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Geoquimica, UniversidadComp/utense de Madrid, 28040 Madrid, Spain blnstituto Geominero de Espana, 28003 Madrid, Spain cNERC Isotope GeosciencesLaboratory, NG125GG, UK

ABSTRACT

Fluorite and barite-rich veins (+sulphides) of theSierra del Guadarrama (Spanish Central System) are usually hosted by Hercynian granitoids. Zoned from one of these veins ha ve a Sm-N d age of 145 ± 18 Ma. This age is similar to the K-Ar ages obtained in other hydrothermal rocks of the Spanish Central System (156-152 Ma) and confirms the existence of an important hydrothermal event Late Jurassic, probably related to extensional fracturing accom­ Atlantic Ocean. ratio found in and barite (0.7154-0.7207) is in most cases lower than that found in the host granitoids (0.7194 and 0.7209), suggesting more than one source of strontium. This, in turn, supports previous models that involve mixing of two fluids during mineralization, an ascending hot ( > 300 QC) and low-salinity fluid and a shallow, cooler ( < 1000 C) and more saline brine. The ascending fluidprobably reacted to a variable extent with the and the most likely source for the shallow fluid derived from a marine/evaporiticenvironment, with a ( ,.."0.7069 in the Late Jurassic). The calculated fNd composition of the fluorites (-8.8 to -7.6) as wen as the variation found in this (0.153-0.405) do not conflict with the mixing model although they can be largely explained by interaction of the ascending fluidwith the host granitoids.

1 ... },,,,.. ·i" .... 't"" ,...... compositionsof sulphides range between - 3.4 and - 0.8 %0and those ofbarites from +15.5 to + 15.8%0. of the sulphides suggests that their sulphur was derived from the hydrothermal leaching of the asc:enCllD.g fluid. The t534S-value of barite is consistent with Upper Jurassic marine sulphate as the source of sulphate in the shallow The absence of equilibrium between reduced and oxidized sulphur species can be easily interpreted as a consequence of the short residence time of the mixed fluid in the open fractures at temperatures between 250" and 100"C. Although no pre-Cretaceous sediments are found on this part of the Iberian Hercynian basement, the data presented here support the existence of a shallow platform joining central Spain with the Iberian or the Subbetic realm in Late Jurassic times.

1. 1I"",.. _ . ...h".MI·;;,.· ....

