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Surface-derived fluids in basement rocks:inferences from palaeo-hydrothermal systems
Available online at www.sciencedirect.com SCIENCE
ELSEVIER
~DIRECT e
Journal of Geochemical Exploration 78-79 (2003) 61-65
JOURNAL OF
GEOCHEMICAL EXPLORATION www.elsevier.com/locate/jgeoexp
Abstract
Surface-derived fluids in basement rocks: inferences from palaeo-hydrothermal systems S.A. Gleeson a'*, B.W.D. Yardley b "Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Canada T6G 2E3 bSehool of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK
Abstract
Drilling into modem systems such as the KTB borehole in Germany indicates that the upper crest is pervasively fractured and saturated with surface-derived fluids at hydrostatic pressures. The recognition of surface-derived fluids in palaeohydrothermal systems is limited to the effectiveness of geochemical tracers. At low total fluid fluxes, fluids tend to interact with wall rocks and become isotopically, chemically and thermally equilibrated. At high fluid fluxes, such as found in upper crustal mineralisation systems, fluid inclusion halogen systematics can be used to identify the origin of the fluids, even when the fluids have equilibrated isotopically and thermally with deeper parts of the hydrothermal system. Unlike the modem systems, fluid inclusions in vein fills in both mineralised and unmineralised systems show strong evidence for episodic fluid flow and fluctuating fluid pressures, even in upper crustal rocks. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Fluid flow; Hydrostatic; Lithostatic;Hydrothermal;Fluid inclusions
1. Introduction
Although hydrogeologists have long known that surface-derived groundwaters can penetrate deep aquifers, traditionally, studies o f hydrothermal mineralisation in basement rocks have emphasised the movement o f hot fluids from depth to shallower levels, rather than the reverse. Initially, deep penetration o f surface waters was recognised from the resulting meteoric water oxygen isotope signature around batholiths (e.g., Taylor, 1977). Subsequently, oxygen
* Corresponding author. Fax: +1-780-492-2030. E-mail address." [email protected] (S.A. Gleeson).
isotopes have been used to identify extensive meteoric water involvement in retrogression at temperatures to about 350 °C and depths to about 8 km in metamorphic core complexes (e.g., Morrison, 1994), and in middle crustal shear zones (Read and Cartwright, 2000). Isotopic studies on mesothermal gold deposits from the Canadian Cordillera suggest that meteoric waters involved in ore formation have circulated to depths o f 1 2 - 1 5 km (Nesbitt and Muehlenbachs, 1989). In many instances, however, it is likely that low fluxes o f penetrating surface waters are not tracked by the oxygen isotope record. For example, Smith et al. (1998) showed that fluids flowing through fractures in granite at ca. 2 km depth in the Soultz experimental
0375-6742/03/$ - see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0375-6742(03)00147-X
62
Abstract
HDR borehole, France had an oxygen isotopic composition resulting from equilibration with the host granite at the local temperature. In this paper, we seek to examine some different flow regimes where there is evidence for the movement of sedimentary brines into basement rock, unrelated to any temperature gradients created by magmatism.
2. Permeability and fluid pressures in basement rocks While the importance of fractures on the bulk permeability of shallow crystalline rocks has been understood for some time (Brace, 1984), one of the recent advances in the understanding the permeability structure of the crystalline upper crust has come from tests in deep bore holes, e.g., KTB, Urach-3, Soultz and Kola Peninsula (e.g., M611er et al., 1997; Stober and Bucher, 2000). These data have shown that crystalline basement rocks can have significant fracture permeability to depths of 9 km, although to some degree this is a function of the rock type. For example, although the total permeability of the metamorphic rocks in the Black Forest is 2.14 x 10 - 7 m/s, for granites and gabbros, the values are ~ 9.55 x 10 7 and 5.01 x 10 8 m/s, respectively (Stober and Bucher, 2000). The data from the KTB borehole confirmed the view that the crystalline upper crust is close to a state of brittle failure at depths of 9 km (e.g., Zoback and Harjes, 1997). Overpressured systems are well known from oil fields, and many other settings have been inferred to have been characterised by near-lithostatic fluid pressures, including most prograde metamorphic environments, some mid-crustal hydrothermal systems (e.g., lode gold deposits) and many shear and thrust fault systems (e.g., Parry and Bruhn, 1990; Sibson, 1990; McCaig et al., 2000). In some hydrothermal gold deposits, there is also evidence for vein formation at supra-hydrostatic pressures (Kontak et al,, 1996). In the deep crust, Yardley and Valley (1997) pointed out that fluid overpressures are normally present in settings where fluid is expelled as rocks are buried and heated; in contrast, due to absorption by retrograde reactions, fluid pressures are characteristically very low in cooled crystalline crust, except transiently, along pathways of fluid infiltration.
