The latest Post-Variscan fluids in the Spanish Central System: evidence from fluid inclusion and stable isotope data Tornas Martin Crespoa,*, Antonio Delgadob,1, Elena Vindel Catenaa, Jose Angel Lopez Garcfaa, Cecile Fabrec "Departamento de Cristalografla y Mineralogia, Facultad de Ciencias Geol6gicas, Complutense University, 28040 Madrid, Spain hDepartamento de Ciencias de la Tierra y Quimica Ambiental, Estaci6n Experimental del Zaidin (CS/C), 18008 Granada, Spain 'CREGU-G2R, BP 23, 54501 Vandeouvre les Nancy, France Abstract The Spanish Central System has been subjected to repeated fluid incursions, which were responsible for a variety of mineralizing episodes including W-Sn, Cu-Zn-Pb-As-(Ag), F-Ba and barren quartz veins. These hydrothermal fluids occurred over a 200 Ma time period and the latest hydrothermal event is recorded in barren quartz veins. This study is a multidisciplinary approach leading to the characterization of the hydrothermal fluids preserved in barren quartz veins, which are spatially but not temporally related to Hercynian upper crustal granites. The veins were dated by the 39 Ar/4o Ar method, and the fluids were examined using petrographic, microthermometric, chemical and isotopic methods. Fluid inclusions in barren quartz veins indicate that two fluids were related to this hydrothermal event. The main part of the quartz veins were formed from an early low salinity « 1 wt% N aCI) H20-NaCl fluid. This fluid was trapped at around 270 ± 25 QC and 0.1-1 kbar under sublithostatic to hydrostatic conditions. a180 (-9 to 2%0) and aD (-70 to - 34.5%0) values indicate a meteoric origin for water, with significant water/rock interactions. The latest H20-NaCI-CaCI2 fluid is found in two types of fluid inclusions: a primary liquid-vapour type (16-24 wt% NaCl and 1-12 wt% CaClz) and secondary hypersaline type (7-15 wt% NaCI and 21-27 wt% CaCI2). Significant Li concen­ trations in this fluid were confirmed. This late Ca-bearing fluid formed quartz crystals in the central part of the veins, and was trapped at 70- 140 QC, at a maximum pressure of 0.5 kbar. The low al80 (-20 to -6%0) and aD (-137 to -116%0) values suggest a meteoric origin for this fluid, however its high salinity probably requires a source from Triassic evaporite basins located in the NE tip of the Spanish Central System. Anomalously low isotopic values have been previously reported from kaolinites of Lower Cretaceous age. Anomalous climatic conditions during the Cretaceous appear to be the main reason to explain this very negative meteoric water. Strong isotopic depletion in meteoric water has been observed in modern areas with monsoonal climates. The hydrothermal evolution of barren quartz veins in the Spanish Central System is comparable to other hydrothermal Post-Variscan events in central and south-western Europe related to the opening of the North­ Atlantic during Cretaceous time. Keywords: Fluid inclusions: Stable isotopes: Hydrothermal fluids: Post-Variscan: Spanish Central System 1. Introduction The Spanish Central System has been subjected to repeated fluid incursions, which are responsible for different types of granite-hosted and metamorphic-hosted minerali­ zation: W-Sn, Cu-Zn-Pb-As(Ag) sulphides, F-Ba and barren quartz veins. Thus, the Spanish Central System offers a good opportunity to analyse a variety of hydrothemal fluids, which may be compared to other hydrothermal * Corresponding author. Fax: +34-91-394-48-72. E-mail addresses:tmartin@geo.ucm.es (T. Martin Crespo), antodel@eez.csic.es (A. Delgado), evinde1@geo.ucm.es (E. Vinde1 Catena), jangel@geo.ucm.es (J.A. Lopez Garcia). 1 Fax: +34-95-812-96-00. Post-Variscan events in Europe. These hydrothermal fluids span a time interval of around 200 Ma. Fluids trapped in the barren quartz veins represent the latest hydrothermal event (Caballero et aI., 1992; Tornos, Delgado, Casquet, & Galindo, 2000; Vindel, Lopez, Martin Crespo, & Garcia, 2000). Tungsten base-metal transport were related to aqueous-carbonic fluids (Garcia, Vindel, & Lopez Garcia, 1999a,b; Vindel, Lopez, Boiron, Cathelineau, & Prieto, 1995), fluorite-barite ores to aqueous fluids (Galindo, Tornos, Darbyshire, & Casquet, 1994; Tornos, Casquet, Locutura, & Collado, 1991) and later barren quartz veins to CaCI2 brines (Martin Crespo, Lopez Garcia, Banks, Vindel, & Garcia, 1999). This study encompasses for the first time the whole evolution of hydrothermal events. Ca­ bearing fluids have not been previously clearly defined in the Spanish Central System, and could be compared to the Ca-rich brines found in other Post-Variscan hydrothermal mineralizations in Europe (Behr & Gerler, 1987; Behr, Horn, Frentzel-Beyme, & Reutel, 1987; Canals & Cardellach, 1993; Charef & Sheppard, 1988; Lodemann et aI., 1998; Muchez & Sintubin, 1998; Muchez, Slobodnik, Viaene, & Keppens, 1995; Munoz, Boyce, Courjault-Rade, Fallick, & Tollon, 1994, 1999; O'Connor, Hogelsberger, Feely, & Rex, 1993; O'Reilly, Jenkin, Feely, Alderton, & Fallick, 1997; Wilkinson, Jenkin, Fallick, & Foster, 1995). Fluid migration during Mesozoic time in the Spanish Central System comprise a flow system involving meteoric waters that increased in salinity because of interaction with evaporitic-bearing sequences. 