Journal of the Geological Society, London, Vol. 153, 1996, pp. 265-275, 10 figs. 2 tables. Printed in Northern Ireland

P-T conditions in the South Wales Coalfield: evidence from coexisting hydrocarbon and aqueous fluid inclusions

D. H. M. ALDERTON’ & R. E. BEVINS2 I Department of Geology, Royal Holloway (University of London), Egham, Surrey TW20 OEX, UK ’Department of Geology, National Museum of Wales, Cardiff CFl 3NP, UK

Abstract: Siderite nodules in the Carboniferous Coal Measures of South Wales contain cavities which are often infilled with quartz, carbonates, sulphides, and hydrocarbons. The quartz contains a mixture of hydrocarbon and aqueous fluid inclusions. The aqueous fluid inclusions consist of a dilute brine (3 wt % NaCl equivalent) and have homogenization temperatures in the range 97-212 “C (mean 143 “C). The hydrocarbon fluid inclusions are dominated by methane with a small component of higher order hydrocarbons;their homogenization temperatures are in therange 35-78°C (mean54°C). It is assumed that the two fluids were trapped simultaneously during growth of the quartz and thus a P-T estimate of entrapment can be obtained by graphical intersection of the hydrocarbon isochores and the aqueous fluid bubble point (homogenization) temperatures assuming hydrocarbon saturation. This method gives temperaturesbetween 130 and160”C, and pressures between 40 and 55 MPa. The timing of mineralization is uncertain, but it is suggested that it took place during burial and low grade metamorphism of the subsiding sedimentary basin (i.e. in the Upper Carboniferous). The hydrother- mal fluids were probably derived from evolved meteoric or connate waters expelled during subsidence and sediment compaction.

Keywords: South Wales coalfield, burial metamorphism, fluid inclusions, hydrocarbons, ironstone.

Fluid inclusions have long been used to place constraints on al. 1979). The cause of this variation has been the subject of the temperature and pressure of hydrothermal fluid activity numerous debates (see White 1992). and the origin of the fluids in a diverse range of geological Siderite-rich nodules are common in the Westphalian of environments. Traditionally this usage has concentrated on South Wales and have, in the past, been exploited on a large the study of hydrothermalmineral deposits, but more scale; up tothe middle of the nineteenthCentury South recently attention has been directed towards environments Wales ‘ironstones’ furnished the bulk of Britain’s iron ores of sedimentburial, hydrocarbon accumulation, and low- (Thomas 1961). The ironstones occur as nodules which often grademetamorphism, and alarge amount of information are ‘septarian’ in nature and contain geodes with a variety of concerning P-T conditions andgeothermal gradientshas well-crystallized mineral species. accrued (see for example Mullis 1987, McLimans 1987 and Carbonates were first to form in the geodes; brown Mullis et al. 1994). Here we utilize coexisting hydrocarbon ankerite forms the initial void coating, but white dolomite andaqueous fluid inclusions present in quartz from and calcite are volumterically more abundant. Sulphides ‘ironstone’(siderite) nodules in the Westphalian(Coal form the second stage in the mineral paragenesis. Millerite Measures) from South Wales todetermine the P-T (NiS) occurs as fine acicular crystals (up to 8 cm in length) conditions during diagenesis and low-grade metamorphism, but, in addition, thereare occurrences of galena, and to speculate on the origin of the hydrothermal fluids. chalcopyrite,sphalerite, pyrite, possible pyrrhotite,and siegenite ((Ni, Co),S4) (North & Howarth 1928; Firth 1971, 1973; Bevins & Horrik 1985) (note that the identifications of Geological setting linnaeite by North & Howarth (1928) and Howarth (1954) were probably erroneous and referred to siegenite). Growth TheSouth Wales Coalfield formed in one of several of sulphides was followed by quartz crystallization. The sedimentary basins that developed across northwest Europe quartz is particularly well-formed andclear, and often in late Carboniferous times. Itforms an E-W-trending occurs in doubly-terminated forms up to 5 cm in length. It is syncline of Variscan age, with dimensions of approximately known locally as ‘Merthyr diamond’ and is superficially very 100 by 30 km. The coal-bearing strataare chiefly of similar to other examples of gem-quality quartz crystals (e.g. Westphalianage (315-290 Ma) andrepresent sediments Herkimer or Marmarosh ‘diamonds’, see below). The quartz which accumulated in a foreland basin onthe northern crystals occur throughout the Coalfield, but tend to be less margins of the Variscan orogen in response to northward- abundant in the anthracite-bearing regions (Firth 1971). propagatingthrusting (Jones 1991). Thissedimentary Brown waxy hydrocarbons are also present in abundance in succession comprises cyclothems (alternations of mudstone, the nodules and typically envelop the quartz crystals. This siltstone, sandstone, coal, andseat-earth) deposited in hydrocarbonhas been termed ‘hatchettite’ and is charac- predominantly estuarine-freshwater conditions in a rapidly terized as an n-alkane which has undergone a high degree of subsiding basin. Coal rank shows a marked variation across maturation (carbon preference index = 0.96-1.06) (Firth & the Coalfield, varying from high volatile bituminous coals in Eglinton 1972; Bevins 1994). the east and southeast, to anthracitein the northwest (Gill et Several samples of quartz-bearing ironstone nodule were 265

