Gold in the Ochil Hills, Scotland

Mineral reconnaissance drainage sampling in the late 1970s identified gold in heavy mineral concentrates collected from a number of localities in the Ochil Hills. Subsequent detailed sampling of these localities showed that alluvial gold is present over a large area of the central Ochils and extends eastwards to the Firth of Tay. The most anomalous catchment, Borland Glen, near Glen Devon, was the focus for further integrated geological, geochemical, and geophysical studies. The geology of Borland Glen comprises a series of Lower Devonian andesitic lavas and pyroclastics intruded by a diorite body and porphyry dykes. Minor hydrothermal alteration is visible at surface. A large induced polarisation anomaly was found near the watershed between Borland Glen and Coul Burn and was interpreted as a steep-sided zone of disseminated pyrite with associated hydrothermal alteration. Overburden sampling proved gold and mercury anomalies over the same area. Seven boreholes were drilled to a maximum depth of 102 m to investigate the source of the IP and overburden geochemical anomalies. Intense hydrothermal alteration and brecciation were found to have affected the lavas and pyroclastics in the central, IP-anomalous zone and were accompanied by major pyritisation with associated minor base metal sulphides. Gold values in the drillcore reach a maximum of 505 ppb Au and it is concluded that the bedrock source of the alluvial gold has not been proved. However, the intense hydrothermal alteration in the setting of an evolved calcalkaline volcanic complex is indicative of a large epithermal system, and a more fertile source may yet be discovered in the area. Other gold sources are indicated in the central Ochils and further detailed investigations are thought to be warranted.