Carbonatation processes at the El Berrocal natural analogue granitic system (Spain): inferences from mineralogical and stable isotope studies

Chemical Geology 150 Ž1998. 293–315

Carbonatation processes at the El Berrocal natural analogue granitic system žSpain /: inferences from mineralogical and stable isotope studies E. Reyes a

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, L. Perez ´ del Villar b, A. Delgado a, G. Cortecci c , R. Nunez ´ ˜ a, M. Pelayo b, J.S. Cozar ´ b

Departamento Ciencias de la Tierra y Quımica Ambiental, Estacion ´ ´ Experimental del Zaidın ´ (CSIC), Prof. Albareda 1. 18008 Granada, Spain b (CIEMAT), IDAE, C.H.E. AÕda. Complutense, 22, 28040 Centro de InÕestigaciones Energeticas, Medio Ambientales y Tecnologicas ´ ´ Madrid, Spain c Dipartimento Science della Terra e Geologico Ambientale, UniÕersita` di Bologna, Piazza Porta S. Donato, 1, 40126 Bologna, Italy Received 28 November 1997; revised 24 June 1998; accepted 24 June 1998

Abstract The El Berrocal graniterU-bearing quartz vein system has been studied as a natural analogue of a high-level radioactive waste repository. The main objective is to understand the geochemical behaviour of natural radionuclides occurring under natural conditions. In this framework, the carbonatation processes have been studied from a mineralogical and isotopic Ž d13C and d18 O. point of view, since carbonate anions are powerful complexing agents for UŽVI. under both low-temperature hydrothermal and environmental conditions. The carbonatation processes in the system are identified by the presence of secondary ankerite, with minor calcite, scattered in the hydrothermally altered granite, and Mn calcite in fracture filling materials. The isotopic signatures of these carbonates lead us to conclude that ankerite and calcite from the former were formed at the end of the same hydrothermal process that altered the granite, at a temperature range of between 728 and 618C for ankerite, and between 528 and 358C for calcite. The effect of edaphic CO 2 on both carbonates, greater on calcite than on ankerite, is demonstrated. Calcites from fracture fillings are, at least, binary mixtures, in different proportions, of hydrothermal calcite, formed between 258 and - 1008C, and supergenic calcite, formed at F 258C. According to their d13C signatures, the effect of edaphic CO 2 in both calcites is also evident. It is assumed that: Ži. hydrothermal calcite from fracture fillings and ankerite from the hydrothermally altered granite are the result of the same hydrothermal process, their chemical differences being due to the intensity of the waterrrock interaction which was stronger in the altered granite than in the fractures; and Žii. all of these carbonatation processes are responsible for ancient and recent migrationrretention of uranium observed in the hydrothermally altered granite and fracture fillings. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Granite; Fracture fillings; Carbonates; Isotopes; Carbon; Oxygen; Natural analogue

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Corresponding author. Fax: q34 58 12 96 00; e-mail: [email protected]

0009-2541r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 9 8 . 0 0 1 1 1 - 9

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1. Introduction The study on radionuclide migration and transport is a controversial subject in environmental and earth sciences and the studies made have mainly focused on laboratory experiments using simple physicochemical systems ŽChoppin and Wong, 1996.. This methodology does not, however further our knowledge of the complex processes occurring in natural systems or their long-term effects. Consequently, there is a need to study the behaviour of radionuclides under natural conditions and in suitable geological formations to host a high level radioactive waste repository ŽHLRWR. ŽParneix, 1992; Smellie et al., 1997; Hidaka and Holliger, 1998.. The El Berrocal site has indeed satisfied this requirement ŽPerez ´ del Villar et al., 1993a.. The isotopic study of secondary carbonates in granitic formations is useful since carbonate anions are powerful complexing agents for UŽVI. and the sources of carbon and oxygen usually lie outside the granitic formations. Thus, the sources of oxygen of secondary carbonates may be percolating meteoric waters, ancient seawaters, and the water from mineral dehydration or magmatic dewatering ŽWhite, 1974.. However, in some magmatic systems, where the waterrrock ratio is very low, magmatic minerals must also supply small amounts of oxygen to the groundwaters ŽCriss and Taylor, 1986.. The occurrence of carbon usually absent in granitic formations, as secondary carbonates can be explained by degradation of the plant cover, magmatic degassing of CO 2 , or decarbonization of sedimentary rocks, ŽCraig, 1953; Arnorsson and Barnes, 1983; Barnes et al., 1988.. Therefore, the isotopic study of secondary carbonates in granitic formations can supply information on potential sources of carbonate solutions, their circulation paths, rockrwater interaction processes and the temperature of formation of these carbonates ŽCraig, 1953; Truesdell and Hulston, 1980; Clauer et al., 1989; Fritz et al., 1989; White et al., 1990.. Both secondary carbonate disseminated in the hydrothermally altered granite and carbonates from fracture fillings of the El Berrocal granitic site have been studied in order to establish: Ž1. their formation temperature, Ž2. the relative sequence and chronological evolution of the carbonatation processes within

the geothermal history of the El Berrocal system, and Ž3. a conceptual model involving ancient and recent waterrrock interaction phenomena.

