1379 Tile Canadian Mineralogist Vol 35. pp. 1379-1390(1997)

VEIN-TYPE GRAPHITE IN JURASSIC VOLCANIC ROCKS OF THE EXTERNAL ZONE OF THE BETIC CORDILLERA, SOUTHERN SPAIN

JOSE F. BARRENECHEA', FRANCISCO JAVIER LUQUE1 and MAGDALENA RODAS1 Departamento de Cristalograffa y Mineralogla, Facultadde Geologfa, Universidad Complutensede Madrid, E-28040 Madrid, Spain

JILL D. PASTERIS' Department ofEarth and Planetary Sciences, Washington University, CampusBox 1169,One BrookingsDrive, St. Louis, Missouri 63130-4899, U.SA.

Abstract

We have investigated volcanic-rock-hostedgraphitemineralization in the Huelma area ofthe Betic Cordillera, in southern Spain, in order to establish the mineralogical characteristics of thegraphite andthe mechanism of its formation. The graphite is highlycrystalline, asrevealed by XRD andRaman spectroscopic data, andrelatively pure chemically (Ccontent >90wt.%). XRD data demonstrate that some of themineralized bodies contain both hexagonal and ihombohcdral graphite. Carbon isotopic compositions (-23.0 <5"C <-20.7%e) andothergeological observations indicate that thecarbon inthegraphite wasderived from organic matter included inthemetamorphic basement. Cooling of aC-O-H fluid appears tobethemostlikelymechanism for precipitation of the graphite, although hydration reactions alsomayhaveplayed a role. The presence of rhombohcdral graphiteis discussed with reference to the kinetics ofgraphiteformation. Keywords: graphite, fluid deposition, volcanic rocks. X-ray diffraction, Raman spectroscopy, Betic Cordillera, Spain.

SOMMAIRE

Nous avons dtudic le graphite des roches volcaniques de la region de Huelma, dans la Cordillerc Bctiquc du sudde I'Espagne, afind*en ftablir lescaractenstiqucs mindralogiques et le mecanisme de formation. II s'agitde graphite a haute perfection cristallinc, comme le revile unecuidc par diffraction X et par spectroscopic de Raman, et ayant unccomposition chimique rclativcmcnt pure (>90% deC, par poids). Scion lesdonnees diffractomCtriques, certains desgiscments contiennent a la fois les formes hexagonale et rhombocdriquc. Les compositions isotopiques (-23.0 <8"C <-20.7%c) et autrcs criteres glologiqucs indiquent quelecarbone dugraphite serait ddrive" dematicrc organique presente dans lcsocle miStamorphiquc. Le relroidisscment d'une phase fluide contenant C-O-H scmblc le mecanisme le plus propice aexpliquer laprecipitation du graphite, quoiquc des reactions d'hydratation ontaussi pu jouer un role. Lapresence degraphite rhomboddriquc s'expliquc par reTcrcnce a la cindtique de formation du graphite.

Keywords: graphite, deposition d'une phase fluide, roches volcaniques, diffraction X, spectroscopic de Raman, Cordillerc Betiquc, Espagne.

Introduction These authors, among others, have suggested deposition from C-bearing fluids as the mechanism of formation. Graphite mineralization is commonly associated Carbonic compounds in these fluids may come from with meiasedimentary rocks,in whichit results from thematuration of organic matter, thedevolatilization of the transformation of organic matter by regional or carbonate-rich materials, and mantle-derived carbon, contact metamorphism. Most syngenetic deposits of Until now, the only known minable concentrationof graphite originate by this process of graphitization graphite associated with volcanic rocks occurs at (Diessel &Offler 1975,Wadaef a/. 1994). Graphite is Borrowdale, England, where the graphite replaces also found inmany igneous rocks, from felsic to mafic primary minerals within a series of hydrothcrmally orultramaflc, asvein-like epigenetic deposits (Duke & altered andesites of Ordovician age, forming irregular Rumble 1986, Luqueet al. 1992, Dissanayake 1994). tonodular concentrations (Strens 1965, Weis etal. 1981).

1E-mailaddresses: [email protected], [email protected], [email protected], [email protected]