2. Geological setting The Spanish Central System is a northeast trending mountain range located in the inner zone of the Hercynian Belt of Spain. Its central part, the so-called 'Sierra de Guadarrama', consists of granitoids and high to medium grade metamorphic rocks, mostly pre-Hercynian ortho­ gneisses and some pre-Ordovician rnetasedirnentary rocks. Scattered relicts of small Perrnian detrital basins, and of a Triassic and Jurassic cover, are also preserved in the north­ east of the Spanish Central System. Late Hercynian grani­ toids were emplaced from 345 to 285 Ma (Casillas, Vialette, Peinado, Duthou, & Pin, 1991; Ibarrola et aI., 1987; Perez del Villar, Crespo, Pardillo, Pelayo, & Galan, 1996a; Perez del Villar et aI., 1996b; Vialette, Bellido, Fuster, & Ibarrola, 1981; Villaseca, Eugercios, Snelling, Huertas, & Castellon, 1995) after the main Hercynian orogeny. The granitoids are mostly peralurninolls rnonzogranites and leucogranites, with minor intrusions of more mafic composition. The main groups of granitoids generate typical contact aureoles in the surrounding wall rocks indicating a high emplacement level, between 5 and 8 km depth (Villaseca, Barbero, & Rodgers, 1998). The Sierra de Guadarrama has experienced numerous fluid-rock interaction events. :Mineralization is of minor economic importance, although a large number of mineralised veins accompanied by hydrothemal alteration occur throughout the batholith and country rocks (Vindel et aI., 1995). The oldest hydrothermal event (300-290 Ma) was associated with veins containing wolframite and sulphides in greisenized granites, whereas barren quartz veins are the most recent event. An age of 100.6 ± 4.3 Ma was obtained from a granite-hosted barren quartz vein by K-Ar dating of sericite from phyllic alteration (Caballero et aI., 1992). 3. Barren quartz veins A large number of veins have been identified throughout the granitic (Colmenarejo, Cerceda, Manzanares el Real, La Cabrera) and metamorphic outcrops (Colrnenarejo, Colmenar Viejo) of Sierra de Guadarrama (Fig. 1). Their essential features are given in Table 1. 3.1. Granite-hosted barren quartz veins These veins range in thickness from 0.3 to 5 rn, have strike lengths of up to 2 km and have subvertical dips. The two most frequent strike directions are N20"E and NllO"E, which follow the main extensional directions between Lower and Upper Cretaceous cover (Alonso & Mas, 1982). Host rock features and the main characteristics of the veins are outlined below: Colmenarejo (COL): barren quartz veins crosscut the Zarzalejo-Valdemorillo biotitic monzogranite. The texture of this granite is typically hypidiomorphic and equigranular, however, a porphyritic facies with a medium-grained matrix has also been observed. The veins are located in the SE of the area and comprise a group of 10 N20"E veins. Several later fractures striking E-W have affected the quartz veins. Cerceda (CER): veins occur throughout two granite types, a fine-grained leucogranite and a porphyritic coarse-grained monzogranite (El Carmn type; Villaseca et aI., 1998). Four quartz veins have been recognised along the contact between the two granites, which display a predominantly strike N-S. The Manzanares el Real (MAN) quartz vein also cross­ cuts the El Carmn granite (Villaseca et aI., 1998). A single N1l6"E barren quartz vein has been recognised in this area. The La Cabrera (CAB) massifrepresents the eastern-most granitic intrusion in the Spanish Central System. It comprises of several varieties of granite. The main facies comprises medium to coarse-grained biotitic granites and monzogranites with subordinate porphyritic varieties. Medium to fine-grained, biotitic, highly evolved leuco­ granites are related to this main fades (Bellido et al., 1981). The studied quartz vein occur in the biotitic granite, striking N20"E, and is located in the SW part of the batholith. Three morphological and textural types of quartz can be distinguished in most of the veins: (1) massive quartz (QI) at the vein margins. Suitable fluid inclusions for microthermo­ metric analyses (>5 fJ.m) are not recognised in this type of quartz; (2) central part of clear quartz crystals (QII) located in the centre of the veins, and (3) margin of QII crystal (QID). Both QII and QID appear in vug cavities and contain abundant fluid inclusions. Hydrothermal muscovite, minor amounts of fluorite and iron oxides have been recognised between quartz crystals (Table 1). Hydrothermal alteration is restricted to the proximity of the lodes (around 0.5 m width), and is poorly developed. Muscovitization and chloritization are the usual and most • 4' Segovia 0 5 IO Km IB ERIAN N A 40"30' MADRID ~ ~ •••• ·11 ~5 ~ + + + .12 ~6 PEN INSU LA B 3 07 4 (i) 8 Fig. 1. Simplified map of the smdied area showing the barren quartz vein locations. Legend: (1) leucogranite, (2) coarse-grained monzogranite, (3) porphyritic monzogranite, (4) not assigned granite, (5) monzogranite, (6a) coarse-grained leucogranite, (6b) leucogranite, (7) metamorphic rock, (8) barren quartz vein (after Villaseca et al., 1998). important alterations. Muscovitization is generated by alteration of feldspars, while chloritization is restricted to the alteration of biotites within the granites. 3.2. Gneiss-hosted barren quartz veins Gneiss-hosted quartz veins (Colmenarejo and Colrnenar Viejo) crosscut pre-Hercynian augen orthogneisses. They show a similar structural setting, paragenesis and wall­ rock alteration to the granite-hosted veins. At Colmenarejo, the N200E veins cut both a monzogranite and a augen orthogneiss. Brecciation of wallrock and veins is character­ istic, showing typically massive veins and crosscutting rnicroveins, like stockworks. Brecciated fragments of wall-rock occur in several veins. Only QI and QII quartz types are recognised in these veins. Hydrother­ mal alteration is restricted to a narrow zone (~ 10 cm) adjacent to the veins and is characterised by musco­ vitization of feldspars. Five barren quartz veins striking N20-40"E have been studied at Colmenar Viejo. Silicifica­ tion and muscovitization were recognised as hydrothermal alterations. 