Weardale Granite Research Articles

Regional variations in fluid formation and metal sources in MVT mineralization in the Pennine Orefield, UK: Implications from rare earth element and yttrium distribution, Sr-Nd isotopes and fluid inclusion compositions of hydrothermal vein fluorites

The Pennine Orefield is one of the most important ore fields for Pb-Zn-Ba-F mineralization in Great Britain. It is subdivided into the Northern Pennine Orefield (NPO), consisting of the Alston and Askrigg Blocks, and the Southern Pennine Orefield (SPO). The Alston Block is underlain by the early Devonian Weardale Granite and the Askrigg Block by the coeval Wensleydale Granite. The potential relationship between the batholiths and the mineralization is a matter of debate. We here studied the rare earth elements and Y (REY) geochemistry, Sr-Nd isotopes and fluid inclusion (FI) compositions of fluorites from the two structural blocks in the NPO and found that the fluorite mineralization in these blocks differ substantially. The REY in Askrigg fluorites show features that are characteristic for leaching of adjacent Lower Carboniferous limestones. In contrast, Alston fluorites have significantly higher REY concentrations, lack REYSN limestone signatures and show a decoupling of redox-sensitive Eu from its trivalent REY ‘neighbours’. Neodymium isotopes indicate a similar crustal source of REY in both blocks, but higher REY concentrations and lower Y/Ho ratios suggest Lower Carboniferous shales as potential REY source in the Alston Block. The fluids that precipitated the Alston fluorites experienced temperatures > 250 °C prior to mineral formation, as evidenced by Eu geothermometry. Fluorite formation, however, occurred at much lower temperatures, as suggested by homogenization temperatures in FI, that fall within ranges of 105–159 °C in Alston and 99–160 °C in Askrigg fluorites. Mineralization of the Mississippi-Valley Type usually lack association with igneous activity. We show that some of the fluids responsible for the NPO mineralization were influenced by magmatic sources. The REY systematics in Alston fluorites may be linked to an interaction of the Permian-age Whin Sill dolerite with the basement granite, which heated fluids and focussed fluid flow into the overlying sedimentary rocks. In the Askrigg Block, where such a dolerite intrusion was not described, fluorites lack any positive EuSN anomalies, indicating that these fluids had never been subjected to temperatures exceeding 200–250 °C.

Emplacement of oil in the Devonian Weardale Granite of northern England

Oil residues occur as solid bitumen in mineralized zones within the Devonian Weardale Granite of the northern Pennines, northern England. Comparable residues are present in the overlying Mississippian rocks and were probably derived from a Carboniferous source, i.e. during later mineralization of the granite. The bitumen was already solidified during fluorite mineralization, which does not contain oil inclusions. The residues do not show the high thermal maturity of organic matter in the region altered by the earliest Permian Whin Sill. Like the sulphide-fluorite mineralization, oil emplacement post-dated intrusion of the sill. Pyrite associated with the oil residues is enriched in trace elements including lead, silver, gold, selenium and tellurium, which suggests that mineralizing fluids at least shared pathways with migrating hydrocarbons and possibly also suggests undiscovered valuable metal resources.

The role of the Devonian Weardale Granite in the emplacement of the North Pennine mineralization

