A gabbro body on northern Yakobi Island is geologically permissive for nickel-copper deposits similar to those that occur at Takanis and Bohemia Basin in the Sitka D-8 quadrangle, about two miles south of the southern boundary of the Mt. Fairweather A-2 quadrangle. (The gabbro body on Mt. Fairweather A-2 quadrangle is about 6 miles northwest of the Takanis nickel-copper deposit.) The structural setting on the northwest part of Yakobi Island is the same as at Bohemia Basin where barren and ore- bearing gabbro occur within the same large dike-like intrusion of non-foliated light colored sodic quartz diorite of Tertiary age that hosts the gabbro body on Fairweather A-2. The dike-like quartz diorite mass crops out for about seven miles on the Mt. Fairweather quadrangle; to the south, it thickens, becomes more complex, and extends into the Sitka quadrangle for about ten more miles (Rossman, 1959, plate 12; Loney and others, 1975, plates 1 and 2).
The eastern part of the gabbro body on northern Yakobi Island is on the mainland of the island; much of the body is on an offshore island complex.Nickel-bearing norite gabbro is unknown at the occurrence, but the intrusion is large enough to host nickel-copper deposits in the body, especially the part exposed on small islands and largely covered by sea water.