2. Geological and geochemical background 2.1. Geological setting The El Berrocal site is located approximately 90 km southwest of Madrid, near the small town of Nombela, in the province of Toledo ŽSpain.. The site takes its name from the El Berrocal granitic pluton, which forms the hill upon which the site is located, at an altitude of 900 m a.s.l. The El Berrocal pluton is located at the central part of the Centro-Iberian Zone, in the Spanish Hercynian Massif ŽJulivert et al., 1972., near the contact between the Tajo River Tertiary basin and the Sierra de Gredos ŽFig. 1.. The main granitic facies of the El Berrocal pluton, the so-called El Berrocal facies, is the host rock of a U-mineralised quartz vein ŽUQV. mined in the 1960s and also known as the El Berrocal U-mine ŽArribas, 1965.. Together, the host-rock and the UQV make up the El Berrocal system. From a mineralogical point of view, the reference granite was classified as a weakly altered alkalinefeldspar granite with two micas, with muscovite being dominant relative to biotite ŽPerez ´ del Villar and De la Cruz, 1989.. Among the accessory minerals, ilmenite, zircon, monazite, xenotime, apatite, uraninite, cassiterite and primary sulphides are highlighted. Muscovite II, fluorite, sericite, chlorite, rutile, anatase, potassium feldspar and albite II are secondary minerals formed during deuteric andror early postmagmatic hydrothermal processes. The geochemical features indicate a highly evolved hypocalcic granite, rich in silica and phosphorus and peraluminous. This granite is ‘fertile in U’ and belongs to the ilmenite granite series or to the S-type granites ŽPerez ´ del Villar and De la Cruz, 1989.. The UQV is composed of quartz and sulphides Žfirst mineralization phase. and pitchblende, pyrite, minor barite and carbonates, the latter being detected by the presence of dissolution moulds filled with FeŽMn. oxyhydroxides Žsecond mineralization phase..

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Fig. 1. Location and geological map of the El Berrocal granitic pluton. Žafter Varea and Iglesias, 1981. Ža. Two-mica porphiritic granite Žthe El Berrocal facies.. Žb. Two-mica leucogranite, fundamentally muscovitic. Žc. Biotitic granite of San Vicente Type. Žd. Almorox–Navamorcuende aplitic dyke. Že. U-mineralised quartz vein. Žf. Nondifferentiated Tertiary sediments Žsands, clays and conglomerates..

For a detailed study of the El Berrocal x:system, the following geological materials have been taken into account: Ži. the El Berrocal facies or reference granite; Žii. the hydrothermally altered granite, with scattered secondary carbonates; Žiii. the hydrother-

mally altered granite affected by weathering, Živ. the weathered-reference granite, Žv. the UQV, including its clayey walls, and Žvi. the fracture infill materials, mainly formed by clays and carbonates ŽFig. 2. ŽPerez ´ del Villar et al., 1996b..

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Fig. 2. N–S cross section of the El Berrocal system, showing the boreholes drilled, the location of samples and the hydraulically conductive zones.

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2.2. Geothermal history Based on a Rb–Sr isochron as well as structural and textural relationships among the materials of the El Berrocal system and the mineral paragenesis, the geothermal history of this system has been established. The El Berrocal pluton intruded 297 " 1 Ma ago, with an initial 87 Srr86 Sr ratio of 0.7175 " 0.0029 which is a typical value for parent magma derived from the continental crust ŽFaure and Powell, 1972.. Further, the El Berrocal facies was affected by a first set of hydrothermal events, at high temperature ŽT ) 3508C., and defined as deuteric andror early postmagmatic alteration processes which transform the original facies into the so-called reference granite. These processes are weak but pervasive since they affect the whole granitic mass. The internal isochron obtained from apatite, albite, potassium feldspar, biotite and bulk-rock sample of the El Berrocal facies suggests that the bulk rock remained a closed system for the Rb–Sr pair during late alteration processes, and that the minerals were closed for Sr isotopes around 289 " 1 Ma ago. Consequently, the difference between the intrusion age and the age obtained from the internal isochron Ž9 Ma. represents the time during which the first set of hydrothermal alteration processes took place in the El Berrocal facies ŽPerez ´ del Villar et al., 1996a.. The second hydrothermal event recorded in the site is related to N80E and N110E fracture families. This process transforms the reference granite into the hydrothermally altered granite adjacent to these fractures. The alteration is evidenced by the sericitization of feldspars, mainly albite, the presence of secondary carbonates scattered through the rock, and the yellow-green colour of the rock ŽPerez ´ del Villar et al., 1995.. The minimum temperature of formation of secondary sericite, determined from its d18 O values, ranges between 808 and 1208C ŽPerez ´ del Villar et al., 1996b.. In relation to this process and using up the silica released by feldspar alteration, sulphide-bearing quartz veins and their clayey walls were formed, filling the abovementioned fractures ŽPerez ´ del Villar et al., 1996b.. According to the d18 O values of quartz from quartz veins and values found for illite from their clayey walls, both quartz veins and clayey walls were formed at about 1008C ŽPerez ´ del Villar

297

et al., 1993b.. Somewhat later, the U-mineralization took place mainly in N110E quartz veins, formed by pitchblende, pyrite, carbonates and minor barite. Fracture fillings, mainly composed of illite, kaolinite, beidellite, carbonates and minor secondary quartz, are the result of the interaction between the granitic fracture gouges and hydrothermal solutions, with overlapping weathering effects ŽPerez ´ del Villar et al., 1996c.. Thus, the d18 O values of illite and kaolinite indicate that these two clay minerals were formed at the same temperature Ž1008C. as the quartz veins and secondary sericite in the hydrothermally altered granite. Finally, the intensity and depth of the weathering effects on the El Berrocal system are related to the degree of fracturing and hydrothermal alteration of the granite, as well as the mineralogical composition of the hydrothermally altered granite and UQV and fracture fillings Žsee Fig. 2.. As far as the fracture fillings are concerned, beidellite and a second generation of kaolinite were formed during these processes ŽPerez ´ del Villar et al., 1997..