After initial mapping of the fluorite/barite zones (1926) then discovery of the underlying unexposed Weardale Granite (1953), and proving its early Devonian age by the Rookhope borehole, extensive geochemical/mineralogical investigations indicate a likely alkali magmatic contribution to the post-Whin Sill orefield, contrasting with most other Hercynian Mississippi-type orefields in UK. Creaney's mapping of rank of early Namurian coals above the two central cupolas of the granite implied high temperatures of over 200°C just prior to the Whin Sill emplacement (297.4 Ma) in concord with an underlying magmatic source lasting 1 – 5 Ma. Their sharp boundaries unequivocally indicate that heat rose through the granite by hydrothermal convection, not conduction. A Stephanian source of quartz-tholeiitic magma is consistent with the Scottish Midland Valley succession (302 – 295 Ma) and contemporaneous Whin magmatism. The magma underplated the granite because of its higher density. It prepared conditions for the subsequent post-Whin Sill emplacement of the mineral deposits, producing the hot convection cell which drew in saline fluids from adjacent deep Carboniferous troughs. Following Whin Sill emplacement, highly pressurized early Permian alkali basaltic magma also underplated the granite. This was contemporaneous with the succession in the Scottish Midland Valley (age 298 – 292 Ma). Hot mineralizing fluids were forced upwards into the established convection cell in the granite and then into the Carboniferous of the Fluorite Zone, forming the mineral deposits, and then outwards beyond the granite cupola regions to deposit barite at c . 50°C.

Development of deep geothermal energy resources in the UK

Deep geothermal exploration in the UK in the 1980s resulted in only one operative borehole (in Southampton). In 2004, a 995 m deep borehole was drilled into the Weardale granite in County Durham. This project differed markedly from the earlier ‘hot dry rock’ investigations in the granites of Cornwall, in that it deliberately targeted possible high natural permeability derived from large fault zones. The new approach met with success, discovering what is believed to be the highest natural permeability ever found in granite anywhere in the world. Drilling of a second borehole at Eastgate in 2010 confirmed that high permeability is preferentially associated with major west–east geological faults. Further deep drilling is now targeting similar faults cutting nearby sedimentary sequences. Meanwhile, renewed interest is being shown in Cornish prospects, as well as in previously unexplored resources in Scotland and Northern Ireland. There is also significant interest in the possible geothermal exploitation of hot brines, which are co-produced with hydrocarbons in existing oilfields. Technological advances in directional drilling, reservoir stimulation and power generation using binary-cycle turbine plants are finally making deep geothermal energy a realistic target for full-scale development in the UK.

Discussion on ‘Hyper-permeable granite: lessons from test pumping in the Eastgate Geothermal Borehole, Weardale, UK’ by P.L. Younger & D.A.C. Manning

The authors have described some unexpected properties from a mineralized fracture system associated with Slitt Vein in the Weardale Granite. They ask whether the presence of open fissures might be more commonplace in granites than is currently supposed. Their suggestion that the main cavity, intersected at 410–413 m in the granite, might result from dissolution by deep weathering at the Carboniferous–Devonian unconformity is interesting, but other possibilities exist. The borehole was sited on a complex quartz–fluorspar mineral vein, and had passed through several mineral-filled veins. Examples of dissolution of one phase of ‘Pennine Style’ mineralization and replacement by …

The North Pennine batholith (Weardale Granite) of northern England: new data on its age and form

SUMMARY A three-dimensional gravity model of the North Pennine batholith is interpreted to show that it comprises five contiguous steep-sided plutons. The North Pennine batholith was previously referred to as the Weardale Granite, but this term is now restricted to the largest, most central pluton, which was proved by the Rookhope Borehole in 1961. The surrounding plutons are the Tynehead and Scordale plutons in the west, and the Rowlands Gill and Cornsay plutons in the east. A new U–Pb zircon age of 399.3±0.7 Ma demonstrates an unequivocal Early Devonian (Emsian) age of emplacement for the Weardale Pluton. The similarity of the forms of the four unsampled plutons to the body beneath Rookhope, and to other Devonian granites in northern England, strongly suggests that they are also of Devonian age. Seismic reflection evidence supports a contrast between relatively deep-seated Devonian granites and tabular Ordovician granites in northern England. The most conspicuous magnetic anomalies over the North Pennine batholith are associated with the Early Permian Great Whin Sill, but there is also evidence of magnetized basement rocks or denser magnetic intrusive phases on the flanks of the non-magnetic, low density plutons. A long-wavelength magnetic low can be explained in part by the granite puncturing a deep magnetic basement, although the demagnetized zone may extend beneath the batholith. A spatial correlation between the vein systems in the Northern Pennine Orefield and the form of the batholith suggests that the granite masses played a significant role in focussing mineralizing fluids into the overlying Carboniferous rocks. The highest temperature mineralization occurs over the Weardale and Tynehead plutons, where there is also the clearest evidence for the channelling of mineralizing fluids through the batholith. The other plutons are generally associated with a lower intensity of mineralization, which occurs on their flanks rather than above their roofs. The Rookhope and Tynehead areas may therefore mark the location of convective ‘chimneys’ which were important in focussing the fluid circulation responsible for the mineralization. The coincidence of the ‘chimney’ zones with the minimum of the long wavelength magnetic low suggests that the hydrothermal system may also have played a role in modifying crustal magnetization.

