300 Million years of episodic hydrothermal activity: stable isotope evidence from hydrothermal rocks of the Eastern Iberian Central System

The Eastern Iberian Central System has abundant ore showings hosted by a wide variety of hydrothermal rocks; they include Sn-W, Fe and Zn-(W) calcic and magnesian skarns, shear zone- and episyenitehosted Cu-Zn-Sn-W orebodies, Cu-W-Sn greisens and W-(Sn), base metal and fluorite-barite veins. Systematic dating and fluid inclusion studies show that they can be grouped into several hydrothermal episodes related with the waning Variscan orogeny. The first event was at about 295 Ma followed by younger pulses associated with Early Alpine rifting and extension and dated near 277, 150 and 100 to 20 Ma, respectively (events n IV). The δ18O-δD and δ34S studies of hydrothermal rocks have elucidated the hydrological evolution of these systems. The event 1 fluids are of mixed origin. They are metamorphic fluids (H20-COrCH4-NaCl; δ18SO = 4.7 to 9.3‰; δD ab.-34‰) related to W-(Sn) veins and modified meteoric waters in the deep magnesian Sn-W skarns (H20-NaCl, 4.5 6.4 wt% NaCl eq.; δI8O = 7.3 7.8‰; δD = -77 to -74‰) and epizonal shallow calcic Zn-(W) and Fe skarns (H20-NaCl, < 8 wt% NaCl eq.; δ18O = -0.4 to 3.4‰; δD = -75 to -58‰). They were probably formed by local hydrothermal cells that were spatially and temporally related to the youngest. Variscan granites, the metals precipitating by fluid unmixing and fluid-rock reactions. The minor influence of magmatic fluids confirms that the intrusion of these granites was essentially water-undersaturated, as most of the hydrothermal fluids were external to the igneous rocks. The fluids involved in the younger hydrothermal systems (events n nI) are very similar. The waters involved in the formation of episyenites, chlorite-rich greisens, retrograde skarns and phyllic and chlorite-rich alterations in the shear zones show no major chemical or isotopic differences. Interaction of the hydrothermal fluids with the host rocks was the main mechanism of ore formation. The composition (H20-NaCl fluids with original salinities below 6.2 wt% NaCl eq.) and the δ18O (-4.6 to 6.3‰) and δD (-51 to -40‰) values are consistent with a meteoric origin, with a δ18O-shift caused by the interaction with the, mostly igneous, host rocks. These fluids circulated within regional-scale convective cells and were then channelled along major crustal discontinuities. In these shear zones the more easily altered minerals such as feldspars, actinolite and chlorite had their δ18O signatures overprinted by low temperature younger events while the quartz inherited the original signature. In the shallower portions of the hydrothermal systems, basement-cover fluorite-barite-base metal veins formed by mixing of these deep fluids with downwards percolating brines. These brines are also interpreted as of meteoric origin (δ18O< ≈ -4‰; δD = -65 to -36‰) that leached the solutes (salinity >14 wt% NaCl eq.) from evaporites hosted in the post -Variscan sequence. The δD values are very similar to most of those recorded by Kelly and Rye in Panasqueira and confirm that the Upper Paleozoic meteoric waters in central Iberia had very negative δD values (≤-52‰) whereas those of Early Mesozoic age ranged between -65 and -36‰

The Aguablanca Cu-Ni(PGE) intraplutonic ore deposit (Extremadura, Spain). Isotope (Sr, Nd, S) consfraints on the source and evolution of magmas and sulfides

The Aguablanca Cu-Ni(PGE) ore is a case of an intraplutonic ore deposito It is hosted by mafic and ultramafic cumulates of the Aguablanca Stock, which is part of the larger calcalkaline Santa Olalla Plutonic Complex of 359±18Ma age (Rb-Sr whole rock determination). Primary mineralization consists of pyrrothite, pentlandite and chalcopyrite and resulted from the crystallization of an inmiscible sulfiderich liquid. Several stages of hydrothermal alteration are superimposed. Isotope work on the host igneous rocks (Sr, Nd) and the ore (S), suggests that contamination with a crustalsource took place at some depth before final emplacement of the plutons (ε Nd360= -5.8 to -7.2; Sr(360) = 0.7082 to 0.7103; (δ14S(sulfides) near + 7.4‰). Assimilation - fractional crystallization (AFC) processes are invoked to explain de chemical variability of magmas

Geocronologia Ar-Ar de flogopitas del stock de Aguablanca (Badajoz). Implicaciones sobre la edad del plutón y de la mineralización de Ni-(Cu) asociada.