3. Systems with low fluxes Where fluid fluxes are very low, surface-derived brines may be present in crystalline rocks, but react so extensively with their hosts that they lose the chemical signature of their source, while the host undergoes mineralogical changes (hydration) without necessarily experiencing large changes in bulk composition. For example, calcic brines encountered at depth at the KTB borehole (M611er et al., 1997) occur in isolated fractures and do not contribute to the regional electric conductivity pattern (Emmermann and Lauterjung, 1997). It is notable that, despite the low fluid fluxes, fluid pressures within fractures in crystalline basement rocks accessed by drilling do not exceed hydrostatic. Nevertheless, it is possible that fluid overpressures could be attained, at least transiently, as the walls of isolated fractures collapse onto an aqueous fill. Hence, many fluid inclusion studies have reported evidence for high fluid pressures in vein quartz, even where it has grown at low temperatures (< 300 °C) (Parry and Bruhn, 1990; Grant et al., 1990; Munz et al., 1995). Fluid overpressuring is clearly an indication of very low flow rates (Neuzil, 1995). Even where there is significant fluid flow, rates may be sufficiently slow to allow fluids to approach equilibrium with their wall rocks, and a simple and pragmatic distinction between high- and low-flux palaeofluid systems is based on whether or not the oxygen isotopic composition of the fluid is locally buffered. At the Soultz experimental HDR borehole, Smith et al. (1998) reported that the latest stages of vein quartz growth had grown from a fluid that was in equilibrium with the feldspar in the host granite, even though this growth occurred in a fracture that was a conduit of focussed fluid flow.
4. Systems with large fluxes Mineralisation due to downward-penetrating fluids is a major process at mid-ocean ridges, but may also be a feature of continental settings. In order to form most hydrothermal ore deposits, large volumes of hot, metal-bearing fluids are required, and these systems are indicative of high fluid fluxes. The Z n - P b deposits of the Irish Midlands are a good example of large fluid flux systems.
Abstract
4.1. Irish Zn-Pb deposits In the Irish deposits, fluid inclusions from carbonates, quartz and sulphide minerals suggest two fluids were present at the site of mineralisation: a lowtemperature high-salinity brine and a hotter (250 °C) more dilute (15 wt.%) brine. The metal-carrying capacity of base metal ore fluids is linked to their salinity. Bulk fluid inclusion halogen analyses (e.g., Viets et al., 1996; Gleeson and Yardley, 2002 and references therein) show that most base metal-mineralizing fluids are formed by the subaerial evaporation of seawater, past the point of halite dissolution. This has also shown to be the case for both fluids in the Irish deposits (Gleeson et al., 1999). At source, therefore, these fluids have densities of greater than 1.2 g/ cm3. These fluids can, therefore, pond on the seafloor, or at the base of aquifers. Despite the evidence that the mineralising fluids were sourced at the surface, Pb-isotope studies have demonstrated these brines have leached metals from Lower Palaeozoic basement rocks. This is spectacularly demonstrated by the Irish P b - Z n ore field, where Carboniferous limestones and their deposits straddle the trace of the Caledonian Iapetus suture across Ireland. The nature of the basement is quite different across the suture, and the resulting contrast in Pb-isotope signature is faithfully recorded in the overlying ore bodies (LeHuray et al., 1987). The metals were transported by the high-temperature fluid to the deposit site (Everett et al., 1999). Fluid inclusions in veins hosted by the Lower Palaeozoic basement trap fluids that have isotopically and geochemically equilibrated with the basement rocks at temperatures 200 °C at low water-rock ratios (Everett et al., 1999). Assuming thermal equilibration as well, this suggests the fluids are mineralizing fluids that come from depths of at least 5 km. Since the metals in the ore deposits appear to be associated with a high-temperature fluid at relatively shallow depths, one of the major requirements for the formation of the deposits is the focusing of large volumes of hot fluids from depth into a restricted part of the upper crust. To explain this phenomenon, Russell (1978) suggested that the deposits were formed by downwardpropagating hydrothermal cells in an extensional environment. He suggested that water-rock interac-
63
tion and the extraction of heat from the basement increased permeability and, aided by further extension, allowed the convective cells to deepen. For such a system to work, the permeability of the basement needs to be high (Russell suggests 10 16 cm2), and this is compatible with the in situ data from the modem boreholes. A problem with this model is the heat requirement: unless the system is exposed to hot rocks, convection will cease as the heat is extracted from the basement (Strens et al., 1987).
