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

Abstract

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