In Renfrew County corundum is erratically distributed through a banded complex of rocks rich in scapolite, andeclase, and nepheline but is more abundantly developed and uniformly distributed in contact zones between this banded complex and a younger hybrid alkaline syenite.
The scapolite-andeclase-nepheline gneiss complex may have formed by (1) the metasomatic alteration of intercalated calcareous and aluminous metasediments, or (2) the intrusion of sill-like masses of an anorthositic magma into such metasediments. No positive evidence supporting either of these hypotheses was obtained in this study. In both hypotheses it is assumed that some corundum formed by the transformation of alumina-rich parts of the metasediments into corundum-bearing rock either by metasomatism or by magmatic assimilation.
The hybrid alkaline syenite appears to have developed by metasomatism of the rocks of the earlier complex. In this process potassium and silica were added, and lime, soda, and alumina were released from the scapolite, andeclase, and nepheline rocks. Much of the corundum in the contact zones is attributed to the crystallization of some alumina released from the older rocks during the transformation into syenite. Some of this alumina may have migrated, with the hydrothermal solutions responsible for the metasomatism, to loci favorable for the development of corundum-rich pegmatitic facies of the hybrid alkaline syenite.