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In this work, we describe an example of volcanic- rocks of the middle and upper units represent basalts rock-hosted graphite in vein-type occurrences located differentiated by the following processes: fractional in the external zone of the Betic Cordillera, in crystallization, assimilation of pelitic rocks, and southeastern Spain. We document the mineralogical, incorporation of a K-rich fluid phase (which accounts crystallochemical, and isotopic characteristics of this for the anomalously high contents of K found in graphite in order to elucidate the possible source ofthe most rocks of the upper unit). These authors further carbon, and the mechanism of formation ofthe graphite suggested that recrystallization and partial melting of in relation to the evolution of the volcanic host-rocks. the pelitic fragments occurred in two stages (silicate and oxide), with temperatures reaching 600°C and Geological Setting 1360°C, respectively, whereas the pressure remained less than 1 kilobar. The study area is located within the Subbetic / Domain of the External Zone of the Betic Cordillera, Analytical Methods which represents the westernmost range of the Mediterranean Alpine Belt (Fig. I). During the The structural features and chemical composition Mesozoic, the External Zone of the Betic Cordillera of graphite and its thermal behavior were studied in formed a passive continental margin in southern Iberia concentratesobtained by acid treatment as described by (Garcia-Hcrnandez et al. 1980), from Middle to Late Luque et al. (1993). Structural parameters, including Jurassic time (155-166 ± 4 Ma; Portugal et al. 1995). intcrplanar spacings (dml), crystallite size along Submarine volcanic activity occurred in relation to a the c axis (Lc), and the degree of graphitization (u), complex system of fractures having a regional NE-SW were determined by X-ray diffraction (XRD) on bulk trend (Comas et al. 1986). Magmatism in this area has concentrates of graphite. Diffraction analyses were been linked to a transtensional regime associated with carried out with a Philips PW 1729 X-ray the opening of the Atlantic Ocean (Garcia Ducnas & diffractomcter using CuKa. radiation at 40 kV and Comas 1983). 30 mA, a graphite monochromator, a step size of0.01 The volcanic rocks occur in discontinuous outcrops (°29), time per step of0.5 s, scan speed of0.02 (°29/s), along a NE-SW belt about 250 km long and 5-10 km and receiving slit of 0.1 mm. Samples were run from wide (Fig. 1). Principal rock-types are pillow lava 20 to 65°26. Silicon was used as an internal standard. and pillow-breccia flows, with local intercalationsof Physical characteristics on the scale of individual hyaloclastite (Puga et al. 1989). The flows range in grains were obtained by means of Raman microprobe total thickness from several dozens of meters to spectroscopy. This technique provides information about 1 km (Puga & Portugal 1989), with individual on the crystallite size within the (001) basal plane of outcrops comprising a few squarekilometers. Sills and the graphite (thatis, the crystallitesize alongthea axis. dykes also are common. Major differences in bulk La); the degree of crystallinity increases as La chemicalcomposition,mineral assemblage, andmineral increases.The characteristicsofthe Raman spectraas a composition are related to complex processes of function of graphite crystallinity were discussed and differentiation (see below). All ofthe igneous rocks arc illustrated in Pasteris & Wopenka (1991), Wopenka & interbedded with a sequence of pelagic carbonate Pasteris (1993), and references therein. Raman sedimentary rocks, dominantly marl and limestone. spectroscopy was performed on grain separates, on On the basis of distribution patterns of major and unpolished, carefully broken fragments of veins of incompatible trace-elements, three units (lower,middle, millimeter-scale thickness, and on small nodule-like and upper) can be distinguished within the volcanic concentrations, in order to test for variations in pile; these show an increasing degree ofcontamination crystallinity from the inner to the outer parts of the with crustal material from bottom to top (Puga & graphite occurrences (see below). Raman microprobe Portugal 1989).These members arecorrelated with the analyses were acquired using a Jobin-Yvon triplc- interlayered sedimentary facies. In particular, crustal monochromator S3000 laser Raman microprobe. All contamination of the igneous rocks of the upper unit three stages of the monochromator have holographic originated in conjunction with a period of tectonic gratingswith 600 grooves/mm. The sample was excited activity in the basement that affected the walls of fairly by an argon-ion laser using the 514.5 nm green line, shallow magma-chambers, triggering the assimilation with about lOmW power at the sample surface. The ofmetapelitic rocks. same Olympus MS Plan 80x ultralong-working- According to Puga et al. (1989), rocks ofthe lower distance microscope objective (N.A. =0.75) was used unit are poorly differentiated alkali olivine basalts. At to image the sample optically, focus the laser beam some sites, the middle and upper units contain xenoliths to approximately I Jim in diameter, and capture of pelitic rocks (with partially replaced crystals of the Raman-scattered radiation for detection by a andalusite, chloritoid, and staurolite), which arc inter 1024-channcl intensified optical diode array detector preted as fragments from a subcropping metamorphic (2.54 cm, proximity-focused). Operated in this mode, basement. Puga & Portugal (1989) concluded that the the system has a spectral resolution of about 7 cm-1.

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FT! Ibcriin Maiiif B9 Pyrenees C3 Belie Cocdilbra HI Iberianand Ca&Iaman CD Ifcniliy tastal

Quaternary alluvial SV-fg deposits Jurassic limestones m Tertiarydeposits Jurassic dolostones Late Cretaceous-Early Triassic clays and Tertiary marls sandstones Cretaceous limestones Fault and marls Anticline p Jurassic igneous Syncline rocks Thrust K Graphite mineralization

Fio. I. Geological settingof the study area. ModiHcd afterDfazde Neiraet al. (1991).