00 NO I I ~ ~ 66 o 0 11 88 N I ~ o ,,­ I ~ o "" o 11 00,,- 66 I I ~ -66 N,,­ I I ~ ~ 66 4. Analytical methods A microthermometric study (~1000 inclusions from 44 samples) was carried out on doubly polished wafers «300 fLm in thickness) using a Linkarn THMSG 600 heat­ ing-freezing stage (MacDonald & Spooner, 1981). The stage was calibrated with melting-point of solid standards at T> 25°C, and natural and synthetic inclusions at T < 0 QC. The rate of heating was monitored in order to get an accuracy of ±O.2 QC during freezing, ± 1 QC when heating over the 25-400 °C range, and ±4 °C over the 400- 600 °C range. Salinity of H,O-NaCI inclusions, expressed as equivalent weight percent NaCl, was calculated from rnicrotherrnornetric data using equations from Bodnar (1993). Salinity and composition of H,O-NaCI-CaCI, inclusions was established from ice, hydrohalite and halite melting temperatures using a :Microsoft Excel Add-in devel­ oped by 1. Naden (Naden, 1996). P-Tproperties for the H,o-NaCI-CaCl, system are not available, but data from Zhang and Frantz (1987) for the H20-NaCl system can be used to approximate the inclusion isochores. Determination of ion ratios in individual fluid inclusions was done by Laser Induced Breakdown Spectroscopy (LIDS), coupling laser ablation with an optical emission spectrometer at CREGU, Nancy. Recent developments using LIDS, previously described by Boiron et a1. (1991, 1997), Fabre, Boiron, Dubessy, and Moissette (1999) and Moissette et a1. (1997), have shown that this method can be adapted for the analysis of the ion content in indi vidual fluid inclusions. A 5 ns laser pulse is delivered by a Nd-YAG laser (266 nm) and focused onto the sample through a Cassegrain objective. A plasma is created by the interaction of the laser and matter. Emission lines of elements present in the plasma are directly analysed by an optical emission spectrometer equipped with a pulsed and gated multichan­ nel detector. The intensity of the emission lines is propor­ tional to the concentration of the elements. The repeatability of LIDS for net intensity ratios is around 10 and 20% for glasses and fluid inclusions, respectively (Fabre et al., 1999). Such results are quite acceptable for the validation of the analytical data. The detection limits are calculated for major elements in fluid inclusions and are for Na and Li 10 ppm, Ca 20 ppm and K 750 pprn. These detection limits are those required for the determination of ions in a majority of fluid inclusions. Bulk crush-leach analysis was performed on samples (between 0.5 and 1 g) prepared and analysed using the procedures set out in Bottrell, Yardley, and Buckley (1988), and modified by Yardley, Banks, Bottrell, and Diamond (1993). The anions Cl and Br were analysed by ion chromatography on double distilled water leaches using a Dionex 4500I HPLC. Na was determined on the same solution leached with an acidified LaCI} solution by Flame Emission Spectroscopy (PES). Irradiation and stepwise heating of micas was done according to Kamber, >Blenkinsop, Villa, and Dahl (1995). 10 rng of mica were handpicked to achieve visual purity of ~ 100%. The selected sample was irradiated in the Ris~ reactor (Denmark), and step-heated in a double-vacuum resistance oven connected to a MAP 215-50B mass spectro­ meter. The analyses were carried out at the 11ineralogisches Institut, Bern (Switzerland). Thermal decrepitation was chosen as the method for extracting fluid inclusions from quartz for the hydrogen isotope analyses. Values of OD fluid were determined using the uranium technique, with a similar methodology to that described by Godfrey (1962). Samples were degassed over­ night by heating at 70 °C under high vacuum. The platinum crucible was then heated (by a radio-frequency induction furnace) to approximately 1200°C. The released water was converted to hydrogen by passing over uranium metal at about 800 °C. 8018 was determined in quartz (10-15 mg) reacting with a stoichiometric excess of ClF} at 650 QC for 12 h (Borthwick & Harmon, 1982; Vennemann & Smith, 1990). Released oxygen was converted to CO, by reaction with a hot platinized graphite rod (Clayton & Mayeda, 1963). The isotope ratios were measured in a FilUligan MAT 251 mass spectrometer. Commercial CO2 was used as the internal standard for the oxygen analyses of silicates contrasted with the V -SMOW, SLAP and GIPS water .. • • ~ • , ". , '" , .. • • ' .. , • .. • .. -. , .. . , • ,.." - -. ... -~, ... ..~ . ..... . .... ... .: \.. ~..: : .. • standards, glVlng a value of 8180 ~ +9.6 ± 0.15%, (V-SMOW) for the international NBS-28 (quartz) standard. The analyses were carried out using a Finnigan l\1AT 251 spectrometer at the Stable Isotope Laboratory, Estacion Experimental del Zaimn (Granada, Spain). 