Rosa María Prol-Ledesma

Techniques for enhancing the spectral response of hydrothermal alteration minerals in Thematic Mapper images of Central Mexico

The use of satellite images during the early stages of mineral exploration has been very successful in pointing out the presence of hydrothermally altered rocks and the main structural features that can be related with ore deposition. Here we compare several methods used for spectral enhancement of the images in order to detect rocks containing hydrothermal alteration in an area in the western section of the Transmexican Volcanic Belt, where recent unaltered volcanic and sedimentary rocks and Tertiary age altered volcanic rocks outcrop. The study area is covered by different types of vegetation: crops, bushes and pine trees. The techniques presented here allow the identification of hydrothermally altered rocks in areas covered with different vegetation types that do not have a similar spectral response. The methods that proved to be the most efficient were the Object Oriented Principal Component Analysis (Crosta Technique) using four TM bands combined with the HSI transformation. The results obtained with...

Methane-related carbonates formed at submarine hydrothermal springs: a new setting for microbially-derived carbonates?

Abstract Numerous small calcite mounds, up to 2.5 m in diameter and 0.75 m in height, accompanied by Ba, Hg and Tl mineralisation, occur in shallow submarine hydrothermal vents on the sea bottom, at 10 m depth, near Punta Mita, on the western coast of Mexico. The hydrothermal activity consists in water and gas (mainly nitrogen and methane) venting at 85°C, through a 100-m-long fissure hosted in basaltic rocks and partially covered by a thin layer of unconsolidated detrital sediments. The mounds consist of travertine-like calcite aggregates that develop around the main submarine hot springs amidst a hydrothermally altered basaltic host rock. Two main calcite generations are texturally recognisable: the first generation shows a radial-fibrous texture; the second is fine-grained calcite, which cements detrital grains and fills the pore spaces. The δ 13 C analyses of calcite reveal a strong depletion in 13 C, with values as low as −39.2‰ (Vienna PeeDee Belemnite), which suggest that microbial communities may have induced calcite precipitation through microbial methane oxidation. Barite, sulphides (mainly pyrite and cinnabar) and phosphates (carbonate–hydroxylapatite) are also present in the mounds in lower concentrations and form by direct precipitation from the hydrothermal fluid. The Punta Mita hydrothermal carbonate mounds represent a potentially novel environment for microbially induced carbonate mineralisation, which is characterised by high temperatures not encountered in areas of cold seep carbonate formation. Stable isotope results suggest that microorganisms responsible for the oxidation of methane may be present and active at temperatures near 85°C at the Punta Mita vents.

Evaluation of the reconnaissance results in geothermal exploration using GIS

Geographical Information System (GIS) is used to determine the spatial association between geophysical and geological evidence and production zones in a well-known geothermal field (Los Azufres, Mexico). Surface observations in Los Azufres were used to delineate areas characterized by high permeability and hot fluid transport from the reservoir: main faults, superficial fracture density, surface manifestations, contacts with the most recent rhyolite domes, and low values in the apparent resistivity surveys. Three knowledge-driven models were constructed based on a conceptual model of the field: a hydrothermal system in rough terrain with secondary permeability. Boolean, Index Overlay and Fuzzy scheme models were proposed and the results obtained show a good correlation with the location of the producing and non-producing wells that have been drilled in the field. The results obtained are useful for well siting (Boolean and Low-Risk Fuzzy models) or for planning further detailed exploration (Index Overlay and High-Risk Fuzzy models).

Benthic foraminifera show some resilience to ocean acidification in the northern Gulf of California, Mexico.

Extensive CO2 vents have been discovered in the Wagner Basin, northern Gulf of California, where they create large areas with lowered seawater pH. Such areas are suitable for investigations of long-term biological effects of ocean acidification and effects of CO2 leakage from subsea carbon capture storage. Here, we show responses of benthic foraminifera to seawater pH gradients at 74-207m water depth. Living (rose Bengal stained) benthic foraminifera included Nonionella basispinata, Epistominella bradyana and Bulimina marginata. Studies on foraminifera at CO2 vents in the Mediterranean and off Papua New Guinea have shown dramatic long-term effects of acidified seawater. We found living calcareous benthic foraminifera in low pH conditions in the northern Gulf of California, although there was an impoverished species assemblage and evidence of post-mortem test dissolution.

Use of cartographic data and Landsat TM images to determine land use change in the vicinity of Mexico City

Evaluation of change in land use is important for planning further development in populated areas. Here we attempt to determine the growth of urban areas in the vicinity of Mexico City, using a 1993 Landsat Thematic Mapper (TM) image and cartographic data contained in maps published by the Instituto Nacional de Estadistica Geografia e Informatica (INEGI 1975, 1983). The area occupied by urban areas in 1975 and 1983 was quantified using raster images generated by scanning the maps. Supervised classification processes were applied to a 1993 Landsat TM image in bands 1, 2, 3, 4, 5 and 7, of the area of Chalco. The image was pre-processed and then processed to enhance the spectral response of the surface materials. The different land cover types that characterise distinct land uses in the study area were identified in the image and an overall classification accuracy of 82% was estimated using aerial photographs from the Chalco area. The resulting evaluation of the land use changes in the Chalco urban area was plotted, and a growth greater than 14% per year was estimated.