2.3. Relationships between alteration processes and U, Th, Zr and REE mobilisation All alteration processes recorded in the El Berrocal system produced the remobilization, migration and retention, mainly by precipitation, of UŽIV. and ThŽIV., as analogues for PuŽIV. and NpŽIV., UŽVI., for PuŽV and VI. and NpŽV., light rare earth elements ŽLREE., for Am, Cm and PuŽIII. ŽChapman and Smellie, 1986., and Zr as a fission product ŽPerez ´ del Villar et al., 1994, 1995, 1996a,b,c and Perez ´ del Villar et al., 1996d.. Thus, during the first set of deuteric andror hydrothermal events, uraninite, monazite, xenotime and apatite in the granite were partially destabilized by F-rich fluids in such a way that when these minerals are surrounded by fluorite, uraninite is totally or partially pseudomorphized by complex U silicophosphates; monazite is totally or partially depleted in REE, xenotime shows U and REE-poor altered zones and apatite is partially dissolved. The presence of U silicophosphates coating chloritized biotite and filling the cleavage planes of muscovite

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and microfissures in quartz support the migration of U away from uraninite and its retention in the rock matrix, respectively. On the other hand, Th from monazite remains immobile and no secondary REEbearing minerals have been found. Based on these textural features, F is probably responsible for the alteration of the U, Th and REE-bearing minerals, as well as for the transport of U and REE, probably as fluoride complexes. This hypothesis was confirmed by the presence of secondary fluorite and U silicate compounds filling dissolution voids in albite. The second hydrothermal process, responsible for the hydrothermally altered granite and quartz veins, mobilised and also reprecipitated U, Th, REE and Zr. The mineralogical evidences of these processes are: the occurrence of U silicophosphates lining and filling microfissures in secondary pyrite; the precipitation of botryoidal ‘mineraloids’, whose composition varies from U ) Th to Th ) U silicophosphates; the presence of Th and Zr silicates filling microfissures in quartz; the neoformation of florencite and the general increase of U, Th, Ce, Y, and Zr contents in the hydrothermally altered granite. In this case, the chemical agent that mobilised U, Th, REE and Zr is not as clearly defined as in the former case. In relation to this second hydrothermal event, the uranium-ore body hosted in the main quartz vein was originated, formed by pitchblende, carbonates, pyrite and minor barite. This paragenesis suggests that U scattered in the granite was oxidised, remobilized and transported, downwards and through the fractures as uranyl–carbonate complexes. These solutions, reheated at depth by the tectonic events that fractured the quartz veins, moved upwards, probably by convective flow. After destabilisation of uranyl– carbonate complexes caused by rock–water interaction processes and a drop in temperature and pressure, uranyl cations were probably reduced by H 2 S, precipitating pitchblende, carbonates and pyrite. The coexistence of pyrite and barite suggests that the precipitation took place between y200 and y300 mV ŽKrivovichev, 1979.. The same process also affected the hydrothermally altered granite close to the UQV and, to a lesser extent, the intra-granitic fractures. The effects of weathering on U of each part of the system may be summarised as follows. v The effects on the uraninite of the reference

granite vary according to the nature and fracturing degree of its host mineral. Thus, if uraninite is included in feldspars, the former is usually dissolved and only the outlines of the crystals remain, with residual Th–Ca silicophosphates inside. The U from uraninite, forming U silicophosphates, is usually filling adjacent microfissures, or is adsorbed, together with P, onto Fe oxyhydroxides. v Weathering on the hydrothermally altered granite mainly caused a distribution of U, which forms large autunite crystals andror is adsorbed onto secondary sericite and Fe oxyhydroxides. v The effects of weathering on the U-ore body are represented by strong oxidation of pitchblende and sulphides, dissolution of carbonates and neoformation of secondary U minerals such as uranotyle, autunite, in secular equilibrium, torbernite and uranocircite. Furthermore, U is also adsorbed onto Fe oxyhydroxides. v Similar weathering effects have been observed on the fracture filling materials. Thus, uranium forms uranyl silicates and phosphates, such as uranotyle, autunite and phosphuranylite, and is adsorbed onto Fe oxyhydroxides. At present, the uranyl carbonate complexes are the dominant soluble species in oxidising groundwaters in the system ŽGomez et al., ´ 1996..

3. Sampling and methods Twelve samples analysed from the carbonatebearing hydrothermally altered granite were taken from borehole S-13, at both sides of the UQV. The average sample size is shaped like a cylinder, 1 cm long and 8.6 cm in diameter. Samples from fracture fillings come from boreholes ŽS. 13, 14, 15, 16 and 17, amounting to 122 samples, and distributed as follows: S-13 Ž21.; S-14 Ž9.; S-15 Ž16.; S-16 Ž54. and S-17 Ž22.. The sampling is representative of both high-and-low hydraulic conductivity fracture zones ŽFig. 2.. Due to the scarcity of infill material above a depth of 50 m, the sampling in S-13, 15 and 16 started below this level. Samples were usually scraped from both sides of the fractures or loosened when the fracture fillings were thicker. Consequently, the available amount of

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Fig. 3. Ža. Patches of secondary carbonates on totally sericitised ŽSer.. albite Žsample from hydrothermally altered granite, photography under polarised light, X65.. Žb. Back-scattered electron image of secondary carbonates Ž1., surrounded by secondary TiO 2 . Žc. EDX pattern of carbonates Ž1. in which Mg, Ca, Mn and Fe peaks are reported.

