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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/2/265/4889061/gsjgs.153.2.0265.pdf by guest on 01 October 2021 266 D. H. M. ALDERTON & R. E. BEVINS examined in the collection of the NationalMuseum of under incident 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