Heat flow variations along the Middle America Trench

A detailed heat flow study of some areas in the Middle America Trench is attempted. Forty six measurements were obtained in the region between the Tres Marias Islands and the Tehuantepec Ridge. The stations were concentrated in three detailed survey areas and 4 profiles. The obtained data show a steep decrease in the heat flow values towards the southern portion of the trench. The detailed survey area, located in the northern end of the trench (Area 20–1) has the highest heat flow average (122 mW m-2), however a characteristical pattern was observed: most data within the Rivera Plate have higher than average heat flow due to the young age of this plate and contrast with the low values associated with the continental lithosphere of the North America Plate. Areas 20–3 and 20–4 have lower averages (50 and 27 mW m-2 respectively) and they coincide with portions of the Guadalupe Plate, proposed by Klitgord and Mammerickx (1982) and assumed to be older than the Cocos Plate, though magnetic lineations have not been directly correlated with age in those areas.

Three decades of land use variations in Mexico City

Uncontrolled urbanization is one of the most important problems of our changing world. Land use change evaluation is an important tool in planning further development in populated areas. This paper reports the results of an investigation on integration of remote sensing and geographic information systems (GIS) to detect urban growth. Here, we attempt to determine urban area growth in the SW part of Mexico City by using multi‐temporal MSS NALC images (1973), Landsat TM images (1992) and a 2000 ETM+ image. Techniques on change detection evaluation were employed to identify areas of urban encroachment. Statistics on development stages of the expanded urban areas were generated and various thematic maps were produced. Remote sensing techniques were used to carry out land use/land cover change detection by using multi‐temporal Landsat data. Urban growth patterns were analysed by using a GIS‐based modelling approach. The images were processed to enhance the spectral response of the surface materials and define land cover types that characterize distinct land uses in the study area. After processing, the signatures typical of the most important land use types were obtained. Supervised classification procedures were applied to all images and the resulting evaluation of land use changes in the Mexico City urban area was plotted. Quantitative results of changes, which were computed with a post‐classification method, were used to analyse the change pattern in urban land classes. Ground data were gathered from aerial photographs taken in 1973, 1992 and ortho‐photographs taken in 2000 to determine the accuracy of the classification method. Results show that the urbanized area surrounding the SW part of Mexico City has expanded at a maximum rate of 36.29% during the 1980s. The integration of remote sensing and GIS was found to be effective in monitoring and analysing urban growth patterns.

Pre- and post-exploitation variations in hydrothermal activity in Los Humeros geothermal field, Mexico

Abstract Los Humeros geothermal field is a remarkable example of the lack of water–rock equilibrium. Significant variations of hydrothermal activity have occurred before and after exploitation started. Presently, discharged water is not in equilibrium with the alteration suite observed in the reservoir rocks. Hydrothermal minerals identified in core and cuttings define the occurrence of several stages of hydrothermal activity. Cooling at depth is inferred from fluid inclusion and alteration mineralogy data from the wells located nearby Los Humeros fault. Most wells produce a two-phase fluid with excess enthalpy, this accounts for the high CO2 content observed in the discharged fluid. Sulfur and carbon isotopic data indicate that volatile species in the geothermal fluid have magmatic as well as sedimentary components, while strontium isotopic composition shows that calcium is provided by the andesitic rocks that form the reservoir. As exploitation of the field started, concentration of HCl increased in the discharged fluids. This shows that recharge of the reservoir is not enough to balance the output for production, and drying out of the field may be taking place at depth. The lack of chemical equilibrium and the presence of gases of magmatic origin suggest that Los Humeros is a relatively young geothermal field related to a recent magmatic intrusion.

Terrestrial Heat Flow in Mexico

Mexico is a country characterized by very intense and complex tectonic activity. This activity should be certainly reflected in the surface heat flow. At present, relatively scarce (53) heat flow measurements have been done in mines or in exploration wells, and their location is dependent on the availability of wells. On the other hand, in the oceanic areas heat flow measurements are more abundant (Pacific Ocean, Gulf of California, Gulf of Mexico and Caribbean Sea). Lately, it has been more frequent to find indirect estimations of continental heat flow using geochemical parameters (silica temperatures and helium isotope ratios).

Morphology of pyrite in particulate matter from shallow submarine hydrothermal vents

Abstract Here we present the first study of pyrite textures observed in particulate matter from an active shallow submarine vent site, which is characterized by large spatial and temporal variations in the physico-chemical conditions caused by a combination of geochemical processes and microbial activity. Morphologic characterization of pyrite crystals from suspended particulate matter in the discharged fluid of a shallow submarine vent system near Punta Mita, Nayarit, Mexico, showed diverse crystal morphologies and aggregates; well-defined framboids are only observed within the deposits of finelayered calcite (calcareous tuff) formed around the vents, whereas particulate matter contains diverse pyrite crystal forms as globular, cubic, octahedral, and pyritohedral but no framboids. Available sulfur isotopic data indicate that pyrite is formed as a consequence of microbial sulfate reduction in a hydrothermal reducing environment. The results of our study in a natural system provide evidence of the effect of variations in key parameters, such as redox state, on the pyrite morphologies and framboid size distribution, and support the conclusions of numerous studies that have attempted to explain pyrite crystallization processes that generate different morphologies as a result of large variations in the physico-chemical conditions.