Hyper-permeable granite: lessons from test-pumping in the Eastgate Geothermal Borehole, Weardale, UK

Abstract Extremely high permeability has been found at 410 m depth in the Weardale Granite, UK, during exploratory drilling for low-enthalpy geothermal resources. The Eastgate Borehole was sunk down the axis of an ancient, sub-vertical hydrothermal vein structure (the Slitt Vein), proving a transmissivity in excess of 4000 darcy m (= 3 × 10 −9 m 2 m), which is one of the highest values ever measured deep within a granite intrusion. Sustained hydraulic testing over 24 h showed that this transmissivity was not local to the borehole: sustained flows and steadying of drawdown suggest that this transmissivity persists along the strike of the Slitt Vein structure. Although this finding is encouraging in relation to the prospects for geothermal energy development in this and similar granites, it raises disquieting issues in relation to assumptions commonly made during planning for radioactive waste isolation concerning the feasible maximum permeability likely to be encountered in granite.

Geochronology (Re–Os and U–Pb) and fluid inclusion studies of molybdenite mineralisation associated with the Shap, Skiddaw and Weardale granites, UK

Late Devonian magmatism in Northern England records key events associated with the Acadian phase of the Caledonian-Appalachian Orogen (C-AO). Zircon U–Pb and molybdenite Re–Os geochronology date emplacement and mineralisation in the Shap (405·2±1·8 Ma), Skiddaw (398·8±0·4 and 392·3±2·8 Ma) and Weardale granites (398·3±1·6 Ma). For the Shap granite, mineralisation and magmatism are contemporaneous, with mineralisation being directly associated with the boiling of CO2-rich magmatic fluids between 300 and 450°C, and 440 and 620 bars. For the Skiddaw granite, the Re–Os age suggests that sulphide mineralisation occurred post-magmatism (398·8±0·4 Ma) and was associated with the boiling (275 and 400°C and at 375–475 bars) of a non-magmatic fluid, enriched in N2, CH4 and S, which is isotopically heavy. In contrast, the co-magmatic molybdenite mineralisation of the Weardale granite formed from non-fluid boiling at 476 to 577°C at 1–1·7 kbars. The new accurate and precise ages indicate that magmatism and Mo-mineralisation occurred during the same period across eastern Avalonia (cf. Ireland). In addition, the ages provide a timing of tectonism of the Acadian phase of the C–AO in northern England. Based on the post-tectonic metamorphic mineral growth associated with the Shap and Skiddaw granite aureoles, Acadian deformation in the northern England continued episodically (before ∼405 Ma) throughout the Emsian (∼398 Ma).

