Geophysical investigation of chromite-bearing ultrabasic rocks in the Baltasound-Hagdale area, Unst, Shetland Islands

Economic deposits of chromite were discovered in Unst, Shetland Islands in the early part of the nineteenth century and extraction continuedi ntermittentlyu ntil exhaustiono f the known near-surface deposits in 1945. Since it is likely that further comparable deposits exist at shallow depth, detailed geophysical smeys employing 2 magnetic and electrical methods were carried out over ? km of the area of known mineralisationt o test the feasabilityo f detectinga nd delineatingt hem. Seven of 16 small positive gravity arumaJ.ie&se tested by shallow boreholes but only two were attributed to chromite concentrations.T he gravity anomaliesa t the other borehole sites remain unexplained; they may be due to unidentified variations in bedrock density at depth; they may be of weathering and the thickness of in overburden thickness.