5. Episodic fluid flow In contrast to thermally driven convection systems such as those observed at mid-ocean ridges, many flow systems in continental basement rocks appear to have operated episodically, for example, in response to tectonic processes, and may display features of both low- and high-flux systems.
5.1. Modum, Norway In the Precambrian high-grade basement rocks of southern Norway, a series of Lower Palaeozoic quartz + albite _+ carbonate + bitumen veins are found hosted by meta-gabbros and metasediments (Munz et al., 1995, 2002). These veins are not laterally extensive, and at any one sample site, connectivity only occurs on scales of tens of meters. Fluid inclusion studies on these veins indicate they were formed by the infiltration of sedimentary waters and hydrocarbons from the overlying Caledonian foreland basin. Chlorite geothermometry and bitumen reflectance data suggest that ambient wall rock temperature at the time of veining was ca. 220 °C. Although the bulk of the fluid inclusions indicate the fluids were thermally equilibrated with the wall rocks, the veins also contain inclusions that clearly indicate local trapping of both hotter (< 300 QC) and cooler fluids (> 100 ° C). The hydrocarbon and aqueous fluid inclusions likewise show evidence for trapping at hydrostatic pressures and lithostatic pressures. Oxygen isotopic data from the veins (Gleeson et al., 2003) suggest that the fluids responsible for bulk quartz growth did not equilibrate with the wall rocks but with cooler overlying rocks.
64
Abstract
5.2. Irish Z n - P b deposits? Notwithstanding the importance o f large fluid fluxes in the generation o f these ores, in some deposits, there is also evidence for episodic flow. Fluids must have interacted significantly with the basement to acquire a distinctive isotopic signature. Modelling by Sanderson and Zhang (1999) demonstrates that as the crust hosting a fault-fracture network changes from low differential stresses to the critical stress state, initially diffuse flow changes to highly localised flow along larger fractures, with an increase in hydraulic conductivity. Such a model allows for the sinking o f brines into fractures that are not connected on a very large scale, and then storage o f the fluids allowing chemical, thermal and isotopic equilibrium to be attained. Any subsequent tectonism could then initiate large-scale focussed fluid flow to the surface in major structures and/or could allow episodic convection to occur.
6. Discussion and conclusions Hydrogeological studies o f fluids in sedimentary aquifers clearly show that aqueous fluid flow driven by gravity (topography) has the ability to move large volumes o f fluids over great distances, permeability permitting. Evidence for meteoric waters in deep shear zones may suggest the effectiveness of topographic fluid flow in transporting surface fluids into the middle crust. The requirement for the formation of hydrothermal mineral deposits is the focusing o f large volumes o f hot, metal-bearing, aqueous fluids into a restricted discharge area. By their nature, therefore, many deposits are formed by relatively fast episodic flows and fluid inclusions, and structural studies show evidence of pressure fluctuations in many deep crustal ore systems. In contrast, studies o f many P b - Z n deposits, hosted by basins and by basements, suggest that these mineralising systems are characterised by hydrostatic or close to hydrostatic pressures. Fluid inclusion analyses indicate that the mineralising fluids originate as evaporated seawater brines, suggesting that gravity driven meteoric waters dissolving halite on their flow paths are not important for the formation of these deposits. However, the importance o f topo-
graphic flow may lie in the contrasting densities o f the mineralising brines and other fluids present in the basins. Less saline fluids may allow for the displacement o f the brines into the basement without significant mixing as suggested by fluid inclusion analyses. The chemistry and temperatures o f the fluids suggest that the fluids have interacted extensively with the basement, either in deep convection cells or were stored in fractures in the basement before being expelled rapidly as a result o f local tectonic activities in episodic events.