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The thermal properties ofgraphite were determined flows in which carbonate forms the inter-pillow material. by simultaneous recording of differential thermal Graphite is very localized, occurring exclusively within analysis (DTA) and thermogruvimetric (TG) curves, the sill. No other deposits ofgraphite have been found using a Stanton STA 781 apparatus. The DTA curves in the area. The sill has the composition of an alkali of graphite were obtained by heating the sample basalt, and an intergranular to intersertal texture. at lO'C/min in the range 20°C to 1000°C, with a Phenocrysts include titaniferous augite, olivine, continuous air supply of 50 mL/min. Under these plagioclase,and alkali feldspar, with corundum, spinel, analytical conditions, the exothermic maximum due to and rutile us the main accessory phases. Representative carbon combustion can be interpreted in terms of compositions of these minerals are listed in Table 1. graphite crystallinity: DTA maxima shift to higher The groundmass is made up of plagioclase and minor temperatures as crystallinity increases (Kwiecinska augite and Fe-Ti oxides. 1980, Wada et al. 1994). The DTA maximum is accom panied by a weight loss (as recorded in the TG curve) TABLE 1. CHEMICAL COMPOSITION* OF PRIMARY MINERALS whose magnitude depends on the carbon content of IN THE IGNEOUS HOST-ROCK AND OF SPINEL DERIVED FROM THE ASSIMILATION OF METAPEUTtC BASEMENT the graphite sample. Estimated contents of carbon based on the TG curves were verified by elemental analysis of the samples using a Perkin-Elmer CHN Cpx(M) PI(19) _Naf«(9) Kft{6) _Spl(7) X 0 X o X a X 0 X O 2400 gas analyzer for the determination ofC, H, and N concentrations. The analyses were done at 925"C, with SiO, 49.21 1.64 52.44 1.79 66.60 0.83 64.67 0.46 0.00 acctanilide used as a standard. AIA 3.12 0.7$ 29.90 1.64 20.56 0.51 18.67 0.12 65.64 031 Geochemical characterization of the graphite also FeOH 9.16 2.62 0.24 0.08 0.13 0.06 0.09 0.01 17.58 0.22 MnO 0.19 0.06 0.00 0.00 000 0.18 0.04 included determination ofthe ratio ofthe stable carbon MgO 15.13 2.74 0.12 0.03 0.00 0.01 0.00 16.23 0.16 isotopes on selected samples; the analyses were CtO 21.10 1.05 12.49 1.54 029 0.13 0.01 0.00 000 performed at Geochron Laboratories(Cambridge, Mas Na.0 0.34 014 4.39 0.82 11.30 1.05 0.31 0.10 000 Kfl 0.00 0.21 0.08 0.67 0.13 1654 0.51 000 sachusetts). Samples selected for isotopic analysis TiO, I.S2 0.63 0.07 0.03 0.00 0.01 000 0.20 0.05 include 1) bulk concentrates of graphite obtained by NiO 0.01 0.02 0.01 0.01 0.00 0.00 0.06 0.01 Cf,0, 0.38 032 0.00 0.00 0.00 0.03 0.01 acid treatment, and 2) powdered samples obtained from Toul 100.16 99.S7 9955 100.31 99.92 small holes (1 to 2 cm apart) excavated using a steel needle on unweathered surfaces of some of the largest * Tht proportion of major-damn: oxidesU reported in «.*/• 1 TotalFeit veins and nodules; although these powders contain reported uFeO Average values, expressed by x,arederived from results of the silicate impurities, their graphite contents allowed for number of analyses shown in parentheses. The standard deviation isgivenby o. Symbols: Cpx dinopyroxent, PI plagioclase, Nafs sodiim-dominjnl alkali reproducible analyses. Graphite samples were burned feldspar, Kts pouiaurrh^otrrinam alkali feldspar, Spl spinel in pure oxygen in the presence of CuO at 850°C to produce C02 for analysis. The C02 gas was purified cryogenically by passing it through a dry ice - alcohol trap to remove H20, and then trapped in two liquid The most abundant ferromagnesian phenocryst is nitrogen traps. The trapped C02 was transferred to a titaniferous augite, which forms subhedral to anhedral sample flask, and the »C/I2C of the C02 was determined crystals ranging from 0.5 to 1.5 mm across; they on a VG Micromass gas-source mass spectrometer have a uniform composition. Olivine, a less abundant (model 903). Results are given in the standard §"C phenocryst, is mostly replaced by smectitc-group notation relative to the PDB standard. The overall minerals and chlorite-group minerals; its composition precision of the method, including preparation and ranges from Fo78 to FoM (Comas et al. 1986, Pugaet al. analysis, is about ±0.2%t>. 1989). Plagioclase forms euhedral phenocrysts 0.1 to Chemical compositions of primary minerals in the 0.8 mm long, and is locally replaced by calcite. Within volcanic host-rocks were obtained by electron-microprobe a single thin section, plagioclase may range in compo analysis with a JEOL Superprobe JXA 8900-M, under sition from labradoritc to albite. The grains of alkali the following instrumental conditions: accelerating feldspar arecompositionally zoned from Na- to K-rich voltage 15 kV, current 20 nA, and beam diameter 5 Jim. members (Table 1). Spinel occurs as aggregates of Standards of similar compositions to the analyzed small (up to 0.1 mm) purple grains; we believe that it minerals were used [see Jarosewich et al. (1980) for formed by reaction of basaltic magma with details on the standard samples]. aluminosilicate minerals (Puga & Portugal 1989). Rutile forms isolated acicular crystals or dendritic Graphite Mineralization aggregates, varying in size from fine-grained in the mesostasis to 0.5-mm-long phenocrysts. Secondary The mineralization studied (Fig. 1) is associated minerals in the basalts include chlorite- and smectite- with one volcanic outcrop of the upper unit located group minerals that mainly formed after olivine, and southeast of Huelma, in Ja6n Province. The host rock is calcite that fills small voids and cracks, both in the a basaltic sill, overlain by a thick layer of pillow-lava phenocrysts and the mesostasis.

