Alder Island is underlain by complex geology, including folded limestone and argillite of the Jurassic to Triassic Kunga Group, Middle Jurassic Yakoun Group volcanics, Lower Cretaceous Longarm Formation sandstones and Tertiary Masset Formation basalts. Granitoid intrusives related to the Middle to Late Jurassic Burnaby Island plutonic suite intrude both Kunga and Yakoun Group rocks.

Visible gold with minor sphalerite occurs in brecciated, silicified, thin-bedded, black limestone of the Peril Formation of the Kunga Group. The best assay obtained was 10.9 grams per tonne gold over 15 centimetres (Assessment Report 8251).