The showing is located within the Central Quesnel Belt of the Quesnellia Terrane. The region is underlain by Upper Triassic to Lower Jurassic Nicola Group sedimentary and volcanic rocks intruded by Lower Jurassic alkalic stocks and a calc-alkalic pluton of probable Cretaceous age. Within this Cretaceous stock, rafts and faulted blocks of Upper Triassic sedimentary and volcanic rocks are preserved.

The Tarn showing is underlain by one of these fault blocks, intruded by biotite diorite, biotite-hornblende diorite and some lamprophyre dikes. The country rocks have been silicified and hornfelsed by diorite intrusion.

Mineralization consists of copper, molybdenum, silver and gold within the contact aureoles and disseminated pyrite and pyrrhotite with minor chalcopyrite, chalcocite and molybdenite within both the country rocks and the dioritic intrusions. Minor skarn-type altera- tion is associated with fractures containing coarsely crystalline chalcopyrite and rosettes of molybdenite. A 30 centimetre sample across a contact skarn zone yielded 2.44 per cent copper, 0.003 per cent molybdenum, less than 0.01 per cent lead, 0.01 per cent zinc, 1.2 grams per tonne gold and 15.7 grams per tonne silver (Assessment Report 9891).