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Fig. 2. Reflected-light photomicrographs showing textural characteristics of Ihe graphite, a) Flakes ofgraphite (light grey) surrounding breccia fragments of the host rock (dark grey). Note that the flakes parallel the contacts, b) Aggregate of randomly oriented flakes of graphite (light grey) forming a nodule-like concentration. The width of the field of view in both cases is 2.5 mm. c) Natural surfaces ofcarefully broken vein of graphite. Differences in brightness within a light region indicate deviations from horizontally of the crystal faces. Largest dimension of photo is 125 Urn.

Graphite mineralization occurs dominantly as breccia fragments (Fig. 2a). Within the nodules, fracture-filling veins, although small nodules and graphite flakes are randomly distributed, and lack a rounded to irregular pocket-like bodies of graphite preferred orientation (Figs. 2b, c). Graphite has not have been observed where two or more ofthe veins cut been found disseminated in the host rock; together with each other. The veins vary in thickness from a few the presence of the veins, this finding suggests an millimeters up to 20 cm, with graphite as the only epigenetic character for the mineralization. mineral in them. Nodules arc typically 0.5 to 3 cm in The structural parameters of these samples of diameter, whereas pocket-like bodies are up to 20-25 graphite obtained by XRD, namely c„, the stacking cm along the greatest dimension. Some areaswithin the height along the [001 ]direction (Lc), and the degree of graphite veins show signs ofbrecciation, with graphite graphitization (u), are summarized in Table 2. Gener being distributed around breccia fragments. In general, ally speaking, the narrower and more symmetrical the the contact between the graphite and the host rock is (002) peak, the greater the Lc and u values. In addition, sharp, although in some cases it is gradational. Calcite t/002 values increase from well-ordered ( 3.35 A).Values of these millimeters to several centimeters occur in association parameters indicate a perfect three-dimensional order with the graphite mineralization. No signs ofgraphite for the graphite studied here. The non-basal reflections were found in the surrounding carbonate sediments or of hexagonal graphite are also recorded in the XRD in the pillow lavas. patterns (Fig. 3). In addition, the ratio of height to In polished slabs, the graphite forms flakes ranging width ofthe (002) reflection, the intensity ratio of the in size from 50 to 800 um, and is more strongly (002) and (004) diffraction peaks, and the symmetry of reflectant than kerogen, coalified organic matter, and the (002) reflection, deduced from the values of^at poorly crystalline graphite. Within the veins, the flakes half height and at one-third ofthe peak height, are also arc oriented predominantly parallel to the vein walls; within the range expected for fully ordered graphite graphite crystals typically occur as coatings around (Landis 1971).

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TABLE 2. X-RAY-DIFFRACTION PARAMETERS OF GRAPHITE FROM THE JURASSIC VOLCANIC HOST-ROCKS. COMPARED TO THOSE OF FULLY ORDEREDGRAPHITE

Betic Cordillera FUDy ordered

3.351 - 3.356 3.35-3.36

15-25 —

DC (u) >0.8S — U(A.) >«50 Proportion (%) of rhombohedral polytype 0-25 No. samples 23

The puameterc offullyorderedgrephilearetaken ftom Lendb (1971) H: peak height. W: peak width. D.G. (u): degree ofgrtphiliiMion, given by (<4g-3440 -0086(1 -p')]. wherep-(l -u)(Kwiecinska 1980).