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Table 1 d13 C and d18 O values and temperature of formation of the calcitic fractions of carbonates from the hydrothermally altered granite SAMPLE

d 13 C ŽPDB .

d 18 OŽPDB .

T Ž8C.

ZA.23 ZA.24 ZA.25 ZA.26 ZA.27 ZA.31 ZA.32 ZA.34 ZA.36 ZB.22 ZB.25 ZB.30

y7.4 y7.6 y7.2 y7.3 y7.3 y7.2 y7.5 y7.3 y7.1 y7.7 y7.2 y10.7

y13.3 y12.8 y12.6 y12.2 y11.7 y12.6 y13.2 y12.7 y11.1 y14.2 y12.1 y13.0

47 44 43 41 39 43 47 44 35 52 40 46

the infill material varied as a function of its thickness and orientation of the fractures in relation to the core axis Žsee Fig. 2..

Carbonates from hydrothermally altered granite were detected and examined by polarising microscopy ŽPM. and scanning electron microscopy, coupled to an energy dispersive X-ray analyser ŽSEM q EDX.. The identification and semiquantification of carbonates from fracture filling materials were performed by SEM q EDX, X-ray diffraction ŽXRD. and differential thermal and thermogravimetric analyses ŽDTA and TGA.. For isotopic measurements, all samples were ground to - 200 mesh, then treated with 100% phosphoric acid for 12 h in a thermostatic bath at 258C ŽMcCrea, 1950.. Samples containing calcite and ankeritic carbonates, as major, minor or trace components Žabout 50, including all the samples of the hydrothermally altered granite., were also treated according to the Al-Aasm et al. Ž1990. method. This method is as follows: CO 2 is removed after a 2-h reaction with phosphoric acid at 258C and this sam-

Fig. 4. Temperature range in which the secondary carbonates Žcalcitic fractions. of the hydrothermally altered granite were formed, obtained from the equation of Anderson and Arthur Ž1983..

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ple is labeled ‘calcite’. The vessel is then closed, and the reaction is continued for 24 h at 258C, after which time the released CO 2 is discarded. The sample may then be allowed to react at 508C for 10 days and the CO 2 subsequently removed is used to determine the isotopic composition of ankerite. Furthermore, nine groundwater samples from

301

borehole S-16, taken at a depth of 400 m, were used to determine the d13 C values in the dissolved inorganic carbon ŽDIC.. Carbon was collected by precipitation of SrCO 3 following the method of Carothers and Kharaka Ž1980.. Strontium carbonate was reacted with phosphoric acid to produce CO 2 for C-isotope analysis ŽMcCrea, 1950..

Fig. 5. The sources of carbon in the ‘El Berrocal’ area. SURFACE PROCESSES: Only C3 plants and atmospheric CO 2 have been considered, since CAMP plants are insignificant in most ecosystems and C4 plants are usually present in arid, hot environments ŽDeines, 1980; Cerling, 1991.. The shaded area shows that the most frequent range of C 3 plants is y30‰ to y25‰ ŽDeines, 1980.. The pre-industrial atmospheric CO 2 has a d13 C value of y6.5‰ ŽFriedli et al., 1986. Žy8‰ at present.. The soil CO 2 is about 4.5‰ heavier than the plant biomass ŽCerling, 1984, 1991.. The isotopic difference between CO 2 and dissolved inorganic carbon ŽDIC. depends on the pH and temperature. This value will be near to 0‰ at value close to pH 5, but is relatively independent of pH between 7.5 and 8 ŽRomanek et al., 1992.. For the isotopic theoretical calculation ŽDIC and calcites. we considered a calcite–bicarbonate enrichment of 1‰ Žindependent of the temperature. and the calcite–CO 2 equation described by Romanek et al. Ž1992. for temperatures of 08, 158 and 308C. DEEP PROCESSES: Highly negative calcites Žmethane-derived cements. and highly positive calcites Žmethanogenic cements. must be discounted, because the values obtained in this study Ž2. are far from these fields. On the other hand, the current DIC Ž1. range between y20.7‰ and y 17.4‰ Ž9 samples, depth s 400 m. is in agreement with the theoretical considerations. Finally, a number of relevant isotope compositional fields from the literature are also presented for comparison: 3 s range of pedogenic calcite ŽTalma and Netterberg, 1983; Salomons and Mook, 1986., 4 s carbonatites ŽHoefs, 1980.; 5 s mantle C ŽKyser, 1986.; 6 s fracture calcites from the Stripa granite ŽClauer et al., 1989; Fritz et al., 1989..

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Table 2 d13 C and d18 O values and temperature of formation of the ankeritic fractions of carbonates SAMPLE

d 13 C ŽPDB .

d 18 OŽPDB .

T Ž8C.

