PRECAMBRIAN RESEARCH CENTER PROFESSIONAL WORKSHOP SERIES Field, Petrographic, and Mineralization Characteristics of Mafic Layered Intrusions

The Sonju Lake intrusion (SLI) is a 1200-m-thick, shallow-dipping, sheet-like intrusion that forms part of the Beaver Bay Complex (RI-58, Chapter 7). Although its exposed strike-length is only about 3 km (Fig. 3-1), the SLI has a distinctive aeromagnetic signature that can be traced for at least 20 km beneath a cover of glacial drift. The nearly constant width of its aeromagnetic anomaly suggests that the intrusive sheet retains a fairly uniform thickness and dip angle over most of this distance. The eastern margin of the SLI is abruptly truncated by the Beaver River diabase dike. The base of the SLI is in intrusive contact with a complex mixture of gabbroic to dioritic rocks, granophyre, and volcanic hornfels. The top of the SLI is in gradational contact with a granophyric quartz ferromonzodiorite phase of the Finland granite. The granite is thought to have acted as a density barrier that cause the SLI mafic magma to underplate it (Miller and Chandler, 1997).