to a extent ...... 1,,"- hosted barite-fluorite vein s with small amounts of base-metal sulphides (galena, and ) are common in the the Sierra del European and North African Hercynian realm. In some cases, the veins cut across Permo­ Tnassic or Lower Jurassic rocks, thus exclud­ ing any relationship with the Hercynian mag­ matism (e.g., Thibieroz, 1987; Canals et al., 1992). In the Catalonian Coastal Ranges (NW Spain) veins as young as the Miocene can be found (Canals and Cardellach, 1993). Ages of veins, hosted by granitoids or pre-Hercynian rocks, are more difficult to assess on geological grounds only. Avalaible radiometric data in­ dicate a broad span of ages: 266-259 Ma in , U.K. (Darbyshire and Shepherd, 199 0; Ches1ey et aI., 1991 ), 206 Ma in the Pen­ 0, nine , U.K. (Halliday et al., 1986), �"'8, 29 5-205 Ma in southern Germany (Dill and Nielsen, 1987), 170-140 Ma in the south French Massif Central and Germany (Bon­ , 0 homme et al., 1987) and 152 Ma in Panas­ ,� � • queira, Portugal (Kelly and Wagner, 1977) . - , , , .. , " " " This suggests that the barite-fluorite veins present episodic hydrothermal events ex­ tended over a long span of time from the Fig. 1. Geological setting of the mineralizations [l=Cainozoic sediments; 2=schists and gneisses (Pre­ Permo-Triassicto Tertiary. cambrian?); 3=femic paragneisses and augen-gneisses This work provides new isotopic evidence (Precambrian?); 4=orthogneisses (Cambrian-Ordovi­ (S7Sr/S6Sr, 147Sm/144Nd and �34S) that we be­ cian); 5=adamellites and granites; 6=barite-rich veins; lieve strengthens the unconfonnity-related 7=f1uorite-richveins] . model for the genesis of the veins and confirms the relative youth of them in the Sierra del Lillo et al. (1992) consider hydrothermal Guadarrama, leading to new palaeogeographic fluids to be directly related to the Hercynian consequences. granitic magmatism. Lillo et al. (1992) argue that the veins were formed under two succes­ 2. Geologicalsetting sive late Variscan tectonic regimes involving extension, concomitant with the emplacement The Sierra del Guadarrama forms part of the of granites, and later transcurrent faulting. An Spanish Central System (SCS) which is a block alternative genetic model, based on geochemi­ of Hercynian basement bounded to the north ca! studies, was proposed by Locutura and and south by Tertiary-Quaternary basins. The Tomos ( 1985, 1987) and Tomos et al. ( 1991) uplift took place for the most part along NE­ who favored an unconfonnity-relatedorigin for SW-trending high- to low-angle thrusts, during the veins that implies mixing of an ascending the Alpine tectogenesis (Lower Miocene). reduced, hot, low-salinitymeteoric fluid and an These faults divide the Sierra del Guadarrama oxidized, more saline brine of basinal origin itself, into several longitudinal blocks with a during late to post-Hercynian times. "pop-up" geometry (e.g., De Vicente et al., The problem of the age of the F-Ba (Pb-Zn) 1992). Most of the barite-fluorite veins are veins has also been addressed elsewhere in the found in the southernmost block, which is also faults. the one that underwent the least uplift and The vertical extension of the veirisis therefore shows only minor erosion. The rocks short (100-200 m) and the thickness gener� ally < 3 hosting the barite-fluorite veins are for the m. Length of the veins vary froni sev­ most part granitoids and to a lesser extent high­ eral metres to few kilometres, a "pinch a�d. grade metamorphic rocks. These rocks form the swell" �like geometry being common. The veins largest part of the Sierra del Guadarrama, display a wide variety of textures, with band­ which has been interpreted as representing the ing, massive open space fillings and different lower plate of an Hercynian "core complex-like generations of breccias, that reflect repeated structure" bounded on its sides by extensional activity of the fractures. Breccia fragments shear zones (Casquet et aI., 1988). consist of reworked vein material as well as The granitoids are mainly peraluminous, host rocks. Fragments of episyenites (i.e. hy� biotite± cordierite porphyritic to medium­ drothermally dequartzified granitoids along grained adameUites and minor, two-mica, fine­ diffuse zones) correspond to an hy­ grained to pegmatitic leucogranites. They were drothermal regional episode dated at 274±6 emplaced following the Hercynian compres­ Ma (Caballero et aI., 1992b; Casquet et al., sive phases in the time span from 345 to 290 1992). Mylonites with a strong phyllic altera­ Ma (Ibarrola et aI., 1987). Older, small bodies tion are also found, and probably represent the of -diorite to tonalites are also found lo­ alteration along the shear zones bounding the cally, as well as later massifs of amphibole­ veins. K-Ar ages of 152±4 and 156±6 Ma bearing adamellites and granites with a mon­ have been obtained for a similar alteration zonitic affinity, which may be as young as 275 consisting of chlorite+sericite+quartz +sul� Ma (Ibarrola et aI., 1987). The majority of phides in the Sierra del Guadarrama. (Cas� metamorphic rocks are orthogneisses (augen­ quet et aI., 1991; Caballero et aI., 1992a) gneisses and leucogneisses) that have yielded In spite of local complexities, the veins dis­ Late Cambrianto Early Ordovician Rb-Sr ages play a gross vertical and lateral zoning. They (Vialette et aI., 1987). Azoic, pre-Ordovician show a deeper and outer quartz zone, an inter­ sediments are more scarce. A striking feature mediate fluorite zone and a shallower and in� of the Sierra del Guadarrama is the presence ner barite zone. Minor amounts of , of dyke swarms of regional extent. They con­ ankerite and , are found as void fillings sist for the most part of granitoid porphyries in the central and upper sections of the veins and minor microdiorites (Huertas, 1991 ) and (Tomos et aI., 1991). Sometimes a colour crosscut all the Hercynian granitic plutons. zoning is displayed by fluorite with an outer They must be older than 275 Ma (Caballero et yellow type, an intermediate purple variety and aI., 1992 a, b). an inner green zone in contact with the barite. Other found in the veins are chlorite, sericite and elay minerals. The ore minerals are galena, sphalerite and chalcopy­ 3. Mineralization and controls rite with minor amounts of , freibergite, pearcite, , native and Bi- Ag sulpho salts. Alteration at the selvedges in� The barite-fluorite veins have a lens-shaped eludes silicification, sericitization and, to a geometry and follow two systems of fractures, lesser extent, chloritization, argillitization and N60-700E and NIl 0-125 °E. The first corre­ adularitization. This host-rock alteration alsO. " . .• sponds to tension joints, often with an "en ech­ appears to display a vertical zonation with elon" arrangement, and the second to open ularitization being more intense at depth, zones along major strike-slip right-lateral the argillitization restricted to the TABLE I

