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



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Tornos Arroyo, Fernando and Delgado Huertas, Antonio and Casquet, César and Galindo Francisco, Mª del Carmen (2000) 300 Million years of episodic hydrothermal activity: stable isotope evidence from hydrothermal rocks of the Eastern Iberian Central System. Mineralium deposita, 35 (6). pp. 551-569. ISSN 0026-4598

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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‰.

Item Type:Article
Subjects:Sciences > Geology > Geochemistry
ID Code:11645
Deposited On:13 Dec 2010 12:03
Last Modified:13 Dec 2010 12:03

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