73 research outputs found
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‰
Assessment of MODIS spectral indices for determining rice paddy agricultural practices and hydroperiod
The aims of this study were to assess the dynamics of the Normalized Difference Vegetation Index (NDVI), Normalized Difference Water Index (NDWI(1) and NDWI(2)) and Shortwave Angle Slope Index (SASI) in relation to rice agricultural
practices and hydroperiod, and (2) to assess the capability for these indices to detect phenometrics in rice under different flooding regimes
Episodic palaeo-hydrothermal activity in the Sierra de Guadarrama (Spanish Central System): new K-Ar ages and tectonic correlation with the Iberian chain
Episodic hydrothermal alterations in the Sierra de Guadarrama took place between ca. 300 Ma and
10 Ma. New K-Ar ages for hydrothermal minerals have been obtained that along with previous data allow
to better constrain the late- to post-Variscan history of tectono-hydrothermal activity. This activity can
further be correlated with Alpine tectonic events in central Iberia recorded in Permian to Cainozoic
sedimentary basins. Dating of hydrothermal events in the basement is thus a useful tool to better constrain
the chronology of tectono-sedimentary events in the cove
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
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
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
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
Assessing regional crop water demand using a satellite-based combination equation with a land surface temperature componen
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