Towards Prediction Of Geological Drill Core Logs Using Geophysical Logs In Volcanic Stratigraphy

In base metal exploration projects in greenstone belts, geological logging of the drill core can be<br>difficult because different volcanic and volcaniclastic rocks are often visually similar. Geophysical<br>logging tools measure physical and chemical properties that are not visible and therefore complement<br>observations made in geological logs. The geophysical data can provide a more complete (e.g. no<br>missing drillcore) and objective geologically-related interpretation of the lithology intersected by the<br>drillholes and can be used as an aid to geological logging of the drill core.<br>The Borehole Geophysics Section of the Geological Survey of Canada has conducted multiparameter<br>geophysical logging in areas of massive sulphide exploration, for more than ten years. Excellent<br>correlation of geophysical logs with geological logs has been observed in boreholes associated with<br>volcanogenic massive sulphide deposits, at locations from Newfoundland to British Columbia. The<br>application of borehole geophysical logs to the problem of core logging in areas of volcanogenic<br>massive sulphide deposits, is illustrated with examples.<br>The first example is a geological hole-to-hole correlation problem at the McConnell Nickel Deposit<br>near Sudbury. A simple case of the obvious relation between geophysical logs and geological logs<br>at the Mudhole base metal prospect near Buchans, Newfoundland, is presented next. The third<br>example is from the Brazil Lake area of Nova Scotia, where the potential for development of a<br>pseudo-geological log based on the use of multiparameter logs is shown. In another example, in the<br>Kam-kotia Mine area near Timmins, Ontario, the pseudo-geological log was derived by using three<br>geophysical logs (gamma-ray, magnetic susceptibility and density). In all cases, the geological<br>interpretation from the geophysical logs was subjective, However, initial investigations suggest an<br>objective, computer-based method of deriving the pseudo-geological log, as a tool for the geologist,<br>is feasible. Some early trials of this semi-automated ‘pre-picking’ of the geology by computer, based<br>on the geophysical logs at the McConnell deposit, are presented, illustrating the difficulties and the<br>possibilities.