Bulk anaJysis of the gas fraction of fluid inclusions in quartz, Iluorite and barite (in mole%) by mass specU'Ometry (Bri, GeologicalSurvey)

------

Quartz Fluorite Barite ------. CB-BI SE-} SE·3" AS-IOJ AS-.s3" AS-104' CB·

. -- ,--- -_._---

H,O 94.28 98.53 98.28 99.10 97.55 95,96 96.1 CO, 5.22 0.85 0.68 0.s5 0,66 lSl I.; CH, 0.12 0.08 0.05 0.06 0.09 0.06 0.( CO 0.19 0.35 0.01 0.21 0.00 0.19 O.� H, 0.04 O.lD 0.77 0.09 1.41 0,28 L' N, 0.16 0.00 0.22 0.00 0.29 0.00 0.(

------. --- aMeanoftwo anaJyses.

sections of the vein system (Locutura and Tor­ 4. Isotope work nos,1987;Mayor et al.,1989). Fluid-inclusion data reflectthe internal zon­ 4.1. Sampling and analytical procedure ing of the veins. Homogenization tempera­ tures in primary inclusions in quartz, fluorite Sampling was restricted to two of the n and barite decrease regularly from 3000 to important barite-fluorite veins in the a 250'C. 270' to 150'C and 200' to 125'C. These veins comprise the Asturiana and probably reflecting progressively lower precip­ Eusebio Mines (AS Mine and SE Mine, itation temperatures (Locutura and Tomos, spectively; Fig. 1) both of which are hoste, 1987). Fluid-inclusion studies, chemical anal­ porphyritic biotite adamellites belonging t( yses of decrepitates and thermodynamic con­ same intrusive unit. Samples of barite, I straints have led to a model of vein evolution rite, carbonates and sulphides were separ consisting of the mixing of an ascending, re­ by hand-picking. Twelve samples of flUCl duced CO2-poor and low-salinity fluid « 0.6 nine from the SE Mine, where the colour m) Na-K fluid,with a shallow, oxidized and ation is best displayed, were analyzed for more saline (>2.8 m) Na-K-Ca-Mg brine Nd and 87Sr ;S6Sr. Four barites, three of t (Tornos et aL, 1991). Fluid pressure was from the ASMine, were analyzed for 87Sr / probably < 100 bar. Analyses of volatiles con­ Barites from the AS Mine were also anal tained in fluid inclusions in fluorites and bar­ for 034S, along with two and ites by decrepitation under vacuum and mass galena from the same mine. 0180 and 013C spectrometry (Shepherd et al.,1991) show that also measured on two calcites from th fluids were almost pure water (XH2o>0.95) Mine. Stable isotope measurements wen with small amounts of CO2 (XC02 <0.05) and ried out at the Scottish Universities Res! traces of H2, CH4 and CO (Table 1). Precipi­ and Reactor Centre and the Salamanca tation of the ore took place in a near�neutral versity Isotope Laboratory. Results are pH environment and close to the pyrite-hae­ played in Tables 2 and 3. matite- buffer. Geochemical calcu­ Concentrations of Rb and Sr as well a! lations show that mixing of two such fluids ac­ Sr atomic ratios for the whole-rock Sat counts well for the observed zonal arrangement were determined by X-ray fluorescence (Tornos et al.. 1991). trometry (Analytical Geochemistry G eral (e.g., Maller and Morteani, 1983). Appli· cationof the Sm-Nd isotope data has recently proved to be a very useful technique to con­ strainthe sourceof the hydrothermal fluids and to date fluorite deposits (Halliday et aI., 1986, 10 1990; Darbyshire and Shepherd, 1990; Ches- 