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.

Trace element concentrations in igneous rocks are frequently lognormally distributed, and Shaw (1961) suggested that minor minerals in igneous rocks might also be lognormally distributed. There is accumulating evidence to show that bulk magnetic susceptibility (BMS) often closely follows a lognormal distribution in fresh igneous rocks but, because BMS is dependent upon oxide-grain size and mineralogy, it is not obvious why this should be so. We have adopted Shaw's simple theoretical model as an argument to account for the lognormal distribution of BMS. The obvious prevailing conditions must be that the original melt was uniform, that pressure, temperature and oxygen fugacity were constant and that the minerals were formed at equilibrium. In addition, because BMS is also a function of grain size and mineralogy, the distribution of these latter parameters must be insensitive to changes in P , T , etc. at equilibrium. It appears that there must be a limit to the lognormal law in trying to apply it to minerals of greater concentration. However, BMS proxies for oxide content, and because it does closely follow the lognormal distribution it thus appears that, in crystalline igneous rocks, the lognormal law is obeyed by minerals of concentrations at least up to a few percent. Knowledge of the mean and variance of such distributions can be useful in a variety of petrological applications, especially in drillcore logging; a simple example is presented.

Bulk magnetic susceptibility (BMS) measurements have been made on granite drill cores from the St. George batholith (New Brunswick), the South Mountain batholith (Nova Scotia), and the Wedgeport pluton (Nova Scotia). The primary magnetite concentrations of the two Nova Scotia cores are statistically indistinguishable, thus lending support to the hypothesis that the Wedgeport pluton, despite being 50 Ma younger, is a satellite of the South Mountain batholith.The St. George core has a primary magnetite concentration over 30 times greater than the Nova Scotia cores, but low-temperature alteration (attributable to subsurface weathering) has greatly reduced its magnetite content. The two Nova Scotia S-type granites are shown to fall into the ilmenite-series category, whereas the St. George granite, which is either S- or A-type, is transitional between the magnetite and ilmenite series.The general observation of intergranular hematite and reduced BMS in the outcrops of some granites is suggested to have important consequences for primary oxidation studies and aeromagnetic interpretation.