The area around Attwood Bay is underlain by Jurassic to Cretaceous Coast Plutonic Complex quartz monzonite and hornblende quartz monzonite. Contained within the monzonite is a wedge of meta- morphosed Lower Cretaceous Gambier Group mafic volcanic rocks trending 120 degrees.

The monzonite is massive, medium to coarse-grained, equigranular and is specifically a hornblende quartz monzonite. The volcanic rocks are dark grey to green, fine-grained and of mafic (andesitic) composition. Minor tuff, porphyry and cherty bands are evident. The volcanic rocks show both a faulted and intrusive contact with the hornblende quartz monzonite. Kaolinization, mylonitization and sili- cification are evident in the rock surrounding the faults. In many places the monzonite is cut by narrow (less than 5 metres thick) multi-directional fine-grained green to grey diorite dykes.

A diorite dyke-monzonite contact contains a small quartz vein with pyrite and chalcopyrite. A 2 to 3 kilogram chip sample from the quartz vein assayed 0.93 per cent copper, 12.0 grams per tonne silver, 0.080 grams per tonne gold, 0.0015 per cent lead, 0.0147 per cent zinc, 0.0003 per cent molybdenum and less than 0.0002 per cent arsenic (Assessment Report 10806).