The pseudo-geological log: using geophysical logs as an aid to geological logging in volcanogenic massive sulphides

Abstract

Geophysical borehole logs, which measure the physical properties of the rocks surrounding the hole, can be used as an aid to geological core logging, because they are continuous with no missing sections, and because they show changes that are not detectable with the naked eye. Three examples of the application of borehole geophysical logs to the problem of core logging in areas of volcanogenic massive sulphide deposits, are discussed. The Mudhole prospect in Newfoundland is a simple case of the obvious relation between geophysical logs and geological logs. In the Buttle Lake area of British Columbia, the experience gained from the Mudhole prospect was used to derive a pseudo-geological log based on the gamma-ray log. The pseudo-geological log was improved further in the Kam-kotia mine area in Ontario by using three geophysical parameters. In all three cases, the geological interpretation from the geophysical logs was subjective. An objective, computer-based method of deriving the pseudo-geological log, as a tool for the geologist, is being investigated.

Combined 3‐D Interpretation of Airborne, Surface, and Borehole Vector Magnetics at the McConnell Nickel Deposit

“…Each lithology has a magnetic signature defined by the composition, grain size, and concentration of magnetic minerals. Lithologies such as basic flows and diabase dikes have higher concentrations of magnetic minerals and so can be easily detected by their increase in magnetic susceptibility (Killeen et al, 1995). In addition, anomalously low magnetic susceptibilities within a lithological unit are often indicative of hydrothermal alteration in which minerals such as magnetite are chemically altered to less magnetic phases by oxygen rich percolating fluids (Morris et al, 1984).…”

Combined 3‐D interpretation of airborne, surface, and borehole vector magnetics at the McConnell Nickel deposit

“…Particular lithologies have a magnetic signature defined by the composition, grain size, and concentration of magnetic minerals. Lithologies such as basic flows and diabase dikes have higher concentrations of magnetic minerals and so can be easily detected by their increase in magnetic susceptibility (Killeen et al, 1995). In addition, anomalously low magnetic susceptibilities within a lithological unit are often indicative of hydrothermal alteration in which minerals such as magnetite are chemically altered to less magnetic phases by oxygen rich percolating fluids (Morris et al, 1984).…”