Ar-Ar dating of intercumulus phlogopite for both a websterite fragment within the mineralized breccia pipe of the Ni-(Cu) Aguablanca magmatic deposit; and the host gabbronorite has yielded ages of 335±2 to 338±3 Ma, i.e., Visean (Mississippian). These values are within error and suggest that the mineralization and the host rock are Variscan in age. Moreover this age is compatible with a previous model which interprets the sulfide mineralization as intrusive (vertical pipe) into the Aguablanca gabbronorites

Palaeostress and geotectonic interpretation of the Alpine Cycle onset in the Sierra del Guadarrama (eastern Iberian Central System), based on evidence from episyenites

Several episodes of hydrothermal activity related to periods of fracturing and/or reactivation of previous structures took place from 300 to - at least - 100 Ma, in the Sierra del Guadarrama, which is part of the crystalline axis of the Iberian Hercynian Fold Belt (Central-Iberian Zone). One of these episodes led to the formation of episyenites, which are de-quartzified and alkalinized granites. Episyenite formation took place on a regional scale and in a short period (approx. at 277 Ma). The episyenites were formed by the action of fluids at temperatures between 350°C and 650°C, at depths of about 6.5 km, and in microfractured dilatancy zones developed under a regional extensional regime. These zones are crosscut by normal faults, developed during the progressive deformation process accompanying the formation of the episyenites. The calculated regional palaeostress tensor has ~r I close to vertical and σ3 between NI0-20E and an average value of the stress ratio (Ф) of 0.19 [Ф = (σ2 - σ3)/(σ1 - σ3)]. Because σ1 is close to vertical the stress tensor is compatible with an extensional deformation field. The analysis also shows that most of the faults that slip under this stress field have an average coefficient of friction of 0.8. This extensional regime was probably accompanied by a regional thermal anomaly, as suggested by the high temperature of the fluids involved, which are amagmatic. This thermo-tectonic episode is interpreted as representative of the generalized extensional regime corresponding to the onset of the Alpine Cycle. The episode was preceded by a wrench-faulting event, equivalent to the Late Variscan event of Arthaud and Matte (1977), for which an age of - at least - 300-290 Ma is indicated by recent radiometric data. In its turn, this event was preceded by the regional extensional gravitative collapse of the Hercynian orogen. A correlation between evidence from the cover (stratigraphy and volcanism) and evidence from the basement (hydrothermal alterations, dyke injection episodes and granitic magmatism) is attempted on the basis of new available radiometric data

The carbonate-hosted Zn-Pb deposits of NW Spain: stratabound and discordant deposits relates to the variscan deformation

The carbonate Vegadeo Fm (Lower-Middle Cambrian, NW Spain) hosts abundant Zn-Pb mineralization. Minor stratiform ore showings occur in mineralogically and geochemically undisturbed limestones of the Lower Member. They are interpreted as related to diagenetic processes, with sulfur derived from the abiogenic reduction of sulfates interbedded in the carbonates and lead leached from the nearby detrital rocks. The major mineralization, stratabound deposits in the Upper Vegadeo Fm and discordant replacements (Rubiales-type), occurs inrelationship with pervasive epigenetic silicification and minor hydrothermal breccias. They are interpreted as of Late Variscan age. Here, the mineralization seems to be formed by reaction of the hosting carbonates with low-saline (<7 wt% NaCI eq.) water-rich fluids that circulated along major lithologic contacts or extensional faults. Isotopic composition of the hydrotherrnal fluids falls within the field of low-grade metamorphic or basinal waters ( δ180=-l.0 to +6.3 ‰; δD=-43 to -3l ‰). The ore-fonning process occurred at temperatures between 150 and 250ºC and at low fluid pressures (<500 b). Lead isotopes suggest tha_t most of the base metals were derived from the hydrothermal remobilization of the diagenetic ores in a "lead-frozen" system. Sorne of the sulfur appears to have had the same source, but there was also a significant input from biogenically -reduced sulfur from the endosing sbales. These Variscan ores have many of the descriptive features of carbonate-hosted deposits, but the fluid compositions, P-T conditions of formation, and tectonic setting were fundamentally different. They can be interpreted as equivalent to MVT deposits, but formed in the intemal zones of orogenic belts

