The O'Donnel occurrence is located east of Atlin Lake about 29 kilometres south of the community of Atlin.

Carbonate sediments of the upper Mississippian to Permian Horsefeed Formation (Cache Creek Complex) forms a gentle ridge between the O'Donnel and Pike rivers. The formation consists mainly of pale grey, clean limestone approximately 900 metres thick. The limestone is commonly massive and in part recrystallized so that primary textures are obliterated (Monger, Geological Survey of Canada Paper 74-47).

Although limestone comprises most of the stratigraphy, exposures at the O'Donnel River site have been described as high-grade magnesite. The occurrence of local hot springs in the carbonates suggests a source of fluids for the alteration of limestones to magnesium carbonate.