The distribution of volcanogenic massive sulphide deposits and implications for new base metal discoveries
Kimberley Heinneman and Evan Morris
Abstract of a paper presented at the Geological Association of Canada annual meeting, 2002
ABSTRACT
We have examined the distribution of Canadian VMS deposits according to depth, reserves and principal method of discovery. Our analysis is based on deposits that are or have at one time been actively mined. The largest VMS deposits that have been discovered either outcrop or are found extremely close to the surface. Approximately ninety percent of all VMS reserves are contained in such near-surface discoveries. It is highly unlikely very large VMS deposits are only found at the current erosional surface of the earth. A more reasonable inference is that very large deposits exist at moderate depths, but have not yet been discovered. These deeper deposits have not been located due to the inadequacy of current exploration tools.
Geophysical methods have been relatively unimportant in the discovery of outcropping deposits. Almost all of these deposits were found by prospecting. Most buried VMS deposits have been found by geophysics. Geophysical methods have been most successful in locating near-surface deposits, but relatively unsuccessful at detecting deeper deposits. Most deep VMS deposits have been discovered by geological means. Where deeper deposits have been discovered by geophysics, the principal method has been magnetics. However VMS deposits are not always accompanied by a concentration of magnetic minerals.
Our results indicate that airborne electromagnetics are useful in finding VMS deposits that are buried under a thin layer of overburden. They have been far less successful at discovering medium or large deposits at moderate depths. We believe that the greatest opportunity for discovering large, buried VMS deposits is through airborne gravity and gravity gradiometer surveys. Gravity surveys have a much greater depth penetration than EM surveys. When used jointly with an EM survey, gravity data can be used to eliminate conductive targets due to graphite and other non-economical sources.