Literature reports indicate that in most natural occurrences, graphite is hexagonal, and that where the rhombohedral phase is present, it seems to decrease in abundance as temperature of formation increases. The two polytypes of graphite (hexagonal and rhombohedral) can be distinguished by XRD analysis, since the diffraction pattern of each type shows some diagnostic reflections. Relative amounts ofeach phase can be evaluated by means of the intensity ratio of specific peaks (Kwiecinska 1980). The XRD patterns of some of the graphite samples studied here show Fig. 3. XRD patterns of both hexagonal (H) and hexagonal two peaks, at 2.08 A and 1.97 A, which correspond to + rhombohedral (R) graphite in samples from two the (101) and (012) reflections, respectively, of the mineralized vein-like bodies. The spacing and symmetry rhombohedral polytype (Fig. 3). On the basis of ofthe (002) and (002 +003) peaks indicate a high degree the intensity ratio of the (101) diffraction peaks for of order. both hexagonal and rhombohedral graphite, the content of the rhombohedral phase within these samples is estimated to be up to 25%. The partially overlapping (101) peaks for both hexagonal and rhombohedral ordered graphite (D:0 = 0; Fig. 4). However, slightly graphite were deconvolved using the Philips PC-APD disordered graphite also was found, having a mean D:0 (version 3.6) software, which is based on the ratioof0.07 (one grain shows a D:0 ratio of0.16). The mathematical model described by Schreiner & Jenkins overall spectral characteristics of these samples of (1983). graphite reveal a high degree oforder along the a axis. At the XRD scale, graphite appears to have a fairly According to the estimation of Wopenka & Pasteris homogeneous degree of order from one mineralized (1993), the size of such in-plane crystallites along body to another. Moreover, within any individual vein the [100] direction (La) is larger than 2000 A, or pocket-like body, no significant variations in and corresponds to that of graphite crystallites in crystallinity or content ofrhombohedral phase has been carbonaceous metapclites in the granulitc facies. found in the XRD patterns of samples taken from the In addition, Raman spectra of individual flakes inner to the outer parts ofthese occurrences. The main analyzed in situ in samples from the inner to the outer difference among mineralized bodies lies in the partsof veins and "nodules" (Fig. 2c) indicate the same abundance of the rhombohedral phase, which may be degree of order and homogeneity as the analyses absent in some areas. reported above on grain separates. Determination of structural features by Raman The DTAcurve shows a strong exothermic peak in microprobe spectroscopy was carriedout on both grain the range 660-730°C due to graphite combustion (Fig. separates and freshly broken, unpolished fragments. A 5). Such temperatures for the DTA maximum suggest a total of 82 grains from three different mineralized high degree of order, comparable to that observed in bodies were analyzed, and ratios ofthe "disorder peak" graphite from amphibolite-facies terrancs(Kwiecinska (D) to the "order peak" (0) in the spectra were calcu 1980). Carbon contents calculated from the weight loss lated. About 70% ofthe spectra correspond to perfectly on the TG curves are greater than 90 wt.%, in good

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agreement with the results obtained by elemental 680°C analysis. The carbon isotope signature of the graphite samples is quite homogeneous and falls in a narrow range of S'3C values from -23.0 to -20.7%o. Samples collected at different distances from the contact V) v> between mineralized bodies and host rocks on a scale O of 1-2 cm apart lack isotopic zoning, and show very constant 813C values within single veins or pocket-like bodies. Even in the relatively large mineralized bodies that arc cut by calcite vcinlets, no significant isotopic variations are observed in the vicinity ofthe carbonates (Fig. 6). 200 400 600 800 T(*C) Origin ofthe Graphite Fig. 5. Differential thermal analysis (DTA) and thcrmogravimctry (TG) curves for graphite, showing Any discussion on the formation of fluid-deposited the exothermic effcel and the weight loss due lo the graphite must deal with aspects of the source of the combustion of carbon. Initial weight: 9.45 mg. Heating carbonand mechanisms ofits mobilization and precipi rate: 10°C/min. tation. In this section, we discuss these issues with reference to the system C-O-H, in which most igneous and metamorphic fluids reasonably can be represented. Relationships in this system provide a good model for the deposition ofgraphite from fluids. Q Hoatrock The most reliable information on the provenance jjT A -23.3 | Graphite of carbon contained in the graphite is provided by the \-23.0. £43.2 • ~/r carbon isotopic ratio (l3C/'2C) of the graphite itself, it Calcite vcinlets although other geological and mineralogical evidence • -23.0 *^K> must be taken into account. There are three main potential sources ofcarbon in fluids:a) Carbon-bearing gases may be released during maturation of organic 0 5 cm 1 1

Fig.6. Sketch ofa hand sample ofa graphite nodule showing 5"C values(in %c). Dots represent sample locations for InSitu Roman Analysis different parts of the nodule. Host rock corresponds compositionally to an alkali basalt.

SECOND ORDER matter. Such biogenically derived, reduced carbon is enriched in l2C, with 8'3Cvalues in the range-40 to -I5%0 (Hahn-Weinheimer & Hirner 1981, Schidlowski et al. 1983, Crawford & Valley 1990). b) Carbon may be produced by the devolatilization of carbonate minerals. Such oxidized carbon is enriched in "C. 1200.00 1600.00 2000.00 2*00.00 2800.00 Ramon Shift in cm' (A Wavemimbois) Once incorporated into a fluid, this type of carbon should lead to the formation of an isotopically heavier graphite than that in a), i.e., 5I3C -5 to +5%o Fit:. 4. Raman spectra of grains of graphite from Huelma (Hahn-Weinheimer & Hirner 1981, Weis el al. 1981). analyzed in situ in broken nodules of graphite. Bottom c) The carbon may be mantle-derived. Graphite spectrum is representative of the highly ordered graphite precipitated from such carbon-bearing fluids show thai is characteristic of these deposits;the upper spectrum represents slightly disordered graphite that is much less intermediate isotopic ratios, with 513C values ranging abundant. Both first-order and second-order spectral between about -9.8 and -4.8%o (Pearson et al. 1990). features shown. As intensity ofdisorder-induced peak(D) The interpretationof isotopic data, however, usually increases in the first-order spectrum, the size of the is complicated by the influence of other processes crystallites measured along the basal plane decreases. that may modify the original isotopic signature.