A deep geothermal exploration well at Eastgate, Weardale, UK: a novel exploration concept for low-enthalpy resources

The first deep geothermal exploration borehole (995 m) to be drilled in the UK for over 20 years was completed at Eastgate (Weardale, Co. Durham) in December 2004. It penetrated 4 m of sandy till (Quaternary), 267.5 m of Lower Carboniferous strata (including the Whin Sill), and 723.5 m of the Weardale Granite (Devonian), with vein mineralization occurring to 913 m. Unlike previous geothermal investigations of UK radiothermal granites that focused on the hot dry rock concept, the Eastgate Borehole was designed to intercept deep fracture-hosted brines associated with the major, geologically ancient, hydrothermal vein systems. Abundant brine (≤46 °C) was encountered within natural fracture networks of very high permeability (transmissivity c . 2000 darcy m) within granite. Evidence for the thermal history of the Carboniferous rocks from phytoclast reflectance measurements shows very high values (≥3.3%) indicating maximum temperatures of 130 °C prior to intrusion of the Whin Sill. Geochemical analysis of cuttings samples from the Eastgate Borehole suggests radiothermal heat production rates for unaltered Weardale Granite averaging 4.1 μW m −3 , with a mean geothermal gradient of 38 °C km −1 . The Eastgate Borehole has significant exploitation potential for direct heat uses; it demonstrates the potential for seeking hydrothermal vein systems within radiothermal granites as targets for geothermal resources.

Aberrations in the coalification patterns of the offshore coalfields of Northumberland and Durham, United Kingdom

The offshore extensions of the Northumberland and Durham Westphalian coalfields are formed of continuous and linked sequences of sediments which lie in contrasting structural regimes, respectively ‘Basin’ and ‘Block’. The coalfields have suffered different geothermal histories. Offshore Northumberland (‘Basin’) sediments endured low heating rates and low maximum temperatures to give seams which, even at the base of the succession, hardly reached vitrinite reflectances of Roil=0.60%, apart from raised values due to igneous activity. The offshore Durham (‘Block’) succession was exposed to higher temperatures and heating rates due to the proximity of the Lower Devonian Weardale granite underlying the ‘Block’, which was re-activated during Carboniferous times. These geothermal conditions not only led to higher levels of reflectance and steeper maturation gradients offshore in the ‘Block’ succession than offshore in coals of the ‘Basin’, but also produced contrasting relationships between correlations of vitrinite oil reflectance and volatile-matter yield of coals for the two structural zones—and probably also for other property relationships as well. Reflectance profiles have been modified due to three circumstances: first, considerable variation locally in the raising of reflectance levels caused by igneous activity; second, lowering of vitrinite reflectance values due to arenaceous sediments being deposited immediately on top of vitrinite precursors within the coal-forming organic matter; and third, possible interference from liptinite-rich accumulations within coal seams or near to them in the succession, which exuded aliphatic materials relatively early in coalification which were then absorbed by vitrinites or, more likely, deposition of the vitrinite precursors in a marine/near marine and calcium-rich environment, so reducing reflectance values on coalification. Similar contrasting property relationships to those described here may have been produced by Lower Devonian granitoid bodies which have been identified on the North Sea High, if they also were re-activated during the Carboniferous.

Aberrations in the coalification patterns of the offshore coalfields of Northumberland and Durham, United Kingdom

The offshore extensions of the Northumberland and Durham Westphalian coalfields are formed of continuous and linked sequences of sediments which lie in contrasting structural regimes, respectively ‘Basin’ and ‘Block’. The coalfields have suffered different geothermal histories. Offshore Northumberland (‘Basin’) sediments endured low heating rates and low maximum temperatures to give seams which, even at the base of the succession, hardly reached vitrinite reflectances of Roil=0.60%, apart from raised values due to igneous activity. The offshore Durham (‘Block’) succession was exposed to higher temperatures and heating rates due to the proximity of the Lower Devonian Weardale granite underlying the ‘Block’, which was re-activated during Carboniferous times. These geothermal conditions not only led to higher levels of reflectance and steeper maturation gradients offshore in the ‘Block’ succession than offshore in coals of the ‘Basin’, but also produced contrasting relationships between correlations of vitrinite oil reflectance and volatile-matter yield of coals for the two structural zones—and probably also for other property relationships as well. Reflectance profiles have been modified due to three circumstances: first, considerable variation locally in the raising of reflectance levels caused by igneous activity; second, lowering of vitrinite reflectance values due to arenaceous sediments being deposited immediately on top of vitrinite precursors within the coal-forming organic matter; and third, possible interference from liptinite-rich accumulations within coal seams or near to them in the succession, which exuded aliphatic materials relatively early in coalification which were then absorbed by vitrinites or, more likely, deposition of the vitrinite precursors in a marine/near marine and calcium-rich environment, so reducing reflectance values on coalification. Similar contrasting property relationships to those described here may have been produced by Lower Devonian granitoid bodies which have been identified on the North Sea High, if they also were re-activated during the Carboniferous.

