Resumen: La actividad hidrotermal tardía que se produjo en el plutón granítico de La Cabrera desarrolló en algunas cavidades miarolíticas y pegmatitas una serie de asociaciones
minerales de temperatura decreciente. Entre éstas, son de especial interés las cálcicas de media-baja temperatura, que resultan únicas en el Sistema Central Español. Los minerales más característicos que aparecen en ellas,
además de calcita, son: prehnita, laumontita, estilbita-(Ca), heulandita-(Ca), chabasita-(Ca), datolita, fluorapofilita y bavenita. Todos ellos han sido caracterizados por difracción de rayos X y análisis térmico. Se contemplan
también sus características químicas, incluyendo las de la babingtonita que aparece de manera accesoria.
Las condiciones de máxima actividad hidrotermal tuvieron lugar hacia 250 ñ 50° C, disminuyendo a partir de los 140° C. Paralelamente al enfriamiento, se produjo un aumento de la feo, que provocó la formación de calcita.
[ABSTRACT]
A late hydrothermal activity in the La Cabrera granitic pluton developed in sorne miarolitic cavities and pegmatites several mineral associations according to the temperature decreasing. Among these associations, the calcic medium-Iow temperature have a special interest. They are unique in the Spanish Central System. The characteristic minerals, beside calcite, are prehnite, laumontite, heulandite-(Ca), stilbite-(Ca), chabazite(
Ca), fluorapophyllite, datolite and bavenite. All minerals are characterized by X-ray difraction and thermal analysis. Their chemical compositions are studied, including babingtonite, an accesory mineral in these associations.
The maximum hydrothermal activity occurred at 250 ñ 50° C decreasing towards 140° C. During the cooling an increase of fco. produced calcite.
Palabras clave: Pegmatitas graníticas, Alteración hidrotermal granítica, Zeolitas, Babingtonita, Bavenita, Chabasita-(Ca), Datolita, Estilbita-(Ca), Fluorapofllita, Heulandita-(Ca), Laumontita,
Prehnita, La Cabrera, Sistema Central Español, Granitic pegmatites, Hydrothermal granitic alteration, Zeolites, Babingtonite, Bavenite,
Chabazite-(Ca) , Datolite, Fluorapophyllite, Heulandite-(Ca), Laumontite, Prehnite, Stilbite-(Ca), La
Cabrera, Spanish Central Massif.
Resumen: The northwest of the Iberian Massif is well known to host numerous gold mineralizations which were
exploited during Roman times. This study presents a multidisciplinary approach leading to the mineralogical,
fluid inclusions, and geochemical characterization of the recently rediscovered
Roman gold district of Llamas
de Cabrera. Gold occurs in extensional quartz veins hosted in the upper layers of the Lower to Middle
Ordovician Armorican quartzite and occasionally in the overlaying Middle Ordovician slates from Luarca
Formation. The veins are characterized by multistage ore deposition developed along three hydrothermal
stages: As–Fe (I), As–Fe (II), and Au–Zn–Cu–Pb, followed by late supergene alteration processes. The fluid inclusion
study undertaken on mineralized quartz revealed the presence of three types of fluids: (1) CO2–(CH4)
fluid inclusions, (2) aqueous-carbonic fluid inclusions dominated by CO2 and subordinate CH4 related to
quartz–arsenopyrite–pyrite deposition, and (3) aqueous fluid inclusions related to the Au-sulfide deposition.
In conjunction with arsenopyrite geothermometry and fluid-inclusion data an attempt was made to determine
the P–T conditions of ore formation. Changes occurred in the P–T conditions fromarsenopyrite and pyrite deposition
in quartz veins from aqueous-carbonic fluids at 300–390 °C and 200–220 MPa towards 180–310 °C and
b200 MPa at the stage of gold from aqueous fluids.Mixing of two aqueous fluids of relatively contrasted salinity
were favorable factors for decreasing gold solubility and could allow gold deposition. The δ34S values are similar
for the two As–Fe stages, +8.0‰ to +16.3‰ and +9.0‰ to +19.5‰ respectively, and for the post-S1 pyrites
from siliciclastic Luarca Formation suggesting a comparable sulfur source. Combining calculated δ18O values of
fluids of+6.7 to+9.4‰with fluid inclusion data indicates that fluids of unknown origin are largely equilibrated
withmetamorphic lithologies at medium–high temperatures. The ascent of some late Variscan magmatic bodies,
not exposed at the present day erosion level, provides the heat source for convective water circulation. Gold
quartz veins from Llamas de Cabrera display similar features and processes to related deposits of similar type
elsewhere in the Variscan belt of western Europe (northwest and central/western Iberia, French Massif Central,
BohemianMassif). However Llamas de Cabrera shows certain differences, such as the non-appearance of igneous
rocks in the area and the absence of Sb-rich minerals compared to some Variscan Sb–Au deposits, including
northern, central, and western Iberia. This last could probably be due to a greater depth of formation for the
As–Au veins in comparison to Sb–Au veins.