ZA.23 ZA.24 ZA.25 ZA.27 ZA.31 ZA.32 ZA.34 ZA.36 ZB.25

y6.5 y6.9 y2.7 y5.8 y6.9 y7.9 y7.7 y7.0 y4.3

y14.0 y13.9 y13.4 y13.9 y14.9 y15.1 y14.9 y14.1 y13.6

65 64 61 64 71 72 71 66 63

Both d13 C and d18 O were determined using a Finnigan Mat 251 mass spectrometer. The experimental error found was "0.1‰ for d13 C and d18 O, using Carrara and EEZ-1 internal standards, previously compared with NBS-18 and NBS-19 ŽReyes et al., 1989.. All the samples were compared to a reference carbon dioxide obtained from a calcite standard prepared at the same time. Thus, oxygen isotope ratios for ankerite were recalculated taking into account the fractionation factor for acid decomposition at 508C: 1.01057 for ankerite ŽRosenbaum and Sheppard, 1986. and 1.0090 for calcite ŽFriedman and O’Neil, 1977.. Only the carbonates from the hydrothermally altered granite presented a significant amount of ankerite. Usually, the amounts of CO 2 from ankeritic carbonates in fracture filling samples were too scarce to be analysed by mass spectrometry. However, in samples whose CO 2 content was high enough, the isotopic results for carbon and oxygen were very similar to those obtained using the method of McCrea Ž1950.. Indeed, the differences were less than 0.2‰ and 0.3‰ for d13 C and d18 O, respectively. Consequently, only the data obtained by McCrea’s method will be included and discussed in this work. Since the differences in isotopic signatures of carbonates from the hydrothermally altered granite

Fig. 6. Temperature range in which the secondary carbonates Žankeritic fractions. of the hydrothermally altered granite were formed, obtained from the Carothers et al. Ž1988. equation for siderite.

were significant, due to the chemical method used, the results will be discussed separately.

4. Results and discussion 4.1. Mineralogy, d18 O and d13C of secondary carbonates from the hydrothermally altered granite According to SEM q EDX data, the carbonatation process that affects the hydrothermally altered granite is mainly characterised by the occurrence of ankerite with minor calcite. Based on the textural relationships between sericite and carbonates ŽFig. 3., the latter could have been formed either simultaneously or somewhat later than sericite. The isotopic results obtained from calcitic fractions of these carbonates ŽTable 1. show the following features: Ža. A striking homogeneity of the d13 C values, which range from y7.7‰ to y7.1‰, except for sample ZB-30, Žy10.8‰.. Žb. The d18 O values

Fig. 7. Ža. Correlation between the carbonate percentages obtained by DTA-TGA and XRD analyses. Žb. Plot of the chemical composition of carbonates from fracture fillings in a ŽCaO. – ŽMgO. – ŽFeO q MnO. ternary diagram.

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303

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E. Reyes et al.r Chemical Geology 150 (1998) 293–315

Fig. 8. Distribution of the main neoformed minerals along the boreholes studied.

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vary to a larger extent than the d13 C values, ranging from y14.2‰ to y11.1‰. Žc. The temperatures obtained, taking into account the equation of Anderson and Arthur Ž1983. and the annual average value of d18 O in the El Berrocal current meteoric waters Žy7.6‰., vary within a narrow range, between 358 and 528C ŽFig. 4.. The relative negative d13 C values of these calcitic fractions indicate that the main source of carbon was the edaphic cover ŽSalomons and Mook, 1986; Cerling, 1991.. This effect was more evident in sample ZB-30, with a typical edaphic d13 C ŽTable 1 and Fig. 5.. Furthermore, the temperatures obtained are consistent with low-temperature hydrothermal processes. However, calcite from sample ZB-30, which presented an anomalous d13 C value, probably caused by the existence of a mixture of hydrothermal and meteoric calcites, might have partially formed at higher temperatures. In this respect, the location of this sample close to the UQV, where the intense circulation of meteoric waters is currently taking place as it did in the past, would suggest the coexistence of old Žhydrothermal. and recent Ženvironmental. calcites, the latter being enriched in light carbon. The isotopic signatures of the ankeritic fractions ŽTable 2. follow a similar trend to those observed in calcite, though they also present the following differences: Ža. Generally, the effect of edaphic CO 2 in ankeritic carbonates is slightly lower than on calcitic fractions ŽTable 2 and Fig. 5.. In samples ZA-25 and ZB-25, The effect is even lower. This enrichment in heavy carbon could be explained by dissolution–precipitation processes. Žb. The temperatures obtained between 618 and 728C ŽFig. 6. were calculated using the equation of Carothers et al. Ž1988. for siderite, because no equation has been found for ankerite in the specialised literature. These temperature values are higher than those of calcitic fractions. Žc. Only three samples, ZA-26, ZB-22 and ZB-30 contain no ankeritic fraction. In short, the hydrothermally altered granite was affected by a very-low-temperature hydrothermal carbonatation process, during which ankeritic carbonates were firstly precipitated, followed by calcite, at lower temperature. It also appears that the influence of edaphic CO 2 in both carbonates followed the cooling of the system. This carbonatation event probably took place after the sericitization process, ac-