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examined in the collection of the NationalMuseum of underincident ultra-violet (UV) light to test for the presence of Wales, some of thesebeing derived from the original hydrocarbons using a Zeiss USMPSO UV microscope; the excitation collection of Firth (1971). Unfortunately, most of the wavelength was set at 365 nm, and the emission was measured over sampleswere collected from mine tips, so the precise a wavelengthrange of 400-700 nm. Other microscope-based horizons where they came from are not known. However, techniqueswere used in anattempt to characterise further the the horizons can be constrained to a certain extent through a nature of thehydrocarbon phases; these included LaserRaman knowledge of theworking coal seams in the individual spectroscopy, Fourier-transform infra-red microscopy (FTIR), and mines. Particularlygood examples of quartz-filledgeodes Fourier-transform Raman microspectroscopy. comefrom the Wyndham Colliery, at Ogmore Vale, Mid The thermometric behaviour of the fluid inclusions was studied Glamorgan [Grid Reference SS 933692071 and the results using a Linkam TH600 microscope heating-freezing stage. This was calibrated using synthetic H,O- and CO,-richfluid inclusions in presented in this paperare based onsamples from this quartz; the accuracy of the resulting data is approximately rt0.l "C locality. The colliery lies near the centre of the Coalfield, at temperatures below -2O"C, +0.2 "C in the range -20 to +50°C, within anarea of low volatilebituminous coals. These and +0.5 "C in the range +50 to 220 "C. Most of the studies were specimenshave been lodged in the collections of the concentrated on phase changes at temperatures above 0 "C. NationalMuseum of Walesunder accession number Minerals were analysed for 0- and C-isotopic compositions in NMW95.38G. thestable isotope laboratories atRoyal Holloway. CO2 was released from carbonates by reaction with phosphoric acid. Quartz wasmixed with an olivine standardand attacked by laser Techniques fluorination using CIF,, and the oxygen released converted to CO, by reaction with heatedgraphite (Mattey & Macpherson 1993). Samples of quartzwere prepared as doubly-polishedsections, Isotope ratios were measured on a VG Prism mass spectrometer. approximately 300pm in thickness. These were examined using a standard petrographic microscope. The inclusions were also studied Fluid inclusion observations at room temperature Most of thequartz is remarkablytransparent and well-crystallized, thus the local name of 'diamonds'. It is also generally free from visible defects and fluid inclusions, but where the latter are present they are extremely abundant and large. Under the petrological microscope the majority of . the fluid inclusions are seen to occur in distinct planes (Fig. la & b); as these follow growth zones parallel to the prism faces in the quartz (Fig. la), they are likely to be of primary .. origin. Microscopic observationsat room temperature allow three types of fluid inclusion to be distinguished (Types 1-3; Table 1):

. \ -;. Type 1 inclusions These are the most abundant inclusions and they occur in large, elongate,tubular shapes, often atleast 50pm in length (Fig. 2a). Many take the form of negative hexagonal bipyramids (Fig. 2c) but they may also occur in oblate and 0 irregular forms (Fig. 3). The vast majority appear opaque, due to total internal reflection, and it thus appears that the bulk of the inclusion consists of a low density, gaseous phase. However, it is also apparentthat the rim of the inclusion consists of a small volume of a separate (liquid) phase (Fig. 2). Because of the often regular shape of these inclusions, fairly reliable estimates of relative phase volumes could be made. Several images of inclusions were magnified andprojected on ato screen to allow accurate measurements of thearea covered,and these were then converted to volumes based on assumptions concerning the inclusion shape.These estimates suggest that the re. . -. -. 0 -'... liquid/(liquid + vapour) volumetric ratios (the 'degrees of - - -..- D m ..-- - fill') are less than 0.1, and typically around 0.05. Fig. 1. (a) Planes of hydrocarbon-rich fluid inclusions aligned In some rare cases, particularly where the shape of the parallel to hexagonal faces of quartz, attesting to their primary inclusion is not rounded, a third colourless (liquid) phase is nature. Length of plate is 250pm. (b) Planes of hydrocarbon-rich also visible (Fig. 4). In the few examples in which it could be fluid inclusions, showing negative hexagonal prism and oblate measured, this liquid phase appears to makeup a volumetric shapes. Length of plate is 200 pm. proportion of about 3-6 X 10-4.

