Interpretation of resistivity and magnetic anomalies from the Fox River Sill, Trans Hudson Orogen, Canada

Abstract

The Fox River Belt is a sequence of rocks at the margin of the Proterozoic Trans Hudson Orogen in Canada that have been intruded by the Fox River Sill, a stratiform ultramafic-mafic sill. An earlier 2-D magnetotelluric (MT) study of the sill revealed a conductor that is spatially correlated with a sheared serpentinite unit in the Lower Central Layered Zone of the sill. Re-analysis of the data from 10 MT sites lying on a 1.4 km north-south profile, approximately perpendicular to geological strike, across a 1 km wide portion of the sill produced a resistivity model containing a conductor with an average resistivity of <1 ohm.m. Using aeromagnetic data from a profile subparallel to the MT profile, a geologically constrained magnetic model of the sill was constructed. Empirical susceptibility-magnetic mineral content relationships were used to estimate the magnetite content of the different geological units from the magnetic model. The results indicated a susceptibility of 0.2 SI for the sheared serpentinite unit, suggesting a magnetite content of ~5% which compares with petrological estimates of up to 10%. The bulk resistivity of geological units in the resistivity model was interpreted in terms of metallic mineral content using published resistivity relationships and a range of connectivity models. Integration of these results with magnetic and geological analyses suggests the enhanced conductivity in the sheared serpentinite is a result of a higher degree of magnetite interconnectivity due to the shear fabric. The analysis also reveals that although portions of the adjacent Marginal Zone in the sill contain concentrations of magnetite similar to those in the sheared serpentinite, the significantly higher resistivity of the Marginal Zone can be explained by a lower degree of magnetite interconnectivity.