305

cording to the temperature of formation and the textural relationships between these carbonates and sericite. 4.2. Mineralogy, d13C and d18 O of carbonates from fracture fillings The carbonate content obtained by XRD ranges from 94% to below the detection limit. These data were confirmed by TGA and both methods are in good agreement ŽFig. 7a.. Calcite is the main carbonate species and was identified in most of the samples. Only in S-13 Ž54 m. and S-15 Ž49 to 75 m., did calcite and dolomite coexist. The semiquantitative chemical analyses by EDX show the constant presence of Mn in both carbonates. The percentage of Mn is so variable in calcite that the Ca–Mncarbonate series Žcalcite–kutnahorite–rhodochrosite. can be defined ŽFig. 7b.. However, kutnahorite and rhodochrosite are always below the XRD detection limit. From a textural and structural point of view, carbonates occur as veinlets of xenomorphic calcite, but more frequently they appear as - 1-mm-thick layers coating both fracture walls. In the latter case, calcite forms a mosaic in which pit etchings are frequent. Sometimes, in some fracture walls, calcite is more developed and shows the typical rhombohedral habit. Wherever calcite and dolomite coexist, their textural relationships are not clear enough for their crystallization sequence to be established. However, both carbonates are more recent than neoformed quartz, because they fill voids between the quartz crystals. Kutnahorite and rhodochrosite seem to be more recent than calcite and smectite, since they always appear as small single rhombohedral crystals on calcite or smectite. In the first case, the crystals are usually oriented parallel to the cleavage planes of calcite. The mineralogical distribution of the five boreholes studied ŽFig. 8. shows that: Ži. in boreholes S-14 and S-17, carbonates appear below 60 and 80 m, respectively. This is due to the dissolution of carbonates by meteoric waters in the upper zones of the site. In these fractures, residual Fe–Mn oxyhydroxides are present instead of carbonates. Žii. No relationship between carbonate concentration and depth is observed in any of the boreholes. Neverthe-

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Table 3 d13 C and d18 O values of carbonates from fracture fillings intersected by boreholes S-13, 14, 15, 16 and 17 Depth Žm.

d 13 C PD B

d 18 O PDB

BOREHOLE 13 50.55 53.43 53.86 84.29 90.75 91.47 91.69 109.16 113.31 113.53 126.77 151.66 154.02 155.15 156.38 157.91 158.25 158.61 158.64 159.21 159.25

y10.4 y6.6 y9.2 y10.6 y12.9 y12.5 y12.5 y11.1 y11.2 y13.1 y13.0 y12.9 y11.7 y9.6 y8.8 y11.1 y11.3 y11.7 y11.4 y11.9 y11.8

y12.4 y15.1 y12.0 y13.2 y9.4 y10.0 y9.8 y15.8 y13.9 y15.3 y9.8 y11.0 y12.0 y14.7 y15.1 y13.6 y12.8 y12.7 y14.2 y13.4 y12.4

BOREHOLE 14 61.77 61.91 74.51 94.70 100.41 141.78 153.04 209.55 209.78

y12.1 y10.9 y11.4 y12.9 y14.0 y14.0 y13.4 y13.1 y13.5

y10.0 y10.6 y10.6 y10.2 y7.5 y8.9 y9.3 y11.6 y11.6

BOREHOLE 15 70.05 89.74 98.47 105.64 108.28 110.66 115.13 116.01 119.29 123.25 123.78 137.61 138.67 151.94 151.95 154.54

y12.0 y12.4 y13.0 y8.3 y10.2 y12.9 y12.8 y12.5 y12.8 y12.9 y11.4 y12.6 y8.4 y12.8 y8.4 y9.6

y8.5 y8.7 y8.2 y14.5 y13.4 y8.7 y9.2 y9.7 y9.0 y8.9 y11.5 y11.0 y13.1 y10.2 y14.7 y11.4

Table 3 Žcontinued. Depth Žm.

d 13 C PD B

d 18 O PDB

BOREHOLE 16 72.65 84.10 86.70 100.65 102.53 115.32 126.51 140.37 163.68 215.27 230.34 232.17 233.02 234.24 248.02 249.31 249.49 249.98 279.59 313.54 334.24 350.50 386.45 442.09 445.60 446.14 463.42 465.57 465.64 467.70 470.73 471.10 474.85 475.03 476.45 476.97 514.16 526.96 527.64 528.64 531.22 549.98 550.13 354.85 558.76 565.02 568.32 576.62 589.02 589.34 590.32 593.78

y12.8 y14.3 y12.1 y13.2 y14.5 y14.3 y12.0 y13.5 y11.1 y14.6 y12.7 y13.2 y13.8 y10.2 y13.7 y13.2 y13.2 y13.1 y15.3 y11.3 y10.9 y14.9 y9.1 y13.8 y14.0 y13.8 y15.6 y13.0 y16.2 y14.5 y14.9 y14.4 y13.6 y14.8 y13.7 y16.4 y13.2 y14.0 y13.7 y12.8 y12.6 y13.5 y13.4 y13.1 y13.3 y13.6 y13.4 y12.4 y12.8 y11.4 y11.2 y9.7

y8.9 y7.5 y11.5 y9.0 y7.5 y8.1 y12.5 y10.4 y13.2 y8.4 y12.7 y12.1 y11.0 y7.9 y9.7 y13.2 y13.3 y13.4 y11.6 y11.3 y7.9 y8.7 y10.7 y10.1 y9.8 y10.0 y11.8 y10.4 y11.0 y11.6 y11.3 y11.4 y12.6 y14.2 y13.7 y13.4 y11.0 y11.7 y12.4 y13.8 y12.4 y12.2 y12.3 y12.6 y12.8 y12.2 y11.8 y12.5 y13.4 y12.7 y14.5 y13.8

E. Reyes et al.r Chemical Geology 150 (1998) 293–315

307

Table 3 Žcontinued. Depth Žm.