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Changes in the temperature of the fluid during the during recrystallization ofthe pelitic rocks. This fluid deposition of graphite (Bottinga 1969), mixing of may have been enriched in carbon-bearing species as a fluids from different provenances (Rumble & Hoering consequence of the decomposition of carbonaceous 1986), and changes in the ratio C02/CH4 of the fluid matter dispersed in the assimilated metamorphic rocks. (Duke & Rumble 1986) affect the magnitude of the Such thermal breakdown would be an ongoing process, isotopic fractionation. Thus, the isotopic signature of and would maintain carbon enrichment in the magma fluid-deposited graphiterecordsboth the sourceofcarbon chamber throughout the period of contamination. and modifications of the fluids during different stages Unfortunately, the metamorphic basement does not ofgraphite precipitation. crop out, and there is only indirect evidence on the Graphite mineralization ofthe Betic Cordillera has nature of the organic matter content of these rocks, as a fairly uniform 8I3Csignature (-23.0 to -20.7%o) that inferred from the "chiastolite"-bearing xenoliths. suggests an organic origin for the carbon species As shown in the experimental work of Ziegenbein contained in the graphite-depositing fluids. It is likely & Johannes (1980), in a C-O-H system, carbon will be that the carbon was derived from the devolatilization incorporated in the fluid phase as H20-soluble mobile oforganic matter within the non-outcropping metamor- gases (mainly CH, and CO,). The transfer of carbon phic basement, which was partially assimilated by the from this homogeneous C-O-H fluid into an aqueous igneous rocks. The presence of carbonaceous matter fluid that coexists with graphite takes place through in these metamorphic rocks is shown by opaque molecular reactions of the type C02 + CH4 = 2C + inclusions in the andalusite xenocrysts ("chiaslolite") in 2H20. Such reactions depict the transport of carbon volcanic rocks of the upper unit. Values of 8I5C for via gaseous species and its eventual precipitation as such carbonaceous inclusions arc not available. In graphite (Walther & Althaus 1993). addition, the small isotopic differences observed The size ofthe stability field ofgraphite is sensitive among graphite samples taken from various bodies to geologically controlled intensive parameters. suggest that deposition occurred over a narrowrange of The graphite + fluid stability field becomes larger temperature and from fluids that had essentially a as temperature decreases at constant pressure, or as uniform composition. For each mineralized body, the pressure increases at a fixed temperature (Fig. 7). Both lack of isotopic zoning and the tcxtural homogeneity of these processes favor the deposition of graphite, as point to a process of precipitation developed during a the graphite field overtakes and incorporatesa specific single stage. Furthermore, the observed homogeneity of C-O-H bulk composition of interest (Ohmoto & the graphite isotopic signature in proximity to calcite Kerrick 1977, Ziegenbein etal. 1989). Isobariccooling vcinlets indicates that the carbonate-precipitating fluids probably is the most effective way to promote or the conditions under which they precipitated were precipitation ofgraphite from a fluid in any geological unsuitable for the formation ofgraphite. Indeed, these context, and especially in the veins studied here. In homogeneous 8I3C values suggest a lack of reaction addition to these isochemical changes that cause the between the previously formed graphite and later deposition ofgraphite from fluids, there are (isobaric- carbonate-bearing fluids. This observation indicates isothernial) changes in fluid chemistry that can induce that isotopic exchange did not proceed once graphite its precipitation. Forexample, hydration reactions can was fully crystallized, probably because of the very deplete a fluid in H20, thereby enriching it in carbon sluggish kinetics of diffusion of carbon in graphite and promoting graphite precipitation (Rumble et al. (Chacko et al. 1991, Dunn & Valley 1992, Scheelc & 1986). Hoefs 1992). Reference to Figure 7 and the above discussion Additional evidence exists for fluid-rock-melt indicates the difficulty in determining the specific interaction in this rock suite. Petrographic studies ofthe P-T-X conditions under which graphite was deposited. volcanic host-rock reveal a hydrothermal-metasomatic However, additional information is available from stage, in which potassium-rich feldspar overgrew the crystallographic form and degree of crystallinity plagioclase crystals, and a volatile-rich fluid phase of the precipitate. Most natural examples of graphite, (with H,0 and some C02) promoted replacement of originating both from the metamorphism of some mafic phases (mainly olivine and pyroxene) by carbonaceous matter and from deposition by fluids, are chlorite, a smectite-group mineral, iron oxides, and of the hexagonal form. The percentage of the carbonates (Puga & Portugal 1989). The effects ofthis rhombohedral phase in "metamorphic" graphite late metasomatism are widespread over most of the decreases with increasing gradeofmetamorphism. The rocks of the upper unit, and it has affected the host rhombohedral polytype in such graphite is considered rocks intensely. This overprint would explain why to have formed during growth of the hexagonal graphite veining is restricted to this outcrop. Puga & structure (Kwiecinska 1980). In contrast with the Portugal (1989) further suggested that the potassium- wide-ranging crystallinity of metamorphic graphite, rich fluid resulted from dehydration and breakdown fluid-deposited graphite commonly displays a uniform of the major phases of the metamorphic xenoliths crystallinity in occurrences worldwide. This observation (especially muscovite and biotite) that took place stands in contrast to experimental results, showing that