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Table 1. Fluid inclusion characteristics

Type Composition Shape and size andComposition Shape Type DF T, mode Th (“C) CommentsT* (“C)

LiV Tubularlobate;to 0.05-0.08 WC) 55, 3578 to (12) - Very abundant, hydrocarbon negative L+V>V liquid 1 hexagonal prisms hydrocarbon up to 50 pm in often difficult to length see L+V (HC + water) 184, 152 to 202 (3) Notmeasured Aqueous phase hydrocarbon plus L+LiV>V not commonly very minor (L) observed aqueous phase

L iV aqueous (a) Irregular,0.9-1.0 up (Water) 143, 97 to 212 (50) -1.6, -0.8 to Quite abundant 2 to 100pm in L+V>L 3.0 (12) and often large; length, or (b) as some solid Type 1 inclusions phases. Necking common. L aqueous (1.0) - measured Not Quite abundant. Necking common

3 Mixed L Irregular;approx. Variable observed Not - - Rare; no hydrocarbon and 20 pm measurements aqueous (L + V) made

L, liquid; V, vapour; HC, hydrocarbon phase. Th = temperature of homogenization: T,, = temperature of ice melting. DF = degree of fill ( = volume ratios, L/L + V). Values are mean values and ranges; number of measurements in brackets.

Type 2 inclusions a pale brown colouration and is surrounded by an aqueous The secondmost abundant type of inclusion consists of phase. The morphology of the type 3 inclusions is fairly either monophase liquid, or liquid with a small vapour large, irregular, and flattened (e.g. Fig. 6). bubble. Freezingstudies (see below) show that these inclusions are made up of an aqueous phase. At least two Nature of the Puids present subtypes of this inclusion group havebeen observed and they probably each represent a separate generation of fluid. The small rim of liquid in the Type 1 inclusions fluoresces Some have a shape and size similar to those of the type 1 under UV radiation (Fig. 2b & d) and is thus presumed to inclusions andappear cogenetic(see below). Othersare bea hydrocarbon. The fluorescence is of apurple-blue much larger (they can sometimes be observed with a hand colour, with amajor peak at 430-470nm. This would lens) and have an irregular shape. These could be secondary correlate with a very light hydrocarbon, typical of a in origin. In this latter type necking is very common (Fig. condensate (API around 40-45) (Hagemann & Hollerbach 5a) and no doubt explains the presence of monophase liquid 1986). When small grains of inclusion-bearing quartzare inclusions, andthe range in degree of fill (typically crushed under refractive index oil, bubbles of gas are 0.95-1.0). In some of these large inclusions there are also released, indicating that the volumetrically dominant vapour several, small grains of a solid phase (Fig. 5b). These have phase in these inclusions consists of a compressed gas. The notas yet been identified but are presumed torepresent third colourless phase in these inclusions does not fluoresce trapped phases. They are colourless, have a high relief, and (Fig. 4), and is presumed to be an aqueous solution. sometimes appear rhombic in shape, so could bea The other microscopic techniques were not particularly carbonate. helpful, mainly because of the predominance of low-density Many of these inclusions described above doalso contain vapour in the inclusions. However, the FTIR scan showed a very small amount of immiscible material,as after four peaks in the range 2800-3000 cm-’, possibly relating to homogenization of the vapour bubble on heating the sample the stretching bonds CH, and CH2, and thus pointing to the a ‘bleb’ of colourlessmaterial is left behind in the liquid presence of alkanes. Aromatic bonds do not appear to be water. This phase is not visible in the aqueous inclusions at presentbut unfortunately the useful area (<2000cm-’) room temperature because it is attached to, andobscured could not bestudied because of interference from Si-0 by, the vapour bubble. stretching in the quartz host. The Laser Raman study was hindered by fluorescence in the inclusions. Only one peak was observed (at 2917 cm-’) and this indicated the presence Type 3 inclusions of methane. The third type of inclusion visible at room temperature is Several attempts were made to extract the gases in bulk relatively rareand consists of mixturea of variable samples of the inclusions for mass spectrometric analysis proportions of three fluids (two liquids and a vapour bubble) but, because of the small volumes available, this was very (Fig. 6). The liquid which wets the vapour bubble often has difficult. Several grains of inclusion-rich quartz were crushed

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(c

0 0 .5,

0. - '8

Fig. 2. (a) Elongate, tubular hydrocarbon inclusion containing vapour and a small volume of liquid. Length of plate is 50 pm. (b) The same inclusion as in (a) but under incident UV light. The liquid hydrocarbon fluoresces. Note however that the shape of the fluorescing liquid does not exactly match the shape of the liquid portion; this is because of a small rim of water. Length of plate is 50 pm. (c) Close up of Fig. l(b), showing detail of shape of hydrocarbon inclusions. Length of plate is 50 pm. (d) The same view as (c) under incident UV illumination. Note how the shape of the fluorescent liquid hydrocarbon in the central inclusion does not match the shape of the liquid part in (c), because of the presence of a rim of water. Length of plate is 50 pm.

invacuum and the contents directly fed into a mass spectrometer. The resulting mass spectrumconfirmed the presence of methane and some water. Higher order- hydrocarbonswere not detected by mass spectrometry

Fig. 4. Vapor-rich hydrocarbon inclusion under both normal illumination and incident W light. Fluorescence (white; 'L') is from the liquid hydrocarbon, but the 'tail' contains (non-fluorescing) Fig. 3. Irregular shape of vapour-rich hydrocarbon inclusions. water ('W). Remainder of the inclusion ('V') is made up of Length of plate is 50 pm. hydrocarbon vapour. Length of plate is 20 pm.

