Investigations for Cu-Ni and PGE in the Hill of Barra area, near Oldmeldrum, Aberdeenshire

Following promising indications from commercial surveys, limited investigations for Cu-Ni mineralisation have been conducted in the vicinity of the Hill of Barra at the eastern end of the Insch intrusion. By analogy with recent findings in the Huntly area, this sheared and disrupted zone of olivine-bearing cumulates was regarded as potentially favourable for the occurrence of PGE enrichment in association with the base-metal mineralisation. The first phase of work, comprising overburden and soil sampling, confirmed the results a the previous commercial survey and defined areas with enhanced Cu-Ni levels. The second phase of investigation was concerned additionally with the evaluation of the PGE potential of the Barra area. Basal overburden sampling and a ground magnetic survey were conducted to elucidate the structure of the area and to examine the trace element distributions and their controls. The magnetic survey data provided important information on the structure of the survey area, clearly delineating the southern contact of the intrusion and highlighting structural discontinuities within the olivine-bearing cumulate rocks which predominate in the survey area. Enhanced levels of Cu and Cu/Ni ratios in overburden were revealed above these structures indicating their potential as zones favourable for the occurrence of hydrothermal base metal enrichment. The precious metal concentrations were uniformly low. It is however significant to note that the single site with an above background Au concentration also possesses the highest As level observed in the survey area, and is located very close to the contact of the intrusion with the Dalradian metasediments. A small suite of rock samples was analysed for a wide range of trace elements, including Pt, Pd, Rh and Au. No significant enrichments in base or precious metals were revealed. The depleted levels of Cu and Ni noted in the Barra olivine-rich cumulates relative to those in Huntly may indicate their derivation from a magma from which significant sulphide segregation had already taken place. A possible site for a magmatic sulphide accumulation produced in this way is suggested by the overburden data at the base of the main peridotite body defined by the magnetic data. The investigations described in this report have failed to identify any significant Cu-Ni or PGE mineralisation in the vicinity of the Hill of Barra. Nevertheless potential remains for PGE enrichment in association with the tectonised pods of ultramafic rocks which are found along the margins of the Insch intrusion further to the west. The integrated use of overburden geochemistry and ground magnetic surveys has been shown to be particularly effective at elucidating the geological structure of poorly-exposed ground covered by exotic overburden. A similar approach is recommended for use elsewhere in north-east Scotland.