Platinum-group elements in the Huntly intrusion, Aberdeenshire, north-east Scotland

This report describes a programme of exploration for the platinum-group elements (PGE) carried out over the Caledonian mafic-ultramafic Huntly intrusion, located in west Aberdeenshire, Grampian region, Scotland. The area was considered as having potential for two principal classes of PGE-bearing mineralisation, namely magmatic reef style in cumulate rocks and hydrothermal type in structurally-controlled settings. Recent re-mapping of the poorly exposed Huntly intrusion by BGS and Aberdeen University has revealed a complex internal structure and clarified the nature of its external contacts. Several discrete bodies of cumulate rocks have been recognised, while much of the intrusion comprises granular or contaminated gabbroic rocks without cumulate characteristics. A comprehensive programme of lithogeochemical sampling was carried out over the intrusion and adjacent country rocks, in order to detect any indications of PGE-bearing mineralisation. Olivine cumulate rocks have the highest background concentrations of Pt and Pd. Maximum values of 50 ppb Pt and 25 ppb Pd were recorded in a peridotite from the West Huntly cumulate body. Deformed ultramafic rocks in a tectonically complex zone near the northern margin of the intrusion around Whitehill are locally enriched in precious metals, up to 28 ppb Pt, 63 ppb Pd and 30 ppb Au. The highest levels of enrichment in precious metals (up to 462 ppb Pt +Pd+Au) were found in discordant mineralised pegmatitic pyroxenites in the West Huntly cumulate body, best exposed in the Bin Quarry. Drilling undertaken in the Bin Quarry showed that these bodies are narrow and impersistent at depth. The West Huntly cumulate block was investigated for reef style PGE mineralisation using a variety of techniques. In the Dunbennan Wood area, detailed ground magnetic surveys were utilised to guide basal overburden sampling along reconnaissance traverses. Sporadic minor PGE enrichment was detected, with levels generally enhanced in the fme fraction (-100 mesh/ -150 micron) samples, relative to panned material. A biogeochemical orientation survey was conducted over the same area using various sample media from the four principal conifer species present, namely larch, Scats pine, Norway spruce and sitka spruce. The twigs of all species sampled showed a positive response to both Pt and Au. Although no conclusive association with bedrock geology was established, a general enhancement of precious metals is noted in larch twigs over ultramafic cumulates in the Dunberman zone. Reconnaissance surveys were conducted over the Central Huntly Shear Zone, a branch of the Portsoy lineament which transects the Huntly intrusion and which is associated with PGE-bearing Cu-Ni sulphide mineralisation on the flank of the Knock intrusion a few kilometres to the north. Magnetic and VLF surveys failed to detect any indications of structurally-controlled sulphide mineralisation in this zone. Soil-gas surveys, involving the measurement of oxygen and carbon dioxide, were conducted along reconnaissance traverses across this structure in order to detect any indications of oxidising sulphide mineralisation. Significant anomalies, with CO2 content locally in excess of lo%, were revealed in several areas. In most cases these anomalies were attributed to pedogenic or biogenic causes. At Cumrie North, however, soil-gas anomalies investigated by pitting are ascribed to the tectonic juxtaposition of contrasting lithologies in a complex shear zone in the northern part of the structure. Only traces of sulphide mineralisation are present within this anomalous zone, but deformed ultramafic lithologies with minor enrichment in PGE were revealed by pitting. Soil sampling has indicated that additional bodies of ultramafic rock are also present in this sector of the shear zone.