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because of inadequate sensitivity, but the colour of the UV fluorescence indicates that some must be present. The small ‘bleb’ of colourless liquid seen in some of the aqueous(Type 2) inclusions also fluoresces in UV illumination, suggesting that it too consists of a liquid hydrocarbon. The brownish liquid in the Type 3 inclusions does fluoresce, so is also a hydrocarbon, but it appears to exhibit differentfluorescence characteristics compared to those for fluids in the Type 1 inclusions. The two dominant types of fluid inclusion (Types 1 and 2) generally to occurseparately. However, some planes appearto contain bothaqueous and hydrocarbon fluid inclusions (Fig. 7). This observation, combined with the fluid inclusion characteristicsoutlined above, points tothe original presence of a heterogenous, immiscible hydrocarbon-aqueous fluid mixture in the hydrothermal fluid responsible for quartz precipitation. Inclusion types 1 and 2 represent trapping of eitherthe hydrocarbon or aqueous portions. The rare three-phase inclusions (Type 3) seem to represent a separate trapping event of a different hydrocarbon-aqueous fluid mixture. Therare examples of hydrocarbon-rich inclusions containingminute amounts of water (e.g. Fig. 2) could either belong to this third category or more likely could represent dissolved water which has exsolved from the hydrocarbon on cooling.

Thermometric studies of the fluid inclusions The results of the thermometric studies are summarized in Table 1 and Fig. 8.

Type 1: hydrocarbon-rich fluid inclusions

Fig. 5. (a) Necking in large aqueous inclusions. Note how the solid On cooling thistype of inclusion, the vapourbubble phases have been retained in an inclusion which is also bubble-free. contracted allowing the liquid phase to become easily Length of plate is 50 pm. (b) Large aqueous inclusion containing visible. The rate of change in phase ratios indicates that the solid phases. Length of plate is 50 pm. liquid phasehas a high coefficient of thermalexpansion, consistent with a hydrocarbon liquid. No clear freezing of

0 m

Y t- 6a

I 0

,: i-;r Fig. 7. Evidence for fluid immiscibility: hydrocarbon-rich inclusions Fig. 6. Three-phase inclusion containing aqueous liquid and an containing a large vapour bubble (‘X) coexisting in the same plane immiscible liquid hydrocarbon surrounding the vapour bubble. with aqueous inclusions, with a small vapour bubble (‘Y’). Length Length of plate is 50 pm. of plate is 100 pm.

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I2 T necking were excluded from the thermometric study, but it is likely that necking still remains a problem, and thus has given rise to some of theextreme values. Alternatively, morethan one episode of hydrothermal activity has occurred (as suggested by the different type 2 mor- phologies), each of slightly different temperature. After freezingthe aqueous inclusions, melting of ice occurs at -1.6"C. Thisvalue suggests that the aqueous

90 Im 110 I20 130 140 150 160 170 l80 190 200 210 220 230 phase has an overall salinity of 2.7wt % equivalent NaCl

Ternperamre ("C) (Bodnar 1993). It is likely that the aqueous fluids are also Fig. 8. Histogram of homogenization temperature data for aqueous saturated with methane (see below). In this case methane fluid inclusions. Mode of homogenization is always to the liquid clathrate will also form when the inclusions are frozen and phase. lead to an increase in the apparent fluid salinity. For the amount of methane present in these fluids (probably about 4000ppm; see below) the resulting effect should be minor and can be neglected here (see Hanor 1980). thecontents was observedbut at temperatures below -100 "C some colourless to pale yellow-brown solids started to appear. Theory of derivation of pressure and temperature On heating, thevapour phaseexpanded, but in most from fluid inclusions inclusions the exact point of total homogenization could not be accurately determined. The temperatures of homogeni- In a system containing two immiscible fluids the zation recorded in Table 1 werebased on12 inclusions thermometric behaviour of the individual fluid phases can be which had small protrusions which were sufficient to allow used to define the P-T conditions of trapping, using the the accurate observation of the final disappearance of the technique of intersecting isochores. The classic example of liquid (e.g. Fig. 3). Homogenization to the vapour phase in such an immiscible system is that of a hydrocarbon these 12 inclusions occurred overthe temperature range ('oil')-water mixture. Figure 9 is a simplified diagram 35-78 'C, with a mean value of 54 "C. illustrating the relevant phase relationships in an immiscible In three, three-phase type 1 inclusions, total homogeni- hydrocarbon-water mixture. zation occurred to the vapour phase by disappearance of the Considerthese fluids (immiscible aqueous liquid and liquid water. This occurred in the range 152-202°C (mean hydrocarbon vapour) trapped as separate fluid inclusions at 184 "C). the temperature and pressure conditions specified by X(& and P2). Theaqueous fluid inclusions must have been trappedon the bubble point curve appropriate for that Type 2: Aqueous inclusions composition (i.e. saturated with hydrocarbondominantly InType 2 inclusions total homogenizationoccurred by methane). On cooling, a bubble will immediately form and disappearance of the small vapor bubble; the solid phases the P-T path will follow a (curved) isochore. On reheating did not dissolve on heating thesamples. Although the modal the sample,these inclusions will homogenize at by temperature of homogenization for the aqueous inclusions is disappearance of the vapour bubble, and this temperature quitemarked at c. 150"C, therange is quite large will represent the temperature of inclusion entrapment. The (97-212°C; see Fig. 8). Inclusions showing any sign of hydrocarbon fluid inclusions will follow a different isochore until the two-phase (liquid + vapour) hydrocarbon field is encountered,and a secondhydrocarbon phase forms. Reheating of these inclusions will cause homogenization at intoone phase by disappearance of eitherthe vapour bubble (on the bubble point curve) or liquid (on the dew pointcurve) depending onthe composition of the fluid. Thus if the compositions of the two fluids are known, and can be modelled, the thermometric behaviour can be used to ---...... construct isochores which intersect and thus give the P-T I conditions of trapping (see for example McLimans 1987). In reality, the hydrocarbon fluid inclusions will also contain a minute amount of dissolved water which will separate into a discrete phase on cooling. This water would modify the phaseboundaries in the hydrocarbon system, although the effects are probably negligible for this exercise. However the addition of hydrocarbon to the aqueous phase is important and must be considered for modelling (Hanor 1980). In theory, the temperature of disappearance of the aqueous phase in the hydrocarbon inclusions will be equal to Temperature thetemperature of homogenisation of the aqueous Fig. 9. Diagram illustrating the possible P-T trapping conditions of inclusions (Burruss 1992). However, in most petroleum water and hydrocarbon fluids in an immiscible mixture. HC, inclusions the aqueous phase will wet the wall of the cavity hydrocarbon; L, liquid; V, vapour; aq, aqueous phase. and the amount will be so small as to be impossible to see.