Constraints on the genesis of the mineralization of the Alston Block, Northern Pennine Orefield, northern England

SUMMARY New evidence from fluid inclusion compositions and simple physical modelling provide important constraints on the genesis of the ores of the Alston Block in northern England. Evidence from fluid inclusions shows that probably at least four, hydrothermal fluids were involved, two of which are formed by evaporation of seawater, in some cases well beyond halite saturation. Some of the high-temperature fluid dissolved evaporites during its evolution. Physical modelling shows that neither compactional dewatering of the adjacent Carboniferous sedimentary basins nor compressional tilting and throughflow within the Northumberland Trough could have contributed significantly to the hydrothermal fluids of the orefield. Most of the high temperature hydrothermal flow must have been generated by penetrative circulation of surface brines to 8 to 10 km depth in the basement, with the upflow focused and accelerated by the high heat production of the Weardale Granite that unconformably underlies the Carboniferous rocks of the Alston Block. The orefield is considered to have formed during the Upper Permian by the following sequence of events. Initially, fluid movement in the basement was prevented by sealing of pre-existing cracks, and temperatures rose at depth. An episode of extension allowed saline water, probably from the Upper Permian Zechstein Sea, to penetrate deep into the basement, reacting with the rocks there, and becoming heated to as much as 200°C. Penetration to 10 km took a few tens of thousands of years. The granite, at 300°C at that depth, up to 100°C hotter than the surrounding basement, acted to focus the flow, and to generate concentric zones of increasing temperature fluid towards the centre. Once the basement had become cooled, the flow slowed, cracks became sealed, and the episode of hydrothermal circulation and ore formation was complete.

Stress And Seismicity In The Lower Continental Crust: A Challenge To Simple Ductility And Implications For Electrical Conductivity Mechanisms

The lower crust is generally considered to be an aseismic, weak zone where fluid distribution might be governed by textural equilibrium geometries. Saline fluids below the transition from brittle to ductile rheology have been advanced as a joint explanation for deep crustal conductivity and seismic reflectivity, the depth of onset of both phenomena being apparently bounded by isotherms in the 300-450 °C temperature range. Some petrologists, meanwhile, contest that the deep crust should be devoid of extensive fluid networks. This review exposes some geophysical exceptions to the statistical norm suggested by global geophysical data compilations and presents counter-arguments that the lower crust in places may be both dry and strong, that fluids if at all present at such depths may not necessarily be connected and that fluid mobility in the lower crust may be more limited and heterogeneous than commonly assumed. Laboratory data on crustal rocks implies that the transition from brittle to ductile rheology actually occurs over a much broader range of temperatures than 300-450 °C, and the apparent association of deep crustal conductive horizons with a temperature field of 300-450 °C may be interpretable in terms of formation temperatures of graphite, rather than fluids and brittle-ductile transition rheology. High v P /v S ratios from a 6 km thick, seismically layered zone below the Weardale granite, NE England can be explained by underplated mafic material. They are unlikely to be explained by fluids in an area where deep crustal conductance has been shown to be relatively low, unless conventional assumptions regarding deep crustal fluid distribution are inadequate or false. Perusal of the literature reveals that lower crustal seismicity is less seldom than generally appreciated. Interpretation of earthquakes nucleating at lower crustal depths is ambiguous, but in some tectonic regimes may indicate preservation of brittle rheology to the Moho and a lower crust that is predominantly mafic and dry. A better understanding of lower crustal deformation mechanisms and history may provide better insight into deep crustal conductivity mechanisms. Recent rock mechanical experiments suggest that permeability (and thus fluid connectivity) may be decreased by ductile shearing, whereas ductile shearing may aid graphitisation at lower crustal temperatures. If the lower crust in some regions is strong, this may explain the apparent preservation of both extant- and palaeostress orientations in interpretations involving electrical anisotropy.