5. Fluid inclusion data 5.1. Fluid inclusion petrography and chronology Three types of fluid inclusion were identified in the quartz veins. They are all aqueous and no C-N-S species were detected by micro-Raman analysis (at G2R, Nancy, France). Notation of the fluid inclusion types follows the nomencla­ ture of Boiron, Essarraj, Sellier, Cathelineau, Lespinasse, and Poty (1992), which takes into account the nature of the dominant chemical phases and the type of phase change. The relative chronology of the fluid inclusions is deduced from textural observations. Lwl: idiomorphic inclusions (liquid + vapour) have been located as primary in the centre of the quartz crystals (QII) following growth planes parallel to the crystal faces (Fig. 2a). Some irregular inclusions have been recog­ nised as pseudosecondary in fluid inclusion planes which •• Fig. 2. (a) Primary Lwl fluid inclusions following growth planes. (b) Primary Lw2 fluid inclusions. (c) Secondary Lw2 inclusions scattered along healed frachues. (d) Lw·h fluid inclusion showing a daughter cubic crystal (halite) and lamellar muscovite as trapped mineral. Table 2 Fluid inclusion microthennometric data for barren quartz veins sllllllllarised by their host rock (granites and augen gneisses) (T FM: first ice melting; TIDI : last hidrohalite melting; T MICE: last ice melting; Ts: last solid melting; TH : homogenization to liquid; [2J = number of samples; (970) = number of measlUements) Host rock Granites Locality Augen gneisses Cobnenarej 0 [7J, Cobnenar Viejo [3J fuclusion type Components Phases at room temperahue % VapolU Cobnenarejo [12J, Cerceda [lOJ, La Cabrera [4 J, Manzanares El Real [8J Lwl H20-NaCl Two phases Lw2 H20-NaCI-CaCI2 Two phases Lw-h H20-CaCI2-NaCI Three phases 5-10 Lwl H20-NaCI Two phases 10-20 5-30 5-10 TeM Cc) (30) - 67/-55, mode: -61 THH Cc) (215) - 18.2/-2, mode: -8 T",c, Cc) (970) - 0.6/0 mode: -0.4 - 26.7/-21.5, mode: -25 - 47.5/-34, mode: -46 120/220 mode: 150 601160 mode: 130 - 0.110, mode: 0 T, Cc) (110) TH Cc) (935) Silinity Bulk salinity (equiv. wt% NaCl) NaCIINaCI + CaCl2 140/300 mode: 250 011 mode: 0.6 701160 mode: 120 16/24 wt% NaCl 1112 wt% CaCh 24/27.3 mode: 26 0.6/0.95 7115 wt% NaO 21127 wt% Ca02 30/38.1 mode: 37 0.210.35 140/300 mode: 260 0/0.2 mode: 0 terminate abruptly within the crystal. Lw 1 inclusions occur in all vein types. Lw2: inclusions are primary and located in the margins of the crystals (QIII) (Fig. 2b). They can also be secondary, scattered along healed fractures in the centre of the crystals (QII) (Fig. 2c). They contain liquid + vapour at room temperature. Lw-h: inclusions contain at least three phases (liquid + vapour + solid) at room temperature including a daughter crystal (Fig. 2d). They occur as pseudosecondary inclusions in QID and, as trails of secondary inclusions cross-cutting Lw 1 and Lw2 fluid inclusion populations. This inclusion type represents the last hydrothermal event in the barren quartz veins. Lw2 and Lw-h fluid inclusions are absent in the gneiss-hosted quartz veins. 5.2. Microthemometric results Results are summarised in Table 2, together with all microthermometric abbreviations used in the text. 5.2.1. Lw1 inclusions The first observable melting of ice takes place at around -25 QC. These eutectics are consistent with the interpreta­ tion of an aqueous solution belonging to the H,O-NaCl system (Shepherd, Rankin, & Alderton, 1985). The TM1CE values (-0.6 to O°C; mode: -OA) indicate low salinity, lower than 1 wt% NaCl. TH L-V are in the range of 140-300 °C, with 250 °C as the modal value (Fig. 3). 5.2.2. Lw2 inclusions Values of TFM , ranging from -67 to -55°C (mode: -61 °C) and the brown-coloured character of crystals are both consistent with the presence of CaCl2 in the fluid (Shepherd et aI., 1985). However the temperatures are lower than the theoretical eutectic temperature in the H,O-NaCI-CaCl, system, ~ - 52 °C (Borisenko, 1977; Yanatieva, 1946). Such low TFM values have been attributed to (i) a melting sequence according to the model univariant curve with a metastable eutectic point of around -70 QC (Davis, Lowenstein, & Spencer, 1990; Spencer, Moller, & Weare, 1990), and/or (ii) to the presence of adrlitional components such LiCl (Zwart & Touret, 1994). Micro­ thermometric analyses are based on measuring solid­ liquid transition temperatures (T FNI, T MICE, or THH)' However, first-melting temperatures and identification of freezing-phases are indeed difficult to recognise, and are prone to error and misinterpretation. Raman spectra data from frozen solution in the system H20- NaCI-CaCI2 indicate that at least some phase transitions between about -70 and -50 QC represent a crystalliza­ tion event (hydrohalite and antarcticite crystallization) and not a metastable melting event (Samson & Walker, 2000). The demostrated existence of these crystallization events in natural fluid inclusions could lead lll1warranted the interpretation of phase transitions below - 50 °C (Sarnson & Walker, 2000). Hydrohalite melting THJl (-18.2 to -2 °C; mode: -8 °C) occurs after ice melting TMICE (-26.7 to -21.5 °C; mode: -25 °C). The composition of these fluids has been calcu­ lated using an Excel-macro (Naden, 1996), and lies in the high salinity part of the H,O-NaCI-CaCl, system, below the ice-hydrohalite cotectic curve around 16-24 wt% NaCl and 1-12 wt% CaCl,. The bulk salinity ranges between 24 and 27.3 equiv. wt% NaCl (mode: 26 equiv. wt% NaCl). Lw2 inclusions homogenise to liquid and show lower temperatures (70-160 and 120°C as the modal value) than the Lw 1 fluids. 45 - 40 35 ~ Ci 30 '" z ~ 25 ~ 0- ~ 20 o :5 .. 