Episienitas de la Sierra de Guadarrama: un proceso hidrotermal regional de edad Pérmico Inferior ligado al inicio de la extensión alpina

Las episienitas de la Sierra de Guadarrama (Sistema Central español), las cuales son resultado de la alteración hidrotermal (decuarcificación y alcalinización) de granitoides, constituyen un conjunto litológico desarrollado en el Pérmico Inferior, con un carácter básicamente isócrono (277 Ma), a escala regional. Estructuralmente definen un marco geodinámico extensional, con dirección de extensión preferente NNE-SSO para este sector del basamento hercínico. Este régimen extensional, asociado a gradientes geotérmicos anormalmente altos, se enmarca en el comienzo de la distensión Alpina (rifting). Ello se debe a la detección en este sector de la Cadena Hercínica de una tectónica de desgarres previa, con una edad comprendida entre 300 y 290 Ma, y que es, a su vez, posterior a los episodios extensionales tardihercínicos asociados al colapso gravitacional de la cadena colisional. Esta etapa de desgarres corresponde a la etapa <Tardi-Hercínica» de Arthaud & Matte (1977)

El skarn de W-Sn del Carro del Diablo (Sistema Central Español)

El skarn del Carro del Diablo es un tipo desconocido en el Sistema Central Español. Se forma por interacción hidrotermal tardi y postmagmática entre los mármoles dolomíticos de la Serie Inferior y la parte apical del stock granítico de Rascafría-El Paular, aunque también están afectadas por la alteración, el propio granito y un nivel de leucogneis. El proceso, jalonado por episodios breves de deformación frágil, da lugar a skarns en venas y laminares sobre el mármol y reemplazamientos en el granito y leucogneis. El skarn sobre mármoles es de tipo magnésico con contenidos bajos en Fe y que evoluciona en condidones de muy baja fS2 (pirrotina) y fO2 (grafito). Asimismo, juegan un papel importante el F y B, que pueden alcanzar altas concentraciones en diversos minerales (fluoborita, condroditas, flogopita, fluorita, etc.). Es significativa la existencia de recurrencias cálcicas con idocrasa en la evolución del skarn, con las que se asocian concentraciones significativas de soheelita no molibdénica. El Sn vinculado al skarn magnésico se presenta como estannina (skarn en venas) o casiterita (skarn laminar). Asimismo, son frecuentes los sulfuros asociados a la etapa de temperatura media (pirrotina, calcopirita, esfalerita, loellingita, arsenopirita). El skarn comienza a formarse a temperaturas superiores a la «solidus» del granito (660º C) y continúa hasta temperaturas muy bajas desarrollando microzonaciones que reflejan cambios en la composición del fluido. La presión litostática (probablemente equivalente a la de fluidos) se evalúa entre 2 y 3,5 Kb. (skarn mesozonal). Los leucogneises se transforman en albititas (ab+cta+esf+ap), sobre las que crecen venas y nódulos de minerales de skarn con zonaciones correlacionables con las de los mármoles. A esta etapa sigue una evolución a tipos greisen poco acentuada. El granito apical, que es una adamellita peralumínica (bt, ms, and), sufre también una albitización en forma de chimeneas o «pipas». A esta fase sigue una de corrosión ácida con formación de oquedades y precipitación de pirita

Caracterización de gneises glandulares del alto valle del Lozoya (Sistema Central Español) en base al quimismo de las biotitas y estado estructural del feldespato potásico

En el presente trabajo se estudian las litologías gneísicas glandulares que afloran en la ladera este del Macizo de Peñalara (Sistema Central Español), en base a la interpretación del quimismo de las biotitas de glándulas y mesostasia y del estado estructural del feldespato potásico que forma la mayoría de estas glándulas. Se deduce a partir del estudio de las biotitas, que las glándulas se comportan como sistema cerrado para el Ti y abierto para el Al durante el metamorfismo regional. El contenido en Ti de las biotitas en el interior de las glándulas es, por tanto, heredado de la historia premetamórfica de éstas, y puede relacionarse con las temperaturas de cristalización ígnea. La relación Fe/Mg de la biotita depende de la composición de la roca total. La variación estructural del feldespato potásico viene impuesta por la litología premetamórfica y por varia­ ciones en el campo de esfuerzos. A partir de estos datos se confirma un origen detrítico para los gneises glandulares de la Serie Calderuelas, mientras que los gneises del Reventón y Morcuera son ortoderivados