the granulite facies metamorphism did not significantly reset the common Pb isotopic composition. These data are comparable with those from other ~1 Ga inliers in Mexico and is consistent with a single Oaxaquia terrane by ~1 Ga.

Fronts of the Sierra Madre Oriental expose Cretaceous rocks in the Actopan Platform allowing the study of the paleokarst developed by the exposure of the rocks in the Albian. The development and position of the paleovadose and paleophreatic zones are studied by geochemistry and petrology. The petrologic evidence in the outcrops show dissolutional, depositional, fracturing, brecciation and collapse features extending over the platform. The transgressive cover of the paleokarst does not show any physical (unconformity) or petrologic (conglomerate or breccia) evidence to delineate the contact with the paleokarst. The isotopic composition (δ18OVPDB and δ13CVPDB) and cation chemistry of the paleometeoric water together with the petrologic evidence of the paleokarstic breccias all support the identification of the paleovadose and paleophreatic zones and the limits to those zones. Delineation of paleokarst and of the overlying sedimentary cover can be based entirely on their distinctive geochemical signatures. Cretaceous paleokarstic rocks are important for the prospection of aquifer, petroleum and mineral resources, as well as in civil engineering projects, like dams.  

Citlaltépetl is an active volcano presently in a dormant state, belonging to the easternmost part of the Mexican Volcanic Belt. Although the volcano is characterized by long periods of volcanic repose, some explosive plinian eruptions occurred in Holocene time. In particular, an eruptive period between 8.5–9.0 ka B.P. originated an alternated sequence of pumice and scoria flows called Citlaltépetl Ignimbrite (0.26 km3 DRE) and a few ash and lapilli fallout deposits (Citlaltépetl Pumice) with a wide dispersion range around the cone. In this work we present a detailed reconstruction of the fallout Citlaltépetl Pumice stratigraphy based on the combined study of 107 vertical sections, grain-size and component analysis of each layer. The eruptive sequence comprises a succession of pyroclastic deposits, including four major eruptive episodes. New radiocarbon dating on charcoal fragments interbedded in the flow deposits beneath the fallout and in the fallout itself show ages younger than 9,475±160 yr. B.P. and older than 8,505±50 yr. B.P., respectively, which is in accordance with previous dates. Stratigraphy and dating allowed a precise stratigraphic correlation of the most representative fallout deposits of the Holocene history of Citlaltépetl volcano and their relationship with associated pyroclastic flows. Such data provide new insights on the formation and eruptive history of the Citlaltépetl Ignimbrite that is of basic importance for further eruptive column modeling with important implications in the volcanic hazard assessment.

We describe the geological features and the extraction methods historically applied in the Guanajuato Mining District, in order to propose hypothesis about the geochemical conditions that prevail in the tailings. Information about geographical conditions of the district, ore type, production history, processing methods, and the recovered amount of gold and silver was compiled. We estimate the approximate volume of tailings and the concentration of calcite and pyrite in relation with the location on the source deposit. According with the production history, approximately 95 million tons of mineralized rock have been extracted and processed by different methods: smelting and amalgamation (from 1548 to 1905), cyanidation (from 1905 to present), and flotation (from 1946 to present). The elements present in the tailings that may be hazardous because of their high toxicity are: As, Cd, Cu, Pb, Se, and Zn, in addition to Hg and Cyanide, which have been added during processing. Due to the heterogeneity of the deposits, the sulfide content in the tailings is highly variable. The sulfide concentrations are higher when the ore occurs as veins and breccias than when it is contained in stockworks. In the areas with argillic alteration, pyrite is present in high concentration. When the ore is oxidized, its concentration of sulfide is very low. Therefore, the sulfide concentration in the tailings varies in accordance with the source mineralogy in the following relation: Veta Madre center, deep (Rayas) > La Sierra » La Luz >Veta Madre south (stockwork) > Veta Madre center, shallow (oxidation). The amount of calcite in the tailings, and therefore the capacity for acidity neutralization, depends on the calcite concentration in the gangue, which is controlled by the host-rock mineralogy and the predominant alteration type in each section of the deposit. The only type of alteration that favors the neutralization capacity is the propylitic alteration. This alteration type is widely spread in the deposit (farther than that 10 km from the veins). The content of calcite in the tailings varies with the location of the source deposit as follows: La Luz > La Sierra » Veta Madre south > Veta Madre center. Some tailings have a high acidity generation capacity, namely those that come from the deeper section of the deposit.