15 '" 10 - 5 o 50 lOO 150 200 250 300 350 L:; Lw-h OLw2 D Lw l T H I, \, ('C) Fig. 3. rH L-V vs salinitity plot for Lwl , Lw2 and Lw-h fluid inclusions. Lwl type display two populations showing rH L-V ranges between 140-220 and 230- 300°C. 5.2.3. Lw-h inclusions These inclusions contain at least three phases at room temperature including a daughter halite crystal. The other solids are interpreted as trapped crystals because they do not dissolve or start to dissolve during heating, and are anom­ alously large compared to their host inclusions and constant liquid/solid volume ratios are not giving. Some of these have been identified as plagioc1ase and lamellar muscovite using SEM + EDS combining the morphology of the solids with qualitative chemical analysis. TH L-V homogenisation temperatures range from 60 to 160 °C (mode: 130 °C), and dissolution temperatures of halite from 120 to 220 °C (mode: 150 °C). An approximation to the high bulk salinity has been calculated for the Lw-h inclusions at between 30 and 38.1 equiv. wt% NaCI (mode: 37 equiv. wt% NaCI). The estimated compositions (calculated by a Microsoft Excel add-in, Naden, 1996) are 7-15 wt% NaCI and 21-27 wt% CaCl,. Lw-h inclusions show dissolution temperatures of halite that are higher than homogenisation temperatures (Ts> TH)' It is likely that Lw-h inclusions have a possible mixture of additional components such as KCI, LiCI or H,S (Zwart & Touret, 1994). A TH-salinity plot (Fig. 4a) indicates the presence of two fluids of different salinity but with a similar TH range (70-140 °C). Lw2 inclusions are less saline than pseudo­ secondary and secondary Lw-h inclusions, thus indi­ cating a progression to higher salinity with quartz crystallization. The Lw-h fluid is enriched in Ca relative to the Lw2 type (Fig. 4b). Slight differences in the NaCllNaCI + CaCl, ratio can be observed between the different quartz veins. The La Cabrera vein, which is located in the northern part of the area, contains Lw2 inclusions with high NaClINaCI + CaCl, ratios; no Lw-h inclusions were observed. Lw2 fluid inclusions from Colmenarejo quartz veins, which are located in the southern part of the region, are more calcic. Inclusions from the other veins, Manzanares and Cerceda, show a wide range in Na/Ca rations and typically have lower TH values. 5.3. Ion analyses The atomic ratios Na/Ca and NalLi were measured in Lw2 and Lw-h inclusions, however ratios in Lwl inclusions could not be estimated due to their low salinity. Mole ratios for Lw2 inclusions are: Na/Ca ~ 7 and NalLi ~ 14. Lw-h inclusions show an enrichment in Ca and Li (Na/Ca = 1.5, NalLi ~ 3) with respect to Lw2 inclusion. These ratios confirm the presence of significant amounts of Li in Lw2 and Lw-h inclusions. Moreover, an enrichment in Li and Ca content is observed from the Lw2 to the Lw-h inclusions. Li concentrations have been measured in quartz crystals and range from 0 to 400 pprn. Previously published data on bulk crush-leach analyses (Martin Crespo et aI., 1999) are considered in this study. Samples chosen from the barren quartz veins for bulk chemical analysis offluidinclusion leachates are characterised by a single inclusion type, representative of the higher salinity Lw2 fluids. The ClIBr molar ratio (703 to 753) and 40 38 36 ::; 34 , z "'" 32 ; 30 ,;. ,:, C· 28 '2 -;; 26 '" 24 22 20 40 0.9 0.8 - 0.7 ~ u 0.6 + ~ 0.5 z a 60 b .. .. .. 80 100 120 TIl L-V eq o Lw2 Col mennrejo X Lw2 Man7..anares • Lw2 la Cabrera • Lw2 Cerceda lJ. Lw-h Colmenarejo .. Lw-h Manzanares .. 140 160 .. I/) Lw2 Colrnenarejo 180 lJ. A~ XLw2 Manznnares ... ~. Lw2 La Cabrcra ::; 0.4 • z 0.3 i 0.2 • Lw2 Cerceda O. t 11 Lw-h CulmenuTt':ju .. Lw-b M anzanarcs O +-----.----,-----.-----.--~~====~==~ 40 60 80 100 120 140 160 180 Fig. 4. Lw2 and Lw-h inclusions (H20-NaCI-CaC12). (a) rH L-V vs salinity plot. (b) rH L-V vs NaOlNaO + CaCI2. NalBr molar ratio (508 to 1054) is close to the ratio for sea water (ClfBr: 655 and NalBr: 562, after Horita, Friedman, Lazar, & Holland, 1991), but slightly impoverished in Br. 6. P-T conditions A reconstruction of the P-T conditions that prevailed during the entrapment of the fluids is given in Fig. 5. The latest stage of quartz deposition is characterised by the circulation of H,O-NaCI-CaCI, fluids. Since Lw2 and Lw-h inclusions show no evidence of boiling, and a mineral geotherrnorneter is not available, fluid pressure is assumed to be greater than or equal to hydrosthatic pressure (Ph). Vuggy textures and the brittle nature of the host rock are taken as indicators of a hydrostatic pressure regime for this fluid. Maximum fluid pressures were constrained by the maximum depth of burial. Late Hercynian granites in the Spanish Central System were emplaced at depths of between 5 and 8 km, under a lithostatic pressure at ~2 kbar (Villaseca et aI., 1998). If the pressure regime was entirely hydrostatic, a depth of 5 km indicates a P',id of less than 0.5 kbar. For this pressure range, the CaCI, bearing fluids were trapped at 70-140 "C (Fig. 5). The early stage of hydrothermal circulation is charac­ terised by a H,O-NaCI fluid represented by low salinity inclusions. The mineral assemblage provides no additional constraints on the P-T conditions. Although TH ranges between 140-220 and 230-300 "C, the most probable conditions for this stage are derived for modal isochores having TH in the 230-300"C range. The TH range between 140 and 220 "C has been only measured in some fluid inclu­ sions from the single quartz vein of the La Cabrera, and has not been considered representative data for P-T estima­ tions. Trapping pressures are difficult to estimate precisely, but a maximum depth of 5 km suggests pressures < 1 kbar, assuming a transition from lithostatic to hydrostatic pressure 1.5 "T""---~-T!"""