Qualitative geophysical interpretation of the Sudbury Structure

Abstract

The Sudbury Structure is one of the most studied geologic structures in the world due to its enigmatic nature and mineral wealth. The available geologic work from the literature and mining industry operations accumulated for more than a century was recently assessed and compiled into a bedrock geologic map. Most regional geophysical investigations of the Sudbury Structure have been quantitative — modeling and depth estimation without a clear definition of surface control. Airborne total magnetic intensity data over the Sudbury Structure were compiled, processed, and interpreted, to define magnetic stratigraphy boundaries and near-surface lineaments. Traditional directional and normalized derivatives were computed to enhance the high-frequency information in the magnetic field. Available airborne frequency-domain electromagnetic (EM) data were also interactively interpreted along profiles and in a gridded format to isolate conductive structures. On-screen geographic information system-based information extraction from multiple derivatives was used to interpret the magnetic contacts, dykes, and lineaments. The magnetic interpretation was compared with published bedrock maps of the Sudbury Structure. Magnetic contacts based on the qualitative classification of the magnetic texture did not always correspond to the geologic boundaries on the existing maps. Some magnetic lineaments corresponded with well-defined geologic structures, some were further extensions of partially mapped structures, and others are newly identified linear structures. Conductive locations identified from the EM profiles were probably due to responses from conductive ore bodies, faults, dykes, lithological contacts, and cultural objects.

Regional 3D geophysical investigation of the Sudbury Structure

“…Previous work (Gupta et al, 1994;Hearst et al, 1994;McGrath and Broome, 1994) and recent qualitative geophysical interpretation of high-resolution airborne geophysical data (Olaniyan et al, 2013) have not provided an adequate explanation of some extensive geophysical anomalies observed within the SIC (Figure 1a and 1b). These include (1) the abrupt discontinuity of the high magnetic intensity in the north range around the Sandcherry Fault, (2) the low magnetic intensity zone that extends from the north range to the south range at their junction with the east range, and (3) the linear gravity high observed in the center of the Sudbury Structure.…”

“…Magnetic data used in this study were extracted along the six profiles ( Figure 1a and 1b) from a new regional compilation (Olaniyan et al, 2013), which brings together data collected in more than 40 different surveys in a single database (courtesy Sudbury Integrated Nickel Operations and Wallbridge Mining). The resolution of the gridded magnetic data varies from 20 m within the SIC to 50 m in a 10-km-wide area to the north of the SIC.…”

“…Olaniyan et al (2014) highlight the major challenges in the merging of these two lines as modeling along an east-west line shows changes in depth and thickness of lithologic units and in the spatial location of the SICfootwall contact. The variations in the SIC lithologic units and the location of the footwall contact along the latitude from the eastern end of the SIC to the western end introduce some inconsistencies into the contacts earlier defined by the Milkereit merged Lithoprobe sections (Olaniyan et al, 2013).…”

“…The 2.5D geologic model for profile B-B′ (bottom) and the corresponding magnetic (top) from the compilation ofOlaniyan et al (2013) and airborne gravity data (middle) provided by Vale. The measured data are the thick dotted line, and the forward model data are the thin solid line.…”

“…The 2.5D geologic interpretation along the longitudinal section F-F′ (bottom) and the corresponding magnetic (top) and gravity (middle) data from the compilation ofOlaniyan et al (2013) and the airborne gravity data provided by Vale. The measured data in thick dotted line broadly form a dome shape and have higher intensities in the western part of the SIC.…”