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Modelling and interpretation of results from the These have the following alkane component compositions: present study (A) C, 85%, C, 5%, C, l%, C, l%, C, 1% C, l%, C8 l%, C,o 5%; (B) C, SO%, C, 10%, C3 S%, C, 3%, C,, 2%. These The results of this thermometricstudy have been used to compositions are supported by the UV fluorescence study construct a P-T diagram illustrating conditions of trapping which indicated thatthe hydrocarbon hasan overall low of the fluid inclusions during quartz growth (Fig. 10). densitycharacter. Likely isochoresfor these compositions Throughout it is assumed that the majority of the inclusions using the mean values of Th are shown in Fig. 10. are representative of one phase of heterogeneous fluid It is assumed that thecontemporaneous trapping of which was trapped at one P-T condition. methane-rich and aqueous fluids occurred. In this case the One of the majorproblems in any attemptto model aqueous phase in thesesamples would be saturated with hydrocarbon fluids is thedetermination of their exact hydrocarbons (mainly methane). Thusthe aqueous fluids composition. Mass spectrometric analysis indicates that would have been trapped on the bubble point curve for a methane is the dominant phase present, but this does not hydrocarbon (dominantly methane)-saturated aqueous fluid correlate with the observation that liquid hydrocarbon was and the measured Th values for these inclusions are equal to visible at room temperature (the critical point of methane is the trapping temperatures. at -82 'C, so liquid methane cannotbepresent at The P-T datafor the hydrocarbon andaqueous temperatures higher than this). inclusions derived using the aboveapproaches are For this study the commercial modelling program summarized in Fig. 10. A certain amount of uncertainty is 'Equi-90' was used. This uses a Peng-Robinson equation of associated with the generation of the hydrocarbon isochores state (Peng & Robinson 1976) togenerate two-phase (due to uncertainty in the exact composition), the Th of the envelopes and isochores for hydrocarbon fluids of fixed aqueous inclusions (due to the large range in values), and compositions (see Burruss 1992). A trial and error approach the assumptions made in the modelling. However, it is was adopted here, using the following information: thought that the shaded area in Fig. 10 represents the likely (1) the mode of Th is always to the vapour phase; fluid temperature and pressure at the time of quartz growth, (2) Th values are in the range 35-78 "C; from 130 to 160°C, and 400 to 550 bars (40-55 MPa) (3) the 'degree of fill' (L/L+ V)is approximately 0.05 at respectively. Of the three available measurements for total 25 "C; homogenization of three-phase inclusions, only one (152 "C) (4) the hydrocarbon phase is dominated by methane. would plot in this range, the others (190 and 202°C) being Only a limited choice of fluid compositions satisfies the slightly higher. If these three values truly represent total abovecriteria andthe twomost likely extremes of these homogenization for this system, then it may be thatthe compositions are shown by the curves A and B in Fig. 10. proposed temperatures of the fluids are 20-30 "C too low. Alternatively theaqueous phase could representa small amount of heterogeneously trapped aqueous liquid, the presence of which would raise thetotal homogenization values above the (true) trapping temperatures. 70 At these P-T conditions, the amount of water dissolved in methane shouldbe approximately 20000 ppm (McCain 1990). Assuming that the water present in the hydrocarbon-