References Brace, W.E, 1984. Permeability of crystalline rocks: new in-situ measurements. J. Geophys. Res. 89, 4327-4330. Emmermann, R., Lauterjung, J., 1997. The German continental deep drilling programKTB: overviewand major results. J. Geophys. Res. 89, 4327-4330. Everett, C.E., Wilkinson, J.J., Boyce, A.J., Ellam, R., Rye, D.M., Fallick, A.E., Gleeson, S.A., 1999. The genesis of Irish-type base metal deposits: characteristics and origins of the principal ore fluid. In: Stanley, C.J. (Ed.), Mineral Deposits; Processes to Processing. A.A. Balkema, Rotterdam, pp. 845-848. Gleeson, S.A., Yardley, B.W.D., 2002. Veins and mineralisation in basement rocks:the role of penetration of sedimentary fluids. In: Stober, I., Bueher, K. (Eds.), Water-Rock Interaction. Kluwer Academic Publishing, Dordrecht, pp. 189-205. Gleeson, S.A., Banks, D.A., Everett, C.E., Wilkinson, J.J., Samson, I.M., Boyce, A.J., 1999. Origin of mineralising fluids in Irishtype deposits: constraints from halogen analyses. In: Stanley, C.J. (Ed.), Mineral Deposits; Processes to Processing. A.A. Balkema, Rotterdam, pp. 857-860. Gleeson, S.A., Yardley, B.W.D., Mtmz, I.A., Boyce, A.J., 2003. Infiltration of basinal fluids into high-grade basement, South Norway: sources and behaviour of waters and brines. Geofluids 3, 1-16. Grant, N.T., Banks, D.A., McCaig, A.M., Yardley, B.W.D., 1990. Chemistry, source, and behavior of fluids involved in alpine thrusting of the central Pyrenees. J. Geophys. Res. 95, 9123-9131. Kontak, D.J., Home, R.J., Smith, EK., 1996. Hydrothermal characterization of the West Gore Sb-Au deposit, Megumaterrane, Nova Scotia, Canada. Econ. Geol. 91, 1239-1262. LeHuray, A.E, Caulfleld, J.B.D., Rye, D.M., Dixon, ER., 1987. Basement controls on sediment-hosted Zn-Pb deposits: a Pb isotope study of Carboniferousmineralisation in Central Ireland. Econ. Geol. 82, 1695-1709. McCaig, A.M., Tritlla, J., Banks, D.A., 2000. Fluid mixing and recyclingduring Pyreneanthrusting: evidencefrom fluid inclusion halogen ratios. Geochim.Cosmochim.Acta 64, 3395-3412. Mrller, P., Weise, S.M., Althaus, E., Bach,W., Behr, H.J., Borchardt, R., Br~iuer,K., Dreseher, J., Erzinger, J., Faber, E., Hansen, B.T.,
Abstract
Hom, E.E., Heunges, E., K~impf, H., Kessels, W., Kirsten, T., Landwehr, D., Lodemaun, M., Machon, L., Pekdeger, A., Pielow, H.-U., Reutel, C., Simon, K., Walther, J., Weinlich, F.H., Zimmer, M:, 1997. Paleofluids and recent fluids in the upper continental crust: results from the German Continental Deep Drilling Program (KTB). J. Geophys. Res. 102, B18233 B18254. Morrison, J., 1994. Meteoric water-rock interaction in the lower plate of the Whipple mountain metamorphic core complex, California. J. Metamorph. Geol. 12, 827-840. Mtmz, I.A., Yardley, B.W.D., Banks, D.A., Wayne, D., 1995. Deep penetration of sedimentary fluids in basement rocks from southeru Norway: evidence from hydrocarbon