Interpretation of magnetic susceptibility: a new approach to geophysical evaluation of the degree of rock alteration

Magnetic susceptibility measurements on cores from boreholes in the Eye–Dashwa lakes pluton, an Archean granitic pluton near Atikokan, Ontario, give a quantitative evaluation of the alteration level of these rocks. The histogram of the 5406 magnetic susceptibility measurements taken from 3.3 km of cores shows a multimodal distribution. In order to differentiate these subpopulations and to recognize their geological significance a numerical approach is employed. The magnetic susceptibility, when plotted on a log scale versus cumulative frequency on a probability scale, successfully differentiates the interval associated with each respective subpopulation. In the granite zone cut by the five boreholes, three distinct subpopulations are identified.The variation of magnetic susceptibility in the Eye–Dashwa lakes granitic cores studied is almost entirely associated with the oxidation state of the iron–titanium oxide minerals, since the variation in the total iron oxide content and the variation in grain size do not vary sufficiently to explain the large variation detected in the magnetic susceptibility logs. Hence, the variation in magnetic susceptibility is interpreted to be directly related to the alteration level seen in the rock (highly altered rock leads to lower magnetic susceptibility signal). Furthermore, the rocks characterized by the epidote group, the chlorite group, and the iron hydroxides, clays, and carbonate group each have a distinct magnetic susceptibility signature.By identifying the different levels of magnetic susceptibility values at this specific research area, the associated levels of alteration can be defined. Using this approach, a geologically interpreted geophysical log is produced. A three-dimensional representation of the five magnetic susceptibility bore-core logs is interpreted on the basis of similar magnetic susceptibility signature.The proposed approach has also been successfully applied to surface studies and is potentially useful in mineral exploration, especially in areas where mineralization is associated with fractured and altered rocks.