n rich inclusions was originally dissolved in the hydrocarbon phase, the volumeestimates indicate that 3000-9000ppm water was originally dissolved in the hydrocarbon. These gW 40 values were based on only a few estimatesand will be p! subject to large errors, forexample from the problems in 1 30 estimating such small relative volumes, the exact nature v) CP v) 7 (particularly density) of the hydrocarbonphase, and the 2 20 I salinity of the aqueous phase. However, they do appear to a be of the right magnitude and indicate that these small 2-phase hydrocarbon field )B )* 9 volumes of aqueous fluid could represent original dissolved water in the hydrocarbon which exsolved on cooling. The Th 01 0 25 50 10075 125 150 175 200225 (total) of these inclusions support this interpretation of trapping of an aqueous-saturated hydrocarbon phase. Temperature ("C) This P-T range is coincident with the bubble point curve of an aqueous fluidwhich containsapproximately Fig. 10. Proposed P-T conditions under which the fluid inclusions 4000-5000 ppm methane (Fig. 10) (Haas 1978). These values in the quartz were trapped. The isochores and two-phase for P, T, and methane content of the fluid are very similar hydrocarbon fields for two fluid compositions (A and B, see text) to those of fluids found in hydrocarbon-rich, deep were generated from a Peng-Robinson equation of state; dashed sedimentary basins such as in the Gulf Coast of the USA lines enclose the likely range of isochore positions due to the range in Th of the hydrocarbon inclusions. Also shown are the bubble (Hanor 1980). point curves for an aqueous fluid containing 3000, 4000, and 5000 ppm methane (from data of Haas 1978). The critical point Stable isotopes (CP) for fluid A is at +16"C, and for fluid B at -20°C. The boundary of the two-phase hydrocarbon field to the right of the The stable isotopic compositions of minerals can often be critical point represents the dew point curve for that fluid used to deriveinformation aboutthe source of the composition. coexisting fluid phase, and, if two coexisting minerals are

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present, the compositions can be used for geothermometry. Discussion In this study both quartz and calcite from the nodules were analysed to investigatewhether their 0- or C-isotopic Euhedral, clear crystals of quartzare oftenfound in compositions could provideadditional andindependent sedimentary rocks, where they are characteristically information onthe temperature of quartz growth, orthe associated with occurrences of hydrocarbonsand meth- source of the hydrothermal fluids. The results are presented ane-rich fluids, for example the Herkimer 'diamonds' from in Table 2. New York State,the Marmarosh 'diamonds' from the Mineral-water oxygen isotopicfractionation relation- Ukraineand the 'cleft' quartz from several localities in ships were obtained from Zheng (1993) for quartz, and the the Swiss Alps (Touray & Jauzein 1967; Touray & compilation of Longstaffe (1989) for carbonates.Because Sagon 1967; Touray et al. 1970; Roedder 1984; Mullis 1987; thecarbonate is not a simple end-member phasesome Chamberlain 1988). There thus appearsto be ageneral compromise was adopted, but this doesnot affect the association betweenhydrocarbons and well-formed quartz interpretation greatly. The carbonate values are higher than crystals. those for the quartz and therefore it is clear that the two Roedder (1984) has concluded that such crystals must minerals were not in isotopic equilibrium with the same fluid have formed very slowly over an extended period of time phase(see Table 2). This is notentirely unexpected although Mullis (1987) has put forward the opposite view - considering thatthe quartz formsovergrowths onthe that some quartz crystals in such environmentsformed carbonate, but the two minerals clearly represent different rapidly. Silica solubility is enhanced by the presence of mineralizing events. hydrocarbons (Bennett 8~ Siege1 1987) and Chamberlain Assuminga temperature of quartz growth of 160°C (1988) has proposed that the Herkimer quartz crystals could (from the fluid inclusion studies), the 6"O composition of have formed as the increasing temperatures during sediment the fluid in equilibrium with thequartz would be burial slowly broke down organic-silica complexes and approximately +5.6%0. Unfortunately this value alone is not progressively released low levels of silica for growth. This sufficient to characterise the hydrothermal fluid and it could mechanism for quartz formation would imply an extended represent waters derived from a variety of sources: modified period of mineralization and a relatively wide range of meteoric,connate, or seawater, metamorphic fluid, temperatures (as observed for the fluid inclusion data). magmatic fluid, or organic fluid (Sheppard 1986). The White (1992) carriedout adetailed study of palae- temperature and environment of these fluids, and their low ogeotherrnalindicators in the South Wales basin, using salinity (less thanthat of seawater), would bemost volatile content, vitrinite reflectance, illite crystallinity, and consistent with a fluid originating as deep,a high basin modelling. All vitrinite reflectance data fromSouth temperature, formationwater, which was derived from Wales are in the range 0.8-4% R,,,. The transition from the sediments undergoing burial metamorphism. diageneticzone tothe anchizone occurs at approximately There is no constraint on the temperature of formation 3.0% R,, and most of the coals therefore lie in the of the carbonate. Because the carbonate infillings are earlier diagenetic zone. Only in the anthracitezone in the and more widespread than those of quartz, it is assumed that north-west of the Coalfield is there indication of lowermost they also formed at lower temperatures. Assuming a anchizone conditions. For the Seven-feet and Five-feet coal temperature of 1OO"C, and thatthe carbonate is pure seams at Wyndham Colliery, White (1992) obtained mean dolomite, the water in equilibrium with it would have a random vitrinite reflectance values of R, = 1.75 and composition close to 5.5%0; at lower temperatures it would R, = 1.66 respectively. Assuming an effective heating time have a lower S"0 content (e.g. at 60°C it would be 0.4%0). of 10 Ma and using the formula of Underwood et al. (1989) This fluid could also represent a meteoricor formation these values give temperatures of 196 and 193°C. These water, possibly one which hasundergone less isotopic temperatures are slightly higher than those proposed based exchange with the host rocks due to the lower temperatures. on the aqueous fluid inclusions, but are very close to the few The 6I3Ccomposition of the carbonate is not particularly data for total homogenization of water-hydrocarbon fluid informative because the large uncertainties in temperature inclusions. and unknown pH and f0, do not allow a calculation of the White's (1992) illite crystallinity values for the South fluid composition (Ohmoto 1972). Wales Coalfield are almost all at diagenetic grade (