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Graphite C Graphite C Graphite C

ftp HO ftp Ho ftp H

Fig.7. Isothermal, isobaric sections forthe system C-O-H showingthe two-phase field (tic lines shown) wheregraphite coexists with fluid. Sections a, b, and c show the effect ofincreasingpressureat fixed T. Sections d, e, and f show the effect of increasing temperature at fixed P. Modified fromFerry & Baumgartner (1987, p. 359),reprinted with the permission of the Mincralogical Society ofAmerica.

poorly ordered graphite may precipitate from C-O-H be very sluggish at temperatures below 700°C. Thus, at fluids (Ziegenbein & Johannes 1990, Krauthcim 1993, low temperatures, it may be easier to nucleate poorly Pasteris et al. 1995, Pasteris & Chou 1998). Not only orderedgraphitethan well-crystallized graphite.Recent does the crystallinity of fluid-deposited graphite reflect experimental work by Pasteris & Chou (1998) suggests its temperature of deposition, but the thermochemical that the graphite initially precipitated from C-O-H properties of "disordered" graphite also differ from fluids is very poorly ordered, and that its degree of those ofwell-crystalline graphite.This distinction leads crystallinityincreases to some maximum with increasing to a smaller field of stability for poorly crystalline durationofthe experiment. Fornatural fluid-deposited graphite (Fig. 8; Ziegenbein & Johannes 1990). graphite, it is possible that above some undetermined Graphite from vein mineralization of the Betic temperature, the largersize ofthe stability field of fully Cordillera thereforestandsout becauseit is both highly ordered graphite thanthat ofdisordered graphite would crystalline (suggesting high temperature) and contains be the main controlling factor in this precipitation significant proportions ofthe rhombohedral polytype process, which would favor the crystallization of (suggesting lower temperature). These characteristics well-ordered graphite. also have been found in the volcanic-rock-hosted The relative abundance of the rhombohedral graphite deposit at Borrowdale, where up to 31.9% polytype ofgraphite in the Betic Cordillera occurrences of the rhombohedral polytype has been reported in (and also in the volcanic-rock-hostcd deposit at graphite samples (Kwiecinska 1980). Such features Borrowdale) therefore may represent an intermediate, may be related to the specific process of graphite mctastable case between poorly ordered graphite and formation. Since deposition of a solid phase from a the more common fully ordered hexagonal phase. If fluid involves both nucleationand graingrowth,kinetic true, then occurrence of the rhombohedral polytype factors may affect the precipitation and physical has the same significance in the evolution of the properties of the graphite. Ziegenbein & Johannes graphitization process for fluid-deposited graphite (1980) have shown that graphite-fluid equilibration can as for metamorphic graphite. Preservation of the

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40 / /If / \W Fluid + disordered * co2 // / / graphite * 20/ \ \ V CH4 * // 1Fluid |1

L_n_l 11,0

Fig. 8. Representation of the system C-O-H at fixed P-T conditions (400°C and 2 kilobars). The minimum C-comenl of a fluid in equilibrium with disordered graphite (solid curve) and with fully ordered graphite(dashed curve) is taken from Ziegenbein & Johannes (1990, p. 559), and is reprinted with the permission ofthe authors.

