Disseminated copper-molybdenum mineralisation near Ballachulish, Highland Region

Chalcopyrite-pyrite-molybdenite mineralisation, in disseminated, veinlet and fracture-filling forms, is developed in adamellite and microadamellite in the Ballachulish igneous complex. Minor scheelite is associated with the sulphides, but is mostly confined to the adamellite. The mineralisation occurs sporadically over an area of at least 1800 x 800 m. It is best developed in and around the eastern part of the microadamellite over an area of about 250 x 450 m, where it was observed over a.vertical interval of 250 m from the highest exposure to the base of a borehole. An IP survey showed that chargeability values are slightly higher in this area. The grade is variable. In 10 ft (3 m) lengths of core, the maximum Cu content was 264 ppm and the maximum molybdenum content 501 ppm, but the average tenor over the (250 x 450 m) mineralised area is not more than 50-100 ppm Cu and lo-30 ppm MO. Selected mineralised outcrop samples gave values of up to 2386 ppm Cu, 9257 ppm MO, 2434 ppm W, 0.31 ppm Au and 8 ppm Ag. Rb-Sr isotopic studies indicate that the ore minerals were deposited shortly after emplacement of the host rocks, and it is considered that they were introduced by a hydrothermal system which, compared with those of classic porphyry models, was small in extent and weak in intensity. Sericitic alteration is generally associated with the mineralisation, but there is no potassic alteration evident and the standard zonation of porphyry copper deposits is absent. There is very little K or Rb metasomatism, the best defined chemical change being a loss of Sr in altered rocks. The hydrothermal fluids, as seen in fluid inclusions, were of moderate salinity, unlike the high salinity fluids usually characteristic of porphyry copper deposits. Anomalously low Rb and high K/Rb values in the unaltered microadamellite are attributed to the separation of a Rb-rich aqueous fluid from the microadamellite before or at the time of consolidation of the rock. The mineralised area lies adjacent to and northwest of a NNE-trending shatter belt, which may have provided structural control at depth, although at the present level of exposure the microadamellite body appears to be the structural control.