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conversion of this to depth values is fraught with difficulty. (a) Triassic iron mineralization Much of the problem stems from defining the causes of the pressure, and the timing of nodule infill. In a sedimentary Low temperature Fe-oxide (+quartz and carbonate) environment such as the case here, the pressure can vary mineralization occurs along the south edge of the Coalfield between being hydrostatic or lithostatic. Pressure-depth (e.g. at Llanharry). The ore mineralization is dominated by plots for fluids in sediments from the Central North Sea and hematiteand goethite and some occurs in Variscan faults the American Gulf Coast basin illustrate that pure lithostatic and Triassic sediments; it is thought to have a late Triassic pressure regimes may not be reached until depths of more age. Rankin & Criddle (1985) studied the fluid inclusions in than 4 km are attained (Hanor 1979; Gaarenstroom er a/. quartz associated with this mineralization and showed that 1993). If the pressure of 500 bars is dueto the equal two types of hydrothermal fluid had been important: one of influence of both lithostatic overburdenand hydrostatic low salinity (2-10 wt% NaCl equivalent) and one of high pressure (i.e. p = 1.7), then the depth of formation of the salinity (10-24%). They suggested thatthe mineralization South Wales quartz would be approximately 3 km. If these was generated by mixing of Fe-bearing hydrothermal fluids assumptions are correct then the geothermal gradient at the generated from the coal-bearingsediments with saline time would be roughly 50°Ckm-'. The later the groundwaters derived from the overlying Triassic sediments. mineralization occurred in the history of basin development Galena fromseptarian nodules at Wyndham Colliery has the more the confining pressure would tend towards being been dated at 240 Ma by Fletcher et al. (1993). Furthermore, lithostatic in nature. This would tendto decrease the Harrison (1975) has shown thatconcretions in the necessary thickness of overburdenand increase the Permo-Triassic red beds of southwestEngland are often geothermalgradient (upto 75"C km-' under purely enriched in a characteristic suite of elements, such as Ni, CO, lithostatic conditions). White (1992) was uncertain about the As, V, U, and Cu. Thus it is possible that some of the metals geothermal gradient in the South Wales Coalfield, but in these geode sulphides were derived by leaching of suggested that it could havebeen between 20 and red-bed Triassic sediments by (post-Triassic) hydrothermal 37 "C km-' (but probably closer to the lower figure) based fluids. on aforeland basin settingfor the coalfield, and a thick Westphalian/Stephanian sedimentary cover. It is not clear which values are correct but similar high geothermal gradients have been noted in coal basin environments akin (b)Mississippi Valley type mineralization to thosehere. For instanceTeichmuller & Teichmuller (1986) suggested 60-80"C km-' , forEuropean Variscan Galena-bearing veins have been observed cuttingCar- foredeeps in a similar position to that of the South Wales boniferousLimestones tothe south of the Coalfield, and Coalfield. Furthermore, the pressures indicated by the fluid these sometimes also contain fluorite and solid hydrocarbons inclusions, are not in accord with burial under a thick (e.g. Bevins 1994; Parnell 1983). The age of this sediment pile. This is in keeping with the presence of mineralization is not known, but presumably the hydrocar- WestphalianA coals in WestphalianC and Dsandstones bons were derivedfrom the adjacent coal-rich Car- (Gayer & Pesek 1992), implying a very rapid (4Ma) boniferous sediments during hydrothermal activity. burial, compaction, and heating event in Westphalian times, Ni and S amongst other elements are particularly which would have occurredunder a relatively thin enriched in both coal and oil (e.g. Swaine 1990; Filby 1993) sedimentary cover. This in itself, implies a contemporaneous and millerite is often associated with coal-bearing sequences high heat flux. (e.g. Young & Nancarrow 1988; Dearman & Jones 1967). It is generallyagreed thatcarbonate nodules initially Parnell (1983) also found relatively high contents of formed in the top few metres of unconsolidated sediments vanadium in hydrocarbonsfrom South Wales ironstone but that growth was protracted and continued possibly down nodules. Goldaccompanied by various sulphides and to a depth of several hundred metres (Raiswell 1971; Curtis selenidesand carbonates has also recently been found et al. 1986; Oertel & Curtis 1986). There is some debate coatingfractures in coal from the northwestpart of the over the causes of the septarian cracks (e.g. Desrochers & Coalfield (Gayer & Rickard 1993) and could thus be related AI-Aasm 1993) although Astin (1986) hasput forward a tothe hydrothermal fluids responsible fornodule filling. strong argument in favour of theirformation as tensile This mineralization along fractures in coal ('cleats') is fractures in response to heightened pore fluid pressure. thought to be derived from overpressuredhydrothermal Astin (1986) suggested that such cracks could form at fluids mobilized and expelled during thrusting in the shallow sediment depths (as little as 50m)and that sedimentary basins (Gayer et al. 1991; Gayer 1993). We formation would be favoured during times of rapid sediment cannot explain theapparent Pb-isotopic age forthe burial. However,although much work hasbeen carried Wyndham galena, but note that the dating of galena is not out on the timing of nodule growth and crack propagation, always straightforwardand the results can often be it is less easy to establish the timing of mineral infilling. interpreted in different ways, depending on the model Indeed Astin (1986) and Astin & Scotchman (1988) invoked. have demonstratedthat more than one phase of calcite We therefore propose that the hydrocarbons developed infillings of septarian cracks can occur, and that these can as a result of thematuration of organic matter in the sometimes be related to the history of sediment burial and adjacent sediments and that the sulphides were generated by uplift. the associated leaching of these sediments by hydrothermal Although we favoura model for mineralization which fluids. The logical conclusion is thatthe quartz was also is closely associated with the burial of the coal-bear- related tothe dewatering of the organic-rich sedimentary ing sediments, two other alternatives must also be consid- pile. If correct then the age of the mineralization would be ered. Carboniferous, and would reflect rapid burial and heating of

