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 wi de variety of hydrothermal rocks: they include Sn-W, Fe and Zn-(W) calcic a nd magnesian skarns, shear zone- and episyenite-hosted Cu-Zn-Sn-W orebodies , CLI-W-Sn greisens and W-(Sn), base metal and fluorire-barite veins. Syste matic dating and fluid inclusion studies show that they can be grouped into several hydrothermal episodes related with the waning Variscan orogeny. Th e first event was at about 295 Ma followed by younger pulses associated wit h Early Alpine rifting and extension and dated near 277, 150 and 100 to 20 Ma, respectively (events II-IV). The delta(18)O-delta D and delta(34)S stud ies of hydrothermal rocks have elucidated the hydrological evolution of the se systems. The event I fluids are of mixed origin. They are metamorphic fl uids (H2O-CO2-CH4-NaCl; delta(18)O = 4.7 to 9.3 parts per thousand; delta D ab.-34 parts per thousand) related to W-(Sn) veins and modified meteoric w aters in the deep magnesian Sn-W skarns (H2O-NaCl, 4.5-6.4 wt% NaCl eq., de lta(18)O = 7.37-7.8 parts per thousand; delta D = -77 to -74 parts per thou sand) and epizonal shallow calcic Zn-(W) and Fe skarns (H2O-NaCl, < 8 wt% N aCl eq.; delta(18)O = -0.4 to 3.4 parts per thousand; delta D = -75 to -58 parts per thousand). They were probably formed by local hydrothermal cells that were spatially and temporally related to the youngest Variscan granite s, the metals precipitating by fluid unmixing and fluid-rock reactions. The minor influence of magmatic fluids confirms that the intrusion of these gr anites was essentially water-undersaturated, as most of the to hydrothermal fluids were external to the igneous rocks. The fluids involved in the youn ger hydrothermal systems (events II-III) are very similar. The waters invol ved in the formation of episyenites, chlorite-rich greisens, retrograde ska rns and phyllic and chlorite-rich alterations in the shear zones show no ma jor chemical or isotopic differences. Interaction of the hydrothermal fluid s with the host rocks was the main mechanism of ore formation. The composit ion (H2O-NaCl fluids with original salinities below 6.2 wt% NaCl eq.) and t he delta(18)O (-4.6 to 6.3 parts per thousand) and delta D (-51 to -40 part s per thousand) values are consistent with a meteoric origin, with a delta( 18)O-shift caused by the interaction with the, mostly igneous, host rocks. These fluids circulated within regional-scale convective cells and were the n channelled along major crustal discontinuities. In these shear zones the more easily altered minerals such as feldspars, actinolite and chlorite had their delta(18)O signatures overprinted by low temperature younger events while the quartz inherited the original signature. In the shallower portion s of the hydrothermal systems, basement-cover fluorite-barite-base metal ve ins formed by mixing of these deep fluids with downwards percolating brines . These brines are also interpreted as of meteoric origin (delta(18)O < app roximate to = -4 parts per thousand; delta D = -65 to -36 parts per thousan d) that leached the solutes (salinity >14 wt% NaCl eq.) from evaporites hos ted in the post-Variscan sequence. The delta D values are very similar to m ost of those recorded by Kelly and Rye in Panasqueira and confirm that the Upper Paleozoic meteoric waters in central Iberia had very negative delta D values (less than or equal to-52 parts per thousand) whereas those of Earl y Mesozoic age ranged between -65 and -36 parts per thousand.