Banded Sphalerite from the North Pennine Orefield

A. P. More, D. J. Vaughan and J. R. Ashworth
The Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE
Department of Geology, The University of Manchester, Manchester M13 9PL
School of Earth Sciences, The University of Birmingham, Edgbaston, Birmingham B15 2TT

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.

Keywords: sphalerite • North Pennine orefield • zoning

Mineralogical Magazine; September 1991 v. 55; no. 380; p. 409-416; DOI: 10.1180/minmag.1991.055.380.10
© 1991, The Mineralogical Society
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