Petrology and Geochemistry of the Heron Lake Stock, Superior Province, Wabigoon Subprovince, Northwestern Ontario

Abstract

The Heron Lake Stock is a lenticular shaped, pretectonic granitoid complex intruding the Jutten metavolcanics of the Savant Lake Greenstone Belt, Wabigoon Subprovince, Superior Province. Mapping, petrography, and chemical analyses have revealed that the stock is essentially trondhjemitic, with minor quartz diorite, granodiorite and quartz monzonite. The trondhjemites have undergone a high degree of sericitization and saussuritization. The grandiorite unit is characterized by secondary K-feldspar and the quartz monzonites are characterized by perthite formation as a result of K-autometasomation is the late stage potash-rich fluids. Late faulting has imposed a secondary foliation along the stock's southern boundary. K/Rb ratios suggest partial melting of lower crust/ upper mantle material producing a trondhjemitic magma. Chemical variation diagrams suggest a process of magmatic differentiation and fractional crystallization.