Microchemical characterization of alluvial gold from Scotland

Alluvial gold grains frequently contain microscopic mineral inclusions, usually in the size range 5 to <1 mu m, comprising opaque minerals, silicates, carbonates and phosphates. Quantitative point analyses and microchemical mapping undertaken with an automated electron microprobe serve to identify the minerals, and in a study of Scottish gold grains 52 different types of opaque mineral were recognized. These inclusions, many of which are unstable in surficial conditions, are inherited from the bedrock source mineralization and are preserved despite processes of weathering and transport. A combination of grain chemistry, the nature of internal chemical heterogeneity and the identity of the inclusions provides a gold grain signature that reflects the type and history of the source mineralization. It is necessary to investigate several grains from an alluvial site to take account of compositional variation in bedrock mineralization and because polished sections typically reveal inclusions in only 10-20% of grains. Most gold particles are concentrated in the lower part of a stream bed and in cracks and fissures within bedrock, where present. Bedrock traps may restrict the mobility of gold grains in streams and specialist collection techniques have been developed to recover gold grains from these environments. A total of 1530 gold grains were obtained from 30 localities in the Southern Uplands, the Ochil Hills and throughout the Highlands of Scotland. On the basis of,gold chemistry and inclusions the Scottish gold studied has been divided into ten types, two of which are minor variants. These varieties can be related to different types of source mineralization or mineralization events. In the Scottish Highlands two types of gold are possibly related to centres of Caledonian igneous activity, but the most widespread type is probably of deep-seated origin and structurally controlled. In the Southern Uplands two types probably reflect separate episodes of structurally controlled mesothermal lode gold mineralization and a further variety is derived from mineralization associated with Permian red-bed basins and their associated mafic volcanics. A fourth type of Southern Uplands gold has a signature that suggests a mafic igneous association and is possibly related to dykes of Tertiary age.