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The role of metamorphic fluids in the development of the Cornubian orefield: fluid inclusion evidence from south Cornwall

Published online by Cambridge University Press:  05 July 2018

J. J. Wilkinson
J. J. Wilkinson*
Affiliation:
Department of Geology, The University of Southampton, Highfield, Southampton SO9 5NH
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Abstract

Veins developed during contact metamorphism associated with the emplacement of the Cornubian granite batholith contain both H2O-rich and CO2-rich fluid inclusions. Microthermometric data indicate that unmixing of a low-CO2, low-salinity fluid occurred at 400–200°C and 1000–500 bars to produce low-density CO2-rich vapour and saline aqueous fluids (8–42 wt. % NaCl equivalent). Decrepitation-linked ICP analyses show that the cation composition of the brines is dominated by Na, K and Ca, but that significant amounts of Li, Sr, Ba, Fe, Mn, Zn and B are also present. Bulk volatile analyses confirm the dominance of CO2 over N2 and CH4 in the vapour phase, with CO2/N2 molar ratios of 15.3–28.7 and CO2/CH4 molar ratios of 66.9–292. The relative abundance of nitrogen suggests an aureole-derived ‘organic’ component is present.

The source of the fluids is ambiguous as they are intermediate in composition between ideal ‘magmatic’ and ‘metamorphic’ end-members. It is proposed that this is due to mixing of the two types of fluid in the contact aureole during granite intrusion. A model is suggested in which magmatic-metamorphic circulation occurred synchronously with granite emplacement and subsequently evolved to a meteoric-dominated system with the bulk of the ore deposits forming in response to the influx of meteoric fluids.

Keywords

metamorphic fluidsfluid inclusionsCornubian orefieldCornwall
Type
Magmatic/metamorphic environment
Information
Mineralogical Magazine , Volume 54 , Issue 375 , June 1990 , pp. 219 - 230
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1990

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Footnotes

*

Present address: Dept of Geology, Royal School of Mines, Prince Consort Road, London SW7 2BP.