-~~:---~r"'T"'---""'T"--"'" 1 0.5 o o 100 200 300 400 500 T("C) Fig. 5. Pressure-temperature diagram showing P-Tconstraints for different inclusion types. The isochores for the fluids have been drawn for each inclusion type: Lwl: solid lines; Lw2: dashed lines; Lw-h: dotted lines. The field for Lwl inclusions (light grey) is determined by the isochores (modal range) and a assumed maximun PlI"id ~ 1 kbar. The field for Lw2 inclusions (dark grey) is determined by intersection of isochores (modal range), and calculated maximum P fluid under hydrostatic regime. regime during the earlier hydrothermal circulation stages. The overall fluid evolution shows a decreasing trend in the temperature and a progressive enrichment in NaCI and CaCl2 content. 7. 39 Ar/40 Ar geochronology To constrain the evolution of the fluids, hydrothermal muscovites from granite-hosted quartz veins (Colmenarejo) were analysed by the 39 Ar/40Ar method. The spectrum obtained has a staircase shape (Fig. 6). A 'plateau' age of 274 ± 5 Ma is suggested by three of the four late steps between 45 and 95% fractional 39 Ar release. The steps with the lowest age could be explained by diffusion processes and by some post-crystallization heating, mainly at the border of the samples because of the later hydrother­ mal heating events. Several Mesozoic hydrothermal events have been recognised in the Spanish Central System, and Villa (1998) has shown that fluid-mineral interaction is more relevant for isotopic exchange compared to the thermal history of a mineral. Anyhow, the later hydrothermal 320.------------------------------------------, 280 _ 240 ~ ~ '-' Q,j ell ~ 200 160 1·- ---- ----- -----+1 120+-------~------~------_r------~------~ o 20 40 60 80 100 Fig. 6. Age spectrum of hydrothermal white mica. The spectrum obtained has a staircase shape and the 'plateau' is suggested by three of the four late steps. Table 3 Stable isotope data. Measured (Meas.) fluid aD and alRo are bulk values for the total fluid inclusion population and quartz, respectively. Calculated (Calc.) fluid alSo values are derived from the bulk mineral data by means of fractionation factors at the appropriate temperature (Clayton et aI., 1972) (N.D.: not determined) Sample Fluid type Inclusion Minimum Modal salinity of Meas. quartz Calc. Meas. bulk type fluid trapping fluid inclusions a 1S0SMOW (%0) quartz-fluid fluid H2O temperature CC)' (wt% NaCl equiv.) a I ROSMOW (%0) h aDsMow (%0) Col. G3 H2O-NaCl Lwl 170-295 0.4 6.8 - 7/-1 ( -105.5)' Cer. G3 Lwl 140-290 8.1 - 8/1 - 55 Man.Gl Lwl 230-300 0.3 6.9 - 7.311.5 - 38/-59 Cab.G2 Lwl 140-230 0.5 10.6 - 5.3/1.5 - 71/-73.5 Col. Ml Lwl 140-260 0.15 7.6 - 9/-1 - 34.5/-64 Colv.M6 Lwl 140-290 0.15 9.7 - 712 - 45/-60 Col. G4 H20-NaCl-CaCh Lw2 90-160 27 6.7 - 20/-7.8 - 116 Cer. G2 Lw2 75-90 26.2 N.D. N.D. N.D. Man.G4 Lw2 70-115 25.5 8.5 - 17/-6 - 137 Cab.GI Lw2 75-130 25.7 N.D. N.D. N.D. , Fluid inclusions temperature range. b Calculated using equations of Clayton et al. (1972) at the appropiate temperature. C Anomalous data because of probably mixing with Lw2 inclusions. temperatures have not raised the closure temperature for muscovites (Villa, 1998). The age determined in the present study is higher than previously reported for sericite from phyllic alteration surrounding barren quartz veins, which was dated by the K-Ar method at 100 ± 4 Ma (Caballero et aI., 1992). The large difference between the two ages suggests that two different hydrothermal events have been dated. The musco­ vite age determined in this work may correspond to the first hydrothermal stage. Alteration events around 270 Ma coincide with Permian extensional tectonics, and with a major hydrothermal phase in the Spanish Central System. The hydrothermal stage dated around 100 Ma corresponds to Lower Cretaceous, could be related to the later Ca-bear­ ing fluids present in the barren quartz veins and considered as a younger reactivation of the faults. -10 -30 .~ -50 .2 ~ -70 ... ~ o -90 ~ ~-110 ~ 00 -130 Present rain water --. 8. Stable isotope results The isotopic composition of the fluid have been measured (Meas.) for the 8D values and calculated (Calc.) for the 8 180 values (Table 3). The Lwl fluid in granite-hosted veins shows a range of bulk 8D (-70 to -38%0) and 8 180 (-8 to 1%0), comparable to values in gneiss-hosted veins (- 64 to -34.5%0 for 8D and -9 to 2%0 for 8 180). The low 8 180 values indicate a meteoric origin for the water, although the more positive values can be related to the interactions water-rocks (Fig. 7). An origin from sea water can be excluded since evaporated seawater will be close to halite saturation (~26.3 equiv. wt% NaCI; Holser, 1979), and the Lwl fluid contains only 0.15- 1 equiv. wt% NaCI. Low salinity fluids TH Range IG-Lwl ••••••• IM-Lwl - • - IG-Lw2 Modal TH o Lwl E@l Lwl-2 • Lw2 -150 L.. ...... _ .................... -.I. ............ _ .................................. ..... -20 -10 o 10 8180 (SMOW) Water Fig. 7. a 180-aD plot of Lwl and Lw2 inclusions (GRH: granite-hosted veins, GNH: gneiss-hosted veins). Range in alRo corresponds to maximum and minimum trapped temperatures (Table 3). The Lw2 fluid inclusions are only present in a granite­ hosted quartz vein. The Lw2 fluid inclusion oD values clus­ ter between -137 and -116%0 and 0180 values between -20 and -6%0. The low 0180 and oD range estimated for Lw2 fluid indicate a meteoric origin for the fluid. However, its high salinity (25.7-27 equiv. wt% NaCI) could indicate dissolution of evaporites. oD values are lower than those reported for other hydrothermal events involving meteoric fluids in the Spanish Central System (Tornos, Delgado, Casquet, & Galindo, 2000). Several possible origins for anomalously low oD values such as those reported here have been argued (Gleeson, Wilkinson, Boyce, Fallick, & Stuart, 1999): (i) water-organic matter interactions; (ii) contributions of hydrogen with different isotopic signature from mica trapped in fluid inclusions; (iil) post-entrapment hydrogen diffusion; and (iv) high-latitude and high-altitude meteoric precipitation. The first possibility can be ruled out because of the lack of organic matter. A source of hydrogen from micas and hydrogen diffusion could be considered. Trapped micas have been recognised in Lw 1 and Lw2 fluid inclusions, however anomalously oD values in Lw 1 fluid inclusions have not been recognised. Therefore, the contribution of hydrogen from trapped mica could be considered as negligible. Post-entrapment hydrogen diffusion from hosted quartz to fluid inclusion could be taken into account. Low oD values have been reported from quartz interstitial sites relative to molecular water, although the hydrogen isotope fractionation factors for the incorporation of hydrogen into quartz interstitial sites are unknown (Gleeson, Grant, & Roberts, 2000). Anomalously low oD are often interpreted to indicate the involvement of high latitude/altitude meteoric waters in palaeoflow systems, which seems to be the main reason to explain such low oD values in Lw2 fluids. Few data lie away from the isotopically well defined Lw 1 group. These intermediate values between Lw 1 and Lw2 oD data can be attributed to a mixture of Lw 1 and Lw2 fluid inclusions. In the analysed sample the inclusions were not exclusively Lwl and some Lw2 inclusions were also included. 9. Discussion The early (Lwl) H,O-NaCI fluid inclusions represent a high oD, 0 180, high temperature and low salinity fluid consistent with an origin related to meteoric waters. Fluids characterised as H,O-NaCI have been recognised as being associated with earlier hydrothermal events of several age at the Sierra de Guadarrarna: W -(Sn)-sulphide veins (Garcfa et aI., 1999a; Vindel et aI., 1995), epysienites (Caballero, 1993), As-(Ag) mineralizations (Garcfa et aI., 1999b) and barite-fluorite (pb-Zn) veins (Tom os et aI., 1991). Therefore this type of fluid was present during all hydrothemal activity in the area. The H,O-NaCI-CaCI, fluids represent a late and minor event relative to the H20-NaCI fluids, and are restricted to granite-hosted veins. The absence of Ca-fluid inclusions in the gneiss-hosted quartz veins could be related to the hosted lithology. Foliation of gneisses could favour the fluid migra­ tion out of the veins. Stable isotope data indicate a meteoric origin for the H,O-NaCI-CaCI, fluid, but the high salinity was probably derived from dissolution of evaporites. The age of the late fluid is <;100 Ma, allowing the possibility that the fluid percolated through Triassic evaporites. Signif­ icant Mesozoic evaporitic basins are located in the northeast of the Spanish Central System (SCS) (Utrilla, Om, Pueyo, & Pierre, 1989; Utrilla, Pierre, Om, & Pueyo, 1992). The northern (Atlantic domain) and southern (Tethys domain) parts of the SCS were connected during the Cretaceous via NE-SW and NW -SE faults (Casas et aI., 1998). Both sedimentary basins were palaeogeographically linked (Gil & Garcfa, 1996). Li represent a good tracer of the evaporitic evolution of primary solutions. It behaves conservatively during seawater concentration and evaporite precipitation (Fontes & Matray, 1993). Significant Li concentrations in quartz crystals and in the H,O-NaCI-CaCI, fluid were measured. High Li concentration have been typically reported from evaporitic sequences and attributed to highly concentrated brines (Fontes & Matray, 1993). Na/Br and CIlBr ratios can be used to distinguish ions from different sources (Kesler et aI., 1995). Halogens (CIlBr and Na/Br ratios) from crush­ leach analysis could represent their original seawater signature in a marine/evaporitic environment. Moreover, the CaCl2 content is also consistent with disso­ lution of evaporites, thus other Ca sources are not necessary. Sedimentary carbonates (higher 0180 ) and lower water­ rock ratios are concordant with diagenetic water enriched in 180 but not in D (Fig. 7). It is important to stress that during Cretaceous time, this area was a low latitude (Ziegler, Scotese, & Barret, 1983). At present, the oxygen isotopic composition of meteoric water in latitudes between 0 and 200 is typified by values ranging between + 1.5 and -7%0 (SMOW) (Rozanski, Araguas, & Gonfiantini, 1993). Ocean waters during the Mesozoic Era were between 2.3 and 1.2%0 lighter than at present (Scherer, 1977; Shackleton & Kennett, 1975). Therefore, a meteoric water derived from the evaporation of marine waters at that time could fall between -0.8 and -9.3%0 (SMOW). Even so, the measured (-137%0< oD < -116%0) and calculated (-20%0 < 0180 < -6%0) isotopic composition of the fluids are too negative to be explained by differences in seawater composition. Most explanations for H,O-NaCI-CaCI, fluids with anomalously low oD values are related to high-latitude and/or high altitude meteoric precipitation (Gleeson et aI., 1999). However, palaeogeographic reconstructions of the Lower Cretaceous in this part of the Spanish Central System (Alonso, Floquet, Melendez, & Salom6n, 1982) indicate intermediate latitude close to