1eyet al.,1991). � GREEN Total REE content and chondrite-normal­ 1 I I I I I I I I I ! I 1 I 1 ized profiles of the fluorites (SE Mine) show a �""'_,",",=�1"'a"='==*,�, SE-P clear relationship with colour. Total contents 1 00 SE-BSE-F vary from 12-39 ppm in the yellow type to 182-192 ppm in the greentype, with interme­ 10 diate values in the purple varieties (89-105 ppm). Thus, the REE content increases in­ PURPLE ward from the vein wall. The yellow and pur­ ple fluorites show a slight relative enrichment in the intermediate and heavy REE+ Y rela­ 100 tive to the light ones (negative fractionation) SE-Y and neither have a Eu anomaly (or at the most �SE-A 10 �/ SH a very small one). Instead, the green fluorites � SE-4 exhibit relativelyflat patternswith a large pos­ YELLOW itiveEu anomaly (Fig. 2). Allthe fluorite REE r I I I I I I I r I r TJ 1..0 Ce SmEu Y Lu profiles are very different to that shown by the Nd Tb Yb host adamellite. Sm-Nd data of the yellow, green and purple varietiesof fluorite from the SE mine and yel­ low and green-purplefluorite fromthe AS Mine ADAM ELUTES fluorite are presented in Table 2. The host ada­ 100T-""'_ mellite from both mines was also analyzed for comparison. The 147Smj144Nd ratio varies \...?"""'==2cj:� SE-8 from 0.-153 to 10 ] ... 0.405 in the fluorites. Within a single vein (SE Mine), the 147Smj144Nd range .I.S-12 can be as large as 0.252. Within any colour type (e.g., the SE Mine purple fluorite), the 147Smj 1 hT,"-",,-,---. ,--,---:---,---,--, 1..0 Ce SmEu Tb Y 'It> lu Nd 144Nd range is smaller. The SE Mine fluorites define an isochron Fig. 2. REE patterns influoritesfromthe SEMine,and in (MSWD=0.5, Fig. 3) yielding an age of host rocks (adamellites)from the SE and AS Mines. 145 ± 18 Ma, with an initial 143Ndj144Nd ratio ofO.5l205±0.00003. The mean age value cor­ ASMine fluorites. The data plot slightly below responds to the Late Jurassic (Harland et al., the SE Mine isochron but the significance of 1982). If the error bar is considered, the age this deviation is not yet certain. can range from the boundary of the Middle­ The good fit of the isochron to the data Late Jurassic to the Early Cretaceous (Neo­ points (MSWD= 0.5), and the coincidence of comian). The number of samples and the lim­ the age with the Kj Ar ages of the associated ited range of 147Smj144Nd ratios preclude the phyllic alteration (see below), are here taken determination ofa Sm-Nd isochron age for the as evidence of a fairly uniform 143Ndj144Nd

TABLE 3

Sulphur isotopic composition ("J"'5cvr) of sulphides and sulphates from San Eusebio (SE) and Asturiana (AS) Mines, and "IS0SMOWand "IJCpDlI of carbonates from SE Mine

Sample "J"'5 (%0vs. CDT)

barite galena sphalerite chalcopyrite

AsturianaMine (Fresnedillas):

AS-102 -1.0 AS-106 -0.8 A5-1oo -3.4 AS-105 +15.8 A5-B +15.8 AS-10l +15.5 SCS-36 +14.9

SanEusebio Mine (Colmenar de Arroyo).

SC£.J2 +15.6 -3.6 -1.6 -1.1 SCS-J8 +15.1 -1.8 -1.9 SCS-28 +15.5 -4.4 seS-29 +16.5 _3.4

Sample "ISO ol3e (%0vs. SMOW) (0/00�·S. PDB)

AS-Ce +23.5 -7.8 AS-108a +21.1 -7.8 AS-lOBb +22.8 -8.6

SCSsamples and the name of locations in brackets are from Lillo et al. (1992).