rhombohedral graphite within mineralization of in the veins is readily attributable to the ability of fluids the Betic Cordillera and at Borrowdale may be induced to pass through and even enlarge open fractures. The by a high rate of cooling, as inferred for volcanic occurrence ofgraphite veins in subvolcanic rock-types environments. (i.e., sills) would be more likely, since submarine Fluid deposition ofgraphite in the Betic Cordillera volcanic rocks (i.e., pillow-lava flows) aredramatically also could have been achieved through hydration outgassed during eruption, as seen at recent spreading reactions. The replacement of primary mafic minerals centers on the sea floor. ofthe igneous host-rock (mainly olivine and pyroxene) by hydrous phases like chlorite indicates that hydration Acknowledgements reactions occurred in the volcanic rocks through hydrothermal activity. Such reactions would remove The authors are indebted to R. Rojas and J. Pan-as H20 from the fluid, which would cause the latter to for assistance during the thermal study and elemental become enriched in carbon, thus favoring graphite analysis of graphite, respectively, and to B. Wopenka precipitation (Rumble et al. 1986, Ziegenbein & for assistance with Raman analyses. Technicians of Johannes 1990). the Centra de Microscopi'a Electr6nica de la U.C.M. In conclusion, the graphite mineralization studied in are also acknowledged for assistance during electron- this paperis believed to have formed as a carbon-bearing microprobe analysis. Thanks are also due to M.L. Otter fluid passed through the subvolcanic host-rocks. (Geochron Laboratories),who provided information on Carbon in the fluid was derived from devolatilization the analytical equipment and conditions for carbon reactions occurring in the assimilated metamorphic isotopic analysis. We are indebted to S. Castaflo for rocks. Graphite precipitated as the fluids cooled and help with line drawings. The final version ofthis paper underwent H20-consuming reactions that caused the has been improved by reviews from I-Ming Chou and formation ofa secondary, hydratcd mineral assemblage. J. Farquhar. This study was supported by the Spanish The presence of both hexagonal and rhombohedral DGICYT through project PB93-0064, and by the U.S. graphite was controlled by the kinetics ofprecipitation. National Science Foundation through grant EAR91- The distribution of the graphite (predominantly in 16967 to JDP. veins) may reflect circulation of fluids through technically weakened or altered zones. Hydraulic References fracturing can be promoted by fluid pressure in such zones, leading to the formation of epigenetic veins of Bottinga, Y. (1969): Calculated fractionation factors for graphite, as indicated by fragments of volcanic rocks carbon and hydrogen isotope exchange in the system (hydraulic brccciation; JCbrak 1992) within the miner calcite - carbon dioxide - graphite - methane - hydrogen alized bodies. Therefore, the concentration ofgraphite - water vapor. Geochim. Cosmochim. Acta 33,49-64.

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Chacko, T., Mayeda, T.K., Clayton, R.N. & Goldsmith, Jebrak, M. (1992): Les lexlures inlra-filoniennes, marqucurs J.R. (1991): Oxygen and carbon isotope fractionation des conditions hydrauliqucs el tcctoniques. Chroniquede between CO, and calcite. Geochim. Cosmochim. Acta 55, la Recherche Miniere 506. 25-35. 2867-2882. Krautiieim. G. (1993): Kristallinilaetsbestimmungen an Comas. M.C., Puga. E., Bargossi. G.M., Morten, L. & Schitchtmineraten ///if und Graphit. Ph.D. thesis. Rossi, P.L. (1986): Paleogcography, sedimentation and University of Hannover, Hannover, Germany. volcanism of (he central Subbctic Zone, Betic Cordilleras, southern Spain. Neues Jahrb. Geol. Palaont., Monatsh., Kwiecinska, B. (1980): Mineralogy of natural graphites. 385-404. Polska Akad. Nauk, Prace Mineralogiczne 67.5-79.

Crawford, W.A. & Valley, J.W. (1990): Origin of graphite Landis, C.A. (1971): Graphitization of dispersed in the Pickering gneiss and the Franklin marble. Honey carbonaceous material in metamorphic rocks. Contrib. Brook Upland, Pennsylvania Piedmont. Geol. Soc. Am., Mineral. Petrol. 30, 34-45. Bull. 102.807-811. Luque, F.J., Barreneciiea. J.F. & Rodas, M. (1993):Graphite DIaz de Neira. J.A.. Enrile. A. & Lopez Olmedo, F. (1991): gcoihcrmometry in low and high temperature regimes: Hoja n. 970 (Huelma) a escala 1:50.000 del Mapa two case studies. Geol. Mag. 130,501-511. Geoldgico de Nacional (MAGNA). InstitutoTccnologico Gcomincro de Espafia, Madrid. Spain. , Rodas. M. & GalAn. E. (1992): Graphite vein mineralizationin the ultramaflc rocks of southern Spain: Diessel, CF.K. & Offler, R. (1975): Change in physical mineralogy and genetic relationships. Mineral. Deposita properties ofcoalified and graphitised phytoclasls wilh 27. 226-233. grade of metamorphism. Neues Jahrb. Mineral., Monatsh.. 11-26. Ohmoto. H. & Kerrick, D.M. (1977): Devolatilization equilibria in graphitic systems. Am. J. Sci. 277, Dissanayake, C.B. (1994): Origin of vein graphite in 1013-1044. high-grade metamorphic terrains. Role of organic matter and sediment subduction. Mineral. Deposita 29, 57-67. Pasteris, J.D. & Chou, I-Ming (1998): Fluid-deposited graphitic inclusions in quartz: comparison between KTB Duke, E.F. & Rumble, D., HI (1986): Tcxtural and isotopic (German Continental Deep Drilling) core samples and variations in graphite from plutonic rocks, south central artificially re-equilibrated natural inclusions. Geochim. New Hampshire. Contrib. Mineral. Petrol. 93.409-419. Cosmochim. Acta (in press).

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Waltiier, J. & Althaus, E. (1993): Graphite deposition in tcctonically mobilized fault planesofthe KTB-pilot drill hole. In Kontinentalcs Ticfbohrprogramm der Bundes- ReceivedMarch 7, 1997, revisedmanuscriptacceptedJune I, republik Deutschland (R. Emmermann, J. Lauterjung 1997.

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