References

Alderton, D. H. M., Thompson, M., Rankin, A. H. and Chryssoulis, S. L. (1982) Developments of the ICP-linked decrepitation technique for the analysis of fluid inclusions in quartz. Chem. Geol. 37, 203-13.CrossRefGoogle Scholar
Badham, J. P. N. (1980) Late magmatic phenomena in the Cornish batholith—useful guides for tin mineralisation. Proc. Ussher Soc. 5, 44-53.Google Scholar
Barnes, R. P. and Andrews, J. R. (1981) Pumpellyite-actinolite grade regional metamorphism in south Cornwall. Ibid. 5, 139-46.Google Scholar
Bottrell, S. H. and Yardley, B. W. D. (1988) The composition of a primary granite-derived ore fluid from S.W. England, determined by fluid inclusion analysis. Geochim. Cosmochim. Acta, 52, 585-8.CrossRefGoogle Scholar
Bowers, T. S. and He[geson, H. C. (1983) Calculation of the thermodynamic and geochemical consequences of non-ideal mixing in the system H2O-CO2-NaCl on phase relations in geologic systems: Equation of state for H2O-CO2-NaC1 fluids at high pressures and temperatures. Ibid. 47, 124-75.Google Scholar
Bowman, J. R. (1987) Stable isotope (C,O) evidence for the infiltration of water-rich fluids during the development of the periclase zone, Alta stock thermal aureole, Utah. Geol. Soc. Am. Prog. Abs. 19, 596.Google Scholar
Darbyshire, D. P. F. and Shepherd, T. J. (1987) Chronology of magmatism in S.W. England: the minor intrusions. Proc. Ussher Soc. 4, 431-8.Google Scholar
Dodson, M. H. and Rex, D. C. (1971) Potassium-argon ages of slates and phyllites from south-west England. Q. J. Geol. Soc. London, 126, 465-99.Google Scholar
Exley, C. S., Stone, M. and Floyd, P. A. (1983) Composition and petrogenesis of the Cornubian batholith and post-orogenic rocks in S.W. England. In The Variscan fold belt in the British Isles (Hancock, P. L., ed.), Adam Higer Ltd., Bristol, 153-77.Google Scholar
Fehn, U. (1985) Post-magmatic convection related to high heat production in granites of S.W. England: a theoretical study. In High Heat Production (HHP) Granites, Hydrothermal Circulation and Ore Genesis. Inst. Min. Metall., 99-112.Google Scholar
Ferry, J. M. (1989) Contact metamorphism of roof pendants at Hope Valley, Alpine County, California, USA. Contrib. Mineral. Petrol. 101, 402-17.CrossRefGoogle Scholar
Gehrig, M. (1980) Phasengleichgewichte und PVT-Dawn ternarer Mischungen aus Wasser, Kohlendioxide, und Natriumchlorid bis 3 kbar und 550°C Doctoral dissertation, University of Karlsruhe, Hochschulverlag, Freiburg.Google Scholar
Giggenbach, W. F. (1988) Geothermal solute equilibria. Derivation of Na-K-Mg-Ca geoindicators. Geochim. Cosrnochirn. Acta, 52, 2749-65.CrossRefGoogle Scholar
Hall, A. (1988) The distribution of ammonium in granites from S.W. England. J. Geol. Soc. London, 145, 37-41.CrossRefGoogle Scholar
Jackson, N. J., Halliday, A. N., Sheppard, S. M. F. and Mitchell, J. G. (1982) Hydrothermal activity in the St. Just mining district, Cornwall, England. In Metallization associated with acid magmatism (Evans, A. M., ed.), Wiley, Chichester, pp. 137-79.Google Scholar
Labotka, T. C., White, C. E. and Papike, J. J. (1985) The evolution of water in the contact-metamorphic aureole of the Duluth complex, northeastern Minnesota. Geol. Soc. Am. Bull. 95, 788-804.2.0.CO;2>CrossRefGoogle Scholar
Lagache, M. and Weisbrod, A. (1977) The system two alkali feldspars KCl-NaCl-H2O at moderate to high temperatures and low pressures. Contrib. Mineral. Petrol. 62, 77-101.CrossRefGoogle Scholar
Miller, M. F., Shepherd, T. J., Yucheng, L. and Halls, C. (1989) The source and significance of carbon isotopes in fluid inclusions from mineralised areas of S.W. England and S. China. (Abstract) ECROFIX, Imperial College, London. Google Scholar
Norton, D. and Knight, J. (1977) Transport phenomena in hydrothermal systems: cooling plutons. Am. J. Sci. 277, 937-81.Google Scholar
Primmer, T. J. (1985) Discussion of the possible contribution of metamorphic water to the mineralizing fluid of south-west England: preliminary stable isotope evidence. Proc. Ussher Soc. 6, 224-8.Google Scholar
Ramboz, C., Pichavant, M. and Weisbrod, A. (1982) Fluid immiscibility in natural processes: use and misuse of fluid inclusion data. II. Interpretation of fluid inclusion data in terms of immiscibility. Chem. Geol. 37, 29-48.CrossRefGoogle Scholar
Rankama, K. and Sahama, T. G. (1950) Geochemistry. University of Chicago Press, Chicago.Google Scholar
Rankin, A. H. and Alderton, D. H. M. (1985) Fluids in granites from southwest England. In High Heat Production (HHP) Granites, Hydrothermal Circulation and Ore Genesis. Inst. Min. Metall., 285-99.Google Scholar
Rattey, R. P. (1980) Deformation in south-west Cornwall. Proc. Ussher Soc. 5, 39-43.Google Scholar
Roedder, E. (1981) Origin of fluid inclusions and changes that occur after trapping. In Short Course in Fluid Inclusions: Applications to Petrology (Hollister, L. S. and Crawford, M. L., eds.), Min. Assoc. Canada, Short Course Notes, Vol. 6, 101-37.Google Scholar
Shepherd, T. J., Rankin, A. H. and Alderton, D. H. M. (1985) A practical guide to fluid inclusion studies. Blackie, London.Google Scholar
Shepherd, T. J., Miller, M. F., Scrivener, R. C. and Darbyshire, D. P. F. (1985) Hydrothermal fluid evolution in relation to mineralization in southwest England with special reference to the Dartmoor-Bodmin area. In High Heat Production (HHP) Granites, Hydrothermal Circulation and Ore Genesis. Inst. Min. Metall., 345-64.Google Scholar
Sheppard, S. M. F. (1977) The Cornubian batholith, S.W. England: D/H and 18O/160 studies of kaolinite and other alteration minerals. J. Geol. Soc. London, 133, 573-91.CrossRefGoogle Scholar
Stone, M. (1975) Structure and petrology of the Tregonning- Godolphin Granite, Cornwall. Proc. Geol. Assoc. 86, 155-70.CrossRefGoogle Scholar
Taylor, H. P. Jr (1977) Water/rock interaction and the origin of H2O in granitic batholiths. J. Geol. Soc. London, 133, 509-58.CrossRefGoogle Scholar
Thompson, M., Rankin, A. H., Walton, S. J., Halls, C. and Foo, B. N. (1980) The analysis of fluid inclusion decrepitate by inductively-coupled plasma atomic emission spectroscopy: an exploratory study. Chem. Geol. 30, 121-33.CrossRefGoogle Scholar
Willis-Richards, J. and Jackson, N. J. (1989) Evolution of the Cornubian orefield, S.W. England. Part I: Batholith modelling and ore distribution. Econ. Geol. 84, 1078-100.CrossRefGoogle Scholar
Wlotzka, F. (1972) Nitrogen. In Handbook of Geochemistry (Wedepohl, K. H., ed.), Vol. II, Springer-Verlag, Berlin, Chap. 7B-O.Google Scholar
Yardley, B. W. D., Banks, D. A. and Campbell, L. S. (1989) Chemical evolution of ore fluids from Cornwall, U.K. (Abstract) ECROFI X, Imperial College, London. Google Scholar