d 13 C PD B

d 18 O PDB

BOREHOLE 16 598.71 602.82

y8.4 y13.9

y15.1 y12.1

BOREHOLE 17 85.60 98.83 100.73 108.61 112.03 138.64 141.40 159.55 160.84 161.31 166.08 174.53 188.45 192.02 192.71 227.04 228.42 231.54 233.08 235.91 240.98 278.49

y12.1 y12.6 y12.5 y13.9 y13.3 y13.2 y13.4 y12.9 y13.7 y13.3 y13.6 y12.1 y13.0 y11.4 y12.6 y13.2 y13.4 y11.6 y12.8 y12.6 y12.1 y13.2

y10.4 y9.6 y9.8 y8.2 y8.8 y11.3 y10.2 y12.3 y10.8 y10.7 y9.2 y11.7 y10.8 y12.5 y12.0 y9.8 y9.5 y12.0 y9.8 y10.9 y10.6 y10.0

less, in brecciated zones of the UQV, intersected by boreholes S-13 and S-15, carbonates are absent due to dissolution by acid waters of meteoric origin produced by the oxidation of sulphides existing in the UQV. The d13 C and d18 O values ŽTable 3, Fig. 9. vary broadly from y6.6‰ to y16.4‰ and from y7.5‰ to y15.8‰ respectively. The effect of edaphic CO 2 in the formation of carbonates is generally greater than that of atmospheric CO 2 ŽFig. 5.. This suggests that the CO 2 of these carbonates mainly derives from the plant cover of the site, essentially constituted by C 3-type plants, with a d13 C value of y26‰ ŽCerling, 1991.. The variations of d13 C and d18 O values with depth in each borehole ŽFig. 10. show a striking symmetry of both isotopic signatures. Thus, the depletion of 18 O in carbonates, which may indicate a higher temperature of formation, is accompanied by an enrichment in 13 C. This indicates either a lower edaphic contribution in the total carbon or an in-

Fig. 9. Histograms of d13 C and d18 O values of carbonates from fracture filling samples.

creased dissolution of preexisting carbonates ŽFig. 5.. All these results indicate that carbonates located down to a depth of 250 m correspond to a mixture of various carbonate phases, formed under different Table 4 Temperatures of formation of carbonates from fracture filling samples T Ž8C.

B-13

B-14

B-15

B-16

B-17

Total

15–25 25–35 35–45 45–55 55–65 T Ž8C max.. T Ž8C min..

5% 19% 38% 29% 10% 59 24

30% 60% – 10% – 35 15

44% 31% 12% 12% – 52 19

17% 31% 41% 11% – 54 16

23% 54% 23% – – 40 19

20% 36% 30% 12% 2% 59 15

308 E. Reyes et al.r Chemical Geology 150 (1998) 293–315

Fig. 10. Variations of d13 C and d18 O values with depth in each borehole.

E. Reyes et al.r Chemical Geology 150 (1998) 293–315

309

Fig. 11. Temperature intervals in which the carbonates from fracture filling samples were formed, obtained from the equation of Anderson and Arthur Ž1983..

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Fig. 12. Ža. Regression lines between d18 O and d13 C considering all the samples analysed Ž r s 0.46. and considering only the samples located between 0 m and 215 m Ž r s 0.74.. Žb. Theoretical quaternary diagram obtained from d18 O and d13 C experimental data, in which the vertices represent the isotopic signatures of the different carbonate components of the mixed carbonate assemblage.

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conditions. Only in S-16, below 215 m, does the symmetrical distribution of both isotopic signatures seem to be broken. This exception may be due to the existence of carbonates precipitated from a mixed solution of deep and shallow bicarbonate waters. Furthermore, some d13 C values are more negative than those of the edaphic typical carbonates ŽSalomons et al., 1978; Talma and Netterberg, 1983; Salomons and Mook, 1986.. This could be explained by taking into account that in granitic recharge waters are usually weakly acidic, and under such conditions the d13 C values of DIC are very close to those of the soil-CO 2 ŽFritz et al., 1989; Romanek et al., 1992.. This is in agreement with the d13 C values obtained from the El Berrocal DIC, ranging between y20.7‰ and y17.4‰. The geothermometric study was carried out based on the equation T Ž8C. s 16 y 4.14Ž dc y d w . q 0.13Ž dc y d w . 2 ŽAnderson and Arthur, 1983. and the d18 O annual average value of the El Berrocal current meteoric water Žy7.56‰. ŽReyes et al., 1995.. The results ŽTable 4, Fig. 11. demonstrate that 80% of these carbonates were formed at temperatures between 258C and at least 658C, while the remaining 20% were formed at ambient temperature. These data would seem to point towards a mixture between carbonates formed at ambient conditions and carbonates formed at temperatures higher than those calculated. Consequently, at least two generations of carbonates are superimposed in the system. In order to confirm this hypothesis, d18 O values of all the samples were plotted vs. d13 C values ŽFig. 12a.. The regression line obtained has a correlation coefficient of 0.46 and a negative slope, suggesting a mixture of carbonates formed under different conditions. Likewise, if only data from samples located between 0 m and 250 m are considered, a regression line with a higher correlation coefficient of 0.74 is obtained ŽFig. 12a.. In order to determine the isotopic signature of each carbonate component, a quaternary diagram was plotted, where the vertices represent the isotopic signatures of carbonates that could have been mixed ŽFig. 12b.. Vertex C was defined taking into account d18 O and d13 C values of a theoretical hydrothermal carbonate formed at 1008C, scarcely influenced by edaphic CO 2 and similar to carbonates studied by Feux and Baker Ž1973. in granitic environments. By joining this vertex to the

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outermost experimental data lines CD and CB were obtained. Lines AD and AB were plotted parallel to the x and y axes respectively, starting from samples richest in 13 C and 18 O. Vertices B and D almost coincide with the experimental values. Therefore, the vertices of this quaternary diagram would indicate the theoretical isotopic signatures of the carbonate end-members that formed the mixtures. Consequently, the carbonates of fracture fillings correspond to mixtures of carbonates of edaphic origin Ž d13 C between y10‰ and y16.5‰. formed at ambient temperature, and of other hydrothermal carbonates with predominant edaphic carbon and precipitated at a temperature between 508 to below 1008C.

