The Gretna Green occurrence is located near Clemens Creek, approximately 1.3 kilometres north of Henderson Lake.

The area is underlain by mafic to intermediate volcanic rocks of the Upper Triassic Vancouver Group, Karmutsen Formation. These are overlain by limestone of the Quatsino Formation, also of the Vancouver Group. A stock of dioritic rock of the Early to Middle Jurassic Island Intrusions occurs along the northeast side of Henderson Lake.

Locally, a skarn zone with chalcopyrite ore occurs in limestone cliffs near a contact with dioritic rock. Masses of epidote and garnet occur near, but usually not with, pods and bunches of chalcopyrite.

The ore was reported to be of high grade but of limited quantity. One selected sample assayed 48.00 grams per tonne gold, 51.43 grams per tonne silver and 17.8 per cent copper Minister of Mines Annual Report 1921).