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 fluorites 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 fluorite 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 mineral (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, sphalerite and chalcopyrite) 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 Cornwall, U.K. (Darbyshire and Shepherd, 199 0; Ches1ey et aI., 1991 ), 206 Ma in the Pen­ 0, nine ore field, 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 fracture 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 quartz-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 carbonates, of dyke swarms of regional extent. They con­ ankerite and calcite, 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 gangue minerals 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 pyrite, freibergite, pearcite, bismuthinite, native bismuth 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.( -- ------ - -.