-3.40/00. c534S-values of barites range from did not form in isotopic equilibrium. This dis­ + 15.5 to + 15.80/00.These values are in agree­ equilibrium is to be expectedin these low-tem­ ment with the ranges of 034S found by Lillo et perature veins due to the lack of exchange be­ al. (1992) for a larger set of Ba-rich veins (in­ tween aqueous sulphide and sulphate after cluded those studied here), of the SCS (ga­ mixing (Ohmoto and Rye, 1979; Ohmoto and lenas -4.4 to -1.80/00: sphalerites -1.9 to Lasaga, 1982; Field and Fifarek, 1985; -1.6%0; barites + 14.9 to + 16.50/00;see Table Ohmoto,1986). 3). Similar values in sulphides and sulphates are found all over the world for the so-called 5. Chronological considerations zoned polymetallic hydrothermal veins (Field and Fifarek,1985). The age of the fluorite-barite veins coin­ Sulphide-sulphate isotope thermometry, cides within the limits of error, with the K-Ar based on data from Lillo et al. (1992) for the ages of l52±4 and 156 ±6 Ma found for the SE Mine and equations of Field and Fifarek spatially associated phyllic alteration (Cas­ (1985,table 6.5),gives unreasonably high val­ quet et al., 1991). Both types of hydrothermal ues (324-381° C) that are above the upper rocks are probably the consequence of a com­ thermal limits found for the precipitation of mon tectonicevent. These results are clearly at fluorite on the basis of fluid-inclusion studies odds with the Hercynian magmatic-relatedhy­ (270-300°C). It suggests that these minerals potheses proposed for these veins by Vindel (1980). Ortega et a1. (1988). and Lillo et al. water) are low (present-day value is 0.70907). (1992). However, this ratiobas varied during geologi­ Late Jurassic ages have also been found for cal timeas a consequence of pI ate interactions other barile veins in other parts of tbe Euro­ and sea-floor spreading (Burke et al., 1982). pean Hercynian realm. Kell)' and Wagner Tbese autbors provide a I1Sr/'ltgr vs. time ( 1977; see also Tbadeu. 1982) obtained a fis­ curve for tbe Pbanerozoic, fitting values o� sion-track age of 152 Ma for the Pb-Zn-Ba tained from marine sediments collected from veins at .Bonh ommeet al. (1987) all over the world. A value of -0.7068, wbicb bavefound K-Ar ages of 170-150 Ma for c1ay is also the lowest Phanerozoic ratio, is found minerals associated with barite occurrences in for tbe Late Jurassic seawater. Koepnick et al. the south French Central Massif and Ger­ (1990), in a tborough review of the Triassic­ many.This supports the hypothesis ofZiegler Jurassic portion of the curve, show that this ( 1988) tbat the European Hercynian base­ should be considered more properly a band ment underwentwidespread fracturingduring with values of the Sr isotope ratio varying b� the late Mesozoic, probably related to the tween 0.70608 and 0.70702 at 145 Ma. witb a opening oftbe North Atlantic. mean value of -0.70691. Mixing of different proportions of seawater (or meteoric water) 6. Constraints on the pro"enance� tbe Ouids and an ascending bot fluid baving interacted witb the bost adamellites would explain the Sr The data presented bere stronglysupport the isotope compositions found in barites and conclusions of Tornas et al. (1991) that two fluorites as well as their variations in time and contrasting fluids were probably involved in space. Because of the probable presence of the genesis of tbe barite-fluorite veins. The highly radiogenic meta morphi c pre-Hercynian possible provenance of tbese fluids are now rocks in the emerged areas, the Sr isotope ratio considered of the shallow water was probably higher thus The occurrence of a pervasive bydrothermal reducing the importance of the shallow contri­ alteration at the selvedges of tbe veins, sug­ bution. This shallow contribution was proba­ gests tbat the hydrothermal fluids interacted bly very small for the case of the two green witb tbe bost rocks. The Sr isotope composi­ fluorites of tbe SE Mine that show (I1Sr/ tionof tbe adamellites calculated at t= 145 Ma R6Sr) 145M:o. values close to 0.720. is 0.7194 and 0.7209 at tbe ES and AS Mines. Baumann and Hofmann (1988) bave r� respectively.Tbese values are bighertban