Disseminated molybdenum mineralisation in the Etive plutonic complex in the western Highlands of Scotland

Molybdenite, mainly in quarts veinlets, ocours over an area about 5 km in diameter within the Central Starav Granite. Although selected samples of mineralised rock have been shown to contain up to 0.9% MO, incidence of sulphide mineral is too sparse, even in the best areas, for a meaningful estimate of tenor to be given. Molybdenite is usually accompanied by pyrite. Chalcopyrite and scheelite are also widespread, though less common. Mild hydrothermal alteration accompanies the mineralisation, but there is no pervasive or zoned alteration, nor is there any K or Rb metasomatism. It is suggested that the ore minerals were deposited from hydrothermal fluids which, for lack of any structural or physico-chemical constraints, circulated freely throughout a large volume of rock with the consequence that the ore minerals are widely dispersed. Within the central mineralised area, the Central Starav Granite is more evolved in composition below 650 m than above that altitude, suggesting that present exposure may be near the original roof of the intrusion. Most of the molybdenite mineralisation lies below 650 m and the Co2 contents of fluid inclusions are generally higher at higher altitudes. A drainage geochemical survey, covering most of the Etive plutonic complex, involved the collection and chemical analysis of 428 stream sediments and panned concentrates. The highest concentrations of MO in stream sediment (40 - 120 ppm) came from streams draining the central mineralised area and only this area was examined in detail. The investigation provided no evidence for the existence of exploitable mineral deposits at surface or for their prediction in depth. The possible extent of the mineralised body at depth can only be tested by drilling, but it is suggested that further examination of exposures, collection of additional samples of vein quarts for fluid inclusion studies, and an Induced Polarisation survey would provide more information about mineralisation at and near the surface and might help to define a target for drilling.