Coincident magnetotelluric,P-wave andS-wave images of the deep continental crust beneath the Weardale granite, NE England: seismic layering, low conductance and implications against the fluids paradigm

SUMMARY It has been postulated that deep crustal zones of both enhanced electrical conductivity and seismic layering may be ascribed a single physical explanation, and more specifically that both result from the presence of free, interconnected fluids around the brittle‐ ductile transition zone. We analyse three complementary geophysical data sets— broadband (10’2‐104 s) magnetotelluric (MT) and P-wave and S-wave deep seismic reflection—from coincident profiles traversing the Weardale granite in northeast England and demonstrate that the lowermost crust in this tectonically stable region contains insuY cient free fl uid to explain the highv P /v S ratios associated with very bright, subhorizontal lamellae seen within the lower crust (below 22 km depth) beneath the granite. Temperature‐depth profiles are presented, and the apparent global correlation between the depths to the tops of zones of enhanced conductivity, seismic reflectivity and inferred brittle‐ductile temperatures of 300‐450°C is also re-examined. The seismic layering demonstrably corresponds to higher temperatures than those usually attributed to the brittle‐ductile transition zone. A priori information of diVerent scalelengths is applied to the problem of static-shift correction of the MT data. Gravity models constrain the depth extent of the granite batholith, whilst pre-existing DC measurements and a borehole drilled into the granite provide information concerning the resistivity structure of the uppermost 500 m. A 3-D model study investigates the significance of possible 3-D induction eVects on the MT data, as implied following application of decomposition. Our results challenge the widespread and little-tested assumption that deep crustal conductors and seismic layering are physically interrelated and occur at the same depth. With static shift and 3-D eVects in the MT data accounted for, we demonstrate that below the Weardale granite the onset of any enhanced conductivity is shallower than the top of the seismic layering, and furthermore that the observed deep crustal conductance of less than 200 S is significantly less than that to be expected (>2000 S) if the high v P /v S ratios calculated from the seismic data are to be explained entirely by fluids. The total conductance of the mid to lower crustal region combined is too low to allow for the significant quantities of free fluids required to explain lower crustal seismic layering through this medium without challenging current hypotheses concerning pore geometries and interconnectivity.

Bismuth-bearing assemblages from the Northern Pennine Orefield

AbstractBismuthinite-bearing quartz veins from the Alston Block of the North Pennine Orefield are all close to, or above, the Rookhope and Tynehead cupolas of the buried Weardale Granite. They are uniform in composition and paragenesis and are earlier than the main fluorite-baryte-galena-sphalerite mineralization of the orefield. Rhythmical crystallization of quartz, chalcopyrite and minor pyrite is followed by fluorite-quartz-chalcopyrite-minor sphalerite-altered pyrrhotite mineralization. Early tin-bearing (up to 0.29 wt.% Sn) chalcopyrite encloses trace amounts of bismuthinite (Bi2S3), synchysite (CaREE(CO3)F2), argentopentlandite (Ag(FeNi)8S8) (close to being stoichiometric), pyrrhotite, cubanite and cosalite (Pb2Bi2S5), while early pyrite carries monoclinic pyrrhotite (close to Fe7S8) and tungsten-bearing cassiterite (up to 1.03 wt.% WO3). Bismuthinite is macroscopically visible and is associated with native bismuth and small, fine-grained, spherical aggregates that qualitative analysis suggests may be cosalite crystals. Synchysite and more rarely monazite, xenotime and adularia are intergrown with bismuthinite. These mineralogical data form part of the basis for an increasing awareness of the contribution of the Weardale Granite to the early phases of mineralization in the Alston Block.

