Antoni Camprubí

Kinematics of the Guerrero terrane accretion in the Sierra de Guanajuato, central Mexico: new insights for the structural evolution of arc–continent collisional zones

The Guerrero terrane comprises Middle Jurassic–Early Cretaceous arc successions that were accreted to the North American craton in the late Early Cretaceous, producing closure of the Arperos oceanic basin and the formation of an approximately 100 km-wide fold–thrust belt. Such a suture is key to investigating the structural evolution related to Guerrero terrane accretion and, in general, to arc–continent collisional zones. The Sierra de Guanajuato is an exposure of the Guerrero terrane suture belt and consists of a complex tectonic pile that formed through at least three major shortening phases: D1SG, D2SG, and D3SG (SG, Sierra de Guanajuato). During the D1SG and D2SG phases, the Upper Jurassic–Lower Cretaceous successions of the Arperos Basin piled up, forming a doubly vergent imbricate fan of thrust sheets that accommodated substantial NE–SW shortening. Mylonite microtextures, as well as syntectonic minerals, indicate that the D1SG and D2SG deformation events took place under low greenschist-facies metamorphic conditions. We relate these deformation phases to the progressive NE migration of the Guerrero terrane, which triggered the collapse and closure of the Arperos Basin. During D3SG, the El Paxtle arc assemblage of the Guerrero terrane was tectonically emplaced onto the previously deformed successions of the Arperos Basin. However, D3SG structures indicate that during this deformational stage, the main shortening direction was oriented NW–SE and that contraction was accommodated mostly by SE-vergent ductile thrusts formed under low greenschist-facies metamorphic conditions. We suggest that the top-to-the-SE emplacement of the El Paxtle assemblage may be a result of the tectonic escape of the arc produced by the continuous NE impingement of the Guerrero terrane during its collisional addition to the Mexican mainland.

Fluid characteristics of the world-class, carbonate-hosted Las Cuevas fluorite deposit (San Luis Potosi´, Mexico)

Abstract Las Cuevas is a world-class high-grade fluorite district that accounts for over 7% of the world total fluorite production. This district is mainly hosted in the Cretaceous limestones of the El Doctor Formation, and is in fault contact with Tertiary rhyolites. This preliminary study is focused on the “G” orebody, a mass of fine-grained fluorite, with abundant cavities lined up by fluorite, sometimes stalactitic, late calcite and clays. Fluid inclusions in cavity filling minerals have salinities up to 0.18 wt.% NaCl eq. with homogenization temperatures ranging from 60 to 110 °C, with the T h decreasing consistently from early cavity filling fluorite to late calcite. δ 18 O and δ 13 C values suggest that both an organic matter maturation and a decarbonation process might have occurred during the formation of the deposit. All the characteristics of the deposit suggest an MVT related origin rather than a magmatic-hydrothermal one as previously proposed by other authors.

Genetic implications of fluid inclusions in skarn chimney ore, Las Animas Zn–Pb–Ag(–F) deposit, Zimapán, Mexico

Three tectonic and physiographic provinces are present in the study area: (1) the Sierra Madre Oriental (SMOR; Suter, 1987; Carrillo-Martinez and Suter, 1982) formed by the succession of anticlines and synclines with a consistent NNW– SSE trend; (2) the Basin and Range (BR), formed by horst and grabens oriented NE–SW and NW– SE; and (3) the Trans-Mexican Volcanic Belt (TMBV; Aranda-Gomez et al., 2000), a continental volcanic arc on the southwest margin of the NorthAmerican plate resulting from the subduction of the Rivera and Cocos plates along the Acapulco trench (Fig. 1). The SMOR hosts a huge variety of ore deposits: Skarn Pb +Zn +Ag+(Hg–Sb) type ore deposits of Paleocene–Eocene age are found mostly at the El Pinon anticline; low sulfidation epithermal Au–Ag

Fluid origin of the Ixtacamaxtitlan hydrothermal deposits, Puebla State, Mexico

The hydrothermal deposit of Ixtacamaxtitlan (Puebla, Mexico) is made up by a succession, from bottom to top, of quartz veins and stockwork enclosed in a porphyritic subvolcanic body, a kaolinitized rhyolitic tuff and a layered opal deposit. This vertical arrangement coupled with the distribution of the different alteration assemblages lead to the interpretation of the whole as a low-sulfidation epithermal deposit. The fluid inclusion study carried on the veins and the stockwork along with the stable isotopic analyses performed on the kaolinitized bodies helped us to propose two major hydrothermal events that occurred in the area: an early event, characterized by hot, hypersaline fluids (up to 280 jC and 36 wt.% NaCl eq.) closely associated with a potassic alteration episode; and a late event, distinguished by cooler and dilute fluids (up to 150 jC and 4 wt.% NaCl eq.), associated with propylitic and quartz-sericite alterations at depth and acid-sulfate alteration close to the paleosurface due to steam-heated phreatic waters. Post-trapping changes found affecting the primary fluid inclusions (hook-shaped morphologies) suggest that there was an uplifting period between the two stages. D 2003 Elsevier Science B.V. All rights reserved.