tbose viewed in detail the problem oftbe fluid source of the fluorite witb tbe exception of two sam­ for barite-fluorite veins in different geological ples of tbe green variety fromtbe SE Mine that settings of the HeICynian basement in Ger­ are slightly higher, but still within tbe limits of many. High Sr isotope compositionsthat over­ error. On the basis of this limited sampling an lap our data., Le. 0.715-0.720, are .found in exclusive contribution to tbe fluid compo­ fluorites and barites from the Central and nents by tbe crystalline basement is unlikely Southern Black Forest, southwest Germany, and anotber less radiogenic source is required. where most of tbe veins are also hosted by In the absence of other suitable litbologies in granitoids and orlbogneisses. Baumann and tbe area (ortbogneisses are more evolved ra­ Horrman conclude thatno direct relation$ip diogenically tban tbe same granitoids; (87Sr/ exists between tbe Sr isotope compositioos·in,.· ·· R6Sr)14SMa for the Abantos Gneiss range from tbe vein minerals and tbe bost rocks. In .tl1eg- :,l... : 0.7383 to 1.0064; Vialetteet al.• 1987].Srmust opinion, the I1Sr/86Sr ratios represent �he 'aYfj� compositions of.. tRc;.;: bave been derived from externalfluids. eiaged time-integrated :,: Sr isotope ratios of seawater (and meteoric underlying rocks. Taking into account: { th,"". I barite-fluorite veinsare almost everywhere re­ This seems particularly true for the two green lated to unconformities (e.g., Dilland Nielsen, fluorite samples discussed abo ve, which on the 1987), the influence of shallow marine or me­ basis of Sr isotopes suggest a minimum shal­ teoric waters should not be disregarded in this low fluid contribution. In spite of this, these case. fluorites show very low ENd-values (-6.9 for Mixing of relatively high- and low-ENd com­ the adamellite vs. -8.3 and -8.0 in the ponents, from the host adamellites and from a fluorites) . bypothetical seawater or meteoric water, re­ Low Sm/Nd ratiosof fel.dsparsrelative to the spectively, could explain tbe calculated com­ accessory minerals such as apatite, zirconand position of the fluid as well as the observed sphene (Hanson, 1980), along with mass-bal­ 147Sm/I44Nd variations. ance considerations, could explain tbe isotopic Stille et al. (1989) bave defined tbe Nd iso­ composition of fluorites.The incongruent dis­ tope composition of Jurassic seawater on the solution of feldspars during phyllic alteration ! basis of data from metasediments and MD (quartz+sericite+chlorite) is probably the l collected at the Pennine realm in the Alps, main process controlling the isotopic compo­ I Central Europe. They obtained an average sition of the ascending fluid. Most of the REE 143Ndjl44Nd initial value of 0.512105 ± contained in accessoryminerals and biotite are 0.000018 (EN,= -6.7:!: 0.6) calculated at 150 probably retained in spite of chloritization Ma. This value is high relative to the SE Mine (Alderton et aL, 1980) and therefore do not fluid,in spite oftbe fact that extreme fIguresof contribute to a large extent to the fluid com­ fNd(150Ma.) as low as -7.7 are found in the Mn position. This can explain the fairly uniform­ ores. However,these values correspond to deep ity of the initiall43Nd/144Nd argued above, as seawater with no detrital influence. In our case well as the low ENd-values,on account of the ifseawater existed in Jurassic times above the 10werSm/Nd ratios of feldspars relati ve to the Sierra del Guadarrama realm,the depth of the whole rock, largely controlledby the accessory water had to be very shallow. Large amounts minerals and biotite (Alderton et al., 1980). of metamorphic rocks. were probably exposed Variations in the Sm/Nd ratio in the fluorite at these times in the sourceareas, on account can be the consequence of a combination of of the erosion level, and could to de­ factors like Sm/Nd fractionation among fluid creased EN.rvalues compared with deep sea­ and feldspars as a functionof variationsin the water (Piepgras and Wasserburg, 1980). In fluid/rock ratio (Rosing, 1990), or among fact,Michard et at. ( 1985) have found values feldspars and sericite, as well as of kineticcon­