Hydrothermal orefields and ore fluids

Abstract The development of Hydrothermal Theories of Ore Genesis during the past 150 years owes much to the pioneering work of two eminent British geologists, H. C. Sorby and K. C. Dunham, who published benchmark papers in the Quarterly Journal of the Geological Society on the classic mining districts of Cornwall and the English Pennines. Sorby’s paper of 1858 laid the foundation for fluid inclusion studies. Despite considerable scepticism or indifference that has lasted well into the twentieth century, fluid inclusions are now widely regarded as the best way of establishing the nature and composition of ancient mineral-forming fluids. Dunham’s paper of 1934 provides a classic account of mineral zonation away from a focus of mineralization in the North Pennine orefield. By analogy with the Cornish deposits this focus was considered to be the result of ascending mineralizing fluids from a hidden granite at depth. Although subsequent drilling confirmed the existence of granite (the Weardale granite) directly beneath the North Pennine Orefield it was older than the Carboniferous Limestone which hosts the mineralization and therefore cannot be its direct source. With the ‘coming of age’ of fluid inclusion techniques in the 1960s, widespread and systematic studies were carried out on samples from the Cornubian and Pennine orefields and other classic mining districts of the world over the succeeding three decades. These, together with stable isotope studies, established that hydrothermal, ore-forming fluids cover a wide temperature range (50° to >500°C) and compositional range (0 to >60 wt % dissolved salts), and are of diverse origin. The composite and protracted nature of hydrothermal events in Cornwall, and indeed in areas of the world where mineralization is spatially associated with granites, is now evident. Most authors agree that, whilst components of the early mineralization may be due to metalliferous fluids directly evolved from cooling granite bodies, much of the later base-metal mineralization is due to thermal convection and pulsation of meteoric fluids and basinal brines within the intrusion and surrounding country rocks. The basinal brine expulsion theory is generally favoured for carbonate-hosted, epigenetic, base metal deposits of the Mississippi valley type. The nature, geological setting and fluid inclusion characteristics place the Pennine ores clearly in this broad class. However, most recent work suggests that the mineral zonation patterns in the northern part and temperature differences between the northern and southern part are still best explained by the hydrogeological and geochemical influence of granite at depth.

Carboniferous biostratigraphy of the Rookhope Borehole, Co. Durham

AbstractA deep borehole drilled at Rookhope in Weardale, Co. Durham in 1960–61 proved the Weardale Granite below Carboniferous sediments at a depth of 390 m; an account of the Carboniferous rocks is presented. The boring commenced in the basal Namurian Great Limestone and entered the Dinantian at the bottom of the limestone at 25 m. Most of the ten Brigantian cyclothems sectioned in the borehole are compound and average 30 m thick. They consist of an initial cycle often more than 20 m thick and overlying minor cycles that are usuallyless than 6 m thick. Each of the cycles, both initial and minor, are coarsening upwards sequences from marine limestone or mudstohe to deltaic and fluvial mudstone, sandstone, seatearth and occasionally a thin coal seam. The underlying Asbian consists of seven cyclothems averaging 8 m thick. By contrast they are simple coarsening upwards sequences similar to the Brigantian initial cycles, but no minor cycles are developed.Marine fossils are abundant in the majority of cyclothems and are listed against their stratigraphical horizon in the borehole. Special studies of miospores, conodonts and foraminiferans from the borehole are presented. A consensus of biostratigraphical evidence shows that the base of the Carboniferous succession in the borehole is of early Asbian age.

