Alteration
and mineralisation at the Island Copper deposit is related to the intrusion of
Early Jurassic quartz feldspar porphyry dykes into surrounding comagmatic
Bonanza volcanic rocks. The pyrophyilite alteration zone forms an upward flaring
cone, largely hosted within a breccia body that cross-cuts the Co, Mo, and
Au-rich ore shell. This alteration zone contains pyrophyllite, dumortierite and
diaspore and resembles alteration occurring in some high-sulfidation deposits,
but which is barren. Fluid-mineral equilibria and fluid inclusion studies
indicate formation at approximately 300 C from fluids dominated by steam and
gases. These fluids appear highly acidic, but sulfur poor, leading to extreme
base leaching within the breccia body and surrounding
rocks.