Platinum-group elements in ultramafic rocks of the Upper Deveron Valley, near Huntly, Aberdeenshire

A programme of exploration for the platinum-group elements (PGE) in the upper part of the Deveron valley south-west of Huntly is described. The geology of this area, the Upper Deveron Belt, is reviewed in the light of recent mapping carried out by the multi-disciplinary East Grampian Project. The main focus of attention was a series of deformed mafic-ultramafrc Caledonian intrusive bodies located within a major regional shear-zone, the Portsoy Lineament. The importance in geochemical exploration for the PGE of obtaining high quality analyses to detection limits of a few ppb is stressed. This is particularly significant in view of the lack of knowledge regarding PGE distribution in the secondary environment and to the low background levels present in geochemical sample media. Limited reconnaissance surveys were conducted in order to define the most promising target zones. The drainage data from partially panned concentrate samples showed a good correlation with the known geology but failed to identify any source of PGE or Cu-Ni mineralisation. In one area, however, a grain of sperrylite (PtAs2) was recovered during panning at a site within a poorlyexposed serpentinite near Bridgend. Detailed follow-up using overburden and rock sampling, guided by a detailed ground magnetic survey, defined a zone of Pt enrichment in basal overburden close to the margin of the serpentinite. Drilling was conducted to investigate a zone of potential in a similar setting near Kelman Hill. In one borehole a section of PGE-enriched serpentinite (up to 280 ppb Pt + Pd) was identified. The host rock has elevated Cr levels with sporadic enrichment in As. A new technique of automated searching for rare phases using the electron microprobe has been applied to this drillcore, resulting in the detection of several complex PGE-bearing grains intergrown with nickel arsenide. A hydrothermal origin for these grains and the observed PGE enrichment in the serpentinite is proposed. Drilling was also undertaken in the Kelman Hill area to investigate a series of Dahadian (Argyll Group) mafic-ultramafic volcanic rocks which crop out in the southern half of the Belt. These were considered to have potential for the occurrence of PGE-bearing Cu-Ni mineralisation and, in altered sections, for the occurrence of Au. The results of the drilling were not encouraging for the precious metals, but provided useful sections through these poorly-exposed unusual lithologies. The reconnaissance survey also demonstrated elevated PGE contents in clinopyroxene-bearing ultramafic rocks in the Succoth - Brown Hill intrusion in the north-east of the Belt. Investigation by drilling in one zone near Red Burn confirmed these enhanced background levels, with Pt/Pd around 1. Higher concentrations were revealed in some sections, up to a maximum of about 270 ppb Pt + Pd, with many values in excess of 100 ppb. The highest levels are often accompanied by an increase in Pd relative to Pt, and sometimes by elevated Au values. Automated searching on the microprobe revealed several grains of Au, together with Pt-Cu and Pd-Sb minerals, in association with base-metal sulphides in sheared and altered host rocks. A possible hydrothermal origin for this mineralisation is indicated.