of �d as low as -11.2 to - 8.8 for Jurassic straints.Very low total REE contents expected sediments in Lorraine, France, a paleogeo­ in the shallow fluidwill not modify to a signif­ graphicen.vironment similar to the Sierra del icant extent the above considerations. There­ Guadarrama. These values attest to an impor­ fore the Sm/Nd data do not conflict with the tant metamorphic component in the sedi­ mixing model, although taken alone cannot be ments and therefore to correspondingly low considered concl usive. fNd-ValueS in the waters. Meteoricwaters of the The fluid sulphur isotope signatures calcu­ same age within the Tethys realm could also lated from the sulphides fracti.onation curves show the same isotopic signature as deduced at temperature between 300° and 150°C vary for seawater. in the range -1.5 to +0.1%0,which is within However,a more realisticexplanation ofthe the range of igneous derived sulphur in re­ low fN.rvaluesshown by the fluorites involves .duced magmas (�34=_3 to+3%o; Ohmoto, the dissolution behaviour of the host-rock 1986). Since there is no evidence of magmatic minerals during the hydrothermal alteration. fluids involved intbesystem, sulphur was most could well lead to the observed sulphur iso­ 3:iI�::i b��ri;,��f��:hand �:�;;�1979). the topic compositions. Short residence time of .the Rye,sulphur l isotope fluids, as expected in these puis antfracture­ �'t�� �l!�d' is� Jessof. straightforward. In controlled hydrothermal systems, would in­ �f..a system some oxidation of the hibit a significant oxidation of the aqueous H2S to sulphate might be ex- in the ascending fluid (Ohmoto and Rye, 1979; sulphidea variable range of o34S-val­ Ohmoto and Lasaga, 1982), thus preserving - u�in barites aDd more negativeo34S-values in the original "igneous" signature in the sul­ s�phides (Obmoto and Rye, 1979). How­ phides. On the other hand, "per descensum" ever, the restricted range of o34g in barites, sulphates would retain the original marine/ found in the SE and AS Mines as well as in evaporitic isotopic signatures other similar veins in the Siena de Guadar­ The oxygen isotope values of carbonates rama, argue for a very limited oxidation and found in the uppermostzones of the veins are supports derivation of sulphate strictly from consistent with an evolved marine or evapo­ marine sources. ritic origin. The a]3c·values are relatively low The isotopic compositions of barites from and suggest a derivation of the carbon from the SE and AS Mines closely match the 0348 marine water. Similar values of £5l3C and £5]80 signature of Middle Triassic to Late Jurassic have also been found by Morad et al. (1992) seawater (+ 16 � 1.5%0; Claypoo! et al., 1982). in diagenetic in Permian to These 034S signatures are distinctly different Triassic sandstones east of the SCS. However, from Permian sulphates (0348 = these authors argue for a meteoric origin for + 10.5 :t:1.0%0) and younger sulphates (Cre­ the fluid, the observed isotopic compositions taceous seawater is heavier except for a mini­ resulting from complex changes during diagen­ mum of __ + 14%0 at the Aptian-Albian; Clay­ esis and controlled by climatic conditions. pool et aI., 1982). However, Utrilla (1989) has recently found that the range of 634g..values ofn 7. Palaeogeographic implications Triassic-Liassic sulphates in northeastern Spain is very broad (from + 10.2 to + 19.2%0) The data presented in this work demon· and very variable even at the localscale. This strate that at least some of the barite-fluorite opens the possibility of the sulphate in barites veins of the Sierra del Guadarrama are proba­ being derived from dissolution of pre-Late bly Late Jurassic in age. The 87Sr/86Sr and sul­ Jurassic evaporites. In fact, Upper Triassic phur isotopic composition argue for mixing of (K.euper) and Liassic evaporites are common an ascending hot meteoric fluid, isotopically in the Iberian realm, to the east of the studied modified by reaction with the host rocks, with area. However, 034S composition of sulphate a shallow brine probably of marine/ evaporitic in fluids provenance. derived from the leaching of these evaporites should probably display a variabil� The sediments resting unconfonnably on the ity at the regional scale that is not observed in Hercynian basement in the Sierra del GUadar­ the barites of the SCS. Also, out of 23 deter­ rama are Upper Cretaceous continental toma.: minations of K.euper evaporites by Utrilla rine materials. To the east and northeast of the (1989),20 are lower than + 15%0,i.e. below Sierra del Guadarrama, mostly continental the values found in the barite veins. Therefore, Triassic sediments, including impbrtantthick� a provenance of the sulphur by dissolution of nesses of Keuper evaporites, and LOwer. Jure. older evaporites seems unlikely. assic marine carbonates are found uncoi:tfgnIi� Mixing of two solutions without subsequent ably overlain by the Cretaceous, An Upper S isotope exchange between aqueous S species Jurassic coastline has been well·defitied faT" to

