Stratabound arsenic and vein antimony mineralisation in Silurian greywackes at Glendinning, south Scotland

Stratiform and disseminated pyrite-arsenopyrite concentrations are overprinted by fracturecontrolled polymetallic mineralisation including stibnite through at least tens of metres of Silurian sediments at Glendinning, near Langholm. Three shallow boreholes were drilled on an anomaly defined by VLF-EM and II? surveys and by antimony values X0 ppm in thin B-C horizon soils. A parallel conductive zone with an accompanying soil anomaly but lacking an IP response was investigated by a fourth hole. The stratabound sulphides form disseminations and bands parallel to the bedding and are particularly concentrated in intraformational breccia units regarded as debris flows, which, together with the presence of smaIl scale slump folds in the greywackes, testify to the existence of an unstable slope during sedimentation. The thickest such unit has a true thickness of 4 m and together with 8 m of adjoining greywackes grades 0.7% As. Phases of fracture-controlled Fe-As-Sb-Pb- Zn-Cu-(?)Hg mineralisation associated with widespread dolomite and quartz veinlets and narrow breccia veins are superimposed on the stratabound mineralisation. Their spatial association with the stratabound mineralisation, the presence of up to 0.33% Sb in the stratiform arsenopyrite and as much as 5% As in the stratiform pyrite, favour a common source for the arsenic and antimony. This source was probably a synsedimentary metal accumulation in a mid or lower fan environment where euxinic conditions periodically developed.