Relevance of aeromagnetic data to revision of geological maps, Purcell Anticlinorium

Regional aeromagnetic data provide one of the few data sets having adequate resolution and distribution to contribute to geological mapping at "conventional" scales such as 1:250,000 or 1:50,000. In southeastern British Columbia compilation of a block of 12 geological maps at a scale of 1:50,000 is presently being undertaken as part of the Targeted Geoscience Initiative Program. These cover a large segment of the Purcell anticlinorium between 49 and 50. The compilation incorporates information acquired principally from earlier government mapping (generally at 1:50,000 scale), and by Teck Cominco Ltd. during an extended period of exploration for SEDEX deposits. Aeromagnetic data contribute to the compilation by examination of correlations between geologically mapped units and magnetic signatures, and by providing detailed magnetic images where the terrain is poorly mapped. New geological information thus obtained can be integrated into the new maps. Resolution is key to whether a magnetic data set can provide meaningful input into a geological map at a specific scale. About 25% of the compilation area is covered by high resolution aeromagnetic data (400 m line-spacing: 60 m flight elevation) yielding detailed images of the magnetic field that in turn reveal fine details of the geology in three survey areas known as Findlay Creek, St. Mary River and Yahk, identified as areas 1, 2 and 3 in the figure to the left. Lower resolution aeromagnetic data collected as part of Canada's National Aeromagnetic Mapping Program (805 m line-spacing, 305 m flight elevation) cover the entire area. Reprocessing of these data yields good quality magnetic images that can contribute to geological mapping, particularly when derivative images are used. Both the high and low resolution magnetic data sets provide insight into different aspects of the geology, and also raise questions, which can only be answered by ground follow up. Magnetic signatures reflect compositional zoning and marginal phases in various igneous intrusions, define unmapped faults, point to unmapped subunits within broader units, and indicate repositioning of certain geologically mapped contacts.