and brine inclusions in quartz rocks. Geochim. Cosmochim. Acta 59, 239-254. Munz, I.A., Yardley, B.W.D., Gleeson, S.A., 2002. Petroleum infiltration of high-grade basement. Southem Norway: pressuretemperature-time composition ( P T - t - X ) constraints. Geofluids 2, 1-13. Nesbitt, B.E., Muehlenbachs, K., 1989. Origins and movements of fluids during deformation and metamorphism in the Canadian Cordillera. Science 245, 733-736. Neuzil, C.E., 1995. Abnormal pressures as hydrodynamic phenomena. Am. J. Sci. 295, 742-786. Parry, W.T., Brubal, R.L., 1990. Fluid pressure transients on seismogenic normal faults. Tectonophysics 179, 335 344. Read, C.M., Cartwright, I., 2000. Meteoric fluid infiltration in the middle crust during shearing: examples from the Anmta Inlier, central Australia. J. Geochem. Explor. 69, 333 337. Russell, M.J., 1978. Downward excavating hydrothermal cells and Irish type ore deposits: importance of an underlying thick Caledonian prism. Trans. Inst. Min. Metall. 87, B168 B171. Sanderson, D.J., Zhang, X., 1999. Critical stress localization of flow associated with deformation of well-fractured rock masses,
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with implications for mineral deposits. In: McCaffrey, K.J.W., Lonergan, L., Wilkinson, J.J. (Eds.), Fractures, Fluid Flow and Mineralisation. Geol. Soc. Spec. Publ., vol. 155, pp. 69-81. Sibson, R.H., 1990. Faulting and fluid flow. Min. Soc. Can. Short Course 18, 93 132. Smith, M.P., Savary, V., Yardley, B.W.D., Valley, J.W., Royer, J.J., Dubois, M., 1998. The evolution of the deep flow regime at Soultz-sous-For~ts, Rhine Graben, Eastem France: evidence from a composite quartz vein. J. Geophys. Res. 103, 27223 -27237. Stober, I., Bucher, K., 2000. Hydraulic properties of the Upper Continental Crust: data from the Urach 3 geothermal well. In: Stober, I., Bucher, K. (Eds.), Hydrogeology of Crystalline Rocks. Kluwer Academic Publishing, The Netherlands. Strens, M.R., Cann, D.L., Cann, J.R., 1987. A thermal balance model of the formation of sedimentary exhalative lead-zinc deposits. Econ. Geol. 82, 1192-1203. Taylor, H.P., 1977. Water/rock interactions and the origin of H20 in granitic batholiths. The application of oxygen and hydrogen isotope studies to problems of hydrothermal alteration and ore deposition. J. Geol. Soc. Lond. 133, 509-558. Viets, J.G., Hofstra, A.H., Emsbo, P., 1996. Solute compositions of fluid inclusions in sphalerite from North American and European Mississippi Valley-Type ore deposits: ore fluids derived from evaporated seawater. Soc. Econ. Geol. Spec. Publ. 4, 465 -482. Yardley, B.W.D., Valley, J.W., 1997. The petrologic case for a dry lower crust. J. Geophys. Res. 102, 12173-12185. Zoback, M.D., Harjes, H.-P., 1997. Injection-induced earthquakes and crustal stress at 9 km depth at the KTB deep drilling site Germany. J. Geophys. Res. 102, 18477-18491.
