Marine deposits of chromite and olivine, Inner Hebrides of Scotland

A reconnaissance survey was carried out of near-shore marine deposits considered to have been derived from Tertiary ultrabasic rocks in SW Skye and southern Rhum, rocks known to be enriched in chrome spine1 and forsteritic olivine. Dive sampling close to the rocky coastlines supplemented grab sampling at surveyed locations in water depths of 50 m or le s. In the bay off Harris, southern Rhum, a heavy mineral sand deposit 3 km 9 in a ea occurs within 2 km of the coast in waters 20-25 m in average depth. A 1 km !? deposit is present up to 1 km off Dibidi Using a wet density of 2.2 g.cm 3 in an average water depth of about 20 m. some 9 million tonnes of sand are calculated to be present in the topmost 1 m of the deltas. Shell calcite forming about 20% was removed prior to chemical analysis. The analytical results indicate that the surficial 1 m of sand contains some 70 000 tonnes of chrome spine1 averaging 32% Cr20 zi at a grade of nearly 1%. Also present are 1.5 - 2 million tonnes of olivine veraging 47% MgO at 25% grade. Accompanying minerals are ilmenite and vanadiferous magnetite and traces of PGE have been detected. The minerals occur in sand-size fractions (125-500 m from which concentrates of 86% chromite and 78% olivine at recoveries of 60% and 50% respectively have been achieved in the laboratory. Most seabed samples from Loch Scavaig and the Soay Sound, SW Skye (63) are grey glacial sandy clays averaging only 0.05% Cr 0 , 2% Mg after carbonate dissolution. Heavy mineral sands derived from the isu? llins igneous centre may nevertheless underly the glacial deposits. Before glaciation, the 60 million year old ultrabasic rocks of Skye and Rhum were deeply eroded and their detritus supplied to the Sea of the Hebrides. The Harris and Dibidil deposits, detected in a survey of the Rhum coastline from A'Bhrideanach in the extreme west to Loch Scresort in the east (99 samples), formed less than 10 000 years ago.