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the basin prior to the onset of the Variscan orogeny. It is BEVINS,R.E. 1994. A Mineralogy of Wales. National Museum of Wales, proposed that the nodules were able to protect the enclosed Geological Series no. 16, Cardiff. - & HORAK,J. M.1985. Siegenite in clay-ironstonenodules from the mineralization fromsubsequent geological events,and the South Wales Coalfield. Journal of the Russell Society, 1, 83-85. effects of Variscan metamorphism and deformation were not BODNAR,R.J. 1993. Revised equation and table for determining the freezing therefore felt in the nodules. pointdepression of H,O-NaCI solutions. Geochimica et Cosmochimica Acta, 57,683-684. BLJRRUSS,R.C. 1992. Phasebehavior in petroleum - water(brine) systems applied to fluid inclusion studies. 4th biennial pan-American conferences Conclusions on research on fluidinclusions. University of California Riverside, Programme and abstracts, 116-118. This study has shown that quartz in siderite nodules from CHAMBERLAIN, S.C. 1988. On the origin of “Herkimer diamonds”. Rocks and the South Wales Coalfield probably grew at P-T conditions Minerals, 63, 454-455. CURTIS,C.D., COLEMAN,M.L. & LOVE, L.G. 1986 Porewater evolution of around150°C and 500 bars (fluid pressure). The duringsediment burial from isotopic andmineral chemistry of calcite, hydrothermal fluids responsiblefor the mineralization dolomite and siderite concretions. Geochimica et Cosmochimica Acta, 50, consisted of an immiscible mixture of methane-rich 2321-2334. hydrocarbon gas and low salinity aqueous brine. The DEARMAN.W.R. & JONES,J.M. 1967. Millerite from Boldon Colliery, County Durham. Transactions of the Natural History Society of Northumberland, aqueouscomponent of the fluids was probably originally 16, 193-196. meteoric orconnate in origin and was expelled from the DESROCHERS,A. & AL-AASM,IS. 1993. Theformation of septarian compactingsediments during rapid burial and subsequent concretions in Queen Charlotte Islands, B.C.: Evidence for microbially dewatering. At present we are uncertain about the precise and hydrothermally mediated reactions at shallow burial depth. Journal of Sedimentary Petrology, 63,282-294. age and therefore the genesis of the nodule mineralization. FILBY.R.H. 1993. Originand trace element species in crude oils, bitumens However, the majority of the evidence tends to favoura andkerogens: implications for correlation studies. In: PARNELL,J., syn-metamorphic,burial-related mineralization because of RUFFELL, A.H.& MOLES,N.R. (eds) Geofluids ‘93 (extended abstracts of the morphology of the quartz; its formation within ironstone conference), 404-409 nodules; the dominance of Ni- and CO-sulphides over base FIRTH,J. N. M. 1973. The mineralogy of the South WalesCoalfield. PhD thesis, University of Bristol. metalsulphides; the presence of methane-rich fluid - 1972. 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Received 14 March 1995; revised typescript accepted 26 October 1995. Scientific editing by Kate Brodie.

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