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 |
| |
Authors: | R E Clayton B Spiro |
| |
Institution: | (1) School of Earth Sciences, Birkbeck College, University of London, London WC1E 7HX, UK e-mail: r.clayton@geology.bbk.ac.uk, GB;(2) NERC Isotope Geosciences Laboratory, Keyworth, Nottingham, NG12 5GG, UK, GB |
| |
Abstract: | The strongly deformed Middle Devonian-Lower Carboniferous metasedimentary-volcanic successions of the Trevone Basin (SW England)
contain stratiform and Pb-Sb vein deposits that reveal a wide variation in δ34S and δ13C, reflecting mineral deposition during diagenesis, regional metamorphism and basin inversion. Pre-Variscan metasedimentary
sulphide (δ34S=−33.7 to −26.7‰) and metabasite sulphide (δ34S=+4.0 to +10.8‰) suggest two accessible source reservoirs for sulphur which were available for Sb-As-(Au) and Pb-Zn-(Ag)
mineralisation (δ34S=−3.3 to −15.0‰) during late Variscan semiductile-brittle shear. On the basis of pressure-corrected fluid inclusion temperatures,
the calculated composition of fluid sulphur reveals an enrichment in δ34SH2S in the individual vein parageneses and depletion of the fluid sulphur reservoir during evolution of the vein systems. Carbonates
in the same veins are partly contemporaneous with Pb-Sb mineralisation and late tensional deformation; their isotopic composition
(δ13C=−3.2 and −13.4‰) appears strongly influenced by the host formation. Fluid inclusions in post-tensional quartz show a marked
reduction in CO2, suggesting that episodes of CO2 degassing in response to punctuated reductions in pressure during uplift and brittle deformation was an important mechanism
for vein carbonation. An origin for the Pb-Sb mineralisation involving local remobilisation of sulphur from the mixed metasedimentary-volcanic
succession is probably inseparable from processes connected with Variscan metamorphism and deformation. Although the N Cornish
Variscan deformation is part of a spatially large-scale event, the isotopic evidence suggests compartmentalisation of sulphur
and carbon isotope features and short distances between sources and sinks.
Received: 15 August 1998 / Accepted: 8 October 1999 |
| |
Keywords: | |
本文献已被 SpringerLink 等数据库收录! |
|