Late and post-Hercynian low temperature veins in the Catalonian Coastal Ranges

Many of the veins enclosed within the Paleozoic basement of the Catalonian Coastal Ranges show several common characteristics: low temperature of formation (between 75 and 200°C), the presence of complex polisaline fluids and a certain relationship to the pretriassic paleosurface. Mineralogical composition and age are variable, ranging from Pb-Zn veins with carbonate gangue of late Hercynian age through metal poor fluorite rich veins to barite rich veins of Triasssic age. Mineralizing fluids are not related to late Hercynianmagmatism and deposition took place in active fractures developed either in extensional as in compressive regimes.La mayoria de mineralizaciones filonianas encajadas en el zócalo Paleozoica de la Cadenas Costeras Catalanas, presentan una serie de características comunes: baja temperatura de formación (entre 75 a 200°C); presencia de fluídos polisalinos complejos y una cierta relación con la paleosuperficie pretriásica. La composición mineralógica y la edad son variables, comprendiendo desde filones de edad tardiHercínica con Pb-Zn y ganga carbonatada, filones de fluorita, pobres en metales, hasta filones con baritina dominante de edad Triásica. Los fluidos mineralizantes no tienen relación con el magmatismo tardi-Hercinico, y la deposición tuvo lugar en fracturas activas, tanto en regímenes compresivos como distensivos

Transitional adakite-like to calc-alkaline magmas in a continental extensional setting at La Paz Au-Cu skarn deposits, Mesa Central, Mexico: metallogenic implications

"The granodiorite intrusions with associated Cu-Au skarn mineralization of La Paz district are located in the east part of the Mesa Central of Mexico. The skarn developed at the contact between a middle Cretaceous calc-argillaceous sedimentary sequence and the magmatic intrusions. A Ag-Pb-Zn vein system postdates the intrusive-skarn assemblage. Two well defi ned fault systems (N-S and E-W) divide the La Paz district. The N-S Dolores fault, with a normal vertical displacement estimated between 500 to 1000 m, separates the western Au-Cu skarn zone from the eastern hydrothermal Ag-Pb-Zn vein system. This fault is considered to be part of the Taxco-San Miguel de Allende fault system. The U-Pb dating of the intrusives at the La Paz district clearly indicates a single emplacement event dated at ca. 37 Ma (monocrystal zircon age). This age probably represents the last post-Laramide orogenic mineralizing event known to occur in the Sierra de Catorce district. Also, four calculated discordant ages suggest the presence of greenvilian basement underneath a a thick crust (35-45 km). The chemistry of the intrusive show a certain variability in composition, but they mostly belong to the high-K calc-alkaline magmatic series. Major and trace elements relationships for the intrusives show a chemical evolution from the adakite to the island arc fi elds, and from mineralized to barren intrusives, repectively. They also suggest the importance of crustal delamination processes, and the necessity of deep cortical drains to transfer oxidized magmas and metals to surface.""Las intrusiones granodioríticas que dieron origen a un depósito de Au-Cu tipo skarn en el distrito minero de La Paz, S.L.P., se localizan en la parte oriental de la Mesa Central. El skarn se desarrolló en el contacto entre una secuencia sedimentaria calco-argílica del Cretácico medio y los intrusivos. Un sistema de vetas mineralizadas en Ag-Pb-Zn post-datan el Skarn. El distrito de La Paz está dividido por dos sistemas de fallas muy bien defi nidas (N-S y E-W). La falla Dolores, de dirección N-S , muestra un desplazamiento normal vertical estimado entre 500 a 1000 m y separa la zona occidental de skarn de Au-Cu de la zona oriental que contiene al sistema hidrotermal de vetas de Ag-Pb-Zn. Esta falla se considera como parte del sistema de fallas Taxco-San Miguel de Allende. El fechamiento de los intrusivos mediante el método U-Pb en circones indica claramente un único evento de emplazamiento alrededor de 37 Ma. Esta fecha representa el último de los pulsos mineralizantes, posteriores a la orogenia Laramide,reconocido en el distrito de la Sierra de Catorce. Asimismo se reportan cuatro edades discordantes que sugieren la presencia de rocas greenvilianas en la base de una corteza gruesa (35–45 km). La geoquímica de los intrusivos muestra algunas diferencias en su composición, pero pertenecen a la serie magmática calco-alcalina con alto contenido de K. Los estudios de elementos mayores y traza muestran una evolución desde el campo adakítico hasta el campo de arco de islas, desde los intrusivos mineralizados a los estériles, respectivamente. Estos datos también sugieren la importancia del proceso de delaminación cortical y la necesidad de fallas profundas para transferir dicho magma y metales hacia la superficie.

