Tuesday, March 21st
601 Booth Street
This meeting doubles as a KEGS Foundation fundraiser for geophysical scholarships.
Individuals who donate $20 or more will receive a tax receipt.
Results of a Magnetotelluric Survey across the enigmatic Darnley Bay anomaly,
Brock Inlier, Northwest Territories
Jim Craven and Rob Rainbird, GSC
The Darnley Bay gravity anomaly near Paulatuk, Northwest Territories is one of the largest discrete geophysical anomalies in the world and likely the largest in North America. It has a measured amplitude of 132 mGal, with dimensions of 80 km x 100 km and has a coincident 1,600 nT magnetic anomaly. The source of the anomaly is unknown and has been an enigma since its discovery by INCO geologists more than 60 years ago. It is buried beneath several kilometres of sedimentary rocks of age varying from Proterozoic to Quaternary and exploration drilling to date has been unsuccessful in penetrating this cover. The potential of the region around the Darnley Bay anomaly to host large-scale economic mineralization has been recognized for many years and it has long-been speculated that its source might be a deeply buried mafic magmatic body hosting Ni-Cu-PGE sulphides on the scale of Sudbury or Noril’sk. Previous exploration of the area was conducted by Inco and Cominco in 1991 and one, 1800m-deep, drill hole was sponsored by Falconbridge in 2000. Darnley Bay Resources drilled three shorter holes in 2010 over base metals targets interpreted from extensive airborne surveys, with follow-up ground geophysics, but did not reach the targets owing to permafrost, thick glacial drift and porous bedrock beneath it.
GSC’s current work in the region is intended to upgrade geoscience knowledge in the Brock Inlier, a large window of mainly Proterozoic and early Paleozoic sedimentary rocks with affinities to the Minto Inlier on Victoria Island and the Mackenzie Mountains to the southwest. The inlier overlaps the eastern edge of the Darnley Bay anomaly so it was thought that a better understanding of the exposed bedrock geology to the east of the anomaly would help us to understand what happens to sedimentary strata as they disappear under younger cover in the area to the west, over the anomaly.
Magnetotelluric (MT) data were acquired along an east-west transect to trace sedimentary rocks layers westward from the Brock Inlier into the subsurface and to improve our understanding of the nature, size and depth of the Darnley Bay anomaly. Co-located audio- (AMT) and broadband (BBMT) magnetotelluric sites were collected at 17 locations during the 2015 summer field. The transect crosses the eastern margin of the Darnley Bay anomaly and into the exposed Brock Inlier sedimentary rocks. The Darnley Bay MT data have been processed using modern, robust methods and have been analyzed for non-uniform source fields, dimensionality, effects of distortion, geo-electric strike angles, and ocean sea water effects. Electrical resistivity profiles derived from rigorous 2D inversions of the MT impedance tensor highlight areas of low resistivity. These low resistivities are commonly associated with rock types containing conductive mineral assemblages or may represent a proxy for zones of deformation, fault zones, and fossil fluid pathways. Here we present and discuss two-dimensional resistivity models along the MT transect.
Jim Craven is a physical scientist with the Geological Survey of Canada in Ottawa and an adjunct professor at the University of Manitoba’s Department of Earth Sciences. He graduated with a M.Sc. in geophysics from the University of Toronto in 1998. He has worked with magnetotelluric datasince being a summer student at the Earth Physics Branch in Ottawa in 1984. His main research interests are: developing new tools for analyzing and interpreting magnetotelluric data and led the first ever project to attempt to use ambient mine noise for seismological mineral exploration. Jim enjoys soccer and music, but finds he can’t play either anywhere as well as his kids.
Rob Rainbird is a research scientist with the Geological Survey of Canada in Ottawa and an adjunct professor at Carleton University’s Department of Earth Sciences. He graduated with a PhD in geology from the University of Western Ontario in 1991. He has worked in the Canadian Arctic as a field geologist for more than 30 years doing geological mapping, resource assessment and thematic research on Proterozoic sedimentary basins. His main research interests are: developing new tools for interpreting geological information from remotely sensed imagery, regional and global-scale correlation of Proterozoic stratigraphy and its utility for reconstructing supercontinents and
understanding Precambrian Earth history. He likes to play hockey, listen to music and take photographs of all the great places he’s been to.