Metasomatic Alteration Control of Petrophysical Properties in the Great Bear Magmatic Zone (Northwest Territories, Canada)

The Great Bear magmatic zone in the Northwest Territories of Canada contains large iron-oxide alkali alteration systems noted for having high potential for iron oxide-apatite, iron oxide-copper-gold, and affiliated ore deposits. Physical properties (density, magnetic, and electrical) were measured on 824 rocks samples selected to represent the range of metasomatic alteration types in the region. Mineralogical and geochemical classification of the prograde iron oxide and alkali alteration facies reveals large variations in physical properties through the evolution of the metasomatic systems. In particular, deep and early sodic alteration produced rocks having low densities and low magnetic susceptibilities. During calcium and iron precipitation, rocks gain extremely high density and susceptibility, due to crystallization of amphibole and especially magnetite. Subsequent hightemperature, potassic- and iron-altered rocks are marked by cocrystallization of magnetite with K-feldspar or biotite, and as the transition from magnetite to hematite takes place, K-feldspar crystallizes instead of iron oxides leading ultimately to potassic felsites having low densities and susceptibilities. Subsequent cooler, shallower, and more oxidized potassic-iron alteration produced high densities but moderate susceptibilities owing to crystallization of hematite. Understanding these major variations in physical properties of rocks enables detailed geophysical mapping of alteration zones, which builds and improves the regional context for integrated mineral exploration vectors to potential ore deposits.