Magnetotelluric studies in and adjacent to the Northumberland Basin, Northern England

During the past decade broadband magnetotelluric (MT) soundings, with d.c. resistivity soundings at some sites, have been undertaken in three separate field studies in and around the Northumberland Basin, a region of great interest to earth scientists on account of the proposed location there of the Iapetus Suture. As a result of an increase in cultural noise during this period, the data from the last two studies have been processed using a new robust constrained impedance tensor estimation program. The resulting apparent resistivity and phase data from these studies, together with those from the first broadband study and some earlier MT responses from the region, have now all been modelled using an interpretative modelling procedure. New information has been provided by the MT models on basement depths and, by integrating these new estimates with those from gravity modelling and seismic studies both on land and offshore, a detailed basement topography map has been compiled for the region. The deep eletrical resistivity structure has been modelled along a NW-SE traverse from the Weardale Granite of the Alston Block across the Northumberland Basin to the Southern Uplands of Scotland. Underlying the more conductive sedimentary rocks, the basement rock is found to have resistivities which range from about 100 μ m in the Northumberland Basin to more than 1000 μ m in the Alston Block and probably of the same order in the Southern Uplands. A mid-crustal conductor exists along the whole traverse, which is well resolved and has a southward dip beneath the Weardale Granite. Under the Northumberland Basin, the conductor is less well resolved and thus an apparent northward dip can only be regarded as tentative. Comparison of the pseudo-2D and full 2D models resulting from this study and from earlier MT and magnetovariational (MV) studies in Southern Scotland with new MT and joint MT and MV inversions of Livelybrooks et al. (Phys. Earth Planet. Inter., 81: 67–84 (1993)) for a subset of the same response estimates reveals both comparable and disparate features. The features common to the models presented in this paper and at least one of the inversions of Livelybrooks et al. include differences in the resistivity and thickness of the upper crust in each of the Alston Block, the Northumberland Basin and the Southern Uplands, and a conducting zone at mid-crustal depths which vary along the traverse from about 10 km near the southern edge of the Northumberland Basin to more than 30 km under the Alston Block and about 30 km in the southern part of the Southern Uplands. The main difference between the pseudo-2D and the 2D forward models and the 2D inversions of Livelybrooks et al. is in the resistivity of the Alston Block, the former being more than 1000 μ m whereas the latter is about 400–500 μ m, a value which is unexpectedly low for a crust of known granitic composition.

Interpretation of Gravity Anomalies Over an Inclined Fault of Finite Strike Length With Quadratic Density Function

The equation for the gravity anomaly of an inclined fault of finite strike length with quadratic density function is derived. The coefficients of the density function and the strike length are assumed to be known. Synthetic anomaly profiles of the fault model for the half strike lengths Y=2, 5, 10, 25, 50 and 100 km and for various values of dip and depths to top and bottom, are treated as unknown parameters. These parameters are solved by the Marquardt's algorithm. The convergence of the method is shown by plotting the values of the objective function, the damping parameter lambda and the various parameters, with respect to iteration number. The same anomaly profiles are interpreted for different initial models. The method converges for all cases. A field profile across the Weardale granite, in northeast England, is interpreted as two faults by the present method.

Migration of biogenic hydrocarbons into granites: a review of hydrocarbons in British plutons

The occurrence of hydrocarbons (bitumens senso lato) in granite plutons and other basement rocks is not per se evidence of an abiogenic origin for the hydrocarbons. On the contrary, plutons are thermal foci and often highly fractured, and therefore are hosts to the migration of fluids from outside the plutons, including water and biogenic hydrocarbons. Hydrocarbons are associated with several British granites. In all cases they can be attributed to biogenic sedimentary source rocks. The occurrences are particularly related to hydrothermal mineralized veins in granites, including the Cornish Hercynian granites, the Mountsorrel Granodiorite and the Weardale Granite.