The Bay of Islands Igneous Complex is composed of layered intrusions that have thick ultrabasic zones overlain by zones of banded gabbroic rocks. Chromite concentrations are localized in the upper parts of the ultrabasic zones. An explanation of the localization is offered, based upon an increase in the alumina content of certain layers near the gabbroic rocks, and the tendency of spinel to form in the system anorthite-forsterite-silica. The chromite concentrations are considered to be the result of an increased rate of crystallization of spinel, due to a higher alumina content, accompanied by the diffusion of chromium through the surrounding interstitial fluid, without a prior concentration of chromium in the magma. The barren nature of the lower horizons is due to a deficiency of alumina to aid in the segregation of spinel. The deposits are late magmatic segregations.

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