ELSEVIER
~DIRECT e
Journal of Geochemical Exploration 78-79 (2003) 61-65
JOURNAL OF
GEOCHEMICAL EXPLORATION www.elsevier.com/locate/jgeoexp
Abstract
Surface-derived fluids in basement rocks: inferences from palaeo-hydrothermal systems S.A. Gleeson a'*, B.W.D. Yardley b "Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Canada T6G 2E3 bSehool of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK
Abstract
Drilling into modem systems such as the KTB borehole in Germany indicates that the upper crest is pervasively fractured and saturated with surface-derived fluids at hydrostatic pressures. The recognition of surface-derived fluids in palaeohydrothermal systems is limited to the effectiveness of geochemical tracers. At low total fluid fluxes, fluids tend to interact with wall rocks and become isotopically, chemically and thermally equilibrated. At high fluid fluxes, such as found in upper crustal mineralisation systems, fluid inclusion halogen systematics can be used to identify the origin of the fluids, even when the fluids have equilibrated isotopically and thermally with deeper parts of the hydrothermal system. Unlike the modem systems, fluid inclusions in vein fills in both mineralised and unmineralised systems show strong evidence for episodic fluid flow and fluctuating fluid pressures, even in upper crustal rocks. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Fluid flow; Hydrostatic; Lithostatic;Hydrothermal;Fluid inclusions
1. Introduction
Although hydrogeologists have long known that surface-derived groundwaters can penetrate deep aquifers, traditionally, studies o f hydrothermal mineralisation in basement rocks have emphasised the movement o f hot fluids from depth to shallower levels, rather than the reverse. Initially, deep penetration o f surface waters was recognised from the resulting meteoric water oxygen isotope signature around batholiths (e.g., Taylor, 1977). Subsequently, oxygen
* Corresponding author. Fax: +1-780-492-2030. E-mail address." [email protected] (S.A. Gleeson).
isotopes have been used to identify extensive meteoric water involvement in retrogression at temperatures to about 350 °C and depths to about 8 km in metamorphic core complexes (e.g., Morrison, 1994), and in middle crustal shear zones (Read and Cartwright, 2000). Isotopic studies on mesothermal gold deposits from the Canadian Cordillera suggest that meteoric waters involved in ore formation have circulated to depths o f 1 2 - 1 5 km (Nesbitt and Muehlenbachs, 1989). In many instances, however, it is likely that low fluxes o f penetrating surface waters are not tracked by the oxygen isotope record. For example, Smith et al. (1998) showed that fluids flowing through fractures in granite at ca. 2 km depth in the Soultz experimental
0375-6742/03/$ - see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0375-6742(03)00147-X
62
Abstract
HDR borehole, France had an oxygen isotopic composition resulting from equilibration with the host granite at the local temperature. In this paper, we seek to examine some different flow regimes where there is evidence for the movement of sedimentary brines into basement rock, unrelated to any temperature gradients created by magmatism.
2. Permeability and fluid pressures in basement rocks While the importance of fractures on the bulk permeability of shallow crystalline rocks has been understood for some time (Brace, 1984), one of the recent advances in the understanding the permeability structure of the crystalline upper crust has come from tests in deep bore holes, e.g., KTB, Urach-3, Soultz and Kola Peninsula (e.g., M611er et al., 1997; Stober and Bucher, 2000). These data have shown that crystalline basement rocks can have significant fracture permeability to depths of 9 km, although to some degree this is a function of the rock type. For example, although the total permeability of the metamorphic rocks in the Black Forest is 2.14 x 10 - 7 m/s, for granites and gabbros, the values are ~ 9.55 x 10 7 and 5.01 x 10 8 m/s, respectively (Stober and Bucher, 2000). The data from the KTB borehole confirmed the view that the crystalline upper crust is close to a state of brittle failure at depths of 9 km (e.g., Zoback and Harjes, 1997). Overpressured systems are well known from oil fields, and many other settings have been inferred to have been characterised by near-lithostatic fluid pressures, including most prograde metamorphic environments, some mid-crustal hydrothermal systems (e.g., lode gold deposits) and many shear and thrust fault systems (e.g., Parry and Bruhn, 1990; Sibson, 1990; McCaig et al., 2000). In some hydrothermal gold deposits, there is also evidence for vein formation at supra-hydrostatic pressures (Kontak et al,, 1996). In the deep crust, Yardley and Valley (1997) pointed out that fluid overpressures are normally present in settings where fluid is expelled as rocks are buried and heated; in contrast, due to absorption by retrograde reactions, fluid pressures are characteristically very low in cooled crystalline crust, except transiently, along pathways of fluid infiltration.
