Relative elemental mobility during hydrothermal alteration of a basic sill, Isle of Skye, N.W. Scotland

In Sconser quarry, Isle of Skye, a thin Tertiary basic sill is cut by vertical veins which formed fluid conduits in a major meteoric-hydrothermal system. In order to study the relative mobility of different elements during hydrothermal metamorphism, sill material adjacent to a large (6 mm wide) vein was cut into slices and subjected to geochemical and isotopic analysis.Isotopic evidence indicates that the basic magma which formed the sill was contaminated by Lewisian (Archaean) gneisses at depth, while material from Torridonian (Proterozoic) sandstone country rocks was introduced by hydrothermal fluids after crystallisation.The behaviour of the different elements during hydrothermal metamorphism divides them into 4 groups.Large-ion low-charge elements Ba, K, Rb and Cs were strongly leached from the wall rock in the vicinity of the vein.Other elements including Sr and Pb were depleted near the vein, but isotopic evidence indicates addition of some material from the fluids. This two-way nuclide transport forms an exchange process.Many high-field-strength elements including the REE are slightly enriched near the vein, but Nd isotope evidence reveals no addition of material from fluids. These elements must have been relatively enriched by the removal of other elements (mainly Si and Al).Ca and Na were added to the wall rock from fluid. The variable mobility of these elements is explained by the differing ease with which they could be incorporated into a new albite-calcite-chlorite-epidote mineralogy. The constitution of this hydrothermal mineralogy was largely determined by the primary igneous mineralogy, though the composition of hydrothermal fluids had a subordinate influence.