The Mexican Volcanic Belt (MVB) is a region of great interest, mainly due to its unique interaction of geodynamic phenomena in combination with the concentration of population and its activities. The knowledge of the MVB has increased through the integration of maps in different scales, from regional to local scale. This study presents a quality assessment of the cartography of the MVB published from 1980 to 2000. In particular, we analyzed those maps included in thesis from undergraduate and graduate levels, as well as all the cartography published by earth sciences research institutes at the Universidad Nacional Autónoma de México (UNAM). The data were obtained from a bibliographic review of thesis, institutional cartography and journals. The assessment also served as a feasibility study for the integration of already published cartography into a spatial database derived from digitized maps. Geological, geomorphological and geophysical maps were evaluated using the MVB as a representative sample of current cartographic production. Thus, a quantitative evaluation was carried out by means of a statistical analysis and comparison against a pre-defined set of cartographic attributes, which were grouped into four different categories: identification, mathematic basis, representation and special elements. Some particular combinations of these attributes were used for basic and rigorous assessments, and served as quality indicators. The mathematic basis group showed the lowest grade. Absence of these cartographic attributes crates a serious problem for the integration of a spatial database, because they are key elements for any re-projections or coordinate transformations, as well as for its geographic registration. In general, those maps included in thesis dissertations had a lower quality grade compared to those published by research institutes. Potential problems that may be faced during map digitalization, spatial database integration, and map production are described. Finally, some solutions related to projections are proposed, which can be implemented to systematize processing task into new cartographic products.

  The sediments with loessial characteristics of the Pampean plain are called “loess y limos pampeanos” (Pampean loess and silts), to distinguish those of purely eolian origin (loess) from those reworked by water (silts). In the continental border of La Plata River, mainly loess-like sediments of the late and middle Pleistocene outcrop, approximately at 20 km from the shoreline. Between that border and the shoreline they underlie at 2–5 m depth the Holocene sediments deposited by the regressive events in a successive lowering of sea level during the late 6,000 years. The objectives of this work are: a) to integrate previous and new information on several sedimentary successions, b) to study the micromorphological features of the paleosols and c) to carry out a regional correlation of paleosols between the localities of San Pedro and La Plata based on field and micromorphological features. The profiles are between 18.5 and 12.5 m a.s.l. with macroscopic pedological features which show partially continuous development. The buried paleosols are superposed and welded, with different degrees of pedogenesis indicated by macro- and micromorphological features of illuviation and hydromorphism. In some cases, these features would have formed simultaneously with the deposition of the eolian dust trapped by grass. The degree of pedogenesis would depend on the ratio between the intensity of the accretion and reworking processes, and pedogenesis. The sedimentary units were affected partially or totally by pedogenesis. Two zones rich in volcanic glass were detected at San Pedro profile; new information reveals the same situation near La Plata city. The uppermost zone, with 20–30% volcanic glass, constitutes in some cases the parent material of the present soil; the deeper zone, contains 50–70% volcanic glass. Micromorphological observations confirm the presence of scarce to abundant clasts of old illuvial horizons and loess embedded in the matrix of most of the paleosols. Pedological processes are most evident in the deeper part of the profiles, indicated by a strong grade of structure and abundant laminated and juxtaposed textural and amorphous features. The lower paleosols can be considered pedostratigraphic units useful for correlation in the continental border in northeastern Buenos Aires Province, Argentina.