Sulphur, carbon and oxygen isotope studies of early Variscan mineralisation and Pb-Sb vein deposits in the Cornubian orefield: implications for the scale of fluid movements during Variscan deformation

The strongly deformed Middle Devonian-Lower Carboniferous metasedimentary-v olcanic successions of the Trevone Basin (SW England) contain stratiform an d Pb-Sb vein deposits that reveal a wide variation in delta(34)S and delta( 13)C, reflecting mineral deposition during diagenesis, regional metamorphis m and basin inversion. Pre-Variscan metasedimentary sulphide (delta(34)S=-3 3.7 to -26.7 parts per thousand,) and metabasite sulphide (delta(34)S = +4. 0 to + 10.8 parts per thousand,) suggest two accessible source reservoirs f or sulphur which were available for Sb-As(Au) and Pb-Zn-(Ag) mineralisation (delta(34)S = -3.3 to -15.0 parts per thousand,) during late Variscan semi ductile-brittle shear. On the basis of pressure-corrected fluid inclusion t emperatures, the calculated composition of fluid sulphur reveals an enrichm ent in delta(34)SH(2)s in the individual vein parageneses and depletion of the fluid sulphur reservoir during evolution of the vein systems. Carbonate s in the same veins are partly contemporaneous with Pb-Sb mineralisation an d late tensional deformation; their isotopic composition (delta(13)C = -3.2 and -13.4 parts per thousand,) appears strongly influenced by the host for mation. Fluid inclusions in post-tensional quartz show a marked reduction i n CO2, suggesting that episodes of CO2 degassing in response to punctuated reductions in pressure during uplift and brittle deformation was an importa nt mechanism for vein carbonation. An origin for the Pb-Sb mineralisation i nvolving local remobilisation of sulphur from the mixed metasedimentary-vol canic succession is probably inseparable from processes connected with Vari scan metamorphism and deformation. Although the N Cornish Variscan deformat ion is part of a spatially large-scale event, the isotopic evidence suggest s compartmentalisation of sulphur and carbon isotope features and short dis tances between sources and sinks.