Heavy Metal Distribution in Rocks, Sediments, Mine Tailings, Leaching Experiments, and Groundwater from the Mineral de Pozos Historical Mining Site, North-Central Mexico

Silver and gold mineralization in epithermal veins was discovered during the 1600s in the Mineral de Pozos area, north-central Mexico. The main period of mining was between 1880 and 1926. Exploitation in the district is estimated at 1,200,000 tons of ore, with average grades of 1.35 kg/ton Ag, and 8.5 g/ton Au. Tailings were deposited (less than 1.2 million tons) along the main creek in the area, under semi-desert conditions, and widely dispersed by fluvial water during rainy seasons. Concentrations of Cr, Co, and Ni in rocks, sediments, and tailings are below average concentrations in the Earths crust, but Cu, Zn, As, Cd, and Pb concentrations are above crustal averages. Concentrations of As and Pb in groundwater (As = 0.011 to 0.090 mg/l; and Pb = 0.025 to 0.035 mg/l) generally exceed the World Health Organization (WHO) standards for drinking water. Results from humidity cell and leaching column experiments indicate that the potential for leaching As is relatively low (average <0.050 mg/l); Pb concentrations in the experiments range from 0.001 to 0.180 mg/l (average of 0.010 mg/l), and Zn is relatively concentrated (up to 80 mg/l). These results suggest that, given the relatively small amount of mine tailings, the As and Pb concentrations in groundwater may be mostly derived from natural sources rather than from mine waste. The high concentration of Zn in tailings and leachates (up to 306 mg/kg and 80 mg/l, respectively), suggest that zinc could be leaching from the tailings; however, under geochemical conditions of the shallow aquifer (near neutral pH, oxidizing conditions, high Fe content), Zn in solution is mostly controlled by adsorption onto Fe-oxyhydroxides.

Evidence for fluid sources by quadrupole mass spectrometry in the La Guitarra AgAu epithermal deposit, Temascaltepec district, Mexico

The Temascaltepec district is formed by several sets of Ag–Au bearing low-sulfidation epithermal veins, including the La Guitarra vein system. The vein stratigraphy of the La Guitarra deposit is defined by three mineralization stages. Samples from all stages were analyzed by quadrupole mass spectrometry (QMS) to quantify the volatiles contained in fluid inclusions. The results show the occurrence of three fluid types in most mineralization stages: magmatic, crustal meteoric, and surficial meteoric fluids, supporting the conclusions from previous fluid inclusion and stable isotope studies. All the analyzed samples display N2/ Ar ratios (0 to 2526) indicating a general mixing between andesitic or rhyolitic and meteoric fluids. The strongest magmatic fluid contributions were found in the metallic associations of the deposit (stages I, IIB and III). D 2003 Elsevier Science B.V. All rights reserved.

Una nueva etapa para el Boletín de la Sociedad Geológica Mexicana

Tras la edición exitosa de los cuatro volúmenes del Boletín de la Sociedad Geológica Mexicana conmemorativos del centenario de la SGM, editados entre 2005 y 2006, es preciso replantearnos el desarrollo futuro del Boletín. Éste pasa por hallar un espacio propio, distinto y distintivo del que ya cubre la Revista Mexicana de Ciencias Geológicas (RMCG). Desde el Comité Editorial actual y pasado del Boletín tenemos la convicción que las dos revistas no tienen por qué entrar en una competencia y que, lejos de ello, pueden complementarse. En la RMCG se vienen publicando desde hace varios años trabajos de investigación original en todos los campos de las Ciencias de la Tierra, generalmente de alcance regional y en que se presentan datos obtenidos mediante un conjunto de técnicas o procedimientos cuyo objeto es la obtención de explicaciones unitarias, esto es, que sirven a un propósito concreto. La consecuencia inmediata de la inclusión de esta revista en el Science Citation Index ha sido el aumento considerable de artículos sometidos a ésta, absorbiendo artículos que otrora hubieran sido sometidos tanto a revistas internacionales como a revistas nacionales (como Geofísica Internacional o el propio Boletín). Sin embargo, el Boletín no ha resentido tal impacto en forma disminución de artículos sometidos debido a la edición del Volumen del Centenario y del presente volumen especial sobre estudios de Geología Urbana. Sin embargo, ¿cuál es el panorama que le espera al Boletín después de éste? El Boletín de la Sociedad Geológica Mexicana es una revista científica arbitrada, publicada desde el año 1904, lo que la convierte en una de las revistas científicas activas en publicación más antiguas de México y de las Américas. A pesar de su inactividad entre 1913 y 1936, se han editado 58 volúmenes. Sigue siendo una necesidad, sin embargo, garantizar la periodicidad del Boletín año tras año sin rebajar de calidad sus contenidos y permaneciendo como un punto de referencia útil en el estudio de las Ciencias de la Tierra en México. M XICANA A .C . SO CI ED AD GEOGIC A