the Cancer Tropic. Therefore, none of the previously documented hypotheses for the formation of anomalously low SD fluids can adequately explain the occurrence of this type of fluids in the Spanish Central System during the Lower Cretaceous. However, other authors such as Dutta and Suttner (1986) and Marfil , Delgado, Rossi, La Iglesia, and Ramseyer (2000), sllldying kaolinites from this period, have also reported anomalously low isotopic values (compared to recent kaolinites that were formed in weath­ ering profi les at low latillldes). A possible explanation could be the so called 'amount effect', that leads to more negative rain waters than expected at these latitudes (Fontes, 1980; Rozanski et aI., 1993). At middle and lower latitudes, the isotopic content of precipitation is found to be higher in small amounts of rain but never near the poles. Highly depleted oxygen isotope values of actual and recent sedi­ ments indicate an origin related with wet periods charac­ terised by intense tropical summer (monsoonal) rainfall with heavy thunderstorms (Luckge, Doose-Rolinski , Khan , Schulz, & Von Rad, 2001). Therefore, we should expect strong isotopic depletion in meteoric waters, such as those observed in modem areas with monsoonal climates (Feng, Cui, Tang, & Conkey, 1999; Marfil et aI. , 20(0). Thus, periods of high precipitation and dissolution of evaporites may have promoted the formation of Lw2 fluids in equili­ brium with very negative and high salinity waters. This data are concordant with the general climate of the Late Jurassic and Lower Cretaceous characterised by high aunospheric CO2 levels and by a monsoonal rainfall pattern (Weissert & Mohr, 1996). CaCl2-rich brines with moderate to high salinity often occur around the margins of Mesozoic basins of central Europe, particularly in the vicinity of older granites (Heijlen, Muchez, Banks, & Nielsen, 2000; Lodemann et aI., 1998; Muchez et aI., 1995; Munoz et aI., 1994; O'Reilly et aI. , 1997; WiLkinson et aI. , 1995), and have been related to the early north Atlantic rifling and associated to the Triassic - Jurassic evaporites (HaJliday & Mitchell , 1984; Mitchell & HaJJiday, 1976; Munoz et aJ ., 1999; O'Connor et aI. , 1993). Mineralizations can be spatially correlated to the granites formed by post-orogenic collapse in relation to intense fraclllring. It is proposed that high salinity H20 - NaCI- CaCI2 solutions, that originated from residual evaporite brines and formation waters, infiltrated into the basement along ex tensional structures (Behr & Gerler, 1987; Behr, Reutel, Horn, & Van den Kerkhof, 1994, Reutel, Behr, Horn, Van den Kerkhof, 1994). Other models propose a fluid flow system involving meteoric water that increased in salinity because of interaction with evaporitic­ bearing sequences, related to the Mesozoic extensional events (Muchez & Sintubin, 1998; Munoz et aI. , 1999). Fluid migration during Mesowic time in the Spanish Central System belongs to a general history in the Variscan range, and can be compared to the model described in Munoz et a1. (1999) for fluids related to major Mesowic extensional events, coinciding with the opening of the Atlantic and Tethys oceans. 10. Conclusions Barren quartz veins represent the latest event of the hydrothermal evolution in the Sierra de Guadarrama (Span­ ish Central System) and are characterised by two different fluids. The first fluid is represented by the H20-NaCI system and characterised by low salinity. The isotopic data indicate a meteoric origin for this Huid. Tills early fluid is related to older hydrothermal events in the area: W- Sn- sulphide veins, episienites, As- Ag mineralizations and barite-fluorite veins. The youngest fluid belongs to the H20 - NaCI-CaCh system and has high salinity. Ca-brines have not been found in the older hydrothermal events in the Sierra de Guadarrama. This later fluid represents meteoric water salt-enriched by dissolution of Triassic evaporites. PaJaeogeographic reconstructions of the Lower Cretaceous in this part of the Spanish Central System do not suggest low SD fluids commonly related to high-latitude and/or high­ altitude. The most plausible explanation could be the ' amOlD1t effect', that leads to more negative rain waters than expected at tropical latitudes, such as those observed in monsoonal climates. F luid composition and evolution in barren quartz veins of the Spanish CentraJ System are similar to other hydrother­ mal Post-Variscan events in centra l and south-western Europe. In all cases mineralization occurs around the margins of Mesozoic basins in the vicinity of granites and has been attributed to early Cretaceous Atlantic rifling. However, geochronological data suggest that several hydro­ thermal phases could be recorded in barren quartz veins up to Cretaceous. The first one (- 270 Ma) is ubiquitous throughout the Spanish Central System, and represents a major hydrothermal circulation event, coincident with the Mid-Permian transition to extensional pre-rift tectonics. Other hydrothermal phases continued during the Mesowic time until the Cretaceous. The late hydro­ thermaJ stage (-100 Ma) could be related to Ca-bearing fluids. Acknowledgements M. Christine Boiron (CREGU-G2R, Nancy, France) and I. Villa (Bern University, Switzerland) are thanked for comments and technical support with LIBS and geochronol­ ogy analysis, respectively. R. Oyarzun is thanked for his careful reading and improvement of the English. 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