3. Systems with low fluxes Where fluid fluxes are very low, surface-derived brines may be present in crystalline rocks, but react so extensively with their hosts that they lose the chemical signature of their source, while the host undergoes mineralogical changes (hydration) without necessarily experiencing large changes in bulk composition. For example, calcic brines encountered at depth at the KTB borehole (M611er et al., 1997) occur in isolated fractures and do not contribute to the regional electric conductivity pattern (Emmermann and Lauterjung, 1997). It is notable that, despite the low fluid fluxes, fluid pressures within fractures in crystalline basement rocks accessed by drilling do not exceed hydrostatic. Nevertheless, it is possible that fluid overpressures could be attained, at least transiently, as the walls of isolated fractures collapse onto an aqueous fill. Hence, many fluid inclusion studies have reported evidence for high fluid pressures in vein quartz, even where it has grown at low temperatures (< 300 °C) (Parry and Bruhn, 1990; Grant et al., 1990; Munz et al., 1995). Fluid overpressuring is clearly an indication of very low flow rates (Neuzil, 1995). Even where there is significant fluid flow, rates may be sufficiently slow to allow fluids to approach equilibrium with their wall rocks, and a simple and pragmatic distinction between high- and low-flux palaeofluid systems is based on whether or not the oxygen isotopic composition of the fluid is locally buffered. At the Soultz experimental HDR borehole, Smith et al. (1998) reported that the latest stages of vein quartz growth had grown from a fluid that was in equilibrium with the feldspar in the host granite, even though this growth occurred in a fracture that was a conduit of focussed fluid flow.
4. Systems with large fluxes Mineralisation due to downward-penetrating fluids is a major process at mid-ocean ridges, but may also be a feature of continental settings. In order to form most hydrothermal ore deposits, large volumes of hot, metal-bearing fluids are required, and these systems are indicative of high fluid fluxes. The Z n - P b deposits of the Irish Midlands are a good example of large fluid flux systems.
Abstract
4.1. Irish Zn-Pb deposits In the Irish deposits, fluid inclusions from carbonates, quartz and sulphide minerals suggest two fluids were present at the site of mineralisation: a lowtemperature high-salinity brine and a hotter (250 °C) more dilute (15 wt.%) brine. The metal-carrying capacity of base metal ore fluids is linked to their salinity. Bulk fluid inclusion halogen analyses (e.g., Viets et al., 1996; Gleeson and Yardley, 2002 and references therein) show that most base metal-mineralizing fluids are formed by the subaerial evaporation of seawater, past the point of halite dissolution. This has also shown to be the case for both fluids in the Irish deposits (Gleeson et al., 1999). At source, therefore, these fluids have densities of greater than 1.2 g/ cm3. These fluids can, therefore, pond on the seafloor, or at the base of aquifers. Despite the evidence that the mineralising fluids were sourced at the surface, Pb-isotope studies have demonstrated these brines have leached metals from Lower Palaeozoic basement rocks. This is spectacularly demonstrated by the Irish P b - Z n ore field, where Carboniferous limestones and their deposits straddle the trace of the Caledonian Iapetus suture across Ireland. The nature of the basement is quite different across the suture, and the resulting contrast in Pb-isotope signature is faithfully recorded in the overlying ore bodies (LeHuray et al., 1987). The metals were transported by the high-temperature fluid to the deposit site (Everett et al., 1999). Fluid inclusions in veins hosted by the Lower Palaeozoic basement trap fluids that have isotopically and geochemically equilibrated with the basement rocks at temperatures 200 °C at low water-rock ratios (Everett et al., 1999). Assuming thermal equilibration as well, this suggests the fluids are mineralizing fluids that come from depths of at least 5 km. Since the metals in the ore deposits appear to be associated with a high-temperature fluid at relatively shallow depths, one of the major requirements for the formation of the deposits is the focusing of large volumes of hot fluids from depth into a restricted part of the upper crust. To explain this phenomenon, Russell (1978) suggested that the deposits were formed by downwardpropagating hydrothermal cells in an extensional environment. He suggested that water-rock interac-
63
tion and the extraction of heat from the basement increased permeability and, aided by further extension, allowed the convective cells to deepen. For such a system to work, the permeability of the basement needs to be high (Russell suggests 10 16 cm2), and this is compatible with the in situ data from the modem boreholes. A problem with this model is the heat requirement: unless the system is exposed to hot rocks, convection will cease as the heat is extracted from the basement (Strens et al., 1987).
