Abstract

Aeromagnetic mapping of the English River gneissic belt (northwestern Ontario) reveals a pattern of sinuous east–west trending anomalies that help distinguish the gneissic belt from the more typical Archean metavolcanic–plutonic terrain. To study the significance of the pattern, a ground follow-up study was made where Lac Seul provides good access and outcrop. Field work consisted of (1) magnetometer profiling on the lake; (2) detailed searching for outcrop of anomaly-causing material and sampling; and (3) regional geological mapping and sampling. Thin section studies and physical property measurements were made in the laboratory as well as an interpretative study of the aeromagnetics using an "apparent susceptibility" data transformation.The main results are: (1) metamorphism is in upper amphibolite to granulite facies; (2) on average, rocks of the area have higher susceptibilities than are typical of the metavolcanic–plutonic regions; (3) the aeromagnetic anomalies are caused by bands of pyroxene–amphibolite gneiss; (4) the study area may be interpreted as a volcanic and sedimentary sequence deformed into a large, east–west striking, nearly isoclinal fold structure which has been intruded and highly metamorphosed; and (5) aeromagnetic data can be used to extend and complete geological mapping in the area.

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