5. Conclusions In the El Berrocal hydrothermally altered granite, two types of disseminated carbonates have been identified according to their chemical composition, temperature of formation and C and O isotopic composition. The first is ankeritic and precipitated at a temperature range between 728 and 618C, whereas the second is calcitic and formed between 528 and 358C. The effect of edaphic CO 2 has been demonstrated, and is stronger in calcite than in ankerite. Consequently, in both cases, carbonate-bearing groundwaters have a meteoric origin and were reheated at depth and moved upwards by convective flow. ŽFig. 13.. Furthermore, the influence of edaphic CO 2 in these carbonates has increased during the cooling of the system. The difference between the temperature of formation of ankerite and calcite suggest that both phases formed during the same low-temperature hydrothermal event. This carbonatation process can also be considered as the thermal tail of the same hydrothermal event that originated the hydrothermally altered granite, the quartz veins and the most important process of mobilizationrmigration of U from uraninite in the granite, that originated the U-ore body. This conclusion seems to be confirmed by the temperature of formation of sericite, which ranges from 808 to 1208C, quartz veins, at about 1008C, the textural relationships between sericite and carbonates in the hydrothermally altered granite and by the

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Fig. 13. Idealised conceptual model of the mobilisation, transport and precipitation of uranium in the El Berrocal system. Ž1. Biotitic granite of San Vicente type. Ž2. The El Berrocal granitic facies or reference granite. Ž3. Pegmoaplites. Ž4. Hydrothermally altered granite, with scattered secondary carbonates. Ž5. The UQV, including its clayey walls. Ž6. The fracture infill materials. Ž7. Granitic soil and weathered-reference granite. Ž8. Percolating meteoric waters. Ž9. Intrusive contact between the El Berrocal pluton and the San Vicente-type granite. ŽA–E. Idealised convective flow of meteoric waters. ŽA. Oxidising and bicarbonate meteoric waters, enriched in light carbon of edaphic origin ŽS. type plants.. Oxidation of uraninite scattered in the granite occurs in this zone. ŽB. Uranium transport as uranyl carbonate complexes. ŽC–D. Reheating of uranyl solutions at T - 100qC by fracturing effects. ŽD. Upward flow, uranyl complexes destabilisation, reducing conditions and precipitation of pitchblende, pyrite and carbonates, mainly in the UQV. No scale considered.

structural relationships between the main quartz vein and the U-ore body. As for the hydrothermally altered granite, at least two types of carbonates have been identified in fracture filling materials, according to their temperatures of formation. The first carbonate was formed under hydrothermal conditions, between 258 and at most 1008C, and the second at ambient temperature F 258C. Almost all the samples correspond to mixtures of both components. In addition, the isotopic signatures of carbon in both carbonates indicate the influence of edaphic carbon and, consequently, the meteoric origin of the water. To explain the precipitation of hydrothermal carbonates, we suggest a reheating at depth and upward movements of carbonate solutions.

According to the temperatures of formation of ankeritic carbonates in the hydrothermally altered granite and hydrothermal carbonates from fracture fillings, it is suggested that both types are the result of the hydrothermal process described above. Their chemical differences may be due to different intensities of waterrrock interactions. Thus, the hydrothermally altered granite is closely related to a major tectonic accident which affected a very important granitic mass, whereas the fracture filling materials are linked to minor tectonic accidents, whose effects are only affective in thin granitic bands at both sides of the fractures. In this way, significant amounts of Fe, Mg and Mn were released during the alteration of biotite and transferred to carbonate solutions. This caused the precipitation of ankerite in the hydrother-

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mally altered granite, whereas in the fractures only Ca and minor Mn were supplied to form Mn-calcite. As for the U-ore body, the hydrothermal carbonatation processes that affected the intra-granitic fractures could also be responsible for the ancient migrationrretention of uranium, mainly by precipitation of minor pitchblende, observed in fracture infill materials. Currently, this UŽIV. oxide is almost totally oxidized by weathering, with the uraniun retained as uranyl minerals and adsorbed onto Fe oxyhydroxides. To explain the latter process, we suggest that bicarbonate meteoric waters, enriched in light carbon, oxidized and dissolved pitchblende, transporting UŽVI. as uranyl–carbonate complexes. After destabilization of these complexes by rock–water interaction processes, uranyl cations were precipitated as uranyl silicates and phosphates, or adsorbed onto Fe oxyhydroxides. Carbonate anions were precipitated as calcite, with minor Mn. In this sense, the uranyl carbonate complexes are, at present, the dominant soluble species in the oxidising groundwaters of the El Berrocal system.

Acknowledgements Financial support for this work was provided by ENRESA, CIEMAT and CEC. We are grateful to Mr. Francisco Sanchez, Francisco Orden and ´ Jeronimo Navea for the preparation of the samples. ´ The authors would also like to thank the Chemical Geology reviewers ŽDr. R. Bros and the anonymous reviewer. for their comments and constructive criticisms which have notably improved the paper.

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