Resumen: En este trabajo se estudian los macizos situados en el Anticlinorio de Olivenza-Monesterio, prestando especial atención al macizo Cerro Cabrera. Este cuerpo contiene
mineralizaciones de Cr, Ni, Cu y Elementos del Grupo del Platino de características similares a las existentes en otras partes de Ossa Morena. En la actualidad, estas
mineralizaciones son objeto de especial interés para la prospección de yacimientos de
interés económico. El objetivo principal de este trabajo es conocer las características de la deformación dúctil y la estructura del macizo peridotítico Cerro Cabrera y de sus rocas encajantes. Para alcanzar este objetivo principal ha sido necesario afrontar una serie de objetivos parciales a lo largo de la investigación: en primer lugar, el estudio de la macroestructura en el área de Calzadilla de los Barros, y en segundo lugar, el estudio de la deformación dúctil asociada a la Zona de Cizalla de los Llanos.
Palabras clave: Milonitas, Peridotitas, Zona de cizalla, Calzadilla de los Barros
Resumen: Fluids related to the epidote+chlorite+quartz and calcite (+laumontite) alterations present in the
cores of pegmatites from La Cabrera pluton have been determined. Two types of fluids are in volved: a)
A low salinity (0.4-4.5 wt% NaCl equiv.) NaCl-H20 fluid is the responsable for the first type of alteration.
Temperature was probably close to 3000ú b) A moderate to
high salinity (15.3-24.5 wt% NaCl equiv.)
heterogeneous (liquid + gas) CaCI2-NaCI-H20 fluid was in volved in the second alteration under
conditions of Pfluid probably very close to hydrostatic. These alterations probably took place as discrete
hydrothermal events of different ages.
Palabras clave: Pegmatites, Ca-alterations, Fluid inclusions, Spanish Central System.
Resumen: ALCUDIA is a 230 km long, vertical incidence deep seismic reflection transect
acquired in spring 2007 across the southern
Central Iberian Zone (part of the pre-Mesozoic
Gondwana paleocontinent) of the Variscan Orogen of Spain. The carefully designed
acquisition parameters resulted in a 20 s TWTT deep, 60–90 fold, high-resolution
seismic reflection transect. The processed image shows a weakly reflective upper crust
(the scarce reflectivity matching structures identified at surface), a thick, highly reflective
and laminated lower crust, and a flat Moho located at 10 s TWTT (30 km depth).
The transect can be divided into three segments with different structural styles in the lower
crust. In the central segment, the lower crust is imaged by regular, horizontal and parallel
reflectors, whereas in the northern and southern segments it displays oblique reflectors
interpreted as an important thrust (north) and tectonic wedging involving the mantle
(south). The ALCUDIA seismic image shows that in an intracontinental orogenic crust,
far from the suture zones, the upper and lower crust may react differently to shortening
in different sectors, which is taken as evidence for decoupling. The interpreted structures,
as deduced from surface geology and the seismic image, show that deformation was
distributed homogeneously in the upper crust, whereas it was concentrated in wedge/thrust
structures at specific sectors in the lower crust. The seismic image also shows the location
of late Variscan faults in spatial association with the lower crustal thickened areas.
Palabras clave: Variscan Orogen of Spain; Gondwana; Sismology; Crustal structure
Resumen: Crystal growth inhibition by non-incorporating impurities has been described and quantified since 1958 by
the so-called step pinning model by Cabrera and Vermilyea [1]. In the original model, as well as in its recent
improvements by Weaver et al. in 2006 and 2007 [2,3], only the inhibition by the adsorption of impurities on
crystal surfaces with fixed compositions is considered. However, most of the
crystals found in nature are solid
solutions with more or less wide chemical variability. Therefore, in order to provide more realistic models of
crystal growth inhibition in natural systems, it is fundamental to study in detail the inhibition of surfaces of
solid solutions by non-incorporating impurities. In this paper, the Cabrera–Vermilyea model has been
generalised for the case of growth inhibition in solid solution–aqueous solution (SS–AS) systems. This
generalisation was made by considering that supersaturation and the physicochemical properties of the solid
solutions are functions of the solid composition. The main implication of the model is that a progressive
inhibition of growth of a solid solution by increasing the concentration of an adsorbed impurity results in
compositional changes on the growing surfaces.
Palabras clave: Crystal growth inhibition, Impurities, Solid solution–aqueous solution systems
Resumen: The Los Llanos Shear Zone bounds the Cabeza Gorda and Zafra units located in the Ossa-Morena Zone (SW Iberian Massif). Within the Cabeza Gorda unit two peridotitic massifs crop out, named Sierra Cabeza Gorda and Cerro Cabrera. The mylonitic rocks consist of metamorphic and
peridotitic rocks
extensively deformed that exhibit a penetrative foliation. Mylonitic fabrics are characterized by s and d structures and S-C and S-C’ band. Some examples show a crenulation cleavage that points out the existence of at least two deformation stages. Kinematic criteria are widespread, but dextral-reverse movement prevails both in outcrop and thin section. Los Llanos Shear Zone orientation is NW-SE, similar to variscan structures trend in the area, and deforms precambrian and paleozoic rocks. Los Llanos Shear Zone represents a dextral-reverse structure of Variscan age.
Palabras clave: Mylonites, Peridotitic massifs, Shear zone, Calzadilla de los Barros