5. Episodic fluid flow In contrast to thermally driven convection systems such as those observed at mid-ocean ridges, many flow systems in continental basement rocks appear to have operated episodically, for example, in response to tectonic processes, and may display features of both low- and high-flux systems.
5.1. Modum, Norway In the Precambrian high-grade basement rocks of southern Norway, a series of Lower Palaeozoic quartz + albite _+ carbonate + bitumen veins are found hosted by meta-gabbros and metasediments (Munz et al., 1995, 2002). These veins are not laterally extensive, and at any one sample site, connectivity only occurs on scales of tens of meters. Fluid inclusion studies on these veins indicate they were formed by the infiltration of sedimentary waters and hydrocarbons from the overlying Caledonian foreland basin. Chlorite geothermometry and bitumen reflectance data suggest that ambient wall rock temperature at the time of veining was ca. 220 °C. Although the bulk of the fluid inclusions indicate the fluids were thermally equilibrated with the wall rocks, the veins also contain inclusions that clearly indicate local trapping of both hotter (< 300 QC) and cooler fluids (> 100 ° C). The hydrocarbon and aqueous fluid inclusions likewise show evidence for trapping at hydrostatic pressures and lithostatic pressures. Oxygen isotopic data from the veins (Gleeson et al., 2003) suggest that the fluids responsible for bulk quartz growth did not equilibrate with the wall rocks but with cooler overlying rocks.
64
Abstract
5.2. Irish Z n - P b deposits? Notwithstanding the importance o f large fluid fluxes in the generation o f these ores, in some deposits, there is also evidence for episodic flow. Fluids must have interacted significantly with the basement to acquire a distinctive isotopic signature. Modelling by Sanderson and Zhang (1999) demonstrates that as the crust hosting a fault-fracture network changes from low differential stresses to the critical stress state, initially diffuse flow changes to highly localised flow along larger fractures, with an increase in hydraulic conductivity. Such a model allows for the sinking o f brines into fractures that are not connected on a very large scale, and then storage o f the fluids allowing chemical, thermal and isotopic equilibrium to be attained. Any subsequent tectonism could then initiate large-scale focussed fluid flow to the surface in major structures and/or could allow episodic convection to occur.
6. Discussion and conclusions Hydrogeological studies o f fluids in sedimentary aquifers clearly show that aqueous fluid flow driven by gravity (topography) has the ability to move large volumes o f fluids over great distances, permeability permitting. Evidence for meteoric waters in deep shear zones may suggest the effectiveness of topographic fluid flow in transporting surface fluids into the middle crust. The requirement for the formation of hydrothermal mineral deposits is the focusing o f large volumes o f hot, metal-bearing, aqueous fluids into a restricted discharge area. By their nature, therefore, many deposits are formed by relatively fast episodic flows and fluid inclusions, and structural studies show evidence of pressure fluctuations in many deep crustal ore systems. In contrast, studies o f many P b - Z n deposits, hosted by basins and by basements, suggest that these mineralising systems are characterised by hydrostatic or close to hydrostatic pressures. Fluid inclusion analyses indicate that the mineralising fluids originate as evaporated seawater brines, suggesting that gravity driven meteoric waters dissolving halite on their flow paths are not important for the formation of these deposits. However, the importance o f topo-
graphic flow may lie in the contrasting densities o f the mineralising brines and other fluids present in the basins. Less saline fluids may allow for the displacement o f the brines into the basement without significant mixing as suggested by fluid inclusion analyses. The chemistry and temperatures o f the fluids suggest that the fluids have interacted extensively with the basement, either in deep convection cells or were stored in fractures in the basement before being expelled rapidly as a result o f local tectonic activities in episodic events.
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