The Sudbury Structure

by Richard Busch

Lithosphere (February 1993); Fallbrook Gem and Mineral Society, Inc.; Fallbrook, CA

The Sudbury district, located in Ontario, Canada, is the site of one of the largest and richest nickel-copper sulfide deposits in the world. The deposit itself is contained within an enigmatic geologic feature called the Sudbury Structure -- a kidney-shaped basin that measures approximately 25 by 40 miles and whose age is estimated to be nearly 2 billion years.

Debate over the Sudbury Structure's origin has raged for over a century. When the structure was first discovered, geologists thought that the structure was a batholith (a mass of magma that rose from the Earth's mantle and solidified before breaking through the surface of the Earth) and that the ores were concentrated through a process called magmatic differentiation in which the different constituents of the magma separated before the magma hardened. Over time, and with additional study, it was discovered that the composition of the ore at Sudbury did not exactly match that which would be expected from a simple magmatic differentiation, and a hydrothermal origin, in which hot solutions dissolve, concentrate, and redeposit the ore material, was considered by some geologists instead. Since neither theory completely explained all of the details that were observed in the Sudbury Structure, controversy between the proponents of the respective theories ensued.

In the early 1960's, with emerging interest in lunar- and astro-geology, geophysicists entertained the theory that the Sudbury Structure might have been formed by a meteorite impact. A glancing blow, it was believed, would account for the elongate shape of the structure. As part of the United States' space program, experiments were conducted in many aspects of astrophysics, including some in crater formation. One seemingly counter-intuitive result of these studies was the discovery that all projectile impacts -- even those with low angles of attack -- produce nearly circular impact craters.

In retrospect, this result shouldn't have been so surprising. All one has to do is look at the moon. The moon certainly contains a fair share of impact craters and, it is safe to assume, the meteors that produced those craters must have struck the lunar surface at all angles; however, an examination of the moon's surface reveals that almost all of the craters are circular, or nearly so.

Studies of the Arizona Meteor Crater, located near Winslow, have lent support to the experimental observation that even oblique meteoric impacts produce nearly circular craters. Prior to NASA's experiments, an attempt was made to find the meteorite that produced Meteor Crater. Since the crater was almost perfectly circular, it was reasoned that the meteorite must have come straight down -- perpendicular to the surface of the Earth -- and therefore the meteorite should be found directly beneath the center of the crater.

Drilling apparatus was carted to the bottom of Meteor Crater and drilling commenced; but after drilling down over one thousand feet to the underlying rock, no trace of any large meteorite was found. Based upon the results of several other bore holes plus an analysis of the distribution of ejected material outside the crater, scientists concluded that the meteorite had struck the Earth at an angle of about 45 degrees and then exploded, scattering fragments up to six miles outside the crater.

So, what does all this have to do with the Sudbury Structure? Well, if oblique meteorite impacts produce circular craters, then the Sudbury Structure, with its length nearly twice the measure of its width, could not be the result of a meteorite impact. Or could it?

As reported in the September 1992 issue of Geology magazine, a seismic profile undertaken by the Geological Survey of Canada has revealed the presence of numerous subsurface faults in the immediate area of the Sudbury Structure. The movement of these faults over the past two billion years has significantly deformed the structure from what the Canadian geophysicists believe may have been a circular shape. This finding, coupled with the occurrence of shocked minerals in the structure, lends new evidence to the theory that the Sudbury Structure and the ore deposits contained therein are the remains of an ancient meteorite crater. If this theory is true, it is estimated that the original crater measured approximately 125 miles in diameter, making the Sudbury Structure part of the largest impact crater discovered on the face of the Earth.

Copyright © 1993 by Fallbrook Gem and Mineral Society, Inc.

The preceding article was originally published in the February 1993 issue of Lithosphere, the official bulletin of the Fallbrook [California] Gem and Mineral Society, Inc; Richard Busch (Editor).

Permission to reproduce and distribute this material, in whole or in part, for non-commercial purposes, is hereby granted provided the sense or meaning of the material is not changed and the author's notice of copyright is retained.

Last updated: 18 September 2002