-', is

sedimentation taceous was by an important period uplift and erosion corresponding to the Ne okimmerian phase. impor- tance the erosion is still unknown in Sierra Guadarrama realm, and the possi- bility Triassic and probably Lower sic sediments once here cannot be ruled out. With regard to the Middle to Upper rassic, a sea entrance fr om the south remains a possibility. of a shallow plat­ fo rm joining central Spain with the Iberian or the Subbetic realm in the south is not in con­ flict with our data.

8. Conclusions

Barite-flu 0 rite del are of Late age. topic data support previous conclusions by Tomos et al. (1991) that show that fluid mix­ ing is an important mechanism in the genesis D.H.M., 1980. Rare ofthe ''unconformity-related'' The earth element ULUUJ.ULY isotopic composition of the first fluid resulted dence from southwest "-'.o.J.,E...... '..... fr om fluid-host rock interaction at depth; no 49: 149-165. evidence fo r magmatic or deeper mantle deri­ vation has been fo und. This fluid ascended along the opened fr actures. The second fluid 1984. Strontium was shallow and contained seawater sulphate. of the barite de- This water probably derived fr om a shallow 79: 1360-1365. 1987. platform realm or a close-to-the-shore evapor­ Use of strontium isotopes to determine the sources of itic environment. A re-entrant of the Upper hydlrothenmal fluorite and barite from northwestern Jurassic coastline fr om the south, stretching at Chem. Geol. Geosci. 66: to the Sierra del Guadarrama, constitutes 273-278. Baumann, A. and Hofinann, R., 1988. Strontium isotope a fe asible palaeogeographic explanation. OHlyd:rotilennalvein minerals in ae}:,oslts ...... Lb of the two was a 747-762. rium process fo r the sulphur isotopes, proba­ ...... nJ.u v .•uUJL'-' , M.G., J.c. and Jebrak, 1987. bly because ofshort residence times in the open Mineralogie, geochimie, terres rares et ilge K-Ar des associees aux mineralisations filoniennes. In: N. fr actures� The isotope signatures ofthe ascend- Clauer and S. Chaudhuri in the ing and the fluids have been preser ved Chem. Geol. (Isot. Geosci. Sect.), the sulphides sulphates, respecti vely. 65: 321-339 (special "n'.on;- RE., Hethcringlon. EA., Koep­ Dill, H. and Nielsen. H .• 1987. Geochemical and geologi­ and ouo, J.B., 1982. Varia­ caL constraints on the formation of unconfonnity-re­ througnout Phanerozoic lated vein deposits of Central Europe. J. Geol. 10: 516-jI9. Soc. London, 144: 97-105. e. Galindo, c.. Gonzalez-Cas.­ Field,C.W. and Fifarek. Rlf., 1985. ligth stable-isotope and Tomos. F., J992a. Dating systematics inibe environment. epiibennal Rev. £ron. eventsin the Sierra del Guadarrama, Oeol.,2: 99-128. .tfercynian Belt (Spain), Googaceta. 11: 18- HallicIay, A.N., Shepherd,T J., Diclcin,A.P., Maclaren, F. and Darbyshire, F., 1986. Srn-Nd elating and fm­

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