Is there a Grenvillian basement in the Guerrero-Morelos Platform of Mexico?

In the Guerrero-Morelos platform (Guerrero State, Mexico) the adakitic rocks of Early Tertiary age contain abundant hornblende-rich tonalite xenoliths. Zircon crystals have been concentrated from both adakites and xenoliths, and dated using in-situ U-Pb ion microprobe analyses. These analyses indicate the presence of inherited Grenvillian and early Triassic/late Permian crust ages, as well as a Paleocene age related with the adakite rock intrusion. This range of inherited ages is reported for the first time in the Guerrero-Morelos platform, suggesting that a continous Grenvillian crust exists b

Opaline chert nodules in maar lake sediments from Camp dels Ninots (La Selva Basin, NE Spain)

Chert nodule samples from three different well cores (CC, CP1 & CA) from the lacustrine infill of the Camp dels Ninot maar-diatreme (La Selva Basin) have been studied by means of X-ray diffraction, and optical and electron microscope technique. The chert nodules replace diatomites and carbonates layers, and varies in mineralogy between opal-A to opal-A/CT. The microtexture of the opal-A rich nodules is characterized by smooth microspheres of ~2μm in diameter that forms aggregates of amalgamated microspheres. Commonly, the nodules contain diatoms and their moulds when replacing diatomites, and dolomite or ankerite crystals and their moulds when replacing carbonates. The opal-A/CT rich nodules exhibit a microtexture consisting of microspheres of ~8μm in diameter that form aggregates with botryoidal and finger-like morphologies. Results indicate that the early diagenetic transformation of opal-A to opal-CT is not complete in the studied sediments.Peer Reviewe

L\u27evolution historique de la privilege au premier saisissant et la conception francais de la egalite des creanciers

The Mayari-Baracoa Ophiolitic Belt (MBOB, eastern Cuba) is composed of two large, chromite-rich massifs: Mayari-Cristal and Moa-Baracoa. The chromitites and hosting dunites were firstly affected by a regional serpentinization event, a subsequent episode of hydrothermal alteration (chloritization mainly) and, finally, these already altered bodies were crosscut by thin calcite-dominated veins. Analysed serpentines from serpentinized chromitites and dunites present very similar isotopic compositions (delta(18)O= + 4.7 parts per thousand to + 6.3 parts per thousand and deltaD = - 67 parts per thousand to - 60 parts per thousand), suggesting that the serpentinization process took place at moderate temperatures, in an oceanic environment. Serpentine formation by interaction with ocean water is also supported by the isotopic composition of chlorite and calcite. These results suggest that the serpentinization, chloritization and fracture filling processes of the Mayari-Baracoa Ophiolite Belt took place in a subocean floor scenario and, thus, that the Mayari-Baracoa serpentines represent a good example of serpentine formed during interaction with seawater. The oceanic origin of the serpentines from serpentinized chromitites and dunites from the MBOB indicate that the serpentinization of the mantle sequence occurred pre-thrusting (pre-emplacement in age). (C) 2003 Elsevier Science B.V. All rights reserved

Dolomitization at the base of the «Landete Dolostones» formation in Sierra de Espadán (Castellón province): isotopic geochemistry (C, O and Sr) and fluid inclusion data

The dolomitization affecting the basal facies of the «Landete Dolostone» Formation in Sierra de Espadán (Castellón Province, Iberian Ranges) has been studied using fluid inclusions, stable (C and O) and radiogenic (Sr) isotopes. Two different processes have been found: (1) a pervasive epigenetic dolomitization originated by mixing of two different fluids of contrasted composition, and (2) the generation of «zebra»-llke rhythmic textures formed from hot (215 °C), high salinity (20-24 wt% eq NaCI) connate fluids equilibrated with the enclosing dolostone ( 8 18O H2O= + 13.0%o). A link between this high temperature event and the presence of a Hg deposit in the area is propose