The origin and evolution of Hercynian crustal fluids, South Cornwall, England

Wilkinson, Jamie John (1989) The origin and evolution of Hercynian crustal fluids, South Cornwall, England. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

Crustal fluids, mobilised in Devonian metasediments during and subsequent to the Hercynian orogeny in South Cornwall, have been investigated by a detailed study of fluid inclusions present in vein material. Four stages of vein formation are distinguished, related to the three main deformation events which affected the area. Within each stage, up to four different vein types are recognised, occurring in distinct structural and/or lithological settings. Fluid inclusion data, in combination with pumpellyite-actinolite mineral assemblages and vitrinite reflectance data indicate that low grade regional metamorphism in south Cornwall occurred at 330 ± 30^oC and 3.4 ± 0.5 kbar. Dense (0.95 ± 0.3 g cm^-3), low salinity (1-4.5 wt% NaCl equivalent), H_2O-rich (X_H2O> 0.986), peak metamorphic fluids were evolved via prograde devolatilisation reactions, contemporaneous with the first compressional deformation event (D1). The major element compositions of these fluids were buffered by quartz + albite + K-mica + chlorite ± paragonite assemblages in greywacke and slate host rocks. Thermodynamic calculations suggest that the peak metamorphic fluids could have transported significant (ppm) quantities of base metals and silver as chloro-complexes, and tin as hydroxy-complexes. High temperature metamorphic fluids migrated up imbricate thrusts to shallower crustal levels during the second deformation event (D2). These fluids caused limited sericitic and chloritic alteration of wallrocks, and became increasingly saline (up to 8 wt% NaCl equivalent) as a result of hydration reactions. CO₂ and N₂ contents increased, possibly as a result of oxidation of organic carbon and ammonium in the wallrocks due to cooling and decreasing oxygen fugacity. Lower ore mineral solubilities in these fluids may have resulted in precipitation, producing low-grade metal enrichment in the upper crust. Fluid inclusion data indicate that fluid temperatures and pressures did not fall below 200^oC and 500 bars respectively, during D2. CO₂(+ N₂₊ CH₄)-bearing fluids, possibly generated by mixing of approximately equal proportions of magmatic and contact metamorphic fluids, were mobilised up to 5 km laterally from the roof zone of the Cornubian granite batholith during emplacement. These fluids unmixed at 400-200^oC and 1000-500 bars to produce low density CO₂-rich vapour and high density, moderately to highly saline, aqueous fluids (8-42 wt% NaCl equivalent). Up to 28 ppm tin could have been transported in the hypersaline fluids, primarily as chloro-complexes. High density, CO₂-rich fluids (up to 85 mol% CO₂) were produced in distal parts of the contact aureole, probably as a result of condensation of low density vapours. Low salinity (1-5 wt% Na Cl equivalent), H₂O-rich fluids separated from the Megilliggar pegmatites during crystallisation. These fluids unmixed at ≈400^oC to produce a low density, CO_2(±CH_4±N_2)-bearing vapour and moderately saline (8-10 wt% NaCl equivalent) aqueous liquid. Magmatic fluids of this type, possibly in combination with contact metamorphic fluids, were responsible for high temperature mineralisation phenomena. The initiation of convective fluid circulation, and dilution of magmatic±metamorphic fluids with meteoric water at or below ≈300^oC, resulted in the formation of the bulk of the Sn-W-polymetallic sulphide deposits observed in the province. Basinal brines migrated along wrench faults into the Cornubian peninsula and mixed with convecting meteoric fluids at structural intersections on the southern flank of the granite batholith at 140-200^oC and < 500 bars. Mixing caused base metal deposition by increasing pH and decreasing salinity.