Abstract: Optical microscopy of doubly polished thin sections of North Pennine sphalerite has revealed a range of previously unrecognised textures for the Alston Block mineralisation. Delicate growth zoning, interrupted by numerous solution disconformities, was seen in transmitted light. Two principal varieties of growth-banded sphalerite are recognised; the earlier (Type 1) is characterised by the development of thin opaque bands. Type 2 has colour bands between yellow and brown, correlated with iron content. In Type 1, iron levels (up to 3 wt.%) are not sufficient to account for the observed opacity. Ultra-violet and infra-red techniques failed to detect any organic inclusions. Electron microscopy revealed locally high concentrations of sub-micrometre inclusions, both beam-stable and beam-unstable, and a variety of growth-related crystal defects.

Fluid inclusion thermometry in both sphalerite varieties and the accompanying quartz gangue implies a saline mineralising fluid (20–25 wt.% equiv. NaCl) at a relatively low temperature (100° to 140°C). Tubular inclusions are conspicuous. A deformation-induced lamelliform optical anisotropy is superimposed on a growth-related grid-iron anisotropy. Growth band offset is apparent where the deformation fabric cross-cuts the growth banding. Deformation on {111} twin and slip planes was indicated by electron microscopy.